[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (RFC 3920) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 RFC 6120

Network Working Group                                     P. Saint-Andre
Internet-Draft                                                     Cisco
Obsoletes: 3920 (if approved)                           October 23, 2009
Intended status: Standards Track
Expires: April 26, 2010


        Extensible Messaging and Presence Protocol (XMPP): Core
                       draft-ietf-xmpp-3920bis-03

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on April 26, 2010.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   This document defines the core features of the Extensible Messaging
   and Presence Protocol (XMPP), a technology for streaming Extensible



Saint-Andre              Expires April 26, 2010                 [Page 1]

Internet-Draft                  XMPP Core                   October 2009


   Markup Language (XML) elements for the purpose of exchanging
   structured information in close to real time between any two or more
   network-aware entities.  XMPP provides a generalized, extensible
   framework for incrementally exchanging XML data, upon which a variety
   of applications can be built.  The framework includes methods for
   stream setup and teardown, channel encryption, authentication of a
   client to a server and of one server to another server, and
   primitives for push-style messages, publication of network
   availability information ("presence"), and request-response
   interactions.  This document also specifies the format for XMPP
   addresses, which are fully internationalizable.

   This document obsoletes RFC 3920.


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   9
     1.1.   Overview . . . . . . . . . . . . . . . . . . . . . . . .   9
     1.2.   Functional Summary . . . . . . . . . . . . . . . . . . .  10
     1.3.   Conventions  . . . . . . . . . . . . . . . . . . . . . .  11
     1.4.   Acknowledgements . . . . . . . . . . . . . . . . . . . .  12
     1.5.   Discussion Venue . . . . . . . . . . . . . . . . . . . .  12
   2.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .  12
     2.1.   Global Addresses . . . . . . . . . . . . . . . . . . . .  13
     2.2.   Presence . . . . . . . . . . . . . . . . . . . . . . . .  13
     2.3.   Persistent Streams . . . . . . . . . . . . . . . . . . .  13
     2.4.   Structured Data  . . . . . . . . . . . . . . . . . . . .  14
     2.5.   Distributed Network  . . . . . . . . . . . . . . . . . .  14
   3.  Addresses . . . . . . . . . . . . . . . . . . . . . . . . . .  15
     3.1.   Overview . . . . . . . . . . . . . . . . . . . . . . . .  15
     3.2.   Domain Identifier  . . . . . . . . . . . . . . . . . . .  16
     3.3.   Localpart  . . . . . . . . . . . . . . . . . . . . . . .  18
     3.4.   Resource Identifier  . . . . . . . . . . . . . . . . . .  18
     3.5.   Determination of Addresses . . . . . . . . . . . . . . .  19
   4.  TCP Binding . . . . . . . . . . . . . . . . . . . . . . . . .  20
     4.1.   Scope  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     4.2.   Hostname Resolution  . . . . . . . . . . . . . . . . . .  20
     4.3.   Client-to-Server Communication . . . . . . . . . . . . .  21
     4.4.   Server-to-Server Communication . . . . . . . . . . . . .  22
     4.5.   Reconnection . . . . . . . . . . . . . . . . . . . . . .  22
     4.6.   Reliability  . . . . . . . . . . . . . . . . . . . . . .  23
     4.7.   Other Bindings . . . . . . . . . . . . . . . . . . . . .  23
   5.  XML Streams . . . . . . . . . . . . . . . . . . . . . . . . .  23
     5.1.   Overview . . . . . . . . . . . . . . . . . . . . . . . .  23
     5.2.   Stream Negotiation . . . . . . . . . . . . . . . . . . .  26
       5.2.1.   Overview . . . . . . . . . . . . . . . . . . . . . .  26
       5.2.2.   Stream Features Format . . . . . . . . . . . . . . .  26



Saint-Andre              Expires April 26, 2010                 [Page 2]

Internet-Draft                  XMPP Core                   October 2009


       5.2.3.   Restarts . . . . . . . . . . . . . . . . . . . . . .  28
       5.2.4.   Resending Features . . . . . . . . . . . . . . . . .  28
       5.2.5.   Completion of Stream Negotiation . . . . . . . . . .  29
       5.2.6.   State Chart  . . . . . . . . . . . . . . . . . . . .  29
     5.3.   Closing a Stream . . . . . . . . . . . . . . . . . . . .  31
       5.3.1.   With Stream Error  . . . . . . . . . . . . . . . . .  31
       5.3.2.   Without Stream Error . . . . . . . . . . . . . . . .  31
       5.3.3.   Handling of Idle Streams . . . . . . . . . . . . . .  31
     5.4.   Stream Attributes  . . . . . . . . . . . . . . . . . . .  32
       5.4.1.   from . . . . . . . . . . . . . . . . . . . . . . . .  33
       5.4.2.   to . . . . . . . . . . . . . . . . . . . . . . . . .  34
       5.4.3.   id . . . . . . . . . . . . . . . . . . . . . . . . .  35
       5.4.4.   xml:lang . . . . . . . . . . . . . . . . . . . . . .  36
       5.4.5.   version  . . . . . . . . . . . . . . . . . . . . . .  37
       5.4.6.   Summary of Stream Attributes . . . . . . . . . . . .  39
     5.5.   Namespace Declarations . . . . . . . . . . . . . . . . .  39
       5.5.1.   Declaration of Streams Namespace . . . . . . . . . .  39
       5.5.2.   Declaration of Default Namespace . . . . . . . . . .  39
       5.5.3.   Declaration of Other Namespaces  . . . . . . . . . .  40
     5.6.   Stream Errors  . . . . . . . . . . . . . . . . . . . . .  41
       5.6.1.   Rules  . . . . . . . . . . . . . . . . . . . . . . .  41
         5.6.1.1.  Stream Errors Are Unrecoverable . . . . . . . . .  41
         5.6.1.2.  Stream Errors Can Occur During Setup  . . . . . .  41
         5.6.1.3.  Stream Errors When the Host is Unspecified or
                   Unknown . . . . . . . . . . . . . . . . . . . . .  42
         5.6.1.4.  Where Stream Errors Are Sent  . . . . . . . . . .  43
       5.6.2.   Syntax . . . . . . . . . . . . . . . . . . . . . . .  43
       5.6.3.   Defined Stream Error Conditions  . . . . . . . . . .  44
         5.6.3.1.  bad-format  . . . . . . . . . . . . . . . . . . .  44
         5.6.3.2.  bad-namespace-prefix  . . . . . . . . . . . . . .  45
         5.6.3.3.  conflict  . . . . . . . . . . . . . . . . . . . .  46
         5.6.3.4.  connection-timeout  . . . . . . . . . . . . . . .  46
         5.6.3.5.  host-gone . . . . . . . . . . . . . . . . . . . .  47
         5.6.3.6.  host-unknown  . . . . . . . . . . . . . . . . . .  47
         5.6.3.7.  improper-addressing . . . . . . . . . . . . . . .  48
         5.6.3.8.  internal-server-error . . . . . . . . . . . . . .  48
         5.6.3.9.  invalid-from  . . . . . . . . . . . . . . . . . .  49
         5.6.3.10. invalid-id  . . . . . . . . . . . . . . . . . . .  49
         5.6.3.11. invalid-namespace . . . . . . . . . . . . . . . .  50
         5.6.3.12. invalid-xml . . . . . . . . . . . . . . . . . . .  50
         5.6.3.13. not-authorized  . . . . . . . . . . . . . . . . .  51
         5.6.3.14. policy-violation  . . . . . . . . . . . . . . . .  52
         5.6.3.15. remote-connection-failed  . . . . . . . . . . . .  53
         5.6.3.16. resource-constraint . . . . . . . . . . . . . . .  53
         5.6.3.17. restricted-xml  . . . . . . . . . . . . . . . . .  54
         5.6.3.18. see-other-host  . . . . . . . . . . . . . . . . .  54
         5.6.3.19. system-shutdown . . . . . . . . . . . . . . . . .  55
         5.6.3.20. undefined-condition . . . . . . . . . . . . . . .  56



Saint-Andre              Expires April 26, 2010                 [Page 3]

Internet-Draft                  XMPP Core                   October 2009


         5.6.3.21. unsupported-encoding  . . . . . . . . . . . . . .  56
         5.6.3.22. unsupported-feature . . . . . . . . . . . . . . .  57
         5.6.3.23. unsupported-stanza-type . . . . . . . . . . . . .  57
         5.6.3.24. unsupported-version . . . . . . . . . . . . . . .  58
         5.6.3.25. xml-not-well-formed . . . . . . . . . . . . . . .  59
       5.6.4.   Application-Specific Conditions  . . . . . . . . . .  60
     5.7.   Simplified Stream Examples . . . . . . . . . . . . . . .  60
   6.  STARTTLS Negotiation  . . . . . . . . . . . . . . . . . . . .  62
     6.1.   Overview . . . . . . . . . . . . . . . . . . . . . . . .  63
     6.2.   Rules  . . . . . . . . . . . . . . . . . . . . . . . . .  63
       6.2.1.   Mandatory-to-Negotiate . . . . . . . . . . . . . . .  63
       6.2.2.   Restart  . . . . . . . . . . . . . . . . . . . . . .  63
       6.2.3.   Data Formatting  . . . . . . . . . . . . . . . . . .  63
       6.2.4.   Order of Negotiation . . . . . . . . . . . . . . . .  64
     6.3.   Process  . . . . . . . . . . . . . . . . . . . . . . . .  64
       6.3.1.   Exchange of Stream Headers and Stream Features . . .  64
       6.3.2.   Initiation of STARTTLS Negotiation . . . . . . . . .  65
         6.3.2.1.  STARTTLS Command  . . . . . . . . . . . . . . . .  65
         6.3.2.2.  Failure Case  . . . . . . . . . . . . . . . . . .  65
         6.3.2.3.  Proceed Case  . . . . . . . . . . . . . . . . . .  66
       6.3.3.   TLS Negotiation  . . . . . . . . . . . . . . . . . .  66
         6.3.3.1.  Rules . . . . . . . . . . . . . . . . . . . . . .  66
         6.3.3.2.  TLS Failure . . . . . . . . . . . . . . . . . . .  67
         6.3.3.3.  TLS Success . . . . . . . . . . . . . . . . . . .  67
   7.  SASL Negotiation  . . . . . . . . . . . . . . . . . . . . . .  68
     7.1.   Overview . . . . . . . . . . . . . . . . . . . . . . . .  68
     7.2.   Rules  . . . . . . . . . . . . . . . . . . . . . . . . .  68
       7.2.1.   Mandatory-to-Negotiate . . . . . . . . . . . . . . .  68
       7.2.2.   Restart  . . . . . . . . . . . . . . . . . . . . . .  68
       7.2.3.   Mechanism Preferences  . . . . . . . . . . . . . . .  69
       7.2.4.   Mechanism Offers . . . . . . . . . . . . . . . . . .  69
       7.2.5.   Data Formatting  . . . . . . . . . . . . . . . . . .  69
       7.2.6.   Security Layers  . . . . . . . . . . . . . . . . . .  70
       7.2.7.   Simple Usernames . . . . . . . . . . . . . . . . . .  70
       7.2.8.   Authorization Identities . . . . . . . . . . . . . .  70
       7.2.9.   Realms . . . . . . . . . . . . . . . . . . . . . . .  71
       7.2.10.  Round Trips  . . . . . . . . . . . . . . . . . . . .  71
     7.3.   Process  . . . . . . . . . . . . . . . . . . . . . . . .  72
       7.3.1.   Exchange of Stream Headers and Stream Features . . .  72
       7.3.2.   Initiation . . . . . . . . . . . . . . . . . . . . .  73
       7.3.3.   Challenge-Response Sequence  . . . . . . . . . . . .  73
       7.3.4.   Abort  . . . . . . . . . . . . . . . . . . . . . . .  74
       7.3.5.   Failure  . . . . . . . . . . . . . . . . . . . . . .  74
       7.3.6.   Success  . . . . . . . . . . . . . . . . . . . . . .  75
     7.4.   SASL Errors  . . . . . . . . . . . . . . . . . . . . . .  76
       7.4.1.   aborted  . . . . . . . . . . . . . . . . . . . . . .  76
       7.4.2.   account-disabled . . . . . . . . . . . . . . . . . .  77
       7.4.3.   credentials-expired  . . . . . . . . . . . . . . . .  77



Saint-Andre              Expires April 26, 2010                 [Page 4]

Internet-Draft                  XMPP Core                   October 2009


       7.4.4.   encryption-required  . . . . . . . . . . . . . . . .  77
       7.4.5.   incorrect-encoding . . . . . . . . . . . . . . . . .  77
       7.4.6.   invalid-authzid  . . . . . . . . . . . . . . . . . .  78
       7.4.7.   invalid-mechanism  . . . . . . . . . . . . . . . . .  78
       7.4.8.   malformed-request  . . . . . . . . . . . . . . . . .  78
       7.4.9.   mechanism-too-weak . . . . . . . . . . . . . . . . .  79
       7.4.10.  not-authorized . . . . . . . . . . . . . . . . . . .  79
       7.4.11.  temporary-auth-failure . . . . . . . . . . . . . . .  79
       7.4.12.  transition-needed  . . . . . . . . . . . . . . . . .  80
     7.5.   SASL Definition  . . . . . . . . . . . . . . . . . . . .  80
   8.  Resource Binding  . . . . . . . . . . . . . . . . . . . . . .  81
     8.1.   Overview . . . . . . . . . . . . . . . . . . . . . . . .  81
     8.2.   Rules  . . . . . . . . . . . . . . . . . . . . . . . . .  82
       8.2.1.   Mandatory-to-Negotiate . . . . . . . . . . . . . . .  82
       8.2.2.   Restart  . . . . . . . . . . . . . . . . . . . . . .  82
     8.3.   Advertising Support  . . . . . . . . . . . . . . . . . .  82
     8.4.   Generation of Resource Identifiers . . . . . . . . . . .  82
     8.5.   Server-Generated Resource Identifier . . . . . . . . . .  83
       8.5.1.   Success Case . . . . . . . . . . . . . . . . . . . .  83
       8.5.2.   Error Cases  . . . . . . . . . . . . . . . . . . . .  83
         8.5.2.1.  Resource Constraint . . . . . . . . . . . . . . .  84
         8.5.2.2.  Not Allowed . . . . . . . . . . . . . . . . . . .  84
     8.6.   Client-Submitted Resource Identifier . . . . . . . . . .  84
       8.6.1.   Success Case . . . . . . . . . . . . . . . . . . . .  84
       8.6.2.   Error Cases  . . . . . . . . . . . . . . . . . . . .  85
         8.6.2.1.  Bad Request . . . . . . . . . . . . . . . . . . .  85
         8.6.2.2.  Conflict  . . . . . . . . . . . . . . . . . . . .  85
       8.6.3.   Retries  . . . . . . . . . . . . . . . . . . . . . .  86
   9.  XML Stanzas . . . . . . . . . . . . . . . . . . . . . . . . .  86
     9.1.   Common Attributes  . . . . . . . . . . . . . . . . . . .  87
       9.1.1.   to . . . . . . . . . . . . . . . . . . . . . . . . .  87
         9.1.1.1.  Client-to-Server Streams  . . . . . . . . . . . .  87
         9.1.1.2.  Server-to-Server Streams  . . . . . . . . . . . .  88
       9.1.2.   from . . . . . . . . . . . . . . . . . . . . . . . .  88
         9.1.2.1.  Client-to-Server Streams  . . . . . . . . . . . .  88
         9.1.2.2.  Server-to-Server Streams  . . . . . . . . . . . .  89
       9.1.3.   id . . . . . . . . . . . . . . . . . . . . . . . . .  89
       9.1.4.   type . . . . . . . . . . . . . . . . . . . . . . . .  90
       9.1.5.   xml:lang . . . . . . . . . . . . . . . . . . . . . .  90
     9.2.   Basic Semantics  . . . . . . . . . . . . . . . . . . . .  91
       9.2.1.   Message Semantics  . . . . . . . . . . . . . . . . .  91
       9.2.2.   Presence Semantics . . . . . . . . . . . . . . . . .  91
       9.2.3.   IQ Semantics . . . . . . . . . . . . . . . . . . . .  92
     9.3.   Stanza Errors  . . . . . . . . . . . . . . . . . . . . .  93
       9.3.1.   Rules  . . . . . . . . . . . . . . . . . . . . . . .  93
       9.3.2.   Syntax . . . . . . . . . . . . . . . . . . . . . . .  94
       9.3.3.   Defined Conditions . . . . . . . . . . . . . . . . .  95
         9.3.3.1.  bad-request . . . . . . . . . . . . . . . . . . .  95



Saint-Andre              Expires April 26, 2010                 [Page 5]

Internet-Draft                  XMPP Core                   October 2009


         9.3.3.2.  conflict  . . . . . . . . . . . . . . . . . . . .  96
         9.3.3.3.  feature-not-implemented . . . . . . . . . . . . .  96
         9.3.3.4.  forbidden . . . . . . . . . . . . . . . . . . . .  97
         9.3.3.5.  gone  . . . . . . . . . . . . . . . . . . . . . .  97
         9.3.3.6.  internal-server-error . . . . . . . . . . . . . .  98
         9.3.3.7.  item-not-found  . . . . . . . . . . . . . . . . .  98
         9.3.3.8.  jid-malformed . . . . . . . . . . . . . . . . . .  98
         9.3.3.9.  not-acceptable  . . . . . . . . . . . . . . . . .  99
         9.3.3.10. not-allowed . . . . . . . . . . . . . . . . . . .  99
         9.3.3.11. not-authorized  . . . . . . . . . . . . . . . . . 100
         9.3.3.12. not-modified  . . . . . . . . . . . . . . . . . . 100
         9.3.3.13. payment-required  . . . . . . . . . . . . . . . . 101
         9.3.3.14. policy-violation  . . . . . . . . . . . . . . . . 102
         9.3.3.15. recipient-unavailable . . . . . . . . . . . . . . 102
         9.3.3.16. redirect  . . . . . . . . . . . . . . . . . . . . 103
         9.3.3.17. registration-required . . . . . . . . . . . . . . 104
         9.3.3.18. remote-server-not-found . . . . . . . . . . . . . 104
         9.3.3.19. remote-server-timeout . . . . . . . . . . . . . . 104
         9.3.3.20. resource-constraint . . . . . . . . . . . . . . . 105
         9.3.3.21. service-unavailable . . . . . . . . . . . . . . . 105
         9.3.3.22. subscription-required . . . . . . . . . . . . . . 106
         9.3.3.23. undefined-condition . . . . . . . . . . . . . . . 106
         9.3.3.24. unexpected-request  . . . . . . . . . . . . . . . 107
       9.3.4.   Application-Specific Conditions  . . . . . . . . . . 108
     9.4.   Extended Content . . . . . . . . . . . . . . . . . . . . 109
     9.5.   Stanza Size  . . . . . . . . . . . . . . . . . . . . . . 111
   10. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . . 111
     10.1.  Client-to-Server . . . . . . . . . . . . . . . . . . . . 111
       10.1.1.  TLS  . . . . . . . . . . . . . . . . . . . . . . . . 112
       10.1.2.  SASL . . . . . . . . . . . . . . . . . . . . . . . . 113
       10.1.3.  Resource Binding . . . . . . . . . . . . . . . . . . 114
       10.1.4.  Stanza Exchange  . . . . . . . . . . . . . . . . . . 115
       10.1.5.  Close  . . . . . . . . . . . . . . . . . . . . . . . 116
     10.2.  Server-to-Server Examples  . . . . . . . . . . . . . . . 116
       10.2.1.  TLS  . . . . . . . . . . . . . . . . . . . . . . . . 116
       10.2.2.  SASL . . . . . . . . . . . . . . . . . . . . . . . . 118
       10.2.3.  Stanza Exchange  . . . . . . . . . . . . . . . . . . 119
       10.2.4.  Close  . . . . . . . . . . . . . . . . . . . . . . . 120
   11. Server Rules for Processing XML Stanzas . . . . . . . . . . . 120
     11.1.  No 'to' Address  . . . . . . . . . . . . . . . . . . . . 120
       11.1.1.  Overview . . . . . . . . . . . . . . . . . . . . . . 120
       11.1.2.  Message  . . . . . . . . . . . . . . . . . . . . . . 121
       11.1.3.  Presence . . . . . . . . . . . . . . . . . . . . . . 121
       11.1.4.  IQ . . . . . . . . . . . . . . . . . . . . . . . . . 121
     11.2.  Local Domain . . . . . . . . . . . . . . . . . . . . . . 121
       11.2.1.  Mere Domain  . . . . . . . . . . . . . . . . . . . . 122
       11.2.2.  Domain with Resource . . . . . . . . . . . . . . . . 122
       11.2.3.  Localpart at Domain  . . . . . . . . . . . . . . . . 122



Saint-Andre              Expires April 26, 2010                 [Page 6]

Internet-Draft                  XMPP Core                   October 2009


         11.2.3.1. No Such User  . . . . . . . . . . . . . . . . . . 122
         11.2.3.2. Bare JID  . . . . . . . . . . . . . . . . . . . . 122
         11.2.3.3. Full JID  . . . . . . . . . . . . . . . . . . . . 123
     11.3.  Remote Domain  . . . . . . . . . . . . . . . . . . . . . 123
       11.3.1.  Existing Stream  . . . . . . . . . . . . . . . . . . 123
       11.3.2.  No Existing Stream . . . . . . . . . . . . . . . . . 123
       11.3.3.  Error Handling . . . . . . . . . . . . . . . . . . . 124
   12. XML Usage . . . . . . . . . . . . . . . . . . . . . . . . . . 124
     12.1.  Restrictions . . . . . . . . . . . . . . . . . . . . . . 124
     12.2.  XML Namespace Names and Prefixes . . . . . . . . . . . . 125
     12.3.  Well-Formedness  . . . . . . . . . . . . . . . . . . . . 125
     12.4.  Validation . . . . . . . . . . . . . . . . . . . . . . . 125
     12.5.  Inclusion of XML Declaration . . . . . . . . . . . . . . 126
     12.6.  Character Encoding . . . . . . . . . . . . . . . . . . . 126
     12.7.  Whitespace . . . . . . . . . . . . . . . . . . . . . . . 126
     12.8.  XML Versions . . . . . . . . . . . . . . . . . . . . . . 126
   13. Internationalization Considerations . . . . . . . . . . . . . 126
   14. Security Considerations . . . . . . . . . . . . . . . . . . . 127
     14.1.  High Security  . . . . . . . . . . . . . . . . . . . . . 127
     14.2.  Certificates . . . . . . . . . . . . . . . . . . . . . . 127
       14.2.1.  Certificate Generation . . . . . . . . . . . . . . . 128
         14.2.1.1. General Considerations  . . . . . . . . . . . . . 128
         14.2.1.2. Server Certificates . . . . . . . . . . . . . . . 129
         14.2.1.3. Client Certificates . . . . . . . . . . . . . . . 131
         14.2.1.4. ASN.1 Object Identifier . . . . . . . . . . . . . 131
       14.2.2.  Certificate Validation . . . . . . . . . . . . . . . 132
         14.2.2.1. Server Certificates . . . . . . . . . . . . . . . 132
         14.2.2.2. Client Certificates . . . . . . . . . . . . . . . 134
         14.2.2.3. Use of Certificates in XMPP Extensions  . . . . . 135
     14.3.  Client-to-Server Communication . . . . . . . . . . . . . 135
     14.4.  Server-to-Server Communication . . . . . . . . . . . . . 135
     14.5.  Order of Layers  . . . . . . . . . . . . . . . . . . . . 136
     14.6.  Mandatory-to-Implement Technologies  . . . . . . . . . . 136
     14.7.  Hash Function Agility  . . . . . . . . . . . . . . . . . 137
     14.8.  SASL Downgrade Attacks . . . . . . . . . . . . . . . . . 137
     14.9.  Lack of SASL Channel Binding to TLS  . . . . . . . . . . 137
     14.10. Use of base64 in SASL  . . . . . . . . . . . . . . . . . 138
     14.11. Stringprep Profiles  . . . . . . . . . . . . . . . . . . 138
     14.12. Address Spoofing . . . . . . . . . . . . . . . . . . . . 139
       14.12.1. Address Forging  . . . . . . . . . . . . . . . . . . 139
       14.12.2. Address Mimicking  . . . . . . . . . . . . . . . . . 140
     14.13. Firewalls  . . . . . . . . . . . . . . . . . . . . . . . 141
     14.14. Denial of Service  . . . . . . . . . . . . . . . . . . . 141
     14.15. Presence Leaks . . . . . . . . . . . . . . . . . . . . . 142
     14.16. Directory Harvesting . . . . . . . . . . . . . . . . . . 143
   15. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 143
     15.1.  XML Namespace Name for TLS Data  . . . . . . . . . . . . 143
     15.2.  XML Namespace Name for SASL Data . . . . . . . . . . . . 143



Saint-Andre              Expires April 26, 2010                 [Page 7]

Internet-Draft                  XMPP Core                   October 2009


     15.3.  XML Namespace Name for Stream Errors . . . . . . . . . . 144
     15.4.  XML Namespace Name for Resource Binding  . . . . . . . . 144
     15.5.  XML Namespace Name for Stanza Errors . . . . . . . . . . 144
     15.6.  Nodeprep Profile of Stringprep . . . . . . . . . . . . . 145
     15.7.  Resourceprep Profile of Stringprep . . . . . . . . . . . 145
     15.8.  GSSAPI Service Name  . . . . . . . . . . . . . . . . . . 145
     15.9.  Port Numbers . . . . . . . . . . . . . . . . . . . . . . 145
   16. Conformance Requirements  . . . . . . . . . . . . . . . . . . 146
   17. References  . . . . . . . . . . . . . . . . . . . . . . . . . 156
     17.1.  Normative References . . . . . . . . . . . . . . . . . . 156
     17.2.  Informative References . . . . . . . . . . . . . . . . . 158
   Appendix A.  Nodeprep . . . . . . . . . . . . . . . . . . . . . . 163
     A.1.   Introduction . . . . . . . . . . . . . . . . . . . . . . 163
     A.2.   Character Repertoire . . . . . . . . . . . . . . . . . . 163
     A.3.   Mapping  . . . . . . . . . . . . . . . . . . . . . . . . 163
     A.4.   Normalization  . . . . . . . . . . . . . . . . . . . . . 163
     A.5.   Prohibited Output  . . . . . . . . . . . . . . . . . . . 164
     A.6.   Bidirectional Characters . . . . . . . . . . . . . . . . 164
     A.7.   Notes  . . . . . . . . . . . . . . . . . . . . . . . . . 164
   Appendix B.  Resourceprep . . . . . . . . . . . . . . . . . . . . 165
     B.1.   Introduction . . . . . . . . . . . . . . . . . . . . . . 165
     B.2.   Character Repertoire . . . . . . . . . . . . . . . . . . 166
     B.3.   Mapping  . . . . . . . . . . . . . . . . . . . . . . . . 166
     B.4.   Normalization  . . . . . . . . . . . . . . . . . . . . . 166
     B.5.   Prohibited Output  . . . . . . . . . . . . . . . . . . . 166
     B.6.   Bidirectional Characters . . . . . . . . . . . . . . . . 166
   Appendix C.  XML Schemas  . . . . . . . . . . . . . . . . . . . . 166
     C.1.   Streams Namespace  . . . . . . . . . . . . . . . . . . . 167
     C.2.   Stream Error Namespace . . . . . . . . . . . . . . . . . 168
     C.3.   STARTTLS Namespace . . . . . . . . . . . . . . . . . . . 170
     C.4.   SASL Namespace . . . . . . . . . . . . . . . . . . . . . 170
     C.5.   Resource Binding Namespace . . . . . . . . . . . . . . . 173
     C.6.   Stanza Error Namespace . . . . . . . . . . . . . . . . . 173
   Appendix D.  Contact Addresses  . . . . . . . . . . . . . . . . . 175
   Appendix E.  Account Provisioning . . . . . . . . . . . . . . . . 175
   Appendix F.  Differences From RFC 3920  . . . . . . . . . . . . . 176
   Appendix G.  Copying Conditions . . . . . . . . . . . . . . . . . 177
   Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . 178












Saint-Andre              Expires April 26, 2010                 [Page 8]

Internet-Draft                  XMPP Core                   October 2009


1.  Introduction

1.1.  Overview

   The Extensible Messaging and Presence Protocol (XMPP) is an
   application profile of the Extensible Markup Language [XML] for
   streaming XML data in close to real time between any two (or more)
   network-aware entities.  XMPP is typically used to exchange messages,
   share presence information, and engage in structured request-response
   interactions.  The basic syntax and semantics of XMPP were developed
   originally within the Jabber open-source community, mainly in 1999.
   In late 2002, the XMPP Working Group was chartered with developing an
   adaptation of the core Jabber protocol that would be suitable as an
   IETF instant messaging (IM) and presence technology.  As a result of
   work by the XMPP WG, [RFC3920] and [RFC3921] were published in
   October 2004, representing the most complete definition of XMPP at
   that time.

   As a result of extensive implementation and deployment experience
   with XMPP since 2004, as well as more formal interoperability testing
   carried out under the auspices of the XMPP Standards Foundation
   (XSF), this document reflects consensus from the XMPP developer
   community regarding XMPP's core XML streaming technology.  In
   particular, this document incorporates the following backward-
   compatible changes from RFC 3920:

   o  Incorporated corrections and errata
   o  Added examples throughout
   o  Clarified and more completely specified matters that were
      underspecified
   o  Modified text to reflect updated technologies for which XMPP is a
      using protocol, e.g., Transport Layer Security (TLS) and the
      Simple Authentication and Security Layer (SASL)
   o  Defined several additional stream, stanza, and SASL error
      conditions
   o  Removed the deprecated DIGEST-MD5 SASL mechanism [DIGEST-MD5] as a
      mandatory-to-implement technology
   o  Added the TLS plus the SASL PLAIN mechanism [PLAIN] as a
      mandatory-to-implement technology
   o  Defined of optional support for multiple resources over the same
      connection
   o  Transferred historical documentation for the server dialback
      protocol from this specification to a separate specification

   Therefore, this document defines the core features of XMPP 1.0, thus
   obsoleting RFC 3920.





Saint-Andre              Expires April 26, 2010                 [Page 9]

Internet-Draft                  XMPP Core                   October 2009


      Note: [XMPP-IM] defines the XMPP features needed to provide the
      basic instant messaging and presence functionality that is
      described in [IMP-REQS].

1.2.  Functional Summary

   This non-normative section provides a developer-friendly, functional
   summary of XMPP; refer to the sections that follow for a normative
   definition of XMPP.

   The purpose of XMPP is to enable the exchange of relatively small
   pieces of structured data (called "XML stanzas") over a network
   between any two (or more) entities.  XMPP is implemented using a
   client-server architecture, wherein a client needs to connect to a
   server in order to gain access to the network and thus be allowed to
   exchange XML stanzas with other entities (which can be associated
   with other servers).  The process whereby a client connects to a
   server, exchanges XML stanzas, and ends the connection is:

   1.  Determine the hostname and port at which to connect
   2.  Open a TCP connection
   3.  Open an XML stream
   4.  Complete TLS negotiation for channel encryption (recommended)
   5.  Complete SASL negotiation for authentication
   6.  Bind a resource to the stream
   7.  Exchange an unbounded number of XML stanzas with other entities
       on the network
   8.  Close the XML stream
   9.  Close the TCP connection

   Within XMPP, one server can optionally connect to another server to
   enable inter-domain or inter-server communication.  For this to
   happen, the two servers need to negotiate a connection between
   themselves and then exchange XML stanzas; the process for doing so
   is:

   1.  Determine the hostname and port at which to connect
   2.  Open a TCP connection
   3.  Open an XML stream
   4.  Complete TLS negotiation for channel encryption (recommended)
   5.  Complete SASL negotiation for authentication *
   6.  Exchange an unbounded number of XML stanzas both directly for the
       servers and indirectly on behalf of entities associated with each
       server (e.g., connected clients)
   7.  Close the XML stream
   8.  Close the TCP connection





Saint-Andre              Expires April 26, 2010                [Page 10]

Internet-Draft                  XMPP Core                   October 2009


      * Note: Depending on local service policies, it is possible that a
      deployed server will use the older server dialback protocol to
      provide weak identity verification in cases where SASL negotiation
      would not result in strong authentication (e.g., because TLS
      negotiation was not mandated by the peer server, or because the
      certificate presented by the peer server during TLS negotiation is
      self-signed and thus provides only weak identity); for details,
      see [XEP-0220].

   In the sections following discussion of XMPP architecture and XMPP
   addresses, this document specifies how clients connect to servers and
   specifies the basic semantics of XML stanzas.  However, this document
   does not define the "payloads" of the XML stanzas that might be
   exchanged once a connection is successfully established; instead,
   those payloads are defined by various XMPP extensions.  For example,
   [XMPP-IM] defines extensions for basic instant messaging and presence
   functionality.  In addition, various specifications produced in the
   XSF's XEP series [XEP-0001] define extensions for a wide range of
   more advanced functionality.

1.3.  Conventions

   The following capitalized keywords are to be interpreted as described
   in [TERMS]: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT";
   "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY",
   "OPTIONAL".

   Certain security-related terms are to be understood in the sense
   defined in [SECTERMS]; such terms include, but are not limited to,
   "assurance", "attack", "authentication", "authorization",
   "certificate", "certification authority", "confidentiality",
   "credential", "downgrade", "encryption", "fingerprint", "hash value",
   "identity", "integrity", "signature", "security perimeter", "self-
   signed certificate", "sign", "spoof", "tamper", "trust", "trust
   anchor", "trust chain", "validate", "verify".  Other security-related
   terms (for example, "denial of service") are to be understood in the
   sense defined in the referenced specifications.

   The term "whitespace" is used to refer to any character that matches
   production [3] content of [XML], i.e., any instance of SP, HT, CR,
   and LF.

   Following the "XML Notation" used in [IRI] to represent characters
   that cannot be rendered in ASCII-only documents, some examples in
   this document use the form "&#x...." as a notational device to
   represent Unicode characters (e.g., the string "ř" stands for
   the Unicode character LATIN SMALL LETTER R WITH CARON).




Saint-Andre              Expires April 26, 2010                [Page 11]

Internet-Draft                  XMPP Core                   October 2009


   In examples, lines have been wrapped for improved readability,
   "[...]" means elision, and the following prepended strings are used
   (these prepended strings are not to be sent over the wire):

   o  C: = a client
   o  E: = any XMPP entity
   o  I: = an initiating entity
   o  P: = a peer server
   o  R: = a receiving entity
   o  S: = a server
   o  S1: = server1
   o  S2: = server2

1.4.  Acknowledgements

   The editor of this document finds it impossible to appropriately
   acknowledge the many individuals who have provided comments regarding
   the protocols defined herein.  However, thanks are due to those who
   have who have provided implementation feedback, bug reports, requests
   for clarification, and suggestions for improvement since the
   publication of the RFC this document supersedes.  The editor has
   endeavored to address all such feedback, but is solely responsible
   for any remaining errors and ambiguities.

1.5.  Discussion Venue

   [[ RFC Editor: please remove this section. ]]

   The document editor and the broader XMPP developer community welcome
   discussion and comments related to the topics presented in this
   document.  The primary and preferred venue is the <xmpp@ietf.org>
   mailing list, for which archives and subscription information are
   available at <https://www.ietf.org/mailman/listinfo/xmpp>.  Related
   discussions often occur on the <standards@xmpp.org> mailing list, for
   which archives and subscription information are available at
   <http://mail.jabber.org/mailman/listinfo/standards>.


2.  Architecture

   XMPP provides a technology for the asynchronous, end-to-end exchange
   of structured data by means of direct, persistent XML streams among a
   distributed network of globally-addressable, presence-aware clients
   and servers.  Because this architectural style involves ubiquitous
   knowledge of network availability and a conceptually unlimited number
   of concurrent information transactions in the context of a given
   client-to-server or server-to-server session, we label it
   "Availability for Concurrent Transactions" (ACT) to distinguish it



Saint-Andre              Expires April 26, 2010                [Page 12]

Internet-Draft                  XMPP Core                   October 2009


   from the "Representational State Transfer" [REST] architectural style
   familiar from the World Wide Web. Although the architecture of XMPP
   is similar in important ways to that of email (see [EMAIL-ARCH]), it
   introduces several modifications to facilitate communication in close
   to real time.  The salient features of this ACTive architectural
   style are as follows.

2.1.  Global Addresses

   As with email, XMPP uses globally-unique addresses (based on the
   Domain Name System) in order to route and deliver messages over the
   network.  All XMPP entities are addressable on the network, most
   particularly clients and servers but also various additional services
   that can be accessed by clients and servers.  In general, server
   addresses are of the form "domain.tld" (e.g., "im.example.com"),
   accounts hosted at a server are of the form "localpart@domain.tld"
   (e.g., "juliet@im.example.com"), and a particular connected device or
   resource that is currently authorized for interaction on behalf of an
   account is of the form "localpart@domain.tld/resource" (e.g.,
   "juliet@im.example.com/balcony").  XMPP addresses are defined under
   Section 3.

2.2.  Presence

   XMPP includes the ability for an entity to advertise its network
   availability or "presence" to other entities.  Such availability for
   communication is signalled end-to-end via dedicated communication
   primitives in XMPP (the <presence/> stanza).  Although knowledge of
   network availability is not strictly necessary for the exchange of
   XMPP messages, it facilitates real-time interaction because the
   originator of a message can know before initiating communication that
   the intended recipient is online and available.  XMPP presence is
   defined in [XMPP-IM].

2.3.  Persistent Streams

   Availability for communication is also built into point-to-point
   connections (e.g., a discrete client-to-server or server-to-server
   connection) through the use of direct, persistent XML streams between
   the entity that initiated the connection (either a client or a
   server) and the entity that received the connection (a server).  Thus
   either party to a stream knows that it can immediately push data to
   the other party for immediate routing or delivery.  XML streams are
   defined under Section 5.







Saint-Andre              Expires April 26, 2010                [Page 13]

Internet-Draft                  XMPP Core                   October 2009


2.4.  Structured Data

   The basic unit of meaning in XMPP is not an XML stream (which simply
   provides the transport for point-to-point communication) but an XML
   "stanza", which is essentially a fragment of XML that is sent over a
   stream.  The root element of a stanza includes routing attributes
   (such as "from" and "to" addresses) and the child elements of the
   stanza contain a payload for delivery to the intended recipient.  XML
   stanzas are defined under Section 9.

2.5.  Distributed Network

   In practice, XMPP consists of a network of clients and servers that
   inter-communicate (however, communication between any two given
   deployed servers is strictly OPTIONAL).  Thus, for example, the user
   <juliet@im.example.com> associated with the server <im.example.com>
   might be able to exchange messages, presence, and other structured
   data with the user <romeo@example.net> associated with the server
   <example.net>.  This pattern is familiar from messaging protocols
   that make use of global addresses, such as the email network (see
   [SMTP] and [EMAIL-ARCH]).  As a result, end-to-end communication in
   XMPP is logically peer-to-peer but physically client-to-server-to-
   server-to-client, as illustrated in the following diagram.

     example.net ---------------- im.example.com
        |                                |
        |                                |
   romeo@example.net           juliet@im.example.com


      Note: Architectures that employ XML streams (Section 5) and XML
      stanzas (Section 9) but that establish peer-to-peer connections
      directly between clients using technologies based on [LINKLOCAL]
      have been deployed, but such architectures are not described in
      this specification and are best described as "XMPP-like"; for
      details, see [XEP-0174].  In addition, XML streams can be
      established end-to-end over any reliable transport, including
      extensions to XMPP itself; however, such methods are out of scope
      for this specification.

   The following paragraphs describe the responsibilities of clients and
   servers on the network.

   A CLIENT is an entity that establishes an XML stream with a server by
   authenticating using the credentials of a local account and that then
   completes resource binding (Section 8) in order to enable delivery of
   XML stanzas between the server and the client over the negotiated
   stream.  The client then uses XMPP to communicate with its server,



Saint-Andre              Expires April 26, 2010                [Page 14]

Internet-Draft                  XMPP Core                   October 2009


   other clients, and any other entities on the network, where the
   server is responsible for delivering stanzas to local entities or
   routing them to remote entities.  Multiple clients can connect
   simultaneously to a server on behalf of the same local account, where
   each client is differentiated by the resource identifier portion of
   an XMPP address (e.g., <localpart@domain/home> vs.
   <localpart@domain/work>), as defined under Section 3 and Section 8.

   A SERVER is an entity whose primary responsibilities are to:

   o  Manage XML streams (Section 5) with local clients and deliver XML
      stanzas (Section 9) to those clients over the negotiated streams;
      this includes responsibility for ensuring that a client needs to
      authenticate with the server before being granted access to the
      XMPP network.
   o  Subject to local service policies on server-to-server
      communication, manage XML streams (Section 5) with remote servers
      and route XML stanzas (Section 9) to those servers over the
      negotiated streams.

   Depending on the application, the secondary responsibilities of an
   XMPP server can include:

   o  Storing XML data that is used by clients (e.g., contact lists for
      users of XMPP-based instant messaging and presence applications as
      defined in [XMPP-IM]); in this case, the relevant XML stanza is
      handled directly by the server itself on behalf of the client and
      is not routed to a remote server or delivered to a local entity.
   o  Hosting local services that also use XMPP as the basis for
      communication but that provide additional functionality beyond
      that defined in this document or in [XMPP-IM]; examples include
      multi-user conferencing services as specified in [XEP-0045] and
      publish-subscribe services as specified in [XEP-0060].


3.  Addresses

3.1.  Overview

   An ENTITY is anything that is network-addressable and that can
   communicate using XMPP.  For historical reasons, the native address
   of an XMPP entity is called a JABBER IDENTIFIER or JID.  A valid JID
   contains a set of ordered elements formed of an XMPP localpart,
   domain identifier, and resource identifier.

   The syntax for a JID is defined as follows using the Augmented
   Backus-Naur Form as specified in [ABNF].




Saint-Andre              Expires April 26, 2010                [Page 15]

Internet-Draft                  XMPP Core                   October 2009


      jid             = [ localpart "@" ] domain [ "/" resource ]
      localpart       = 1*(nodepoint)
                        ; a "nodepoint" is a UTF-8 encoded Unicode code
                        ; point that satisfies the Nodeprep profile of
                        ; stringprep
      domain          = fqdn / address-literal
      fqdn            = *(ldhlabel ".") toplabel
      ldhlabel        = letdig [*61(ldh) letdig]
      toplabel        = ALPHA   *61(ldh) letdig
      letdig          = ALPHA / DIGIT
      ldh             = ALPHA / DIGIT / "-"
      address-literal = IPv4address / IPv6address
                        ; the "IPv4address" and "IPv6address" rules are
                        ; defined in RFC 3986
      resource        = 1*(resourcepoint)
                        ; a "resourcepoint" is a UTF-8 encoded Unicode
                        ; code point that satisfies the Resourceprep
                        ; profile of stringprep

   All JIDs are based on the foregoing structure.  One common use of
   this structure is to identify a messaging and presence account, the
   server that hosts the account, and a connected resource (e.g., a
   specific device) in the form of <localpart@domain/resource>.
   However, localparts other than clients are possible; for example, a
   specific chat room offered by a multi-user conference service (see
   [XEP-0045]) could be addressed as <room@service> (where "room" is the
   name of the chat room and "service" is the hostname of the multi-user
   conference service) and a specific occupant of such a room could be
   addressed as <room@service/nick> (where "nick" is the occupant's room
   nickname).  Many other JID types are possible (e.g., <domain/
   resource> could be a server-side script or service).

   Each allowable portion of a JID (localpart, domain identifier, and
   resource identifier) MUST NOT be more than 1023 bytes in length,
   resulting in a maximum total size (including the '@' and '/'
   separators) of 3071 bytes.

      Note: While the format of a JID is consistent with [URI], an
      entity's address on an XMPP network MUST be represented as a JID
      (without a URI scheme) and not a [URI] or [IRI] as specified in
      [XMPP-URI]; the latter specification is provided only for
      identification and interaction outside the context of the XMPP
      wire protocol itself.

3.2.  Domain Identifier

   The DOMAIN IDENTIFIER portion of a JID is that portion after the '@'
   character (if any) and before the '/' character (if any); it is the



Saint-Andre              Expires April 26, 2010                [Page 16]

Internet-Draft                  XMPP Core                   October 2009


   primary identifier and is the only REQUIRED element of a JID (a mere
   domain identifier is a valid JID).  Typically a domain identifier
   identifies the "home" server to which clients connect for XML routing
   and data management functionality.  However, it is not necessary for
   an XMPP domain identifier to identify an entity that provides core
   XMPP server functionality (e.g., a domain identifier can identity an
   entity such as a multi-user conference service, a publish-subscribe
   service, or a user directory).

      Note: A single server can service multiple domain identifiers,
      i.e., multiple local domains; this is typically referred to as
      virtual hosting.

   The domain identifier for every server or service that will
   communicate over a network SHOULD be a fully qualified domain name
   (see [DNS]); while the domain identifier MAY be either an Internet
   Protocol (IPv4 or IPv6) address or a text label that is resolvable on
   a local network (commonly called an "unqualified hostname"), it is
   possible that domain identifiers that are IP addresses will not be
   acceptable to other services for the sake of interdomain
   communication.  Furthermore, domain identifiers that are unqualified
   hostnames MUST NOT be used on public networks but MAY be used on
   private networks.

      Note: If the domain identifier includes a final character
      considered to be a label separator (dot) by [IDNA] or [DNS], this
      character MUST be stripped from the domain identifier before the
      JID of which it is a part is used for the purpose of routing an
      XML stanza, comparing against another JID, or constructing an
      [XMPP-URI]; in particular, the character MUST be stripped before
      any other canonicalization steps are taken, such as application of
      the [NAMEPREP] profile of [STRINGPREP] or completion of the
      ToASCII operation as described in [IDNA].

   A domain identifier MUST be an "internationalized domain name" as
   defined in [IDNA], that is, "a domain name in which every label is an
   internationalized label".  When preparing a text label (consisting of
   a sequence of Unicode code points) for representation as an
   internationalized label in the process of constructing an XMPP domain
   identifier or comparing two XMPP domain identifiers, an application
   MUST ensure that for each text label it is possible to apply without
   failing the ToASCII operation specified in [IDNA] with the
   UseSTD3ASCIIRules flag set (thus forbidding ASCII code points other
   than letters, digits, and hyphens).  If the ToASCII operation can be
   applied without failing, then the label is an internationalized
   label.  An internationalized domain name (and therefore an XMPP
   domain identifier) is constructed from its constituent
   internationalized labels by following the rules specified in [IDNA].



Saint-Andre              Expires April 26, 2010                [Page 17]

Internet-Draft                  XMPP Core                   October 2009


      Note: The ToASCII operation includes application of the [NAMEPREP]
      profile of [STRINGPREP] and encoding using the algorithm specified
      in [PUNYCODE]; for details, see [IDNA].  Although the output of
      the ToASCII operation is not used in XMPP, it MUST be possible to
      apply that operation without failing.

3.3.  Localpart

   The LOCALPART of a JID is an optional identifier placed before the
   domain identifier and separated from the latter by the '@' character.
   Typically a localpart uniquely identifies the entity requesting and
   using network access provided by a server (i.e., a local account),
   although it can also represent other kinds of entities (e.g., a chat
   room associated with a multi-user conference service).  The entity
   represented by an XMPP localpart is addressed within the context of a
   specific domain.

   A localpart MUST NOT be zero bytes in length and, as for all portions
   of a JID, MUST NOT be more than 1023 bytes in length.

   A localpart MUST be formatted such that the Nodeprep profile of
   [STRINGPREP] can be applied without failing (see Appendix A).  Before
   comparing two localparts, an application MUST first ensure that the
   Nodeprep profile has been applied to each identifier (the profile
   need not be applied each time a comparison is made, as long as it has
   been applied before comparison).

3.4.  Resource Identifier

   The RESOURCE IDENTIFIER portion of a JID is an optional identifier
   placed after the domain identifier and separated from the latter by
   the '/' character.  A resource identifier can modify either a
   <localpart@domain> address or a mere <domain> address.  Typically a
   resource identifier uniquely identifies a specific connection (e.g.,
   a device or location) or object (e.g., a participant in a multi-user
   conference room) belonging to the entity associated with an XMPP
   localpart at a local domain.

   When an XMPP address does not include a resource identifier (i.e.,
   when it is of the form <domain> or <localpart@domain>), it is
   referred to as a BARE JID.  When an XMPP address includes a resource
   identifier (i.e., when it is of the form <domain/resource> or
   <localpart@domain/resource>), is referred to as a FULL JID.

   A resource identifier MUST NOT be zero bytes in length and, as for
   all portions of a JID, MUST NOT be more than 1023 bytes in length.

   A resource identifier MUST be formatted such that the Resourceprep



Saint-Andre              Expires April 26, 2010                [Page 18]

Internet-Draft                  XMPP Core                   October 2009


   profile of [STRINGPREP] can be applied without failing (see
   Appendix B).  Before comparing two resource identifiers, an
   application MUST first ensure that the Resourceprep profile has been
   applied to each identifier (the profile need not be applied each time
   a comparison is made, as long as it has been applied before
   comparison).

      Note: For historical reasons, the term "resource identifier" is
      used in XMPP to refer to the optional portion of an XMPP address
      that follows the domain identifier and the "/" separator
      character; this use of the term "resource identifier" is not to be
      confused with the meanings of "resource" and "identifier" provided
      in Section 1.1 of [URI].

   XMPP entities SHOULD consider resource identifiers to be opaque
   strings and SHOULD NOT impute meaning to any given resource
   identifier.  In paticular, the use of the '/' character as a
   separator between the domain identifier and the resource identifier
   does not imply that resource identifiers are hierarchical in the way
   that, say, HTTP addresses are hierarchical; thus for example an XMPP
   address of the form <localpart@domain/foo/bar> does not identify a
   resource "bar" that exists below a resource "foo" in a hierarchy of
   resources associated with the entity "localpart@domain".

3.5.  Determination of Addresses

   After the parties to an XML stream have completed the appropriate
   aspects of stream negotiation (typically SASL negotiation (Section 7)
   and, if appropriate, resource binding (Section 8)) the receiving
   entity for a stream MUST determine the initiating entity's JID.

   For server-to-server communication, the initiating server's JID MUST
   be the authorization identity (as defined by [SASL]), either (1) as
   directly communicated by the initiating server during SASL
   negotiation (Section 7) or (2) as derived by the receiving server
   from the authentication identity if no authorization identity was
   specified during SASL negotiation (Section 7).  (For information
   about the determination of addresses in the absence of SASL
   negotiation when the older server dialback protocol is used, see
   [XEP-0220].)

   For client-to-server communication, the client's bare JID
   (<localpart@domain>) MUST be the authorization identity (as defined
   by [SASL]), either (1) as directly communicated by the client during
   SASL negotiation (Section 7) or (2) as derived by the server from the
   authentication identity if no authorization identity was specified
   during SASL negotiation (Section 7).  The resource identifier portion
   of the full JID (<localpart@domain/resource>) MUST be the resource



Saint-Andre              Expires April 26, 2010                [Page 19]

Internet-Draft                  XMPP Core                   October 2009


   identifier negotiated by the client and server during resource
   binding (Section 8).

   The receiving entity MUST ensure that the resulting JID (including
   localpart, domain identifier, resource identifier, and separator
   characters) conforms to the rules and formats defined earlier in this
   section; to meet this restriction, the receiving entity MAY replace
   the JID sent by the initiating entity with the canonicalized JID as
   determined by the receiving entity.


4.  TCP Binding

4.1.  Scope

   As XMPP is defined in this specification, an initiating entity
   (client or server) MUST open a Transmission Control Protocol [TCP]
   connection at the receiving entity (server) before it negotiates XML
   streams with the receiving entity.  The parties then maintain that
   TCP connection for as long as the XML streams are in use.  The rules
   specified in the following sections apply to the TCP binding.

4.2.  Hostname Resolution

   Before opening the TCP connection, the initiating entity first MUST
   resolve the Domain Name System (DNS) hostname associated with the
   receiving entity and determine the appropriate TCP port for
   communication with the receiving entity.  The process is:

   1.  Attempt to resolve the hostname using (a) a [DNS-SRV] Service of
       "xmpp-client" (for client-to-server connections) or "xmpp-server"
       (for server-to-server connections) and (b) a Proto of "tcp",
       resulting in resource records such as "_xmpp-
       client._tcp.example.net." or "_xmpp-server._tcp.im.example.com.".
       The result of the SRV lookup will be one or more combinations of
       a port and hostname, which hostnames the initiating entity MUST
       resolve according to returned SRV record weight (if the result of
       the SRV lookup is a single RR with a Target of ".", i.e. the root
       domain, the initiating entity MUST abort SRV processing but
       SHOULD attempt a fallback resolution as described below).  The
       initiating entity uses the IP address(es) from the first
       successfully resolved hostname (with the corresponding port
       number returned by the SRV lookup) in order to connect to the
       receiving entity.  If the initiating entity fails to connect
       using one of the IP addresses, the initiating entity uses the
       next resolved IP address to connect.  If the initiating entity
       fails to connect using all resolved IP addresses, then the
       initiating entity repeats the process of resolution and



Saint-Andre              Expires April 26, 2010                [Page 20]

Internet-Draft                  XMPP Core                   October 2009


       connection for the next hostname returned by the SRV lookup.
   2.  If the SRV lookup aborts or fails, the fallback SHOULD be a
       normal IPv4 or IPv6 address record resolution to determine the IP
       address, where the port used is the "xmpp-client" port of 5222
       for client-to-server connections or the "xmpp-server" port 5269
       for server-to-server connections.
   3.  For client-to-server connections, the fallback MAY be a [DNS-TXT]
       lookup for alternative connection methods, for example as
       described in [XEP-0156].

      Note: If the initiating entity has been explicitly configured to
      associate a particular hostname (and potentially port) with the
      original hostname of the receiving entity (say, to "hardcode" an
      association between an original hostname of example.net and a
      configured hostname and of webcm.example.com:80), the initiating
      entity SHALL use the configured name instead of performing the
      foregoing resolution process on the original name.

      Note: Many XMPP servers are implemented in such a way that they
      can host additional services (beyond those defined in this
      specification and [XMPP-IM]) at hostnames that are subdomains of
      the hostname of the main XMPP service (e.g.,
      conference.example.net for a [XEP-0045] service associated with
      the example.net XMPP service) or subdomains of the first-level
      domain of the underlying host (e.g., muc.example.com for a
      [XEP-0045] service associated with the im.example.com XMPP
      service).  If an entity from a remote domain wishes to use such
      additional services, it would generate an appropriate XML stanza
      and the remote domain itself would attempt to resolve the
      service's hostname via an SRV lookup on resource records such as
      "_xmpp-server._tcp.conference.example.net." or "_xmpp-
      server._tcp.muc.example.com.".  Therefore if a service wishes to
      enable entities from remote domains to access these additional
      services, it needs to advertise the appropriate "_xmpp-server" SRV
      records in addition to the "_xmpp-server" record for its main XMPP
      service.

4.3.  Client-to-Server Communication

   Because a client is subordinate to a server and therefore a client
   authenticates to the server but the server does not necessarily
   authenticate to the client, it is necessary to have only one TCP
   connection between client and server.  Thus the server MUST allow the
   client to share a single TCP connection for XML stanzas sent from
   client to server and from server to client (i.e., the inital stream
   and response stream as specified under Section 5).





Saint-Andre              Expires April 26, 2010                [Page 21]

Internet-Draft                  XMPP Core                   October 2009


4.4.  Server-to-Server Communication

   Because two servers are peers and therefore each peer MUST
   authenticate with the other, the servers MUST use two TCP
   connections: one for XML stanzas sent from the first server to the
   second server and another (initiated by the second server) for XML
   stanzas from the second server to the first server.

   This rule applies only to XML stanzas (Section 9).  Therefore during
   STARTTLS negotiation (Section 6) and SASL negotiation (Section 7) the
   servers would use one TCP connection, but after stream setup that TCP
   connection would be used only for the initiating server to send XML
   stanzas to the receiving server.  In order for the receiving server
   to send XML stanzas to the initiating server, the receiving server
   would need to reverse the roles and negotiate an XML stream from the
   receiving server to the initiating server.

4.5.  Reconnection

   It can happen that an XMPP server goes offline while servicing TCP
   connections from local clients and from other servers.  Because the
   number of such connections can be quite large, the reconnection
   algorithm employed by entities that seek to reconnect can have a
   significant impact on software and network performance.  The
   following guidelines are RECOMMENDED:

   o  The time to live (TTL) specified in Domain Name System records
      MUST be honored, even if DNS results are cached; if the TTL has
      not expired, an entity that seeks to reconnect MUST NOT re-resolve
      the server hostname before reconnecting.
   o  The time that expires before an entity first seeks to reconnect
      MUST be randomized (e.g., so that all clients do not attempt to
      reconnect exactly 30 seconds after being disconnected).
   o  If the first reconnection attempt does not succeed, an entity MUST
      back off increasingly on the time between subsequent reconnection
      attempts, e.g. in accordance with the exponential backoff
      principle.

      Note: Because it is possible that a disconnected entity cannot
      determine the cause of disconnection (e.g., because there was no
      explicit stream error) or does not need a new stream for immediate
      communication (e.g., because the stream was idle and therefore
      timed out), it SHOULD NOT assume that is needs to reconnect
      immediately.







Saint-Andre              Expires April 26, 2010                [Page 22]

Internet-Draft                  XMPP Core                   October 2009


4.6.  Reliability

   The use of long-lived TCP connections in XMPP implies that the
   sending of XML stanzas over XML streams can be unreliable, since the
   parties to a long-lived TCP connection might not discover a
   connectivity disruption in a timely manner.  At the XMPP application
   layer, long connectivity disruptions can result in undelivered
   stanzas.  Although the core XMPP technology defined in this
   specification does not contain features to overcome this lack of
   reliability, there exist XMPP extensions for doing so (e.g.,
   [XEP-0198]).

4.7.  Other Bindings

   There is no necessary coupling of an XML stream to a TCP connection.
   For example, two entities could connect to each other via another
   transport, such as [HTTP] as specified in [XEP-0124] and [XEP-0206].
   Although this specification neither encourages nor discourages other
   bindings, it defines only a binding of XMPP to TCP.


5.  XML Streams

5.1.  Overview

   Two fundamental concepts make possible the rapid, asynchronous
   exchange of relatively small payloads of structured information
   between presence-aware entities: XML streams and XML stanzas.  These
   terms are defined as follows.

   Definition of XML Stream:  An XML STREAM is a container for the
      exchange of XML elements between any two entities over a network.
      The start of an XML stream is denoted unambiguously by an opening
      STREAM HEADER (i.e., an XML <stream> tag with appropriate
      attributes and namespace declarations), while the end of the XML
      stream is denoted unambiguously by a closing XML </stream> tag.
      During the life of the stream, the entity that initiated it can
      send an unbounded number of XML elements over the stream, either
      elements used to negotiate the stream (e.g., to complete TLS
      negotiation (Section 6) or SASL negotiation (Section 7)) or XML
      stanzas.  The INITIAL STREAM is negotiated from the initiating
      entity (typically a client or server) to the receiving entity
      (typically a server), and can be seen as corresponding to the
      initiating entity's "connection" or "session" with the receiving
      entity.  The initial stream enables unidirectional communication
      from the initiating entity to the receiving entity; in order to
      enable information exchange from the receiving entity to the
      initiating entity, the receiving entity MUST negotiate a stream in



Saint-Andre              Expires April 26, 2010                [Page 23]

Internet-Draft                  XMPP Core                   October 2009


      the opposite direction (the RESPONSE STREAM).
   Definition of XML Stanza:  An XML STANZA is a discrete semantic unit
      of structured information that is sent from one entity to another
      over an XML stream, and is the basic unit of meaning in XMPP.  An
      XML stanza exists at the direct child level of the root <stream/>
      element; the start of any XML stanza is denoted unambiguously by
      the element start tag at depth=1 of the XML stream (e.g.,
      <presence>), and the end of any XML stanza is denoted
      unambiguously by the corresponding close tag at depth=1 (e.g.,
      </presence>).  The only XML stanzas defined herein are the
      <message/>, <presence/>, and <iq/> elements qualified by the
      default namespace for the stream, as described under Section 9;
      for example, an XML element sent for the purpose of TLS
      negotiation (Section 6) or SASL negotiation (Section 7) is not
      considered to be an XML stanza, nor is a stream error or a stream
      feature.  An XML stanza MAY contain child elements (with
      accompanying attributes, elements, and XML character data) as
      necessary in order to convey the desired information, which MAY be
      qualified by any XML namespace (see [XML-NAMES] as well as
      Section 9.4 herein).

   Consider the example of a client's connection to a server.  In order
   to connect to a server, a client initiates an XML stream by sending a
   stream header to the server, optionally preceded by a text
   declaration specifying the XML version and the character encoding
   supported (see Section 12.5 and Section 12.6).  Subject to local
   policies and service provisioning, the server then replies with a
   second XML stream back to the client, again optionally preceded by a
   text declaration.  Once the client has completed SASL negotiation
   (Section 7) and resource binding (Section 8), the client can send an
   unbounded number of XML stanzas over the stream.  When the client
   desires to close the stream, it simply sends a closing </stream> tag
   to the server as further described under Section 5.3.

   In essence, then, an XML stream acts as an envelope for all the XML
   stanzas sent during a connection.  We can represent this in a
   simplistic fashion as follows.














Saint-Andre              Expires April 26, 2010                [Page 24]

Internet-Draft                  XMPP Core                   October 2009


   +--------------------+
   | <stream>           |
   |--------------------|
   | <presence>         |
   |   <show/>          |
   | </presence>        |
   |--------------------|
   | <message to='foo'> |
   |   <body/>          |
   | </message>         |
   |--------------------|
   | <iq to='bar'>      |
   |   <query/>         |
   | </iq>              |
   |--------------------|
   | <iq from='bar'>    |
   |   <query/>         |
   | </iq>              |
   |--------------------|
   | [ ... ]            |
   |--------------------|
   | </stream>          |
   +--------------------+

      Note: Those who are accustomed to thinking of XML in a document-
      centric manner might view a client's connection to a server as
      consisting of two open-ended XML documents: one from the client to
      the server and one from the server to the client.  On this
      analogy, the two XML streams can be considered equivalent to two
      "documents" (matching production [1] content of [XML]) that are
      built up through the accumulation of XML stanzas, the root
      <stream/> element can be considered equivalent to the "document
      entity" for each "document" (as described in Section 4.8 of
      [XML]), and the XML stanzas sent over the streams can be
      considered equivalent to "fragments" of the "documents" as
      described in [XML-FRAG].  However, this perspective is merely an
      analogy; XMPP does not deal in documents and fragments but in
      streams and stanzas.

   The remainder of this section defines the following aspects of XML
   streams:

   o  The stream negotation process
   o  How to close a stream
   o  The XML attributes of a stream
   o  The XML namespaces of a stream





Saint-Andre              Expires April 26, 2010                [Page 25]

Internet-Draft                  XMPP Core                   October 2009


5.2.  Stream Negotiation

5.2.1.  Overview

   Because the receiving entity for a stream acts as a gatekeeper to the
   domains it services, it imposes certain conditions for connecting as
   a client or as a peer server.  At a minimum, the initiating entity
   needs to authenticate with the receiving entity before it is allowed
   to send stanzas to the receiving entity, typically using SASL as
   described under Section 7.  However, the receiving entity can
   consider conditions other than authentication to be mandatory, such
   as encryption using TLS as described under Section 6.  The receiving
   entity informs the initiating entity about such conditions by
   communicating STREAM FEATURES: the set of particular protocol
   interactions that are mandatory for the initiating entity to complete
   before the receiving entity will accept XML stanzas from the
   initiating entity (e.g., authentication), as well as any protocol
   interactions that are voluntary but that might improve the handling
   of an XML stream (e.g., establishment of application-layer
   compression).

   The existence of conditions for connecting implies that streams need
   to be negotiated, and the order of layers (TCP, then TLS, then SASL,
   then XMPP; see Section 14.5) implies that stream negotiation is a
   well-defined process.  Furthermore, stream features can be either
   mandatory-to-negotiate or voluntary-to-negotiate; this imposes
   further structure on the negotiation process.  Finally, for security
   reasons the parties to a stream need to discard knowledge that they
   gained during the negotiation process after successfully completing
   the protocol interactions defined for certain features (e.g., TLS in
   all cases and SASL in the case when a security layer might be
   established); this is done by flushing the old stream context and
   exchanging new stream headers over the existing TCP connection.

5.2.2.  Stream Features Format

   If the initiating entity includes the 'version' attribute set to a
   value of at least "1.0" in the initial stream header, after sending
   the response stream header the receiving entity MUST send a
   <features/> child element (prefixed by the streams namespace prefix)
   to the initiating entity in order to announce any conditions for
   continuation of the stream negotiation process.  Each condition takes
   the form of a child element of the <features/> element, qualified by
   a namespace that is different from the streams namespace and the
   default namespace.

   If a particular stream feature is or can be mandatory-to-negotiate,
   the definition of that feature needs to either declare that the



Saint-Andre              Expires April 26, 2010                [Page 26]

Internet-Draft                  XMPP Core                   October 2009


   feature is always mandatory-to-negotiate (e.g., this is true of
   resource binding for XMPP clients) or specify a way for the receiving
   entity to flag the feature as mandatory-to-negotiate for this
   interaction (e.g., this is done for TLS by including an empty
   <required/> element in the advertisement for that stream feature).

   For security reasons, certain stream features necessitate the
   initiating entity to send a new initial stream header upon successful
   negotiation of the feature (e.g., TLS in all cases and SASL in the
   case when a security layer might be established).  If this is true of
   a given stream feature, the definition of that feature needs to
   declare that a stream restart is expected after negotiation of the
   feature.

   A <features/> element that contains at least one mandatory feature
   indicates that the stream negotiation is not complete and that the
   initiating entity MUST negotiate further features.

   R: <stream:features>
        <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
          <required/>
        </starttls>
      </stream:features>

   A <features/> element MAY contain more than one mandatory feature.
   This means that the initiating entity can choose among the mandatory
   features.  For example, perhaps a future technology will perform
   roughly the same function as TLS, so the receiving entity might
   advertise support for both TLS and the future technology.

   A <features/> element that contains only voluntary features indicates
   that the stream negotiation is complete and that the initiating
   entity is cleared to send XML stanzas, but that the initiating entity
   MAY negotiate further features if desired.

   R: <stream:features>
        <session xmlns='urn:ietf:params:xml:ns:xmpp-session'/>
        <compression xmlns='http://jabber.org/features/compress'>
          <method>zlib</method>
          <method>lzw</method>
        </compression>
      </stream:features>

   A <features/> element that contains both mandatory and voluntary
   features indicates that the negotiation is not complete but that the
   initiating entity MAY complete the voluntary feature(s) before it
   attempts to negotiate the mandatory feature(s).




Saint-Andre              Expires April 26, 2010                [Page 27]

Internet-Draft                  XMPP Core                   October 2009


   R: <stream:features>
        <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
        <compression xmlns='http://jabber.org/features/compress'>
          <method>zlib</method>
          <method>lzw</method>
        </compression>
      </stream:features>

   An empty <features/> element indicates that the stream negotiation is
   complete and that the initiating entity is cleared to send XML
   stanzas.

   R: <stream:features/>

      Note: The order of child elements contained in any given
      <features/> element is not significant.

5.2.3.  Restarts

   On successful negotiation of a feature that necessitates a stream
   restart, both parties MUST consider the previous stream to be
   replaced but MUST NOT terminate the underlying TCP connection;
   instead, the parties MUST reuse the existing connection, which might
   be in a new state (e.g., encrypted as a result of TLS negotiation).
   The initiating entity then MUST send a new initial stream header,
   which SHOULD be preceded by an XML declaration as described under
   Section 12.5.  When the receiving entity receives the new initial
   stream header, it MUST generate a new stream ID (instead of re-using
   the old stream ID) before sending a new response stream header (which
   SHOULD be preceded by an XML declaration as described under
   Section 12.5).

   For the sake of backward compatibility, the receiving entity MUST
   accept stream restarts at any stage in the stream negotiation process
   even if the receiving entity has not indicated that a stream restart
   is mandatory at that stage.

5.2.4.  Resending Features

   After completing negotiation of any stream feature (even stream
   features that do not necessitate a stream restart), the receiving
   entity MUST send an updated list of stream features to the initiating
   entity, where the list MAY be empty if there are no further features
   to be advertised.

   At any time after stream establishment and before closing of the
   stream, the receiving entity MAY send additional or modified stream
   feature advertisements to the initiating entity (e.g., because a new



Saint-Andre              Expires April 26, 2010                [Page 28]

Internet-Draft                  XMPP Core                   October 2009


   feature has been enabled).

5.2.5.  Completion of Stream Negotiation

   The receiving entity indicates completion of the stream negotiation
   process by sending to the initiating entity either an empty
   <features/> element or a <features/> element that contains only
   voluntary features.  Once stream negotiation is complete, the
   initiating entity is cleared to send XML stanzas over the stream for
   as long as the stream is maintained by both parties.

   The initiating entity MUST NOT attempt to send XML stanzas
   (Section 9) to entities other than itself (i.e., the client's
   connected resource or any other authenticated resource of the
   client's account) or the server until stream negotiation has been
   completed.  However, if it does attempt to do so, the receiving
   entity MUST NOT accept such stanzas and MUST return a <not-
   authorized/> stream error.  This rule applies to XML stanzas only
   (i.e., <message/>, <presence/>, and <iq/> elements qualified by the
   default namespace) and not to XML elements used for stream
   negotiation (e.g., elements used to complete TLS negotiation
   (Section 6) or SASL negotiation (Section 7)).

5.2.6.  State Chart

   We summarize the foregoing rules in the following non-normative state
   chart for the stream negotiation process, presented from the
   perspective of the initiating entity.























Saint-Andre              Expires April 26, 2010                [Page 29]

Internet-Draft                  XMPP Core                   October 2009


                        +------------+
                        |  open TCP  |
                        | connection |
                        +------------+
                              |
                       +---------------+
                       | send initial  |<-------------------------+
                       | stream header |                          |
                       +---------------+                          |
                              |                                   |
                      +------------------+                        |
                      | receive response |                        |
                      | stream header    |                        |
                      +------------------+                        |
                              |                                   |
                       +----------------+                         |
                       | receive stream |                         |
   +------------------>| features       |                         |
   |                   +----------------+                         |
   |                          |                                   |
   |       +<-----------------+                                   |
   |       |                                                      |
   |    {empty?} ----> {all voluntary?} ----> {some mandatory?}   |
   |       |      no          |          no         |             |
   |       | yes              | yes                 |             |
   |       |                  |                     |             |
   |       |           +---------------+    +----------------+    |
   |       |           | MAY negotiate |    | MUST negotiate |    |
   |       |           | any or none   |    | one feature    |    |
   |       |           +---------------+    +----------------+    |
   |       |                  |                     |             |
   |   +----------+      +-----------+              |             |
   |   | process  |<-----| negotiate |              |             |
   |   | complete |  no  | a feature |              |             |
   |   +----------+      +-----------+              |             |
   |                          |                     |             |
   |                     yes  |                     |             |
   |                          |                     |             |
   |                          +----->----+----<-----+             |
   |                                     |                        |
   |                                     |                        |
   +<---------------------------[restart mandatory?]------------->+
                  no                                     yes








Saint-Andre              Expires April 26, 2010                [Page 30]

Internet-Draft                  XMPP Core                   October 2009


5.3.  Closing a Stream

   An XML stream between two entities can be closed because a stream
   error has occurred or in some cases in the absence of an error.
   Where feasible, it is preferable to close a stream only if a stream
   error has occurred.

   A stream is closed by sending a closing </stream> tag over the TCP
   connection.

   S: </stream:stream>

   After an entity sends a closing stream tag, it MUST NOT send further
   data over that stream.

5.3.1.  With Stream Error

   If a stream error has occurred, the entity that detects the error
   MUST close the stream as described under Section 5.6.1.

5.3.2.  Without Stream Error

   At any time after XML streams have been negotiated between two
   entities, either entity MAY close its stream to the other party in
   the absence of a stream error by sending a closing stream tag.

   P: </stream:stream>

   The entity that sends the closing stream tag SHOULD wait for the
   other party to also close its stream.

   S: </stream:stream>

   However, the entity that sends the first closing stream tag MAY
   consider both streams to be void if the other party does not send its
   closing stream tag within a reasonable amount of time (where the
   definition of "reasonable" is a matter of implementation or
   deployment).

   After the entity that sent the first closing stream tag receives a
   reciprocal closing stream tag from the other party (or if it
   considers the stream to be void after a reasonable amount of time),
   it MUST terminate the underlying TCP connection or connections.

5.3.3.  Handling of Idle Streams

   An XML stream can become idle, i.e., neither entity has sent XMPP
   traffic over the stream for some period of time.  The idle timeout



Saint-Andre              Expires April 26, 2010                [Page 31]

Internet-Draft                  XMPP Core                   October 2009


   period is a matter of implementation and local service policy;
   however, it is RECOMMENDED to be liberal in accepting idle streams,
   since experience has shown that doing so improves the reliability of
   communications over XMPP networks.  In particular, it is typically
   more efficient to maintain a stream between two servers than it is to
   aggressively timeout such a stream, especially with regard to
   synchronization of presence information as described in [XMPP-IM];
   therefore it is RECOMMENDED to maintain such a stream since
   experience has shown that server-to-server streams are cyclical and
   typically need to be re-established every 24 to 48 hours if they are
   timed out.

   An XML stream can appear idle at the XMPP level because the
   underlying TCP connection has become idle (e.g., a client's network
   connection has been lost).  One common method for preventing a TCP
   connection from going idle or for detecting an idle TCP connection is
   to send a space character (U+0020) over the TCP connection between
   XML stanzas, which is allowed for XML streams as described under
   Section 12.7; the sending of such a space character is properly
   called a WHITESPACE KEEPALIVE (although the term "whitespace ping" is
   often used, in fact it is not a ping since no "pong" is possible).
   Other connection-testing methods include the application-level pings
   described in [XEP-0199] and the more comprehensive stream management
   protocol described in [XEP-0198].  Implementers are advised to
   support whichever connection-testing methods they deem appropriate,
   but to carefully weigh the network impact of such methods against the
   benefits of discovering idle streams in a timely manner.  The length
   of time between the use of any particular connection test is a matter
   of implementation and local service policy; however, it is
   RECOMMENDED that any such test be performed not more than once every
   60 seconds.

   To close an idle stream with a local client or remote server, a
   server MUST close the stream without error as explained under
   Section 5.3.2.

5.4.  Stream Attributes

   The attributes of the root <stream/> element are defined in the
   following sections.

      Note: The attributes of the root <stream/> element are not
      prepended by a namespace prefix because, as explained in
      [XML-NAMES], "[d]efault namespace declarations do not apply
      directly to attribute names; the interpretation of unprefixed
      attributes is determined by the element on which they appear."





Saint-Andre              Expires April 26, 2010                [Page 32]

Internet-Draft                  XMPP Core                   October 2009


5.4.1.  from

   The 'from' attribute communicates an XMPP identity of the entity
   sending the stream element.

      Note: It is possible for an entity to have more than one XMPP
      identity (e.g., in the case of a server that provides virtual
      hosting).  It is also possible that an entity does not know the
      XMPP identity of the principal controlling the entity (e.g.,
      because the XMPP identity is assigned at a level other than the
      XMPP application layer, as in the General Security Service
      Application Program Interface [GSS-API]).

   For initial stream headers in client-to-server communication, if the
   client knows the XMPP identity of the principal controlling the
   client (typically an account name of the form <localpart@domain>),
   then it MAY include the 'from' attribute and set its value to that
   identity; if not, then it MUST NOT include the 'from' attribute.
   Note: Including the XMPP identity before the stream is protected via
   TLS can expose that identity to eavesdroppers.

   I: <?xml version='1.0'?>
      <stream:stream
          from='juliet@im.example.com'
          to='im.example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   For initial stream headers in server-to-server communication, a
   server MUST include the 'from' attribute and MUST set its value to a
   hostname serviced by the initiating entity.

   I: <?xml version='1.0'?>
      <stream:stream
          from='example.net'
          to='im.example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:server'
          xmlns:stream='http://etherx.jabber.org/streams'>

   For response stream headers in both client-to-server and server-to-
   server communication, the receiving entity MUST include the 'from'
   attribute and MUST set its value to a hostname serviced by the
   receiving entity (which MAY be a hostname other than that specified
   in the 'to' attribute of the initial stream header).



Saint-Andre              Expires April 26, 2010                [Page 33]

Internet-Draft                  XMPP Core                   October 2009


   R: <?xml version='1.0'?>
      <stream:stream
          from='im.example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@im.example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   Whether or not the 'from' attribute is included, each entity MUST
   verify the identity of the other entity before exchanging XML stanzas
   with it (see Section 14.3 and Section 14.4).

      Note: It is possible that implementations based on an earlier
      revision of this specification will not include the 'from' address
      on stream headers; an entity SHOULD be liberal in accepting such
      stream headers.

5.4.2.  to

   For initial stream headers in both client-to-server and server-to-
   server communication, the initiating entity MUST include the 'to'
   attribute and MUST set its value to a hostname that the initiating
   entity knows or expects the receiving entity to service.

   I: <?xml version='1.0'?>
      <stream:stream
          from='juliet@im.example.com'
          to='im.example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   For response stream headers in client-to-server communication, if the
   client included a 'from' attribute in the initial stream header then
   the server MUST include a 'to' attribute in the response stream
   header and MUST set its value to the bare JID specified in the 'from'
   attribute of the initial stream header.  If the client did not
   include a 'from' attribute in the initial stream header then the
   server MUST NOT include a 'to' attribute in the response stream
   header.








Saint-Andre              Expires April 26, 2010                [Page 34]

Internet-Draft                  XMPP Core                   October 2009


   R: <?xml version='1.0'?>
      <stream:stream
          from='im.example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@im.example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   For response stream headers in server-to-server communication, the
   receiving entity MUST include a 'to' attribute in the response stream
   header and MUST set its value to the hostname specified in the 'from'
   attribute of the initial stream header.

   R: <?xml version='1.0'?>
      <stream:stream
          from='im.example.com'
          id='g4qSvGvBxJ+xeAd7QKezOQJFFlw='
          to='example.net'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:server'
          xmlns:stream='http://etherx.jabber.org/streams'>

   Whether or not the 'to' attribute is included, each entity MUST
   verify the identity of the other entity before exchanging XML stanzas
   with it (see Section 14.3 and Section 14.4).

      Note: It is possible that implementations based on an earlier
      revision of this specification will not include the 'to' address
      on stream headers; an entity SHOULD be liberal in accepting such
      stream headers.

5.4.3.  id

   The 'id' attribute communicates a unique identifier for the stream.
   This identifier is called a STREAM ID.  The stream ID MUST be
   generated by the receiving entity when it sends a response stream
   header, MUST BE unique within the receiving application (normally a
   server), and MUST be both unpredictable and nonrepeating because it
   can be security-critical (see [RANDOM] for recommendations regarding
   randomness for security purposes).

   For initial stream headers, the initiating entity MUST NOT include
   the 'id' attribute; however, if the 'id' attribute is included, the
   receiving entity MUST silently ignore it.




Saint-Andre              Expires April 26, 2010                [Page 35]

Internet-Draft                  XMPP Core                   October 2009


   For response stream headers, the receiving entity MUST include the
   'id' attribute.

   R: <?xml version='1.0'?>
      <stream:stream
          from='im.example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@im.example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

5.4.4.  xml:lang

   The 'xml:lang' attribute communicates an entity's preferred or
   default language for any human-readable XML character data to be sent
   over the stream.  The syntax of this attribute is defined in Section
   2.12 of [XML]; in particular, the value of the 'xml:lang' attribute
   MUST conform to the NMTOKEN datatype (as defined in Section 2.3 of
   [XML]) and MUST conform to the language identifier format defined in
   [LANGTAGS].

   For initial stream headers, the initiating entity SHOULD include the
   'xml:lang' attribute.

   I: <?xml version='1.0'?>
      <stream:stream
          from='juliet@im.example.com'
          to='im.example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   For response stream headers, the receiving entity MUST include the
   'xml:lang' attribute.  If the initiating entity included an 'xml:
   lang' attribute in its initial stream header and the receiving entity
   supports that language in the human-readable XML character data that
   it generates and sends to the initiating entity (e.g., in the <text/>
   element for stream and stanza errors), the value of the 'xml:lang'
   attribute MUST be an identifier for the initiating entity's preferred
   language; if the receiving entity supports a language that closely
   matches the initiating entity's preferred language (e.g., "de"
   instead of "de-CH"), then the value of the 'xml:lang' attribute
   SHOULD be the identifier for the matching language but MAY be the
   identifier for the default language of the receiving entity; if the
   receiving entity does not support the initiating entity's preferred



Saint-Andre              Expires April 26, 2010                [Page 36]

Internet-Draft                  XMPP Core                   October 2009


   language or a closely matching language (or the initiating entity did
   not include the 'xml:lang' attribute in its initial stream header),
   then the value of the 'xml:lang' attribute MUST be the identifier for
   the default language of the receiving entity.

   R: <?xml version='1.0'?>
      <stream:stream
          from='im.example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@im.example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   If the initiating entity included the 'xml:lang' attribute in its
   initial stream header, the receiving entity SHOULD remember that
   value as the default xml:lang for all stanzas sent by the initiating
   entity.  As described under Section 9.1.5, the initiating entity MAY
   include the 'xml:lang' attribute in any XML stanzas it sends over the
   stream.  If the initiating entity does not include the 'xml:lang'
   attribute in any such stanza, the receiving entity SHOULD add the
   'xml:lang' attribute to the stanza, whose value MUST be the
   identifier for the language preferred by the initiating entity (even
   if the receiving entity does not support that language for human-
   readable XML character data it generates and sends to the initiating
   entity, such as in stream or stanza errors).  If the initiating
   entity includes the 'xml:lang' attribute in any such stanza, the
   receiving entity MUST NOT modify or delete it.

5.4.5.  version

   The inclusion of the version attribute set to a value of at least
   "1.0" signals support for the stream-related protocols defined in
   this specification, including (TLS negotiation (Section 6), SASL
   negotiation (Section 7), Section 5.2.2, and stream errors
   (Section 5.6).

   The version of XMPP specified herein is "1.0"; in particular, XMPP
   1.0 encapsulates the stream-related protocols as well as the basic
   semantics of the three defined XML stanza types (<message/>,
   <presence/>, and <iq/>).

   The numbering scheme for XMPP versions is "<major>.<minor>".  The
   major and minor numbers MUST be treated as separate integers and each
   number MAY be incremented higher than a single digit.  Thus, "XMPP
   2.4" would be a lower version than "XMPP 2.13", which in turn would
   be lower than "XMPP 12.3".  Leading zeros (e.g., "XMPP 6.01") MUST be



Saint-Andre              Expires April 26, 2010                [Page 37]

Internet-Draft                  XMPP Core                   October 2009


   ignored by recipients and MUST NOT be sent.

   The major version number will be incremented only if the stream and
   stanza formats or obligatory actions have changed so dramatically
   that an older version entity would not be able to interoperate with a
   newer version entity if it simply ignored the elements and attributes
   it did not understand and took the actions specified in the older
   specification.

   The minor version number will be incremented only if significant new
   capabilities have been added to the core protocol (e.g., a newly
   defined value of the 'type' attribute for message, presence, or IQ
   stanzas).  The minor version number MUST be ignored by an entity with
   a smaller minor version number, but MAY be used for informational
   purposes by the entity with the larger minor version number (e.g.,
   the entity with the larger minor version number would simply note
   that its correspondent would not be able to understand that value of
   the 'type' attribute and therefore would not send it).

   The following rules apply to the generation and handling of the
   'version' attribute within stream headers:

   1.  The initiating entity MUST set the value of the 'version'
       attribute in the initial stream header to the highest version
       number it supports (e.g., if the highest version number it
       supports is that defined in this specification, it MUST set the
       value to "1.0").
   2.  The receiving entity MUST set the value of the 'version'
       attribute in the response stream header to either the value
       supplied by the initiating entity or the highest version number
       supported by the receiving entity, whichever is lower.  The
       receiving entity MUST perform a numeric comparison on the major
       and minor version numbers, not a string match on
       "<major>.<minor>".
   3.  If the version number included in the response stream header is
       at least one major version lower than the version number included
       in the initial stream header and newer version entities cannot
       interoperate with older version entities as described, the
       initiating entity SHOULD generate an <unsupported-version/>
       stream error.
   4.  If either entity receives a stream header with no 'version'
       attribute, the entity MUST consider the version supported by the
       other entity to be "0.9" and SHOULD NOT include a 'version'
       attribute in the response stream header.







Saint-Andre              Expires April 26, 2010                [Page 38]

Internet-Draft                  XMPP Core                   October 2009


5.4.6.  Summary of Stream Attributes

   The following table summarizes the attributes of the root <stream/>
   element.

   +----------+--------------------------+-------------------------+
   |          | initiating to receiving  | receiving to initiating |
   +----------+--------------------------+-------------------------+
   | to       | JID of receiver          | JID of initiator        |
   | from     | JID of initiator         | JID of receiver         |
   | id       | silently ignored         | stream identifier       |
   | xml:lang | default language         | default language        |
   | version  | XMPP 1.0+ supported      | XMPP 1.0+ supported     |
   +----------+--------------------------+-------------------------+

5.5.  Namespace Declarations

5.5.1.  Declaration of Streams Namespace

   A streams namespace declaration is REQUIRED in all XML stream headers
   and the name of the streams namespace MUST be
   'http://etherx.jabber.org/streams'.  If this rule is violated, the
   entity that receives the offending stream header MUST return a stream
   error to the sending entity, which SHOULD be <invalid-namespace/> but
   MAY be <bad-format/>.

   The element names of the <stream/> element and its <features/> and
   <error/> children MUST be qualified by the streams namespace prefix
   in all instances.  If this rule is violated, the entity that receives
   the offending element MUST return a stream error to the sending
   entity, which SHOULD be <bad-format/>.

   For historical reasons, an implementation MAY accept only the
   'stream:' prefix for these elements.  If an entity receives a stream
   header with a streams namespace prefix it does not accept, it MUST
   return a stream error to the sending entity, which SHOULD be <bad-
   namespace-prefix/> but MAY be <bad-format/>.

5.5.2.  Declaration of Default Namespace

   A default namespace declaration is REQUIRED and defines the allowable
   first-level children of the root stream element.  This namespace
   declaration MUST be the same for the initial stream and the response
   stream so that both streams are qualified consistently.  The default
   namespace declaration applies to the stream and all first-level child
   element sent within a stream unless explicitly qualified by the
   streams namespace or another namespace.




Saint-Andre              Expires April 26, 2010                [Page 39]

Internet-Draft                  XMPP Core                   October 2009


   A server implementation MUST support the following two default
   namespaces:

   o  jabber:client -- this default namespace is declared when the
      stream is used for communication between a client and a server
   o  jabber:server -- this default namespace is declared when the
      stream is used for communication between two servers

   A client implementation MUST support the 'jabber:client' default
   namespace.

   If an implementation accepts a stream that is qualified by the
   'jabber:client' or 'jabber:server' namespace, it MUST support the
   common attributes (Section 9.1) and basic semantics (Section 9.2) of
   all three core stanza types (message, presence, and IQ).

   For historical reasons, an implementation MAY refuse to support any
   other default namespaces.  If an entity receives a stream header with
   a default namespace it does not support, it MUST return an <invalid-
   namespace/> stream error.

   An implementation MUST NOT generate namespace prefixes for elements
   qualified by the default namespace if the default namespace is
   'jabber:client' or 'jabber:server'.

      Note: The 'jabber:client' and 'jabber:server' namespaces are
      nearly identical but are used in different contexts (client-to-
      server communication for 'jabber:client' and server-to-server
      communication for 'jabber:server').  The only difference between
      the two is that the 'to' and 'from' attributes are OPTIONAL on
      stanzas sent over XML streams qualified by the 'jabber:client'
      namespace, whereas they are REQUIRED on stanzas sent over XML
      streams qualified by the 'jabber:server' namespace.

   An implementation MAY support a default namespace other than "jabber:
   client" or "jabber:server".  However, because such namespaces would
   define applications other than XMPP, they are to be defined in
   separate specifications.

5.5.3.  Declaration of Other Namespaces

   Because an XML stanza is the primary unit of meaning in XMPP and
   because an XML stanza can be routed outside the context of the stream
   in which it originated, a stream header MUST NOT include namespace
   declarations for namespaces that are different from the streams
   namespace and the default namespace.





Saint-Andre              Expires April 26, 2010                [Page 40]

Internet-Draft                  XMPP Core                   October 2009


5.6.  Stream Errors

   The root stream element MAY contain an <error/> child element that is
   prefixed by the streams namespace prefix.  The error child shall be
   sent by a compliant entity if it perceives that a stream-level error
   has occurred.

5.6.1.  Rules

   The following rules apply to stream-level errors.

5.6.1.1.  Stream Errors Are Unrecoverable

   Stream-level errors are unrecoverable.  Therefore, if an error occurs
   at the level of the stream, the entity that detects the error MUST
   send a <error/> element with an appropriate child element that
   specifies the error condition and at the same time send a closing
   </stream> tag.

   C: <message><body></message>

   S: <stream:error>
        <xml-not-well-formed
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

   The entity that generates the stream error then SHOULD immediately
   terminate the underlying TCP connection, although it MAY wait until
   after it receives a closing </stream> tag from the entity to which it
   sent the stream error.

   C: </stream:stream>

5.6.1.2.  Stream Errors Can Occur During Setup

   If the error is triggered by the initial stream header, the receiving
   entity MUST still send the opening <stream> tag, include the <error/>
   element as a child of the stream element, and send the closing
   </stream> tag (preferably all at the same time).











Saint-Andre              Expires April 26, 2010                [Page 41]

Internet-Draft                  XMPP Core                   October 2009


   C: <?xml version='1.0'?>
      <stream:stream
          from='juliet@im.example.com'
          to='im.example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://wrong.namespace.example.org/'>

   S: <?xml version='1.0'?>
      <stream:stream
          from='im.example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@im.example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>
      <stream:error>
        <invalid-namespace
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.6.1.3.  Stream Errors When the Host is Unspecified or Unknown

   If the initiating entity provides no 'to' attribute or provides an
   unknown host in the 'to' attribute and the error occurs during stream
   setup, the receiving entity SHOULD provide an authoritative hostname
   in the 'from' attribute of the stream header sent before termination,
   but absent such an authoritative hostname MAY instead simply populate
   the response stream's 'from' attribute with the value of the initial
   stream header's 'to' attribute (where the value of the 'from'
   attribute MAY be empty if the initiating entity provided no 'to'
   attribute).
















Saint-Andre              Expires April 26, 2010                [Page 42]

Internet-Draft                  XMPP Core                   October 2009


   C: <?xml version='1.0'?>
      <stream:stream
          from='juliet@im.example.com'
          to='unknown.host.example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   S: <?xml version='1.0'?>
      <stream:stream
          from='im.example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@im.example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>
      <stream:error>
        <host-unknown
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.6.1.4.  Where Stream Errors Are Sent

   When two XML streams are used between the initiating entity and the
   receiving entity (one in each direction) rather than using a single
   bidirectional stream, stanza errors triggered by stanzas sent over
   the outbound stream are returned on the inbound stream (since they
   are inbound stanzas from the perspective of the entity that sent the
   triggering stanza), whereas stream errors related to the outbound
   stream are returned on the outbound stream (since they are not
   inbound stanzas from the perspective of the entity that sent the
   triggering stanza but strictly related to the outbound stream
   itself); the same is true, naturally, of any stream errors that are
   related to the outbound stream but not triggered by an outbound
   stanza.

5.6.2.  Syntax

   The syntax for stream errors is as follows, where "defined-condition"
   is a placeholder for one of the conditions defined under
   Section 5.6.3.







Saint-Andre              Expires April 26, 2010                [Page 43]

Internet-Draft                  XMPP Core                   October 2009


   <stream:error>
     <defined-condition xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
     [<text xmlns='urn:ietf:params:xml:ns:xmpp-streams'
           xml:lang='langcode'>
       [ ... descriptive text ... ]
     </text>]
     [application-specific condition element]
   </stream:error>

   The <error/> element:

   o  MUST contain a child element corresponding to one of the defined
      stream error conditions (Section 5.6.3); this element MUST be
      qualified by the 'urn:ietf:params:xml:ns:xmpp-streams' namespace.
   o  MAY contain a <text/> child element containing XML character data
      that describes the error in more detail; this element MUST be
      qualified by the 'urn:ietf:params:xml:ns:xmpp-streams' namespace
      and SHOULD possess an 'xml:lang' attribute specifying the natural
      language of the XML character data.
   o  MAY contain a child element for an application-specific error
      condition; this element MUST be qualified by an application-
      defined namespace, and its structure is defined by that namespace
      (see Section 5.6.4).

   The <text/> element is OPTIONAL.  If included, it MUST be used only
   to provide descriptive or diagnostic information that supplements the
   meaning of a defined condition or application-specific condition.  It
   MUST NOT be interpreted programmatically by an application.  It MUST
   NOT be used as the error message presented to a human user, but MAY
   be shown in addition to the error message associated with the defined
   condition element (and, optionally, the application-specific
   condition element).

5.6.3.  Defined Stream Error Conditions

   The following stream-level error conditions are defined.

5.6.3.1.  bad-format

   The entity has sent XML that cannot be processed.

   (In the following example, the client sends an XMPP message that is
   not well-formed XML.)








Saint-Andre              Expires April 26, 2010                [Page 44]

Internet-Draft                  XMPP Core                   October 2009


   C: <message>
        <body>No closing body tag!
      </message>

   S: <stream:error>
        <bad-format
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

   This error MAY be used instead of the more specific XML-related
   errors, such as <bad-namespace-prefix/>, <invalid-xml/>, <restricted-
   xml/>, <unsupported-encoding/>, and <xml-not-well-formed/>.  However,
   the more specific errors are RECOMMENDED.

5.6.3.2.  bad-namespace-prefix

   The entity has sent a namespace prefix that is unsupported, or has
   sent no namespace prefix on an element that needs such a prefix (see
   Section 12.2).

   (In the following example, the client specifies a namespace prefix of
   "foobar" for the XML streams namespace.)

   C: <?xml version='1.0'?>
      <stream:stream
          from='juliet@im.example.com'
          to='im.example.com'
          version='1.0'
          xmlns='jabber:client'
          xmlns:foobar='http://etherx.jabber.org/streams'>

   S: <?xml version='1.0'?>
      <stream:stream
          from='im.example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@im.example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>
      <stream:error>
        <bad-namespace-prefix
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>





Saint-Andre              Expires April 26, 2010                [Page 45]

Internet-Draft                  XMPP Core                   October 2009


5.6.3.3.  conflict

   The server is either (1) closing the existing stream for this entity
   because a new stream has been initiated that conflicts with the
   existing stream, or (2) is refusing a new stream for this entity
   because allowing the new stream would conflict with an existing
   stream (e.g., because the server allows only a certain number of
   connections from the same IP address).

   C: <?xml version='1.0'?>
      <stream:stream
          from='juliet@im.example.com'
          to='im.example.com'
          version='1.0'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   S: <?xml version='1.0'?>
      <stream:stream
          from='im.example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@im.example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>
      <stream:error>
        <conflict
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.6.3.4.  connection-timeout

   The entity has not generated any traffic over the stream for some
   period of time (configurable according to a local service policy) and
   therefore the connection is being dropped.

   P: <stream:error>
        <connection-timeout
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>








Saint-Andre              Expires April 26, 2010                [Page 46]

Internet-Draft                  XMPP Core                   October 2009


5.6.3.5.  host-gone

   The value of the 'to' attribute provided in the initial stream header
   corresponds to a hostname that is no longer serviced by the receiving
   entity.

   (In the following example, the peer specifies a 'to' address of
   "foo.im.example.com" when connecting to the "im.example.com" server,
   but the server no longer hosts a service at that address.)

   P: <?xml version='1.0'?>
      <stream:stream
          from='example.net'
          to='foo.im.example.com'
          version='1.0'
          xmlns='jabber:server'
          xmlns:stream='http://etherx.jabber.org/streams'>

   S: <?xml version='1.0'?>
      <stream:stream
          from='im.example.com'
          id='g4qSvGvBxJ+xeAd7QKezOQJFFlw='
          to='example.net'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:server'
          xmlns:stream='http://etherx.jabber.org/streams'>
      <stream:error>
        <host-gone
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.6.3.6.  host-unknown

   The value of the 'to' attribute provided in the initial stream header
   does not correspond to a hostname that is serviced by the receiving
   entity.

   (In the following example, the peer specifies a 'to' address of
   "example.org" when connecting to the "im.example.com" server, but the
   server knows nothing of that address.)









Saint-Andre              Expires April 26, 2010                [Page 47]

Internet-Draft                  XMPP Core                   October 2009


   P: <?xml version='1.0'?>
      <stream:stream
          from='example.net'
          to='example.org'
          version='1.0'
          xmlns='jabber:server'
          xmlns:stream='http://etherx.jabber.org/streams'>

   S: <?xml version='1.0'?>
      <stream:stream
          from='im.example.com'
          id='g4qSvGvBxJ+xeAd7QKezOQJFFlw='
          to='example.net'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:server'
          xmlns:stream='http://etherx.jabber.org/streams'>
      <stream:error>
        <host-unknown
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.6.3.7.  improper-addressing

   A stanza sent between two servers lacks a 'to' or 'from' attribute,
   the 'from' or 'to' attribute has no value, or the value is not a
   valid XMPP address.

   (In the following example, the peer sends a stanza without a 'to'
   address.)

   P: <message from='juliet@im.example.com'>
        <body>Wherefore art thou?</body>
      </message>

   S: <stream:error>
        <improper-addressing
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.6.3.8.  internal-server-error

   The server has experienced a misconfiguration or an otherwise-
   undefined internal error that prevents it from servicing the stream.





Saint-Andre              Expires April 26, 2010                [Page 48]

Internet-Draft                  XMPP Core                   October 2009


   S: <stream:error>
        <internal-server-error
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.6.3.9.  invalid-from

   The JID or hostname provided in a 'from' address is not a valid JID
   or does not match an authorized JID or validated domain as negotiated
   between servers via SASL or server dialback, or as negotiated between
   a client and a server via authentication and resource binding.

   (In the following example, a peer that has authenticated only as
   "example.net" attempts to send a stanza from an address at
   "example.org".)

   P: <message from='romeo@example.org' to='juliet@im.example.com'>
        <body>Neither, fair saint, if either thee dislike.</body>
      </message>

   S: <stream:error>
        <invalid-from
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.6.3.10.  invalid-id

   The stream ID or server dialback ID is invalid or does not match an
   ID previously provided.

   (In the following example, the server dialback ID is invalid; see
   [XEP-0220].)

   P: <db:verify
          from='example.net'
          to='im.example.com'
          id='unknown-id'
          type='invalid'/>

   S: <stream:error>
        <invalid-id
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>





Saint-Andre              Expires April 26, 2010                [Page 49]

Internet-Draft                  XMPP Core                   October 2009


5.6.3.11.  invalid-namespace

   The streams namespace name is something other than
   "http://etherx.jabber.org/streams" (see Section 12.2) or the default
   namespace is not supported (e.g., something other than "jabber:
   client" or "jabber:server").

   (In the following example, the client specifies a streams namespace
   of 'http://wrong.namespace.example.org/'.)

   C: <?xml version='1.0'?>
      <stream:stream
          from='juliet@im.example.com'
          to='im.example.com'
          version='1.0'
          xmlns='jabber:client'
          xmlns:stream='http://wrong.namespace.example.org/'>

   S: <?xml version='1.0'?>
      <stream:stream
          from='im.example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@im.example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>
      <stream:error>
        <invalid-namespace
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.6.3.12.  invalid-xml

   The entity has sent invalid XML over the stream to a server that
   performs validation (see Section 12.4).

   (In the following example, the peer attempts to send an IQ stanza of
   type "subscribe" but the XML schema defines no such value for the
   'type' attribute.)










Saint-Andre              Expires April 26, 2010                [Page 50]

Internet-Draft                  XMPP Core                   October 2009


   P: <iq from='example.net'
          id='some-id'
          to='im.example.com'
          type='subscribe'>
        <ping xmlns='urn:xmpp:ping'/>
      </iq>

   S: <stream:error>
        <invalid-xml
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.6.3.13.  not-authorized

   The entity has attempted to send XML stanzas before the stream has
   been authenticated, or otherwise is not authorized to perform an
   action related to stream negotiation; the receiving entity MUST NOT
   process the offending stanza before sending the stream error.

   (In the following example, the client attempts to send XML stanzas
   before authenticating with the server.)





























Saint-Andre              Expires April 26, 2010                [Page 51]

Internet-Draft                  XMPP Core                   October 2009


   C: <?xml version='1.0'?>
      <stream:stream
          from='juliet@im.example.com'
          to='im.example.com'
          version='1.0'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   S: <?xml version='1.0'?>
      <stream:stream
          from='im.example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@im.example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'

   C: <message to='romeo@example.net'>
        <body>Wherefore art thou?</body>
      </message>

   S: <stream:error>
        <not-authorized
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.6.3.14.  policy-violation

   The entity has violated some local service policy (e.g., the stanza
   exceeds a configured size limit); the server MAY choose to specify
   the policy in the <text/> element or in an application-specific
   condition element.

   (In the following example, the client sends an XMPP message that is
   too large according to the server's local service policy.)

   C: <message to='juliet@im.example.com' id='foo'>
        <body>[ ... the-emacs-manual ... ]</body>
      </message>

   S: <stream:error>
        <policy-violation
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>

   S: </stream:stream>



Saint-Andre              Expires April 26, 2010                [Page 52]

Internet-Draft                  XMPP Core                   October 2009


5.6.3.15.  remote-connection-failed

   The server is unable to properly connect to a remote entity that is
   needed for authentication or authorization, such as a remote
   authentication database or (in server dialback) the authoritative
   server.

   C: <?xml version='1.0'?>
      <stream:stream
          from='juliet@im.example.com'
          to='im.example.com'
          version='1.0'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   S: <?xml version='1.0'?>
      <stream:stream
          from='im.example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@im.example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>
      <stream:error>
        <remote-connection-failed
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.6.3.16.  resource-constraint

   The server lacks the system resources necessary to service the
   stream.

















Saint-Andre              Expires April 26, 2010                [Page 53]

Internet-Draft                  XMPP Core                   October 2009


   C: <?xml version='1.0'?>
      <stream:stream
          from='juliet@im.example.com'
          to='im.example.com'
          version='1.0'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   S: <?xml version='1.0'?>
      <stream:stream
          from='im.example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@im.example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>
      <stream:error>
        <resource-constraint
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.6.3.17.  restricted-xml

   The entity has attempted to send restricted XML features such as a
   comment, processing instruction, DTD subset, or XML entity reference
   (see Section 12.1).

   (In the following example, the client sends an XMPP message
   containing an XML comment.)

   C: <message to='juliet@im.example.com'>
        <!--<subject/>-->
        <body>This message has no subject.</body>
      </message>

   S: <stream:error>
        <restricted-xml
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.6.3.18.  see-other-host

   The server will not provide service to the initiating entity but is
   redirecting traffic to another host; the XML character data of the
   <see-other-host/> element returned by the server SHOULD specify the



Saint-Andre              Expires April 26, 2010                [Page 54]

Internet-Draft                  XMPP Core                   October 2009


   alternate hostname or IP address at which to connect, which SHOULD be
   a valid domain identifier but MAY also include a port number.  When
   it receives a see-other-host stream error, the initiating entity
   SHOULD cleanly handle the disconnection and then reconnect to the
   host specified in the <see-other-host/> element; if no port is
   specified, the initiating entity SHOULD perform a [DNS-SRV] lookup on
   the provided domain identifier but MAY assume that it can connect to
   that domain identifier at the standard XMPP ports (i.e., 5222 for
   client-to-server connections and 5269 for server-to-server
   connections).

   C: <?xml version='1.0'?>
      <stream:stream
          from='juliet@im.example.com'
          to='im.example.com'
          version='1.0'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   S: <?xml version='1.0'?>
      <stream:stream
          from='im.example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@im.example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>
      <stream:error>
        <see-other-host
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'>
          [2001:41D0:1:A49b::1]:9222
        </see-other-host>
      </stream:error>
      </stream:stream>

5.6.3.19.  system-shutdown

   The server is being shut down and all active streams are being
   closed.

   S: <stream:error>
        <system-shutdown
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>





Saint-Andre              Expires April 26, 2010                [Page 55]

Internet-Draft                  XMPP Core                   October 2009


5.6.3.20.  undefined-condition

   The error condition is not one of those defined by the other
   conditions in this list; this error condition SHOULD be used only in
   conjunction with an application-specific condition.

   S: <stream:error>
        <undefined-condition
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
        <app-error xmlns='http://example.com/ns'/>
      </stream:error>
      </stream:stream>

5.6.3.21.  unsupported-encoding

   The initiating entity has encoded the stream in an encoding that is
   not supported by the server (see Section 12.6) or has otherwise
   improperly encoded the stream (e.g., by violating the rules of the
   [UTF-8] encoding).

   (In the following example, the client attempts to encode data using
   UTF-16 instead of UTF-8.)

   C: <?xml version='1.0' encoding='UTF-16'?>
      <stream:stream
          from='juliet@im.example.com'
          to='im.example.com'
          version='1.0'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   S: <?xml version='1.0'?>
      <stream:stream
          from='im.example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@im.example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
      <stream:error>
        <unsupported-encoding
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>






Saint-Andre              Expires April 26, 2010                [Page 56]

Internet-Draft                  XMPP Core                   October 2009


5.6.3.22.  unsupported-feature

   The receiving entity has advertised a mandatory stream feature that
   the initiating entity does not support, and has offered no other
   mandatory feature alongside the unsupported feature.

   (In the following example, the receiving entity requires negotiation
   of an example feature but the initiating entity does not support the
   feature.)

   R: <stream:features>
        <example xmlns='urn:xmpp:example'/>
      </stream:features>

   I: <stream:error>
        <unsupported-feature
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.6.3.23.  unsupported-stanza-type

   The initiating entity has sent a first-level child of the stream that
   is not supported by the server or consistent with the default
   namespace.

   (In the following example, the client attempts to send an XML stanza
   of <pubsub/> when the default namespace is "jabber:client".)























Saint-Andre              Expires April 26, 2010                [Page 57]

Internet-Draft                  XMPP Core                   October 2009


   C: <pubsub>
        <publish node='princely_musings'>
          <item id='ae890ac52d0df67ed7cfdf51b644e901'>
            <entry xmlns='http://www.w3.org/2005/Atom'>
              <title>Soliloquy</title>
              <summary>
   To be, or not to be: that is the question:
   Whether 'tis nobler in the mind to suffer
   The slings and arrows of outrageous fortune,
   Or to take arms against a sea of troubles,
   And by opposing end them?
              </summary>
              <link rel='alternate' type='text/html'
                    href='http://denmark.example/2003/12/13/atom03'/>
              <id>tag:denmark.example,2003:entry-32397</id>
              <published>2003-12-13T18:30:02Z</published>
              <updated>2003-12-13T18:30:02Z</updated>
            </entry>
          </item>
        </publish>
      </pubsub>

   S: <stream:error>
        <unsupported-stanza-type
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>

5.6.3.24.  unsupported-version

   The value of the 'version' attribute provided by the initiating
   entity in the stream header specifies a version of XMPP that is not
   supported by the server; the server MAY specify the version(s) it
   supports in the <text/> element.

   (In the following example, the client specifies an XMPP version of
   "11.0" but the server supports only version "1.0" and "1.1".)














Saint-Andre              Expires April 26, 2010                [Page 58]

Internet-Draft                  XMPP Core                   October 2009


   C: <?xml version='1.0'?>
      <stream:stream
          from='juliet@im.example.com'
          to='im.example.com'
          version='11.0'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   S: <?xml version='1.0'?>
      <stream:stream
          from='im.example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@im.example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'
      <stream:error>
        <unsupported-version
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
        <text xmlns='urn:ietf:params:xml:ns:xmpp-streams'>
          1.0, 1.1
        </text>
      </stream:error>
      </stream:stream>

5.6.3.25.  xml-not-well-formed

   The initiating entity has sent XML that violates the well-formedness
   rules of [XML] or [XML-NAMES].

   (In the following example, the client sends an XMPP message that is
   not well-formed XML.)

   C: <message>
        <body>No closing body tag!
      </message>

   S: <stream:error>
        <xml-not-well-formed
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>








Saint-Andre              Expires April 26, 2010                [Page 59]

Internet-Draft                  XMPP Core                   October 2009


5.6.4.  Application-Specific Conditions

   As noted, an application MAY provide application-specific stream
   error information by including a properly-namespaced child in the
   error element.  The application-specific element SHOULD supplement or
   further qualify a defined element.  Thus the <error/> element will
   contain two or three child elements.

   C: <message>
        <body>
          My keyboard layout is:

          QWERTYUIOP{}|
          ASDFGHJKL:"
          ZXCVBNM<>?
        </body>
      </message>

   S: <stream:error>
        <xml-not-well-formed
            xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
        <text xml:lang='en' xmlns='urn:ietf:params:xml:ns:xmpp-streams'>
          Some special application diagnostic information!
        </text>
        <escape-your-data xmlns='http://example.com/ns'/>
      </stream:error>
      </stream:stream>

5.7.  Simplified Stream Examples

   This section contains two simplified examples of a stream-based
   connection between a client and a server; these examples are included
   for the purpose of illustrating the concepts introduced thus far.


















Saint-Andre              Expires April 26, 2010                [Page 60]

Internet-Draft                  XMPP Core                   October 2009


   A basic connection:

   C: <?xml version='1.0'?>
      <stream:stream
          from='juliet@im.example.com'
          to='im.example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   S: <?xml version='1.0'?>
      <stream:stream
          from='im.example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@im.example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   [ ... channel encryption ... ]

   [ ... authentication ... ]

   [ ... resource binding ... ]

   C:   <message from='juliet@im.example.com/balcony'
                 to='romeo@example.net'
                 xml:lang='en'>
          <body>Art thou not Romeo, and a Montague?</body>
        </message>

   S:   <message from='romeo@example.net/orchard'
                 to='juliet@im.example.com/balcony'
                 xml:lang='en'>
          <body>Neither, fair saint, if either thee dislike.</body>
        </message>

   C: </stream:stream>

   S: </stream:stream>









Saint-Andre              Expires April 26, 2010                [Page 61]

Internet-Draft                  XMPP Core                   October 2009


   A connection gone bad:

   C: <?xml version='1.0'?>
      <stream:stream
          from='juliet@im.example.com'
          to='im.example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   S: <?xml version='1.0'?>
      <stream:stream
          from='im.example.com'
          id='++TR84Sm6A3hnt3Q065SnAbbk3Y='
          to='juliet@im.example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   [ ... channel encryption ... ]

   [ ... authentication ... ]

   [ ... resource binding ... ]


   C:   <message from='juliet@im.example.com/balcony'
                 to='romeo@example.net'
                 xml:lang='en'>
          <body>No closing body tag!
        </message>

   S: <stream:error>
       <xml-not-well-formed
           xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
      </stream:error>
      </stream:stream>


   More detailed examples are provided under Section 10.


6.  STARTTLS Negotiation






Saint-Andre              Expires April 26, 2010                [Page 62]

Internet-Draft                  XMPP Core                   October 2009


6.1.  Overview

   XMPP includes a method for securing the stream from tampering and
   eavesdropping.  This channel encryption method makes use of the
   Transport Layer Security [TLS] protocol, specifically a "STARTTLS"
   extension that is modelled after similar extensions for the [IMAP],
   [POP3], and [ACAP] protocols as described in [USINGTLS].  The XML
   namespace name for the STARTTLS extension is
   'urn:ietf:params:xml:ns:xmpp-tls'.

   Support for STARTTLS is REQUIRED in XMPP client and server
   implementations.  An administrator of a given deployment MAY
   necessitate the use of TLS for client-to-server communication,
   server-to-server communication, or both.  A deployed client SHOULD
   use TLS to secure its stream with a server prior to attempting the
   completion of SASL negotiation (Section 7), and deployed servers
   SHOULD use TLS between two domains for the purpose of securing
   server-to-server communication.

6.2.  Rules

6.2.1.  Mandatory-to-Negotiate

   If the receiving entity advertises only the STARTTLS feature or if
   the receiving entity includes the <required/> child element, the
   parties MUST consider TLS as mandatory-to-negotiate.

6.2.2.  Restart

   After TLS negotiation, the parties MUST restart the stream.

6.2.3.  Data Formatting

   During STARTTLS negotiation, the entities MUST NOT send any
   whitespace as separators between XML elements (i.e., from the last
   character of the <starttls/> element qualified by the
   'urn:ietf:params:xml:ns:xmpp-tls' namespace at depth=1 of the stream
   as sent by the initiating entity until the last character of the
   <proceed/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls'
   namespace at depth=1 of the stream as sent by the receiving entity).
   This prohibition helps to ensure proper security layer byte
   precision.  Any such whitespace shown in the STARTTLS examples
   provided in this document is included only for the sake of
   readability.







Saint-Andre              Expires April 26, 2010                [Page 63]

Internet-Draft                  XMPP Core                   October 2009


6.2.4.  Order of Negotiation

   If the initiating entity chooses to use TLS, STARTTLS negotiation
   MUST be completed before proceeding to SASL negotiation (Section 7);
   this order of negotiation is necessary to help safeguard
   authentication information sent during SASL negotiation, as well as
   to make it possible to base the use of the SASL EXTERNAL mechanism on
   a certificate (or other credentials) provided during prior TLS
   negotiation.

6.3.  Process

6.3.1.  Exchange of Stream Headers and Stream Features

   The initiating entity resolves the hostname of the receiving entity
   as specified under Section 4, opens a TCP connection to the
   advertised port at the resolved IP address, and sends an initial
   stream header to the receiving entity; if the initiating entity is
   capable of STARTTLS negotiation, it MUST include the 'version'
   attribute set to a value of at least "1.0" in the initial stream
   header.

   I: <stream:stream
        from='juliet@im.example.com'
        to='im.example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'>

   The receiving entity MUST send a response stream header to the
   initiating entity over the TCP connection opened by the initiating
   entity; if the receiving entity is capable of STARTTLS negotiation,
   it MUST include the 'version' attribute set to a value of at least
   "1.0" in the response stream header.

   R: <stream:stream
        from='im.example.com'
        id='t7AMCin9zjMNwQKDnplntZPIDEI='
        to='juliet@im.example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'

   The receiving entity then MUST send stream features to the initiating
   entity.  If the receiving entity supports TLS, the stream features
   MUST include an advertisement for support of STARTTLS negotiation,



Saint-Andre              Expires April 26, 2010                [Page 64]

Internet-Draft                  XMPP Core                   October 2009


   i.e., a <starttls/> element qualified by the
   'urn:ietf:params:xml:ns:xmpp-tls' namespace.

   If the receiving entity considers STARTTLS negotiation to be
   mandatory, the <starttls/> element SHOULD contain an empty
   <required/> child element.

   R: <stream:features>
        <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
          <required/>
        </starttls>
      </stream:features>

6.3.2.  Initiation of STARTTLS Negotiation

6.3.2.1.  STARTTLS Command

   In order to begin the STARTTLS negotiation, the initiating entity
   issues the STARTTLS command (i.e., a <starttls/> element qualified by
   the 'urn:ietf:params:xml:ns:xmpp-tls' namespace) to instruct the
   receiving entity that it wishes to begin a STARTTLS negotiation to
   secure the stream.

   I: <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

   The receiving entity MUST reply with either a <proceed/> element
   (proceed case) or a <failure/> element (failure case) qualified by
   the 'urn:ietf:params:xml:ns:xmpp-tls' namespace.

6.3.2.2.  Failure Case

   If the failure case occurs, the receiving entity MUST return a
   <failure/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls'
   namespace, terminate the XML stream, and terminate the underlying TCP
   connection.

   R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

   R: </stream:stream>

   Causes for the failure case include but are not limited to:

   1.  The initiating entity has sent a malformed STARTTLS command.
   2.  The receiving entity does not offer STARTTLS negotiation either
       temporarily or permanently.
   3.  The receiving entity cannot complete STARTTLS negotiation because
       of an internal error.




Saint-Andre              Expires April 26, 2010                [Page 65]

Internet-Draft                  XMPP Core                   October 2009


      Note: STARTTLS failure is not triggered by TLS errors such as bad
      certificate or unknown certificate authority; those errors are
      generated and handled during the TLS negotiation itself as
      described in [TLS].

   If the failure case occurs, the initiating entity MAY attempt to
   reconnect as explained under Section 4.5.

6.3.2.3.  Proceed Case

   If the proceed case occurs, the receiving entity MUST return a
   <proceed/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-tls'
   namespace.

   R: <proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

   The receiving entity MUST consider the TLS negotiation to have begun
   immediately after sending the closing '>' character of the <proceed/>
   element to the initiating entity.  The initiating entity MUST
   consider the TLS negotiation to have begun immediately after
   receiving the closing '>' character of the <proceed/> element from
   the receiving entity.

   The entities now proceed to TLS negotiation as explained in the next
   section.

6.3.3.  TLS Negotiation

6.3.3.1.  Rules

   In order to complete TLS negotiation over the TCP connection, the
   entities MUST follow the process defined in [TLS].

   The following rules apply:

   1.  The entities MUST NOT send any further XML data until the TLS
       negotiation has either failed or succeeded.
   2.  The receiving entity MUST present a certificate.
   3.  The receiving entity SHOULD send a certificate request to the
       initiating entity so that mutual authentication will be possible.
   4.  The initiating entity MUST validate the certificate to determine
       if the TLS negotiation shall succeed; see Section 14.2.2
       regarding certificate validation procedures.
   5.  The receiving entity SHOULD choose which certificate to present
       based on the 'to' attribute of the initial stream header.






Saint-Andre              Expires April 26, 2010                [Page 66]

Internet-Draft                  XMPP Core                   October 2009


      Note: See Section 14.6 regarding ciphers that MUST be supported
      for TLS; naturally, other ciphers MAY be supported as well.

6.3.3.2.  TLS Failure

   If the TLS negotiation results in failure, the receiving entity MUST
   terminate the TCP connection.

   The receiving entity MUST NOT send a closing </stream> tag before
   terminating the TCP connection, since the receiving entity and
   initiating entity MUST consider the original stream to be replaced
   upon failure of the TLS negotiation.

6.3.3.3.  TLS Success

   If the TLS negotiation is successful, then the entities MUST proceed
   as follows.

   1.  The initiating entity MUST discard any information transmitted in
       layers above TCP that it obtained from the receiving entity in an
       insecure manner before TLS took effect (e.g., the receiving
       entity's from address or the stream ID and stream features
       received from the receiving entity).
   2.  The receiving entity MUST discard any information transmitted in
       layers above TCP that it obtained from the initiating entity in
       an insecure manner TLS took effect (e.g., the initiating entity's
       from address).
   3.  The initiating entity MUST send a new initial stream header to
       the receiving entity over the encrypted connection.

   I: <stream:stream
        from='juliet@im.example.com'
        to='im.example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'>

       Note: The initiating entity MUST NOT send a closing </stream> tag
       before sending the new initial stream header, since the receiving
       entity and initiating entity MUST consider the original stream to
       be replaced upon success of the TLS negotiation.
   4.  The receiving entity MUST respond with a new response stream
       header over the encrypted connection.







Saint-Andre              Expires April 26, 2010                [Page 67]

Internet-Draft                  XMPP Core                   October 2009


   R: <stream:stream
        from='im.example.com'
        id='vgKi/bkYME8OAj4rlXMkpucAqe4='
        to='juliet@im.example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'

   5.  The receiving entity also MUST send stream features to the
       initiating entity, which MUST NOT include the STARTTLS feature
       but which SHOULD include the SASL stream feature as described
       under Section 7.

   R: <stream:features>
        <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
          <mechanism>EXTERNAL</mechanism>
          <mechanism>PLAIN</mechanism>
        </mechanisms>
      </stream:features>


7.  SASL Negotiation

7.1.  Overview

   XMPP includes a method for authenticating a stream by means of an
   XMPP-specific profile of the Simple Authentication and Security Layer
   protocol (see [SASL]).  SASL provides a generalized method for adding
   authentication support to connection-based protocols, and XMPP uses
   an XML namespace profile of SASL that conforms to the profiling
   requirements of [SASL].

   Support for SASL negotiation is REQUIRED in XMPP client and server
   implementations.

7.2.  Rules

7.2.1.  Mandatory-to-Negotiate

   The parties to a stream MUST consider SASL as mandatory-to-negotiate.

7.2.2.  Restart

   After SASL negotiation, the parties MUST restart the stream.






Saint-Andre              Expires April 26, 2010                [Page 68]

Internet-Draft                  XMPP Core                   October 2009


7.2.3.  Mechanism Preferences

   Any entity that will act as a SASL client or a SASL server MUST
   maintain an ordered list of its preferred SASL mechanisms according
   to the client or server, where the list is ordered by the perceived
   strength of the mechanisms.  A server MUST offer and a client MUST
   try SASL mechanisms in the order of their perceived strength.  For
   example, if the server offers the ordered list "PLAIN DIGEST-MD5
   GSSAPI" or "DIGEST-MD5 GSSAPI PLAIN" but the client's ordered list is
   "GSSAPI DIGEST-MD5", the client shall try GSSAPI first and then
   DIGEST-MD5 but shall never try PLAIN (since PLAIN is not on its
   list).

7.2.4.  Mechanism Offers

   If the receiving entity considers TLS negotiation (Section 6) to be
   mandatory before use of a particular SASL authentication mechanism
   will be acceptable, the receiving entity MUST NOT advertise that
   mechanism in its list of available SASL authentication mechanisms
   prior to successful TLS negotiation.

   If during prior TLS negotiation the initiating entity presented a
   certificate that is acceptable to the receiving entity for purposes
   of strong identity verification in accordance with local service
   policies, the receiving entity MUST offer the SASL EXTERNAL mechanism
   to the initiating entity during SASL negotiation (refer to [SASL])
   and SHOULD prefer that mechanism.  However, the EXTERNAL mechanism
   MAY be offered under other circumstances as well.

   See Section 14.6 regarding mechanisms that MUST be supported;
   naturally, other SASL mechanisms MAY be supported as well.  Best
   practices for the use of several SASL mechanisms in the context of
   XMPP are described in [XEP-0175] and [XEP-0178].

7.2.5.  Data Formatting

   The following data formattting rules apply to the SASL negotiation:

   1.  During SASL negotiation, the entities MUST NOT send any
       whitespace as separators between XML elements (i.e., from the
       last character of the <auth/> element qualified by the
       'urn:ietf:params:xml:ns:xmpp-sasl' namespace at depth=1 of the
       stream as sent by the initiating entity until the last character
       of the <success/> element qualified by the
       'urn:ietf:params:xml:ns:xmpp-sasl' namespace at depth=1 of the
       stream as sent by the receiving entity).  This prohibition helps
       to ensure proper security layer byte precision.  Any such
       whitespace shown in the SASL examples provided in this document



Saint-Andre              Expires April 26, 2010                [Page 69]

Internet-Draft                  XMPP Core                   October 2009


       is included only for the sake of readability.
   2.  Any XML character data contained within the XML elements MUST be
       encoded using base64, where the encoding adheres to the
       definition in Section 4 of [BASE64] and where the padding bits
       are set to zero.
   3.  As formally specified in the XML schema for the
       'urn:ietf:params:xml:ns:xmpp-sasl' namespace under Appendix C.4,
       the receiving entity MAY include one or more application-specific
       child elements inside the <mechanisms/> element to provide
       information that might be needed by the initiating entity in
       order to complete successful SASL negotiation using one or more
       of the offered mechanisms; however, the syntax and semantics of
       all such elements are out of scope for this specification.

7.2.6.  Security Layers

   Upon successful SASL negotiation that involves negotiation of a
   security layer, both the initiating entity and the receiving MUST
   discard any application-layer state (i.e, state from the XMPP layer,
   excluding state from the TLS negotiation or SASL negotiation).

7.2.7.  Simple Usernames

   It is possible that provision of a "simple username" is supported by
   the selected SASL mechanism (e.g., this is supported by the DIGEST-
   MD5 and CRAM-MD5 mechanisms but not by the EXTERNAL and GSSAPI
   mechanisms).  The simple username provided during authentication MUST
   be as follows:

   Client-to-server communication:  The initiating entity's registered
      account name, i.e., a user name as described under Section 3.3
      (this is not a bare JID of the form <localpart@domain> but only
      the localpart of the JID).  The simple username MUST adhere to the
      Nodeprep (Appendix A) profile of [STRINGPREP].
   Server-to-server communication:  The initiating entity's sending
      domain, i.e., IP address or fully qualified domain name as
      contained in an XMPP domain identifier.  The simple username MUST
      adhere to the [NAMEPREP] profile of [STRINGPREP].

7.2.8.  Authorization Identities

   An authorization identity is an optional identity specified by the
   initiating entity, which is typically used by an administrator to
   perform some management task on behalf of another user.  If the
   initiating entity wishes to act on behalf of another entity and the
   selected SASL mechanism supports transmission of an authorization
   identity, the initiating entity MUST provide an authorization
   identity during SASL negotiation.  If the initiating entity does not



Saint-Andre              Expires April 26, 2010                [Page 70]

Internet-Draft                  XMPP Core                   October 2009


   wish to act on behalf of another entity, it MUST NOT provide an
   authorization identity.  As specified in [SASL], the initiating
   entity MUST NOT provide an authorization identity unless the
   authorization identity is different from the default authorization
   identity derived from the authentication identity.  If provided, the
   value of the authorization identity MUST be a bare JID of the form
   <domain> (i.e., an XMPP domain identifier only) for servers and a
   bare JID of the form <localpart@domain> (i.e., localpart and domain
   identifier) for clients.

      Note: The authorization identity communited during SASL
      negotiation is used to determine the canonical address for the
      initiating client or server according to the receiving server, as
      described under Section 3.5.

7.2.9.  Realms

   The receiving entity MAY include a realm when negotiating certain
   SASL mechanisms.  If the receiving entity does not communicate a
   realm, the initiating entity MUST NOT assume that any realm exists.
   The realm MUST be used only for the purpose of authentication; in
   particular, an initiating entity MUST NOT attempt to derive an XMPP
   hostname from the realm information provided by the receiving entity.

7.2.10.  Round Trips

   [SASL] specifies that a using protocol such as XMPP can define two
   methods by which the protocol can save round trips where allowed for
   the SASL mechanism:

   1.  When the SASL client (the XMPP "initiating entity") requests an
       authentication exchange, it can include "initial response" data
       with its request if appropriate for the SASL mechanism in use.
       In XMPP this is done by including the initial response as the XML
       character data of the <auth/> element.
   2.  At the end of the authentication exchange, the SASL server (the
       XMPP "receiving entity") can include "additional data with
       success" if appropriate for the SASL mechanism in use.  In XMPP
       this is done by including the additional data as the XML
       character data of the <success/> element.

   For the sake of protocol efficiency, it is RECOMMENDED for XMPP
   clients and servers to use these methods, however they MUST support
   the less efficient modes as well.







Saint-Andre              Expires April 26, 2010                [Page 71]

Internet-Draft                  XMPP Core                   October 2009


7.3.  Process

   The process for SASL negotiation is as follows.

7.3.1.  Exchange of Stream Headers and Stream Features

   If SASL negotiation follows successful STARTTLS negotation
   (Section 6), then the SASL negotiation occurs over the encrypted
   stream that has already been negotiated.  If not, the initiating
   entity resolves the hostname of the receiving entity as specified
   under Section 4, opens a TCP connection to the advertised port at the
   resolved IP address, and sends an initial stream header to the
   receiving entity; if the initiating entity is capable of STARTTLS
   negotiation, it MUST include the 'version' attribute set to a value
   of at least "1.0" in the initial stream header.

   I: <stream:stream
        from='juliet@im.example.com'
        to='im.example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'>

   The receiving entity MUST send a response stream header to the
   initiating entity; if the receiving entity is capable of SASL
   negotiation, it MUST include the 'version' attribute set to a value
   of at least "1.0" in the response stream header.

   R: <stream:stream
        from='im.example.com'
        id='vgKi/bkYME8OAj4rlXMkpucAqe4='
        to='juliet@im.example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'

   The receiving entity also MUST send stream features to the initiating
   entity.  If the receiving entity supports SASL, the stream features
   MUST include an advertisement for support of SASL negotiation, i.e.,
   a <mechanisms/> element qualified by the
   'urn:ietf:params:xml:ns:xmpp-sasl' namespace.

   The <mechanisms/> element MUST contain one <mechanism/> child element
   for each authentication mechanism the receiving entity offers to the
   initiating entity.  The order of <mechanism/> elements in the XML
   indicates the preference order of the SASL mechanisms according to



Saint-Andre              Expires April 26, 2010                [Page 72]

Internet-Draft                  XMPP Core                   October 2009


   the receiving entity; however the initiating entity MUST maintain its
   own preference order independent of the preference order of the
   receiving entity.

   R: <stream:features>
        <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
          <mechanism>EXTERNAL</mechanism>
          <mechanism>PLAIN</mechanism>
        </mechanisms>
      </stream:features>

7.3.2.  Initiation

   In order to begin the SASL negotiation, the initiating entity sends
   an <auth/> element qualified by the
   'urn:ietf:params:xml:ns:xmpp-sasl' namespace and includes an
   appropriate value for the 'mechanism' attribute.  This element MAY
   contain XML character data (in SASL terminology, the "initial
   response") if the mechanism supports or requires it; if the
   initiating entity needs to send a zero-length initial response, it
   MUST transmit the response as a single equals sign character ("="),
   which indicates that the response is present but contains no data.

   I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
            mechanism='PLAIN'>AGp1bGlldAByMG0zMG15cjBtMzA=</auth>

7.3.3.  Challenge-Response Sequence

   If necessary, the receiving entity challenges the initiating entity
   by sending a <challenge/> element qualified by the
   'urn:ietf:params:xml:ns:xmpp-sasl' namespace; this element MAY
   contain XML character data (which MUST be generated in accordance
   with the definition of the SASL mechanism chosen by the initiating
   entity).

   The initiating entity responds to the challenge by sending a
   <response/> element qualified by the
   'urn:ietf:params:xml:ns:xmpp-sasl' namespace; this element MAY
   contain XML character data (which MUST be generated in accordance
   with the definition of the SASL mechanism chosen by the initiating
   entity).

   If necessary, the receiving entity sends more challenges and the
   initiating entity sends more responses.

   This series of challenge/response pairs continues until one of three
   things happens:




Saint-Andre              Expires April 26, 2010                [Page 73]

Internet-Draft                  XMPP Core                   October 2009


   o  The initiating entity aborts the handshake.
   o  The receiving entity reports failure of the handshake.
   o  The receiving entity reports success of the handshake.

   These scenarios are described in the following sections.

7.3.4.  Abort

   The initiating entity aborts the handshake by sending an <abort/>
   element qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl'
   namespace.

   I: <abort xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>

   Upon receiving an <abort/> element, the receiving entity MUST return
   a <failure/> element qualified by the
   'urn:ietf:params:xml:ns:xmpp-sasl' namespace and containing an
   <aborted/> child element.

   R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
        <aborted/>
      </failure>

7.3.5.  Failure

   The receiving entity reports failure of the handshake by sending a
   <failure/> element qualified by the
   'urn:ietf:params:xml:ns:xmpp-sasl' namespace (the particular cause of
   failure MUST be communicated in an appropriate child element of the
   <failure/> element as defined under Section 7.4).

   R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
        <not-authorized/>
      </failure>

   Where appropriate for the chosen SASL mechanism, the receiving entity
   SHOULD allow a configurable but reasonable number of retries (at
   least 2 and no more than 5); this enables the initiating entity
   (e.g., an end-user client) to tolerate incorrectly-provided
   credentials (e.g., a mistyped password) without being forced to
   reconnect.

   If the initiating entity attempts a reasonable number of retries with
   the same SASL mechanism and all attempts fail, it MAY fall back to
   the next mechanism in its ordered list by sending a new <auth/>
   request to the receiving entity.  If there are no remaining
   mechanisms in its list, the initiating entity SHOULD instead send an
   <abort/> element to the receiving entity.



Saint-Andre              Expires April 26, 2010                [Page 74]

Internet-Draft                  XMPP Core                   October 2009


   If the initiating entity exceeds the number of retries, the receiving
   entity MUST return a stream error (which SHOULD be <policy-
   violation/> but MAY be <not-authorized/>).

7.3.6.  Success

   The receiving entity reports success of the handshake by sending a
   <success/> element qualified by the
   'urn:ietf:params:xml:ns:xmpp-sasl' namespace; this element MAY
   contain XML character data (in SASL terminology, "additional data
   with success") if the chosen SASL mechanism supports or requires it;
   if the receiving entity needs to send additional data of zero length,
   it MUST transmit the data as a single equals sign character ("=").

   R: <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>

      Note: The authorization identity communited during SASL
      negotiation is used to determine the canonical address for the
      initiating client or server according to the receiving server, as
      described under Section 3.5.

   Upon receiving the <success/> element, the initiating entity MUST
   initiate a new stream over the existing TCP connection by sending a
   new initial stream header to the receiving entity.

   I: <stream:stream
        from='juliet@im.example.com'
        to='im.example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'

      Note: The initiating entity MUST NOT send a closing </stream> tag
      before sending the new initial stream header, since the receiving
      entity and initiating entity MUST consider the original stream to
      be replaced upon sending or receiving the <success/> element.

   Upon receiving the new initial stream header from the initiating
   entity, the receiving entity MUST respond by sending a new response
   XML stream header to the initiating entity.










Saint-Andre              Expires April 26, 2010                [Page 75]

Internet-Draft                  XMPP Core                   October 2009


   R: <stream:stream
        from='im.example.com'
        id='gPybzaOzBmaADgxKXu9UClbprp0='
        to='juliet@im.example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'>

   The receiving entity MUST also send stream features, containing any
   further available features or containing no features (via an empty
   <features/> element).

   R: <stream:features>
        <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
      </stream:features>

7.4.  SASL Errors

   The syntax of SASL errors is as follows:

   <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
     <defined-condition/>
     [<text xml:lang='langcode'>
         OPTIONAL descriptive text
     </text>]
   </failure>

   Where "defined-condition" is one of the SASL-related error conditions
   defined in the following sections.

   Inclusion of a defined condition is REQUIRED.

   Inclusion of the <text/> element is OPTIONAL, and can be used to
   provide application-specific information about the error condition,
   which information MAY be displayed to a human but only as a
   supplement to the defined condition.

7.4.1.  aborted

   The receiving entity acknowledges an <abort/> element sent by the
   initiating entity; sent in reply to the <abort/> element.

   I: <abort xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>

   R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
        <aborted/>
      </failure>



Saint-Andre              Expires April 26, 2010                [Page 76]

Internet-Draft                  XMPP Core                   October 2009


7.4.2.  account-disabled

   The account of the initiating entity has been temporarily disabled;
   sent in reply to an <auth/> element (with or without initial response
   data) or a <response/> element.

   I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
            mechanism='PLAIN'>AGp1bGlldAByMG0zMG15cjBtMzA=</auth>

   R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
        <account-disabled/>
        <text xml:lang='en'>Call 212-555-1212 for assistance.</text>
      </failure>

7.4.3.  credentials-expired

   The authentication failed because the initiating entity provided
   credentials that have expired; sent in reply to a <response/> element
   or an <auth/> element with initial response data.

   I: <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
        [ ... ]
      </response>

   R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
        <credentials-expired/>
      </failure>

7.4.4.  encryption-required

   The mechanism requested by the initiating entity cannot be used
   unless the underlying stream is encrypted; sent in reply to an
   <auth/> element (with or without initial response data).

   I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
            mechanism='PLAIN'>AGp1bGlldAByMG0zMG15cjBtMzA=</auth>

   R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
        <encryption-required/>
      </failure>

7.4.5.  incorrect-encoding

   The data provided by the initiating entity could not be processed
   because the [BASE64] encoding is incorrect (e.g., because the
   encoding does not adhere to the definition in Section 4 of [BASE64]);
   sent in reply to a <response/> element or an <auth/> element with
   initial response data.



Saint-Andre              Expires April 26, 2010                [Page 77]

Internet-Draft                  XMPP Core                   October 2009


   I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
            mechanism='DIGEST-MD5'>[ ... ]</auth>

   R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
        <incorrect-encoding/>
      </failure>

7.4.6.  invalid-authzid

   The authzid provided by the initiating entity is invalid, either
   because it is incorrectly formatted or because the initiating entity
   does not have permissions to authorize that ID; sent in reply to a
   <response/> element or an <auth/> element with initial response data.

   I: <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
        [ ... ]
      </response>

   R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
        <invalid-authzid/>
      </failure>

7.4.7.  invalid-mechanism

   The initiating entity did not provide a mechanism or requested a
   mechanism that is not supported by the receiving entity; sent in
   reply to an <auth/> element.

   I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
            mechanism='CRAM-MD5'/>

   R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
        <invalid-mechanism/>
      </failure>

7.4.8.  malformed-request

   The request is malformed (e.g., the <auth/> element includes initial
   response data but the mechanism does not allow that, or the data sent
   violates the syntax for the specified SASL mechanism); sent in reply
   to an <abort/>, <auth/>, <challenge/>, or <response/> element.

   (In the following example, the XML character data of the <auth/>
   element contains more than 255 UTF-8-encoded Unicode characters and
   therefore violates the "token" production for the SASL ANONYMOUS
   mechanism as specified in [ANONYMOUS].)





Saint-Andre              Expires April 26, 2010                [Page 78]

Internet-Draft                  XMPP Core                   October 2009


   I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
            mechanism='ANONYMOUS'>[ ... some-long-token ... ]</auth>

   R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
        <malformed-request/>
      </failure>

7.4.9.  mechanism-too-weak

   The mechanism requested by the initiating entity is weaker than
   server policy permits for that initiating entity; sent in reply to an
   <auth/> element (with or without initial response data).

   I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
            mechanism='PLAIN'>AGp1bGlldAByMG0zMG15cjBtMzA=</auth>

   R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
        <mechanism-too-weak/>
      </failure>

7.4.10.  not-authorized

   The authentication failed because the initiating entity did not
   provide proper credentials or the receiving entity has detected an
   attack but wishes to disclose as little information as possible to
   the attacker; sent in reply to a <response/> element or an <auth/>
   element with initial response data.

   I: <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
        [ ... ]
      </response>

   R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
        <not-authorized/>
      </failure>

      Note: This error condition includes but is not limited to the case
      of incorrect credentials or an unknown username.  In order to
      discourage directory harvest attacks, no differentiation is made
      between incorrect credentials and an unknown username.

7.4.11.  temporary-auth-failure

   The authentication failed because of a temporary error condition
   within the receiving entity, and it is advisable for the initiating
   entity to try again later; sent in reply to an <auth/> element or a
   <response/> element.




Saint-Andre              Expires April 26, 2010                [Page 79]

Internet-Draft                  XMPP Core                   October 2009


   I: <response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
        [ ... ]
      </response>

   R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
        <temporary-auth-failure/>
      </failure>

7.4.12.  transition-needed

   The authentication failed because the mechanism cannot be used until
   the initiating entity provides (for one time only) a plaintext
   password so that the receiving entity can build a hashed password for
   use in future authentication attempts; sent in reply to an <auth/>
   element with or without initial response data.

   I: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
            mechanism='CRAM-MD5'>[ ... ]</auth>

   R: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
        <transition-needed/>
      </failure>

      Note: An XMPP client MUST treat a <transition-needed/> error with
      extreme caution, SHOULD NOT provide a plaintext password over an
      XML stream that is not encrypted via Transport Layer Security, and
      MUST warn a human user before allowing the user to provide a
      plaintext password over an unencrypted connection.

7.5.  SASL Definition

   The profiling requirements of [SASL] require that the following
   information be supplied by the definition of a using protocol.

   service name:  "xmpp"
   initiation sequence:  After the initiating entity provides an opening
      XML stream header and the receiving entity replies in kind, the
      receiving entity provides a list of acceptable authentication
      methods.  The initiating entity chooses one method from the list
      and sends it to the receiving entity as the value of the
      'mechanism' attribute possessed by an <auth/> element, optionally
      including an initial response to avoid a round trip.
   exchange sequence:  Challenges and responses are carried through the
      exchange of <challenge/> elements from receiving entity to
      initiating entity and <response/> elements from initiating entity
      to receiving entity.  The receiving entity reports failure by
      sending a <failure/> element and success by sending a <success/>
      element; the initiating entity aborts the exchange by sending an



Saint-Andre              Expires April 26, 2010                [Page 80]

Internet-Draft                  XMPP Core                   October 2009


      <abort/> element.  Upon successful negotiation, both sides
      consider the original XML stream to be closed and new stream
      headers are sent by both entities.
   security layer negotiation:  The security layer takes effect
      immediately after sending the closing '>' character of the
      <success/> element for the receiving entity, and immediately after
      receiving the closing '>' character of the <success/> element for
      the initiating entity.  The order of layers is first [TCP], then
      [TLS], then [SASL], then XMPP.
   use of the authorization identity:  The authorization identity can be
      used in XMPP to denote the non-default <localpart@domain> of a
      client or the sending <domain> of a server; an empty string is
      equivalent to an absent authorization identity.


8.  Resource Binding

8.1.  Overview

   After a client authenticates with a server, it MUST bind a specific
   resource to the stream so that the server can properly address the
   client (see Section 3).  That is, there MUST be an XMPP resource
   identifier associated with the bare JID (<localpart@domain>) of the
   client, so that the address for use over that stream is a full JID of
   the form <localpart@domain/resource>.  This ensures that the server
   can deliver XML stanzas to and receive XML stanzas from the client in
   relation to entities other than the server itself, as explained under
   Section 11 (the client could exchange stanzas with the server itself
   before binding a resource since the full JID is needed only for
   addressing outside the context of the stream negotiated between the
   client and the server, but this is not commonly done).

   After a client has bound a resource to the stream, it is referred to
   as a CONNECTED RESOURCE.  A server SHOULD allow an entity to maintain
   multiple connected resources simultaneously, where each connected
   resource is associated with a distinct XML stream and differentiated
   from the other connected resources by a distinct resource identifier;
   however, a server MUST enable the administrator of an XMPP service to
   limit the number of connected resources in order to prevent certain
   denial of service attacks as described under Section 14.14.

   If, before completing the resource binding step, the client attempts
   to send an XML stanza to an entity other than the client's account or
   the server, the server MUST NOT process the stanza and MUST either
   ignore the stanza or return a <not-authorized/> stream error to the
   client.

   Support for resource binding is REQUIRED in XMPP client and server



Saint-Andre              Expires April 26, 2010                [Page 81]

Internet-Draft                  XMPP Core                   October 2009


   implementations.

8.2.  Rules

8.2.1.  Mandatory-to-Negotiate

   The parties to a stream MUST consider resource binding as mandatory-
   to-negotiate.

8.2.2.  Restart

   After resource binding, the parties MUST NOT restart the stream.

8.3.  Advertising Support

   Upon sending a new response stream header to the client after
   successful SASL negotiation, the server MUST include a <bind/>
   element qualified by the 'urn:ietf:params:xml:ns:xmpp-bind' namespace
   in the stream features it presents to the client.

      Note: Resource binding is mandatory-to-negotiate for clients.

      Note: The server MUST NOT include the resource binding stream
      feature until after the client has authenticated, typically by
      means of successful SASL negotiation.

   S: <stream:stream
          from='im.example.com'
          id='gPybzaOzBmaADgxKXu9UClbprp0='
          to='juliet@im.example.com'
          version='1.0'
          xml:lang='en'
          xmlns='jabber:client'
          xmlns:stream='http://etherx.jabber.org/streams'>

   S: <stream:features>
        <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
      </stream:features>

   Upon being so informed that resource binding is mandatory, the client
   MUST bind a resource to the stream as described in the following
   sections.

8.4.  Generation of Resource Identifiers

   A resource identifier MUST at a minimum be unique among the connected
   resources for that <localpart@domain>.  Enforcement of this policy is
   the responsibility of the server.



Saint-Andre              Expires April 26, 2010                [Page 82]

Internet-Draft                  XMPP Core                   October 2009


   A resource identifier can be security-critical.  For example, if a
   malicious entity can guess a client's resource identifier then it
   might be able to determine if the client (and therefore the
   controlling principal) is online or offline, thus resulting in a
   presence leak as described under Section 14.15.  To prevent that
   possibility, a client can either (1) generate a random resource
   identifier on its own or (2) ask the server to generate a resource
   identifier on its behalf, which MUST be random (see [RANDOM]).  When
   generating a random resource identifier, it is RECOMMENDED that the
   resource identifier be a Universally Unique Identifier (UUID), for
   which the format specified in [UUID] is RECOMMENDED.

8.5.  Server-Generated Resource Identifier

   A server that supports resource binding MUST be able to generate an
   XMPP resource identifier on behalf of a client.

8.5.1.  Success Case

   A client requests a server-generated resource identifier by sending
   an IQ stanza of type "set" (see Section 9.2.3) containing an empty
   <bind/> element qualified by the 'urn:ietf:params:xml:ns:xmpp-bind'
   namespace.

   C: <iq id='bind_1' type='set'>
       <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
      </iq>

   Once the server has generated an XMPP resource identifier for the
   client, it MUST return an IQ stanza of type "result" to the client,
   which MUST include a <jid/> child element that specifies the full JID
   for the connected resource as determined by the server.

   S: <iq id='bind_1' type='result'>
       <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
         <jid>
           juliet@im.example.com/4db06f06-1ea4-11dc-aca3-000bcd821bfb
         </jid>
       </bind>
      </iq>

8.5.2.  Error Cases

   When a client asks the server to generate a resource identifer during
   resource binding, the following stanza error conditions are possible:






Saint-Andre              Expires April 26, 2010                [Page 83]

Internet-Draft                  XMPP Core                   October 2009


   o  The account has reached a limit on the number of simultaneous
      connected resources allowed.
   o  The client is otherwise not allowed to bind a resource to the
      stream.

8.5.2.1.  Resource Constraint

   If the account has reached a limit on the number of simultaneous
   connected resources allowed, the server MUST return a <resource-
   constraint/> error.

   S: <iq id='bind_2' type='error'>
        <error type='wait'>
          <resource-constraint
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>

8.5.2.2.  Not Allowed

   If the client is otherwise not allowed to bind a resource to the
   stream, the server MUST return a <not-allowed/> error.

   S: <iq id='bind_2' type='error'>
        <error type='cancel'>
          <not-allowed
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>

8.6.  Client-Submitted Resource Identifier

   Instead of asking the server to generate a resource identifier on its
   behalf, a client MAY attempt to submit a resource identifier that it
   has generated or that the controlling user has provided.

8.6.1.  Success Case

   A client asks its server to accept a client-submitted resource
   identifier by sending an IQ stanza of type "set" containing a <bind/>
   element with a child <resource/> element containing non-zero-length
   XML character data.

   C: <iq id='bind_2' type='set'>
        <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
          <resource>balcony</resource>
        </bind>
      </iq>



Saint-Andre              Expires April 26, 2010                [Page 84]

Internet-Draft                  XMPP Core                   October 2009


   The server SHOULD accept the client-submitted resource identifier.
   It does so by returning an IQ stanza of type "result" to the client,
   including a <jid/> child element that specifies the full JID for the
   connected resource and contains without modification the client-
   submitted text.

   S: <iq id='bind_2' type='result'>
       <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
         <jid>juliet@im.example.com/balcony</jid>
       </bind>
      </iq>

8.6.2.  Error Cases

   When a client attempts to submit its own XMPP resource identifier
   during resource binding, the following stanza error conditions are
   possible in addition to those described under Section 8.5.2:

   o  The provided resource identifier cannot be processed by the
      server, e.g. because it is not in accordance with the Resourceprep
      (Appendix B) profile of [STRINGPREP]).
   o  The provided resource identifier is already in use.

8.6.2.1.  Bad Request

   If the provided resource identifier cannot be processed by the
   server, the server MAY return a <bad-request/> error (but SHOULD
   instead apply the Resourceprep (Appendix B) profile of [STRINGPREP]
   or otherwise process the resource identifier so that it is in
   conformance).

   S: <iq id='bind_2' type='error'>
        <error type='modify'>
          <bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>

8.6.2.2.  Conflict

   If there is already a connected resource of the same name, the server
   MUST do one of the following:

   1.  Not accept the resource identifier provided by the client but
       instead override it with an XMPP resource identifier that the
       server generates.
   2.  Terminate the current resource and allow the newly-requested
       resource.




Saint-Andre              Expires April 26, 2010                [Page 85]

Internet-Draft                  XMPP Core                   October 2009


   3.  Disallow the newly-requested resource and maintain the current
       resource.

   Which of these the server does is up to the implementation, although
   it is RECOMMENDED to implement case #1.

   S: <iq id='bind_2' type='result'>
       <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
         <jid>
      juliet@im.example.com/balcony 4db06f06-1ea4-11dc-aca3-000bcd821bfb
         </jid>
       </bind>
      </iq>

   In case #2, the server MUST send a <conflict/> stream error to the
   current resource and return an IQ stanza of type "result" (indicating
   success) to the newly-requested resource.

   S: <iq id='bind_2' type='result'/>

   In case #3, the server MUST send a <conflict/> stanza error to the
   newly-requested resource but maintain the XML stream for that
   connection so that the newly-requested resource has an opportunity to
   negotiate a non-conflicting resource identifier before sending
   another request for resource binding.

   S: <iq id='bind_2' type='error'>
        <error type='modify'>
          <conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>

8.6.3.  Retries

   If an error occurs when a client submits a resource identifier, the
   server SHOULD allow a configurable but reasonable number of retries
   (at least 2 and no more than 5); this enables the client to tolerate
   incorrectly-provided resource identifiers (e.g., bad data formats or
   duplicate text strings) without being forced to reconnect.

   After the client has reached the retry limit, the server MUST return
   a <policy-violation/> stream error to the client.


9.  XML Stanzas

   After a client has connected to a server or two servers have
   connected to each other, either party can send XML stanzas over the



Saint-Andre              Expires April 26, 2010                [Page 86]

Internet-Draft                  XMPP Core                   October 2009


   negotiated stream.  Three kinds of XML stanza are defined for the
   'jabber:client' and 'jabber:server' namespaces: <message/>,
   <presence/>, and <iq/>.  In addition, there are five common
   attributes for these stanza types.  These common attributes, as well
   as the basic semantics of the three stanza types, are defined herein;
   more detailed information regarding the syntax of XML stanzas for
   instant messaging and presence applications is provided in [XMPP-IM],
   and for other applications in the relevant XMPP extension
   specifications.

   A server MUST NOT process a partial stanza and MUST NOT attach
   meaning to the transmission timing of any part of a stanza (before
   receipt of the close tag).

   Support for the XML stanza syntax and semantics defined herein is
   REQUIRED in XMPP client and server implementations.

9.1.  Common Attributes

   The following five attributes are common to message, presence, and IQ
   stanzas.

9.1.1.  to

   The 'to' attribute specifies the JID of the intended recipient for
   the stanza.

   <message to='romeo@example.net'>
     <body>Art thou not Romeo, and a Montague?</body>
   </message>

   For information about server processing of inbound and outbound XML
   stanzas based on the nature of the 'to' address, refer to Section 11.

9.1.1.1.  Client-to-Server Streams

   The following rules apply to inclusion of the 'to' attribute in the
   context of XML streams qualified by the 'jabber:client' namespace
   (i.e., client-to-server streams).

   1.  A stanza with a specific intended recipient MUST possess a 'to'
       attribute whose value is an XMPP address.
   2.  A stanza sent from a client to a server for direct processing by
       the server on behalf of the client (e.g., presence sent to the
       server for broadcasting to other entities) MUST NOT possess a
       'to' attribute.





Saint-Andre              Expires April 26, 2010                [Page 87]

Internet-Draft                  XMPP Core                   October 2009


9.1.1.2.  Server-to-Server Streams

   The following rules apply to inclusion of the 'to' attribute in the
   context of XML streams qualified by the 'jabber:server' namespace
   (i.e., server-to-server streams).

   1.  A stanza MUST possess a 'to' attribute whose value is an XMPP
       address; if a server receives a stanza that does not meet this
       restriction, it MUST generate an <improper-addressing/> stream
       error.
   2.  The domain identifier portion of the JID in the 'to' atttribute
       MUST match a hostname serviced by the receiving server; if a
       server receives a stanza that does not meet this restriction, it
       MUST generate a <host-unknown/> or <host-gone/> stream error.

9.1.2.  from

   The 'from' attribute specifies the JID of the sender.

   <message from='juliet@im.example.com/balcony'
            to='romeo@example.net'>
     <body>Art thou not Romeo, and a Montague?</body>
   </message>

9.1.2.1.  Client-to-Server Streams

   The following rules apply to the 'from' attribute in the context of
   XML streams qualified by the 'jabber:client' namespace (i.e., client-
   to-server streams).

   1.  When the server receives an XML stanza from a client and the
       stanza does not include a 'from' attribute, the server MUST add a
       'from' attribute to the stanza, where the value of the 'from'
       attribute is the full JID (<localpart@domain/resource>)
       determined by the server for the connected resource that
       generated the stanza (see Section 3.5), or the bare JID
       (<localpart@domain>) in the case of subscription-related presence
       stanzas (see [XMPP-IM]).
   2.  When the server receives an XML stanza from a client and the
       stanza includes a 'from' attribute, the server MUST either (a)
       validate that the value of the 'from' attribute provided by the
       client is that of a connected resource for the associated entity
       or (b) override the provided 'from' attribute by adding a 'from'
       attribute as specified under Rule #1.
   3.  When the server generates a stanza from the server for delivery
       to the client on behalf of the account of the connected client
       (e.g., in the context of data storage services provided by the
       server on behalf of the client), the stanza MUST either (a) not



Saint-Andre              Expires April 26, 2010                [Page 88]

Internet-Draft                  XMPP Core                   October 2009


       include a 'from' attribute or (b) include a 'from' attribute
       whose value is the account's bare JID (<localpart@domain>).
   4.  When the server generates a stanza from the server itself for
       delivery to the client, the stanza MUST include a 'from'
       attribute whose value is the bare JID (i.e., <domain>) of the
       server.
   5.  A server MUST NOT send to the client a stanza without a 'from'
       attribute if the stanza was not generated by the server (e.g., if
       it was generated by another client or another server); therefore,
       when a client receives a stanza that does not include a 'from'
       attribute, it MUST assume that the stanza is from the server to
       which the client is connected.

9.1.2.2.  Server-to-Server Streams

   The following rules apply to the 'from' attribute in the context of
   XML streams qualified by the 'jabber:server' namespace (i.e., server-
   to-server streams).

   1.  A stanza MUST possess a 'from' attribute whose value is an XMPP
       address; if a server receives a stanza that does not meet this
       restriction, it MUST generate an <improper-addressing/> stream
       error.
   2.  The domain identifier portion of the JID contained in the 'from'
       attribute MUST match the hostname of the sending server (or any
       validated domain thereof) as communicated in the SASL negotiation
       (see Section 7), server dialback (see [XEP-0220], or similar
       means; if a server receives a stanza that does not meet this
       restriction, it MUST generate an <invalid-from/> stream error.

   Enforcement of these rules helps to prevent certain denial of service
   attacks as described under Section 14.14.

9.1.3.  id

   The 'id' attribute is used by the entity that generates a stanza
   ("the originating entity") to track any response or error stanza that
   it might receive in relation to the generated stanza from another
   entity (such as an intermediate server or the intended recipient).

   It is up to the originating entity whether the value of the 'id'
   attribute will be unique only within its current stream (session) or
   unique globally.

   For <message/> and <presence/> stanzas, it is RECOMMENDED for the
   originating entity to include an 'id' attribute; for <iq/> stanzas,
   it is REQUIRED.




Saint-Andre              Expires April 26, 2010                [Page 89]

Internet-Draft                  XMPP Core                   October 2009


   If the generated stanza includes an 'id' attribute then it is
   REQUIRED for the response or error stanza to also include an 'id'
   attribute, where the value of the 'id' attribute MUST match that of
   the generated stanza.

   Note: The semantics of IQ stanzas impose additional restrictions; see
   Section 9.2.3.

9.1.4.  type

   The 'type' attribute specifies the purpose or context of the message,
   presence, or IQ stanza.  The particular allowable values for the
   'type' attribute vary depending on whether the stanza is a message,
   presence, or IQ stanza.  The defined values for message and presence
   stanzas are specific to instant messaging and presence applications
   and therefore are specified in [XMPP-IM], whereas the values for IQ
   stanzas specify the role of an IQ stanza in a structured request-
   response exchange and therefore are specified under Section 9.2.3.
   The only 'type' value common to all three stanzas is "error"; see
   Section 9.3.

9.1.5.  xml:lang

   A stanza SHOULD possess an 'xml:lang' attribute (as defined in
   Section 2.12 of [XML]) if the stanza contains XML character data that
   is intended to be presented to a human user (as explained in
   [CHARSET], "internationalization is for humans").  The value of the
   'xml:lang' attribute specifies the default language of any such
   human-readable XML character data.

   <presence from='romeo@example.net/orchard' xml:lang='en'>
     <show>dnd</show>
     <status>Wooing Juliet</status>
   </presence>

   The value of the 'xml:lang' attribute MAY be overridden by the 'xml:
   lang' attribute of a specific child element.

   <presence from='romeo@example.net/orchard' xml:lang='en'>
     <show>dnd</show>
     <status>Wooing Juliet</status>
     <status xml:lang='cs'>Dvo&#x0159;&#x00ED;m se Julii</status>
   </presence

   If an outbound stanza generated by a client does not possess an 'xml:
   lang' attribute, the client's server SHOULD add an 'xml:lang'
   attribute whose value is that specified for the stream as defined
   under Section 5.4.4.



Saint-Andre              Expires April 26, 2010                [Page 90]

Internet-Draft                  XMPP Core                   October 2009


   C: <presence from='romeo@example.net/orchard'>
        <show>dnd</show>
        <status>Wooing Juliet</status>
      </presence>

   S: <presence from='romeo@example.net/orchard'
                to='juliet@im.example.com'
                xml:lang='en'>
        <show>dnd</show>
        <status>Wooing Juliet</status>
      </presence>

   If an inbound stanza received received by a client or server does not
   possess an 'xml:lang' attribute, an implementation MUST assume that
   the default language is that specified for the stream as defined
   under Section 5.4.4.

   The value of the 'xml:lang' attribute MUST conform to the NMTOKEN
   datatype (as defined in Section 2.3 of [XML]) and MUST conform to the
   format defined in [LANGTAGS].

   A server MUST NOT modify or delete 'xml:lang' attributes on stanzas
   it receives from other entities.

9.2.  Basic Semantics

9.2.1.  Message Semantics

   The <message/> stanza can be seen as a "push" mechanism whereby one
   entity pushes information to another entity, similar to the
   communications that occur in a system such as email.  All message
   stanzas SHOULD possess a 'to' attribute that specifies the intended
   recipient of the message; upon receiving such a stanza, a server
   SHOULD route or deliver it to the intended recipient (see Section 11
   for general routing and delivery rules related to XML stanzas).

9.2.2.  Presence Semantics

   The <presence/> stanza can be seen as a specialized broadcast or
   "publish-subscribe" mechanism, whereby multiple entities receive
   information (in this case, network availability information) about an
   entity to which they have subscribed.  In general, a publishing
   entity (client) SHOULD send a presence stanza with no 'to' attribute,
   in which case the server to which the entity is connected SHOULD
   broadcast that stanza to all subscribed entities.  However, a
   publishing entity MAY also send a presence stanza with a 'to'
   attribute, in which case the server SHOULD route or deliver that
   stanza to the intended recipient.  See Section 11 for general routing



Saint-Andre              Expires April 26, 2010                [Page 91]

Internet-Draft                  XMPP Core                   October 2009


   and delivery rules related to XML stanzas, and [XMPP-IM] for rules
   specific to presence applications.

9.2.3.  IQ Semantics

   Info/Query, or IQ, is a request-response mechanism, similar in some
   ways to the Hypertext Transfer Protocol [HTTP].  The semantics of IQ
   enable an entity to make a request of, and receive a response from,
   another entity.  The data content of the request and response is
   defined by the schema or other structural definition associated with
   the XML namespace that qualifies the direct child element of the IQ
   element (see Section 9.4), and the interaction is tracked by the
   requesting entity through use of the 'id' attribute.  Thus, IQ
   interactions follow a common pattern of structured data exchange such
   as get/result or set/result (although an error can be returned in
   reply to a request if appropriate):

   Requesting                  Responding
     Entity                      Entity
   ----------                  ----------
       |                            |
       | <iq id='1' type='get'>     |
       |   [ ... payload ... ]      |
       | </iq>                      |
       | -------------------------> |
       |                            |
       | <iq id='1' type='result'>  |
       |   [ ... payload ... ]      |
       | </iq>                      |
       | <------------------------- |
       |                            |
       | <iq id='2' type='set'>     |
       |   [ ... payload ... ]      |
       | </iq>                      |
       | -------------------------> |
       |                            |
       | <iq id='2' type='error'>   |
       |   [ ... condition ... ]    |
       | </iq>                      |
       | <------------------------- |
       |                            |

   To enforce these semantics, the following rules apply:

   1.  The 'id' attribute is REQUIRED for IQ stanzas.
   2.  The 'type' attribute is REQUIRED for IQ stanzas.  The value MUST
       be one of the following (if the value is other than one of the
       following strings, the recipient or an intermediate router MUST



Saint-Andre              Expires April 26, 2010                [Page 92]

Internet-Draft                  XMPP Core                   October 2009


       return a stanza error of <bad-request/>):
       *  get -- The stanza requests information, inquires about what
          data is needed in order to complete further operations, etc.
       *  set -- The stanza provides data that is needed for an
          operation to be completed, sets new values, replaces existing
          values, etc.
       *  result -- The stanza is a response to a successful get or set
          request.
       *  error -- The stanza reports an error that has occurred
          regarding processing or delivery of a previously-sent get or
          set request (see Section 9.3).
   3.  An entity that receives an IQ request of type "get" or "set" MUST
       reply with an IQ response of type "result" or "error".  The
       response MUST preserve the 'id' attribute of the request.
   4.  An entity that receives a stanza of type "result" or "error" MUST
       NOT respond to the stanza by sending a further IQ response of
       type "result" or "error"; however, the requesting entity MAY send
       another request (e.g., an IQ of type "set" to provide obligatory
       information discovered through a get/result pair).
   5.  An IQ stanza of type "get" or "set" MUST contain exactly one
       child element, which specifies the semantics of the particular
       request.
   6.  An IQ stanza of type "result" MUST include zero or one child
       elements.
   7.  An IQ stanza of type "error" MAY include the child element
       contained in the associated "get" or "set" and MUST include an
       <error/> child; for details, see Section 9.3.

9.3.  Stanza Errors

   Stanza-related errors are handled in a manner similar to stream
   errors (Section 5.6).  Unlike stream errors, stanza errors are
   recoverable; therefore they do not result in termination of the XML
   stream and underlying TCP connection.  Instead, the entity that
   discovers the error condition returns an ERROR STANZA to the sender,
   i.e., a stanza of the same kind (message, presence, or IQ) whose
   'type' attribute is set to a value of "error" and which contains an
   <error/> child element that specifies the error condition.  The
   specified error condition provides a hint regarding actions that the
   sender can take to remedy the error if possible.

9.3.1.  Rules

   The following rules apply to stanza errors:

   1.  The receiving or processing entity that detects an error
       condition in relation to a stanza SHOULD return an error stanza
       (and MUST do so for IQ stanzas).



Saint-Andre              Expires April 26, 2010                [Page 93]

Internet-Draft                  XMPP Core                   October 2009


   2.  The entity that generates an error stanza MAY include the
       original XML sent so that the sender can inspect and, if
       necessary, correct the XML before attempting to resend.
   3.  If the generated stanza included an 'id' attribute then it is
       REQUIRED for the error stanza to also include an 'id' attribute,
       where the value of the 'id' attribute MUST match that of the
       generated stanza.
   4.  An error stanza MUST contain an <error/> child element.
   5.  An <error/> child MUST NOT be included if the 'type' attribute
       has a value other than "error" (or if there is no 'type'
       attribute).
   6.  An entity that receives an error stanza MUST NOT respond to the
       stanza with a further error stanza; this helps to prevent
       looping.

9.3.2.  Syntax

   The syntax for stanza-related errors is:

   <stanza-kind from='intended-recipient' to='sender' type='error'>
     [OPTIONAL to include sender XML here]
     <error type='error-type'>
       <defined-condition xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
       [<text xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'
             xml:lang='langcode'>
         OPTIONAL descriptive text
       </text>]
       [OPTIONAL application-specific condition element]
     </error>
   </stanza-kind>

   The "stanza-kind" MUST be one of message, presence, or iq.

   The "error-type" MUST be one of the following:

   o  auth -- retry after providing credentials
   o  cancel -- do not retry (the error cannot be remedied)
   o  continue -- proceed (the condition was only a warning)
   o  modify -- retry after changing the data sent
   o  wait -- retry after waiting (the error is temporary)

   The <error/> element:

   o  MUST contain a child element corresponding to one of the stanza
      error conditions defined under Section 9.3.3; this element MUST be
      qualified by the 'urn:ietf:params:xml:ns:xmpp-stanzas' namespace.





Saint-Andre              Expires April 26, 2010                [Page 94]

Internet-Draft                  XMPP Core                   October 2009


   o  MAY contain a <text/> child element containing XML character data
      that describes the error in more detail; this element MUST be
      qualified by the 'urn:ietf:params:xml:ns:xmpp-stanzas' namespace
      and SHOULD possess an 'xml:lang' attribute specifying the natural
      language of the XML character data.
   o  MAY contain a child element for an application-specific error
      condition; this element MUST be qualified by an application-
      specific namespace that defines the syntax and semantics of the
      element.

   The <text/> element is OPTIONAL.  If included, it MUST be used only
   to provide descriptive or diagnostic information that supplements the
   meaning of a defined condition or application-specific condition.  It
   MUST NOT be interpreted programmatically by an application.  It MUST
   NOT be used as the error message presented to a human user, but MAY
   be shown in addition to the error message associated with the defined
   condition element (and, optionally, the application-specific
   condition element).

9.3.3.  Defined Conditions

   The following conditions are defined for use in stanza errors.

9.3.3.1.  bad-request

   The sender has sent a stanza containing XML that does not conform to
   the appropriate schema or that cannot be processed (e.g., an IQ
   stanza that includes an unrecognized value of the 'type' attribute,
   or an element that is qualified by a recognized namespace but that
   violates the defined syntax for the element); the associated error
   type SHOULD be "modify".

   C: <iq from='juliet@im.example.com/balcony'
          id='some-id'
          to='im.example.com'
          type='subscribe'>
        <ping xmlns='urn:xmpp:ping'/>
      </iq>

   S: <iq from='im.example.com'
          id='some-id'
          to='juliet@im.example.com/balcony'
          type='error'>
        <error type='modify'>
          <bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>




Saint-Andre              Expires April 26, 2010                [Page 95]

Internet-Draft                  XMPP Core                   October 2009


9.3.3.2.  conflict

   Access cannot be granted because an existing resource exists with the
   same name or address; the associated error type SHOULD be "cancel".

   C: <iq id='bind_2' type='set'>
        <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
          <resource>balcony</resource>
        </bind>
      </iq>

   S: <iq id='bind_2' type='error'>
        <error type='cancel'>
          <conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>

9.3.3.3.  feature-not-implemented

   The feature represented in the XML stanza is not implemented by the
   intended recipient or an intermediate server and therefore the stanza
   cannot be processed (e.g., the entity understands the namespace but
   does not recognize the element name); the associated error type
   SHOULD be "cancel" or "modify".

   C: <iq from='juliet@im.example.com/balcony'
          id='subscriptions1'
          to='pubsub.example.com'
          type='get'>
        <pubsub xmlns='http://jabber.org/protocol/pubsub'>
          <subscriptions/>
        </pubsub>
      </iq>

   E: <iq from='pubsub.example.com
          id='subscriptions1'
          to='juliet@im.example.com/balcony'
          type='error'>
        <error type='cancel'>
          <feature-not-implemented
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
          <unsupported
              xmlns='http://jabber.org/protocol/pubsub#errors'
              feature='retrieve-subscriptions'/>
        </error>
      </iq>





Saint-Andre              Expires April 26, 2010                [Page 96]

Internet-Draft                  XMPP Core                   October 2009


9.3.3.4.  forbidden

   The requesting entity does not possess the necessary permissions to
   perform the action; the associated error type SHOULD be "auth".

   C: <presence
          from='juliet@im.example.com/balcony'
          to='characters@muc.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>

   E: <presence
          from='characters@muc.example.com/JulieC'
          to='juliet@im.example.com/balcony'
          type='error'>
        <error type='auth'>
          <forbidden xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>

9.3.3.5.  gone

   The recipient or server can no longer be contacted at this address,
   typically on a permanent basis; the associated error type SHOULD be
   "cancel" or "modify" and the error stanza SHOULD include a new
   address as the XML character data of the <gone/> element (which MUST
   be a URI or IRI at which the entity can be contacted, typically an
   XMPP IRI as specified in [XMPP-URI]).

   C: <presence
          from='juliet@im.example.com/balcony'
          to='characters@muc.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>

   E: <presence
          from='characters@muc.example.com/JulieC'
          to='juliet@im.example.com/balcony'
          type='error'>
        <error type='modify'>
          <gone xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
            xmpp:conference.example.com
          </gone>
        </error>
      </presence>






Saint-Andre              Expires April 26, 2010                [Page 97]

Internet-Draft                  XMPP Core                   October 2009


9.3.3.6.  internal-server-error

   The server could not process the stanza because of a misconfiguration
   or an otherwise-undefined internal server error; the associated error
   type SHOULD be "wait" or "cancel".

   C: <presence
          from='juliet@im.example.com/balcony'
          to='characters@muc.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>

   E: <presence
          from='characters@muc.example.com/JulieC'
          to='juliet@im.example.com/balcony'
          type='error'>
        <error type='wait'>
          <internal-server-error
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>

9.3.3.7.  item-not-found

   The addressed JID or item requested cannot be found; the associated
   error type SHOULD be "cancel" or "modify".

   C: <presence from='userfoo@example.com/bar'
                to='nosuchroom@conference.example.org/foo'/>

   S: <presence from='nosuchroom@conference.example.org/foo'
                to='userfoo@example.com/bar'
                type='error'>
        <error type='cancel'>
          <item-not-found xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>

      Note: An application MUST NOT return this error if doing so would
      provide information about the intended recipient's network
      availability to an entity that is not authorized to know such
      information; instead it MUST return a <service-unavailable/>
      error.

9.3.3.8.  jid-malformed

   The sending entity has provided or communicated an XMPP address
   (e.g., a value of the 'to' attribute) or aspect thereof (e.g., an



Saint-Andre              Expires April 26, 2010                [Page 98]

Internet-Draft                  XMPP Core                   October 2009


   XMPP resource identifier) that does not adhere to the syntax defined
   under Section 3; the associated error type SHOULD be "modify".

   C: <presence
          from='juliet@im.example.com/balcony'
          to='ch@r@cters@muc.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>

   E: <presence
          from='ch@r@cters@muc.example.com/JulieC'
          to='juliet@im.example.com/balcony'
          type='error'>
        <error type='modify'>
          <jid-malformed
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>

9.3.3.9.  not-acceptable

   The recipient or server understands the request but is refusing to
   process it because it does not meet criteria defined by the recipient
   or server (e.g., a local policy regarding stanza size limits or
   acceptable words in messages); the associated error type SHOULD be
   "modify".

   C: <message to='juliet@im.example.com' id='foo'>
        <body>[ ... the-emacs-manual ... ]</body>
      </message>

   S: <message from='juliet@im.example.com' id='foo'>
        <error type='modify'>
          <not-acceptable
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </message>

9.3.3.10.  not-allowed

   The recipient or server does not allow any entity to perform the
   action (e.g., sending to entities at a blacklisted domain); the
   associated error type SHOULD be "cancel".








Saint-Andre              Expires April 26, 2010                [Page 99]

Internet-Draft                  XMPP Core                   October 2009


   C: <presence
          from='juliet@im.example.com/balcony'
          to='characters@muc.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>

   E: <presence
          from='characters@muc.example.com/JulieC'
          to='juliet@im.example.com/balcony'
          type='error'>
        <error type='cancel'>
          <not-allowed xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>

9.3.3.11.  not-authorized

   The sender needs to provide proper credentials before being allowed
   to perform the action, or has provided improper credentials; the
   associated error type SHOULD be "auth".

   C: <presence
          from='juliet@im.example.com/balcony'
          to='characters@muc.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>

   E: <presence
          from='characters@muc.example.com/JulieC'
          to='juliet@im.example.com/balcony'>
        <error type='auth'>
          <not-authorized xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>

9.3.3.12.  not-modified

   The item requested has not changed since it was last requested; the
   associated error type SHOULD be "continue".












Saint-Andre              Expires April 26, 2010               [Page 100]

Internet-Draft                  XMPP Core                   October 2009


   C: <iq from='juliet@capulet.com/balcony'
          id='roster2'
          type='get'>
       <query xmlns='jabber:iq:roster'>
          <headers xmlns='http://jabber.org/protocol/shim'>
            <header name='If-None-Match'>
              some-long-opaque-string
            </header>
          </headers>
       </query>
      </iq>

   S: <iq type='error'
          to='juliet@capulet.com/balcony'
          id='roster2'>
        <query xmlns='jabber:iq:roster'>
          <headers xmlns='http://jabber.org/protocol/shim'>
            <header name='If-None-Match'>
              some-long-opaque-string
            </header>
          </headers>
        </query>
        <error type='modify'>
          <not-modified xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>

9.3.3.13.  payment-required

   The requesting entity is not authorized to access the requested
   service because payment is necessary; the associated error type
   SHOULD be "auth".



















Saint-Andre              Expires April 26, 2010               [Page 101]

Internet-Draft                  XMPP Core                   October 2009


   C: <iq from='romeo@example.net/foo'
          id='items1'
          to='pubsub.example.com'
          type='get'>
        <pubsub xmlns='http://jabber.org/protocol/pubsub'>
          <items node='my_musings'/>
        </pubsub>
      </iq>

   E: <iq from='pubsub.example.com'
          id='items1'
          to='romeo@example.net/foo'
          type='error'>
        <error type='auth'>
          <payment-required
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>

9.3.3.14.  policy-violation

   The entity has violated some local service policy (e.g., the stanza
   exceeds a configured size limit); the server MAY choose to specify
   the policy in the <text/> element or in an application-specific
   condition element; the associated error type SHOULD be "modify" or
   "wait" depending on the policy being violated.

   (In the following example, the client sends an XMPP message that is
   too large according to the server's local service policy.)

   C: <message to='juliet@im.example.com' id='foo'>
        <body>[ ... the-emacs-manual ... ]</body>
      </message>

   S: <message from='juliet@im.example.com' id='foo'>
        <error type='cancel'>
          <policy-violation
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </message>

9.3.3.15.  recipient-unavailable

   The intended recipient is temporarily unavailable; the associated
   error type SHOULD be "wait".






Saint-Andre              Expires April 26, 2010               [Page 102]

Internet-Draft                  XMPP Core                   October 2009


   C: <presence
          from='juliet@im.example.com/balcony'
          to='characters@muc.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>

   E: <presence
          from='characters@muc.example.com/JulieC'
          to='juliet@im.example.com/balcony'>
        <error type='wait'>
          <recipient-unavailable
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>

      Note: An application MUST NOT return this error if doing so would
      provide information about the intended recipient's network
      availability to an entity that is not authorized to know such
      information; instead it MUST return a <service-unavailable/>
      error.

9.3.3.16.  redirect

   The recipient or server is redirecting requests for this information
   to another entity, typically in a temporary fashion (the <gone/>
   condition is used for permanent addressing failures); the associated
   error type SHOULD be "modify" and the error stanza SHOULD contain the
   alternate address in the XML character data of the <redirect/>
   element (which MUST be a URI or IRI at which the entity can be
   contacted, typically an XMPP IRI as specified in [XMPP-URI]).

   C: <presence
          from='juliet@im.example.com/balcony'
          to='characters@muc.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>

   E: <presence
          from='characters@muc.example.com/JulieC'
          to='juliet@im.example.com/balcony'
          type='error'>
        <error type='modify'>
          <redirect xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
            xmpp:characters@conference.example.org
          </redirect>
        </error>
      </presence>




Saint-Andre              Expires April 26, 2010               [Page 103]

Internet-Draft                  XMPP Core                   October 2009


9.3.3.17.  registration-required

   The requesting entity is not authorized to access the requested
   service because prior registration is necessary; the associated error
   type SHOULD be "auth".

   C: <presence
          from='juliet@im.example.com/balcony'
          to='characters@muc.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>

   E: <presence
          from='characters@muc.example.com/JulieC'
          to='juliet@im.example.com/balcony'>
        <error type='auth'>
          <registration-required
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>

9.3.3.18.  remote-server-not-found

   A remote server or service specified as part or all of the JID of the
   intended recipient does not exist; the associated error type SHOULD
   be "cancel".

   C: <presence
          from='juliet@im.example.com/balcony'
          to='characters@muc.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>

   E: <presence
          from='characters@muc.example.com/JulieC'
          to='juliet@im.example.com/balcony'>
        <error type='cancel'>
          <remote-server-not-found
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>

9.3.3.19.  remote-server-timeout

   A remote server or service specified as part or all of the JID of the
   intended recipient (or needed to fulfill a request) could not be
   contacted within a reasonable amount of time; the associated error
   type SHOULD be "wait".



Saint-Andre              Expires April 26, 2010               [Page 104]

Internet-Draft                  XMPP Core                   October 2009


   C: <presence
          from='juliet@im.example.com/balcony'
          to='characters@muc.example.com/JulieC'>
        <x xmlns='http://jabber.org/protocol/muc'/>
      </presence>

   E: <presence
          from='characters@muc.example.com/JulieC'
          to='juliet@im.example.com/balcony'>
        <error type='wait'>
          <remote-server-timeout
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </presence>

9.3.3.20.  resource-constraint

   The server or recipient lacks the system resources necessary to
   service the request; the associated error type SHOULD be "wait" or
   "modify".

   C: <iq from='romeo@example.net/foo'
          id='items1'
          to='pubsub.example.com'
          type='get'>
        <pubsub xmlns='http://jabber.org/protocol/pubsub'>
          <items node='my_musings'/>
        </pubsub>
      </iq>

   E: <iq from='pubsub.example.com'
          id='items1'
          to='romeo@example.net/foo'
          type='error'>
        <error type='wait'>
          <resource-constraint
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </iq>

9.3.3.21.  service-unavailable

   The server or recipient does not currently provide the requested
   service; the associated error type SHOULD be "cancel".







Saint-Andre              Expires April 26, 2010               [Page 105]

Internet-Draft                  XMPP Core                   October 2009


   C: <message from='romeo@example.net/foo'
               to='juliet@im.example.com'>
        <body>Hello?</body>
      </message>

   S: <message from='juliet@im.example.com/foo'
               to='romeo@example.net'>
        <error type='cancel'>
          <service-unavailable
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </message>

   An application MUST return a <service-unavailable/> error instead of
   <item-not-found/> or <recipient-unavailable/> if sending one of the
   latter errors would provide information about the intended
   recipient's network availability to an entity that is not authorized
   to know such information.

9.3.3.22.  subscription-required

   The requesting entity is not authorized to access the requested
   service because a prior subscription is necessary; the associated
   error type SHOULD be "auth".

   C: <message
          from='romeo@example.net/orchard'
          to='playbot@shakespeare.example.com'
        <body>help</body>
      </message>

   E: <message
         from='playbot@shakespeare.example.com'
         to='romeo@example.net/orchard'
         type='error'>
        <error type='auth'>
          <subscription-required
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
        </error>
      </message>

9.3.3.23.  undefined-condition

   The error condition is not one of those defined by the other
   conditions in this list; any error type can be associated with this
   condition, and it SHOULD be used only in conjunction with an
   application-specific condition.




Saint-Andre              Expires April 26, 2010               [Page 106]

Internet-Draft                  XMPP Core                   October 2009


   C: <message
          from='northumberland@shakespeare.example'
          id='richard2-4.1.247'
          to='kingrichard@royalty.england.example'>
        <body>My lord, dispatch; read o'er these articles.</body>
        <amp xmlns='http://jabber.org/protocol/amp'>
          <rule action='notify'
                condition='deliver'
                value='stored'/>
        </amp>

   S: <message from='example.org'
               id='amp1'
               to='northumberland@example.net/field'
               type='error'>
        <amp xmlns='http://jabber.org/protocol/amp'
             from='kingrichard@example.org'
             status='error'
             to='northumberland@example.net/field'>
          <rule action='error'
                condition='deliver'
                value='stored'/>
        </amp>
        <error type='modify'>
          <undefined-condition
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
          <failed-rules xmlns='http://jabber.org/protocol/amp#errors'>
            <rule action='error'
                  condition='deliver'
                  value='stored'/>
          </failed-rules>
        </error>
      </message>

9.3.3.24.  unexpected-request

   The recipient or server understood the request but was not expecting
   it at this time (e.g., the request was out of order); the associated
   error type SHOULD be "wait" or "modify".












Saint-Andre              Expires April 26, 2010               [Page 107]

Internet-Draft                  XMPP Core                   October 2009


   C: <iq from='romeo@example.net/foo'
          id='unsub1'
          to='pubsub.example.com'
          type='set'>
        <pubsub xmlns='http://jabber.org/protocol/pubsub'>
           <unsubscribe
               node='my_musings'
               jid='romeo@example.net'/>
        </pubsub>
      </iq>

   E: <iq from='pubsub.example.com'
          id='unsub1'
          to='romeo@example.net/foo'
          type='error'>
        <error type='cancel'>
          <unexpected-request
              xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
          <not-subscribed
              xmlns='http://jabber.org/protocol/pubsub#errors'/>
        </error>
      </iq>

9.3.4.  Application-Specific Conditions

   As noted, an application MAY provide application-specific stanza
   error information by including a properly-namespaced child in the
   error element.  The application-specific element SHOULD supplement or
   further qualify a defined element.  Thus, the <error/> element will
   contain two or three child elements.

   <iq id='some-id' type='error'>
     <error type='modify'>
       <bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
       <too-many-parameters xmlns='http://example.com/ns'/>
     </error>
   </iq>














Saint-Andre              Expires April 26, 2010               [Page 108]

Internet-Draft                  XMPP Core                   October 2009


   <message type='error' id='another-id'>
     <error type='modify'>
       <undefined-condition
             xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
       <text xml:lang='en'
             xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'>
         [ ... application-specific information ... ]
       </text>
       <too-many-parameters xmlns='http://example.com/ns'/>
     </error>
   </message>

   An entity that receives an application-specific error condition it
   does not understand MUST ignore the condition.

9.4.  Extended Content

   Although the message, presence, and IQ stanzas provide basic
   semantics for messaging, availability, and request-response
   interactions, XMPP uses XML namespaces (see [XML-NAMES] to extend the
   basic stanza syntax for the purpose of providing additional
   functionality.

   A message or presence stanza MAY contain one or more optional child
   elements specifying content that extends the meaning of the message
   (e.g., an XHTML-formatted version of the message body as described in
   [XEP-0071]), and an IQ stanza of type "get" or "set" MUST contain one
   such child element.  Such a child element MAY have any name and MUST
   possess a namespace declaration (other than "jabber:client", "jabber:
   server", or "http://etherx.jabber.org/streams") that defines all data
   contained within the child element.  Such a child element is called
   an "extension element".  An extension element can be included either
   at the direct child level of the stanza or in any mix of levels.

   Similarly, "extension attributes" are allowed.  That is: a stanza
   itself (i.e., the <iq/>, <message/>, and <presence/> elements
   qualified by the "jabber:client" or "jabber:server" namespace
   declared as the default namespace for the stream) and any child
   element of such a stanza (whether a child element qualifed by the
   default namespace or an extension element) MAY also include one or
   more attributes that are qualified by XML namespaces that are
   different from the default namespace or the reserved "xml" prefix
   (including the "empty namespace" if the attribute is not prefixed).
   For the sake of backward compatibility and maximum interoperability,
   an entity that generates a stanza SHOULD NOT include such attributes
   in the stanza itself or in child elements of the stanza that are
   qualified by the default namespace (e.g., the message <body/>
   element).



Saint-Andre              Expires April 26, 2010               [Page 109]

Internet-Draft                  XMPP Core                   October 2009


   An extension element or extension attribute is said to be EXTENDED
   CONTENT and the namespace name for such an element or attribute is
   said to be an EXTENDED NAMESPACE.

   To illustrate these concepts, several examples follow.

   The following stanza contains one direct child element whose extended
   namespace is 'jabber:iq:roster':

   <iq from='juliet@capulet.com/balcony'
       id='roster1'
       type='get'>
    <query xmlns='jabber:iq:roster'/>
   </iq>

   The following stanza contains two direct child elements with two
   different extended namespaces.

   <presence from='juliet@capulet.com/balcony'>
     <c xmlns='http://jabber.org/protocol/caps'
        node='http://exodus.jabberstudio.org/caps'
        ver='0.9'/>
     <x xmlns='vcard-temp:x:update'>
       <photo>sha1-hash-of-image</photo>
     </x>
   </presence>

   The following stanza contains two child elements, one of which is
   qualified by the default namespace and one of which is qualified by
   an extended namespace; the extension element in turn contains a child
   elememnt that is qualified by a different extended namespace.

   <message to='juliet@capulet.com'>
     <body>Hello?</body>
     <html xmlns='http://jabber.org/protocol/xhtml-im'>
       <body xmlns='http://www.w3.org/1999/xhtml'>
         <p style='font-weight:bold'>Hello?</t>
       </body>
     </html>
   </message>

   It is conventional in the XMPP community for implementations to not
   generate namespace prefixes for elements that are qualified by
   extended namespaces.  However, if an implementation generates such
   namespace prefixes then it MUST include the namespace declaration in
   the stanza itself or a child element of the stanza, not in the stream
   header (see Section 5.5.3).




Saint-Andre              Expires April 26, 2010               [Page 110]

Internet-Draft                  XMPP Core                   October 2009


   Routing entities (typically servers) SHOULD try to maintain prefixes
   when serializing XML stanzas for processing, but receiving entities
   MUST NOT rely on the prefix strings having any particular value.

   Support for any given extended namespace is OPTIONAL on the part of
   any implementation.  If an entity does not understand such a
   namespace, the entity's expected behavior depends on whether the
   entity is (1) the recipient or (2) an entity that is routing the
   stanza to the recipient.

   Recipient:  If a recipient receives a stanza that contains a child
      element it does not understand, it MUST silently ignore that
      particular XML data, i.e., it MUST NOT process it or present it to
      a user or associated application (if any).  In particular:
      *  If an entity receives a message or presence stanza that
         contains XML data qualified by a namespace it does not
         understand, the portion of the stanza that qualified by the
         unknown namespace MUST be ignored.
      *  If an entity receives a message stanza whose only child element
         is qualified by a namespace it does not understand, it MUST
         ignore the entire stanza.
      *  If an entity receives an IQ stanza of type "get" or "set"
         containing a child element qualified by a namespace it does not
         understand, the entity MUST return an IQ stanza of type "error"
         with an error condition of <service-unavailable/>.
   Router:  If a routing entity (typically a server) handles a stanza
      that contains a child element it does not understand, it MUST
      ignore the associated XML data by routing or delivering it
      untouched to the recipient.

9.5.  Stanza Size

   XMPP is optimized for the exchange of relatively large numbers of
   relatively small stanzas.  A client or server MAY enforce a maximum
   stanza size.  The maximum stanza size MUST NOT be smaller than 10000
   bytes, from the opening "<" character to the closing ">" character.
   If an entity receives a stanza that exceeds its maximum stanza size,
   it MUST return a <not-acceptable/> stanza error or a <policy-
   violation/> stream error.


10.  Examples

10.1.  Client-to-Server

   The following examples show the XMPP data flow for a client
   negotiating an XML stream with a server, exchanging XML stanzas, and
   closing the negotiated stream.  The server is "im.example.com", the



Saint-Andre              Expires April 26, 2010               [Page 111]

Internet-Draft                  XMPP Core                   October 2009


   server requires use of TLS, the client authenticates via the SASL
   PLAIN mechanism as "juliet@im.example.com", and the client binds a
   client-submitted resource to the stream.  It is assumed that before
   sending the initial stream header, the client has already resolved an
   SRV record of _xmpp-client._tcp.im.example.com and has opened a TCP
   connection to the advertised port at the resolved IP address.

      Note: The alternate steps shown are provided only to illustrate
      the protocol for failure cases; they are not exhaustive and would
      not necessarily be triggered by the data sent in the examples.

10.1.1.  TLS

   Step 1: Client initiates stream to server:

   C: <stream:stream
        from='juliet@im.example.com'
        to='im.example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'>

   Step 2: Server responds by sending a response stream header to
   client:

   S: <stream:stream
        from='im.example.com'
        id='t7AMCin9zjMNwQKDnplntZPIDEI='
        to='juliet@im.example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'

   Step 3: Server sends stream features to client (STARTTLS extension
   only at this point):

   S: <stream:features>
        <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
          <required/>
        </starttls>
      </stream:features>

   Step 4: Client sends STARTTLS command to server:

   C: <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>




Saint-Andre              Expires April 26, 2010               [Page 112]

Internet-Draft                  XMPP Core                   October 2009


   Step 5: Server informs client that it is allowed to proceed:

   S: <proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

   Step 5 (alt): Server informs client that STARTTLS negotiation has
   failed and closes both XML stream and TCP connection:

   S: <failure xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

   S: </stream:stream>

   Step 6: Client and server attempt to complete TLS negotiation over
   the existing TCP connection (see [TLS] for details).

   Step 7: If TLS negotiation is successful, client initiates a new
   stream to server:

   C: <stream:stream
        from='juliet@im.example.com'
        to='im.example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'>

   Step 7 (alt): If TLS negotiation is unsuccessful, server closes TCP
   connection.

10.1.2.  SASL

   Step 8: Server responds by sending a stream header to client along
   with any available stream features:

   S: <stream:stream
        from='im.example.com'
        id='vgKi/bkYME8OAj4rlXMkpucAqe4='
        to='juliet@im.example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'

   S: <stream:features>
        <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
          <mechanism>DIGEST-MD5</mechanism>
          <mechanism>PLAIN</mechanism>
        </mechanisms>
      </stream:features>



Saint-Andre              Expires April 26, 2010               [Page 113]

Internet-Draft                  XMPP Core                   October 2009


   Step 9: Client selects an authentication mechanism, in this case
   [PLAIN]:

   C: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
            mechanism='PLAIN'>AGp1bGlldAByMG0zMG15cjBtMzA=</auth>

   The decoded base64 data is "U+0000julietU+0000r0m30myr0m30", i.e., a
   username of "juliet" and a password of "r0m30myr0m30" with the actual
   ASCII 0 (or NUL) character as separator.

   Step 10: Server informs client of success:

   S: <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>

   Step 10 (alt): Server returns error to client:

   S: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
        <not-authorized/>
      </failure>

   Step 11: Client initiates a new stream to server:

   C: <stream:stream
        from='juliet@im.example.com'
        to='im.example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'

10.1.3.  Resource Binding

   Step 12: Server responds by sending a stream header to client along
   with supported features (in this case resource binding):

   S: <stream:stream
        from='im.example.com'
        id='gPybzaOzBmaADgxKXu9UClbprp0='
        to='juliet@im.example.com'
        version='1.0'
        xml:lang='en'
        xmlns='jabber:client'
        xmlns:stream='http://etherx.jabber.org/streams'>

   S: <stream:features>
        <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
      </stream:features>




Saint-Andre              Expires April 26, 2010               [Page 114]

Internet-Draft                  XMPP Core                   October 2009


   Upon being so informed that resource binding is mandatory, the client
   needs to bind a resource to the stream; here we assume that the
   client submits a human-readable text string.

   Step 13: Client binds a resource:

   C: <iq id='bind_1' type='set'>
        <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
          balcony
        </bind>
      </iq>

   Step 14: Server accepts submitted resource identifier and informs
   client of successful resource binding:

   S: <iq id='bind_1' type='result'>
        <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
          <jid>
            juliet@im.example.com/balcony
          </jid>
        </bind>
      </iq>

10.1.4.  Stanza Exchange

   Now the client is allowed to send XML stanzas over the negotiated
   stream.

   C: <message from='juliet@im.example.com/balcony'
               to='romeo@example.net'
               xml:lang='en'>
        <body>Art thou not Romeo, and a Montague?</body>
      </message>

   If necessary, sender's server negotiates XML streams with intended
   recipient's server (see Section 10.2).

   The intended recipient replies and the message is delivered to the
   client.

   E: <message from='romeo@example.net/orchard'
               to='juliet@im.example.com/balcony'
               xml:lang='en'>
        <body>Neither, fair saint, if either thee dislike.</body>
      </message>

   The client can subsequently send and receive an unbounded number of
   subsequent XML stanzas over the stream.



Saint-Andre              Expires April 26, 2010               [Page 115]

Internet-Draft                  XMPP Core                   October 2009


10.1.5.  Close

   Desiring to send no further messages, the client closes the stream.

   C: </stream:stream>

   Consistent with the recommended stream closing handshake, the server
   closes the stream as well:

   S: </stream:stream>

   Client now terminates the underlying TCP connection.

10.2.  Server-to-Server Examples

   The following examples show the data flow for a server negotiating an
   XML stream with another server, exchanging XML stanzas, and closing
   the negotiated stream.  The initiating server ("Server1") is
   im.example.com; the receiving server ("Server2") is example.net and
   it requires use of TLS; im.example.com presents a certificate and
   authenticates via the SASL EXTERNAL mechanism.  It is assumed that
   before sending the initial stream header, Server1 has already
   resolved an SRV record of _xmpp-server._tcp.example.net and has
   opened a TCP connection to the advertised port at the resolved IP
   address.

      Note: The alternate steps shown are provided only to illustrate
      the protocol for failure cases; they are not exhaustive and would
      not necessarily be triggered by the data sent in the examples.

10.2.1.  TLS

   Step 1: Server1 initiates stream to Server2:

   S1: <stream:stream
         from='im.example.com'
         to='example.net'
         version='1.0'
         xmlns='jabber:server'
         xmlns:stream='http://etherx.jabber.org/streams'>











Saint-Andre              Expires April 26, 2010               [Page 116]

Internet-Draft                  XMPP Core                   October 2009


   Step 2: Server2 responds by sending a response stream header to
   Server1:

   S2: <stream:stream
         from='example.net'
         id='hTiXkW+ih9k2SqdGkk/AZi0OJ/Q='
         to='im.example.com'
         version='1.0'
         xmlns='jabber:server'
         xmlns:stream='http://etherx.jabber.org/streams'>

   Step 3: Server2 sends stream features to Server1:

   S2: <stream:features>
         <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
           <required/>
         </starttls>
       </stream:features>

   Step 4: Server1 sends the STARTTLS command to Server2:

   S1: <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

   Step 5: Server2 informs Server1 that it is allowed to proceed:

   S2: <proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

   Step 5 (alt): Server2 informs Server1 that STARTTLS negotiation has
   failed and closes stream:

   S2: <failure xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

   S2: </stream:stream>

   Step 6: Server1 and Server2 attempt to complete TLS negotiation via
   TCP (see [TLS] for details).

   Step 7: If TLS negotiation is successful, Server1 initiates a new
   stream to Server2:

   S1: <stream:stream
         from='im.example.com'
         to='example.net'
         version='1.0'
         xmlns='jabber:server'
         xmlns:stream='http://etherx.jabber.org/streams'>

   Step 7 (alt): If TLS negotiation is unsuccessful, Server2 closes TCP



Saint-Andre              Expires April 26, 2010               [Page 117]

Internet-Draft                  XMPP Core                   October 2009


   connection.

10.2.2.  SASL

   Step 8: Server2 sends a response stream header to Server1 along with
   available stream features (including a preference for the SASL
   EXTERNAL mechanism):

   S2: <stream:stream
         from='example.net'
         id='RChdjlgj/TIBcbT9Keu31zDihH4='
         to='im.example.com'
         version='1.0'
         xmlns='jabber:server'
         xmlns:stream='http://etherx.jabber.org/streams'>

   S2: <stream:features>
         <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
           <mechanism>EXTERNAL</mechanism>
         </mechanisms>
       </stream:features>

   Step 9: Server1 selects the EXTERNAL mechanism, in this case with an
   authorization identity encoded according to [BASE64]:

   S1: <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
             mechanism='EXTERNAL'/>aW0uZXhhbXBsZS5jb20=</auth>

   The decoded authorization identity is "im.example.com".

   Step 10: Server2 determines that the authorization identity provided
   by Server1 matches the information in the presented certificate and
   therefore returns success:

   S2: <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>

   Step 10 (alt): Server2 informs Server1 of failed authentication:

   S2: <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
         <not-authorized/>
       </failure>

   S2: </stream:stream>








Saint-Andre              Expires April 26, 2010               [Page 118]

Internet-Draft                  XMPP Core                   October 2009


   Step 11: Server1 initiates a new stream to Server2:

   S1: <stream:stream
         from='im.example.com'
         to='example.net'
         version='1.0'
         xmlns='jabber:server'
         xmlns:stream='http://etherx.jabber.org/streams'>

   Step 12: Server2 responds by sending a stream header to Server1 along
   with any additional features (or, in this case, an empty features
   element):

   S2: <stream:stream
         from='example.net'
         id='MbbV2FeojySpUIP6J91qaa+TWHM='
         to='im.example.com'
         version='1.0'
         xmlns='jabber:server'
         xmlns:stream='http://etherx.jabber.org/streams'>

   S2: <stream:features/>

10.2.3.  Stanza Exchange

   Now Server1 is allowed to send XML stanzas to Server2 over the
   negotiated stream; here we assume that the transferred stanzas are
   those shown earlier for client-to-server communication, albeit over a
   server-to-server stream qualified by the 'jabber:server' namespace.

   Server1 sends XML stanza to Server2:

   S1: <message from='juliet@im.example.com/balcony'
                to='romeo@example.net'
                xml:lang='en'>
       <body>Art thou not Romeo, and a Montague?</body>
      </message>

   The intended recipient replies and the message is delivered from
   Server2 to Server1.

   Server2 sends XML stanza to Server1:

   S2: <message from='romeo@example.net/orchard'
                to='juliet@im.example.com/balcony'
                xml:lang='en'>
         <body>Neither, fair saint, if either thee dislike.</body>
       </message>



Saint-Andre              Expires April 26, 2010               [Page 119]

Internet-Draft                  XMPP Core                   October 2009


10.2.4.  Close

   Desiring to send no further messages, Server1 closes the stream.  (In
   practice, the stream would most likely remain open for some time,
   since Server1 and Server2 do not immediately know if the stream will
   be needed for further communication.)

   S1: </stream:stream>

   Consistent with the recommended stream closing handshake, Server2
   closes the stream as well:

   S2: </stream:stream>

   Server1 now terminates the underlying TCP connection.


11.  Server Rules for Processing XML Stanzas

   An XMPP server MUST ensure in-order processing of XML stanzas between
   any two entities.  This includes stanzas sent by a client to its
   server for direct processing by the server (e.g., in-order processing
   of a roster get and initial presence as described in [XMPP-IM]).

   Beyond the requirement for in-order processing, each server
   implementation will contain its own logic for processing stanzas it
   receives.  Such logic determines whether the server needs to ROUTE a
   given stanza to another domain, DELIVER it to a local entity
   (typically a connected client associated with a local account), or
   HANDLE it directly within the server itself.  The following rules
   apply.

      Note: Particular XMPP applications MAY specify delivery rules that
      modify or supplement the following rules; for example, a set of
      delivery rules for instant messaging and presence applications is
      defined in [XMPP-IM].

11.1.  No 'to' Address

11.1.1.  Overview

   If the stanza possesses no 'to' attribute, the server MUST handle it
   directly on behalf of the entity that sent it, where the meaning of
   "handle it directly" depends on whether the stanza is message,
   presence, or IQ.  Because all stanzas received from other servers
   MUST possess a 'to' attribute, this rule applies only to stanzas
   received from a local entity (such as a client) that is connected to
   the server.



Saint-Andre              Expires April 26, 2010               [Page 120]

Internet-Draft                  XMPP Core                   October 2009


11.1.2.  Message

   If the server receives a message stanza with no 'to' attribute, it
   MUST treat the message as if the 'to' address were the bare JID
   <localpart@domain> of the sending entity.

11.1.3.  Presence

   If the server receives a presence stanza with no 'to' attribute, it
   MUST broadcast it to the entities that are subscribed to the sending
   entity's presence, if applicable ([XMPP-IM] defines the semantics of
   such broadcasting for presence applications).

11.1.4.  IQ

   If the server receives an IQ stanza with no 'to' attribute, it MUST
   process the stanza on behalf of the account from which received the
   stanza, as follows:

   1.  If the IQ stanza is of type "get" or "set" and the server
       understands the namespace that qualifies the payload, the server
       MUST handle the stanza on behalf of the sending entity or return
       an appropriate error to the sending entity.  While the meaning of
       "handle" is determined by the semantics of the qualifying
       namespace, in general the server shall respond to the IQ stanza
       of type "get" or "set" by returning an appropriate IQ stanza of
       type "result" or "error", responding as if the server were the
       bare JID of the sending entity.  As an example, if the sending
       entity sends an IQ stanza of type "get" where the payload is
       qualified by the 'jabber:iq:roster' namespace (as described in
       [XMPP-IM]), then the server shall return the roster associated
       with the sending entity's bare JID to the particular resource of
       the sending entity that requested the roster.
   2.  If the IQ stanza is of type "get" or "set" and the server does
       not understand the namespace that qualifies the payload, the
       server MUST return an error to the sending entity, which MUST be
       <service-unavailable/>.
   3.  If the IQ stanza is of type "error" or "result", the server MUST
       handle the error or result as appropriate for the request-
       response interaction, responding as if the server were the bare
       JID of the sending entity.

11.2.  Local Domain

   If the hostname of the domain identifier portion of the JID contained
   in the 'to' attribute matches one of the configured hostnames of the
   server itself, the server MUST first determine if the hostname is
   serviced by the server or by a specialized local service.  If the



Saint-Andre              Expires April 26, 2010               [Page 121]

Internet-Draft                  XMPP Core                   October 2009


   latter, the server MUST route the stanza to that service.  If the
   former, the server MUST proceed as follows.

11.2.1.  Mere Domain

   If the JID contained in the 'to' attribute is of the form <domain>,
   then the server MUST either handle the stanza as appropriate for the
   stanza kind or return an error stanza to the sender.

11.2.2.  Domain with Resource

   If the JID contained in the 'to' attribute is of the form <domain/
   resource>, then the server MUST either handle the stanza as
   appropriate for the stanza kind or return an error stanza to the
   sender.

11.2.3.  Localpart at Domain

      Note: For addresses of this type, more detailed rules in the
      context of instant messaging and presence applications are
      provided in [XMPP-IM].

11.2.3.1.  No Such User

   If there is no local account associated with the <localpart@domain>,
   how the stanza shall be processed depends on the stanza type.

   o  For a message stanza, the server MUST return a <service-
      unavailable/> stanza error to the sender.
   o  For a presence stanza, the server SHOULD silently discard the
      stanza.
   o  For an IQ stanza, the server MUST return a <service-unavailable/>
      stanza error to the sender.

11.2.3.2.  Bare JID

   If the JID contained in the 'to' attribute is of the form
   <localpart@domain>, how the stanza shall be processed depends on the
   stanza type.

   o  For a message stanza, if there exists at least one connected
      resource for the account the server SHOULD deliver it to at least
      one of the connected resources.  If there exists no connected
      resource, the server MUST either return an error or store the
      message offline for delivery when the account next has a connected
      resource.





Saint-Andre              Expires April 26, 2010               [Page 122]

Internet-Draft                  XMPP Core                   October 2009


   o  For a presence stanza, if there exists at least one connected
      resource for the account the server SHOULD deliver it to at least
      one of the connected resources.  If there exists no connected
      resource, the server MUST silently discard the stanza.
   o  For an IQ stanza, the server MUST handle it directly on behalf of
      the intended recipient.

11.2.3.3.  Full JID

   If the JID contained in the 'to' attribute is of the form
   <localpart@domain/resource> and there is no connected resource that
   exactly matches the full JID, the stanza shall be processed as if the
   JID were of the form <localpart@domain>.

   If the JID contained in the 'to' attribute is of the form
   <localpart@domain/resource> and there is a connected resource that
   exactly matches the full JID, the server SHOULD deliver the stanza to
   that connected resource.

11.3.  Remote Domain

   If the hostname of the domain identifier portion of the JID contained
   in the 'to' attribute does not match one of the configured hostnames
   of the server itself, the server SHOULD attempt to route the stanza
   to the remote domain (subject to local service provisioning and
   security policies regarding inter-domain communication, since such
   communication is optional for any given deployment).  There are two
   possible cases.

11.3.1.  Existing Stream

   If a server-to-server stream already exists between the two domains,
   the sender's server shall attempt to route the stanza to the
   authoritative server for the remote domain over the existing stream.

11.3.2.  No Existing Stream

   If there exists no server-to-server stream between the two domains,
   the sender's server shall proceed as follows:

   1.  Resolve the hostname of the remote domain (as defined under
       Section 14.4).
   2.  Negotiate a server-to-server stream between the two domains (as
       defined under Section 6 and Section 7).
   3.  Route the stanza to the authoritative server for the remote
       domain over the newly-established stream.





Saint-Andre              Expires April 26, 2010               [Page 123]

Internet-Draft                  XMPP Core                   October 2009


11.3.3.  Error Handling

   If routing of a stanza to the intended recipient's server is
   unsuccessful, the sender's server MUST return an error to the sender.
   If resolution of the remote domain is unsuccessful, the stanza error
   MUST be <remote-server-not-found/>.  If resolution succeeds but
   streams cannot be negotiated, the stanza error MUST be <remote-
   server-timeout/>.

   If stream negotiation with the intended recipient's server is
   successful but the remote server cannot deliver the stanza to the
   recipient, the remote server shall return an appropriate error to the
   sender by way of the sender's server.


12.  XML Usage

12.1.  Restrictions

   The Extensible Messaging and Presence Protocol (XMPP) defines a class
   of data objects called XML streams as well as the behavior of
   computer programs that process XML streams.  XMPP is an application
   profile or restricted form of the Extensible Markup Language [XML],
   and a complete XML stream (including start and end stream tags) is a
   conforming XML document.

   However, XMPP does not deal with XML documents but with XML streams.
   Because XMPP does not require the parsing of arbitrary and complete
   XML documents, there is no requirement that XMPP needs to support the
   full feature set of [XML].  In particular, the following features of
   XML are prohibited in XMPP:

   o  comments (as defined in Section 2.5 of [XML])
   o  processing instructions (Section 2.6 therein)
   o  internal or external DTD subsets (Section 2.8 therein)
   o  internal or external entity references (Section 4.2 therein) with
      the exception of the predefined entities (Section 4.6 therein)

   An XMPP implementation MUST behave as follows with regard to these
   features:

   1.  An XMPP implementation MUST NOT inject characters matching such
       features into an XML stream.
   2.  If an XMPP implementation receives characters matching such
       features over an XML stream, it MUST return a stream error, which
       SHOULD be <restricted-xml/> but MAY be <bad-format/>.





Saint-Andre              Expires April 26, 2010               [Page 124]

Internet-Draft                  XMPP Core                   October 2009


12.2.  XML Namespace Names and Prefixes

   XML namespaces (see [XML-NAMES]) are used within XMPP streams to
   create strict boundaries of data ownership.  The basic function of
   namespaces is to separate different vocabularies of XML elements that
   are structurally mixed together.  Ensuring that XMPP streams are
   namespace-aware enables any allowable XML to be structurally mixed
   with any data element within XMPP.  XMPP-specific rules for XML
   namespace names and prefixes are defined under Section 5.5 for XML
   streams and Section 9.4 for XML stanzas.

12.3.  Well-Formedness

   There are two varieties of well-formedness:

   o  "XML-well-formedness" in accordance with the definition of "well-
      formed" in Section 2.1 of [XML].
   o  "Namespace-well-formedness" in accordance with the definition of
      "namespace-well-formed" in Section 7 of [XML-NAMES].

   The following rules apply.

   An XMPP entity MUST NOT generate data that is not XML-well-formed.
   An XMPP entity MUST NOT accept data that is not XML-well-formed;
   instead it MUST return an <xml-not-well-formed/> stream error and
   close the stream over which the data was received.

   An XMPP entity MUST NOT generate data that is not namespace-well-
   formed.  An XMPP server SHOULD NOT route or deliver data that is not
   namespace-well-formed, and SHOULD return a stanza error of <not-
   acceptable/> or a stream error of <xml-not-well-formed/> in response
   to the receipt of such data.

      Note: Because these restrictions were underspecified in an earlier
      revision of this specification, it is possible that
      implementations based on that revision will send data that does
      not comply with the restrictions; an entity SHOULD be liberal in
      accepting such data.

12.4.  Validation

   A server is not responsible for ensuring that XML data delivered to a
   client or routed to another server is valid, in accordance with the
   definition of "valid" provided in Section 2.8 of [XML].  An
   implementation MAY choose to accept or provide only validated data,
   but such behavior is OPTIONAL.  A client SHOULD NOT rely on the
   ability to send data that does not conform to the schemas, and SHOULD
   ignore any non-conformant elements or attributes on the incoming XML



Saint-Andre              Expires April 26, 2010               [Page 125]

Internet-Draft                  XMPP Core                   October 2009


   stream.

      Note: The terms "valid" and "well-formed" are distinct in XML.

12.5.  Inclusion of XML Declaration

   Before sending a stream header, an implementation SHOULD send an XML
   declaration (matching production [23] content of [XML]).
   Applications MUST follow the rules provided in [XML] regarding the
   format of the XML declaration and the circumstances under which the
   XML declaration is included.

12.6.  Character Encoding

   Implementations MUST support the UTF-8 transformation of Universal
   Character Set [UCS2] characters, as needed for conformance with
   [CHARSET] and as defined in [UTF-8].  Implementations MUST NOT
   attempt to use any other encoding.  If one party to an XML stream
   detects that the other party has attempted to send XML data with an
   encoding other than UTF-8, it MUST return a stream error, which
   SHOULD be <unsupported-encoding/> but MAY be <bad-format/>.

   Note: Because it is mandatory for an XMPP implementation to support
   all and only the UTF-8 encoding and because UTF-8 always has the same
   byte order, an implementation MUST NOT send a byte order mark ("BOM")
   at the beginning of the data stream.  If an entity receives the
   Unicode character U+FEFF anywhere in an XML stream (including as the
   first character of the stream), it MUST interpret that character as a
   zero width no-break space, not as a byte order mark.

12.7.  Whitespace

   Except where explicitly disallowed (e.g., during TLS negotiation
   (Section 6) and SASL negotiation (Section 7)), either entity MAY send
   whitespace as separators between XML stanzas or between any other
   first-level elements sent over the stream.  One common use for
   sending such whitespace is explained under Section 5.3.3.

12.8.  XML Versions

   XMPP is an application profile of XML 1.0.  A future version of XMPP
   might be defined in terms of higher versions of XML, but this
   specification addresses XML 1.0 only.


13.  Internationalization Considerations

   As specified under Section 12.6, XML streams MUST be encoded in



Saint-Andre              Expires April 26, 2010               [Page 126]

Internet-Draft                  XMPP Core                   October 2009


   UTF-8.

   As specified under Section 5.4, an XML stream SHOULD include an 'xml:
   lang' attribute specifying the default language for any XML character
   data that is intended to be presented to a human user.  As specified
   under Section 9.1.5, an XML stanza SHOULD include an 'xml:lang'
   attribute if the stanza contains XML character data that is intended
   to be presented to a human user.  A server SHOULD apply the default
   'xml:lang' attribute to stanzas it routes or delivers on behalf of
   connected entities, and MUST NOT modify or delete 'xml:lang'
   attributes on stanzas it receives from other entities.

   As specified under Section 3, a server MUST support and enforce
   [IDNA] for domain identifiers, the Nodeprep (Appendix A) profile of
   [STRINGPREP] for localparts, and the Resourceprep (Appendix B)
   profile of [STRINGPREP] for resource identifiers; this enables XMPP
   addresses to include a wide variety of Unicode characters outside the
   US-ASCII range.


14.  Security Considerations

14.1.  High Security

   For the purposes of XMPP communication (client-to-server and server-
   to-server), the term "high security" refers to the use of security
   technologies that provide both mutual authentication and integrity
   checking (on the importance of high security, refer to [STRONGSEC]);
   in particular, when using certificate-based authentication to provide
   high security, a trust chain SHOULD be established out-of-band,
   although a shared certification authority signing certificates could
   allow a previously unknown certificate to establish trust in-band.
   See Section 14.2 regarding certificate validation procedures.

   Implementations MUST support high security.  Service provisioning
   SHOULD use high security, subject to local security policies.

   The initial stream and the response stream MUST be secured
   separately, although security in both directions MAY be established
   via mechanisms that provide mutual authentication.

14.2.  Certificates

   Channel encryption of an XML stream using Transport Layer Security as
   described under Section 6, and in some cases also authentication as
   described under Section 7, is commonly based on a digital certificate
   presented by the receiving entity (or, in the case of mutual
   authentication, both the receiving entity and the initiating entity).



Saint-Andre              Expires April 26, 2010               [Page 127]

Internet-Draft                  XMPP Core                   October 2009


   This section describes best practices regarding the generation of
   digital certificates to be presented by XMPP entities and the
   verification of digital certificates presented by XMPP entities.

   Considerations specific to certificate geneneration and validation
   with regard to client certificates or server certificates are
   described in the following sections.

14.2.1.  Certificate Generation

14.2.1.1.  General Considerations

   The following rules apply to public key certificates that are issued
   to XMPP entities:

   1.  The certificate MUST conform to [X509].
   2.  The certificate MUST NOT contain a basicConstraints extension
       with the cA boolean set to TRUE.
   3.  The subject field MUST NOT be null.
   4.  The hash algorithm for the signature SHOULD be SHA-256 as defined
       by [X509-ALGO].

   The following rules apply to attribute certificates that are issued
   to XMPP entities:

   1.  The certificate MUST conform to [X509-AUTH].
   2.  The holder field MUST be the baseCertificateID.
   3.  The attributes field MUST include the Access Identity attribute,
       as specified in Section 4.4.2 of [X509-AUTH].  Both the service
       and ident fields' GeneralName choice MUST be registeredID.  The
       service and ident fields MUST be as defined in Section 5.3 of
       [X509-AUTH].  Other attributes MAY be included.
   4.  The extensions field MUST include the non-critical noRevAvail
       extension, as defined in Section 4.3.6 of [X509-AUTH], to
       indicate that no revocation information is available from the
       attribute certificate issuer.
   5.  The extensions field MAY include the authorityKeyIdentifier
       extension if the issuer has more than one key pair.
   6.  The extensions field MAY include the issuerAltName extension if
       the issuer's certificate includes the subjectAltName extension
       (where the issuerAltName extension MUST be marked non-critical).

   The following rules apply to issuers of XMPP certificates:

   1.  The certificate MUST conform to [X509].
   2.  The certificate MUST NOT contain a basicConstraints extension
       with the cA boolean set to TRUE.




Saint-Andre              Expires April 26, 2010               [Page 128]

Internet-Draft                  XMPP Core                   October 2009


   3.  The certificate MUST contain a keyUsage extension with the
       digitalSignature bit set.
   4.  The subject field MUST NOT be null.
   5.  The hash algorithm for the signature SHOULD be SHA-256 as defined
       by [X509-ALGO].
   6.  For issuers of public key certificates, the issuer's certificate
       MUST contain a basicConstraints extension with the cA boolean set
       to TRUE.
   7.  For issuers of access certificates, the issuer's certificate MUST
       NOT contain a basicConstraints extension with the cA boolean set
       to TRUE.

14.2.1.2.  Server Certificates

   In a digital certificate to be presented by an XMPP server (i.e., a
   SERVER CERTIFICATE), it is RECOMMENDED for the certificate to include
   one or more JIDs (i.e., domain identifiers) associated with domains
   serviced at the server.  The representations described in the
   following sections are RECOMMENDED.  These representations are
   provided in preference order.

14.2.1.2.1.  SRVName

   A server's domain identifier SHOULD be represented as an SRVName,
   i.e., as an otherName field of type "id-on-dnsSRV" as specified in
   [X509-SRV].

14.2.1.2.2.  dNSName

   A server's domain identifier SHOULD be represented as a dNSName,
   i.e., as a subjectAltName extension of type dNSName.

   The dNSName MAY contain one instance of the wildcard character '*'.
   The wildcard character applies only to the left-most domain name
   component and matches any single component (thus a dNSName of
   *.example.com matches foo.example.com but not bar.foo.example.com or
   example.com itself).  The wildcard character is not allowed in
   component fragments (thus a dNSName of im*.example.net is not allowed
   and shall not be taken to match im1.example.net and im2.example.net).

14.2.1.2.3.  XmppAddr

   A server's domain identifier MAY be represented as an XmppAddr, i.e.,
   as a UTF8String within an otherName entity inside the subjectAltName,
   using the [ASN.1] Object Identifier "id-on-xmppAddr" specified under
   Section 14.2.1.4.  In server certificates, this representation is
   included only for the sake of backward-compatibility.




Saint-Andre              Expires April 26, 2010               [Page 129]

Internet-Draft                  XMPP Core                   October 2009


14.2.1.2.4.  Common Name

   A server's domain identifier SHOULD NOT be represented as a Common
   Name; instead, the Common Name field SHOULD be reserved for
   representation of a human-friendly name.

14.2.1.2.5.  Examples

   For our first (relatively simple) example, consider a company called
   "Example Products, Inc."  It hosts an XMPP service at
   "im.example.com" (i.e., user addresses at the service are of the form
   "user@im.example.com"), and SRV lookups for the xmpp-client and xmpp-
   server services at "im.example.com" yield one machine, called
   "x.example.com", as follows:

   _xmpp-client._tcp.im.example.com. 400 IN SRV 20 0 5222 x.example.com
   _xmpp-server._tcp.im.example.com. 400 IN SRV 20 0 5269 x.example.com

   The certificate presented by x.example.com contains the following
   representations:

   o  An otherName type of SRVName (id-on-dnsSRV) containing an
      IA5String (ASCII) string of: "_xmpp-client.im.example.com"
   o  An otherName type of SRVName (id-on-dnsSRV) containing an
      IA5String (ASCII) string of: "_xmpp-server.im.example.com"
   o  A dNSName containing an ASCII string of "im.example.com"
   o  An otherName type of XmppAddr (id-on-xmppAddr) containing a UTF-8
      string of: "im.example.com"
   o  A CN containing an ASCII string of "Example Products, Inc."

   For our second (more complex) example, consider an ISP called
   "Example Internet Services".  It hosts an XMPP service at
   "example.net" (i.e., user addresses at the service are of the form
   "user@example.net"), but SRV lookups for the xmpp-client and xmpp-
   server services at "example.net" yield two machines ("x1.example.net"
   and "x2.example.net"), as follows:

   _xmpp-client._tcp.example.net. 68400 IN SRV 20 0 5222 x1.example.net.
   _xmpp-client._tcp.example.net. 68400 IN SRV 20 0 5222 x2.example.net.
   _xmpp-server._tcp.example.net. 68400 IN SRV 20 0 5269 x1.example.net.
   _xmpp-server._tcp.example.net. 68400 IN SRV 20 0 5269 x2.example.net.

   Example Internet Services also hosts chatrooms at chat.example.net,
   and provides an xmpp-server SRV record for that service as well (thus
   enabling entity from remote domains to access that service).  It also
   might provide other such services in the future, so it wishes to
   represent a wildcard in its certificate to handle such growth.




Saint-Andre              Expires April 26, 2010               [Page 130]

Internet-Draft                  XMPP Core                   October 2009


   The certificate presented by either x1.example.net or x2.example.net
   contains the following representations:

   o  An otherName type of SRVName (id-on-dnsSRV) containing an
      IA5String (ASCII) string of: "_xmpp-client.example.net"
   o  An otherName type of SRVName (id-on-dnsSRV) containing an
      IA5String (ASCII) string of: "_xmpp-server.example.net"
   o  An otherName type of SRVName (id-on-dnsSRV) containing an
      IA5String (ASCII) string of: "_xmpp-server.chat.example.net"
   o  A dNSName containing an ASCII string of "example.net"
   o  A dNSName containing an ASCII string of "*.example.net"
   o  An otherName type of XmppAddr (id-on-xmppAddr) containing a UTF-8
      string of: "example.net"
   o  An otherName type of XmppAddr (id-on-xmppAddr) containing a UTF-8
      string of: "chat.example.net"
   o  A CN containing an ASCII string of "Example Internet Services"

14.2.1.3.  Client Certificates

   In a digital certificate to be presented by an XMPP client controlled
   by a human user (i.e., a CLIENT CERTIFICATE), it is RECOMMENDED for
   the certificate to include one or more JIDs associated with an XMPP
   user.  If included, a JID MUST be represented as an XmppAddr, i.e.,
   as a UTF8String within an otherName entity inside the subjectAltName,
   using the [ASN.1] Object Identifier "id-on-xmppAddr" specified under
   Section 14.2.1.4.

14.2.1.4.  ASN.1 Object Identifier

   The [ASN.1] Object Identifier "id-on-xmppAddr" (also called an
   XmppAddr) is defined as follows.

   id-pkix OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
           dod(6) internet(1) security(5) mechanisms(5) pkix(7) }

   id-on  OBJECT IDENTIFIER ::= { id-pkix 8 }  -- other name forms

   id-on-xmppAddr  OBJECT IDENTIFIER ::= { id-on 5 }

   XmppAddr ::= UTF8String

   As an alternative to the "id-on-xmppAddr" notation, this Object
   Identifier MAY be represented in dotted display format (i.e.,
   "1.3.6.1.5.5.7.8.5") or in the Uniform Resource Name notation
   specified in [URN-OID] (i.e., "urn:oid:1.3.6.1.5.5.7.8.5").

   Thus for example the JID "juliet@im.example.com" as included in a
   certificate could be formatted in any of the following three ways:



Saint-Andre              Expires April 26, 2010               [Page 131]

Internet-Draft                  XMPP Core                   October 2009


   id-on-xmppAddr:
      subjectAltName=otherName:id-on-xmppAddr;UTF8:juliet@im.example.com
   dotted display format:  subjectAltName=otherName:
      1.3.6.1.5.5.7.8.5;UTF8:juliet@im.example.com
   URN notation:  subjectAltName=otherName:urn:oid:
      1.3.6.1.5.5.7.8.5;UTF8:juliet@im.example.com

   Use of the "id-on-xmppAddr" format is RECOMMENDED in the generation
   of certificates, but all three formats MUST be supported for the
   purpose of certificate validation.

   The "id-on-xmppAddr" object identifier MAY be used on conjuction with
   the extended key usage extension specified in Section 4.2.1.12 of
   [X509] in order to explicitly define and limit the intended use of a
   certificate to the XMPP network.

14.2.2.  Certificate Validation

   When an XMPP entity is presented with a server certificate or client
   certificate by a peer for the purpose of encryption or authentication
   of XML streams as described under Section 6 and Section 7, the entity
   MUST validate the certificate to determine if the certificate shall
   be considered a TRUSTED CERTIFICATE, i.e., a certificate that is
   acceptable for encryption and/or authentication in accordance with
   the XMPP entity's local service policies or configured settings.

   For both server certificates and client certificates, the validating
   entity MUST verify the integrity of the certificate, MUST verify that
   the certificate has been properly signed by the issuing Certificate
   Authority, and MUST support certificate revocation messages.  An
   implementation MUST enable a human user to view information about the
   full chain of certificates.

   The following sections describe certificate validation rules for
   server-to-server and client-to-server streams.

14.2.2.1.  Server Certificates

   When an entity (client or server) validates a certificate presented
   by an XMPP server, there are three possible cases, as discussed in
   the following sections.

14.2.2.1.1.  Case #1

   If the server certificate appears to be certified by a chain of
   certificates terminating in a trust anchor (as described in Section
   6.1 of [X509]), the entity MUST check the certificate for any
   instances of the SRVName, dNSName, and XmppAddr (in that order of



Saint-Andre              Expires April 26, 2010               [Page 132]

Internet-Draft                  XMPP Core                   October 2009


   preference) as described under Section 14.2.1.2.1,
   Section 14.2.1.2.2, and Section 14.2.1.2.3.  There are three possible
   sub-cases:

   Sub-Case #1:  The entity finds at least one SRVName, dNSName, or
      XmppAddr that matches the hostname to which it attempted to
      connect; the entity MUST use this represented domain identifier as
      the validated identity of the XMPP server.  The server certificate
      MUST be checked against the hostname as provided by the entity
      (client or server), not the hostname as resolved via the Domain
      Name System; e.g., if a user specifies a hostname of "example.net"
      but a [DNS-SRV] lookup returns "x1.example.net", the certificate
      MUST be checked as "example.net".  A user-oriented client MAY
      provide a configuration setting that enables a human user to
      explicitly specify a hostname to be checked for connection
      purposes.
   Sub-Case #2:  The entity finds no SRVName, dNSName, or XmppAddr that
      matches the hostname to which it attempted to connect and a human
      user has not permanently accepted the certificate during a
      previous connection attempt; the entity MUST NOT use the
      represented domain identifier (if any) as the validated identity
      of the XMPP server.  Instead, if the connecting entity is a user-
      oriented client then it MUST either (1) automatically terminate
      the connection with a bad certificate error or (2) show the
      certificate (including the entire certificate chain) to the user
      and give the user the choice of terminating the connecting or
      accepting the certificate temporarily (i.e., for this connection
      attempt only) or permanently (i.e., for all future connection
      attempts) and then continuing with the connection; if a user
      permanently accepts a certificate in this way, the client MUST
      cache the certificate (or some non-forgeable representation such
      as a hash value) and in future connection attempts behave as in
      Sub-Case #3.  (It is the resposibility of the human user to verify
      the hash value or fingerprint of the certificate with the peer
      over a trusted communication layer.)  If the connecting entity is
      an XMPP server or an automated client, the application SHOULD
      terminate the connection (with a bad certificate error) and log
      the error to an appropriate audit log; an XMPP server or automated
      client MAY provide a configuration setting that disables this
      check, but MUST provide a setting that enables the check.
   Sub-Case #3:  The entity finds no SRVName, dNSName, or XmppAddr that
      matches the hostname to which it attempted to connect but a human
      user has permanently accepted the certificate during a previous
      connection attempt; the entity MUST verify that the cached
      certificate was presented and MUST notify the user if the
      certificate has changed.





Saint-Andre              Expires April 26, 2010               [Page 133]

Internet-Draft                  XMPP Core                   October 2009


14.2.2.1.2.  Case #2

   If the server certificate is certified by a Certificate Authority not
   known to the entity, the entity MUST proceed as under Case #1, Sub-
   Case #2 or Case #1, Sub-Case #3 as appropriate.

14.2.2.1.3.  Case #3

   If the server certificate is self-signed, the entity MUST proceed as
   under Case #1, Sub-Case #2 or Case #1, Sub-Case #3 as appropriate.

14.2.2.2.  Client Certificates

   When an XMPP server validates a certificate presented by a client,
   there are three possible cases, as discussed in the following
   sections.

14.2.2.2.1.  Case #1

   If the client certificate appears to be certified by a chain of
   certificates terminating in a trust anchor (as described in Section
   6.1 of [X509]), the server MUST check the certificate for any
   instances of the XmppAddr as described under Section 14.2.1.4.  There
   are three possible sub-cases:

   Sub-Case #1:  The server finds one XmppAddr for which the domain
      identifier portion of the represented JID matches one of the
      configured hostnames of the server itself; the server SHOULD use
      this represented JID as the validated identity of the client.
   Sub-Case #2:  The server finds more than one XmppAddr for which the
      domain identifier portion of the represented JID matches one of
      the configured hostnames of the server itself; the server SHOULD
      use one of these represented JIDs as the validated identity of the
      client, choosing among them according to local service policies or
      based on the 'to' address of the initial stream header.
   Sub-Case #3:  The server finds no XmppAddrs, or finds at least one
      XmppAddr but the domain identifier portion of the represented JID
      does not match one of the configured hostnames of the server
      itself; the server MUST NOT use the represented JID (if any) as
      the validated identity of the client but instead MUST either
      validate the identity of the client using other means.

14.2.2.2.2.  Case #2

   If the client certificate is certified by a Certificate Authority not
   known to the server, the server MUST proceed as under Case #1, Sub-
   Case #3.




Saint-Andre              Expires April 26, 2010               [Page 134]

Internet-Draft                  XMPP Core                   October 2009


14.2.2.2.3.  Case #3

   If the client certificate is self-signed, the server MUST proceed as
   under Case #1, Sub-Case #3.

14.2.2.3.  Use of Certificates in XMPP Extensions

   Certificates MAY be used in extensions to XMPP for the purpose of
   application-layer encryption or authentication above the level of XML
   streams (e.g., for end-to-end encryption).  Such extensions shall
   define their own certificate handling rules, which at a minimum
   SHOULD be consistent with the rules specified herein but MAY specify
   additional rules.

14.3.  Client-to-Server Communication

   A compliant client implementation MUST support both TLS and SASL for
   connections to a server.

   The TLS protocol for encrypting XML streams (defined under Section 6)
   provides a reliable mechanism for helping to ensure the
   confidentiality and integrity of data exchanged between two entities.

   The SASL protocol for authenticating XML streams (defined under
   Section 7) provides a reliable mechanism for validating that a client
   connecting to a server is who it claims to be.

   Client-to-server communication MUST NOT proceed until the DNS
   hostname asserted by the server has been resolved as specified under
   Section 4.  If there is a mismatch between the hostname to which a
   client attempted to connect (e.g., "example.net") and the hostname to
   which the client actually connects (e.g., "x1.example.net"), the
   client MUST warn a human user about the mismatch and the human user
   MUST approve the connection before the client proceeds; however, the
   client MAY also allow the user to add the presented hostname to a
   configured set of accepted hostnames to expedite future connections.

   A client's IP address and method of access MUST NOT be made public by
   a server, nor are any connections other than the original server
   connection necessary.  This helps to protect the client's server from
   direct attack or identification by third parties.

14.4.  Server-to-Server Communication

   A compliant server implementation MUST support both TLS and SASL for
   inter-domain communication.

   Because service provisioning is a matter of policy, it is optional



Saint-Andre              Expires April 26, 2010               [Page 135]

Internet-Draft                  XMPP Core                   October 2009


   for any given domain to communicate with other domains, and server-
   to-server communication can be disabled by the administrator of any
   given deployment.  If a particular domain enables inter-domain
   communication, it SHOULD enable high security.

   Administrators might want to require use of SASL for server-to-server
   communication to ensure both authentication and confidentiality
   (e.g., on an organization's private network).  Compliant
   implementations SHOULD support SASL for this purpose.

   Server-to-server communication MUST NOT proceed until the DNS
   hostnames asserted by both servers have been resolved as specified
   under Section 4.

14.5.  Order of Layers

   The order of layers in which protocols MUST be stacked is:

   1.  TCP
   2.  TLS
   3.  SASL
   4.  XMPP

   The rationale for this order is that [TCP] is the base connection
   layer used by all of the protocols stacked on top of TCP, [TLS] is
   often provided at the operating system layer, [SASL] is often
   provided at the application layer, and XMPP is the application
   itself.

14.6.  Mandatory-to-Implement Technologies

   At a minimum, all implementations MUST support the following
   mechanisms unless otherwise specified below:

   for authentication only:  the SASL Salted Challenge Response
      mechanism [SCRAM] (preferred) and the SASL PLAIN mechanism [PLAIN]
      (not preferred
   for confidentiality only:  TLS (using the
      TLS_RSA_WITH_AES_128_CBC_SHA cipher)
   for both confidentiality and authentication with passwords:  TLS plus
      the SASL Salted Challenge Response mechanism (see [SCRAM]) and TLS
      plus the SASL PLAIN mechanism (see [PLAIN])
   for both confidentiality and authentication without passwords:  TLS
      plus the SASL EXTERNAL mechanism (see Appendix A of [SASL]) using
      the TLS_RSA_WITH_AES_128_CBC_SHA cipher supporting peer
      certificates (clients SHOULD support this, and servers MUST)

   Naturally, implementations MAY support other ciphers with TLS and MAY



Saint-Andre              Expires April 26, 2010               [Page 136]

Internet-Draft                  XMPP Core                   October 2009


   support other SASL mechanisms.

      Note: The use of TLS plus SASL SCRAM or SASL PLAIN replaces the
      SASL DIGEST-MD5 mechanism as XMPP's mandatory-to-implement
      password-based method for authentication.  For backward-
      compatibility with existing deployed infrastructure,
      implementations are encouraged to continue supporting the SASL
      DIGEST-MD5 mechanism as specified in [DIGEST-MD5], however there
      are known interoperability issues with DIGEST-MD5 that make it
      impractical in the long term.  The use of the SASL SCRAM mechanism
      is strongly preferred over the SASL PLAIN mechanism because of its
      superior security properties, and PLAIN is intended to be a
      fallback only for implementations that do not yet support SCRAM.
      Refer to [SCRAM] and [PLAIN] for important security considerations
      related to these SASL mechanisms.

14.7.  Hash Function Agility

   XMPP itself does not directly mandate the use of any particular hash
   function.  However, technologies on which XMPP depends (e.g., TLS and
   particular SASL mechanisms), as well as various XMPP extensions,
   might make use of hash functions.  Those who implement XMPP
   technologies or who develop XMPP extensions are advised to closely
   monitor the state of the art regarding attacks against cryptographic
   hashes in Internet protocols as they relate to XMPP.  For helpful
   guidance, refer to [HASHES].

14.8.  SASL Downgrade Attacks

   Because the initiating entity chooses an acceptable SASL mechanism
   from the list presented by the receiving entity, the initiating
   entity depends on the receiving entity's list for authentication.
   This dependency introduces the possibility of a downgrade attack if
   an attacker can gain control of the channel and therefore present a
   weak list of mechanisms.  To prevent this attack, the parties SHOULD
   protect the channel using TLS before attempting SASL negotiation.

14.9.  Lack of SASL Channel Binding to TLS

   The SASL framework itself does not provide a method for binding SASL
   authentication to a security layer providing confidentiality and
   integrity protection that was negotiated at a lower layer.  Such a
   binding is known as a "channel binding" (see [CHANNEL]).  Some SASL
   mechanisms provide channel bindings.  However, if a SASL mechanism
   does not provide a channel binding, then the mechanism cannot provide
   a way to verify that the source and destination end points to which
   the lower layer's security is bound are equivalent to the end points
   that SASL is authenticating; furthermore, if the end points are not



Saint-Andre              Expires April 26, 2010               [Page 137]

Internet-Draft                  XMPP Core                   October 2009


   identical, then the lower layer's security cannot be trusted to
   protect data transmitted between the SASL-authenticated entities.  In
   such a situation, a SASL security layer SHOULD be negotiated that
   effectively ignores the presence of the lower-layer security.

14.10.  Use of base64 in SASL

   Both the client and the server MUST verify any base64 data received
   during SASL negotiation (Section 7).  An implementation MUST reject
   (not ignore) any characters that are not explicitly allowed by the
   base64 alphabet; this helps to guard against creation of a covert
   channel that could be used to "leak" information.

   An implementation MUST NOT break on invalid input and MUST reject any
   sequence of base64 characters containing the pad ('=') character if
   that character is included as something other than the last character
   of the data (e.g., "=AAA" or "BBBB=CCC"); this helps to guard against
   buffer overflow attacks and other attacks on the implementation.

   While base 64 encoding visually hides otherwise easily recognized
   information (such as passwords), it does not provide any
   computational confidentiality.

   All uses of base 64 encoding MUST follow the definition in Section 4
   of [BASE64] and padding bits MUST be set to zero.

14.11.  Stringprep Profiles

   XMPP makes use of the [NAMEPREP] profile of [STRINGPREP] for
   processing of domain identifiers; for security considerations related
   to Nameprep, refer to the appropriate section of [NAMEPREP].

   In addition, XMPP defines two profiles of [STRINGPREP]: Nodeprep
   (Appendix A) for localparts and Resourceprep (Appendix B) for
   resource identifiers.

   The Unicode and ISO/IEC 10646 repertoires have many characters that
   look similar.  In many cases, users of security protocols might
   perform visual matching, such as when comparing the names of trusted
   third parties.  Because it is impossible to map similar-looking
   characters without a great deal of context (such as knowing the fonts
   used), stringprep does nothing to map similar-looking characters
   together, nor to prohibit some characters because they look like
   others.

   A localpart can be employed as one part of an entity's address in
   XMPP.  One common usage is as the username of an instant messaging
   user; another is as the name of a multi-user conference room; and



Saint-Andre              Expires April 26, 2010               [Page 138]

Internet-Draft                  XMPP Core                   October 2009


   many other kinds of entities could use localparts as part of their
   addresses.  The security of such services could be compromised based
   on different interpretations of the internationalized localpart; for
   example, a user entering a single internationalized localpart could
   access another user's account information, or a user could gain
   access to a hidden or otherwise restricted chat room or service.

   A resource identifier can be employed as one part of an entity's
   address in XMPP.  One common usage is as the name for an instant
   messaging user's connected resource; another is as the nickname of a
   user in a multi-user conference room; and many other kinds of
   entities could use resource identifiers as part of their addresses.
   The security of such services could be compromised based on different
   interpretations of the internationalized resource identifier; for
   example, a user could attempt to initiate multiple connections with
   the same name, or a user could send a message to someone other than
   the intended recipient in a multi-user conference room.

14.12.  Address Spoofing

   As discussed in [XEP-0165], there are two forms of address spoofing:
   forging and mimicking.

14.12.1.  Address Forging

   In the context of XMPP technologies, address forging occurs when an
   entity is able to generate an XML stanza whose 'from' address does
   not correspond to the account credentials with which the entity
   authenticated onto the network (or an authorization identity provided
   during SASL negotiation (Section 7)).  For example, address forging
   occurs if an entity that authenticated as "juliet@im.example.com" is
   able to send XML stanzas from "nurse@im.example.com" or
   "romeo@example.net".

   Address forging is difficult in XMPP systems, given the requirement
   for sending servers to stamp 'from' addresses and for receiving
   servers to verify sending domains via server-to-server
   authentication.  However, address forging is not impossible, since a
   rogue server could forge JIDs at the sending domain by ignoring the
   stamping requirement.  A rogue server could even forge JIDs at other
   domains by means of a DNS poisoning attack if [DNSSEC] is not used.
   This specification does not define methods for discovering or
   counteracting such rogue servers.

   Note: An entity outside the security perimeter of a particular server
   cannot reliably distinguish between bare JIDs of the form
   <localpart@domain> at that server, since the server could forge any
   such JID; therefore only the domain identifier can be authenticated



Saint-Andre              Expires April 26, 2010               [Page 139]

Internet-Draft                  XMPP Core                   October 2009


   or authorized with any level of assurance.

14.12.2.  Address Mimicking

   Address mimicking occus when an entity provides legitimate
   authentication credentials for and sends XML stanzas from an account
   whose JID appears to a human user to be the same as another JID.  For
   example, in some XMPP clients the address "paypa1@example.org"
   (spelled with the number one as the final character of the localpart)
   might appear to be the same as "paypal@example.org (spelled with the
   lower-case version of the letter "L"), especially on casual visual
   inspection; this phenomenon is sometimes called "typejacking".  A
   more sophisticated example of address mimicking might involve the use
   of characters from outside the US-ASCII range, such as the Cherokee
   characters U+13DA U+13A2 U+13B5 U+13AC U+13A2 U+13AC U+13D2 instead
   of the US-ASCII characters "STPETER".

   In some examples of address mimicking, it is unlikely that the
   average user could tell the difference between the real JID and the
   fake JID.  (Naturally, there is no way to distinguish with full
   certainty which is the fake JID and which is the real JID; in some
   communication contexts, the JID with Cherokee characters might be the
   real JID and the JID with US-ASCII characters might thus appear to be
   the fake JID.)  Because JIDs can contain almost any Unicode
   character, it can be relatively easy to mimic some JIDs in XMPP
   systems.  The possibility of address mimicking introduces security
   vulnerabilities of the kind that have also plagued the World Wide
   Web, specifically the phenomenon known as phishing.

   Mimicked addresses that involve characters from only one character
   set or from the character set typically employed by a particular user
   are not easy to combat (e.g., the simple typejacking attack
   previously described, which relies on a surface similarity between
   the characters "1" and "l" in some presentations).  However, mimicked
   addresses that involve characters from more than one character set,
   or from a character set not typically employed by a particular user,
   can be mitigated somewhat through intelligent presentation.  In
   particular, every human user of an XMPP technology presumably has a
   preferred language (or, in some cases, a small set of preferred
   languages), which an XMPP application SHOULD gather either explicitly
   from the user or implicitly via the operating system of the user's
   device.  Furthermore, every language has a range (or a small set of
   ranges) of characters normally used to represent that language in
   textual form.  Therefore, an XMPP application SHOULD warn the user
   when presenting a JID that uses characters outside the normal range
   of the user's preferred language(s).  This recommendation is not
   intended to discourage communication across language communities;
   instead, it recognizes the existence of such language communities and



Saint-Andre              Expires April 26, 2010               [Page 140]

Internet-Draft                  XMPP Core                   October 2009


   encourages due caution when presenting unfamiliar character sets to
   human users.

   For more detailed recommendations regarding prevention of address
   mimicking in XMPP systems, refer to [XEP-0165].

14.13.  Firewalls

   Communication using XMPP normally occurs over TCP connections on port
   5222 (client-to-server) or port 5269 (server-to-server), as
   registered with the IANA (see Section 15).  Use of these well-known
   ports allows administrators to easily enable or disable XMPP activity
   through existing and commonly-deployed firewalls.

14.14.  Denial of Service

   [DOS] defines denial of service as follows:

      A Denial-of-Service (DoS) attack is an attack in which one or more
      machines target a victim and attempt to prevent the victim from
      doing useful work.  The victim can be a network server, client or
      router, a network link or an entire network, an individual
      Internet user or a company doing business using the Internet, an
      Internet Service Provider (ISP), country, or any combination of or
      variant on these.

   [XEP-0205] provides a detailed discussion of potential denial of
   service attacks against XMPP systems and best practices for
   preventing such attacks.  The recommendations include:

   1.  A server implementation SHOULD enable a server administrator to
       limit the number of TCP connections that it will accept from a
       given IP address at any one time.  If an entity attempts to
       connect but the maximum number of TCP connections has been
       reached, the receiving server MUST NOT allow the new connection
       to proceed.
   2.  A server implementation SHOULD enable a server administrator to
       limit the number of TCP connection attempts that it will accept
       from a given IP address in a given time period.  (While it is
       possible to limit the number of connections at the TCP layer
       rather than at the XMPP application layer, this is not advisable
       because limits at the TCP layer might result in an inability to
       access non-XMPP services.)  If an entity attempts to connect but
       the maximum number of connections has been reached, the receiving
       server MUST NOT allow the new connection to proceed.
   3.  A server MUST NOT process XML stanzas from clients that have not
       yet provided appropriate authentication credentials and MUST NOT
       process XML stanzas from peer servers whose identity it has not



Saint-Andre              Expires April 26, 2010               [Page 141]

Internet-Draft                  XMPP Core                   October 2009


       either authenticated via SASL or weakly verified via server
       dialback (see [XEP-0220]).
   4.  A server implementation SHOULD enable a server administrator to
       limit the number of connected resources it will allow an account
       to bind at any one time.  If a client attempts to bind a resource
       but it has already reached the configured number of allowable
       resources, the receiving server MUST return a <resource-
       constraint/> stanza error.
   5.  A server implementation SHOULD enable a server administrator to
       limit the size of stanzas it will accept from a connected client
       or peer server.  If a connected resource or peer server sends a
       stanza that violates the upper limit, the receiving server SHOULD
       NOT process the stanza and instead SHOULD return a <not-allowed/>
       stanza error.  Alternatively (e.g., if the sender has sent an
       egregiously large stanza), the server MAY instead return a
       <policy-violation/> stream error.
   6.  A server implementation SHOULD enable a server administrator to
       limit the number of XML stanzas that a connected client is
       allowed to send to distinct recipients within a given time
       period.  If a connected client sends too many stanzas to distinct
       recipients in a given time period, the receiving server SHOULD
       NOT process the stanza and instead SHOULD return an <unexpected-
       request/> stanza error.
   7.  A server implementation SHOULD enable a server administrator to
       limit the amount of bandwidth it will allow a connected client or
       peer server to use in a given time period.
   8.  A server implementation MAY enable a server administrator to
       limit the types of stanzas (based on the extended content
       "payload") that it will allow a connected resource or peer server
       send over an active connection.  Such limits and restrictions are
       a matter of deployment policy.
   9.  A server implementation MAY refuse to route or deliver any stanza
       that it considers to be abusive, with or without returning an
       error to the sender.

   For more detailed recommendations regarding denial of service attacks
   in XMPP systems, refer to [XEP-0205].

14.15.  Presence Leaks

   One of the core aspects of XMPP is presence: information about the
   network availability of an XMPP entity (i.e., whether the entity is
   currently online or offline).  A PRESENCE LEAK occurs when an
   entity's network availability is inadvertently and involuntarily
   revealed to a second entity that is not authorized to know the first
   entity's network availability.

   Although presence is discussed more fully in [XMPP-IM], it is



Saint-Andre              Expires April 26, 2010               [Page 142]

Internet-Draft                  XMPP Core                   October 2009


   important to note that an XMPP server MUST NOT leak presence.  In
   particular at the core XMPP level, real-time addressing and network
   availability is associated with a specific connected resource;
   therefore, any disclosure of a connected resource's full JID
   comprises a presence leak.  To help prevent such a presence leak, a
   server MUST NOT return different stanza errors if a potential
   attacker sends XML stanzas to the entity's bare JID
   (<localpart@domain>) or full JID (<localpart@domain/resource>).

14.16.  Directory Harvesting

   When a server generates an error stanza in response to receiving a
   stanza for a user account that does not exist, the use of the
   <service-unavailable/> stanza error condition can help protect
   against dictionary attacks, since this is the same error condition
   that is returned if, for instance, the namespace of an IQ child
   element is not understood, or if offline message storage or message
   forwarding is not enabled for a domain.  However, subtle differences
   in the exact XML of error stanzas, as well as in the timing with
   which such errors are returned, can enable an attacker to determine
   the network presence of a user when more advanced blocking
   technologies are not used (see for instance [XEP-0016] and
   [XEP-0191]).


15.  IANA Considerations

   The following sections update the registrations provided in
   [RFC3920].

15.1.  XML Namespace Name for TLS Data

   A URN sub-namespace for STARTTLS negotiation data in the Extensible
   Messaging and Presence Protocol (XMPP) is defined as follows.  (This
   namespace name adheres to the format defined in [XML-REG].)

   URI:  urn:ietf:params:xml:ns:xmpp-tls
   Specification:  XXXX
   Description:  This is the XML namespace name for STARTTLS negotiation
      data in the Extensible Messaging and Presence Protocol (XMPP) as
      defined by XXXX.
   Registrant Contact:  IETF, XMPP Working Group, <xmppwg@xmpp.org>

15.2.  XML Namespace Name for SASL Data

   A URN sub-namespace for SASL negotiation data in the Extensible
   Messaging and Presence Protocol (XMPP) is defined as follows.  (This
   namespace name adheres to the format defined in [XML-REG].)



Saint-Andre              Expires April 26, 2010               [Page 143]

Internet-Draft                  XMPP Core                   October 2009


   URI:  urn:ietf:params:xml:ns:xmpp-sasl
   Specification:  XXXX
   Description:  This is the XML namespace name for SASL negotiation
      data in the Extensible Messaging and Presence Protocol (XMPP) as
      defined by XXXX.
   Registrant Contact:  IETF, XMPP Working Group, <xmppwg@xmpp.org>

15.3.  XML Namespace Name for Stream Errors

   A URN sub-namespace for stream error data in the Extensible Messaging
   and Presence Protocol (XMPP) is defined as follows.  (This namespace
   name adheres to the format defined in [XML-REG].)

   URI:  urn:ietf:params:xml:ns:xmpp-streams
   Specification:  XXXX
   Description:  This is the XML namespace name for stream error data in
      the Extensible Messaging and Presence Protocol (XMPP) as defined
      by XXXX.
   Registrant Contact:  IETF, XMPP Working Group, <xmppwg@xmpp.org>

15.4.  XML Namespace Name for Resource Binding

   A URN sub-namespace for resource binding in the Extensible Messaging
   and Presence Protocol (XMPP) is defined as follows.  (This namespace
   name adheres to the format defined in [XML-REG].)

   URI:  urn:ietf:params:xml:ns:xmpp-bind
   Specification:  XXXX
   Description:  This is the XML namespace name for resource binding in
      the Extensible Messaging and Presence Protocol (XMPP) as defined
      by XXXX.
   Registrant Contact:  IETF, XMPP Working Group, <xmppwg@xmpp.org>

15.5.  XML Namespace Name for Stanza Errors

   A URN sub-namespace for stanza error data in the Extensible Messaging
   and Presence Protocol (XMPP) is defined as follows.  (This namespace
   name adheres to the format defined in [XML-REG].)

   URI:  urn:ietf:params:xml:ns:xmpp-stanzas
   Specification:  XXXX
   Description:  This is the XML namespace name for stanza error data in
      the Extensible Messaging and Presence Protocol (XMPP) as defined
      by XXXX.







Saint-Andre              Expires April 26, 2010               [Page 144]

Internet-Draft                  XMPP Core                   October 2009


   Registrant Contact:  IETF, XMPP Working Group, <xmppwg@xmpp.org>

15.6.  Nodeprep Profile of Stringprep

   The Nodeprep profile of stringprep is defined under Nodeprep
   (Appendix A).  The IANA has registered Nodeprep in the stringprep
   profile registry.

   Name of this profile:

      Nodeprep

   RFC in which the profile is defined:

      XXXX

   Indicator whether or not this is the newest version of the profile:

      This is the first version of Nodeprep

15.7.  Resourceprep Profile of Stringprep

   The Resourceprep profile of stringprep is defined under Resourceprep
   (Appendix B).  The IANA has registered Resourceprep in the stringprep
   profile registry.

   Name of this profile:

      Resourceprep

   RFC in which the profile is defined:

      XXXX

   Indicator whether or not this is the newest version of the profile:

      This is the first version of Resourceprep

15.8.  GSSAPI Service Name

   The IANA has registered "xmpp" as a GSSAPI [GSS-API] service name, as
   defined under Section 7.5.

15.9.  Port Numbers

   The IANA has registered "xmpp-client" and "xmpp-server" as keywords
   for [TCP] ports 5222 and 5269 respectively.




Saint-Andre              Expires April 26, 2010               [Page 145]

Internet-Draft                  XMPP Core                   October 2009


   These ports SHOULD be used for client-to-server and server-to-server
   communications respectively, but other ports MAY be used.


16.  Conformance Requirements

   This section describes a protocol feature set that summarizes the
   conformance requirements of this specification.  This feature set is
   appropriate for use in software certification, interoperability
   testing, and implementation reports.  For each feature, this section
   provides the following information:

   o  A human-readable name
   o  An informational description
   o  A reference to the particular section of this document that
      normatively defines the feature
   o  Whether the feature applies to the Client role, the Server role,
      or both (where "N/A" signifies that the feature is not applicable
      to the specified role)
   o  Whether the feature MUST or SHOULD be implemented, where the
      capitalized terms are to be understood as described in [TERMS]

   Note: The feature set specified here attempts to adhere to the
   concepts and formats proposed by Larry Masinter within the IETF's
   NEWTRK Working Group in 2005, as captured in [INTEROP].  Although
   this feature set is more detailed than called for by [REPORTS], it
   provides a suitable basis for the generation of implementation
   reports to be submitted in support of advancing this specification
   from Proposed Standard to Draft Standard in accordance with
   [PROCESS].

   Feature:  address-domain-length
   Description:  Ensure that the domain identifier of an XMPP address is
      limited to 1023 bytes in length.
   Section:  Section 3.2
   Roles:  Both MUST.

   Feature:  address-domain-prep
   Description:  Ensure that the domain identifier portion of an XMPP
      address conforms to the Nameprep profile of Stringprep.
   Section:  Section 3.2
   Roles:  Client SHOULD, Server MUST.

   Feature:  address-localpart-length







Saint-Andre              Expires April 26, 2010               [Page 146]

Internet-Draft                  XMPP Core                   October 2009


   Description:  Ensure that the localpart of an XMPP address is limited
      to 1023 bytes in length.
   Section:  Section 3.3
   Roles:  Both MUST.

   Feature:  address-localpart-prep
   Description:  Ensure that the localpart of an XMPP address conforms
      to the Nodeprep profile of Stringprep.
   Section:  Section 3.3
   Roles:  Client SHOULD, Server MUST.

   Feature:  address-resource-length
   Description:  Ensure that the resource identifier of an XMPP address
      is limited to 1023 bytes in length.
   Section:  Section 3.4
   Roles:  Both MUST.

   Feature:  address-resource-prep
   Description:  Ensure that the resource identifier portion of an XMPP
      address conforms to the Resourceprep profile of Stringprep.
   Section:  Section 3.2
   Roles:  Client SHOULD, Server MUST.

   Feature:  bind-gen
   Description:  Generate a random resource on demand.
   Section:  Section 8.5
   Roles:  Client N/A, Server MUST.

   Feature:  bind-mtn
   Description:  Consider resource binding as mandatory-to-negotiate.
   Section:  Section 8.2.1
   Roles:  Client MUST, Server MUST.

   Feature:  bind-restart
   Description:  Do not restart the stream after negotiation of resource
      binding.
   Section:  Section 8.2.2
   Roles:  Client MUST, Server MUST.

   Feature:  bind-support
   Description:  Support binding of client resources to an authenticated
      stream.
   Section:  Section 8
   Roles:  Client MUST, Server MUST.







Saint-Andre              Expires April 26, 2010               [Page 147]

Internet-Draft                  XMPP Core                   October 2009


   Feature:  sasl-errors
   Description:  Support SASL errors during the negotiation process.
   Section:  Section 7.4
   Roles:  Client MUST, Server MUST.

   Feature:  sasl-mtn
   Description:  Consider SASL as mandatory-to-negotiate.
   Section:  Section 7.2.1
   Roles:  Client MUST, Server MUST.

   Feature:  sasl-restart
   Description:  Complete a stream restart after SASL negotiation.
   Section:  Section 7.2.2
   Roles:  Client MUST, Server MUST.

   Feature:  sasl-support
   Description:  Support the Simple Authentication and Security Layer
      for stream authentication.
   Section:  Section 7
   Roles:  Client MUST, Server MUST.

   Feature:  sasl-whitespace
   Description:  Ensure that no whitespace is sent between XML elements
      during SASL negotiation.
   Section:  Section 7.2.5
   Roles:  Client MUST, Server MUST.

   Feature:  security-mti-auth-plain
   Description:  Support the SASL PLAIN mechanism for authentication
      only.
   Section:  Section 14.6
   Roles:  Client MUST, Server MUST.

   Feature:  security-mti-auth-scram
   Description:  Support the SASL Salted Challenge Response mechanism
      for authentication only.
   Section:  Section 14.6
   Roles:  Client MUST, Server MUST.

   Feature:  security-mti-both-external
   Description:  Support TLS with SASL EXTERNAL for confidentiality and
      authentication.
   Section:  Section 14.6
   Roles:  Client SHOULD, Server MUST.







Saint-Andre              Expires April 26, 2010               [Page 148]

Internet-Draft                  XMPP Core                   October 2009


   Feature:  security-mti-both-plain
   Description:  Support TLS with SASL PLAIN for confidentiality and
      authentication.
   Section:  Section 14.6
   Roles:  Client MUST, Server MUST.

   Feature:  security-mti-both-scram
   Description:  Support TLS with SASL SCRAM for confidentiality and
      authentication.
   Section:  Section 14.6
   Roles:  Client MUST, Server MUST.

   Feature:  security-mti-confidentiality
   Description:  Support TLS using the TLS_RSA_WITH_AES_128_CBC_SHA
      cipher for confidentiality only.
   Section:  Section 14.6
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-attribute-from
   Description:  Support the common 'from' attribute for all stanza
      kinds.
   Section:  Section 9.1.1
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-attribute-from-validate
   Description:  Validate the 'from' address of all stanzas received
      from connected clients or peer servers.
   Section:  Section 9.1.2
   Roles:  Client N/A, Server MUST.

   Feature:  stanza-attribute-id
   Description:  Support the common 'id' attribute for all stanza kinds.
   Section:  Section 9.1.3
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-attribute-to
   Description:  Support the common 'to' attribute for all stanza kinds.
   Section:  Section 9.1.1
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-attribute-to-validate
   Description:  Ensure that all stanzas received from peer servers
      include a 'to' address.
   Section:  Section 9.1.1







Saint-Andre              Expires April 26, 2010               [Page 149]

Internet-Draft                  XMPP Core                   October 2009


   Roles:  Client N/A, Server MUST.

   Feature:  stanza-attribute-type
   Description:  Support the common 'type' attribute for all stanza
      kinds.
   Section:  Section 9.1.4
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-attribute-xmllang
   Description:  Support the common 'xml:lang' attribute for all stanza
      kinds.
   Section:  Section 9.1.5
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-error
   Description:  Generate and handle stanzas of type "error" for all
      stanza kinds.
   Section:  Section 9.3
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-error-child
   Description:  Ensure that stanzas of type "error" include an <error/>
      child element.
   Section:  Section 9.3
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-error-id
   Description:  Ensure that stanzas of type "error" preserve the 'id'
      provided in the triggering stanza.
   Section:  Section 9.3
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-error-reply
   Description:  Do not reply to a stanza of type "error" with another
      stanza of type "error".
   Section:  Section 9.3
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-extension
   Description:  Correctly process XML data qualified by an unsupported
      XML namespace, where "correctly process" means to ignore that
      portion of the stanza in the case of a message or presence stanza
      and return an error in the case of an IQ stanza (for the intended
      recipient) or to routeor deliver the stanza (for a routing entity
      such as a server).






Saint-Andre              Expires April 26, 2010               [Page 150]

Internet-Draft                  XMPP Core                   October 2009


   Section:  Section 9.4
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-iq-child
   Description:  Include exactly one child element in an <iq/> stanza of
      type "get" or "set", zero or one child elements in an <iq/> stanza
      of type "result", and one or two child elements in an <iq/> stanza
      of type "error".
   Section:  Section 9.2.3
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-iq-id
   Description:  Ensure that all <iq/> stanzas include an 'id'
      attribute.
   Section:  Section 9.2.3
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-iq-reply
   Description:  Reply to an <iq/> stanzas of type "get" or "set" with
      an <iq/> stanzas of type "result" or "error".
   Section:  Section 9.2.3
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-iq-type
   Description:  Ensure that all <iq/> stanzas include an 'type'
      attribute whose value is "get", "set", "result", or "error".
   Section:  Section 9.2.3
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-kind-iq
   Description:  Support the <iq/> stanza.
   Section:  Section 9.2.3
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-kind-message
   Description:  Support the <message/> stanza.
   Section:  Section 9.2.1
   Roles:  Client MUST, Server MUST.

   Feature:  stanza-kind-presence
   Description:  Support the <presence/> stanza.
   Section:  Section 9.2.2
   Roles:  Client MUST, Server MUST.








Saint-Andre              Expires April 26, 2010               [Page 151]

Internet-Draft                  XMPP Core                   October 2009


   Feature:  stream-attribute-initial-from
   Description:  Include a 'from' attribute in the initial stream
      header.
   Section:  Section 5.4.1
   Roles:  Both SHOULD.

   Feature:  stream-attribute-initial-lang
   Description:  Include an 'xml:lang' attribute in the initial stream
      header.
   Section:  Section 5.4.4
   Roles:  Both SHOULD.

   Feature:  stream-attribute-initial-to
   Description:  Include a 'to' attribute in the initial stream header.
   Section:  Section 5.4.2
   Roles:  Both SHOULD.

   Feature:  stream-attribute-response-from
   Description:  Include a 'from' attribute in the response stream
      header.
   Section:  Section 5.4.1
   Roles:  Client N/A, Server MUST.

   Feature:  stream-attribute-response-id
   Description:  Include an 'id' attribute in the response stream
      header.
   Section:  Section 5.4.3
   Roles:  Client N/A, Server MUST.

   Feature:  stream-attribute-response-id-unique
   Description:  Ensure that the 'id' attribute in the response stream
      header is unique within the context of the receiving entity.
   Section:  Section 5.4.3
   Roles:  Client N/A, Server MUST.

   Feature:  stream-attribute-response-to
   Description:  Include a 'to' attribute in the response stream header.
   Section:  Section 5.4.2
   Roles:  Client N/A, Server SHOULD.

   Feature:  stream-error-generate
   Description:  Generate a stream error (followed by a closing stream
      tag and termination of the TCP connection) upon detecting a
      stream-related error condition.







Saint-Andre              Expires April 26, 2010               [Page 152]

Internet-Draft                  XMPP Core                   October 2009


   Section:  Section 5.6
   Roles:  Client MUST, Server MUST.

   Feature:  stream-hostname-resolution
   Description:  Resolve hostnames before opening a TCP connection.
   Section:  Section 4.2
   Roles:  Both MUST.

   Feature:  stream-negotiation-complete
   Description:  Do not consider the stream negotiation process to be
      complete until the receiving entity sends a stream features
      advertisement that is empty or that contains only voluntary-to-
      negotiate features.
   Section:  Section 5.2.5
   Roles:  Client MUST, Server MUST.

   Feature:  stream-negotiation-features
   Description:  Send stream features after sending a response stream
      header.
   Section:  Section 5.2.2
   Roles:  Client N/A, Server MUST.

   Feature:  stream-negotiation-restart
   Description:  Consider the previous stream to be replaced upon
      negotiation of a stream feature that necessitates a stream
      restart, and send or receive a new initial stream header after
      negotiation of such a stream feature.
   Section:  Section 5.2.3
   Roles:  Client MUST, Server MUST.

   Feature:  stream-reconnect
   Description:  Reconnect with intelligently (e.g., with exponential
      backoff) if a TCP connection is terminated unexpectedly.
   Section:  Section 4.5
   Roles:  Both MUST.

   Feature:  stream-tcp-binding
   Description:  Bind an XML stream to a TCP connection.
   Section:  Section 4
   Roles:  Both MUST.

   Feature:  tls-certs
   Description:  Check the identity specified in a certificate that is
      presented during TLS negotiation.







Saint-Andre              Expires April 26, 2010               [Page 153]

Internet-Draft                  XMPP Core                   October 2009


   Section:  Section 14.2.2
   Roles:  Client MUST, Server MUST.

   Feature:  tls-mtn
   Description:  Consider TLS as mandatory-to-negotiate if STARTTLS is
      the only feature advertised or ir the STARTTLS feature includes an
      empty <required/> element.
   Section:  Section 6.2.1
   Roles:  Client MUST, Server MUST.

   Feature:  tls-restart
   Description:  Complete a stream restart after TLS negotiation.
   Section:  Section 6.2.2
   Roles:  Client MUST, Server MUST.

   Feature:  tls-support
   Description:  Support Transport Layer Security for stream encryption.
   Section:  Section 6
   Roles:  Client MUST, Server MUST.

   Feature:  tls-whitespace
   Description:  Ensure that no whitespace is sent between XML elements
      during TLS negotiation.
   Section:  Section 6.2.3
   Roles:  Client MUST, Server MUST.

   Feature:  xml-namespace-default
   Description:  Ensure that there is a default namespace for the stream
      (other than the streams namespace).
   Section:  Section 5.5.2
   Roles:  Both MUST.

   Feature:  xml-namespace-default-client
   Description:  Support 'jabber:client' as a default namespace.
   Section:  Section 5.5.2
   Roles:  Both MUST.

   Feature:  xml-namespace-default-server
   Description:  Support 'jabber:server' as a default namespace.
   Section:  Section 5.5.2
   Roles:  Client N/A, Server MUST.

   Feature:  xml-namespace-streams-declaration
   Description:  Ensure that there is a namespace declaration for the
      'http://etherx.jabber.org/streams' namespace.






Saint-Andre              Expires April 26, 2010               [Page 154]

Internet-Draft                  XMPP Core                   October 2009


   Section:  Section 5.5.1
   Roles:  Both MUST.

   Feature:  xml-namespace-streams-prefix
   Description:  Ensure that all elements qualified by the
      'http://etherx.jabber.org/streams' namespace are prefixed by the
      prefix defined in the namespace declaration.
   Section:  Section 5.5.1
   Roles:  Both MUST.

   Feature:  xml-restriction-comment
   Description:  Do not generate or accept XML comments.
   Section:  Section 12.1
   Roles:  Client MUST, Server MUST.

   Feature:  xml-restriction-dtd
   Description:  Do not generate or accept internal or external DTD
      subsets.
   Section:  Section 12.1
   Roles:  Client MUST, Server MUST.

   Feature:  xml-restriction-pi
   Description:  Do not generate or accept XML processing instructions.
   Section:  Section 12.1
   Roles:  Client MUST, Server MUST.

   Feature:  xml-restriction-ref
   Description:  Do not generate or accept internal or external entity
      references with the exception of the predefined entities.
   Section:  Section 12.1
   Roles:  Client MUST, Server MUST.

   Feature:  xml-wellformed-xml
   Description:  Do not generate or accept data that is not XML-well-
      formed.
   Section:  Section 12.3
   Roles:  Client MUST, Server MUST.

   Feature:  xml-wellformed-ns
   Description:  Do not generate or accept data that is not namespace-
      well-formed.
   Section:  Section 12.3
   Roles:  Client MUST, Server MUST.


17.  References





Saint-Andre              Expires April 26, 2010               [Page 155]

Internet-Draft                  XMPP Core                   October 2009


17.1.  Normative References

   [ABNF]     Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [BASE64]   Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

   [CHARSET]  Alvestrand, H., "IETF Policy on Character Sets and
              Languages", BCP 18, RFC 2277, January 1998.

   [DNS]      Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [DNS-SRV]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [IDNA]     Faltstrom, P., Hoffman, P., and A. Costello,
              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, March 2003.

   [LANGTAGS]
              Phillips, A. and M. Davis, "Tags for Identifying
              Languages", BCP 47, RFC 4646, September 2006.

   [NAMEPREP]
              Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
              Profile for Internationalized Domain Names (IDN)",
              RFC 3491, March 2003.

   [PLAIN]    Zeilenga, K., "The PLAIN Simple Authentication and
              Security Layer (SASL) Mechanism", RFC 4616, August 2006.

   [RANDOM]   Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.

   [SASL]     Melnikov, A. and K. Zeilenga, "Simple Authentication and
              Security Layer (SASL)", RFC 4422, June 2006.

   [SCRAM]    Menon-Sen, A., Melnikov, A., Newman, C., and N. Williams,
              "Salted Challenge Response (SCRAM) SASL and GSS-API
              Mechanism", draft-ietf-sasl-scram-10 (work in progress),
              October 2009.

   [STRINGPREP]
              Hoffman, P. and M. Blanchet, "Preparation of
              Internationalized Strings ("stringprep")", RFC 3454,



Saint-Andre              Expires April 26, 2010               [Page 156]

Internet-Draft                  XMPP Core                   October 2009


              December 2002.

   [TCP]      Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

   [TERMS]    Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [TLS]      Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [UCS2]     International Organization for Standardization,
              "Information Technology - Universal Multiple-octet coded
              Character Set (UCS) - Amendment 2: UCS Transformation
              Format 8 (UTF-8)", ISO Standard 10646-1 Addendum 2,
              October 1996.

   [UNICODE]  The Unicode Consortium, "The Unicode Standard, Version
              3.2.0", 2000.

              The Unicode Standard, Version 3.2.0 is defined by The
              Unicode Standard, Version 3.0 (Reading, MA, Addison-
              Wesley, 2000.  ISBN 0-201-61633-5), as amended by the
              Unicode Standard Annex #27: Unicode 3.1
              (http://www.unicode.org/reports/tr27/) and by the Unicode
              Standard Annex #28: Unicode 3.2
              (http://www.unicode.org/reports/tr28/).

   [UTF-8]    Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [UUID]     Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              July 2005.

   [URI]      Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [X509]     Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

   [X509-AUTH]
              Housley, R., Farrell, S., and S. Turner, "An Internet
              Attribute Certificate Profile for Authorization",
              draft-ietf-pkix-3281update-05 (work in progress),



Saint-Andre              Expires April 26, 2010               [Page 157]

Internet-Draft                  XMPP Core                   October 2009


              April 2009.

   [X509-ALGO]
              Jonsson, J. and B. Kaliski, "Public-Key Cryptography
              Standards (PKCS) #1: RSA Cryptography Specifications
              Version 2.1", RFC 3447, February 2003.

   [X509-SRV]
              Santesson, S., "Internet X.509 Public Key Infrastructure
              Subject Alternative Name for Expression of Service Name",
              RFC 4985, August 2007.

   [XML]      Paoli, J., Maler, E., Sperberg-McQueen, C., Yergeau, F.,
              and T. Bray, "Extensible Markup Language (XML) 1.0 (Fourth
              Edition)", World Wide Web Consortium Recommendation REC-
              xml-20060816, August 2006,
              <http://www.w3.org/TR/2006/REC-xml-20060816>.

   [XML-NAMES]
              Layman, A., Hollander, D., Tobin, R., and T. Bray,
              "Namespaces in XML 1.1 (Second Edition)", World Wide Web
              Consortium Recommendation REC-xml-names11-20060816,
              August 2006, <http://www.w3.org/TR/REC-xml-names>.

17.2.  Informative References

   [ACAP]     Newman, C. and J. Myers, "ACAP -- Application
              Configuration Access Protocol", RFC 2244, November 1997.

   [ANONYMOUS]
              Zeilenga, K., "Anonymous Simple Authentication and
              Security Layer (SASL) Mechanism", RFC 4505, June 2006.

   [ASN.1]    CCITT, "Recommendation X.208: Specification of Abstract
              Syntax Notation One (ASN.1)", 1988.

   [CHANNEL]  Williams, N., "On the Use of Channel Bindings to Secure
              Channels", RFC 5056, November 2007.

   [DIGEST-MD5]
              Leach, P. and C. Newman, "Using Digest Authentication as a
              SASL Mechanism", RFC 2831, May 2000.

   [DNSSEC]   Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [DNS-TXT]  Rosenbaum, R., "Using the Domain Name System To Store



Saint-Andre              Expires April 26, 2010               [Page 158]

Internet-Draft                  XMPP Core                   October 2009


              Arbitrary String Attributes", RFC 1464, May 1993.

   [DOS]      Handley, M., Rescorla, E., and IAB, "Internet Denial-of-
              Service Considerations", RFC 4732, December 2006.

   [EMAIL-ARCH]
              Crocker, D., "Internet Mail Architecture", RFC 5598,
              July 2009.

   [GSS-API]  Linn, J., "Generic Security Service Application Program
              Interface Version 2, Update 1", RFC 2743, January 2000.

   [HASHES]   Hoffman, P. and B. Schneier, "Attacks on Cryptographic
              Hashes in Internet Protocols", RFC 4270, November 2005.

   [HTTP]     Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [IMAP]     Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, March 2003.

   [IMP-REQS]
              Day, M., Aggarwal, S., and J. Vincent, "Instant Messaging
              / Presence Protocol Requirements", RFC 2779,
              February 2000.

   [INTEROP]  Masinter, L., "Formalizing IETF Interoperability
              Reporting", draft-ietf-newtrk-interop-reports-00 (work in
              progress), October 2005.

   [IRI]      Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, January 2005.

   [LINKLOCAL]
              Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
              Configuration of IPv4 Link-Local Addresses", RFC 3927,
              May 2005.

   [MAILBOXES]
              Crocker, D., "MAILBOX NAMES FOR COMMON SERVICES, ROLES AND
              FUNCTIONS", RFC 2142, May 1997.

   [POP3]     Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, May 1996.

   [PROCESS]  Bradner, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, October 1996.



Saint-Andre              Expires April 26, 2010               [Page 159]

Internet-Draft                  XMPP Core                   October 2009


   [PUNYCODE]
              Costello, A., "Punycode: A Bootstring encoding of Unicode
              for Internationalized Domain Names in Applications
              (IDNA)", RFC 3492, March 2003.

   [REPORTS]  Dusseault, L. and R. Sparks, "Guidance on Interoperation
              and Implementation Reports for Advancement to Draft
              Standard", BCP 9, RFC 5657, September 2009.

   [REST]     Fielding, R., "Architectural Styles and the Design of
              Network-based Software Architectures",  2000.

   [RFC3920]  Saint-Andre, P., Ed., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 3920, October 2004.

   [RFC3921]  Saint-Andre, P., Ed., "Extensible Messaging and Presence
              Protocol (XMPP): Instant Messaging and Presence",
              RFC 3921, October 2004.

   [SECTERMS]
              Shirey, R., "Internet Security Glossary, Version 2",
              RFC 4949, August 2007.

   [SMTP]     Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

   [STRONGSEC]
              Schiller, J., "Strong Security Requirements for Internet
              Engineering Task Force Standard Protocols", BCP 61,
              RFC 3365, August 2002.

   [URN-OID]  Mealling, M., "A URN Namespace of Object Identifiers",
              RFC 3061, February 2001.

   [USINGTLS]
              Newman, C., "Using TLS with IMAP, POP3 and ACAP",
              RFC 2595, June 1999.

   [XEP-0001]
              Saint-Andre, P., "XMPP Extension Protocols", XSF XEP 0001,
              January 2008.

   [XEP-0016]
              Millard, P. and P. Saint-Andre, "Privacy Lists", XSF
              XEP 0016, February 2007.

   [XEP-0030]
              Hildebrand, J., Millard, P., Eatmon, R., and P. Saint-



Saint-Andre              Expires April 26, 2010               [Page 160]

Internet-Draft                  XMPP Core                   October 2009


              Andre, "Service Discovery", XSF XEP 0030, June 2008.

   [XEP-0045]
              Saint-Andre, P., "Multi-User Chat", XSF XEP 0045,
              July 2007.

   [XEP-0060]
              Millard, P., Saint-Andre, P., and R. Meijer, "Publish-
              Subscribe", XSF XEP 0060, September 2008.

   [XEP-0071]
              Saint-Andre, P., "XHTML-IM", XSF XEP 0071, September 2008.

   [XEP-0077]
              Saint-Andre, P., "In-Band Registration", XSF XEP 0077,
              January 2006.

   [XEP-0124]
              Paterson, I., Smith, D., and P. Saint-Andre,
              "Bidirectional-streams Over Synchronous HTTP (BOSH)", XSF
              XEP 0124, April 2009.

   [XEP-0156]
              Hildebrand, J. and P. Saint-Andre, "Discovering
              Alternative XMPP Connection Methods", XSF XEP 0156,
              June 2007.

   [XEP-0165]
              Saint-Andre, P., "Best Practices to Prevent JID
              Mimicking", XSF XEP 0165, December 2007.

   [XEP-0174]
              Saint-Andre, P., "Link-Local Messaging", XSF XEP 0174,
              November 2008.

   [XEP-0175]
              Saint-Andre, P., "Best Practices for Use of SASL
              ANONYMOUS", XSF XEP 0175, November 2007.

   [XEP-0178]
              Saint-Andre, P. and P. Millard, "Best Practices for Use of
              SASL EXTERNAL with Certificates", XSF XEP 0178,
              February 2007.

   [XEP-0191]
              Saint-Andre, P., "Simple Communications Blocking", XSF
              XEP 0191, February 2007.




Saint-Andre              Expires April 26, 2010               [Page 161]

Internet-Draft                  XMPP Core                   October 2009


   [XEP-0198]
              Karneges, J., Hildebrand, J., Saint-Andre, P., and F.
              Forno, "Stream Management", XSF XEP 0198, June 2009.

   [XEP-0199]
              Saint-Andre, P., "XMPP Ping", XSF XEP 0199, June 2009.

   [XEP-0205]
              Saint-Andre, P., "Best Practices to Discourage Denial of
              Service Attacks", XSF XEP 0205, January 2009.

   [XEP-0206]
              Paterson, I., "XMPP Over BOSH", XSF XEP 0206,
              October 2008.

   [XEP-0220]
              Saint-Andre, P. and J. Miller, "Server Dialback", XSF
              XEP 0220, October 2008.

   [XEP-0271]
              Saint-Andre, P., "XMPP Nodes", XSF XEP 0271, June 2009.

   [XML-FRAG]
              Grosso, P. and D. Veillard, "XML Fragment Interchange",
              World Wide Web Consortium CR CR-xml-fragment-20010212,
              February 2001,
              <http://www.w3.org/TR/2001/CR-xml-fragment-20010212>.

   [XML-REG]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              January 2004.

   [XML-SCHEMA]
              Thompson, H., Maloney, M., Mendelsohn, N., and D. Beech,
              "XML Schema Part 1: Structures Second Edition", World Wide
              Web Consortium Recommendation REC-xmlschema-1-20041028,
              October 2004,
              <http://www.w3.org/TR/2004/REC-xmlschema-1-20041028>.

   [XMPP-IM]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Instant Messaging and  Presence",
              draft-ietf-xmpp-3921bis-02 (work in progress),
              October 2009.

   [XMPP-URI]
              Saint-Andre, P., "Internationalized Resource Identifiers
              (IRIs) and Uniform Resource Identifiers (URIs) for the
              Extensible Messaging and Presence Protocol (XMPP)",
              RFC 5122, February 2008.



Saint-Andre              Expires April 26, 2010               [Page 162]

Internet-Draft                  XMPP Core                   October 2009


Appendix A.  Nodeprep

A.1.  Introduction

   This appendix defines the "Nodeprep" profile of stringprep.  As such,
   it specifies processing rules that will enable users to enter
   internationalized localparts in the Extensible Messaging and Presence
   Protocol (XMPP) and have the highest chance of getting the content of
   the strings correct.  (An XMPP localpart is the optional portion of
   an XMPP address that precedes an XMPP domain identifier and the '@'
   separator; it is often but not exclusively associated with an instant
   messaging username.)  These processing rules are intended only for
   XMPP localparts and are not intended for arbitrary text or any other
   aspect of an XMPP address.

   This profile defines the following, as required by [STRINGPREP]:

   o  The intended applicability of the profile: internationalized
      localparts within XMPP
   o  The character repertoire that is the input and output to
      stringprep: Unicode 3.2, specified in Section 2 of this Appendix
   o  The mappings used: specified in Section 3
   o  The Unicode normalization used: specified in Section 4
   o  The characters that are prohibited as output: specified in Section
      5
   o  Bidirectional character handling: specified in Section 6

A.2.  Character Repertoire

   This profile uses Unicode 3.2 with the list of unassigned code points
   being Table A.1, both defined in Appendix A of [STRINGPREP].

A.3.  Mapping

   This profile specifies mapping using the following tables from
   [STRINGPREP]:

      Table B.1
      Table B.2

A.4.  Normalization

   This profile specifies the use of Unicode normalization form KC, as
   described in [STRINGPREP].







Saint-Andre              Expires April 26, 2010               [Page 163]

Internet-Draft                  XMPP Core                   October 2009


A.5.  Prohibited Output

   This profile specifies the prohibition of using the following tables
   from [STRINGPREP].

      Table C.1.1
      Table C.1.2
      Table C.2.1
      Table C.2.2
      Table C.3
      Table C.4
      Table C.5
      Table C.6
      Table C.7
      Table C.8
      Table C.9

   In addition, the following additional Unicode characters are also
   prohibited:

      U+0022 (QUOTATION MARK), i.e., "
      U+0026 (AMPERSAND), i.e., &
      U+0027 (APOSTROPHE), i.e., '
      U+002F (SOLIDUS), i.e., /
      U+003A (COLON), i.e., :
      U+003C (LESS-THAN SIGN), i.e., <
      U+003E (GREATER-THAN SIGN), i.e., >
      U+0040 (COMMERCIAL AT), i.e., @

A.6.  Bidirectional Characters

   This profile specifies checking bidirectional strings, as described
   in Section 6 of [STRINGPREP].

A.7.  Notes

   Because the additional characters prohibited by Nodeprep are
   prohibited after normalization, an implementation MUST NOT enable a
   human user to input any Unicode code point whose decomposition
   includes those characters; such code points include but are not
   necessarily limited to the following (refer to [UNICODE] for complete
   information).

   o  U+2100 (ACCOUNT OF)
   o  U+2101 (ADDRESSED TO THE SUBJECT)
   o  U+2105 (CARE OF)





Saint-Andre              Expires April 26, 2010               [Page 164]

Internet-Draft                  XMPP Core                   October 2009


   o  U+2106 (CADA UNA)
   o  U+226E (NOT LESS-THAN)
   o  U+226F (NOT GREATER-THAN)
   o  U+2A74 (DOUBLE COLON EQUAL)
   o  U+FE13 (SMALL COLON)
   o  U+FE60 (SMALL AMPERSAND)
   o  U+FE64 (SMALL LESS-THAN SIGN)
   o  U+FE65 (SMALL GREATER-THAN SIGN)
   o  U+FE6B (SMALL COMMERCIAL AT)
   o  U+FF02 (FULLWIDTH QUOTATION MARK)
   o  U+FF06 (FULLWIDTH AMPERSAND)
   o  U+FF07 (FULLWIDTH APOSTROPHE)
   o  U+FF0F (FULLWIDTH SOLIDUS)
   o  U+FF1A (FULLWIDTH COLON)
   o  U+FF1C (FULLWIDTH LESS-THAN SIGN)
   o  U+FF1E (FULLWIDTH GREATER-THAN SIGN)
   o  U+FF20 (FULLWIDTH COMMERCIAL AT)


Appendix B.  Resourceprep

B.1.  Introduction

   This appendix defines the "Resourceprep" profile of stringprep.  As
   such, it specifies processing rules that will enable users to enter
   internationalized resource identifiers in the Extensible Messaging
   and Presence Protocol (XMPP) and have the highest chance of getting
   the content of the strings correct.  (An XMPP resource identifier is
   the optional portion of an XMPP address that follows an XMPP domain
   identifier and the '/' separator.)  These processing rules are
   intended only for XMPP resource identifiers and are not intended for
   arbitrary text or any other aspect of an XMPP address.

   This profile defines the following, as required by [STRINGPREP]:

   o  The intended applicability of the profile: internationalized
      resource identifiers within XMPP
   o  The character repertoire that is the input and output to
      stringprep: Unicode 3.2, specified in Section 2 of this Appendix
   o  The mappings used: specified in Section 3
   o  The Unicode normalization used: specified in Section 4
   o  The characters that are prohibited as output: specified in Section
      5
   o  Bidirectional character handling: specified in Section 6







Saint-Andre              Expires April 26, 2010               [Page 165]

Internet-Draft                  XMPP Core                   October 2009


B.2.  Character Repertoire

   This profile uses Unicode 3.2 with the list of unassigned code points
   being Table A.1, both defined in Appendix A of [STRINGPREP].

B.3.  Mapping

   This profile specifies mapping using the following tables from
   [STRINGPREP]:

      Table B.1

B.4.  Normalization

   This profile specifies the use of Unicode normalization form KC, as
   described in [STRINGPREP].

B.5.  Prohibited Output

   This profile specifies the prohibition of using the following tables
   from [STRINGPREP].

      Table C.1.2
      Table C.2.1
      Table C.2.2
      Table C.3
      Table C.4
      Table C.5
      Table C.6
      Table C.7
      Table C.8
      Table C.9

B.6.  Bidirectional Characters

   This profile specifies checking bidirectional strings, as described
   in Section 6 of [STRINGPREP].


Appendix C.  XML Schemas

   Because validation of XML streams and stanzas is optional, the
   following XML schemas are provided for descriptive purposes only.
   These schemas are not normative.

   The following schemas formally define various XML namespaces used in
   the core XMPP protocols, in conformance with [XML-SCHEMA].  For
   schemas defining the 'jabber:client' and 'jabber:server' namespaces,



Saint-Andre              Expires April 26, 2010               [Page 166]

Internet-Draft                  XMPP Core                   October 2009


   refer to [XMPP-IM].

C.1.  Streams Namespace

   <?xml version='1.0' encoding='UTF-8'?>

   <xs:schema
       xmlns:xs='http://www.w3.org/2001/XMLSchema'
       targetNamespace='http://etherx.jabber.org/streams'
       xmlns='http://etherx.jabber.org/streams'
       elementFormDefault='unqualified'>

     <xs:import namespace='jabber:client'/>
     <xs:import namespace='jabber:server'/>
     <xs:import namespace='urn:ietf:params:xml:ns:xmpp-sasl'/>
     <xs:import namespace='urn:ietf:params:xml:ns:xmpp-streams'/>
     <xs:import namespace='urn:ietf:params:xml:ns:xmpp-tls'/>

     <xs:element name='stream'>
       <xs:complexType>
         <xs:sequence xmlns:client='jabber:client'
                      xmlns:server='jabber:server'>
           <xs:element ref='features' minOccurs='0' maxOccurs='1'/>
           <xs:any namespace='urn:ietf:params:xml:ns:xmpp-tls'
                   minOccurs='0'
                   maxOccurs='unbounded'/>
           <xs:any namespace='urn:ietf:params:xml:ns:xmpp-sasl'
                   minOccurs='0'
                   maxOccurs='unbounded'/>
           <xs:choice minOccurs='0' maxOccurs='1'>
             <xs:choice minOccurs='0' maxOccurs='unbounded'>
               <xs:element ref='client:message'/>
               <xs:element ref='client:presence'/>
               <xs:element ref='client:iq'/>
             </xs:choice>
             <xs:choice minOccurs='0' maxOccurs='unbounded'>
               <xs:element ref='server:message'/>
               <xs:element ref='server:presence'/>
               <xs:element ref='server:iq'/>
               <xs:element ref='db:result'/>
               <xs:element ref='db:verify'/>
             </xs:choice>
           </xs:choice>
           <xs:element ref='error' minOccurs='0' maxOccurs='1'/>
         </xs:sequence>
         <xs:attribute name='from' type='xs:string' use='optional'/>
         <xs:attribute name='id' type='xs:string' use='optional'/>
         <xs:attribute name='to' type='xs:string' use='optional'/>



Saint-Andre              Expires April 26, 2010               [Page 167]

Internet-Draft                  XMPP Core                   October 2009


         <xs:attribute name='version' type='xs:decimal' use='optional'/>
         <xs:attribute ref='xml:lang' use='optional'/>
       </xs:complexType>
     </xs:element>

     <xs:element name='features'>
       <xs:complexType>
         <xs:any namespace='##other'/>
       </xs:complexType>
     </xs:element>

     <xs:element name='error'>
       <xs:complexType>
         <xs:sequence  xmlns:err='urn:ietf:params:xml:ns:xmpp-streams'>
           <xs:group   ref='err:streamErrorGroup'/>
           <xs:element ref='err:text'
                       minOccurs='0'
                       maxOccurs='1'/>
           <xs:any     namespace='##other'
                       minOccurs='0'
                       maxOccurs='1'/>
         </xs:sequence>
       </xs:complexType>
     </xs:element>

   </xs:schema>

C.2.  Stream Error Namespace

   <?xml version='1.0' encoding='UTF-8'?>

   <xs:schema
       xmlns:xs='http://www.w3.org/2001/XMLSchema'
       targetNamespace='urn:ietf:params:xml:ns:xmpp-streams'
       xmlns='urn:ietf:params:xml:ns:xmpp-streams'
       elementFormDefault='qualified'>

     <xs:element name='bad-format' type='empty'/>
     <xs:element name='bad-namespace-prefix' type='empty'/>
     <xs:element name='conflict' type='empty'/>
     <xs:element name='connection-timeout' type='empty'/>
     <xs:element name='host-gone' type='empty'/>
     <xs:element name='host-unknown' type='empty'/>
     <xs:element name='improper-addressing' type='empty'/>
     <xs:element name='internal-server-error' type='empty'/>
     <xs:element name='invalid-from' type='empty'/>
     <xs:element name='invalid-id' type='empty'/>
     <xs:element name='invalid-namespace' type='empty'/>



Saint-Andre              Expires April 26, 2010               [Page 168]

Internet-Draft                  XMPP Core                   October 2009


     <xs:element name='invalid-xml' type='empty'/>
     <xs:element name='not-authorized' type='empty'/>
     <xs:element name='policy-violation' type='empty'/>
     <xs:element name='remote-connection-failed' type='empty'/>
     <xs:element name='resource-constraint' type='empty'/>
     <xs:element name='restricted-xml' type='empty'/>
     <xs:element name='see-other-host' type='xs:string'/>
     <xs:element name='system-shutdown' type='empty'/>
     <xs:element name='undefined-condition' type='empty'/>
     <xs:element name='unsupported-encoding' type='empty'/>
     <xs:element name='unsupported-stanza-type' type='empty'/>
     <xs:element name='unsupported-version' type='empty'/>
     <xs:element name='xml-not-well-formed' type='empty'/>

     <xs:group name='streamErrorGroup'>
       <xs:choice>
         <xs:element ref='bad-format'/>
         <xs:element ref='bad-namespace-prefix'/>
         <xs:element ref='conflict'/>
         <xs:element ref='connection-timeout'/>
         <xs:element ref='host-gone'/>
         <xs:element ref='host-unknown'/>
         <xs:element ref='improper-addressing'/>
         <xs:element ref='internal-server-error'/>
         <xs:element ref='invalid-from'/>
         <xs:element ref='invalid-id'/>
         <xs:element ref='invalid-namespace'/>
         <xs:element ref='invalid-xml'/>
         <xs:element ref='not-authorized'/>
         <xs:element ref='policy-violation'/>
         <xs:element ref='remote-connection-failed'/>
         <xs:element ref='resource-constraint'/>
         <xs:element ref='restricted-xml'/>
         <xs:element ref='see-other-host'/>
         <xs:element ref='system-shutdown'/>
         <xs:element ref='undefined-condition'/>
         <xs:element ref='unsupported-encoding'/>
         <xs:element ref='unsupported-stanza-type'/>
         <xs:element ref='unsupported-version'/>
         <xs:element ref='xml-not-well-formed'/>
       </xs:choice>
     </xs:group>

     <xs:element name='text'>
       <xs:complexType>
         <xs:simpleContent>
           <xs:extension base='xs:string'>
             <xs:attribute ref='xml:lang' use='optional'/>



Saint-Andre              Expires April 26, 2010               [Page 169]

Internet-Draft                  XMPP Core                   October 2009


           </xs:extension>
         </xs:simpleContent>
       </xs:complexType>
     </xs:element>

     <xs:simpleType name='empty'>
       <xs:restriction base='xs:string'>
         <xs:enumeration value=''/>
       </xs:restriction>
     </xs:simpleType>

   </xs:schema>

C.3.  STARTTLS Namespace

   <?xml version='1.0' encoding='UTF-8'?>

   <xs:schema
       xmlns:xs='http://www.w3.org/2001/XMLSchema'
       targetNamespace='urn:ietf:params:xml:ns:xmpp-tls'
       xmlns='urn:ietf:params:xml:ns:xmpp-tls'
       elementFormDefault='qualified'>

     <xs:element name='starttls'>
       <xs:complexType>
         <xs:choice minOccurs='0' maxOccurs='1'>
           <xs:element name='required' type='empty'/>
         </xs:choice>
       </xs:complexType>
     </xs:element>

     <xs:element name='proceed' type='empty'/>

     <xs:element name='failure' type='empty'/>

     <xs:simpleType name='empty'>
       <xs:restriction base='xs:string'>
         <xs:enumeration value=''/>
       </xs:restriction>
     </xs:simpleType>

   </xs:schema>

C.4.  SASL Namespace

   <?xml version='1.0' encoding='UTF-8'?>

   <xs:schema



Saint-Andre              Expires April 26, 2010               [Page 170]

Internet-Draft                  XMPP Core                   October 2009


       xmlns:xs='http://www.w3.org/2001/XMLSchema'
       targetNamespace='urn:ietf:params:xml:ns:xmpp-sasl'
       xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
       elementFormDefault='qualified'>

     <xs:element name='mechanisms'>
       <xs:complexType>
         <xs:sequence>
           <xs:element name='mechanism'
                       minOccurs='1'
                       maxOccurs='unbounded'
                       type='xs:NMTOKEN'/>
           <xs:any namespace='##other'
                   minOccurs='0'
                   minOccurs='unbounded'/>
         </xs:sequence>
       </xs:complexType>
     </xs:element>

     <xs:element name='abort' type='empty'/>

     <xs:element name='auth'>
       <xs:complexType>
         <xs:simpleContent>
           <xs:extension base='xs:string'>
             <xs:attribute name='mechanism'
                           type='xs:NMTOKEN'
                           use='required'/>
           </xs:extension>
         </xs:simpleContent>
       </xs:complexType>
     </xs:element>

     <xs:element name='challenge' type='xs:string'/>

     <xs:element name='response' type='xs:string'/>

     <xs:element name='success' type='xs:string'/>

     <xs:element name='failure'>
       <xs:complexType>
         <xs:sequence>
           <xs:choice minOccurs='0'>
             <xs:element name='aborted' type='empty'/>
             <xs:element name='account-disabled' type='empty'/>
             <xs:element name='credentials-expired' type='empty'/>
             <xs:element name='encryption-required' type='empty'/>
             <xs:element name='incorrect-encoding' type='empty'/>



Saint-Andre              Expires April 26, 2010               [Page 171]

Internet-Draft                  XMPP Core                   October 2009


             <xs:element name='invalid-authzid' type='empty'/>
             <xs:element name='invalid-mechanism' type='empty'/>
             <xs:element name='malformed-request' type='empty'/>
             <xs:element name='mechanism-too-weak' type='empty'/>
             <xs:element name='not-authorized' type='empty'/>
             <xs:element name='temporary-auth-failure' type='empty'/>
             <xs:element name='transition-needed' type='empty'/>
           </xs:choice>
           <xs:element ref='text' minOccurs='0' maxOccurs='1'/>
         </xs:sequence>
       </xs:complexType>
     </xs:element>

     <xs:element name='text'>
       <xs:complexType>
         <xs:simpleContent>
           <xs:extension base='xs:string'>
             <xs:attribute ref='xml:lang' use='optional'/>
           </xs:extension>
         </xs:simpleContent>
       </xs:complexType>
     </xs:element>

     <xs:simpleType name='empty'>
       <xs:restriction base='xs:string'>
         <xs:enumeration value=''/>
       </xs:restriction>
     </xs:simpleType>

   </xs:schema>





















Saint-Andre              Expires April 26, 2010               [Page 172]

Internet-Draft                  XMPP Core                   October 2009


C.5.  Resource Binding Namespace

   <?xml version='1.0' encoding='UTF-8'?>

   <xs:schema
       xmlns:xs='http://www.w3.org/2001/XMLSchema'
       targetNamespace='urn:ietf:params:xml:ns:xmpp-bind'
       xmlns='urn:ietf:params:xml:ns:xmpp-bind'
       elementFormDefault='qualified'>

     <xs:element name='bind'>
       <xs:complexType>
         <xs:choice>
           <xs:choice>
             <xs:element name='resource' type='resourceType'/>
             <xs:element name='jid' type='fullJIDType'/>
           </xs:choice>
           <xs:any namespace='##other'
                   minOccurs='0'
                   minOccurs='unbounded'/>
         </xs:choice>
       </xs:complexType>
     </xs:element>

     <xs:simpleType name='fullJIDType'>
       <xs:restriction base='xs:string'>
         <xs:minLength value='8'/>
         <xs:maxLength value='3071'/>
       </xs:restriction>
     </xs:simpleType>

     <xs:simpleType name='resourceType'>
       <xs:restriction base='xs:string'>
         <xs:minLength value='1'/>
         <xs:maxLength value='1023'/>
       </xs:restriction>
     </xs:simpleType>

   </xs:schema>

C.6.  Stanza Error Namespace

   <?xml version='1.0' encoding='UTF-8'?>

   <xs:schema
       xmlns:xs='http://www.w3.org/2001/XMLSchema'
       targetNamespace='urn:ietf:params:xml:ns:xmpp-stanzas'
       xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'



Saint-Andre              Expires April 26, 2010               [Page 173]

Internet-Draft                  XMPP Core                   October 2009


       elementFormDefault='qualified'>

     <xs:element name='bad-request' type='empty'/>
     <xs:element name='conflict' type='empty'/>
     <xs:element name='feature-not-implemented' type='empty'/>
     <xs:element name='forbidden' type='empty'/>
     <xs:element name='gone' type='xs:string'/>
     <xs:element name='internal-server-error' type='empty'/>
     <xs:element name='item-not-found' type='empty'/>
     <xs:element name='jid-malformed' type='empty'/>
     <xs:element name='not-acceptable' type='empty'/>
     <xs:element name='not-allowed' type='empty'/>
     <xs:element name='not-authorized' type='empty'/>
     <xs:element name='not-modified' type='empty'/>
     <xs:element name='payment-required' type='empty'/>
     <xs:element name='policy-violation' type='empty'/>
     <xs:element name='recipient-unavailable' type='empty'/>
     <xs:element name='redirect' type='xs:string'/>
     <xs:element name='registration-required' type='empty'/>
     <xs:element name='remote-server-not-found' type='empty'/>
     <xs:element name='remote-server-timeout' type='empty'/>
     <xs:element name='resource-constraint' type='empty'/>
     <xs:element name='service-unavailable' type='empty'/>
     <xs:element name='subscription-required' type='empty'/>
     <xs:element name='undefined-condition' type='empty'/>
     <xs:element name='unexpected-request' type='empty'/>
     <xs:element name='unknown-sender' type='empty'/>

     <xs:group name='stanzaErrorGroup'>
       <xs:choice>
         <xs:element ref='bad-request'/>
         <xs:element ref='conflict'/>
         <xs:element ref='feature-not-implemented'/>
         <xs:element ref='forbidden'/>
         <xs:element ref='gone'/>
         <xs:element ref='internal-server-error'/>
         <xs:element ref='item-not-found'/>
         <xs:element ref='jid-malformed'/>
         <xs:element ref='not-acceptable'/>
         <xs:element ref='not-authorized'/>
         <xs:element ref='not-allowed'/>
         <xs:element ref='not-modified'/>
         <xs:element ref='payment-required'/>
         <xs:element ref='policy-violation'/>
         <xs:element ref='recipient-unavailable'/>
         <xs:element ref='redirect'/>
         <xs:element ref='registration-required'/>
         <xs:element ref='remote-server-not-found'/>



Saint-Andre              Expires April 26, 2010               [Page 174]

Internet-Draft                  XMPP Core                   October 2009


         <xs:element ref='remote-server-timeout'/>
         <xs:element ref='resource-constraint'/>
         <xs:element ref='service-unavailable'/>
         <xs:element ref='subscription-required'/>
         <xs:element ref='undefined-condition'/>
         <xs:element ref='unexpected-request'/>
         <xs:element ref='unknown-sender'/>
       </xs:choice>
     </xs:group>

     <xs:element name='text'>
       <xs:complexType>
         <xs:simpleContent>
           <xs:extension base='xs:string'>
             <xs:attribute ref='xml:lang' use='optional'/>
           </xs:extension>
         </xs:simpleContent>
       </xs:complexType>
     </xs:element>

     <xs:simpleType name='empty'>
       <xs:restriction base='xs:string'>
         <xs:enumeration value=''/>
       </xs:restriction>
     </xs:simpleType>

   </xs:schema>


Appendix D.  Contact Addresses

   Consistent with [MAILBOXES], an organization that offers an XMPP
   service SHOULD provide an Internet mailbox of "XMPP" for inquiries
   related to that service, where the host portion of the resulting
   mailto URI MUST be the organization's domain, not the domain of the
   XMPP service itself (e.g., the XMPP service might be offered at
   im.example.com but the Internet mailbox would be <xmpp@example.com>).


Appendix E.  Account Provisioning

   Account provisioning is out of scope for this specification.
   Possible methods for account provisioning include account creation by
   a server administrator and in-band account registration using the
   'jabber:iq:register' namespace as documented in [XEP-0077].






Saint-Andre              Expires April 26, 2010               [Page 175]

Internet-Draft                  XMPP Core                   October 2009


Appendix F.  Differences From RFC 3920

   Based on consensus derived from implementation and deployment
   experience as well as formal interoperability testing, the following
   substantive modifications were made from RFC 3920.

   o  Corrected the ABNF syntax for JIDs to prevent zero-length
      localparts, domain identifiers, and resource identifiers.
   o  To avoid confusion with the term "node" as used in [XEP-0030] and
      [XEP-0060] (see also [XEP-0271]), changed the term "node
      identifier" to "localpart" (but retained the name "Nodeprep" for
      backward compatibility).
   o  Corrected the nameprep processing rules to require use of the
      UseSTD3ASCIIRules flag.
   o  Recommended or mandated use of the 'from' and 'to' attributes on
      stream headers.
   o  More fully specified stream closing handshake.
   o  Specified recommended stream reconnection algorithm.
   o  Specified return of <restricted-xml/> stream error in response to
      receipt of prohibited XML features.
   o  Specified that the SASL SCRAM mechanism is a mandatory-to-
      implement technology.
   o  Specified that TLS plus the SASL PLAIN mechanism is a mandatory-
      to-implement technology for client-to-server connections.
   o  Specified that support for the SASL EXTERNAL mechanism is required
      for servers but only recommended for clients (since end-user X.509
      certificates are difficult to obtain and not yet widely deployed).
   o  More clearly specified the certificate profile for both public key
      certificates and attribute certificates.
   o  Added the <account-disabled/>, <credentials-expired/>,
      <encryption-required/>, <malformed-request/>, and <transition-
      needed/> SASL error conditions to handle error flows mistakenly
      left out of RFC 3920 or discussed in RFC 4422 but not in RFC 2222.
   o  Added the <not-modified/> stanza error condition to enable
      potential ETags usage.
   o  Removed unnecessary requirement for escaping of characters that
      map to certain predefined entities, which do not need to be
      escaped in XML.
   o  Clarified the process of DNS SRV lookups and fallbacks.
   o  Clarified the handling of SASL security layers.
   o  Clarified the handling of stream features, regularized use of the
      <required/> child element, and defined use of the <optional/>
      child element.
   o  Clarified the handling of data that violates the well-formedness
      definitions for XML 1.0 and XML namespaces.
   o  Specified the security considerations in more detail, especially
      with regard to presence leaks and denial of service attacks.




Saint-Andre              Expires April 26, 2010               [Page 176]

Internet-Draft                  XMPP Core                   October 2009


   o  Moved historical documentation of the server dialback protocol
      from this specification to a separate specification maintained by
      the XMPP Standards Foundation.

   In addition, numerous changes of an editorial nature were made in
   order to more fully specify and clearly explain XMPP.


Appendix G.  Copying Conditions

   Regarding this entire document or any portion of it, the author makes
   no guarantees and is not responsible for any damage resulting from
   its use.  The author grants irrevocable permission to anyone to use,
   modify, and distribute it in any way that does not diminish the
   rights of anyone else to use, modify, and distribute it, provided
   that redistributed derivative works do not contain misleading author
   or version information.  Derivative works need not be licensed under
   similar terms.


Index

   B
      Bare JID  18

   C
      Connected Resource  81

   D
      Domain Identifier  16

   E
      Entity  15
      Error Stanza  93
      Extended Content  109

   F
      Full JID  18

   I
      Initial Stream  23
      IQ Stanza  92

   J
      Jabber Identifier  15

   L
      Localpart  18



Saint-Andre              Expires April 26, 2010               [Page 177]

Internet-Draft                  XMPP Core                   October 2009


   M
      Message Stanza  91

   P
      Presence Stanza  91

   R
      Resource Identifier  18
      Response Stream  23

   S
      Stream ID  35

   W
      Whitespace Keepalive  32

   X
      XML Stanza  24
      XML Stream  23


Author's Address

   Peter Saint-Andre
   Cisco

   Email: Peter.SaintAndre@WebEx.com
























Saint-Andre              Expires April 26, 2010               [Page 178]


Html markup produced by rfcmarkup 1.109, available from https://tools.ietf.org/tools/rfcmarkup/