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Versions: 00 01 02 03

Network Working Group                                          M.T. Rose
Internet-Draft                                    Invisible Worlds, Inc.
Expires: August 26, 2001                                        G. Klyne
                                            Content Technologies Limited
                                                            D.H. Crocker
                                                  Brandenburg Consulting
                                                       February 25, 2001


                     The Application Exchange Core
                        draft-mrose-apex-core-03

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   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 August 26, 2001.

Copyright Notice

   Copyright (C) The Internet Society (2001). All Rights Reserved.

Abstract

   This memo describes APEX, an extensible, asynchronous message
   relaying service for application layer programs.










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Table of Contents

   1.      Introduction . . . . . . . . . . . . . . . . . . . . . . .  4
   1.1     Overview . . . . . . . . . . . . . . . . . . . . . . . . .  5
   1.2     Architecture at a Glance . . . . . . . . . . . . . . . . .  6
   2.      Service Principles . . . . . . . . . . . . . . . . . . . .  8
   2.1     Modes of Operation . . . . . . . . . . . . . . . . . . . .  8
   2.2     Naming of Entities . . . . . . . . . . . . . . . . . . . .  9
   3.      Service Provisioning . . . . . . . . . . . . . . . . . . . 10
   3.1     Connection Establishment . . . . . . . . . . . . . . . . . 10
   3.2     Authentication . . . . . . . . . . . . . . . . . . . . . . 10
   3.3     Authorization  . . . . . . . . . . . . . . . . . . . . . . 10
   3.4     Confidentiality  . . . . . . . . . . . . . . . . . . . . . 10
   3.5     Relaying Integrity . . . . . . . . . . . . . . . . . . . . 11
   3.6     Traffic Analysis . . . . . . . . . . . . . . . . . . . . . 11
   4.      The APEX . . . . . . . . . . . . . . . . . . . . . . . . . 12
   4.1     Use of XML and MIME  . . . . . . . . . . . . . . . . . . . 12
   4.2     Profile Identification and Initialization  . . . . . . . . 14
   4.3     Message Syntax . . . . . . . . . . . . . . . . . . . . . . 14
   4.4     Message Semantics  . . . . . . . . . . . . . . . . . . . . 15
   4.4.1   The Attach Operation . . . . . . . . . . . . . . . . . . . 15
   4.4.2   The Bind Operation . . . . . . . . . . . . . . . . . . . . 17
   4.4.3   The Terminate Operation  . . . . . . . . . . . . . . . . . 19
   4.4.4   The Data Operation . . . . . . . . . . . . . . . . . . . . 20
   4.4.4.1 Relay Processing of Data . . . . . . . . . . . . . . . . . 22
   4.4.4.2 Application Processing of Data . . . . . . . . . . . . . . 23
   4.5     APEX Access Policies . . . . . . . . . . . . . . . . . . . 24
   4.5.1   Access Policies in the Endpoint-Relay Mode . . . . . . . . 25
   4.5.2   Access Policies in the Relay-Relay Mode  . . . . . . . . . 26
   5.      APEX Options . . . . . . . . . . . . . . . . . . . . . . . 27
   5.1     The statusRequest Option . . . . . . . . . . . . . . . . . 29
   6.      APEX Services  . . . . . . . . . . . . . . . . . . . . . . 34
   6.1     Use of the APEX Core DTD . . . . . . . . . . . . . . . . . 35
   6.1.1   Transaction-Identifiers  . . . . . . . . . . . . . . . . . 35
   6.1.2   The Reply Operation  . . . . . . . . . . . . . . . . . . . 36
   6.2     The Report Service . . . . . . . . . . . . . . . . . . . . 37
   7.      Registration Templates . . . . . . . . . . . . . . . . . . 38
   7.1     APEX Option Registration Template  . . . . . . . . . . . . 38
   7.2     APEX Service Registration Template . . . . . . . . . . . . 38
   8.      Initial Registrations  . . . . . . . . . . . . . . . . . . 39
   8.1     Registration: The APEX Profile . . . . . . . . . . . . . . 39
   8.2     Registration: The APEX Service-Selector for GSTN . . . . . 39
   8.3     Registration: The statusRequest Option . . . . . . . . . . 40
   8.4     Registration: The Report Service . . . . . . . . . . . . . 40
   9.      DTDs . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
   9.1     The APEX Core DTD  . . . . . . . . . . . . . . . . . . . . 41
   9.2     The Report Service DTD . . . . . . . . . . . . . . . . . . 44
   10.     Reply Codes  . . . . . . . . . . . . . . . . . . . . . . . 45



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   11.     Security Considerations  . . . . . . . . . . . . . . . . . 46
           References . . . . . . . . . . . . . . . . . . . . . . . . 47
           Authors' Addresses . . . . . . . . . . . . . . . . . . . . 47
   A.      Acknowledgements . . . . . . . . . . . . . . . . . . . . . 49
   B.      IANA Considerations  . . . . . . . . . . . . . . . . . . . 50
   C.      Changes from IMXP  . . . . . . . . . . . . . . . . . . . . 51
           Full Copyright Statement . . . . . . . . . . . . . . . . . 52












































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1. Introduction

   Network applications can be broadly distinguished by five operational
   characteristics:

   o  server push or client pull;

   o  synchronous (interactive) or asynchronous (batch);

   o  time-assured or time-insensitive;

   o  best-effort or reliable; and,

   o  stateful or stateless.

   For example:

   o  the world-wide web is a pull, synchronous, time-insensitive,
      reliable, stateless service; whilst

   o  Internet mail is a push, asynchronous, time-insensitive, best-
      effort (without DSN), stateless service.

   Messaging applications vary considerably in their operational
   requirements. For example, some messaging applications require
   assurance of timeliness and reliability, whilst others do not.

   These features come at a cost, in terms of both infrastructural and
   configuration complexity. Accordingly, the underlying service must be
   extensible to support different requirements in a consistent manner.

   This memo defines a core messaging service that supports a range of
   operational characteristics. The core service supports a variety of
   tailored services for both user-based and programmatic exchanges.

















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1.1 Overview

   APEX provides an extensible, asynchronous message relaying service
   for application layer programs.

   APEX, at its core, provides a best-effort datagram service. Each
   datagram, simply termed "data", is originated and received by APEX
   "endpoints" -- applications that dynamically attach to the APEX
   "relaying mesh".

   The data transmitted specifies:

   o  an originating endpoint;

   o  an opaque content (via a URI-reference);

   o  one or more recipient endpoints; and,

   o  zero or more options.

   Options are used to alter the semantics of the the service, may occur
   on a per-recipient or per-data basis, and may be processed by either
   a single or multiple relays.

   Additional APEX services are provided on top of the relaying mesh;
   e.g., access control and presence information.

   APEX is specified, in part, as a BEEP[1] "profile". Accordingly, many
   aspects of APEX (e.g., authentication) are provided within the BEEP
   core. Throughout this memo, the terms "peer", "initiator",
   "listener", "client", and "server" are used in the context of BEEP.
   In particular, Section 2.1 of the BEEP core memo discusses the roles
   that a BEEP peer may perform.

   When reading this memo, note that the terms "endpoint" and "relay"
   are specific to APEX, they do not exist in the context of BEEP.















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1.2 Architecture at a Glance

   The APEX stack:

      +-------------+
      | APEX        |        an APEX process is either:
      |     process |
      +-------------+            - an application attached as an APEX
      |             |              endpoint; or,
      |    APEX     |
      |             |            - an APEX relay
      +-------------+
      |             |        APEX services are realized as applications
      |    BEEP     |        having a special relationship with the APEX
      |             |        relays in their administrative domain
      +-------------+
      |   TCP/IP    |
      +-------------+
      |     ...     |
      +-------------+































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   The APEX entities:

          administrative domain #1          administrative domain #2
       +----------------------------+    +----------------------------+
       |   +------+                 |    |                 +------+   |
       |   |      |                 |    |                 |      |   |
       |   | appl |                 |    |                 | appl |   |
       |   |      |                 |    |                 |      |   |
       |   +......+       +------+  |    |  +------+       +......+   |
       |   |      |       |      |  |    |  |      |       |      |   |
       |   |end-  |       |relay |  |    |  |relay |       |end-  |   |
       |   | point|       |      |  |    |  |      |       | point|   |
       |   +------+       +------+  |    |  +------+       +------+   |
       |   |      |       |      |  |    |  |      |       |      |   |
       |   | APEX |       | APEX |  |    |  | APEX |       | APEX |   |
       |   |      |       |      |  |    |  |      |       |      |   |
       |   +------+       +------+  |    |  +------+       +------+   |
       |        ||         ||  ||   |    |   ||  ||         ||        |
       |        =============  ================  =============        |
       +----------------------------+    +----------------------------+

                      | <---- APEX relaying mesh ----> |



           Note: relaying between administrative domains is configured
                 using SRV RRs.  Accordingly, the actual number of
                 relays between two endpoints is not fixed.























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2. Service Principles

2.1 Modes of Operation

   APEX is used in two modes:

   endpoint-relay: in which the endpoint is always the BEEP initiator of
      the service, whilst relays are always the BEEP listeners. In this
      context, applications attach as endpoints, and then the
      transmission of data occurs.

   relay-relay: in which relays typically, though not necessarily,
      reside in different administrative domains. In this context,
      applications bind as relays, and then the transmission of data
      occurs.

   In the endpoint-relay mode, an endpoint (BEEP initiator) may:

   o  attach as one or more endpoints;

   o  send data to other endpoints;

   o  receive data from other endpoints; and,

   o  terminate any of its attachments.

   A relay (BEEP listener), in addition to servicing requests from a
   BEEP initiator, may:

   o  terminate any of the endpoint's attachments;

   o  deliver data from other endpoints; and,

   o  indicate the delivery status of data sent earlier by the endpoint.

   In the relay-relay mode, a relay (BEEP listener or initiator) may:

   o  bind as one or more administrative domains;

   o  send data;

   o  receive data; and,

   o  terminate any bindings.







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2.2 Naming of Entities

   Endpoints are named using the "addr-spec" syntax specified in Section
   3.4.1 of [2], i.e., the familiar "local@domain" syntax.

   Using the service-selector convention of RFC 2846[3], all endpoint
   identities having a local-part starting with "apex=" are reserved for
   use by APEX services registered with the IANA.

   Relays, although not named, serve of behalf of administrative
   domains, as identified by a FQDN, e.g., "example.com".

   In APEX, "endpoints" and "relays" are the fundamental entities. APEX
   is carried over BEEP, which has the "peer" as its fundamental entity.
   The relationship between BEEP peer entities and APEX endpoint and
   relay entities are defined by APEX's Access Policies (Section 4.5).



































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3. Service Provisioning

3.1 Connection Establishment

   The SRV algorithm[4] is used to determine the IP/TCP addressing
   information assigned to the relays for an administrative domain:

   service: "apex-edge" (for the endpoint-relay mode), or "apex-mesh"
      (for the relay-relay mode);

   protocol: "tcp"; and,

   domain: the administrative domain.


3.2 Authentication

   Authentication is a matter of provisioning for each BEEP peer (c.f.,
   Section 4.5).

   An APEX relay might be provisioned to allow a BEEP peer identity to
   coincide with a given endpoint identity. For example, a relay in the
   "example.com" administrative domain may be configured to allow a BEEP
   peer identified as "fred@example.com" to be authorized to attach as
   the APEX endpoint "fred@example.com".

3.3 Authorization

   Authorization is a matter of provisioning for each BEEP peer (c.f.,
   Section 4.5).

   Typically, a relay requires that its BEEP peer authenticate as a
   prelude to authorization, but an endpoint usually does not require
   the same of its BEEP peer.

3.4 Confidentiality

   Confidentiality is a matter of provisioning for each BEEP peer.

   Typically, any data considered sensitive by an originating endpoint
   will have its content encrypted for the intended recipient
   endpoint(s), rather than relying on hop-by-hop encryption. Similarly,
   an originating endpoint will sign the content if end-to-end
   authentication is desired.







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3.5 Relaying Integrity

   Data are relayed according to SRV entries in the DNS. Accordingly,
   relaying integrity is a function of the DNS and the applications
   making use of the DNS. Additional assurance is provided if the BEEP
   initiator requires that the BEEP listener authenticate itself.

3.6 Traffic Analysis

   Hop-by-hop protection of data transmitted through the relaying mesh
   (endpoint identities and content) is afforded at the BEEP level
   through the use of a transport security profile. Other traffic
   characteristics, e.g., volume and timing of transmissions, is not
   protected from third-party analysis.





































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4. The APEX

   Section 8.1 contains the BEEP profile registration for APEX.

4.1 Use of XML and MIME

   Each BEEP payload exchanged via APEX consists of an XML document and
   possibly an arbitrary MIME content.

   If only an XML document is sent in the BEEP payload, then the mapping
   to a BEEP payload is straight-forward, e.g.,

       C: MSG 1 2 . 111 39
       C: Content-Type: application/beep+xml
       C:
       C: <terminate transID='1' />
       C: END

   Otherwise, if an arbitrary MIME content is present, it is indicated
   by a URI-reference[5] in the XML control document. The URI-reference
   may contain an absolute-URI (and possibly a fragment-identifier), or
   it may be a relative-URI consisting only of a fragment-identifier.
   Arbitrary MIME content is included in the BEEP payload by using a
   "multipart/related"[6], identified using a "cid" URL[7], and the XML
   control document occurs as the start of the "multipart/related",
   e.g.,

       C: MSG 1 1 . 42 1234
       C: Content-Type: multipart/related; boundary="boundary";
       C:               start="<1@example.com>";
       C:               type="application/beep+xml"
       C:
       C: --boundary
       C: Content-Type: application/beep+xml
       C: Content-ID: <1@example.com>
       C:
       C: <data content='cid:2@example.com'>
       C:     <originator identity='fred@example.com' />
       C:     <recipient identity='barney@example.com' />
       C: </data>
       C: --boundary
       C: Content-Type: image/gif
       C: Content-Transfer-Encoding: binary
       C: Content-ID: <2@example.com>
       C:
       C: ...
       C: --boundary--
       C: END



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   Because BEEP provides an 8bit-wide path, a "transformative" Content-
   Transfer-Encoding (e.g., "base64" or "quoted-printable") should not
   be used. Further, note that MIME[8] requires that the value of the
   "Content-ID" header be globally unique.

   If the arbitrary MIME content is itself an XML document, it may be
   contained with the control document directly, and identified using a
   URI-reference consisting of only a fragment-identifier, e.g.,

       C: MSG 1 1 . 42 295
       C: Content-Type: application/beep+xml
       C:
       C: <data content='#Content'>
       C:     <originator identity='fred@example.com' />
       C:     <recipient identity='barney@example.com' />
       C:     <data-content Name='Content'>
       C:         <statusResponse transID='86'>
       C:             <destination identity='barney@example.com'>
       C:                 <reply code='250' />
       C:             </destination>
       C:         <statusResponse>
       C:     </data-content>
       C: </data>
       C: END



























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4.2 Profile Identification and Initialization

   The APEX is identified as

       http://xml.resource.org/profiles/APEX

   in the BEEP "profile" element during channel creation.

   No elements are required to be exchanged during channel creation;
   however, in the endpoint-relay mode, the BEEP initiator will
   typically include an "attach" element during channel creation, e.g.,

       <start number='1'>
           <profile uri='http://xml.resource.org/profiles/APEX'>
               <![CDATA[<attach endpoint='fred@example.com'
                                transID='1' />]]>
           </profile>
       </start>

   Similarly, in the relay-relay mode, the BEEP initiator will typically
   include an "bind" element during channel creation, e.g.,

       <start number='1'>
           <profile uri='http://xml.resource.org/profiles/APEX'>
               <![CDATA[<bind relay='example.com'
                              transID='1' />]]>
           </profile>
       </start>


4.3 Message Syntax

   Section 9.1 defines the BEEP payloads that are used in the APEX.


















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4.4 Message Semantics

4.4.1 The Attach Operation

   When an application wants to attach to the relaying mesh as a given
   endpoint, it sends an "attach" element to a relay, e.g.,

       +-------+                  +-------+
       |       | -- attach -----> |       |
       | appl. |                  | relay |
       |       | <--------- ok -- |       |
       +-------+                  +-------+

     C: <attach endpoint='fred@example.com' transID='1' />
     S: <ok />

   or

       +-------+                  +-------+
       |       | -- attach -----> |       |
       |       |                  |       |
       |       | <--------- ok -- |       |
       | appl. |                  | relay |
       |       | -- attach -----> |       |
       |       |                  |       |
       |       | <--------- ok -- |       |
       +-------+                  +-------+

     C: <attach endpoint='fred@example.com' transID='1' />
     S: <ok />
     C: <attach endpoint='wilma@example.com' transID='2' />
     S: <ok />

   or

       +-------+                  +-------+
       |       | -- attach -----> |       |
       | appl. |                  | relay |
       |       | <------ error -- |       |
       +-------+                  +-------+

     C: <attach endpoint='fred@example.com' transID='1' />
     S: <error code='537'>access denied</error>








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   The "attach" element has an "endpoint" attribute, a "transID"
   attribute, and contains zero or more "option" elements:

   o  the "endpoint" attribute specifies the endpoint that the
      application wants to attach as;

   o  the "transID" attribute specifies the transaction-identifier
      associated with this operation; and,

   o  the "option" elements, if any, specify additional processing
      options (Section 5).

   When a relay receives an "attach" element, it performs these steps:

   1.  If the transaction-identifier refers to a previous, non-
       terminated operation on this BEEP channel, an "error" element
       having code 555 is returned.

   2.  If the relay is in a different administrative domain than this
       endpoint, an "error" element having code 553 is returned.

   3.  If the application is not authorized to attach as this endpoint
       (c.f., Section 4.5.1), an "error" element having code 537 is
       returned.

   4.  If any options are present, they are processed.

   5.  If another application has already attached as this endpoint, an
       "error" element having code 554 is returned.

   6.  Otherwise, the application is bound as this endpoint, and an "ok"
       element is returned.



















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4.4.2 The Bind Operation

   When an application wants to identify itself as a relay, it sends a
   "bind" element to another relay, e.g.,

       +-------+                  +-------+
       |       | -- bind -------> |       |
       | relay |                  | relay |
       |   #1  | <--------- ok -- |   #2  |
       +-------+                  +-------+

     C: <bind relay='example.com' transID='1' />
     S: <ok />

   or

       +-------+                  +-------+
       |       | -- bind -------> |       |
       |       |                  |       |
       |       | <--------- ok -- |       |
       | relay |                  | relay |
       |   #1  | -- bind -------> |   #2  |
       |       |                  |       |
       |       | <--------- ok -- |       |
       +-------+                  +-------+

     C: <bind relay='example.com' transID='1' />
     S: <ok />
     C: <bind relay='rubble.com' transID='1' />
     S: <ok />

   or

       +-------+                  +-------+
       |       | -- bind -------> |       |
       | relay |                  | relay |
       |   #1  | <------ error -- |   #2  |
       +-------+                  +-------+

     C: <bind relay='example.com' transID='1' />
     S: <error code='537'>access denied</error>










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   The "bind" element has a "relay" attribute, a "transID" attribute,
   and contains zero or more "option" elements:

   o  the "relay" attribute specifies the administrative domain on whose
      behalf the application wants to serve;

   o  the "transID" attribute specifies the transaction-identifier
      associated with this operation; and,

   o  the "option" elements, if any, specify additional processing
      options (Section 5).

   When a relay receives an "bind" element, it performs these steps:

   1.  If the transaction-identifier refers to a previous, non-
       terminated operation on this BEEP channel, an "error" element
       having code 555 is returned.

   2.  The relay performs the SRV algorithm[4] for the desired
       administrative domain (i.e., using a service of "apex-mesh" and a
       protocol of "tcp"). For each domain name returned by the
       algorithm, the corresponding IP address(es) are retrieved using
       the DNS. The relay compares the application's IP address and TCP
       port number to the corresponding IP addresses and TCP port
       numbers found using the SRV algorithm. If none match, an "error"
       element having code 537 is returned.

   3.  If the application is not authorized to bind on behalf of this
       administrative domain (c.f., Section 4.5.2), an "error" element
       having code 537 is returned.

   4.  If any options are present, they are processed.

   5.  Otherwise, the application is accepted as serving this
       administrative domain, and an "ok" element is returned.
















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4.4.3 The Terminate Operation

   When an application or relay wants to release an attachment or
   binding, it sends a "terminate" element, e.g.,

       +-------+                  +-------+
       |       | -- terminate --> |       |
       | appl. |                  | relay |
       |       | <--------- ok -- |       |
       +-------+                  +-------+

     C: <terminate transID='1' />
     S: <ok />

   or

       +-------+                  +-------+
       |       | -- terminate --> |       |
       | appl. |                  | relay |
       |       | <------ error -- |       |
       +-------+                  +-------+

     C: <terminate transID='13' />
     S: <error code='550'>unknown transaction-identifier</error>

   or

       +-------+                  +-------+
       |       | <-- terminate -- |       |
       | appl. |                  | relay |
       |       | -- ok ---------> |       |
       +-------+                  +-------+

     C: <terminate transID='1' />
     S: <ok />

   The "terminate" element has a "transID" attribute and no content.

   When an application or relay receives a "terminate" element, it
   performs these steps:

   1.  If the transaction-identifier does not refer to a previous
       unterminated operation on this BEEP channel, an "error" element
       having code 550 is returned.

   2.  Otherwise, the application is no longer bound as an endpoint or a
       relay, and an "ok" element is returned.




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4.4.4 The Data Operation

   When an application or relay wants to transmit data over the relaying
   mesh, it sends a "data" element, e.g.,

       +-------+                  +-------+
       |       | -- data -------> |       |
       | appl. |                  | relay |
       |   #1  | <--------- ok -- |       |
       +-------+                  +-------+

     C: <data content='cid:1@example.com'>
            <originator identity='fred@example.com' />
            <recipient identity='barney@example.com' />
        </data>
     S: <ok />

   or

       +-------+                  +-------+
       |       | -- data -------> |       |
       | appl. |                  | relay |
       |   #1  | <------ error -- |       |
       +-------+                  +-------+

     C: <data content='cid:1@example.com'>
            <originator identity='fred@example.com' />
            <recipient identity='barney@example.com' />
        </data>
     S: <error code='537'>access denied</error>

   or

                                  +-------+                  +-------+
                                  |       | -- data -------> |       |
                                  | relay |                  | appl. |
                                  |       | <--------- ok -- |   #2  |
                                  +-------+                  +-------+

     C: <data content='cid:1@example.com'>
            <originator identity='fred@example.com' />
            <recipient identity='barney@example.com' />
        </data>
     S: <ok />







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   The "data" element has a "content" attribute, and contains an
   "originator" element, one or more "recipient" elements, zero or more
   "option" elements, and, optionally, a "data-content" element:

   o  the "content" attribute is a URI-reference that specifies the
      contents of the data (c.f., Section 4.1);

   o  the "originator" element refers to the endpoint sending the data;

   o  each "recipient" element refers to an endpoint destination for the
      data;

   o  the "option" elements, if any, specify additional processing
      options (Section 5), termed per-data options; and,

   o  the "data-content" element, if present, specifies a nested XML
      entity using a URI fragment-identifier as the value of the
      "content" attribute.

   The "originator" element has an "identity" attribute, and contains
   zero or more option elements:

   o  the "identity" attribute specifies the sending endpoint; and

   o  the "option" elements, if any, specify additional processing
      options for the originator, termed per-originator options.

   Each "recipient" element has an "identity" attribute, and contains
   zero or more option elements:

   o  the "identity" attribute specifies the destination endpoint; and

   o  the "option" elements, if any, specify additional processing
      options for this recipient, termed per-recipient options.

















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4.4.4.1 Relay Processing of Data

   When a relay receives a "data" element, it performs these steps:

   1.  If the BEEP client is not authorized to originate or relay data
       on behalf of the "originator" endpoint (c.f., Section 4.5), an
       "error" element having code 537 is returned.

   2.  If any per-data options are present, they are processed.

   3.  An "ok" element is returned.

   4.  If any per-originator options are present, they are processed.

   5.  For each recipient:

       1.  If any per-recipient options are present, they are processed.

       2.  If the recipient endpoint is not in the administrative domain
           associated with the relay, then an APEX session is
           established to a relay that accepts data for the recipient's
           administrative domain, and a new "data" element, containing
           that "recipient" element and all applicable options, is sent
           to that relay.

           If no errors (e.g., an APEX session can not be established)
           occur during processing, and if the recipient's relay returns
           an "ok" element, then the recipient is considered to be
           successfully processed.

       3.  Otherwise, if the recipient endpoint is in the same
           administrative domain as the relay, the APEX access service
           must check that the originator endpoint is allowed to
           communicate with the recipient endpoint (the recipient's
           access entry[9] must contain a "core:data" token for the
           originator), and the recipient endpoint must be currently
           attached.

           If so, a new "data" element, containing only that "recipient"
           element, is sent to the corresponding application. If the
           recipient's endpoint returns an "ok" element, then the
           recipient is considered to be successfully processed.

   Note that an implementation may choose to optimize its behavior by
   grouping multiple recipients in a single "data" element that is
   subsequently transmitted. It may do so providing that the
   optimization retains these semantics and any other semantics related
   to per-data and per-recipient options.



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   Finally, note that a relay receiving a "data" element from an
   application may be configured to add domain-specific options.

4.4.4.2 Application Processing of Data

   When an application receives a "data" element, it performs these
   steps:

   1.  If any per-data or per-originator options are present, they are
       not processed (but may be noted).

   2.  For each recipient:

       1.  If any per-recipient options are present, they are not
           processed (but may be noted).

       2.  If the application is not attached as the recipient endpoint,
           then an error in processing has occurred.

       3.  Otherwise, the "data" element is further processed in an
           application-specific manner, and the recipient is considered
           to be successfully processed.

   3.  If no recipients could be successfully processed, an "error"
       element is returned; otherwise, an "ok" element is returned.


























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4.5 APEX Access Policies

   Access to APEX is provided by the juxtaposition of:

   o  authenticating as a BEEP peer;

   o  attaching as an APEX endpoint or binding as an APEX relay; and,

   o  being listed as an actor by the APEX access service (c.f., [9]).

   Each of these activities occurs according to the policies of the
   relevant administrative domain:

   o  each administrative domain is responsible for keeping its own
      house in order through "local provisioning"; and,

   o  each administrative domain decides the level of trust to associate
      with other administrative domains.

































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4.5.1 Access Policies in the Endpoint-Relay Mode

   o  When an application wants to attach to the relaying mesh, local
      provisioning maps BEEP peer identities to allowed APEX endpoints
      (c.f., Step 3 of Section 4.4.1).

      Typically, the identity function is used, e.g., if an application
      authenticates itself as the BEEP peer named as "fred@example.com",
      it is allowed to attach as the APEX endpoint named as
      "fred@example.com".

      Using the subaddress-specification convention of RFC 2846[3], an
      application authorized to attach as a given APEX endpoint is also
      authorized to attach as any sub-address of that APEX endpoint,
      e.g., an application authorized to attach as the APEX endpoint
      "fred@example.com" is also authorized to attach as the APEX
      endpoint "fred/appl=wb@example.com".

   o  When an application wants to send data, local provisioning maps
      attached endpoints to allowed originators (c.f., Step 1 of Section
      4.4.4.1).

      Typically, the identity function is used, e.g., if an application
      attaches as the APEX endpoint named as "fred@example.com", it is
      allowed to send data originating from the same APEX endpoint.
      However, other policies are permissible, for example, the
      administrative domain may allow the application attached as the
      APEX endpoint named as "wilma@example.com" to send data
      originating as either "wilma@example.com" or "fred@example.com".

   o  Finally, when a relay is delivering to an endpoint within its own
      administrative domain, it consults the recipient's access entry
      looking for an entry having the originator as an actor (c.f., Step
      5.3 of Section 4.4.4.1).

















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4.5.2 Access Policies in the Relay-Relay Mode

   o  When an application wants to bind as a relay on behalf of an
      administrative domain, in addition to Step 2 of Section 4.4.2,
      local provisioning may map BEEP peer identities to allowed APEX
      relays (c.f., Step 3).

      If so, then typically the identity function is used. e.g., if an
      application authenticates itself as the BEEP peer named as
      "example.com", it is allowed to bind as a relay on behalf of the
      administrative domain "example.com".

   o  When a relay is sending data, no access policies, per se, are
      applied.

   o  When a relay is receiving data, local provisioning maps BEEP peer
      identities to allowed originators (c.f., Step 1 of Section
      4.4.4.1).

      Typically, the identity function is used, e.g., if a relay
      authenticates itself as being from the same administrative domain
      as the originator of the data, then the data is accepted.

      In addition, some relays may also be configured as "trusted"
      intermediaries, so that if a BEEP peer authenticates itself as
      being from such a relay, then the data is accepted.

























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5. APEX Options

   APEX, at its core, provides a best-effort datagram service. Options
   are used to alter the semantics of the core service.

   The semantics of the APEX "option" element are context-specific.
   Accordingly, the specification of an APEX option must define:

   o  the identity of the option;

   o  the context in which the option may appear;

   o  what content, if any, is contained within the option; and,

   o  the processing rules for the option.

   An option registration template (Section 7.1) organizes this
   information.

   An "option" element is contained within either a "data",
   "originator", or "recipient" element, all of which are termed the
   "containing" element. The "option" element has several attributes and
   contains arbitrary content:

   o  the "internal" and the "external" attributes, exactly one of which
      is present, uniquely identify the option;

   o  the "targetHop" attribute specifies which relays should process
      the option;

   o  the "seeNoEvil" attribute specifies whether the option, if
      unrecognized, may be safely ignored;

   o  the "transID" attribute specifies a transaction-identifier for the
      option; and,

   o  the "localize" attribute, if present, specifies one or more
      language tokens, each identifying a desirable language tag to be
      used if textual diagnostics are returned to the originator.

   The value of the "internal" attribute is the IANA-registered name for
   the option. If the "internal" attribute is not present, then the
   value of the "external" attribute is a URI or URI with a fragment-
   identifier. Note that a relative-URI value is not allowed.







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   The "targetHop" attribute specifies which relay(s) should process the
   option:

      this: the option applies to this relay, and must be removed prior
      to transmitting the containing element.

      final: the option applies to this relay, only if the the relay is
      able to transmit the containing element directly to the recipient.

      all: the option applies to this relay and is retained for the
      next.

   Note that a final relay does not remove any options as it transmits
   the containing element directly to the recipient.

   The "seeNoEvil" attribute specifies whether the relay may ignore the
   option if it is unrecognized, and is consulted only if the
   "targetHop" attribute indicates that the option applies to that
   relay. If the option applies, and if the value of the "seeNoEvil"
   attribute is "false", and if the relay does not "understand" the
   option, then this is considered a processing error.






























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5.1 The statusRequest Option

   Section 8.3 contains the APEX option registration for the
   "statusRequest" option.

   If this option is present, then each applicable relay sends a
   "statusResponse" message to the originator. This is done by issuing a
   data operation whose originator is the report service associated with
   the issuing relay, whose recipient is the endpoint address of the
   "statusRequest" originator, and whose content is a "statusResponse"
   element.

   A "statusRequest" option MUST NOT be present in any data operation
   containing a "statusResponse" element.





































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   Consider these examples:

       +-------+                  +-------+
       |       | -- data -------> |       |
       | appl. |                  | relay |
       |   #1  | <--------- ok -- |       |
       +-------+                  +-------+

     C: <data content='cid:1@example.com'>
            <originator identity='fred@example.com' />
            <recipient identity='barney@example.com' />
            <option internal='statusRequest' targetHop='final'
                    seeNoEvil='false' transID='86' />
        </data>
     S: <ok />

                                  +-------+                  +-------+
                                  |       | -- data -------> |       |
                                  | relay |                  | appl. |
                                  |       | <--------- ok -- |   #2  |
                                  +-------+                  +-------+

     C: <data content='cid:1@example.com'>
            <originator identity='fred@example.com' />
            <recipient identity='barney@example.com' />
            <option internal='statusRequest' targetHop='final'
                    seeNoEvil='false' transID='86' />
        </data>
     S: <ok />

       +-------+                  +-------+
       |       | <------- data -- |       |
       | appl. |                  | relay |
       |   #1  | -- ok ---------> |       |
       +-------+                  +-------+

     C: <data content='#Content'>
            <originator identity='apex=report@example.com' />
            <recipient identity='fred@example.com' />
            <data-content Name='Content'>
                <statusResponse transID='86'>
                    <destination identity='barney@example.com'>
                        <reply code='250' />
                    </destination>
                </statusResponse>
            </data-content>
        </data>
     S: <ok />



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   or

       +-------+                  +-------+
       |       | -- data -------> |       |
       | appl. |                  | relay |
       |   #1  | <--------- ok -- |       |
       +-------+                  +-------+

     C: <data content='cid:1@example.com'>
            <originator identity='fred@example.com' />
            <recipient identity='barney@example.com' />
            <option internal='statusRequest' targetHop='final'
                    seeNoEvil='false' transID='86' />
        </data>
     S: <ok />

       +-------+                  +-------+
       |       | <------- data -- |       |
       | appl. |                  | relay |
       |   #1  | -- ok ---------> |       |
       +-------+                  +-------+

     C: <data content='#Content'>
            <originator identity='apex=report@example.com' />
            <recipient identity='fred@example.com' />
            <data-content Name='Content'>
                <statusResponse transID='86'>
                    <destination identity='barney@example.com'>
                        <reply code='550'>unknown endpoint
                                          identity</reply>
                    </destination>
                </statusResponse>
            </data-content>
        </data>
     S: <ok />
















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   or

       +-------+                  +-------+
       |       | -- data -------> |       |
       | appl. |                  | relay |
       |   #1  | <--------- ok -- |   #1  |
       +-------+                  +-------+

     C: <data content='cid:1@example.com'>
            <originator identity='fred@example.com' />
            <recipient identity='barney@rubble.com' />
            <option internal='statusRequest' targetHop='final'
                    seeNoEvil='false' transID='86' />
        </data>
     S: <ok />
                                  +-------+                  +-------+
                                  |       | -- data -------> |       |
                                  | relay |                  | relay |
                                  |   #1  | <--------- ok -- |   #2  |
                                  +-------+                  +-------+

     C: <data content='cid:1@example.com'>
            <originator identity='fred@example.com' />
            <recipient identity='barney@rubble.com' />
            <option internal='statusRequest' targetHop='final'
                    seeNoEvil='false' transID='86' />
        </data>
     S: <ok />

                                  +-------+                  +-------+
                                  |       | -- data -------> |       |
                                  | relay |                  | appl. |
                                  |   #2  | <--------- ok -- |   #2  |
                                  +-------+                  +-------+

     C: <data content='cid:1@example.com'>
            <originator identity='fred@example.com' />
            <recipient identity='barney@example.com' />
            <option internal='statusRequest' targetHop='final'
                    seeNoEvil='false' transID='86' />
        </data>
     S: <ok />









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                                  +-------+                  +-------+
                                  |       | <------- data -- |       |
                                  | relay |                  | relay |
                                  |   #1  | -- ok ---------> |   #2  |
                                  +-------+                  +-------+

     C: <data content='#Content'>
            <originator identity='apex=report@rubble.com' />
             <recipient identity='fred@example.com' />
             <data-content Name='Content'>
                 <statusResponse transID='86'>
                 <destination identity='barney@rubble.com'>
                     <reply code='250' />
                 </destination>
                 </statusResponse>
             </data-content>
         </data>
     S: <ok />

       +-------+                  +-------+
       |       | <------- data -- |       |
       | appl. |                  | relay |
       |   #1  | -- ok ---------> |   #1  |
       +-------+                  +-------+

     C: <data content='#Content'>
            <originator identity='apex=report@rubble.com' />
            <recipient identity='fred@example.com' />
            <data-content Name='Content'>
                <statusResponse transID='86'>
                    <destination identity='barney@rubble.com'>
                        <reply code='250' />
                    </destination>
                </statusResponse>
            </data-content>
        </data>
     S: <ok />

   Note that a trace of a data's passage through the relaying mesh can
   be achieved by setting the "targetHop" attribute to "all".











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6. APEX Services

   APEX, at its core, provides a best-effort datagram service. Within an
   administrative domain, all relays must be able to handle messages for
   any endpoint within that domain. APEX services are logically defined
   as endpoints but given their ubiquitous semantics they do not
   necessarily need to be associated with a single physical endpoint. As
   such, they may be provisioned co-resident with each relay within an
   administrative domain, even though they are logically provided on top
   of the relaying mesh, i.e.,

      +----------+     +----------+    +----------+    +---------+
      |   APEX   |     |   APEX   |    |   APEX   |    |         |
      |  access  |     | presence |    |  report  |    |   ...   |
      | service  |     |  service |    | service  |    |         |
      +----------+     +----------+    +----------+    +---------+
           |                |               |               |
           |                |               |               |
   +----------------------------------------------------------------+
   |                                                                |
   |                            APEX core                           |
   |                                                                |
   +----------------------------------------------------------------+

   That is, applications communicate with an APEX service by exchanging
   data with a "well-known endpoint" (WKE).

   For example, APEX applications communicate with the report service by
   exchanging data with the well-known endpoint "apex=report" in the
   corresponding administrative domain, e.g., "apex=report@example.com"
   is the endpoint associated with the report service in the
   "example.com" administrative domain.

   The specification of an APEX service must define:

   o  the WKE of the service;

   o  the syntax and sequence of messages exchanged with the service;

   o  what access control tokens are consulted by the service.

   A service registration template (Section 7.2) organizes this
   information.

   Finally, note that within a single administrative domain, the
   relaying mesh makes use of the APEX access service in order to
   determine if an originator is allowed to transmit data to a recipient
   (c.f., Step 5.3 of Section 4.4.4.1)



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6.1 Use of the APEX Core DTD

   The specification of an APEX service may use definitions found in the
   APEX core DTD (Section 9.1). For example, the reply operation
   (Section 6.1.2) is defined to provide a common format for responses.

6.1.1 Transaction-Identifiers

   In using APEX's transaction-identifiers, note the following:

   o  In the endpoint-relay and relay-relay modes, transaction-
      identifiers are meaningful only during the lifetime of a BEEP
      channel.

      For example, when an application issues the attach operation, the
      associated transaction-identifier has meaning only within the
      context of the BEEP channel used for the attach operation. When
      the BEEP connection is released, the channel no longer exists and
      the application is no longer attached to the relaying mesh.

   o  In contrast, when an application communicates with an APEX
      service, transaction-identifiers are often embedded in the data
      that is sent. This means that transaction-identifiers are
      potentially long-lived.

      For example, an application may attach as an endpoint, send data
      (containing an embedded transaction-identifier) to a service, and,
      some time later, detach from the relaying mesh. Later on, a second
      application may attach as the same endpoint, and send data of its
      own (also containing embedded transaction-identifiers).
      Subsequently, the second application may receive data from the
      service responding to the first application's request and
      containing the transaction-identifier used by the first
      application.

   To minimize the likelihood of ambiguities with long-lived
   transaction-identifiers, the values of transaction-identifiers
   generated by applications should appear to be unpredictable.













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6.1.2 The Reply Operation

   Many APEX services make use of a reply operation. Accordingly,
   Section 9.1 contains a definition of a "reply" element that can be
   used for this purpose.

   The "reply" element has a "code" attribute, a "transID" attribute, an
   optional "xml:lang" attribute, and may contain arbitrary textual
   content:

   o  the "code" element specifies a three-digit reply code (c.f.,
      Section 10);

   o  the "transID" attribute specifies the transaction-identifier
      corresponding to this reply;

   o  the "xml:lang" attribute, if present, specifies the language that
      the element's content is written in; and,

   o  the textual content is a diagnostic (possibly multiline) which is
      meaningful to implementers, perhaps administrators, and possibly
      even users.





























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6.2 The Report Service

   Section 8.4 contains the APEX service registration for the report
   service:

   o  Within an administrative domain, the service is addressed using
      the well-known endpoint of "apex=report".

   o  Section 9.2 defines the syntax of the operations exchanged with
      the service.

   o  A consumer of the service does not initiate communications with
      the service.

   o  The service initiates communications by sending data containing
      the "statusResponse" operation.

   If a relay processes a "statusRequest" option (Section 5.1), then it
   sends data to the originator containing a "statusResponse" element
   (Section 9.2).

   The "statusResponse" element has a "transID" attribute and contains
   one or more "destination" elements:

   o  the "transID" attribute specifies the value contained in the
      "statusRequest" option; and,

   o  each "destination" element has an "identity" attribute and
      contains a "reply" element:

      *  the "identity" attribute specifies the recipient endpoint that
         is being reported on; and,

      *  the "reply" element (Section 6.1.2) specifies the delivery
         status of that recipient.
















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7. Registration Templates

7.1 APEX Option Registration Template

   When an APEX option is registered, the following information is
   supplied:

   Option Identification: specify the NMTOKEN or the URI that
      authoritatively identifies this option.

   Present in: specify the APEX elements in which the option may appear.

   Contains: specify the XML content that is contained within the
      "option" element.

   Processing Rules: specify the processing rules associated with the
      option.

   Contact Information: specify the postal and electronic contact
      information for the author of the profile.


7.2 APEX Service Registration Template

   When an APEX service is registered, the following information is
   supplied:

   Well-Known Endpoint: specify the local-part of an endpoint identity,
      starting with "apex=".

   Syntax of Messages Exchanged: specify the elements exchanged with the
      service.

   Sequence of Messages Exchanged: specify the order in which data is
      exchanged with the service.

   Access Control Tokens: specify the token(s) used to control access to
      the service (c.f., [9]).

   Contact Information: specify the postal and electronic contact
      information for the author of the profile.










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8. Initial Registrations

8.1 Registration: The APEX Profile

   Profile Identification: http://xml.resource.org/profiles/APEX

   Messages exchanged during Channel Creation: "attach", "bind"

   Messages starting one-to-one exchanges: "attach", "bind",
      "terminate", or "data"

   Messages in positive replies: "ok"

   Messages in negative replies: "error"

   Messages in one-to-many exchanges: none

   Message Syntax: c.f., Section 9.1

   Message Semantics: c.f., Section 4.4

   Contact Information: c.f., the "Authors' Addresses" section of this
      memo


8.2 Registration: The APEX Service-Selector for GSTN

   Service-Selector Name: APEX

   Description of Use: Specifies endpoints for registered APEX services
      on the host indicated by the address' domain name, c.f., Section 6

   Security Considerations: The definition of a service-related endpoint
      does not introduce security concerns, per se; however, because the
      defined endpoints are service control points, the nature of
      messages sent to them may introduce security concerns

   Contact Information: c.f., the "Authors' Addresses" section of this
      memo












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8.3 Registration: The statusRequest Option

   Option Identification: statusRequest

   Present in: APEX's "data" and "recipient" elements

   Contains: nothing

   Processing Rules: c.f., Section 5.1

   Contact Information: c.f., the "Authors' Addresses" section of this
      memo


8.4 Registration: The Report Service

   Well-Known Endpoint: apex=report

   Syntax of Messages Exchanged: c.f., Section 9.2

   Sequence of Messages Exchanged: c.f., Section 6.2

   Access Control Tokens: none

   Contact Information: c.f., the "Authors' Addresses" section of this
      memo

























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9. DTDs

9.1 The APEX Core DTD

   <!--
     DTD for the APEX core, as of 2000-12-12


     Refer to this DTD as:

       <!ENTITY % APEXCORE PUBLIC "-//Blocks//DTD APEX CORE//EN"
                  "http://xml.resource.org/profiles/APEX/apex-core.dtd">
       %APEXCORE;
     -->


   <!ENTITY % BEEP PUBLIC "-//Blocks//DTD BEEP//EN"
              "http://xml.resource.org/profiles/BEEP/beep.dtd">
   %BEEP;


   <!--
     DTD data types:

          entity        syntax/reference     example
          ======        ================     =======
       APEX endpoint
          ENDPOINT      addr-spec,            fred@example.com
                        c.f., [RFC-0822]

       fully-qualified domain-name
           FQDN         c.f., [RFC-1034]      example.com

       seconds
           SECONDS      0..2147483647        600

       unique-identifier
           UNIQID       1..2147483647        42
     -->

   <!ENTITY  % ENDPOINT  "CDATA">
   <!ENTITY  % FQDN      "NMTOKEN">
   <!ENTITY  % SECONDS   "CDATA">
   <!ENTITY  % UNIQID    "CDATA">







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   <!--
     APEX messages, exchanged as application/beep+xml

        role       MSG         RPY         ERR
       ======      ===         ===         ===
         I         attach      ok          error

       I or L      bind        ok          error

       I or L      terminate   ok          error

       I or L      data        ok          error
     -->


   <!ELEMENT attach      (option*)>
   <!ATTLIST attach
             endpoint    %ENDPOINT;        #REQUIRED
             transID     %UNIQID;          #REQUIRED>


   <!ELEMENT bind        (option*)>
   <!ATTLIST bind
             relay       %FQDN;            #REQUIRED
             transID     %UNIQID;          #REQUIRED>


   <!ELEMENT terminate   EMPTY>
   <!ATTLIST terminate
             transID     %UNIQID;          #REQUIRED>


   <!ELEMENT data        (originator,recipient+,option*,data-content?)>
   <!ATTLIST data
             content     %URI;             #REQUIRED>

   <!ELEMENT originator  (option*)>
   <!ATTLIST originator
             identity    %ENDPOINT;        #REQUIRED>

   <!ELEMENT recipient   (option*)>
   <!ATTLIST recipient
             identity    %ENDPOINT;        #REQUIRED>

   <!ELEMENT data-content
                         ANY>
   <!ATTLIST Name        ID                #REQUIRED>




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   <!ELEMENT ok          EMPTY>

   <!ELEMENT error       (#PCDATA)>
   <!ATTLIST error
             code        %XYZ;              #REQUIRED
             xml:lang    %LANG;             #IMPLIED>

   <!ELEMENT reply       (#PCDATA)>
   <!ATTLIST reply
             code        %XYZ;             #REQUIRED
             transID     %UNIQID;          #REQUIRED
             xml:lang    %LANG;            #IMPLIED>


   <!-- either the "internal" or the "external" attribute is present in
        an option -->

   <!ELEMENT option      ANY>
   <!ATTLIST option
             internal    NMTOKEN           ""
             external    %URI;             ""
             targetHop   (this|final|all)  "final"
             seeNoEvil   (true|false)      "true"
             transID     %UNIQID;          #REQUIRED
             localize    %LOCS;            "i-default">


























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9.2 The Report Service DTD

   <!--
     DTD for the APEX report service, as of 2000-12-12


     Refer to this DTD as:

       <!ENTITY % APEXREPORT PUBLIC "-//Blocks//DTD APEX REPORT//EN"
                "http://xml.resource.org/profiles/APEX/apex-report.dtd">
       %APEXREPORT;
     -->


   <!ENTITY % APEXCORE PUBLIC "-//Blocks//DTD APEX CORE//EN"
              "http://xml.resource.org/profiles/APEX/apex-core.dtd">
   %APEXCORE;


   <!--
     Synopsis of the APEX report service

       service WKE: apex=report

       message exchanges:

           service initiates    consumer replies
           =================    ================
           statusResponse       (nothing)

       access control tokens: none
     -->


   <!ELEMENT statusResponse
                         (destination+)>
   <!ATTLIST statusResponse
             transID     %UNIQID;          #REQUIRED>

   <!ELEMENT destination (reply)>
   <!ATTLIST destination
             identity    %ENDPOINT;        #REQUIRED>









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10. Reply Codes

   code    meaning
   ====    =======
   250     transaction successful

   421     service not available

   450     requested action not taken

   451     requested action aborted

   454     temporary authentication failure

   500     general syntax error (e.g., poorly-formed XML)

   501     syntax error in parameters (e.g., non-valid XML)

   504     parameter not implemented

   530     authentication required

   534     authentication mechanism insufficient

   535     authentication failure

   537     action not authorized for user

   538     authentication mechanism requires encryption

   550     requested action not taken

   553     parameter invalid

   554     transaction failed (e.g., policy violation)

   555     transaction already in progress














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11. Security Considerations

   Consult Section 3 and Section 4.5 for a discussion of security
   issues, e.g., relaying integrity. In addition, since APEX is a
   profile of the BEEP, consult [1]'s Section 9 for a discussion of
   BEEP-specific security issues.

   In addition, the statusRequest option (Section 5.1) may be used to
   expose private network topology. Accordingly, administrators may wish
   to choose to disable this option except at the ingress/egress points
   for their domain.








































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References

   [1]   Rose, M.T., "The Blocks Extensible Exchange Protocol Core",
         draft-ietf-beep-framework-11 (work in progress), January 2001.

   [2]   Resnick, P., "Internet Message Format", draft-drums-msg-fmt-09
         (work in progress), September 2000.

   [3]   Allocchio, C., "GSTN Address Element Extensions in E-mail
         Services", RFC 2846, June 2000.

   [4]   Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
         specifying the location of services (DNS SRV)", RFC 2782,
         February 2000.

   [5]   Berners-Lee, T., Fielding, R.T. and L. Masinter, "Uniform
         Resource Identifiers (URI): Generic Syntax", RFC 2396, August
         1998.

   [6]   Levinson, E., "The MIME Multipart/Related Content-type", RFC
         2387, August 1998.

   [7]   Levinson, E., "Content-ID and Message-ID Uniform Resource
         Locators", RFC 2392, August 1998.

   [8]   Freed, N. and N. Borenstein, "Multipurpose Internet Mail
         Extensions (MIME) Part One: Format of Internet Message Bodies",
         RFC 2045, November 1996.

   [9]   Rose, M.T., Klyne, G. and D.H. Crocker, "The APEX Access
         Service", draft-mrose-apex-access-02 (work in progress),
         December 2000.

   [10]  Rose, M.T., Klyne, G. and D.H. Crocker, "The APEX Presence
         Service", draft-mrose-apex-presence-03 (work in progress),
         February 2001.















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Authors' Addresses

   Marshall T. Rose
   Invisible Worlds, Inc.
   1179 North McDowell Boulevard
   Petaluma, CA  94954-6559
   US

   Phone: +1 707 789 3700
   EMail: mrose@invisible.net
   URI:   http://invisible.net/


   Graham Klyne
   Content Technologies Limited
   1220 Parkview
   Arlington Business Park
   Theale, Reading  RG7 4SA
   UK

   Phone: +44 118 930 1300
   EMail: gk@acm.org


   David H. Crocker
   Brandenburg Consulting
   675 Spruce Drive
   Sunnyvale, CA  94086
   US

   Phone: +1 408 246 8253
   EMail: dcrocker@brandenburg.com
   URI:   http://www.brandenburg.com/


















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Appendix A. Acknowledgements

   The authors gratefully acknowledge the contributions of: Harald
   Alvestrand, Eric Dixon, Darren New, Chris Newman, and Scott Pead.















































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Appendix B. IANA Considerations

   The IANA registers "APEX" as a standards-track BEEP profile, as
   specified in Section 8.1.

   The IANA registers "apex" as a GSTN service-selector, as specified in
   Section 8.2.

   The IANA maintains a list of:

   o  APEX options, c.f., Section 7.1; and,

   o  APEX services, c.f., Section 7.2.

   For each list, the IESG is responsible for assigning a designated
   expert to review the specification prior to the IANA making the
   assignment. As a courtesy to developers of non-standards track APEX
   options and services, the mailing list apexwg@invisible.net may be
   used to solicit commentary.

   The IANA makes the registrations specified in Section 8.3 and Section
   8.4.





























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Appendix C. Changes from IMXP

   o  s/IMXP/APEX/g

   o  Clarify the notion of co-residence for APEX services.

   o  Change data's originator from an attribute to an element.

   o  Change addr-spec reference from RFC 822 to [2].

   o  Move the section on "IANA Considerations" to an appendix.








































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Full Copyright Statement

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   The limited permissions granted above are perpetual and will not be
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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