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

Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 RFC 4975

SIMPLE Working Group                                         B. Campbell
Internet-Draft                                              J. Rosenberg
Expires: April 22, 2004                                        R. Sparks
                                                             dynamicsoft
                                                              P. Kyzivat
                                                           Cisco Systems
                                                        October 23, 2003


                   The Message Session Relay Protocol
                 draft-ietf-simple-message-sessions-02

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 April 22, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

   This document describes the Message Session Relay Protocol (MSRP), a
   mechanism for transmitting a series of Instant Messages within a
   session. MSRP sessions are managed using the Session Description
   Protocol (SDP) offer/answer model carried by a signaling protocol
   such as the Session Initiation Protocol (SIP).

   MSRP supports end-to-end Instant Message Sessions, as well as
   sessions traversing one or two relays.




Campbell, et al.         Expires April 22, 2004                 [Page 1]

Internet-Draft                    MSRP                      October 2003


Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.    Motivation for Session-mode Messaging  . . . . . . . . . . .  4
   3.    Scope of this Document . . . . . . . . . . . . . . . . . . .  5
   4.    Protocol Overview  . . . . . . . . . . . . . . . . . . . . .  6
   5.    Architectural Considerations . . . . . . . . . . . . . . . .  7
   5.1   Use of Relays  . . . . . . . . . . . . . . . . . . . . . . .  8
   5.2   Transferring Large Content . . . . . . . . . . . . . . . . .  8
   5.3   Connection Sharing . . . . . . . . . . . . . . . . . . . . .  9
   6.    SDP Offer-Answer Exchanges for MSRP Sessions . . . . . . . . 10
   6.1   Use of the SDP M-line  . . . . . . . . . . . . . . . . . . . 10
   6.2   The Direction Attribute  . . . . . . . . . . . . . . . . . . 11
   6.3   The Accept Types Attribute . . . . . . . . . . . . . . . . . 12
   6.4   MIME Wrappers  . . . . . . . . . . . . . . . . . . . . . . . 13
   6.5   URL Negotiations . . . . . . . . . . . . . . . . . . . . . . 13
   6.6   Example SDP Exchange . . . . . . . . . . . . . . . . . . . . 14
   7.    The Message Session Relay Protocol . . . . . . . . . . . . . 15
   7.1   MSRP URLs  . . . . . . . . . . . . . . . . . . . . . . . . . 15
   7.1.1 MSRP URL Comparison  . . . . . . . . . . . . . . . . . . . . 16
   7.1.2 Resolving MSRP Host Device . . . . . . . . . . . . . . . . . 16
   7.1.3 The msrps URL Scheme . . . . . . . . . . . . . . . . . . . . 17
   7.2   MSRP messages  . . . . . . . . . . . . . . . . . . . . . . . 17
   7.3   MSRP Transactions  . . . . . . . . . . . . . . . . . . . . . 19
   7.4   MSRP Sessions  . . . . . . . . . . . . . . . . . . . . . . . 19
   7.4.1 Initiating an MSRP session . . . . . . . . . . . . . . . . . 19
   7.4.2 Handling VISIT requests  . . . . . . . . . . . . . . . . . . 23
   7.4.3 Sending Instant Messages on a Session  . . . . . . . . . . . 23
   7.4.4 Ending a Session . . . . . . . . . . . . . . . . . . . . . . 25
   7.4.5 Session Inactivity Timer . . . . . . . . . . . . . . . . . . 26
   7.4.6 Managing Session State and Connections . . . . . . . . . . . 27
   7.5   MSRP Relays  . . . . . . . . . . . . . . . . . . . . . . . . 27
   7.5.1 Establishing Session State at a Relay  . . . . . . . . . . . 28
   7.5.2 Removing Session State from a relay  . . . . . . . . . . . . 29
   7.5.3 Sending IMs across an MSRP relay . . . . . . . . . . . . . . 30
   7.5.4 Relay Pairs  . . . . . . . . . . . . . . . . . . . . . . . . 30
   7.5.5 Relay Shutdown . . . . . . . . . . . . . . . . . . . . . . . 31
   7.6   Digest Authentication  . . . . . . . . . . . . . . . . . . . 31
   7.6.1 The SHA1 Algorithm . . . . . . . . . . . . . . . . . . . . . 33
   7.7   Method Descriptions  . . . . . . . . . . . . . . . . . . . . 33
   7.7.1 BIND . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
   7.7.2 SEND . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
   7.7.3 VISIT  . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
   7.8   Response Code Descriptions . . . . . . . . . . . . . . . . . 34
   7.8.1 200  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
   7.8.2 400  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
   7.8.3 401  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
   7.8.4 403  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35



Campbell, et al.         Expires April 22, 2004                 [Page 2]

Internet-Draft                    MSRP                      October 2003


   7.8.5 415  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
   7.8.6 426  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
   7.8.7 481  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
   7.8.8 500  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
   7.8.9 506  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
   7.9   Header Field Descriptions  . . . . . . . . . . . . . . . . . 36
   7.9.1 TR-ID  . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
   7.9.2 Exp  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
   7.9.3 CAuth  . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
   7.9.4 SChal  . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
   7.9.5 Content-Type . . . . . . . . . . . . . . . . . . . . . . . . 37
   7.9.6 S-URL  . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
   8.    Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 37
   8.1   No Relay . . . . . . . . . . . . . . . . . . . . . . . . . . 38
   8.2   Single Relay . . . . . . . . . . . . . . . . . . . . . . . . 40
   8.3   Two Relays . . . . . . . . . . . . . . . . . . . . . . . . . 43
   9.    IANA Considerations  . . . . . . . . . . . . . . . . . . . . 46
   9.1   MSRP Port  . . . . . . . . . . . . . . . . . . . . . . . . . 46
   9.2   MSRP URL Schemes . . . . . . . . . . . . . . . . . . . . . . 47
   9.2.1 Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
   9.2.2 Character Encoding . . . . . . . . . . . . . . . . . . . . . 47
   9.2.3 Intended Usage . . . . . . . . . . . . . . . . . . . . . . . 47
   9.2.4 Protocols  . . . . . . . . . . . . . . . . . . . . . . . . . 47
   9.2.5 Security Considerations  . . . . . . . . . . . . . . . . . . 47
   9.2.6 Relevant Publications  . . . . . . . . . . . . . . . . . . . 47
   9.3   SDP Parameters . . . . . . . . . . . . . . . . . . . . . . . 47
   9.3.1 Direction  . . . . . . . . . . . . . . . . . . . . . . . . . 47
   9.3.2 Accept Types . . . . . . . . . . . . . . . . . . . . . . . . 48
   9.3.3 Wrapped Types  . . . . . . . . . . . . . . . . . . . . . . . 48
   10.   Security Considerations  . . . . . . . . . . . . . . . . . . 48
   10.1  TLS and the MSRPS Scheme . . . . . . . . . . . . . . . . . . 48
   10.2  Sensitivity of the Session URL . . . . . . . . . . . . . . . 49
   10.3  End to End Protection of IMs . . . . . . . . . . . . . . . . 50
   10.4  CPIM compatibility . . . . . . . . . . . . . . . . . . . . . 50
   10.5  PKI Considerations . . . . . . . . . . . . . . . . . . . . . 50
   11.   Changes from Previous Draft Versions . . . . . . . . . . . . 51
   11.1  draft-ietf-simple-message-sessions-02  . . . . . . . . . . . 51
   11.2  draft-ietf-simple-message-sessions-01  . . . . . . . . . . . 51
   11.3  draft-ietf-simple-message-sessions-00  . . . . . . . . . . . 52
   11.4  draft-campbell-simple-im-sessions-01 . . . . . . . . . . . . 52
   12.   Contributors . . . . . . . . . . . . . . . . . . . . . . . . 53
         Normative References . . . . . . . . . . . . . . . . . . . . 53
         Informational References . . . . . . . . . . . . . . . . . . 54
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 55
         Intellectual Property and Copyright Statements . . . . . . . 56






Campbell, et al.         Expires April 22, 2004                 [Page 3]

Internet-Draft                    MSRP                      October 2003


1. Introduction

   The MESSAGE [10] extension to SIP [2] allows SIP to be used to
   transmit instant messages. Instant messages sent using the MESSAGE
   method are normally independent of each other. This approach is often
   called page-mode messaging, since it follows a model similar to that
   used by many two-way pager devices. Page-mode messaging makes sense
   for instant message exchanges where a small number of messages occur.
   Endpoints may treat page-mode messages as if they took place in an
   imaginative session, but there is no formal relationship between one
   message and another.

   There are also applications in which it is useful for instant
   messages to be formally associated in a session. For example, a user
   may wish to join a text conference, participate in the conference for
   some period of time, then leave the conference. This usage is
   analogous to regular media sessions that are typically initiated,
   managed, and terminated using SIP. We commonly refer to this model as
   session-mode messaging.

   One of the primary purposes of SIP and SDP (Section 6) is the
   management of media sessions. Session-mode messaging can be thought
   of as a media session like any other.  This document describes the
   motivations for session-mode messaging, the Message Session Relay
   Protocol, and the use of the SDP offer/answer mechanism for managing
   MSRP session.

2. Motivation for Session-mode Messaging

   Message sessions offer several advantages over page-mode messages.
   For message exchanges that include more than a small number of
   message transactions, message sessions offer a way to remove
   messaging load from intervening SIP proxies. For example, a minimal
   session setup and tear-down requires one INVITE/ACK transaction, and
   one BYE transaction, for a total of 5 SIP messages. Normal SIP
   request routing allows for all but the initial INVITE transaction to
   bypass any intervening proxies that do not specifically request to be
   in the path for future requests. Session-mode messages never cross
   the SIP proxies themselves.

   Each page-mode message involves a complete SIP transaction, that is,
   a request and a response. Any page-mode message exchange that
   involves more than 2 MESSAGE requests will generate more SIP requests
   than a minimal session initiation sequence. Since MESSAGE is normally
   used outside of a SIP dialog, these requests will typically traverse
   the entire proxy network between the endpoints.

   Due to network congestion concerns, the MESSAGE method has



Campbell, et al.         Expires April 22, 2004                 [Page 4]

Internet-Draft                    MSRP                      October 2003


   significant limitations in message size, a prohibition against
   overlapping requests, etc. Much of this has been required because of
   perceived limitations in the congestion-avoidance features of SIP
   itself. Work is in progress to mitigate these concerns.

   However, session-mode messages are always sent over  reliable,
   congestion-safe transports. Therefore, there are no restrictions on
   message sizes. There is no requirement to wait for acknowledgement
   before sending another message, so that message transactions can be
   overlapped.

   Message sessions allow greater efficiency for secure message
   exchanges. The SIP MESSAGE request inherits the S/MIME features of
   SIP, allowing a message to be signed and/or encrypted. However, this
   approach requires public key operations for each message. With
   session-mode messaging, a session key can be established at the time
   of session initiation. This key can be used to protect each message
   that is part of the session. This requires only symmetric key
   operations for each subsequent IM, and no additional certificate
   exchanges are required after the initial exchange. The establishment
   of the session key can be done using standard techniques that apply
   to voice and video, in addition to instant messaging.

   Finally, SIP devices can treat message sessions like any other media
   sessions. Any SIP feature that can be applied to other sorts of media
   sessions can equally apply to message sessions. For example,
   conferencing [12], third party call control [13], call transfer [14],
   QoS integration [15], and privacy [16] can all be applied to message
   sessions.

   Messaging sessions can also reduce the overhead in each individual
   message. In page-mode, each message needs to include all of the SIP
   headers that are mandated by RFC 3261 [2]. However, many of these
   headers are not needed once a context is established for exchanging
   messages. As a result, messaging session mechanisms can be designed
   with significantly less overhead.

3. Scope of this Document

   This document describes the use of MSRP between endpoints, or via one
   or two relays, where endpoints have advance knowledge of the relays.
   It does not provide a mechanism for endpoints to determine whether a
   relay is needed, or for endpoints to discover the presence of relays.

   This document describes the use of MSRP over TCP. MSRP may be used
   over other congestion-controlled protocols such as SCTP. However, the
   specific bindings for other such protocols are outside the scope of
   this document.



Campbell, et al.         Expires April 22, 2004                 [Page 5]

Internet-Draft                    MSRP                      October 2003


4. Protocol Overview

   The Message Session Relay Protocol (MSRP) provides a mechanism for
   transporting session-mode messages between endpoints. MSRP also
   contains primitives to allow the use of one or two relay devices.
   MSRP uses connection oriented, reliable network transport protocols
   only. It is intrinsically NAT and firewall friendly, as it allows
   participants to positively associate message sessions with specific
   connections, and does not depend upon connection source address,
   which may be obscured by NATs.

   MSRP uses the following primitives:

   SEND: Used to send message content from one endpoint to another.

   VISIT: Used by an endpoint to establish a session association to the
      opposite endpoint, or to a relay that was selected by the opposite
      endpoint.

   BIND: Used by an endpoint to establish a session at a relay, and
      allow the opposite endpoint to visit that relay.

   The simplest use case for MSRP is a session that goes directly
   between endpoints, with no intermediaries involved. Assume A is an
   endpoint that wishes to establish a message session, and B is the
   endpoint invited by A. A invites B to participate in a message
   session by sending a URL that represents the session. This URL is
   temporary, and must not duplicate the URL used for any other active
   sessions.

   B "visits" A by connecting to A and sending a VISIT request
   containing the URL that A provided. This associates the connection
   from B with the session. B then responds to the invitation, informing
   A that B has accepted the session. A and B may now exchange messages
   using SEND requests on the connection.

   When either party wishes to end the session, it informs the peer
   party with a SIP BYE request. A terminates the session by
   invalidating associated state, and dropping the connection.

   The end to end case looks something like the following. (Note that
   the example shows a logical flow only; syntax will come later in this
   document.)

   A->B (SDP): offer (msrp://A/123)






Campbell, et al.         Expires April 22, 2004                 [Page 6]

Internet-Draft                    MSRP                      October 2003


   B->A (MSRP): VISIT (msrp://A/123)
   A->B (MSRP): 200 OK
   B->A (SDP): answer(msrp://A/123)
   A->B (MSRP): SEND
   B->A (MSRP): 200 OK
   B->A (MSRP): SEND
   A->B (MSRP): 200 OK

   The session state has an associated inactivity timer. This timer is
   initialized when a successful VISIT request occurs, and is reset each
   time either endpoint sends a SEND request. If this timer expires
   without being reset, the hosting device invalidates the session state
   and terminates all associated connections. Endpoints that are
   otherwise idle may keep a session active by periodically sending SEND
   requests with no content.

   A slightly more complicated case involves a single relay, known about
   in advance by one of the parties. The endpoint that has the
   preexisting relationship with the relay uses the BIND method to
   establish session state in the relay. The relay returns a temporary
   URL, that identifies the session. For endpoints A and B, and relay R,
   the flow would look like the following:


   A->R: MSRP: BIND(msrp://r)
   R->A: MSRP: 200 OK (msrp://r/4uye)
   A->B (SDP): offer (msrp://r/4uye)
   B->R (MSRP): VISIT (msrp://r/4uye)
   R->B (MSRP): 200 OK
   B->A (SDP): answer(msrp://r/4uye)
   A->R (MSRP): SEND
   R->B (MSRP): SEND
   B->R (MSRP): 200 OK
   R->A (MSRP): 200 OK
   B->R (MSRP): SEND
   R->A (MSRP): SEND
   A->R (MSRP): 200 OK
   R->B (MSRP): 200 OK

   The BIND request contains an expiration time. If a successful VISIT
   request does not occur prior to the expiration, the relay will
   destroy the session. Additionally, when tearing down a session, the
   host endpoint invalidates the session state by issuing a BIND request
   with an expiration value of zero.

5. Architectural Considerations

   There are a number of considerations that, if handled in a reasonable



Campbell, et al.         Expires April 22, 2004                 [Page 7]

Internet-Draft                    MSRP                      October 2003


   fashion, will allow more effective use of the protocols described in
   this document.

5.1 Use of Relays

   The primary motivation for relay support in MSRP is to deal with
   situations where, due to issues of network topologies, neither
   endpoint is able to receive an inbound TCP connection from the other.
   For example, both endpoints may be behind separate firewalls that
   only allow outbound connections. Relays may also be needed for policy
   enforcement. For example, parts of the financial industry require the
   logging of all communication.

   However, the use of such relays has a significant impact on the
   scalability of MSRP. Each relay will require two TCP connections for
   each session in use, as well as memory for local session state
   storage. Most general purpose platforms on which one might implement
   MSRP relays will have relatively low limits on the number of
   simultaneous TCP connections they can handle.

   Therefore relays SHOULD NOT be used indiscriminately. In the absence
   of strong reasons to use relays, MSRP endpoints SHOULD be configured
   to set up point-to-point sessions.

   MSRP supports the use of two relays, where each endpoint has a relay
   acting on its behalf. However, most of the network topology issues
   mentioned above can work with a single relay, if that relay is
   reachable by both endpoints. Dual relays are only needed for cases of
   very strict firewall policy, such as when only specific hosts are
   allowed to connect to the outside world; or situations requiring
   strict policy enforcement at both endpoint domains. If a given usage
   scenario can be solved with a single relay, then a second relay
   SHOULD NOT be used.

   In spite of these recommendations, relays serve a real purpose in
   that they increase the likelihood of two arbitrary endpoints being
   able to talk to one another. Therefore if a provider deploys MSRP
   endpoints in a network configuration that prevents them from
   receiving TCP connections from arbitrary peers, and does not wish to
   explicitly prevent MSRP communication with the outside world, then
   the provider SHOULD provide its endpoints with the use of an MSRP
   relay that is reachable from arbitrary peers.

5.2 Transferring Large Content

   MSRP endpoints may attempt to send very long messages in a session.
   For example, most commercial instant messaging systems have a file
   transfer feature. Since MSRP does not impose message size limits,



Campbell, et al.         Expires April 22, 2004                 [Page 8]

Internet-Draft                    MSRP                      October 2003


   there is nothing to prevent endpoints from transferring files over
   it.

   An analysis of whether it makes sense to do this, rather than sending
   such content over FTP, HTTP, or some other such protocol, is beyond
   the scope of this document. However, implementers should be aware of
   the impact of sending very large messages over MSRP. The primary
   impact is, since MSRP is sent over TCP, is that any additional
   messages that the sender wishes to send will be blocked until the
   large transfer is complete. This includes responses to messages sent
   by the peer. Therefore, any SEND transactions initiated by the peer
   are likely to time out, even though they are received without
   problems.

   Further, there is no way to abort the sending of a very large message
   before it is complete. For the sake of efficiency, the framing
   mechanism in MSRP is very simple. There is no clean way to recover
   framing if the complete message is not sent.

   These issues can be mitigated greatly if the endpoint simply
   establishes a separate session for the transfer. This allows the
   transfer to be sent without interfering with any instant messages
   being sent on other sessions. Further, the endpoint can abort the
   transfer by simply tearing down the transfer session. Therefore, if a
   peer wishes to send very large content, it SHOULD establish a
   dedicated session for that purpose.

      Open Issue: Do we need a mechanism to communicate the purpose of
      the session? It has been mentioned that the peer may not realize
      the purpose of the session, and start using it for normal
      messaging. Also, there has been discussion that we need a stronger
      mechanism to avoid transaction timeouts caused by long requests.

5.3 Connection Sharing

   The SIMPLE working group spent quite a bit of effort in the
   consideration of shared TCP connections. Connection sharing would
   offer value whenever a large number of message sessions cross the
   same two adjacent devices. This situation is likely to occur in the
   two relay model. It may also occur in the point-to-point model if the
   endpoints are multiuser devices, as is likely with web-hosted
   messaging services.

   Unfortunately, such connection sharing in TCP created significant
   problems. The biggest problem is it introduced a head-of-line
   blocking problem that spanned sessions. For example, if two different
   pairs of users had sessions that crossed the same shared connection,
   a large message sent on one session would block transfer of messages



Campbell, et al.         Expires April 22, 2004                 [Page 9]

Internet-Draft                    MSRP                      October 2003


   on the other session. The working group considered this an
   unacceptable property of shared connections. One possible solution
   was to put limits on message size, and possibly add mechanisms to
   allow breaking messages into many chunks. However, these solutions
   promised to add a great deal of complexity to the protocol, so the
   work group chose not to go that route.

   It may be possible to relax this requirement using other transport
   protocols, such as SCTP. The lack of connection sharing in this
   document should not be construed to prohibit shared connections on
   other such protocols. However, such specification is beyond the scope
   of this document.

6. SDP Offer-Answer Exchanges for MSRP Sessions

   MSRP sessions will typically be initiated using the Session
   Description Protocol (SDP) [1] offer-answer mechanism, carried in the
   Session Initiation Protocol (SIP) [2] or any other protocol
   supporting it. MSRP borrows the idea of the direction attributes from
   COMEDIA [18], but does not depend on that specification.

6.1 Use of the SDP M-line

   The SDP "m"-line takes the following form:

      m=<media> <port> <protocol> <format list>

   For non-RTP media sessions, The media field specifies the top level
   MIME media type for the session. For MSRP sessions, the media field
   MUST have the value of "message". The port field is normally not
   used, and SHOULD be set to 9999. An exception is when the port field
   value is set to zero, according to normal SDP usage.

   The proto field MUST designate the message session mechanism and
   transport protocol, separated by a "/" character. For MSRP, left part
   of this value MUST be "msrp". For MSRP over TCP, the right part of
   this field MUST take the value "tcp". For MSRP over other transport
   protocols, the field value MUST be defined by the specification for
   that protocol binding.

   The format list list is ignored for MSRP. This is because MSRP
   formats are specified as MIME content types, which are not convenient
   to encode in the SDP format list syntax. Instead, the allowed formats
   are negotiated using "a"-line attributes. For MSRP sessions, the
   format list SHOULD contain a "*" character, and nothing else.

   The port field in the M-line is not normally used to determine the
   port to which to connect. Rather, the actual port is determined by



Campbell, et al.         Expires April 22, 2004                [Page 10]

Internet-Draft                    MSRP                      October 2003


   the contents of the session URL (Section 7.1). However, a port value
   of zero has the normal SDP meaning.

   The following example illustrates an m-line for a message session,
   where the endpoint is willing to accept root payloads of message/
   cpim, plain text or HTML. The second two types could either be
   presented as the root body, or could be contained within message/cpim
   bodies.

      m=message 9999 msrp/tcp *

6.2 The Direction Attribute

   Since MSRP uses connection oriented transport protocols, one goal of
   the SDP negotiation is to determine which participant initiates the
   transport connection. The direction attribute advertises whether the
   offerer or answerer wishes to initiate the connection, wishes the
   peer endpoint to initiate the connection, or doesn't care.

   The endpoint that accepts the connection, or has a relay accept the
   connection on its behalf, is said to "host" the session, and is known
   as the hosting endpoint. The endpoint that initiates the connection
   is said to "visit" the session, and is known as the visiting
   endpoint.

   The direction attribute is included in an SDP a-line, with a value
   taking the following syntax:

               direction       = direction-label ":" role
               direction-label = "direction"
               role            = active / passive / both
               active          = "active"
               passive         = "passive"
               both            = "both" [sp timeout]
               timeout         = 1*DIGIT ; timeout value in seconds

   The values for the role field are as follows:

   passive: The endpoint wishes to host the session

   active: The endpoint wishes the peer to host the session.

   both: The endpoint is willing to act as either host or visitor. If
      "both" is selected, it may contain an optional timeout value. This
      timeout specifies how much time the answerer should wait before
      giving up on a connection and attempting to take over as host
      device.  If the timeout value is not specified, it defaults to 30
      seconds.



Campbell, et al.         Expires April 22, 2004                [Page 11]

Internet-Draft                    MSRP                      October 2003


   The SDP offer for an MSRP session MUST contain a direction attribute,
   which MAY take any of the defined values. If the offerer is capable
   of hosting the session, or can arrange for a relay to host the
   session on its behalf, then it SHOULD select "both". The endpoint
   SHOULD NOT select "active" unless it cannot host the session under
   any circumstances. The endpoint SHOULD NOT select "passive" unless it
   has no option but to host the session.

   The SDP answer also MUST contain a direction attribute, but its value
   choices are limited based on the value in the offer. If the offer
   contained "active", then the answerer MUST either select "passive" or
   reject the offer. Likewise, if the offer contained  "passive", then
   the answerer MUST select "active" or reject the offer. If the offer
   contained "both", the answerer SHOULD select "active", but MAY select
   "passive" if it is unable to reach the host device, or if local
   policy requires it to act as host.

6.3 The Accept Types Attribute

   MSRP can carry any MIME encoded payload. Endpoints specify MIME
   content types that they are willing to receive in the accept types
   "a"-line attribute. This attribute has the following syntax:

               accept-types       = accept-types-label ":" format-list
               accept-types-label = "accept-types"
               format-list        = format-entry *( SP format-entry)
               format-entry       = (type "/" subtype) / ("*")
               type               = token
               subtype            = token

   SDP offers for MSRP sessions MUST include an accept-types attribute.
   SDP answers MUST also include the attribute, which MUST contain
   either the same list as in the offer or a subset of that list.

   A "*" entry in the accept-types attribute indicates that the sender
   may attempt to send messages with media types that have not been
   explicitly listed. If the receiver is able to process the media type,
   it does so. If not, it will respond with a 415. Note that all
   explicit entries SHOULD be considered preferred over any non-listed
   types. This feature is needed as, otherwise, the list of formats  for
   rich IM devices may be prohibitively large.

   The accept-types attribute may include container types, that is, mime
   formats that contain other types internally. If compound types are
   used, the types listed in the accept-types attribute may be used both
   as the root payload, or may be wrapped in a listed container type.
   (Note that the container type MUST also be listed in the accept-types
   attribute.)



Campbell, et al.         Expires April 22, 2004                [Page 12]

Internet-Draft                    MSRP                      October 2003


6.4 MIME Wrappers

   The MIME content-types in the accept-types attribute will often
   include container types; that is, types that contain other types. For
   example, "message/cpim" or "multipart/mixed."  Occasionally an
   endpoint will need to specify a MIME body type that can only be used
   if wrapped inside a listed container type.

   Endpoints MAY specify MIME types that are only allowed to be wrapped
   inside compound types using the "accept-wrapped-types" attribute in
   an SDP a-line. This attribute has the following syntax:

               accept-wrapped-types = wrapped-types-label ":" format-list
               wrapped-types-label  = "accept-wrapped-types"

   The format-list element has the identical syntax as defined for the
   accept-types attribute. The semantics for this attribute are
   identical to those of the accept-types attribute, with the exception
   that the specified types may only be used when wrapped inside
   containers. Only types listed in accept-types may be used as the
   "root" type for the entire body. Since any type listed in
   accept-types may be used both as a root body, and wrapped in other
   bodies, format entries from the m-line SHOULD NOT be repeated in this
   attribute.

   This approach does not allow for specifying distinct lists of
   acceptable wrapped types for different types of containers. If an
   endpoint understands a MIME type in the context of one wrapper, it is
   assumed to understand it in the context of any other acceptable
   wrappers, subject to any constraints defined by the wrapper types
   themselves.

      The approach of specifying types that are only allowed inside of
      containers separately from the primary payload types allows an
      endpoint to force the use of certain wrappers. For example, a CPIM
      gateway device may require all messages to be wrapped inside
      message/cpim bodies, but may allow several content types inside
      the wrapper. If the gateway were to specify the wrapped types in
      the accept-types attribute, its peer could choose to use those
      types without the wrapper.

6.5 URL Negotiations

   An MSRP session is identified by an MSRP URL, which is determined by
   the hosting endpoint, and negotiated in the SDP exchange. Any SDP
   offer or answer that creates a possibility that the sender will host
   the session, that is, it contains a direction value of "passive" or
   "both",  MUST contain an MSRP URL in a session attribute. This



Campbell, et al.         Expires April 22, 2004                [Page 13]

Internet-Draft                    MSRP                      October 2003


   attribute has the following syntax:

   a=session:<MSRP_URL>

   where <MSRP_URL> is an MSRP or MSRPS URL as defined in Section 7.1.

   The visitor will use the session URL established by the host both to
   resolve the host address and port, and to identify the session when
   connecting. For MSRP sessions, the address field in the C-line is not
   relevant, and MUST be ignored. The port field in the M-line MUST be
   ignored if non-zero. Zero values have the normal meaning for SDP.

   The following example shows an SDP offer with a session URL of
   "msrp://example.com:7394/2s93i"

           c=IN IP4 useless.host.name
           m=message 9999 msrp/tcp *
           a=accept-types:text/plain
           a=direction:both
           a=session:msrp://example.com:7394/2s93i


   The session URL MUST be a temporary URL assigned just for this
   particular session. It MUST NOT duplicate any URL in use for any
   other session hosted by the endpoint or relay. Further, since the
   peer endpoint will use the session URL to identify itself when
   connecting, it SHOULD be hard to guess, and protected from
   eavesdroppers. This will be discussed in more detail in Section 10.

6.6 Example SDP Exchange

   Endpoint A wishes to invite Endpoint B to a MSRP session. A offers
   the following session description containing the following lines:

     c=IN IP4 alice.example.com
     m=message 9999 msrp/tcp *
     a=accept-types: message/cpim text/plain text/html
     a=direction:both
     a=session:msrp://alice.example.com:7394/2s93i9

   Endpoint B chooses to participate in the role of visitor, opens a TCP
   connection to alice.example.com:7394, and successfully performs a
   VISIT transaction passing the URL of msrp://alice.example.com:7394/
   2s93i9. B indicates that it has accomplished this by answering with:

     c=IN IP4 dontlookhere
     m=message 9999 msrp/tcp *
     a=accept-types:message/cpim text/plain



Campbell, et al.         Expires April 22, 2004                [Page 14]

Internet-Draft                    MSRP                      October 2003


     a=direction:active

   A may now send IMs to B by executing SEND transactions on the same
   connection on which B sent the VISIT request.

7. The Message Session Relay Protocol

   The Message Session Relay Protocol (MSRP) is a text based, message
   oriented protocol for the transfer of instant messages in the context
   of a session. MSRP uses the UTF8 character set.

   MSRP messages MUST be sent over a reliable, congestion-controlled,
   connection-oriented transport protocol. This document specifies the
   use of MSRP over TCP. Other documents may specify bindings for other
   such protocols.

7.1 MSRP URLs

   MSRP sessions are identified by MSRP URLs. An MSRP URL follows a
   subset of the URL syntax in Appendix A of RFC2396 [4], with a scheme
   of "msrp":

      msrp_url = "msrp" ":" "//" [userinfo] hostport ["/' resource]
      resource = 1*unreserved

   The constructions for "userinfo", "hostport", and "unreserved" are
   detailed in RFC2396 [4].

   An MSRP URL server part identifies the hosting device of an MSRP
   session. If the server part contains a numeric IP address, it MUST
   also contain a port. The resource part identifies a particular
   session at that host device. The absence of the resource part
   indicates a reference to an MSRP host device, but does not
   specifically refer to a particular session resource.

   MSRP has an  IANA registered recommended port defined in Section 9.1.
   This value SHOULD NOT be considered a default, as the URL process
   described herein will always explicitly resolve a port number.
   However, the URLs SHOULD be configured so that the recommended port
   is used whenever appropriate. This makes life easier for network
   administrators who need to manage firewall policy for MSRP.

   The server part will typically not contain a userinfo component, but
   MAY do so to indicate a user account for which the session is valid.
   Note that this is not the same thing as identifying the session
   itself. If a userinfo component exists, MUST be constructed only from
   "unreserved" characters, to avoid a need for escape processing.
   Escaping MUST NOT be used in an MSRP URL. Furthermore, a userinfo



Campbell, et al.         Expires April 22, 2004                [Page 15]

Internet-Draft                    MSRP                      October 2003


   part MUST NOT contain password information.

   The following is an example of a typical MSRP URL:

      msrp://host.example.com:8493/asfd34

7.1.1 MSRP URL Comparison

   MSRP URL comparisons MUST be performed according to the following
   rules:

   1.  The host part is compared as case insensitive.

   2.  If the port exists explicitly in either URL, then it must match
       exactly. An URL with an explicit port is never equivalent to
       another with no port specified.

   3.  The resource part is compared as case insensitive. A URL without
       a resource part is never equivalent to one that includes a
       resource part.

   4.  Userinfo parts are not considered for URL comparison.

   Path normalization is not relevant for MSRP URLs. Escape
   normalization is not required, since the relevant parts are limited
   to unreserved characters.

7.1.2 Resolving MSRP Host Device

   An MSRP host device is identified by the server part of an MSRP URL.

   If the server part contains a numeric IP address and port, they MUST
   be used as listed.

   If the server part contains a host name and a port, the connecting
   device MUST determine a host address by doing an A or AAAA DNS query,
   and use the port as listed.

   If the server part contains a host name but no port, the connecting
   device MUST perform the following steps:

   1.  Construct an SRV [6] query  string by prefixing the host name
       with the service field "_msrp" and the protocol field ("_tcp" for
       TCP). For example, "_msrp._tcp.host.example.com".

   2.  Perform a DNS SRV query using this query string.





Campbell, et al.         Expires April 22, 2004                [Page 16]

Internet-Draft                    MSRP                      October 2003


   3.  Select a resulting record according to the rules in RFC2782 [6].
       Determine the port from the chosen record.

   4.  If necessary, determine a host device address by performing an A
       or AAAA query on the host name field in the selected SRV result
       record. If multiple A or AAAA records are returned, the first
       entry SHOULD be chosen for the initial connection attempt. This
       allows any ordering created in the DNS to be preserved.

   5.  If the connection attempt fails, the device SHOULD attempt to
       connect to the addresses returned in any additional A or AAAA
       records, in the order the records were presented. If all of these
       fail, the device SHOULD attempt to use any additional SRV records
       that may have been returned, following the normal rules for SRV
       record selection.

   Note that in most cases, the transport protocol will be determined
   separately from the resolution process. For example, if the MSRP URL
   was communicated in an SDP offer or answer, the SDP M-line will
   contain the transport protocol. When an MSRP URL is communicated
   outside of SDP, the protocol SHOULD also be communicated. For
   example, a client may be configured to use a particular relay that is
   referenced with an MSRP URL. The client MUST also be told what
   protocol to use. If a device needs to resolve an MSRP URL and does
   not know the protocol, it SHOULD assume TCP.

7.1.3 The msrps URL Scheme

   The "msrps" URL Scheme indicates that each hop MUST be secured with
   TLS. Otherwise, it is used identically as an MSRP URL, except that a
   MSRPS URL MUST NOT be considered equivalent to an MSRP URL. The MSRPS
   scheme is further discussed in Section 10.

7.2 MSRP messages

   MSRP messages are either requests or responses. Requests and
   responses are distinguished from one another by the first line. The
   first line of a Request takes the form of the request-start entry
   below. Likewise, the first line of a response takes the form of
   response-start. The syntax for an MSRP message is as follows:

       msrp-message   = request-start/response-start *(header CRLF)
                                  [CRLF body]
       request-start  = "MSRP" SP length SP  Method CRLF
       response-start = "MSRP" SP length SP Status-Code SP
                                Reason CRLF

       length       = 1*DIGIT  ; the length of the message,



Campbell, et al.         Expires April 22, 2004                [Page 17]

Internet-Draft                    MSRP                      October 2003


                               ;  exclusive of the start line.
       Method       = SEND / BIND / VISIT / other-method
       other-method = token
       header       = Client-Authenticate / Server-Challenge /
                      Transaction-ID / Session-URL/ Content-Type / Expires
       Status-Code  = 200    ;Success
                    / 400    ;Bad Request
                    / 401    ;Authentication Required
                    / 403    ;Forbidden
                    / 415    ;Unsupported Content Type
                    / 426    ;Upgrade Required
                    / 481    ;No session
                    / 500    ;Cannot Deliver
                    / 506    ;duplicate session

       Reason              = token ; Human readable text describing status
       Client-Authenticate = "CAuth" ":" credentials
       Server-Challenge    = "SChal" ":" challenge
       Transaction-ID      = "Tr-ID" ":" token
       Content-Type        = "Content-Type" ":" quoted-string
       Session-URL         = "S-URL" ":" msrp_url
       Expires             = "Exp"":" delta-seconds
       delta-seconds       = 1*DIGIT ; Integer number of seconds

       challenge           = digest-scheme SP digest-challenge *("," digest-challenge)
       digest-scheme       = "Digest"
       digest-challenge    = nonce / algorithm / auth-param
       nonce               = "nonce" "=" nonce-value
       nonce-value         = quoted-string
       algorithm           = "algorithm" "=" ( "SHA1" / token )


       credentials         = "Digest" digest-response *("," digest-response)
       digest-response     = username / nonce / response / algorithm /
                             auth-param
       username            = "username" "=" username-value
       username-value      = quoted-string
       response            = "response" "=" request-digest
       request-digest      = <"> 40LHEX <">
       LHEX                =  "0" / "1" / "2" / "3" /
                              "4" / "5" / "6" / "7" /
                              "8" / "9" / "a" / "b" /
                              "c" / "d" / "e" / "f"




   All requests and responses MUST contain at least a TR-ID header



Campbell, et al.         Expires April 22, 2004                [Page 18]

Internet-Draft                    MSRP                      October 2003


   field. Messages MAY contain other fields, depending on the method or
   response code.

7.3 MSRP Transactions

   An MSRP transaction consists of exactly one request and one response.
   A response matches a transaction if it share the same TR-ID value,
   and arrives on the same connection on which the transaction was sent.

   BIND is always hop by hop. VISIT transactions are usually hop-by-hop,
   but may be relayed in situations where the visiting endpoint uses a
   relay.  However, SEND transactions are end-to-end, meaning that under
   normal circumstances the response is sent by the peer endpoint, even
   if there are intervening relays.

   Endpoints MUST select TR-ID header field values in requests so that
   they are not repeated by the same endpoint in scope of the given
   session. TR-ID values SHOULD be globally unique. The TR-ID space of
   each endpoint is independent of that of its peer. Endpoints MUST NOT
   infer any semantics from the TR-ID header field beyond what is stated
   above. In particular, TR-ID values are not required to follow any
   sequence.

   MSRP Transactions complete when a response is received, or after a
   timeout interval expires with no response. Endpoints MUST treat such
   timeouts in exactly the same way they would treat a 500 response. The
   timeout interval SHOULD be 30 seconds, but other values may be
   established as a matter of local policy.

7.4 MSRP Sessions

   AN MSRP session is a context in which a series of instant messages
   are exchanged, using SEND requests. A session has two endpoints (a
   host and a visitor) and may have one or two relays. A session is
   identified by an MSRP URL.

7.4.1 Initiating an MSRP session

   When an endpoint wishes to engage a peer endpoint in a message
   session, it invites the peer to communicate using an SDP offer,
   carried over SIP or some other protocol supporting the SDP offer/
   answer model. For the purpose of this document, we will refer to the
   endpoint choosing to initiate communication as the offerer, and the
   peer being invited as the answerer.

   The offerer SHOULD volunteer to act as the hosting endpoint if
   allowed by policy and network topology. An endpoint is said to host a
   session if one of two conditions are true. The host either directly



Campbell, et al.         Expires April 22, 2004                [Page 19]

Internet-Draft                    MSRP                      October 2003


   listens for a connection from the peer endpoint, and maintains
   session state itself, or it uses a BIND request to initialize session
   state at a relay that will listen for a connection from the peer. The
   peer that is not the host is designated as the visitor. The offerer
   MAY request the answerer to act as host if it is prevented from
   accepting connections by network topology or policy, and is not able
   to bind to a relay to act on its behalf.

   If the offerer wishes to host the session directly, that is without
   using a relay, it MUST perform the following steps:

   1.  Construct a session MSRP URL . This URL MUST be resolvable to the
       offerer. The URL SHOULD be temporary, SHOULD be hard to guess,
       and MUST not duplicate the URL  of any other session currently
       hosted by the offerer.

   2.  Listen for a connection from the peer.

   3.  Construct an SDP offer as described in Section 6, including the
       list of allowed IM payload formats in the accept-types attribute.
       The offerer maps the session URL to the session attribute, as
       described in Section 6.5.

   4.  Insert a direction attribute. This value SHOULD be "both",
       indicating that the offerer will allow the answerer to override
       the offerer's decision to host. If "both" is selected, the
       offerer SHOULD leave the timeout at the default value (by leaving
       out the value entirely. However, the offerer MAY select a
       different timeout if circumstances warrant it. The direction
       value MAY be "passive" if the offerer is prevented from allowing
       the answerer override this choice.

   5.  Send the SDP offer using the normal processing for the signaling
       protocol.

   If the offerer chooses to force the answerer to host the session, it
   MUST perform the following steps instead:

   1.  Construct an SDP offer as described above, but with no session
       attribute.

   2.  Insert a direction attribute with a value of "active".

   3.  Send the offer using normal processing for the signaling
       protocol.

   When the answerer receives the SDP offer and chooses to participate
   in the session, it must choose whether to act as the host or the



Campbell, et al.         Expires April 22, 2004                [Page 20]

Internet-Draft                    MSRP                      October 2003


   visitor. A direction attribute value of "both" in the offer indicates
   that the offerer prefers to host, but will allow the answerer to
   host.  In this case the answerer SHOULD act as the visitor, but MAY
   choose to host. A value of "passive" means the offerer insists upon
   hosting, in which case the answerer MUST act as visitor or decline
   the offer.

   If the answerer chooses to participate as a visitor, it MUST perform
   the following steps:

   1.  Determine the host address and port from the session URL,
       following the procedures in section Section 7.1

   2.  Connect to the host address and port, using the transport
       protocol from the M-line.

   3.  Construct a VISIT request, which MUST contain the following
       information:

       1.  An S-URL header field containing the session URL.

       2.  A TR-ID header field containing a unique transaction ID.

       3.  A size field containing size of the message subsequent to the
           start-line.

   4.  Send the request and wait for a response

   5.  If the transaction succeeds, send a SDP answer via the signaling
       protocol, according to the following rules:

       1.  The C-line is  copied unmodified from the offer.

       2.  The M-Line contains a dummy port value, the protocol field
           from the original offer, and the accept-types attribute
           contains the SEND payload media types that the answerer is
           willing to accept. The accept-types attribute in the answer
           MUST be either the same as that of the offer, or it MUST be a
           subset.

       3.  A direction attribute containing the value "active".

   6.  If the transaction fails, the answerer MAY choose to act as host,
       if allowed by the direction attribute of the answer. If the
       answerer is unable or unwilling to host, then it should return an
       error response as appropriate for the signaling protocol.

   Some TCP connection failure conditions may ordinarily take some time



Campbell, et al.         Expires April 22, 2004                [Page 21]

Internet-Draft                    MSRP                      October 2003


   to notice. For example, if the offerer is unable to open a TCP
   connection to the host device, this connection attempt may take a
   fairly large number of seconds to timeout. This length of time will
   not be acceptable for many call flow scenarios. Therefore, the
   devices SHOULD limit the time they wait for the TCP connection to a
   shorter timeout value, which will default to 30 seconds. However, the
   offerer MAY supply a different time in the timeout parameter of the
   "both" direction value. If the offerer supplies a value, the answerer
   SHOULD use that value for the TCP connection timeout, interpreted as
   an integer number of seconds.

   If the answerer chooses to host the session, it MUST perform the
   following steps:

   1.  Construct a new session URL . This MUST be a MSRP or MSRPS URL,
       MUST resolve to the answerer, and MUST not be the same as the
       session URL in the offer.  The URL SHOULD be temporary, SHOULD be
       hard to guess, and MUST not duplicate URLs currently identifying
       any active sessions hosted by the answerer.

   2.  Listen for a connection from the peer.

   3.  Construct an SDP answer as described in Section 6, mapping the
       new session URL to the session attribute, and inserting a
       direction attribute with the value of "passive".

   4.  Send the SDP offer using the normal processing for the signaling
       protocol.

   When the offerer receives the SDP answer, it must determine who will
   continue to host the session. If the answer contained a direction
   attribute value of "active", the offerer MUST continue as host. If
   the offer contained "active" or "both" and the answer contains
   "passive", then the offerer MUST allow the answerer to host the
   session.

   If the offerer chooses not to continue as host, it MUST perform the
   following steps:

   1.  Release resources it acquired in expectation of hosting the
       session, if any.

   2.  Determine the host address and port from the session URL of the
       answer, following the procedures in section Section 7.1

   3.  Connect to the host address and port, using the transport
       protocol from the M-line.




Campbell, et al.         Expires April 22, 2004                [Page 22]

Internet-Draft                    MSRP                      October 2003


   4.  Construct a VISIT request, which MUST contain the following
       information:

       1.  A S-URL header field containing the session URL.

       2.  A TR-ID header field containing a unique transaction ID.

       3.  A size field containing size of the message subsequent to the
           start-line.

   5.  Send the request and wait for a response

   6.  If the transaction succeeds, set the actual expiration time to
       the value in the Exp header field in the response, and
       acknowledge the answer via the signaling protocol. If either the
       connection attempt or the VISIT transaction fail, acknowledge the
       answer, then initiate the tear-down of the session using the
       signaling protocol.

7.4.2 Handling VISIT requests

   An MSRP endpoint that is hosting a session will receive a VISIT
   request from the visiting endpoint. When an endpoint receives a VISIT
   request, it MUST perform the following procedures:

   1.  Check if state exists for a session with a URL that matches the
       S-URL of the VISIT request. If so, and if no visitor connection
       has been associated with the session, then return a 200 response,
       and save state designating the connection on which the request
       was received as the visitor leg of the session.

   2.  If the session exists, and the visitor connection has already
       been established, return a 506 response and do not change session
       state in any way.

   3.  If no matching session exists, return a 481 request, and do not
       change session state in any way.

7.4.3 Sending Instant Messages on a Session

   Once a MSRP session has been established, either endpoint may send
   instant messages to its peer using the SEND method. When an endpoint
   wishes to do so, it MUST construct a SEND request according to the
   following process:

   1.  Insert the message payload in the body, and the media type in the
       Content-Type header field. The media type MUST match one of the
       types in the format list negotiated in the SDP exchange. If a "*"



Campbell, et al.         Expires April 22, 2004                [Page 23]

Internet-Draft                    MSRP                      October 2003


       was present in the accept-types attribute, then the media type
       SHOULD match one of the explicitly listed entries, but MAY be any
       other arbitrary value.

   2.  Set the TR-ID header field to a unique value.

   3.  Send the request on the connection associated with the session.

   4.  If a 2xx response code is received, the transaction was
       successful.

   5.  If a 5xx response code is received, the transaction failed, but
       other transactions may still succeed in the future. The endpoint
       MAY attempt to send the message content again in a new request,
       that is, with a new TR-ID value. If the endpoint receives 5xx
       responses more than some threshold number of times in a row, it
       SHOULD assume the session has failed, and initiate tear-down via
       the signaling protocol. The threshold value is a matter of local
       policy.

   6.  If a 415 response is received, this indicates the recipient is
       unable or unwilling to process the media type. The sender SHOULD
       NOT attempt to send that particular media type again in the
       context of this session.

   7.  If any other response code is received, the endpoint SHOULD
       assume the session has failed, and initiate tear-down.

      Normally transaction timeouts are treated the same as transactions
      that receive 5xx responses But, unlike transactions that fail
      explicitly, requests that have been timed out may in fact have
      been delivered to the peer endpoint, and even presented to the
      user. Attempting to resend such messages may result in the peer
      user seeing duplicate messages. Therefore a client implementation
      should take such action carefully, and should clearly indicate the
      situation to the user.

      Open Issue: Do we need to create a duplicate suppression
      mechanism? If retries were sent with with the TR-ID, then the
      recipient could recognize a duplicate message if it occurs in the
      same session.

   When an endpoint receives a SEND request, it MUST perform the
   following steps.

   1.  Determine that it understands the media type in the body, if any
       exists.




Campbell, et al.         Expires April 22, 2004                [Page 24]

Internet-Draft                    MSRP                      October 2003


   2.  If it does, return a 200 response and render the message to the
       user. The method of rendering is a matter of local policy.

   3.  If it does not understand the media type, return a 415 response.

7.4.4 Ending a Session

   When either endpoint in an MSRP session wishes to end the session, it
   first signals its intent using the normal processing for the
   signaling protocol. For example, in SIP, it would send a BYE request
   to the peer. After agreeing to end the session, the host endpoint
   MUST release any resources acquired as part of the session. The
   process for this differs depending on whether the session is hosted
   directly by the host, or by a relay.

   The host MUST destroy local state for the session. This involves
   completely removing the state entry for this session and invalidating
   session URL. If the host is using an MSRP relay, it MUST send a BIND
   containing an expires value of zero. This request MUST be sent on the
   host connection established by the original BIND request. This BIND
   request MUST include the session URL in the S-URL header field.

      Since these host actions completely destroy the session state at
      the hosting device, the visitor is not required to take further
      action beyond cleaning up any local state. If for some reason the
      host fails to destroy session state, the state will be invalidated
      anyway when the inactivity timer expires.

   When an endpoint chooses to close a session, it may have SEND
   transactions outstanding. For example, it may have send SEND requests
   to which it has not yet received a response, or it may have received
   SEND requests that to which it has not responded. Once an endpoint
   has decided to close the connection, it SHOULD wait for such
   outstanding transactions to complete. It SHOULD NOT generate any new
   SEND transactions, and it MAY choose not to respond to any new SEND
   requests that are received after it decides to close the session. It
   SHOULD not respond to any new messages that arrive after it signals
   its intent to close the session.

   When an endpoint is signaled of its peer's intent to close a session,
   it SHOULD NOT initiate any more SEND requests. It SHOULD wait for any
   outstanding transactions that it initiated to complete, and it SHOULD
   attempt respond to any open SEND transactions received prior to being
   signaled.

   It is not possible to completely eliminate the chance of a session
   terminating with incomplete SEND transactions. When this occurs, an
   endpoint SHOULD clearly inform the user that the messages mat not



Campbell, et al.         Expires April 22, 2004                [Page 25]

Internet-Draft                    MSRP                      October 2003


   have been delivered.

7.4.5 Session Inactivity Timer

   State associated with MSRP sessions, either at the host endpoint, or
   a hosting or visiting relay, is soft-state; that is, it expires over
   time if no message activity occurs. Each such device maintains a pair
   of inactivity timer, each with an initial value of 12 minutes. One of
   these timers is assigned for each endpoint.

      All devices use the same, predetermined timer expiration value.
      While there might be some utility in negotiating this timer on a
      per device basis, such negotiation would add a great deal of
      complexity to MSRP.  The choice of 12 minutes is somewhat
      arbitrary, but is intended to balance the bandwidth overhead
      against how quickly a relay can shed stale sessions. Since host
      endpoints will normally explicitly destroy sessions, stale
      sessions should only occur under failure conditions.


      Open Issue: In the 2 relay use case, the visitor does not
      explicitly remove state from the visiting relay. Rather, the
      visiting relay must infer that a session has been removed when the
      host device closes the connection, or when the inactivity timer
      expires.

   When a hosting device or visiting relay returns a successful response
   to a VISIT request, it MUST initialize both timers. The device MUST
   reset a timer anytime the associated endpoint sends a SEND request.
   If either timer expires without being reset, the device MUST
   invalidate the session, using normal procedures depending on the
   device's role in the session.

   Each endpoint MUST keep a similar timer, which it initializes when
   the session is created from its perspective. For the host endpoint,
   this is when it receives a successful response to a BIND request. For
   a visiting endpoint, this is when it sees a successful response to a
   VISIT request. Each endpoint resets its timer whenever it sends a
   SEND request.  If an endpoint inactivity timer approaches expiration,
   and the endpoint wishes to continue participating in the session, it
   MUST send a SEND request. This request MAY be sent without a body if
   there is no user data to send. Endpoints MUST select the timer value
   so that there is sufficient time for the SEND request to traverse to
   the opposite endpoint. If the endpoint waits to the last moment,
   there is a danger that it will not be received by all relevant
   devices in time to prevent session destruction.





Campbell, et al.         Expires April 22, 2004                [Page 26]

Internet-Draft                    MSRP                      October 2003


      Open Issue: There has been list discussion suggesting we should
      have a separate KEEPALIVE method for this purpose, rather than
      using SEND requests.

7.4.6 Managing Session State and Connections

   A MSRP session is represented by state at the host device. As mention
   previously, session state is identified by an MSRP URL. An active
   session also has two associated network connections. The connection
   between the hosting device and the host endpoint is known as the host
   connection. The connection with the visiting endpoint is the visiting
   connection.  Note that when the session state is hosted directly by
   an endpoint, the host connection may not involve a physical network
   connection; rather it is a logical connection the device maintains
   with itself.

   When session state is destroyed for any reason, the hosting device
   SHOULD drop the connection(s).

   If a connection fails for any reason, the session hosting device MUST
   invalidate the session state. This is true regardless of whether the
   dropped connection is the host or visiting connection. Once a
   connection is dropped, the associated session state MUST NOT be
   reused. If the endpoints wish to continue to communicate after a
   connection failure, they must initiate a new session. An endpoint
   detecting a connection failure SHOULD attempt to tear down the
   session using the rules of the signaling protocol.

      It would be nice to allow sessions to be recovered after a
      connection failure, perhaps by allowing the opposite endpoint to
      reconnect, and send a new VISIT or BIND request. However, this
      approach creates a race condition between the time that the
      hosting device notices the failed connection, and the time that
      the endpoint tries to recover the session. If the endpoint
      attempts to reconnect prior to the hosting device noticing the
      failure, the hosting device will interpret the recovery attempt as
      a conflict. The only way around this would be to force the hosting
      device to do a liveness check on the original connection, which
      would create a lot of complexity and overhead that do not seem to
      be worth the trouble.

7.5 MSRP Relays

   MSRP supports the use of message relays. This specification describes
   the use of one or two relays. While more than two relays are not
   forbidden by MSRP, a solution for an arbitary number of relays is
   beyond the scope of this document.




Campbell, et al.         Expires April 22, 2004                [Page 27]

Internet-Draft                    MSRP                      October 2003


7.5.1 Establishing Session State at a Relay

   An endpoint that wishes to host a MSRP session MAY do so by
   initiating session state at a MSRP relay, rather than hosting
   directly. An endpoint may wish to do this because network topology or
   local policy prevents a peer from connecting directly to the
   endpoint. The use of a relay should not be the default case, that is,
   a hosting endpoint that is not prevented from doing so by topology or
   policy SHOULD host the session directly. In order to use a relay, an
   MSRP endpoint MUST have knowledge of that relay's existence and
   location.

   We previously mentioned how an endpoint wishing to host a MSRP
   session constructs the session URL. When using a relay, the endpoint
   delegates that responsibility to the relay.

   To establish session state at a relay, the endpoint MUST perform the
   following steps:

   1.  Open a network connection to the relay at the relays address and
       port. Normally, this information will be resolved from an MSRP
       URL representing the relay, although the relay MAY be configured
       with an explicit address and port, rather than a URL.

   2.  Construct a BIND request with a S-URL that refers to the relay.

   3.  Set the Exp header field to a desired value.

   4.  Send the BIND request on the connection.

   5.  Respond to any authentication request from the relay.

   6.  If the response has a 2xx status code, use the URL in the S-URL
       header field as the session URL. The endpoint uses this URL in
       exactly the same manner as it had constructed it itself.
       Additionally, accept the expires value in the response as
       pre-visit expiration time.

   A MSRP relay listens for connections at all times. When it receives a
   BIND request, it SHOULD authenticate the request, either using
   digest-authentication, TLS authentication, or some other
   authentication mechanism. If authentication succeeds, the relay
   performs the following steps:

   1.  Verify the client is authorized to BIND to this relay. If not,
       return a 403 response and make no state change.





Campbell, et al.         Expires April 22, 2004                [Page 28]

Internet-Draft                    MSRP                      October 2003


   2.  If the client is authorized, construct a session MSRP URL. The
       URL MUST resolve to the relay. It SHOULD be temporary, and hard
       to guess. It MUST not duplicate any URL used in any active
       sessions hosted by the relay. If the relay wishes the visiting
       endpoint to connect over a port other than the MSRP relay
       well-know port, it MUST explicitly add the port number to visitor
       URL.

   3.  Establish the pre-visit expiration time for the session according
       to Section 7.4.5.

   4.  Create state for the session. The relay MUST associate the
       connection on which the BIND request arrived as the host
       connection for the session.

   5.  Return a 200 response, with the session URL in the S-URL header
       field, and the pre-visit session expiration time in the Exp
       header field.

   When an MSRP relay receives a VISIT request, it MUST perform the
   following steps:

   1.  Check the S-URL header field value to see it matches the URL for
       an existing session state entry.

   2.  If not, return a 481 response and make no state changes

   3.  If it matches, but another connection has already been associated
       with the session URL, return a 506 response and make no state
       changes. If the session has been previously associated with this
       connection, treat the request as a refresh.

   4.  If it matches, and no visiting connection has been previously
       associated with the session, then the VISIT succeeds. The relay
       assigns the connection on which it received the VISIT request as
       the visiting connection for the session, and returns a 200
       response.

7.5.2 Removing Session State from a relay

   An MSRP relay SHOULD remove state for a session when any of the
   following conditions occur:

   o  The session inactivity timer expires.

   o  The pre-visit timer expires before a VISIT request has occurred.





Campbell, et al.         Expires April 22, 2004                [Page 29]

Internet-Draft                    MSRP                      October 2003


   o  The host sends a BIND refresh request matching with an expiration
      value of zero.

   o  Either the host or visitor network connection fails for any
      reason.

7.5.3 Sending IMs across an MSRP relay

   Once a session is established at a relay, the host and visitor may
   exchange IMs by sending SEND requests. Under normal circumstances,
   the relay does not respond to SEND requests in any way. Rather, the
   relay MUST  forward the request to the peer connection unchanged.
   Likewise, if the relay receives a response it MUST forward the
   request unchanged on the peer connection.

   If a SEND request arrives on a connection that is not associated with
   a session, the relay MUST return a 481 response.

7.5.4 Relay Pairs

   In rare circumstances, two relays may be required in a session. For
   example, two endpoints may exist in separate administrative domains,
   where each domain's policy insist that all sessions must cross that
   domain's relay. A relay operating on behalf of the visiting endpoint
   is known as a visiting relay. An MSRP relay MAY be capable of acting
   as a visiting relay.

      This document does not describe a mechanism for an endpoint to
      discover that it needs to use a visiting relay. We assume that an
      endpoint is globally configured to use or not use such a relay,
      and does not make this decision on a session-by-session basis.
      This, of course, does not preclude using some other mechanism to
      make such a decision.

   In a two relay scenario, the visitor connects to a relay operating on
   its behalf, rather than connecting directly to the hosting device.
   The visitor sends a VISIT request as it would if it had connected
   directly to the hosting device. The visiting relay then connects to
   the hosting device and performs a VISIT request on behalf of the
   visitor.

   When a relay that is capable of acting as a visiting relay receives a
   VISIT request, it MUST check to see if the S-URL of the request
   matches a domain that the relay hosts. If the URL matches, then the
   visitor is not requesting the relay act as a visiting relay, and it
   SHOULD operate normally. If the URL does not match, then the relay
   SHOULD perform the following steps:




Campbell, et al.         Expires April 22, 2004                [Page 30]

Internet-Draft                    MSRP                      October 2003


   1.  The relay SHOULD authenticate the VISIT request, using digest
       authentication or some other mechanism.

   2.  Determine that the visiting endpoint is authorized to use this
       device as a visiting relay. If not, return a 403 response and
       drop the connection.

   3.  Attempt to open a connection to the hosting device, determining
       the address and port from the S-URL exactly as if it were a
       visiting endpoint connecting directly. If this connection is
       successful, continue with the remaining steps. Otherwise, return
       a 500 response.

   4.  Create local state to associate the connection to the host device
       with the connection to the visiting device.

   5.  Relay the VISIT request unchanged to the hosting device.

   6.  Relay the response to the VISIT request unchanged to the visiting
       endpoint.

   7.  Relay all subsequent requests arriving on one of the associated
       connections to the peer connection.

   If either associated connection fails for any reason, the visiting
   relay MUST invalidate the session state, and MUST drop the peer
   connection.

7.5.5 Relay Shutdown

   Relay administrators will occasionally need to take MSRP relays out
   of service. A relay implementation SHOULD allow a graceful shutdown
   that minimizes the occurrence of "lost", or timed out, messages. When
   a relay effects a graceful shutdown, it SHOULD refuse all new
   connection attempts, and refuse all MSRP requests, returning 481
   responses. In order to allow any open transactions a high chance of
   completion, the relay SHOULD wait at least one transaction timeout
   period (normally 30 seconds) between the time it starts refusing
   requests and the time it closes existing connections and shuts down.

      Open Issue: We have discussed that an endpoint implementation may
      attempt to establish a new session (perhaps using a different
      relay) with its peer. Do we wish to specify anything at all about
      such behavior?

7.6 Digest Authentication

   MSRP relays may use the digest authentication scheme to authenticate



Campbell, et al.         Expires April 22, 2004                [Page 31]

Internet-Draft                    MSRP                      October 2003


   users. MSRP digest authentication is a simplified version of HTTP
   digest authentication [19], but this specification does not
   normatively depend on that document. MSRP digest authentication does
   not support the concept of a protection domain, nor does it support
   integrity protection. Since a user of a relay is expected to have
   credentials for that particular relay, it does not support the realm
   concept. Finally, since digest authentication is only expected for
   the initial BIND or VISIT request, MSRP does not support HTTP digest
   optimizations such as MD5-sess and preemptive credential loading by
   the client.

   Typically, a hosting user that uses a relay will have a preexisting
   relationship with that relay. This relationship SHOULD include
   authentication credentials. An MSRP relay SHOULD authenticate initial
   BIND requests.

   It is less likely that the visiting user will have an account at the
   hosting relay, so in most cases the authentication of VISIT requests
   is not useful. However a relay MAY authenticate initial VISIT
   requests. A visiting relay SHOULD authenticate initial VISIT
   requests, as it is much more likely to share credentials with the
   visiting user.

      There has been some discussion that a hosting relay SHOULD also
      authenticate VISIT requests. However, it will be  common for
      visiting users to have no preexisting relationship with the host
      relay. Using authentication here would require the host endpoint
      to send temporary credentials in the SDP exchange, perhaps as part
      of the session URL. However, these temporary credentials would
      necessarily be transferred via the same channels as the session
      URL itself. If the credentials are sufficiently protected in
      transfer, then so is the session URL. Further, since the session
      URL is intended for a one time use, and is expected to be hard to
      guess, that URL itself should be sufficient for this purpose. Any
      situation where this is not adequate can be covered by the use of
      the MSRPS scheme.

   MSRP relays MUST NOT request authentication for any method other than
   BIND and VISIT.

   If a relay wishes to authenticate a request using digest
   authentication, it MAY challenge the request by responding with a
   401 response, which MUST include a SChal header field.

   If an endpoint wishes to respond to a digest authentication challenge
   received in a 401 response, it MAY do so by sending a new VISIT or
   BIND request, identical to the previous request, but with a CAuth
   header field containing the response to the challenge.



Campbell, et al.         Expires April 22, 2004                [Page 32]

Internet-Draft                    MSRP                      October 2003


7.6.1 The SHA1 Algorithm

   The only digest authentication algorithm defined in this
   specification is SHA1. [9] Other algorithms can be added as
   extensions. SHA1 is the default algorithm if no algorithm directive
   is present in the challenge. All MSRP devices MUST support SHA1.

      Open Issue: Do we need to specify how to offer more than one
      algorithm in a challenge? Do we need multiple algorithms possible
      for a particular challenge, or should we follow the HTTP digest
      approach of multiple challenges. It has been suggested that SHA1
      MUST always be offered, to ensure that the client and server will
      have at least one common algorithm.

   The SHA1 digest is defined as follows:

   Let KD(secret, data) denote the string obtained by  performing the
   digest algorithm to the data "data" with the secret "secret". Let
   H(data) denote the string obtained by performing the checksum
   algorithm on the data "data".

   For the "SHA1" algorithm, H(data) = SHA1(data), and KD(secret,data) =
   H(concat(secret, ":", data)

   Section 7.2 describes the syntax for the request-digest value in a
   CAuth header as 40 digits in lower case hexadecimal notation. The
   actual structure of the field is defined as follows. Note that
   unq(quoted-string) denotes the value of the string with the quotes
   removed.

       request-digest = <"> < KD ( H(A1), unq(nonce-value) ":" H(A2) ) > <">
       A1             = unq(username-value) ":" shared-secret ; "unq" denotes removal of quotes
       A2             = concat(Method,TR-ID,S-URI)

   When the relay receives a CAuth header, it SHOULD check its validity
   by looking up the shared secret, or H(A1), performing the same digest
   operation as performed by the client, and comparing the results to
   the request-digest value.

7.7 Method Descriptions

   This section summarizes the purpose of each MSRP method. All MSRP
   messages MUST contain the TR-ID header fields. All messages MUST
   contain a length field in the start line that indicates the overall
   length of the request, including any body, but not including the
   start line itself. Additional requirements exist depending on the
   individual method. Except where otherwise noted, all requests are hop
   by hop.



Campbell, et al.         Expires April 22, 2004                [Page 33]

Internet-Draft                    MSRP                      October 2003


7.7.1 BIND

   The BIND method is used by a host endpoint to establish or refresh
   session state at a hosting relay. BIND requests SHOULD be
   authenticated. BIND requests MUST contain the S-URL and  Exp header
   fields and MAY contain the CAuth header fields.

   A successful response to a BIND request MUST contain the S-URL and
   Exp header fields.

7.7.2 SEND

   The SEND method is used by both the host and visitor endpoints to
   send instant messages to its peer endpoint. SEND requests SHOULD
   contain a MIME body part. The body MUST be of a media type included
   in the format list negotiated in the SDP exchange. If a body is
   present, the request MUST contain a Content-Type header field
   identifying the media type of the body.

   Unlike other methods, SEND requests are end to end in nature. This
   means the request is consumed only by the opposite endpoint. Under
   normal conditions, any intervening relays merely forward the request
   on towards the peer endpoint.

7.7.3 VISIT

   The visiting endpoint uses the VISIT method to associate a network
   connection with the session state at the hosting device, which could
   be either the host endpoint or a relay operating on behalf of the
   host endpoint. The request MUST contain a S-URL header matching the
   session URL.

      There is normally no authentication operation for the VISIT
      request. This is because the session URL acts as a shared secret
      between host and the visitor. This puts certain requirements on
      the handling of the session URLs that are discussed in Section 10.
      However, if a visiting relay is used, it SHOULD authenticate VISIT
      requests.

7.8 Response Code Descriptions

   This section summarizes the various response codes. Except where
   noted, all responses MUST contain a TR-ID header field matching the
   TR-ID header field of the associated request. Responses are never
   consumed by relays.






Campbell, et al.         Expires April 22, 2004                [Page 34]

Internet-Draft                    MSRP                      October 2003


7.8.1 200

   The 200 response code indicates a successful transaction.

7.8.2 400

   A 400 response indicates a request was unintelligible.

7.8.3 401

   A 401 response indicates authentication is required. 401 responses
   MUST NOT be used in response to any method other than BIND and VISIT.
   A 401 response MUST contain a SChal header field.

7.8.4 403

   A 403 response indicates the user is not authorized to perform the
   action.

7.8.5 415

   A 415 response indicates the SEND request contained a MIME
   content-type that is not understood by the receiver.

7.8.6 426

   A 426 response indicates that the request is only allowed over TLS
   protected connections.


7.8.7 481

   A 481 response indicates that no session exists for the connection.

7.8.8 500

   A 500 response indicates that a relay was unable to deliver a request
   to the target.

7.8.9 506

   A 506 response indicates that a VISIT request occurred in which the
   S-URL indicates a session that is already associated with another
   connection. A 506 response MUST NOT be returned in response to any
   method other than VISIT.






Campbell, et al.         Expires April 22, 2004                [Page 35]

Internet-Draft                    MSRP                      October 2003


7.9 Header Field Descriptions

   This section summarizes the various header fields. MSRP header fields
   are single valued; that is, they MUST NOT occur more than once in a
   particular request or response.

7.9.1 TR-ID

   The TR-ID header field contains a transaction identifier used to map
   a response to the corresponding request. A TR-ID value MUST be unique
   among all values used by a given endpoint inside a given session.
   MSRP elements MUST NOT assume any additional semantics for TR-ID.

7.9.2 Exp

   The Exp header field specifies when the state associated with a BIND
   request will expire, if no successful VISIT request has been
   received. The value is specified as an integer number of seconds from
   the time the request is received. BIND requests MUST contain this
   header field. Furthermore, successful responses to BIND requests MUST
   also contain the Exp header.

   The maximum value for the Exp header field is (2**32)-1 seconds.

   Exp has no meaning if it occurs in MSRP messages other than BIND
   requests, and responses to those requests. MSRP compliant devices
   SHOULD NOT use Exp in other requests or responses, unless that usage
   is defined in an extension to this specification.

7.9.3 CAuth

   The CAuth header field is used by a host endpoint to offer digest
   authentication credentials to a relay, in response to a digest
   authentication challenge. CAuth SHOULD NOT be present in a request of
   any method other than BIND and VISIT.

   The syntax of the CAuth credentials is described in Section 7.2

   The meaning of each value is as follows:

   username: The user's account name.

   nonce: The nonce value copied from the challenge.

   response: A 32 hex digit string that proves user knowledge of the
      shared secret.





Campbell, et al.         Expires April 22, 2004                [Page 36]

Internet-Draft                    MSRP                      October 2003


   algorithm: The algorithm value copied from the challenge.

   auth-param: Additional parameters for the sake of extensibility.

7.9.4 SChal

   The SChal header field is used by a relay to carry the challenge in a
   digest authentication attempt. Exactly one SChal header field MUST
   exist in a 401 response. The SChal header MUST NOT be used in any
   message except for a 401 response. The syntax for the SChal challenge
   is described in Section 7.2

   The meaning of each value is as follows:

   digest scheme: A token to identify the particular authentication
      scheme. Since MSRP only supports digest, this value MUST be set to
      "Digest"

   nonce: A server-specified string, which the relay SHOULD uniquely
      generate each time it sends a 401 response. This string SHOULD
      take the form of base64 or hexadecimal data, to avoid the presence
      of a double-quote character, which is not allowed.

   algorithm: A token indicating the algorithms to be used to generate
      the digest and checksum. This directive exists for the sake of
      extensibility; the only value defined by this document is "SHA1".
      Absence of this directive indicates a value of "SHA1".

7.9.5 Content-Type

   The Content-Type header field is used to indicate the MIME media type
   of the body. Content-Type MUST be present if a body is present.

      Open Issue: We may need to clean up our MIME usage. This includes
      better defining the Content-Type usage possibly moving
      content-type into the body, indicating MIME version, etc.

7.9.6 S-URL

   The S-URL header field is used to identify the session. The S-URI
   header field MUST be present in a BIND request, a successful response
   to a BIND request, or a VISIT request.

8. Examples

   This section shows some example message flows for various common
   scenarios. The examples assume SIP is used to transport the SDP
   exchange. Details of the SIP messages and SIP proxy infrastructure



Campbell, et al.         Expires April 22, 2004                [Page 37]

Internet-Draft                    MSRP                      October 2003


   are omitted for the sake of brevity. In the examples, assume the
   offerer is sip:alice@atlanta.com and the answerer is
   sip:bob@biloxi.com. In any given MSRP message, an "xx" in the length
   field indicates the actual length of the rest of the message.

8.1 No Relay

   In this scenario, the session goes directly between endpoints with no
   MSRP relays involved.

           Alice                     Bob
             |                        |
             |                        |
             |(1) (SIP) INVITE        |
             |----------------------->|
             |(2) (MSRP) VISIT        |
             |<-----------------------|
             |(3) (MSRP) 200 OK       |
             |----------------------->|
             |(4) (SIP) 200 OK        |
             |<-----------------------|
             |(5) (SIP) ACK           |
             |----------------------->|
             |(6) (MSRP) SEND         |
             |----------------------->|
             |(7) (MSRP) 200 OK       |
             |<-----------------------|
             |(8) (MSRP) SEND         |
             |<-----------------------|
             |(9) (MSRP) 200 OK       |
             |----------------------->|
             |(10) (SIP) BYE          |
             |----------------------->|
             |(11) (SIP) 200 OK       |
             |<-----------------------|
             |                        |
             |                        |

   1.   Alice constructs a session URL of msrp://
        alicepc.atlanta.com:7777/iau39 and listens for a connection on
        TCP port 7777.

        Alice->Bob (SIP): INVITE sip:bob@biloxi.com

        c=IN IP4 fillername
        m=message 9999 msrp/tcp *
        a=accept-types:text/plain
        a=direction:both



Campbell, et al.         Expires April 22, 2004                [Page 38]

Internet-Draft                    MSRP                      October 2003


        a=session:msrp://alicepc.atlanta.com:7777/iau39

   2.   Bob opens a TCP connection to alicepc.atlanta.com:7777:

        Bob->Alice (MSRP):

        MSRP xx VISIT
        S-URL:msrp://alicepc.atlanta.com:7777/iau39
        Tr-ID: sie09s

   3.   Alice->Bob (MSRP):

        MSRP xx 200 OK
        Tr-ID: sie09s
        Exp:300

   4.   Bob->Alice (SIP): 200 OK

        c=IN IP4 ignorefield
        m=message 9999 msrp/tcp *
        a=accept-types:text/plain
        a=direction:active

   5.   Alice->Bob (SIP): ACK
   6.   Alice->Bob (MSRP):

        MSRP xx SEND
        TR-ID: 123
        Content-Type: "text/plain"
        Hi, I'm Alice!

   7.   Bob->Alice (MSRP):

        MSRP xx 200 OK
        TR-ID: 123

   8.   Bob->Alice (MSRP):

        MSRP xx SEND
        TR-ID: 456
        Content-Type: "text/plain"

        Hi, Alice! I'm Bob!

   9.   Alice->Bob (MSRP):

        MSRP xx 200 OK
        TR-ID: 456



Campbell, et al.         Expires April 22, 2004                [Page 39]

   10.  Alice->Bob (SIP): BYE

        Alice invalidates session and drops connection.

   11.  Bob invalidates local state for the session.

        Bob->Alice (SIP): 200 OK

8.2 Single Relay

   This scenario introduces an MSRP relay at relay.atlanta.com.

           Alice                    Relay                     Bob
             |                        |                        |
             |                        |                        |
             |(1) (MSRP) BIND         |                        |
             |----------------------->|                        |
             |(2) (MSRP) 200 OK       |                        |
             |<-----------------------|                        |
             |(3) (SIP) INVITE        |                        |
             |------------------------------------------------>|
             |                        |(4) (MSRP) VISIT        |
             |                        |<-----------------------|
             |                        |(5) (MSRP) 200 OK       |
             |                        |----------------------->|
             |(6) (SIP) 200 OK        |                        |
             |<------------------------------------------------|
             |(7) (SIP) ACK           |                        |
             |------------------------------------------------>|
             |(8) (MSRP) SEND         |                        |
             |----------------------->|                        |
             |                        |(9) (MSRP) SEND         |
             |                        |----------------------->|
             |                        |(10) (MSRP) 200 OK      |
             |                        |<-----------------------|
             |(11) (MSRP) 200 OK      |                        |
             |<-----------------------|                        |
             |                        |(12) (MSRP) SEND        |
             |                        |<-----------------------|
             |(13) (MSRP) SEND        |                        |
             |<-----------------------|                        |
             |(14) (MSRP) 200 OK      |                        |
             |----------------------->|                        |
             |                        |(15) (MSRP) 200 OK      |
             |                        |----------------------->|
             |(16) (SIP) BYE          |                        |
             |------------------------------------------------>|
             |(17) (MSRP) BIND        |                        |
             |----------------------->|                        |
             |(18) (MSRP) 200 OK      |                        |



Campbell, et al.         Expires April 22, 2004                [Page 40]

Internet-Draft                    MSRP                      October 2003


             |<-----------------------|                        |
             |(19) (SIP) 200 OK       |                        |
             |<------------------------------------------------|
             |                        |                        |
             |                        |                        |


   1.   Alice->Relay (MSRP): Alice opens a connection to the relay, and
        sends the following:

        MSRP xx BIND
        S-URL:msrp://relay.atlanta.com
        TR-ID: 321
        Exp:600

   2.   Relay->Alice (MSRP):

        MSRP xx 200 OK
        TR-ID: 321
        S-URL: msrp://relay.atlanta.com:7777/iau39
        Exp:300

   3.   Alice->Bob (SIP): INVITE sip:bob@biloxi.com

        c=IN IP4 dummyvalue
        m=message 9999 msrp/tcp *
        a=accept-types:text/plain
        a=direction:passive
        a=session:msrp://relay.atlanta.com:7777/iau39

   4.   Bob->Alice: Open connection to relay.atlanta.com:7777.

        Bob->Relay (MSRP):

        MSRP xx VISIT
        S-URL:msrp://relay.atlanta.com:7777/iau39
        TR-ID: sie09s

   5.   Relay->Bob (MSRP):

        MSRP xx 200 OK
        TR-ID: sie09s
        Exp:300

   6.   Bob->Alice (SIP): 200 OK

        c=IN IP4 nobodybutuschickens
        m=message 9999 msrp/tcp *



Campbell, et al.         Expires April 22, 2004                [Page 41]

Internet-Draft                    MSRP                      October 2003


        a=accept-types:text/plain
        a=direction:active

   7.   Alice->Bob (SIP): ACK
   8.   Alice->Relay (MSRP):

        MSRP xx SEND
        TR-ID: 123
        Content-Type: "text/plain"
        Hi, I'm Alice!

   9.   Relay->Bob (MSRP):

        MSRP xx SEND
        TR-ID: 123
        Content-Type: "text/plain"
        Hi, I'm Alice!

   10.  Bob->Relay (MSRP):

        MSRP xx 200 OK
        TR-ID: 123

   11.  Relay->Alice (MSRP):

        MSRP xx 200 OK
        TR-ID: 123

   12.  Bob->Relay (MSRP):

        MSRP xx SEND
        TR-ID: 456
        Content-Type:"text/plain"

        Hi, Alice! I'm Bob!

   13.  Relay->Alice (MSRP):

        MSRP xx SEND
        TR-ID: 456
        Content-Type: "text/plain"

        Hi, Alice! I'm Bob!

   14.  Alice->relay (MSRP):

        MSRP xx 200 OK
        TR-ID: 456



Campbell, et al.         Expires April 22, 2004                [Page 42]

   15.  Relay->Bob (MSRP):

        MSRP xx 200 OK
        TR-ID: 456

   16.  Alice->Bob (SIP): BYE

   17.  Alice->Relay (MSRP):

        MSRP xx  BIND
        S-URL: msrp://relay.atlanta.com:7777/iau39
        TR-ID: 42
        Exp:0

   18.  Relay->Alice (MSRP): Relay invalidates session state.

        MSRP xx 200 OK
        TR-ID: 42
        Exp:0

   19.  Bob invalidates local state for the session.

        Bob->Alice (SIP): 200 OK

8.3 Two Relays

   In this scenario, both Alice and Bob are each required by local
   policy to route all sessions through a different local relay.

           Alice      AtlantaRelay    BiloxiRelay        Bob
             |              |              |              |
             |              |              |              |
             |(1) (MSRP) BIND              |              |
             |------------->|              |              |
             |(2) (MSRP) 200 OK            |              |
             |<-------------|              |              |
             |(3) (SIP) INVITE             |              |
             |------------------------------------------->|
             |              |              |(4) (MSRP) VISIT
             |              |              |<-------------|
             |              |(5) (MSRP) VISIT             |
             |              |<-------------|              |
             |              |(6) (MSRP) 200 OK            |
             |              |------------->|              |
             |              |              |(7) (MSRP) 200 OK
             |              |              |------------->|
             |(8) (SIP) 200 OK             |              |
             |<-------------------------------------------|
             |(9) (SIP) ACK |              |              |
             |------------------------------------------->|



Campbell, et al.         Expires April 22, 2004                [Page 43]

Internet-Draft                    MSRP                      October 2003


             |(10) (MSRP) SEND             |              |
             |------------->|              |              |
             |              |(11) (MSRP) SEND             |
             |              |------------->|              |
             |              |              |(12) (MSRP) SEND
             |              |              |------------->|
             |              |              |(13) (MSRP) 200 OK
             |              |              |<-------------|
             |              |(14) (MSRP) 200 OK           |
             |              |<-------------|              |
             |(15) (MSRP) SEND             |              |
             |<-------------|              |              |
             |(16) (SIP) BYE|              |              |
             |------------------------------------------->|
             |(17) (MSRP) BIND             |              |
             |------------->|              |              |
             |(18) (MSRP) 200 OK           |              |
             |<-------------|              |              |
             |(19) (SIP) 200 OK            |              |
             |<-------------------------------------------|
             |              |              |              |
             |              |              |              |


   1.   Alice->AtlantaRelay (MSRP): Alice opens a connection to her
        relay, and sends the following:

        MSRP xx BIND
        S-URL: msrp://relay.atlanta.com
        TR-ID: 321
        Exp:600

   2.   AtlantaRelay->Alice (MSRP):

        MSRP xx 200 OK
        TR-ID: 321
        S-URL: msrp://relay.atlanta.com:7777/iau39
        Exp:600

   3.   Alice->Bob (SIP): INVITE sip:bob@biloxi.com

        c=IN IP4 blahblahblah
        m=message 9999 msrp/tcp *
        a=accept-types:text/plain
        a=session:msrp://relay.atlanta.com:7777/iau39
        a=direction:passive





Campbell, et al.         Expires April 22, 2004                [Page 44]

Internet-Draft                    MSRP                      October 2003


   4.   Bob determines that, due to local policy, he must connect
        through his own relay.

        Bob->BiloxiRelay (MSRP): Bob opens a connection to his relay,
        and sends the following:

        MSRP xx VISIT
        S-URL: msrp://relay.atlanta.com:7777/iau39
        TR-ID: 934

   5.   BiloxiRelay->AtlantaRelay (MSRP): BiloxiRelay resolves the URL,
        opens a connection to relay.atlanta.com:7777, and sends the
        following:

        MSRP xx VISIT
        S-URL: msrp://relay.atlanta.com:7777/iau39
        TR-ID: 934

   6.   AtlantaRelay->BiloxiRelay(MSRP):

        MSRP xx 200 OK
        TR-ID: 934

   7.   BiloxiRelay->Bob(MSRP):

        MSRP xx 200 OK
        TR-ID: 934

   8.   Bob->Alice (SIP): 200 OK

        c=IN IP4 stuff
        m=message 9999 msrp/tcp *
        a=accept-types:text/plain
        a=direction: active

   9.   Alice->Bob (SIP): ACK

   10.  Alice->AtlantaRelay (MSRP):

        MSRP xx SEND
        TR-ID: 123
        Content-Type: "text/plain"
        Hi, I'm Alice!

   11.  AtlantaRelay ->BiloxiRelay (MSRP):

        MSRP xx SEND
        TR-ID: 123



Campbell, et al.         Expires April 22, 2004                [Page 45]

Internet-Draft                    MSRP                      October 2003


        Content-Type: "text/plain"
        Hi, I'm Alice!

   12.  BiloxiRelay->Bob (MSRP):

        MSRP xx SEND
        TR-ID: 123
        Content-Type: "text/plain"
        Hi, I'm Alice!

   13.  Bob->BiloxiRelay (MSRP):

        MSRP xx 200 OK
        TR-ID: 123

   14.  BiloxiRelay->AtlantaRelay (MSRP):

        MSRP xx 200 OK
        TR-ID: 123

   15.  AtlantaRelay->Alice (MSRP):

        MSRP xx 200 OK
        TR-ID: 123

   16.  Alice->Bob (SIP): BYE

   17.  Alice->AtlantaRelay (MSRP):

        MSRP xx BIND
        S-URL: msrp://relay.atlanta.com:7777/iau39
        TR-ID: 42
        Exp:0

   18.  Relay->Alice (MSRP): Relay invalidates session state.

        MSRP xx 200 OK
        TR-ID: 42
        Exp:0

   19.  Bob->Alice (SIP): 200 OK

9. IANA Considerations

9.1 MSRP Port

   MSRP uses TCP port XYX, to be determined by IANA after this document
   is approved for publication. Usage of this value is described in



Campbell, et al.         Expires April 22, 2004                [Page 46]

Internet-Draft                    MSRP                      October 2003


   Section 7.1

9.2 MSRP URL Schemes

   This document defines the URL schemes of "msrp" and "msrps".

9.2.1 Syntax

   See Section 7.1.

9.2.2 Character Encoding

   See Section 7.1.

9.2.3 Intended Usage

   See Section 7.1.

9.2.4 Protocols

   The Message Session Relay Protocol (MSRP).

9.2.5 Security Considerations

   See Section 10.

9.2.6 Relevant Publications

   RFCXXXX

   [Note to RFC Editor: Please replace RFCXXXX in the above paragraph
   with the actual number assigned to this document.

9.3 SDP Parameters

   This document registers the following SDP parameters in the
   sdp-parameters registry:

9.3.1 Direction

   Attribute-name:  direction
   Long-form Attribute Name Direction
   Type: Media level
   Subject to Charset Attribute No
   Purpose and Appropriate Values See Section 6.2.






Campbell, et al.         Expires April 22, 2004                [Page 47]

Internet-Draft                    MSRP                      October 2003


9.3.2 Accept Types

   Attribute-name:  accept-types
   Long-form Attribute Name Acceptable MIME Types
   Type: Media level
   Subject to Charset Attribute No
   Purpose and Appropriate Values See Section 6.3.

9.3.3 Wrapped Types

   Attribute-name:  accept-wrapped-types
   Long-form Attribute Name Acceptable MIME Types Inside Wrappers
   Type: Media level
   Subject to Charset Attribute No
   Purpose and Appropriate Values See Section 6.4.

10. Security Considerations

   There are a number of security considerations for MSRP, some of which
   are mentioned elsewhere in this document. This section discusses
   those further, and introduces some new ones.

      Open Issue: There have been suggestions that we need more here
      covering the multiple authentication possibilities, MITM attack
      possibility on digest if not over TLS, and possible bid-down
      attacks on the digest algorithm selection.

10.1 TLS and the MSRPS Scheme

   All MSRP devices must support TLS, with at least the
   TLS_RSA_WITH_AES_128_CBC_SHA [8] cipher suite. Other cipher suites
   MAY be supported.

   MSRP does not define a separate TCP port for TLS connections. This
   means that all MSRP server devices, that is, all devices that listen
   for TCP connections, MUST be prepared to handle both TLS and plain
   text connections on the same port. When a device accepts a TCP
   connection, it MUST watch for the TLS handshake messages to determine
   if a particular connection uses TLS. If the first data received is
   not part of a start TLS request, the device ceases to watch for the
   TLS handshake until it reads the entire message. Once the message has
   been completely received, the device resumes watching for the start
   TLS message.

   An MSRP device MAY refuse to accept a given request over a non-TLS
   connection by returning a 426 response.

   MSRP devices acting in the role of TCP client MAY perform a TLS



Campbell, et al.         Expires April 22, 2004                [Page 48]

Internet-Draft                    MSRP                      October 2003


   handshake at any time, as long as the request occurs between MSRP
   messages. The endpoint MUST NOT send a start TLS request in the
   middle of an MSRP message.

      The working group considered only requiring the endpoint to watch
      for a TLS handshake at the beginning of the session. However, the
      endpoint should be able to determine if a new message is a start
      TLS request or an MSRP request by only reading ahead three bytes.
      Therefore, the working group chose to allow a session to switch to
      TLS in mid-stream, as long as the switch occurs between MRSP
      messages.

   The MSRPS URI scheme indicates that all hops in an MSRP session MUST
   be protected with TLS. Ensuring this implies some additional rules. A
   relay MUST return an MSRPS URL to a BIND request if the request
   arrived over TLS and included a MSRPS URI in the S-URI header field.
   The relay MAY return an MSRPS URI to any BIND request that arrives
   over TLS, but MUST NOT return an MSRP URI to a BIND request that does
   not arrive over TLS. If a relay receives a BIND request with an MSRPS
   S-URI, over a non-TLS connection, it MUST reject the request with a
   426 response. A relay may insist on always using MSRPS by returning a
   426 to any bind received over an unprotected connection, and always
   returning MSRPS URLs to BIND requests over protected connections.

   A VISIT request for an MSRPS URL MUST be sent over a TLS protected
   connection. If a visiting relay receives a VISIT request for an MSRPS
   URL over an unprotected connection, it MUST reject the request with a
   426 response.

10.2 Sensitivity of the Session URL

   The URL of a MSRP session is used by the visiting endpoint to
   identify itself to the hosting device, regardless of whether the
   session is directly hosted by the host endpoint, or is hosted by a
   relay. If an attacker were able to acquire the session URL, either by
   guessing it or by eavesdropping, there is a window of opportunity in
   which the attacker could hijack the session by sending a VISIT
   request to the host device before the true visiting endpoint. Because
   of this sensitivity, the session URL SHOULD be constructed in a way
   to make it difficult to guess, and should be sufficiently random so
   that it is unlikely to be reused. All mechanisms used to transport
   the session URL to the visitor and back to the host SHOULD be
   protected from eavesdroppers and man-in-the-middle attacks.

   Therefore an MSRP device MUST support the use of TLS for at least the
   VISIT request, which by extension indicates the endpoint MUST support
   the use of TLS for all MSRP messages. Further, MSRP connections
   SHOULD actually be protected with TLS. Further, an MSRP endpoint MUST



Campbell, et al.         Expires April 22, 2004                [Page 49]

Internet-Draft                    MSRP                      October 2003


   be capable of using the security features of the signaling protocol
   in order to protect the SDP exchange and SHOULD actually use them on
   all such exchanges. End-to-end protection schemes SHOULD be preferred
   over hop-by-hop schemes for protection of the SDP exchange.

10.3 End to End Protection of IMs

   Instant messages can contain very sensitive information. As a result,
   as specified in RFC 2779 [3], instant messaging protocols need to
   provide for encryption, integrity and authentication of instant
   messages. Therefore MSRP endpoints MUST support the end-to-end
   encryption and integrity of bodies sent via SEND requests using
   Secure MIME (S/MIME) [7].

   Note that while each protected body could use separate keying
   material, this is inefficient in that it requires an independent
   public key operation for each message. Endpoints wishing to invoke
   end-to-end protection of message sessions SHOULD exchange symmetric
   keys in SDP k-lines, and use secret key encryption on for each MSRP
   message. When symmetric keys are present in the SDP, the offer-answer
   exchange MUST be protected from eavesdropping and tampering using the
   appropriate facilities of the signaling protocol. For example, if the
   signaling protocol is SIP, the SDP exchange MUST be protected using
   S/MIME.

10.4 CPIM compatibility

   MSRP sessions may be gatewayed to other CPIM [17]compatible
   protocols. If this occurs, the gateway MUST maintain session state,
   and MUST translate between the MSRP session semantics and CPIM
   semantics that do not include a concept of sessions. Furthermore,
   when one endpoint of the session is a CPIM gateway, instant messages
   SHOULD be wrapped in "message/cpim" [5] bodies. Such a gateway MUST
   include "message/cpim" as the first entry in its SDP accept-types
   attribute. MSRP endpoints sending instant messages to a peer that has
   included 'message/cpim" as the first entry in the accept-types
   attribute SHOULD encapsulate all instant message bodies in "message/
   cpim" wrappers. All MSRP endpoints MUST support the message/cpim
   type, and SHOULD support the S/MIME features of that format.

10.5 PKI Considerations

   Several aspects of MSRP will benefit from being used in the context
   of a public key infrastructure. For example, the MSRPS scheme allows,
   and even encourages, TLS connections between endpoint devices.  And
   while MSRP allows for a symmetric session key to protect all messages
   in a session, it is most likely that session key itself would be
   exchanged in a signaling protocol such as SIP. Since that key is



Campbell, et al.         Expires April 22, 2004                [Page 50]

Internet-Draft                    MSRP                      October 2003


   extremely sensitive, its exchange would also need to be protected. In
   SIP, the preferred mechanism for this would be S/MIME, which would
   also benefit from a PKI.

   However, all of these features may be used without PKI. Each endpoint
   could instead use self signed certificates. This will, of course, be
   less convenient than with a PKI, in that there would be no
   certificate authority to act as a trusted introducer. Peers would be
   required to exchange certificates prior to securely communicating.

   Since, at least for the immediate future, any given MSRP
   implementation is likely to communicate with at least some peers that
   do not have a PKI available, MSRP implementations SHOULD support the
   use of self-signed certificates, and SHOULD support the ability to
   configure lists of trusted certificates.

11. Changes from Previous Draft Versions

   This section to be deleted prior to publication as an RFC

11.1 draft-ietf-simple-message-sessions-02

      Moved all content type negotiation from the "m"-line format list
      into "a"-line attributes. Added the accept-types attribute. This
      is due to the fact that the sdp format-list syntax is not
      conducive to encoding MIME content types values.
      Added "other-method" construction to the message syntax to allow
      for extensible methods.
      Consolidated all syntax definitions into the same section. Cleaned
      up ABNF for digest challenge and response syntax.
      Changed the session inactivity timeout to 12 minutes.
      Required support for the SHA1 alogorithm.
      Required support for the message/cpim format.
      Fixed lots of editorial issues.
      Documented a number of open issues from recent list discussions.

11.2 draft-ietf-simple-message-sessions-01

      Abstract rewritten.
      Added architectural considerations section.
      The m-line format list now only describes the root body part for a
      request. Contained body part types may be described in the
      "accept-wrapped-types" a-line attribute.
      Added a standard dummy value for the m-line port field. Clarified
      that a zero in this field has normal SDP meaning.
      Clarified that an endpoint is globally configured as to whether or
      not to use a relay. There is no relay discovery mechanism
      intrinsic to MSRP.



Campbell, et al.         Expires April 22, 2004                [Page 51]

Internet-Draft                    MSRP                      October 2003


      Changed digest algorithm to SHA1. Added TR-ID and S-URI to the
      hash for digest authentication.
      CMS usage replaced with S/MIME.
      TLS and MSRPS usage clarified.
      Session state timeout is now based on SEND activity, rather than
      BIND and VISIT refreshes.
      Default port added.
      Added sequence diagrams to the example message flows.
      Added discussion of self-signed certificates in the security
      considerations section.

11.3 draft-ietf-simple-message-sessions-00

      Name changed to reflect status as a work group item.
      This version no longer supports the use of multiple sessions
      across a single TCP session. This has several related changes:
      There is now a single session URI, rather than a separate one for
      each endpoint. The session URI is not required to be in requests
      other than BIND and VISIT, as the session can be determined based
      on the connection on which it arrives.
      BIND and VISIT now create soft state, eliminating the need for the
      RELEASE and LEAVE methods.
      The MSRP URL format was changed to better reflect generic URL
      standards. URL comparison and resolution rules were added. SRV
      usage added.
      Determination of host and visitor roles now uses a direction
      attribute much like the one used in COMEDIA.
      Format list negotiation expanded to allow a "prefer these formats
      but try anything" semantic
      Clarified handling of direction notification failures.
      Clarified signaling associated with session failure due to dropped
      connections.
      Clarified security related motivations for MSRP.
      Removed MIKEY dependency for session key exchange. Simple usage of
      k-lines in SDP, where the SDP exchange is protected end-to-end
      seems sufficient.

11.4 draft-campbell-simple-im-sessions-01

   Version 01 is a significant re-write. References to COMEDIA were
   removed, as it was determined that COMEDIA would not allow
   connections to be used bidirectional in the presence of NATs.
   Significantly more discussion of a concrete mechanism has been added
   to make up for no longer using COMEDIA. Additionally, this draft and
   draft-campbell-cpimmsg-sessions (which would have also changed
   drastically) have now been combined into this single draft.





Campbell, et al.         Expires April 22, 2004                [Page 52]

Internet-Draft                    MSRP                      October 2003


12. Contributors

   The following people contributed substantially to this ongoing
   effort:

                                Rohan Mahy
                                Allison Mankin
                                Jon Peterson
                                Brian Rosen
                                Dean Willis
                                Adam Roach
                                Cullen Jennings
                                Aki Niemi
                                Hisham Khartabil
                                Pekka Pessi
                                Chris Boulton

Normative References

   [1]  Handley, M. and V. Jacobson, "SDP: Session Description
        Protocol", RFC 2327, April 1998.

   [2]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
        Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
        Session Initiation Protocol", RFC 3261, June 2002.

   [3]  Day, M., Aggarwal, S. and J. Vincent, "Instant Messaging /
        Presence Protocol Requirements", RFC 2779, February 2000.

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

   [5]  Atkins, D. and G. Klyne, "Common Presence and Instant Messaging
        Message Format", draft-ietf-impp-cpim-msgfmt-08 (work in
        progress), January 2003.

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

   [7]  Ramsdell, B., "S/MIME Version 3 Message Specification", RFC
        2633, June 1999.

   [8]  Chown, P., ""Advanced Encryption Standard (AES) Ciphersuites for
        Transport Layer Security (TLS)", RFC 3268, June 2002.

   [9]  Eastlake, 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
        (SHA1)", RFC 3174, September 2001.



Campbell, et al.         Expires April 22, 2004                [Page 53]

Internet-Draft                    MSRP                      October 2003


Informational References

   [10]  Campbell, B. and J. Rosenberg, "Session Initiation Protocol
         Extension for Instant Messaging", RFC 3428, September 2002.

   [11]  Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
         "RTP: A Transport Protocol for Real-Time Applications", RFC
         1889, January 1996.

   [12]  Mahy, R., Campbell, B., Sparks, R., Rosenberg, J., Petrie, D.
         and A. Johnston, "A Multi-party Application Framework for SIP",
         draft-ietf-sipping-cc-framework-02 (work in progress), May
         2003.

   [13]  Rosenberg, J., Peterson, J., Schulzrinne, H. and G. Camarillo,
         "Best Current Practices for Third Party Call Control in the
         Session Initiation Protocol", draft-ietf-sipping-3pcc-04 (work
         in progress), June 2003.

   [14]  Sparks, R. and A. Johnston, "Session Initiation Protocol Call
         Control - Transfer", draft-ietf-sipping-cc-transfer-01 (work in
         progress), February 2003.

   [15]  Camarillo, G., Marshall, W. and J. Rosenberg, "Integration of
         Resource Management and Session Initiation Protocol (SIP)", RFC
         3312, October 2002.

   [16]  Peterson, J., "A Privacy Mechanism for the Session Initiation
         Protocol (SIP)", RFC 3323 , November 2002.

   [17]  Peterson, J., "A Common Profile for Instant Messaging (CPIM)",
         draft-ietf-impp-im-04 (work in progress), August 2003.

   [18]  Yon, D., "Connection-Oriented Media Transport in SDP",
         draft-ietf-mmusic-sdp-comedia-05 (work in progress), March
         2003.

   [19]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
         Leach, P., Luotonen, A. and L. Stewart, "HTTP Authentication:
         Basic and Digest Access Authentication", RFC 2617, June 1999.











Campbell, et al.         Expires April 22, 2004                [Page 54]

Internet-Draft                    MSRP                      October 2003


Authors' Addresses

   Ben Campbell
   dynamicsoft
   5100 Tennyson Parkway
   Suite 1200
   Plano, TX  75024

   EMail: bcampbell@dynamicsoft.com


   Jonathan Rosenberg
   dynamicsoft
   600 Lanidex Plaza
   Parsippany, NJ  07054

   EMail: jdrosen@dynamicsoft.com


   Robert Sparks
   dynamicsoft
   5100 Tennyson Parkway
   Suite 1200
   Plano, TX  75024

   EMail: rsparks@dynamicsoft.com


   Paul Kyzivat
   Cisco Systems
   Mail Stop LWL3/12/2
   900 Chelmsford St.
   Lowell, MA  01851

   EMail: pkyzivat@cisco.com
















Campbell, et al.         Expires April 22, 2004                [Page 55]

Internet-Draft                    MSRP                      October 2003


Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights. Information on the
   IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11. Copies of
   claims of rights made available for publication and any assurances of
   licenses to be made available, or the result of an attempt made to
   obtain a general license or permission for the use of such
   proprietary rights by implementors or users of this specification can
   be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard. Please address the information to the IETF Executive
   Director.


Full Copyright Statement

   Copyright (C) The Internet Society (2003). All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assignees.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION



Campbell, et al.         Expires April 22, 2004                [Page 56]

Internet-Draft                    MSRP                      October 2003


   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Acknowledgment

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











































Campbell, et al.         Expires April 22, 2004                [Page 57]


Html markup produced by rfcmarkup 1.109, available from https://tools.ietf.org/tools/rfcmarkup/