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SIMPLE WG                                               B. Campbell, Ed.
Internet-Draft                                          Estacado Systems
Expires: April 24, 2005                                     R. Mahy, Ed.
                                                               Airespace
                                                        C. Jennings, Ed.
                                                     Cisco Systems, Inc.
                                                        October 24, 2004


                   The Message Session Relay Protocol
               draft-ietf-simple-message-sessions-09.txt

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on April 24, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

   This document describes the Message Session Relay Protocol (MSRP), a
   protocol for transmitting a series of related instant messages in the
   context of a session.  Message sessions are treated like any other
   media stream when setup via a rendezvous or session setup protocol



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   such as the Session Initiation Protocol (SIP).

Table of Contents

   1.   Conventions  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.   Introduction and Background  . . . . . . . . . . . . . . . .   4
   3.   Applicability of MSRP  . . . . . . . . . . . . . . . . . . .   5
   4.   Protocol Overview  . . . . . . . . . . . . . . . . . . . . .   5
   5.   Key Concepts . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1  MSRP Framing and Message Chunking  . . . . . . . . . . . .   8
     5.2  MSRP Addressing  . . . . . . . . . . . . . . . . . . . . .   9
     5.3  MSRP Transaction and Report Model  . . . . . . . . . . . .   9
     5.4  MSRP Connection Model  . . . . . . . . . . . . . . . . . .  10
   6.   MSRP URLs  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     6.1  MSRP URL Comparison  . . . . . . . . . . . . . . . . . . .  13
     6.2  Resolving MSRP Host Device . . . . . . . . . . . . . . . .  14
   7.   Method-Specific Behavior . . . . . . . . . . . . . . . . . .  14
     7.1  Constructing Requests  . . . . . . . . . . . . . . . . . .  14
       7.1.1  Delivering SEND requests . . . . . . . . . . . . . . .  15
       7.1.2  Sending REPORT requests  . . . . . . . . . . . . . . .  17
       7.1.3  Failure REPORT Generation  . . . . . . . . . . . . . .  18
     7.2  Constructing Responses . . . . . . . . . . . . . . . . . .  19
     7.3  Receiving Requests . . . . . . . . . . . . . . . . . . . .  20
       7.3.1  Receiving SEND requests  . . . . . . . . . . . . . . .  20
       7.3.2  Receiving REPORT requests  . . . . . . . . . . . . . .  22
   8.   Using MSRP with SIP  . . . . . . . . . . . . . . . . . . . .  22
     8.1  SDP Offer-Answer Exchanges for MSRP Sessions . . . . . . .  22
       8.1.1  URL Negotiations . . . . . . . . . . . . . . . . . . .  25
       8.1.2  Path Attributes with Multiple URLs . . . . . . . . . .  26
       8.1.3  Updated SDP Offers . . . . . . . . . . . . . . . . . .  27
       8.1.4  Example SDP Exchange . . . . . . . . . . . . . . . . .  27
       8.1.5  Connection Negotiation . . . . . . . . . . . . . . . .  28
     8.2  MSRP User Experience with SIP  . . . . . . . . . . . . . .  28
   9.   Formal Syntax  . . . . . . . . . . . . . . . . . . . . . . .  28
   10.  Response Code Descriptions . . . . . . . . . . . . . . . . .  31
     10.1   200  . . . . . . . . . . . . . . . . . . . . . . . . . .  31
     10.2   400  . . . . . . . . . . . . . . . . . . . . . . . . . .  31
     10.3   403  . . . . . . . . . . . . . . . . . . . . . . . . . .  31
     10.4   415  . . . . . . . . . . . . . . . . . . . . . . . . . .  31
     10.5   426  . . . . . . . . . . . . . . . . . . . . . . . . . .  31
     10.6   481  . . . . . . . . . . . . . . . . . . . . . . . . . .  31
     10.7   501  . . . . . . . . . . . . . . . . . . . . . . . . . .  32
     10.8   506  . . . . . . . . . . . . . . . . . . . . . . . . . .  32
   11.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . .  32
     11.1   Basic IM session . . . . . . . . . . . . . . . . . . . .  32
     11.2   Message with XHTML Content . . . . . . . . . . . . . . .  34
     11.3   Chunked Message  . . . . . . . . . . . . . . . . . . . .  35
     11.4   System Message . . . . . . . . . . . . . . . . . . . . .  35



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     11.5   Positive Report  . . . . . . . . . . . . . . . . . . . .  36
     11.6   Forked IM  . . . . . . . . . . . . . . . . . . . . . . .  36
   12.  Extensibility  . . . . . . . . . . . . . . . . . . . . . . .  39
   13.  CPIM compatibility . . . . . . . . . . . . . . . . . . . . .  39
   14.  Security Considerations  . . . . . . . . . . . . . . . . . .  40
   15.  IANA Considerations  . . . . . . . . . . . . . . . . . . . .  44
     15.1   MSRP Port  . . . . . . . . . . . . . . . . . . . . . . .  44
     15.2   MSRP URL Schemes . . . . . . . . . . . . . . . . . . . .  44
     15.3   SDP Parameters . . . . . . . . . . . . . . . . . . . . .  44
       15.3.1   Accept Types . . . . . . . . . . . . . . . . . . . .  44
       15.3.2   Wrapped Types  . . . . . . . . . . . . . . . . . . .  44
       15.3.3   Max Size . . . . . . . . . . . . . . . . . . . . . .  44
       15.3.4   Path . . . . . . . . . . . . . . . . . . . . . . . .  45
   16.  Change History . . . . . . . . . . . . . . . . . . . . . . .  45
     16.1   draft-ietf-simple-message-sessions-09  . . . . . . . . .  45
     16.2   draft-ietf-simple-message-sessions-08  . . . . . . . . .  45
     16.3   draft-ietf-simple-message-sessions-07  . . . . . . . . .  46
     16.4   draft-ietf-simple-message-sessions-06  . . . . . . . . .  46
     16.5   draft-ietf-simple-message-sessions-05  . . . . . . . . .  47
     16.6   draft-ietf-simple-message-sessions-04  . . . . . . . . .  47
     16.7   draft-ietf-simple-message-sessions-03  . . . . . . . . .  47
     16.8   draft-ietf-simple-message-sessions-02  . . . . . . . . .  48
     16.9   draft-ietf-simple-message-sessions-01  . . . . . . . . .  48
     16.10  draft-ietf-simple-message-sessions-00  . . . . . . . . .  49
     16.11  draft-campbell-simple-im-sessions-01 . . . . . . . . . .  49
   17.  Contributors and Acknowledgments . . . . . . . . . . . . . .  49
   18.  References . . . . . . . . . . . . . . . . . . . . . . . . .  50
   18.1   Normative References . . . . . . . . . . . . . . . . . . .  50
   18.2   Informational References . . . . . . . . . . . . . . . . .  51
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  52
        Intellectual Property and Copyright Statements . . . . . . .  53




















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1.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC-2119 [5].

   This document consistently refers to a "message" as a complete unit
   of MIME or text content.  In some cases a message is split and
   delivered in more than one MSRP request.  Each of these portions of
   the complete message is called a "chunk".

2.  Introduction and Background

   A series of related instant messages between two or more parties can
   be viewed as part of a "message session", that is, an conversational
   exchange of messages with a definite beginning and end.  This is in
   contrast to individual messages each sent completely independently.
   The SIMPLE Working Group describes messaging schemes that only track
   individual messages as "page-mode" messages, whereas messaging that
   is part of a "session" with a definite start and end is called
   session-mode messaging.

   Page-mode messaging is enabled in SIMPLE via the SIP [4]MESSAGE
   method [18].  Session-mode messaging has a number of benefits [19]
   over page-mode messaging however, such as explicit rendezvous,
   tighter integration with other media types, direct client-to-client
   operation, and brokered privacy and security.

   This document defines a session-oriented instant message transport
   protocol called the Message Session Relay Protocol (MSRP), whose
   sessions can be included in an offer or answer [3] using the Session
   Description Protocol(SDP [2]).  The exchange is carried by some
   signaling protocol, such as the Session Initiation Protocol (SIP
   [4]).  This allows a communication user agent to offer a messaging
   session as one of the possible media types in a session.  For
   instance, Alice may want to communicate with Bob.  Alice doesn't know
   at the moment whether Bob has his phone or his IM client handy, but
   she's willing to use either.  She sends an invitation to a session to
   the address of record she has for Bob, sip:bob@example.com.  Her
   invitation offers both voice and an IM session.  The SIP services at
   example.com forward the invitation to Bob at his currently registered
   clients.  Bob accepts the invitation at his IM client and they begin
   a threaded chat conversation.

   This session model allows message sessions to be integrated into
   advanced communications applications with little to no additional
   protocol development.  For example, during the above chat session,
   Bob decides Alice really needs to be talking to Carol.  Bob can



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   transfer [17] Alice to Carol, introducing them into their own
   messaging session.  Messaging sessions can then be easily integrated
   into call-center and dispatch environments utilizing third-party call
   control [16] and conferencing [15] applications.

3.  Applicability of MSRP

   MSRP is not designed for use as a standalone protocol.  MSRP MUST
   only be used in the context of a rendezvous mechanism meeting the
   following requirements:

      The rendezvous mechanism MUST provide both MSRP URLs associated
      with an MSRP session to each of the participating endpoints.  The
      rendezvous mechanism MUST implement mechanisms to provide these
      URLs securely - they MUST NOT be made available to an untrusted
      third party or be easily discoverable.

      The rendezvous mechanism MUST provide mechanisms for the
      negotiation of any supported MSRP extensions that are not
      backwards compatible.

      The rendezvous mechanism MUST be able to natively transport im:
      URIs or automatically translate  im: URIs [24] into the addressing
      identifiers of the rendezvous protocol.

   To use a rendezvous mechanism with MSRP, an RFC must be prepared
   describing how it exchanges MSRP URIs and meets these requirements
   listed here.  This document provides such a description for the use
   of MSRP in the context of SIP and SDP.

   SIP meets these requirements for a rendezvous mechanism.  The MSRP
   URLs are exchanged using SDP in an offer/answer exchange via SIP.
   The exchanged SDP can also be used to negotiate MSRP extensions.
   This SDP can be secured using any of the mechanisms available in SIP,
   including using the sips mechanism to ensure transport security
   across intermediaries and S/MIME for end-to-end protection of the SDP
   entity.  SIP can carry arbitrary URIs (including im: URIs) in the
   Request-URI, and procedures are available to map im: URIs to sip: or
   sips: URIs.  It is expected that initial deployments of MSRP will use
   SIP as its rendezvous mechanism.

4.  Protocol Overview

   MSRP is a text-based, connection-oriented protocol for exchanging
   arbitrary (binary) MIME content, especially instant messages.  This
   section is a non-normative overview of how MSRP works and how it is
   used with SIP.




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   MSRP sessions are typically arranged using SIP the same way a session
   of audio or video media is setup.  One SIP user agent (Alice) sends
   the other (Bob) a SIP invitation containing an offer
   session-description which includes a session of MSRP.  The receiving
   SIP user agent can accept the invitation and include an answer
   session-description which acknowledges the choice of media.  Alice's
   session description contains an MSRP URL that describes where she is
   willing to receive MSRP requests from Bob, and vice-versa.  (Note:
   Some lines in the examples are removed for clarity and brevity.)

       Alice sends to Bob:

   INVITE sip:alice@atlanta.example.com SIP/2.0
   To: <sip:bob@biloxi.example.com>
   From: <sip:alice@atlanta.example.com>;tag=786
   Call-ID: 3413an89KU
   Content-Type: application/sdp

   c=IN IP4 10.1.1.1
   m=message 9 msrp *
   a=accept-types:text/plain
   a=path:msrp://atlanta.example.com:7654/jshA7we;tcp

       Bob sends to Alice:

   SIP/2.0 200 OK
   To: <sip:bob@biloxi.example.com>;tag=087js
   From: <sip:alice@atlanta.example.com>;tag=786
   Call-ID: 3413an89KU
   Content-Type: application/sdp

   c=IN IP4 10.2.2.2
   m=message 9 msrp *
   a=accept-types:text/plain
   a=path:msrp://biloxi.example.com:12763/kjhd37s2s2;tcp

       Alice sends to Bob:

   ACK sip:alice@atlanta.example.com SIP/2.0
   To: <sip:bob@biloxi.example.com>;tag=087js
   From: <sip:alice@atlanta.example.com>;tag=786
   Call-ID: 3413an89KU

   MSRP defines two request types, or methods.  SEND requests are used
   to deliver a complete message or a chunk (a portion of a complete
   message), while REPORT requests report on the status of an earlier
   SEND request.  When Alice receives Bob's answer, she checks to see if
   she has an existing connection to Bob.  If not, she opens a new



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   connection to Bob using the URL he provided in the SDP.  Alice then
   delivers a SEND request to Bob with her initial message, and Bob
   replies indicating that Alice's request was received successfully.

   MSRP a786hjs2 SEND
   To-Path: msrp://biloxi.example.com:12763/kjhd37s2s2;tcp
   From-Path: msrp://atlanta.example.com:7654/jshA7we;tcp
   Message-ID: 87652
   Content-Type: text/plain

   Hey Bob, are you there?
   -------a786hjs2$

   MSRP a786hjs2 200 OK
   To-Path: msrp://atlanta.example.com:7654/jshA7we;tcp
   From-Path: msrp://biloxi.example.com:12763/kjhd37s2s2;tcp
   Message-ID: 87652
   -------a786hjs2$


   Alice's request begins with the MSRP start line, which contains a
   transaction identifier that is also used as a final boundary marker.
   Next she includes the path of URLs to the destination in the To-Path
   header, and her own URL in the From-Path header.  In this typical
   case there is just one "hop", so there is only one URL in each path
   header field.  She also includes a message ID which she can use to
   correlate responses and status reports with the original message.
   Next she puts the actual content.  Finally she closes the request
   with an end line: seven hyphens, the transaction identifier /
   boundary marker and a "$" to indicate this request contains the end
   of a complete message.

   If Alice wants to deliver a very large message, she can split the
   message into chunks and deliver each chunk in a separate SEND
   request.  The message ID corresponds to the whole message, so the
   receiver can also use it to reassemble the message and tell which
   chunks belong with which message.  Chunking is described in more
   detail in Section 5.1.

   Alice can also specify what type of reporting she would like in
   response to her request.  If Alice requests positive acknowledgments,
   Bob sends a REPORT request to Alice confirming the delivery of her
   complete message.  This is especially useful if Alice sent a series
   of SEND request containing chunks of a single message.  More on
   requesting types of reports and errors is described in Section 5.3.

   Alice and Bob generally choose their MSRP URLs in such a way that is
   difficult to guess the exact URL.  Alice and Bob can reject requests



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   to URLs they are not expecting to service, and can correlate the
   specific URL with the probable sender.  Alice and Bob can also use
   TLS [1] to provide channel security over this hop.  To receive MSRP
   requests over a TLS protected connection, Alice or Bob could
   advertise URLs with the "msrps" scheme instead of "msrp."

   This document specifies MSRP behavior only peer-to-peer sessions,
   that is, sessions crossing only a single hop.  However, work to
   specify behavior for MSRP relay devices [20] (referred to herein as
   "relays") is occuring as a separate effort.  MSRP is designed with
   the expectation that MSRP can carry URLs for nodes on the far side of
   such relays.  For this reason, a URL with the "msrps" scheme makes no
   assertion about the security properties of other hops, just the next
   hop.  The user agent knows the URL for each hop, so it can verify
   that each URL has the desired security properties.

   MSRP URLs are discussed in more detail in Section 6.

   An adjacent pair of busy MSRP nodes (for example two relays) can
   easily have several sessions, and exchange traffic for several
   simultaneous users.  The nodes can use existing connections to carry
   new traffic with the same destination host, port, transport protocol,
   and scheme.  MSRP nodes can keep track of how many sessions are using
   a particular connection and close these connections when no sessions
   have used them for some period of time.  Connection management is
   discussed in more detail in Section 5.4.

5.  Key Concepts

5.1  MSRP Framing and Message Chunking

   Messages sent using MSRP can be very large and can be delivered in
   several SEND requests, where each SEND request contains one chunk of
   the overall message.  Long chunks may be interruped to ensure
   fairness across shared transport connections.  To support this, MSRP
   uses a boundary based framing mechanism.  The start line of an MSRP
   request contains a unique boundary string that is used to indicate
   the end of the request.  Following the boundary string at the end of
   the body data, there is a flag that indicates whether this is the
   last chunk of data for this message or whether the message will be
   continued in a subsequent chunk.  There is also a Byte-Range header
   in the request that indicates the overall position of this chunk
   inside the complete message.

   For example, the following snippet of two SEND requests demonstrates
   a message that contains the text "abcdEFGH" being sent as two chunks.





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    MSRP dkei38sd SEND
    Message-ID: 456
    Byte-Range: 1-4/8
    Content-Type: text/plain

    abcd
    -------dkei38sd+

    MSRP dkei38ia SEND
    Message-ID: 456
    Byte-Range: 5-8/8
    Content-Type: text/plain

    EFGH
    -------dkei38ia$

   This chunking mechanism allows a sender to interrupt a chunk part way
   through sending it.  The ability to interrupt messages allows
   multiple sessions to share a TCP connection, and for large messages
   to be sent efficiently while not blocking other messages that share
   the same connection.

   The ability to interrupt messages is needed so that TCP connections
   can be shared.  Connection sharing is necessary for "fair" allocation
   of bandwidth in congestion situations and for allowing MSRP network
   elements that have a very large number of concurrent connections to
   different users.

5.2  MSRP Addressing

   MSRP entities are addressed using URLs.  The MSRP URL schemes are
   defined in Section 6.  The syntax of the To-Path and From-Path
   headers each allow for a list of URLs.  This was done to allow the
   protocol to work with gateways or relays defined in the future, to
   provide a complete path to the end recipient.  When two MSRP nodes
   communicate directly they need only one URL in the To-Path list and
   one URL in the From-Path list.

5.3  MSRP Transaction and Report Model

   A sender sends MSRP requests to a receiver.  The receiver MUST
   quickly accept or reject the request.  If the receiver initially
   accepted the request, it still may then do things that take
   significant time to succeed or fail.  For example, if the receiver is
   an MSRP to XMPP [28] gateway, it may forward the message over XMPP.
   The XMPP side may later indicate that the request did not work.  At
   this point, the MSRP receiver may need to indicate that the request
   did not succeed.  There are two important concepts here: first, the



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   hop by hop delivery of the request may succeed or fail; second, the
   end result of the request may be successfully processed or not.  The
   first type of status is referred to as "transaction status" and may
   be returned in response to a request.  The second type of status is
   referred to as "request status" and may be returned in a REPORT
   transaction.

   The original sender of a request can indicate if they wish to receive
   reports for requests that fail, and can independently indicate if
   they wish to receive reports for requests that succeed.  A receiver
   only sends a success REPORT if it knows that the request succeeded,
   and the sender requested a success report.  A receiver only sends a
   failure REPORT if the request failed and the sender requested failure
   reports.

      This document describes the behavior of MSRP endpoints.  MSRP
      relays or gateways are likely to have additional conditions that
      indicate a failure REPORT should be sent, such as the failure to
      receive a positive response from the next hop.

   Two header fields control the sender's desire to receive reports.
   The header "Report-Success" can have a value of "yes" or "no" and the
   "Report-Failure" header can have a value of "yes", "no", or
   "partial".

   The combinations of reporting are needed to meet the various
   scenarios of currently deployed IM systems.  Report-Success might be
   "no" in many public systems to reduce load but is used in some
   current enterprise systems, such as systems used for securities
   trading.  A Report-Failure value of "no" is useful for sending system
   messages such as "the system is going down in 5 minutes" without
   causing a response explosion to the sender.  A Report-Failure of
   "yes" is used by many systems that wish to notify the user if the
   message failed but some other systems choose to use a value of
   "partial" to reduce the load on the servers caused by 200 OK
   responses, but still allow error responses to be sent in many cases.

5.4  MSRP Connection Model

   When MSRP wishes to send a request to a peer identified by an MSRP
   URL, it first needs a transport connection, with the appropriate
   security properties, to the host specified in the URL.  If the sender
   already has such a connection, that is, one associated with the same
   host, port, and URL scheme, then it SHOULD reuse that connection.

   When a new MSRP session is created, the element that sent the SDP
   offer MUST immediately issue a SEND request to the answerer.  This
   request MAY have a empty body, or MAY carry content.



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   Likewise, the offerer MUST open the transport connection to the
   answerer, if a new connection is required.  However, this requirement
   may be weakened if standardized mechanisms for negotiating the
   connection direction become available, and is implemented by both
   parties to the connection.

   When an element needs to form a new connection, it looks at the URL
   to decide on the type of connection (TLS, TCP, etc.) then connects to
   the host indicated by the URL, following the URL resolution rules in
   Section 6.2.  Connections using the msrps: scheme MUST use TLS.  The
   SubjectAltName in the received certificate MUST match the hostname
   part of the URL and the certificate MUST be valid, including having a
   date that is valid and being signed by an acceptable certificate
   authority.  At this point the device that initiated the connection
   can assume that this connection is with the correct host.

   If the connection used mutual TLS authentication, and the TLS client
   presented a valid certificate, then the element accepting the
   connection can immediately know the identity of the connecting host.
   When mutual TLS authentication is not used, the listening device MUST
   wait until it receives a request on the connection, at which it
   infers the identity of the connecting device from the associated SDP.

   When the first request arrives, its To-Path header field should
   contain a URL that the listening element handed out in the SDP for a
   session.  The element that accepted the connection looks up the URL
   in the received request, and determines which session it matches.  If
   a match exists, the node MUST assume that the host that formed the
   connection is the host that this URL was given to.  If no match
   exists, the node MUST reject the request with a 481 response.  The
   node MUST also check to make sure the session is not already in use
   on another connection.  If so, it MUST reject the request with a 506
   response.

      If it were legal to have multiple connections associated with the
      same session, a security problem would exist.  If the initial SEND
      request is not protected, an eavesdropper might learn the URL, and
      use it to insert messages into the session via a different
      connection.

   If a connection fails for any reason, then an MSRP endpoint MUST
   consider any sessions associated with the connection as also having
   failed.  When an endpoint notices such a failure, it MAY attempt to
   re-create any such sessions.  If it chooses to do so, it MUST use new
   SDP exchange, for example, in a SIP re-invite or update [11].  If a
   replacement session is successfully created, endpoints MAY attempt to
   resend any content for which delivery on the original session could
   not be confirmed.  If it does this, the Message-ID values for the



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   resent messages MUST match those used in the initial attempts.  If
   the receiving endpoint receives more than one message with the same
   Message-ID.  It SHOULD assume that the messages are duplicates.  It
   MAY take any action based on that knowledge, but SHOULD NOT present
   the duplicate messages to the user without warning of the
   duplication.

   In this situation, the endpoint MUST choose Message-ID values so that
   they are unique in the context of both the original session and the
   replacement session.

   When endpoints create a new session in this fashion, the chunks for a
   given logical message MAY be split across the sessions.  However,
   endpoints SHOULD NOT split chunks between sessions under non-failure
   circumstances.

   If an endpoint attempts to re-create a failed session in this manner,
   it MUST NOT assume that the MSRP URLs in the SDP will be the same as
   the old ones.

   A connection SHOULD not be closed while there are sessions associated
   with it.

6.  MSRP URLs

   URLs using the MSRP and MSRPS schema are used to identify a session
   of instant messages at a particular MSRP device.  MSRP URLs are
   ephemeral; an MSRP device may use a different MSRP URL in a different
   session.  An MSRP URL generally has no meaning outside of the
   associated session.

   An MSRP URL follows a subset of the URL syntax in Appendix A of
   RFC2396bis [9], with a scheme of "msrp" or "msrps".  The syntax is
   described in Section 9.

   The constructions for "userinfo",  and "unreserved" are detailed in
   RFC2396bis [9].  In order to allow IPV6 addressing, the construction
   for hostport is that used for SIP in RFC3261.  URLs designating MSRP
   over TCP MUST include the "tcp" transport parameter.

      Since this document only specifies MSRP over TCP, all MSRP URLs
      herein use the "tcp" transport parameter.  Documents that provide
      bindings on other transports should define respective parameters
      for those transports.

   An MSRP URL hostport field identifies a participant in a particular
   MSRP session.  If the hostport contains a numeric IP address, it MUST
   also contain a port.  The session-id part identifies a particular



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   session the participant.  The absence of the session-id part
   indicates a reference to an MSRP host device, but does not
   specifically refer to a particular session.

   A scheme of "msrps" indicates the underlying connection MUST be
   protected with TLS.

   MSRP has an IANA registered recommended port defined in Section 15.1.
   This value is not a default, as the URL negotiation process described
   herein will always include explicit port numbers.  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, it 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
   part MUST NOT contain password information.

      The limitation of userinfo to unreserved characters is an
      additional restriction to the userinfo definition in RFC2396bis.
      That version allows reserved characters.  The additional
      restriction is to avoid the need for escaping.

   The following is an example of a typical MSRP URL:

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

6.1  MSRP URL Comparison

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

   1.  The scheme must match exactly.

   2.  If the hostpart contains an explicit IP address, and/or port,
       these are compared for address and port equivalency.  Otherwise,
       hostpart is compared as a case insensitive character string.

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

   4.  The session-id part is compared as case sensitive.  A URL without
       a session-id part is never equivalent to one that includes one.



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   5.  URLs with different "transport" parameters never match.  Two URLs
       that are identical except for transport are not equivalent.

   6.  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.

6.2  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 a 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.

      This process assumes that the connection port is always known
      prior to resolution.  This is always true for the MSRP URL uses
      described in this document, that is, URLs exchanged in the SDP
      offer and answer.  The introduction of relays may create
      situations where this is not the case.  For example, the MSRP URL
      that a user enters into a client to configure it to use a relay
      may be intended to be easily remembered and communicated by
      humans, and therefore is likely to omit the port.  Therefore, the
      relay specification [20] may describe additional steps to resolve
      the port number.

   MSRP devices MAY use other methods for discovering other such
   devices, when appropriate.  For example, MSRP endpoints may use other
   mechanisms to discover relays, which are beyond the scope of this
   document.

7.  Method-Specific Behavior

7.1  Constructing Requests

   To form a new request, the sender creates a unique transaction
   identifier and uses this and the method name to create an MSRP
   request start line.  Next, the sender places the target path in a
   To-Path header, and the sender's URL in a From-Path header.  If
   multiple URLs are present in the To-Path, the leftmost is the first
   URL visited; the rightmost URL is the last URL visited.  The



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   processing then becomes method specific.  Additional method-specific
   headers are added as described in the following sections.

   After any method-specific headers are added, processing continues to
   handle a body, if present.  A body in a Non-SEND request MUST NOT be
   longer than 2048 octets.  If the request has a body, it must contain
   a Content-Type header field.  It may contain other MIME specific
   headers.  The Content-Type header MUST be the last header line.  The
   body MUST be separated from the headers with an extra CRLF.

   The boundary marker that terminates the body MUST be preceded by a
   CRLF that is not part of the body and then seven "-" (minus sign)
   characters.  After the boundary marker, there MUST be a flag
   character.  If the chunk represents the data that forms the end of
   the complete message, the flag value MUST be a "$".  If sender is
   abandoning an incomplete message, and intends to send no further
   chunks in that message, it MUST be a "#".  Otherwise it MUST be a
   "+".

   If the request contains a body, the sender MUST ensure that the
   closing sequence (a CRLF, seven hyphens, and the transaction
   identifier) is not present in the body.  If the closing sequence is
   present in the body, the sender MUST choose a new transaction
   identifier that is not present in the body, and add the closing
   sequence, including the "$", "#", or "+" character, and a final CRLF.

   Finally, requests which have no body MUST NOT contain a Content-Type
   header or any other MIME specific header.  Bodiless requests MUST
   contain a closing sequence after the final header.

   Once a request is ready for delivery, the sender follows the
   connection management (Section 5.4) rules to forward the request over
   an existing open connection or create a new connection.

7.1.1  Delivering SEND requests

   When an endpoint has a message to deliver, it first generates a new
   unique Message-ID.  This ID MUST be unique within the scope of the
   session.  If necessary, it breaks the message into chunks.  It then
   generates a SEND request for each chunk, following the procedures for
   constructing requests (Section 7.1).

   Each chunk MUST contain a Message-ID header field containing the
   Message-ID.  If the sender wishes non-default status reporting, it
   MUST insert a Report-Failure and/or Report-Success header field with
   an appropriate value.  All chunks of the same message MUST use the
   same Report-Failure and Report-Success values in their SEND requests.




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   If success reports are requested, i.e.  the value of the
   Report-Success header is "yes", the sending device MAY wish to run a
   timer of some value that makes sense for its application and take
   action if a success Report is not received in this time.  There is no
   universal value for this timer.  For many IM applications, it may be
   2 minutes while for some trading systems it may be under a second.
   Regardless of whether such a timer is used, if the success report has
   not been received by the time the session is ended, the device SHOULD
   inform the user.

   If the value of "Report-Failure" is set to "yes", then the sender of
   the request runs a timer.  If a 200 response to the transaction is
   not received within 30 seconds from the time the last byte of the
   transaction is sent, the element MUST inform the user that the
   request probably failed.  If the value is set to "partial", then the
   element sending the transaction does not have to run a timer, but
   MUST inform the user if receives a non-recoverable error response to
   the transaction.

   If no Report-Success header is present in a SEND request, it MUST be
   treated the same as a Report-Success header with value of "no".  If
   no Report-Failure header is present, it MUST be treated the same as a
   Report-Failure header with value of "yes".  REPORT requests MUST have
   the same Message-ID header value as the request they are reporting
   on.  They MAY also have the Byte-Range of the chunk they are
   reporting on.  If an MSRP element receives a REPORT for a Message-ID
   it does not recognize, it SHOULD silently ignore the REPORT.

   Report-Success and Report-Failure MUST NOT be present for any method
   other than SEND.  MSRP nodes MUST NOT send REPORT requests in
   response to report requests.  MSRP Nodes MUST NOT send MSRP responses
   to REPORT requests.

   The Byte-Range header value contains a starting value (range-start)
   followed by a "-", an ending value (range-end) followed by a "/", and
   finally the total length.  The first byte in the message is indicated
   by a one, rather than a zero.

   The first chunk of the message SHOULD, and all subsequent chunks MUST
   include a Byte-Range header field.  The range-start field MUST
   indicate the position of the first byte in the body in the overall
   message (that is, a value of one).  The range-end field SHOULD
   indicate the position of the last byte in the body, if known.  It
   MUST take the value of "*" if the position is unknown, or if the
   request needs to be interruptible.  The total field SHOULD contain
   the total size of the message, if known.  The total field MAY contain
   a "*" if the total size of the message is not known in advance.  The
   sender MUST send all chunks in Byte-Range order.  (However, the



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   receiver cannot assume the requests will be delivered in order, as
   intervening relays may have changed the order.)

   To insure fairness over a connection, senders MUST NOT send chunks
   with a body larger than 2048 octets unless they are prepared to
   interrupt them (meaning that any chunk with a body of greater than
   2048 octets will have a "*" character in the range-end field).  A
   sender can use one of the following two strategies to satisfy this
   requirement.  The sender is STRONGLY RECOMMENDED to send messages
   larger than 2048 octets using as few chunks as possible, interrupting
   chunks (at least 2048 octets long) when other traffic is waiting to
   use the same connection.  Alternatively, the sender MAY simply send
   chunks in 2048 octet increments until the final chunk.  Note that the
   former strategy results in markedly more efficient use of the
   connection.  All MSRP nodes MUST be able to receive chunks of any
   size from 0 octets to the maximum number of octets they can receive
   for a complete message.  Senders SHOULD NOT break messages into
   chunks smaller than 2048 octets, except for the final chunk of a
   complete message.

   A SEND request is interrupted while a body is in the process of being
   written to the connection by simply noting how much of the message
   has already been written to the connection, then writing out the
   boundary string to end the chunk.  It can then be resumed in a
   another chunk with the same Message-ID and a Byte-Range header range
   start field containing the position of the first byte after the
   interruption occurred.

   SEND requests larger than 2k MUST be interrupted to send pending
   response or REPORT requests.  If multiple SEND requests from
   different sessions are concurrently being sent over the same
   connection, the device SHOULD implement some scheme to alternate
   between them such that each concurrent request gets a chance to send
   some fair portion of data at regular intervals suitable to the
   application.

   The sender MUST NOT assume that a message is received by the peer
   with the same chunk allocation with which it was sent.  An
   intervening relay could possibly break SEND requests into smaller
   chunks, or aggregate multiple chunks into larger ones.

   The default disposition of body is "render".  If the sender wants
   different disposition, it MAY insert a Content-Disposition header.
   Since MSRP is a binary protocol, transfer encoding MUST be "binary".

7.1.2  Sending REPORT requests

   REPORT requests are similar to SEND requests, except that report



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   requests MUST NOT include Report-Success or Report-Failure header
   fields, and MUST contain a Status header field.  REPORT requests MUST
   contain the Message-ID header from the original SEND request.

   If an MSRP element receives a REPORT for a Message-ID it does not
   recognize, it SHOULD silently ignore the REPORT.

   An MSRP endpoint MUST be able to generate success REPORT requests.

   REPORT requests will normally not include a body, as the REPORT
   request header fields can carry sufficient information in most cases.
   However, REPORT requests MAY include a body containing additional
   information about the status of the associated SEND request.  Such a
   body is informational only, and the sender of the REPORT request
   SHOULD NOT assume that the recipient pays any attention to the body.
   Since REPORT requests are not interruptible, the size of such a body
   MUST NOT exceed 2048 octets.

   An endpoint MUST send a success report if it successfully receives a
   SEND request which contained a Report-Success value of "yes" and
   either contains a complete message, or contains the last chunk needed
   to complete the message.  This request is sent following the normal
   procedures (Section 7.1), with a few additional requirements.

   The endpoint inserts a To-Path header field containing the From-Path
   value from the original request, and a From-Path header containing
   the URL identifying itself in the session.  The endpoint then inserts
   a Status header field with a namespace of "000", a short-status of
   "200" and a relevant Reason phrase, and a Message-ID header field
   containing the value from the original request.

   The endpoint MUST NOT send a success report for a SEND request that
   either contained no Report-Success header field, or contained such a
   field with a value of "no".  That is, if no Report-Success header
   field is present, it is treated identically to one with a value of
   "no."

7.1.3  Failure REPORT Generation

   If an MSRP endpoint receives a SEND request that it cannot process
   for some reason, and the Report-Failure header either was not present
   in the original request, or had a value of "yes", it SHOULD simply
   include the appropriate error code in the transaction response.
   However, there may be situations where the error cannot be determined
   quickly, such as when the endpoint is a gateway that must wait for a
   downstream network to indicate an error.  In this situation, it MAY
   send a 200 OK response to the request, and then send a failure REPORT
   request when the error is detected.



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   If the endpoint receives a SEND request with a Report-Failure header
   field value of "no", then it MUST NOT send a failure REPORT request,
   and MUST NOT send a transaction response.  If the value is "partial",
   it MUST NOT send a transaction response to the request, but SHOULD
   send an appropriate non-200 class responsea failure occurs.

   As stated above, if no Report-Failure header is present, it MUST be
   treated the same as a Report-Failure header with value of "yes".

   Construction of failure REPORT requests is identical to that for
   success reports, except the Status header code and reason fields MUST
   contain appropriate error codes.  Any error response code defined in
   this specification MAY also be used in failure reports.

   If a failure report is sent in response to a SEND request that
   contained a chunk, it MUST include a Byte-Range header indicating the
   actual range being reported on.  It can take the range-start and
   total values from the original SEND request, but MUST calculate the
   range-end field from the actual body data.

   Endpoints SHOULD NOT send REPORT requests if they have reason to
   believe the request will not be delivered.  For example, they SHOULD
   NOT send a REPORT request on a session that is no longer valid.

      This section only describes failure report generation behavior for
      MSRP endpoints.  Relay behavior is beyond the scope of this
      document, and will be considered in a separate document.  We
      expect failure reports to be more commonly generated by relays
      than by endpoints.

7.2  Constructing Responses

   If an MSRP endpoint receives a request that either contains a
   Report-Failure header value of "yes", or does not contain a
   Report-Failure header field at all, it MUST immediately generate a
   response.  Likewise, if an MSRP endpoint receives a request that
   contains a Report-Failure header value of "partial", and the receiver
   is unable to process the request, it SHOULD immediately generate a
   response.

   To construct the response, the endpoint first creates the response
   start-line, inserting appropriate response code and reason fields.
   The transaction identifier in the response start line MUST match the
   transaction identifier from the original request.

   The endpoint then inserts an appropriate To-Path header field.  If
   the request triggering the response was a SEND request, the To-Path
   header field is formed by copying the last (right-most) URI in the



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   From-Path header field of the request.  (Responses to SEND requests
   are returned only to the previous hop.)  For responses to all other
   request methods, the To-Path header field contains the full path back
   to the original sender.  This full path is generated by taking the
   list of URLs from the From-Path of the original request, reversing
   the list, and writing the reversed list into the To-Path of the
   response.  (Legal REPORT requests do not request responses, so this
   specification doesn't exercise the behavior described above, however
   we expect that extensions for gateways and relays will need such
   behavior.)

   Finally, the endpoint inserts a From-Path header field containing the
   URL that identifies it in the context of the session, followed by the
   closing sequence after the last header field.  The response MUST be
   transmitted back on the same connection on which the original request
   arrived.

7.3  Receiving Requests

   The receiving endpoint must first check the URL in the To-Path to
   make sure the request belongs to an existing session.  When the
   request is received, the To-Path will have exactly one URL, which
   MUST map to an existing session that is associated with the
   connection on which the request arrived.  If this is not true, and
   the request contained a Report-Failure header value of "no" or
   "partial", then the receiver SHOULD quietly ignore the request.  If
   the Report-Failure header is not present, or had a value of "yes",
   then the receiver MUST return a 481 response.

   Further request processing by the receiver is method specific.

7.3.1  Receiving SEND requests

   When the receiving endpoint receives a SEND request, it first
   determines if it contains a complete message, or a chunk from a
   larger message.  If the request contains no Byte-Range header, or
   contains one with a range-start value of "1", and the closing line
   continuation flag has a value of "$", then the request contained the
   entire message.  Otherwise, the receiver looks at the Message-ID
   value to associate chunks together into the original message.  It
   forms a virtual buffer to receive the message, keeping track of which
   bytes have been received and which are missing.  The receiver takes
   the data from the request and places it in the appropriate place in
   the buffer.  The receiver SHOULD determine the actual length of each
   chunk by inspecting the payload itself; it is possible the body is
   shorter than the range-end field indicates.  This can occur if the
   sender interrupted a SEND request unexpectedly.  It is worth nothing
   that the chunk that has a termination character of "$" defines the



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   total length of the message.

      It is technically illegal for the sender to prematurely interrupt
      a request that had anything other "*" in the last-byte position.
      But having the receiver calculate a chunk length based on actual
      content adds resilience in the face of sender erros.  errors.
      Since this should never happen with compliant senders, this only
      has a SHOULD strength.

   Receivers MUST not assume the chunks will be delivered in order or
   that they will receive all the chunks with "+" flags before they
   receive the chunk with the "$" flag.  In certain cases of connection
   failure, it is possible for information to be duplicated.  If chunks
   data is received that overlaps already received data for the same
   message, the last chunk received takes precedence (even though this
   may not have been the last chunk transmitted).  For example, if bytes
   1 to 100 was received and a chunk arrives that contains bytes 50 to
   150, this second chunk will overwrite bytes 50 to 100 of the data
   that had already been received.  Although other schemes work, this is
   the easiest for the receiver and results in consistent behavior
   between clients.

   The seven "-" before the boundary are used so that the receiver can
   search for the value "----", 32 bits at a time to find the probable
   location of the boundary.  This allows most processors to locate the
   boundaries and copy the memory at the same rate that a normal memory
   copy could be done.  This approach results in a system that is as
   fast as framing based on specifying the body length in the headers of
   the request, but also allows for the interruption of messages.

   What is done with the body is outside the scope of MSRP and largely
   determined by the MIME Content-Type and Content-Disposition.  The
   body MAY be rendered after the whole message is received or partially
   rendered as it is being received.

   If the SEND request contained a Content-Type header field indicating
   an unsupported MIME type, the receiver SHOULD send a 415 response or
   failure report, as appropriate for the Report-Failure header field
   value.  All MSRP endpoints MUST be able to receive the
   multipart/mixed and multipart/alternative MIME types.

   If the Report-Success header was set to "yes", then when a complete
   message has been received, the receiver MUST send a sucess REPORT
   with a byte range covering the whole message.  If the Report-Success
   header is not set to "no", then the receiver MAY generate incremental
   success REPORTs as the chunks are recieved.  These can be sent
   periodically and cover all the bytes that have been received so far
   or they can be sent after a chunk arrives and cover just the part



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   from that chunk.

7.3.2  Receiving REPORT requests

   When an endpoint receives a REPORT request, it correlates it to the
   original SEND request using the Message-ID and the Byte-Range, if
   present.  If it requested success reports, then it SHOULD keep enough
   state about each outstanding sent message so that it can correlate
   REPORT requests to the original messages.

   An endpoint that receives a REPORT request containing a Status header
   with a namespace field of "000", it SHOULD interpret the report in
   exactly the same way it would interpret an MSRP transaction response
   with a response code matching the short-code field.

   It is possible to receive a failure report or a failure transaction
   response for a chunk that is currently being delivered.  In this case
   the entire message corresponding to that chunk should be aborted, by
   including the "#" character in the continuation field of the closing.

   It is possible that an endpoint will receive a REPORT request on a
   session that is no longer valid.  The endpoint's behavior if this
   happens is a matter of local policy.  The endpoint is not required to
   take any steps to facilitate such late delivery, i.e.  it is not
   expected to keep a connection active in case late REPORTs might
   arrive.

   When a device that sent a SEND request receives a failure REPORT
   indicating that a particular byte range was not received,it MUST
   treat the session as failed.  If it wishes to recover,  it MUST first
   re-negotiate the URLs at the signaling level then resend that range
   of bytes of the message on the resulting new session.

   MSRP Modes MUST NOT send a MSRP REPORT in responses to REPORT
   requests.

8.  Using MSRP with SIP

8.1  SDP Offer-Answer Exchanges for MSRP Sessions

   MSRP sessions will typically be initiated using the Session
   Description Protocol (SDP) [2] via the SIP offer-answer mechanism
   [3].

   This document defines a handful of new SDP parameters to setup MSRP
   sessions.  These are detailed below and in the IANA Considerations
   section.




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   The general format of an SDP media-line is:

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

   An offered or accepted MSRP media-line MUST have the following value
   exactly, with the exception that the port field MAY be set to any
   value.  (The port value will be ignored, unless it is zero.
   According to [3], a user agent that wishes to accept an offer, but
   not a specific media-line MUST set the port number of that media-line
   to zero (0) in the response.)

   m=message 9 msrp *

      While MSRP could theoretically carry any media type, "message" is
      appropriate.  For MSRP, the port number is always ignored--the
      actual port number is provided in an MSRP URL.  Instead a dummy
      value is used, which is not meaningful if non-zero.  The protocol
      is always "msrp", and the value of the format list is always a
      single asterisk character ("*").

   An MSRP media-line is always accompanied by a mandatory "path"
   attribute.  This attribute contains a space separated list of URLs
   that must be visited to contact the user agent advertising this
   session-description.  If more than one URL is present, the leftmost
   URL is the first URL that must be visited to reach the target
   resource.  (The path list can contain multiple URLs to allow for the
   deployment of gateways or relays in the future.)  MSRP
   implementations which can accept incoming connections will typically
   only provide a single URL here.

   MSRP media lines MUST also be accompanied by an "accept-types"
   attribute.  This attribute contains a list of MIME types which are
   acceptable to the endpoint.

   A "*" entry in the accept-types attribute indicates that the sender
   may attempt to send content with media types that have not been
   explicitly listed.  Likewise, an entry with an explicit type and a
   "*" character as the subtype indicates that the sender may attempt to
   send content with any subtype of that type.  If the receiver receives
   an MSRP request and is able to process the media type, it does so.
   If not, it will respond with a 415 response.  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



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   as the root payload, or may be wrapped in a listed container type.
   Any container types MUST also be listed in the accept-types
   attribute.

   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 when wrapped
   inside compound types using the "accept-wrapped-types" attribute in
   an SDP a-line.

   The semantics for accept-wrapped-types 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 the accept-types attribute 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
   accept-types 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 [12] 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 might attempt to use
      those types without the wrapper.
      If the recipient of an offer does not understand any of the
      payload types indicated in the offered SDP, it SHOULD indicate
      that using the appropriate mechanism of the rendezvous protocol.
      For example, in SIP, it SHOULD return a SIP 488 response.
      An endpoint MAY indicate the maximum size message they wish to
      receive using the max-size a-line attribute.  Max-size refers to
      the complete message in octets, not the size of any one chunk.
      Senders SHOULD NOT exceed the max-size limit for any message sent
      in the resulting session.  However, the receiver should consider
      max-size value as a hint.







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

           max-size = max-size-label ":" max-size-value
           max-size-label = "max-size"
           max-size-value = 1*(DIGIT) ;max size in octets

      Note: RFC2327 does not allow the hyphen in att-field, which is
      defined as alphanumeric.  However, this is expected to be allowed
      in an update to that specification, which should be available
      shortly.

8.1.1  URL Negotiations

   Each endpoint in an MSRP session is identified by a URL.  These URLs
   are negotiated in the SDP exchange.  Each SDP offer or answer MUST
   contain one or more MSRP URL in a path attribute.  This attribute has
   the following syntax:

   "a=path:" MSRP_URL *(SP MSRP_URL)

   where MSRP_URL is an msrp: or msrps: URL as defined in Section 6.
   MSRP URLs included in an SDP offer or answer MUST include explicit
   port numbers.

   An MSRP device uses the URL to determine a host address, port,
   transport, and protection level when connecting, and to identify the
   target when sending requests and responses.

   The offerer and answerer each selects a URL to represent itself, and
   send it to the peer device in the SDP document.  Each device stores
   the path value received from the peer, and uses that value as the
   target for requests inside the resulting session.  If the path
   attribute received from the peer contains more than one URL, then the
   target URL is the rightmost, while the leftmost entry represents the
   adjacent hop.  If only one entry is present, then it is both the peer
   and adjacent hop URL.  The target path is the entire path attribute
   value received from the peer.

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




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    v=0
    o=alice 2890844526 2890844527 IN IP4 alice.example.com
    s=
    c=IN IP4 alice.example.com
    m=message 9 msrp *
    a=accept-types:text/plain
    a=path:msrp://a.example.com:7394/2s93i;tcp

   The rightmost URI in the path attribute MUST identify the endpoint
   that generated the SDP document, or some other location where that
   endpoint wishes to receive requests associated with the session.  It
   MUST be assigned for this particular session, and MUST NOT duplicate
   any URI in use for any other session in which the endpoint is
   currently participating.  It SHOULD be hard to guess, and protected
   from eavesdroppers.  This is discussed in more detail in Section 14.

8.1.2  Path Attributes with Multiple URLs

   As mentioned previously, this document describes MSRP for
   peer-to-peer scenarios, that is, when no relays are used.  However,
   we expect a separate document to describe the use of relays.  In
   order to allow an MSRP device that only implements the core
   specification to interoperate with devices that use relays, this
   document must include a few assumptions about how relays work.

   An endpoint that uses one or more relays will indicate that by
   putting a URL for each device in the relay chain into the SDP path
   attribute.  The final entry would point to the endpoint itself.  The
   other entries would indicate each proposed relay, in order.  The
   first entry would point to the first relay in the chain from the
   perspective of the peer; that is, the relay to which the peer device,
   or a relay operating on its behalf, should connect.

   Endpoints that do not wish to insert a relay, including those that do
   not support relays at all, will put exactly one URL into the path
   attribute.  This URL represents both the endpoint for the session,
   and the connection point.

   Even though endpoints that implement only this specification will
   never introduce a relay, they need to be able to interoperate with
   other endpoints that do use relays.  Therefore, they MUST be prepared
   to receive more than one URL in the SDP path attribute.  When an
   endpoint receives more than one URL in a path header, only the first
   entry is relevant for purposes of resolving the address and port, and
   establishing the network connection, as it describes the first
   adjacent hop.

   If an endpoint puts more than one URL in a path attribute, the final



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   URL in the path (the peer URL) attribute MUST exhibit the uniqueness
   properties described above.  Uniqueness requirements for other
   entries in the attribute are out of scope for this document.

8.1.3  Updated SDP Offers

   MSRP endpoints may sometimes need to send additional SDP exchanges
   for an existing session.  They may need to send periodic exchanges
   with no change to refresh state in the network, for example, SIP
   Session Timers.  They may need to change some other stream in a
   session without affecting the MSRP stream, or they may need to change
   an MSRP stream without affecting some other stream.

   Either peer may initiate an updated exchange at any time.  The
   endpoint that sends the new offer assumes the role of offerer for all
   purposes.  The answerer MUST respond with a path attribute that
   represents a valid path to itself at the time of the updated
   exchange.  This new path may be the same as its previous path, but
   may be different.  The new offerer MUST NOT assume that the peer will
   answer with the same path it used previously.

   If either party wishes to send an SDP document that changes nothing
   at all, then it MUST have the same o-line as in the previous
   exchange.

8.1.4  Example SDP Exchange

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

    v=0
    o=usera 2890844526 2890844527 IN IP4 alice.example.com
    s=
    c=IN IP4 alice.example.com
    t=0 0
    m=message 9 msrp *
    a=accept-types: message/cpim text/plain text/html
    a=path:msrp://alice.example.com:7394/2s93i9;tcp

   B responds with its own URL:

    v=0
    o=userb 2890844530 2890844532 IN IP4 bob.example.com
    s=
    c=IN IP4 bob.example.com
    t=0 0
    m=message 9 msrp *
    a=accept-types:message/cpim text/plain



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    a=path:msrp://bob.example.com:8493/si438ds;tcp


8.1.5  Connection Negotiation

   Previous versions of this document included a mechanism to negotiate
   the direction for any required TCP connection.  The mechanism was
   loosely based on the COMEDIA [23] work being done in the MMUSIC
   working group.  The primary motivation was to allow MSRP sessions to
   succeed in situations where the offerer could not accept connections
   but the answerer could.  For example, the offerer might be behind a
   NAT, while the answerer might have a globally routable address.

   The SIMPLE working group chose to remove that mechanism from MSRP, as
   it added a great deal of complexity to connection management.
   Instead, MSRP now specifies a default connection direction.

8.2  MSRP User Experience with SIP

   In typical SIP applications, when an endpoint receives an INVITE
   request, it alerts the user, and waits for user input before
   responding.  This is analogous to the typical telephone user
   experience, where the callee "answers" the call.

   In contrast, the typical user experience for instant messaging
   applications is that the initial received message is immediately
   displayed to the user, without waiting for the user to "join" the
   conversation.  Therefore, the principle of least surprise would
   suggest that MSRP endpoints using SIP signaling SHOULD allow a mode
   where the endpoint quietly accepts the session, and begins displaying
   messages.

   SIP INVITE requests may be forked by a SIP proxy, resulting in more
   than one endpoint receiving the same INVITE.  SIP early media [27]
   techniques can be used to establish a preliminary session with each
   endpoint, and canceling the INVITE transaction for any endpoints that
   do not send MSRP traffic after some period of time.

9.  Formal Syntax

   MSRP is a text protocol that uses the UTF-8 [14] transformation
   format.

   The following syntax specification uses the augmented Backus-Naur
   Form (BNF) as described in RFC-2234 [6].


   msrp-req-or-resp = msrp-request / msrp-response



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   msrp-request = req-start headers [content-stuff] end-line
   msrp-response = resp-start headers end-line

   req-start  = pMSRP SP transact-id SP method CRLF
   resp-start = pMSRP SP transact-id SP status-code [SP phrase] CRLF
   phrase = utf8text

   pMSRP = %x4D.53.52.50 ; MSRP in caps
   transact-id = ident
   method = mSEND / mREPORT / other-method
   mSEND = %x53.45.4e.44 ; SEND in caps
   mREPORT = %x52.45.50.4f.52.54; REPORT in caps

   other-method = 1*UPALPHA
   status-code = 3DIGIT ; any code defined in this document
                        ; or an extension document

   MSRP_url = msrp-scheme "://" [userinfo "@"] hostport
              ["/" session-id] ";" transport
                        ; userinfo as defined in RFC2396, except
                        ; limited to unreserved.
                        ; hostport as defined in RFC3261
                        ; [Todo: update with RFC number for 2396bis]

   msrp-scheme = "msrp" / "msrps"
   session-id = 1*unreserved  ; unreserved as defined in RFC2396
   transport = "tcp" / ALPHANUM


   headers = To-Path CRLF From-Path CRLF 1*( header CRLF )

   header =   Message-ID
    / Report-Success
    / Report-Failure
    / Byte-Range
    / Status
    / ext-header

   To-Path = "To-Path:" SP MSRP-url *( SP URL )
   From-Path = "From-Path:" SP MSRP-url *( SP URL )
   Message-ID = "Message-ID:" SP ident
   Report-Success = "Report-Success:" SP ("yes" / "no" )
   Report-Failure = "Report-Failure:" SP ("yes" / "no" / "partial" )
   Byte-Range = "Byte-Range:" SP range-start "-" range-end "/" total
   range-start = 1*DIGIT
   range-end   = 1*DIGIT / "*"
   total       = 1*DIGIT / "*"




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   Status = "Status:" SP namespace SP status-code [SP text-reason]
   namespace = "000"
   text-reason = *(VCHAR / WSP) ; All visible charcters / SP / HTAB
                                   ; (defined in RFC2234 CORE)

   ident = alphanum  3*31ident-char
   ident-char = alphanum / "." / "-" / "+" / "%" / "="


   content-stuff = *(Other-Mime-Header CRLF)
                   Content-Type 2CRLF data CRLF

   Content-Type = "Content-Type:" SP media-type
   media-type = type "/" subtype *( ";" gen-param )
   type = token
   subtype = token

   gen-param = pname [ "=" pval ]
   pname = token
   pval  = token / quoted-string

   token = 1*(%x21 / %x23-27 / %x2A-2B / %x2D-2E
              / %x30-39 / %x41-5A / %x5E-7E)

   quoted-string = DQUOTE *(qdtext / qd-esc) DQUOTE
   qdtext = SP / HTAB / %x21 / %x23-5B / %x5D-7E
               / UTF8-NONASCII
   qd-esc = (BACKSLASH BACKSLASH) / (BACKSLASH DQUOTE)
   BACKSLASH = "\"
   UPALPHA  = %x41-5A
   ALPHANUM = ALPHA / DIGIT



   Other-Mime-Header = (Content-ID
    / Content-Description
    / Content-Disposition
    / mime-extension-field);

       ; Content-ID, and Content-Description are defined in RFC2045.
       ; Content-Disposition is defined in RFC2183
       ; MIME-extension-field indicates additional MIME extension
       ; headers as described in RFC2045


   data = *OCTET
   end-line = "-------" transact-id continuation-flag CRLF
   continuation-flag = "+" / "$" / "#"



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   ext-header = hname ":" SP hval CRLF
   hname = ALPHA *token
   hval = utf8text

   utf8text = *(HTAB / %x20-7E / UTF8-NONASCII)

   UTF8-NONASCII = %xC0-DF 1UTF8-CONT
                 / %xE0-EF 2UTF8-CONT
                 / %xF0-F7 3UTF8-CONT
                 / %xF8-Fb 4UTF8-CONT
                 / %xFC-FD 5UTF8-CONT
   UTF8-CONT     = %x80-BF



10.  Response Code Descriptions

   This section summarizes the semantics of various response codes that
   may be used in MSRP transaction responses.  These codes may also be
   used in the Status header in REPORT requests.

10.1  200

   The 200 response code indicates a successful transaction.

10.2  400

   A 400 response indicates a request was unintelligible.

10.3  403

   The action is not allowed

10.4  415

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

10.5  426

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

10.6  481

   A 481 response indicates that the indicated session does not exist.





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10.7  501

   A 501 response indicates that the recipient does not understand the
   request method.

      The 501 response code exists to allow some degree of method
      extensibility.  It is not intended as a license to ignore methods
      defined in this document; rather it is a mechanism to report lack
      of support of extension methods.

10.8  506

   A 506 response indicates that a request arrived on a session which is
   already bound to another network connection.

11.  Examples

11.1  Basic IM session

   This section shows an example flow for the most common scenario.  The
   example assumes SIP is used to transport the SDP exchange.  Details
   of the SIP messages and SIP proxy infrastructure are omitted for the
   sake of brevity.  In the example, assume the offerer is
   sip:alice@example.com and the answerer is sip:bob@example.com.

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



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   1.  Alice constructs a local URL of
       msrp://alicepc.example.com:7777/iau39;tcp .

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

       v=0
       o=alice 2890844557 2890844559 IN IP4 alicepc.example.com
       s=
       c=IN IP4 alicepc.example.com
       t=0 0
       m=message 9 msrp *
       a=accept-types:text/plain
       a=path:msrp://alicepc.example.com:7777/iau39;tcp

   2.  Bob listens on port 8888, and sends the following response:

       Bob->Alice (SIP): 200 OK

       v=0
       o=bob 2890844612 2890844616 IN IP4 bob.example.com
       s=
       c=IN IP4 bob.example.com
       t=0 0
       m=message 9 msrp *
       a=accept-types:text/plain
       a=path:msrp://bob.example.com:8888/9di4ea;tcp

   3.  Alice->Bob (SIP): ACK

   4.  (Alice opens connection to Bob.) Alice->Bob (MSRP):

       MSRP d93kswow SEND
       To-Path:msrp://bob.example.com:8888/9di4ea;tcp
       From-Path:msrp://alicepc.example.com:7777/iau39;tcp
       Message-ID: 12339sdqwer
       Content-Type:text/plain
       Hi, I'm Alice!
       -------d93kswow$

   5.  Bob->Alice (MSRP):

       MSRP d93kswow 200 OK
       To-Path:msrp://bob.example.com:8888/9di4ea;tcp
       From-Path:msrp://alicepc.example.com:7777/iau39;tcp
       -------d93kswow$

   6.  Bob->Alice (MSRP):




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       MSRP dkei38sd SEND
       To-Path:msrp://alice.example.com:7777/iau39;tcp
       From-Path:msrp://bob.example.com:8888/9di4ea;tcp
       Message-ID: 456
       Content-Type:text/plain
       Hi, Alice! I'm Bob!
       -------dkei38sd$

   7.  Alice->Bob (MSRP):

       MSRP dkei38sd 200 OK
       To-Path:msrp://alice.example.com:7777/iau39;tcp
       From-Path:msrp://bob.example.com:8888/9di4ea;tcp
       -------dkei38sd$

   8.  Alice->Bob (SIP): BYE

       Alice invalidates local session state.

   9.  Bob invalidates local state for the session.

       Bob->Alice (SIP): 200 OK

11.2  Message with XHTML Content

   MSRP dsdfoe38sd SEND
   To-Path:msrp://alice.atlanta.com:7777/iau39;tcp
   From-Path:msrp://bob.atlanta.com:8888/9di4ea;tcp
   Message-ID: 456
   Content-Type:application/xhtml+xml

   <?xml version="1.0" encoding="UTF-8"?>
   <!DOCTYPE html
   PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
   "_http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd_">
   <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
     <head>
       <title>FY2005 Results</title>
   </head>
     <body>
      <p>See the results at<a
   href="http://example.org/">example.org</a>.</p>
     </body>
   </html>
   -------dsdfoe38sd$






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11.3  Chunked Message

   For an example of a chunked message, see the example in Section 5.1.

11.4  System Message

   Sysadmin->Alice (MSRP):

   MSRP d93kswow SEND
   To-Path:msrp://alicepc.example.com:8888/9di4ea;tcp
   From-Path:msrp://example.com:7777/iau39;tcp
   Message-ID: 12339sdqwer
   Report-Failure: no
   Report-Success: no
   Content-Type:text/plain
   This conference will end in 5 minutes
   -------d93kswow$


































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11.5  Positive Report

   Alice->Bob (MSRP):

   MSRP d93kswow SEND
   To-Path:msrp://bob.example.com:8888/9di4ea;tcp
   From-Path:msrp://alicepc.example.com:7777/iau39;tcp
   Message-ID: 12339sdqwer
   Report-Success: yes
   Content-Type:text/html

   <html><body>
   <p>Here is that important link...
   <a href="www.example.com/foobar">foobar</a>
   </p>
   </body></html>
   -------d93kswow$

   Bob->Alice (MSRP):

   MSRP d93kswow 200 OK
   To-Path:msrp://alicepc.example.com:7777/iau39;tcp
   From-Path:msrp://bob.example.com:8888/9di4ea;tcp
   -------d93kswow$

   Bob->Alice (MSRP):

   MSRP dkei38sd REPORT
   To-Path:msrp://alicepc.example.com:7777/iau39;tcp
   From-Path:msrp://bob.example.com:8888/9di4ea;tcp
   Message-ID: 12339sdqwer
   Status: 000 200 OK
   -------dkei38sd$



11.6  Forked IM

   Traditional IM systems generally do a poor job of handling multiple
   simultaneous IM clients online for the same person.  While some do a
   better job than many existing systems, handling of multiple clients
   is fairly crude.  This becomes a much more significant issue when
   always-on mobile devices are available, but when it is desirable to
   use them only if another IM client is not available.

   Using SIP makes rendezvous decisions explicit, deterministic, and
   very flexible; instead "pager-mode" IM systems use implicit
   implementation-specific decisions which IM clients cannot influence.



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   With SIP session mode messaging rendezvous decisions can be under
   control of the client in a predictable, interoperable way for any
   host that implements callee capabilities [29].  As a result,
   rendezvous policy is managed consistently for each address of record.

   The following example shows Juliet with several IM clients where she
   can be reached.  Each of these has a unique SIP Contact and MSRP
   session.  The example takes advantage of SIP's capability to "fork"
   an invitation to several Contacts in parallel, in sequence, or in
   combination.  Juliet has registered from her chamber, the balcony,
   her PDA, and as a last resort, you can leave a message with her
   Nurse.  Juliet's contacts are listed below.  The q-values express
   relative preference (q=1.0 is the highest preference).

      The example uses REGISTER to learn of Juliet's registered
      contacts.  This does not constitute an endorsement of that
      approach; it is used here to avoid cluttering the example with too
      many SIP details.  A more realistic application would be the use a
      SIP proxy or redirect server for this purpose.


      We query for a list of Juliet's contacts by sending a REGISTER:

   REGISTER sip:thecapulets.example.com SIP/2.0
   To: Juliet <sip:juliet@thecapulets.example.com>
   From: Juliet <sip:juliet@thecapulets.example.com>;tag=12345
   Call-ID: 09887877
   CSeq: 772 REGISTER

      The Response contains her Contacts:

   SIP/2.0 200 OK
   To: Juliet <sip:juliet@thecapulets.example.com>
   From: Juliet <sip:juliet@thecapulets.example.com>;tag=12345
   Call-ID: 09887877
   CSeq: 772 REGISTER
   Contact: <sip:juliet@balcony.thecapulets.example.com>
    ;q=0.9;expires=3600
   Contact: <sip:juliet@chamber.thecapulets.example.com>
    ;q=1.0;expires=3600
   Contact: <sip:jcapulet@veronamobile.example.net>;q=0.4;expires=3600
   Contact: <sip:nurse@thecapulets.example.com>;q=0.1;expires=3600


   When Romeo opens his IM program, he selects Juliet and types the
   message "art thou hither?" (instead of "you there?").  His client
   sends a SIP invitation to sip:juliet@thecapulets.example.com.  The
   Proxy there tries first the balcony and the chamber simultaneously.



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   A client is running on both those systems, both of which setup early
   sessions of MSRP with Romeo's client.  The client automatically sends
   the message over the MSRPS to the two MSRP URIs involved.  After a
   delay of a several seconds with no reply or activity from Juliet, the
   proxy cancels the invitation at her first two contacts, and forwards
   the invitation on to Juliet's PDA.  Since her father is talking to
   her about her wedding, she selects "Do Not Disturb" on her PDA, which
   sends a "Busy Here" response.  The proxy then tries the Nurse, who
   answers and tells Romeo what is going on.



    Romeo       Juliet's     Juliet/      Juliet/      Juliet/     Nurse
                 Proxy       balcony      chamber       PDA

      |            |            |            |           |           |
      |--INVITE--->|            |            |           |           |
      |            |--INVITE--->|            |           |           |
      |            |<----180----|            |           |           |
      |<----180----|            |            |           |           |
      |---PRACK---------------->|            |           |           |
      |<----200-----------------|            |           |           |
      |<===Early MSRP Session==>| art thou hither?       |           |
      |            |            |            |           |           |
      |            |--INVITE---------------->|           |           |
      |            |<----180-----------------|           |           |
      |<----180----|            |            |           |           |
      |---PRACK----------------------------->|           |           |
      |<----200------------------------------|           |           |
      |<========Early MSRP Session==========>| art thou hither?      |
      |            |            |            |           |           |
      |            |            |            |           |           |
      |            | .... Time Passes ....   |           |           |
      |            |            |            |           |           |
      |            |            |            |           |           |
      |            |--CANCEL--->|            |           |           |
      |            |<---200-----|            |           |           |
      |            |<---487-----|            |           |           |
      |            |----ACK---->|            |           |           |
      |            |--CANCEL---------------->|           |           |
      |            |<---200------------------|           |           |
      |            |<---487------------------|           |           |
      |            |----ACK----------------->|           |           |
      |            |--INVITE---------------------------->|  romeo wants
      |            |            |            |           |  to IM w/ you
      |            |<---486 Busy Here--------------------|           |
      |            |----ACK----------------------------->|           |
      |            |            |            |           |           |



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      |            |--INVITE---------------------------------------->|
      |            |<---200 OK---------------------------------------|
      |<--200 OK---|            |            |           |           |
      |---ACK------------------------------------------------------->|
      |<================MSRP Session================================>|
      |            |            |            |           |           |
      |                                         Hi Romeo, Juliet is  |
      |                                         with her father now  |
      |                                         can i take a message?|
      |                                                              |
      |  Tell her to go to confession tommorrow....                  |



12.  Extensibility

   MSRP was designed to be only minimally extensible.  New MSRP Methods,
   Headers, and status codes can be defined in standards track RFCs.
   There is no registry of headers, methods, or status codes, since the
   number of new elements and total extensions is expected to be very
   small.  MSRP does not contain a version number or any negotiation
   mechanism to require or discover new features.  If a
   non-interoperable update or extension occurs in the future, it will
   be treated as a new protocol, and must describe how its use will be
   signaled.

   In order to allow extension header fields without breaking
   interoperability, if an MSRP device receives a request or response
   containing a header field that it does not understand, it MUST ignore
   the header field and process the request or response as if the header
   field was not present.  If an MSRP device receives a request with an
   unknown method, it MUST return a 501 response.

   MSRP was designed to use lists of URLs instead of a single URL in the
   To-Path and From-Path headers in anticipation of relay or gateway
   functionality being added.  In addition, msrp: and msrps: URLs can
   contain parameters which are extensible.

13.  CPIM compatibility

   MSRP sessions may go to a gateway to other CPIM [24] 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" [12] 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



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   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.

   If a message is to be wrapped in a message/cpim envelope, the
   wrapping MUST be done prior to breaking the message into chunks, if
   needed.

   All MSRP endpoints MUST recognize the From, To, DateTime, and Require
   headers as defined in RFC3862.  Such applications SHOULD recognize
   the CC header, and MAY recognize the Subject header.  Any MSRP
   application that recognizes any message/cpim header MUST understand
   the NS (name space) header.

   All message/cpim body parts send by an MSRP endpoint MUST include the
   From and To headers.  If the message/cpim body part is protected
   using S/MIME, then it MUST also include the DateTime header.

   The NS, To, and CC headers may occur multiple times.  Other headers
   defined in RFC3862 MUST NOT occur more than once in a given
   message/cpim body part in an MSRP message.  The Require header MAY
   include multiple values.  The NS header MAY occur zero or more times,
   depending on how many name spaces are being referenced.

   Extension headers MAY occur more than once, depending on the
   definition of such headers.

14.  Security Considerations

   Instant Messaging systems are used to exchange a variety of sensitive
   information ranging from personal conversations, to corporate
   confidential information, to account numbers and other financial
   trading information.  IM is used by individuals, corporations, and
   governments for communicating important information.  Like many
   communications systems, the properties of Integrity and
   Confidentiality of the exchanged information, along with the
   possibility of Anonymous communications, and knowing you are
   communicating with the correct other party are required.  MSRP pushes
   many of the hard problems to SIP when SIP sets up the session, but
   some of the problems remain.  Spam and DoS attacks are also very
   relevant to IM systems.

   MSRP needs to provide confidentiality and integrity for the messages
   it transfers.  It also needs to provide assurances the connected host
   is the host that it meant to connect to and that the connection has
   not been hijacked.



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   When using only TCP connections, MSRP security is fairly weak.  If
   host A is contacting B, B passes its hostname and a secret to A using
   SIP.  If the SIP offer or answer is not TLS or S/MIME [26] protected,
   anyone can see this secret.  A then connects to the provided host
   name and passes the secret in the clear across the connection to B.
   A assumes that it is talking to B based on where it sent the SYN
   packet and then delivers the secret in plain text across the
   connections.  B assumes it is talking to A because the host on the
   other end of the connection delivered the secret.  An attacker that
   could ACK the SYN packet could insert itself as a man in the middle
   in the connection.

   When using TLS connections, the security is significantly improved.
   We assume that the host accepting the connection has a certificate
   from a well know certificate authority.  Furthermore, we assume that
   the SIP signaling to set up the session is protected with TLS (using
   sips).  In this case, when host A contacts host B, the secret is
   passed through a SIP confidential channel to A.  A connects with TLS
   to B.  B presents a valid certificate, so A knows it really is
   connected to B.  A then delivers the secret provided by B, so that B
   can verify it is connected to A.  In this case, a rogue SIP Proxy can
   see the secret in the SIP signaling traffic and could potentially
   insert itself as a man-in-the-middle.

   Realistically, using TLS is only feasible when connecting to gateways
   or relays , as the types of hosts that end clients use for sending
   instant messages are unlikely to have a long term stable IP address
   or a stable DNS name that a certificate can bind to.  In addition,
   the cost of server certificates from well known certificate
   authorities is currently too high for the vast majority of end users
   to even consider getting one for each client.

   The only strong security for connections without relays is achieved
   using S/MIME.  This does not require the actual endpoint to have
   certificates from a well known certificate authority.  The Identity
   [21] and Certificates [22] mechanism with SIP provides S/MIME based
   delivery of a secret between A and B.  No SIP intermediary except the
   explicitly trusted authentication service (one per user) can see the
   secret.  The S/MIME encryption of the SDP can also be used by SIP to
   exchange keying material that can be used in MRSP.  The MSRP session
   can then use S/MIME with this keying material to encrypt and sign
   messages sent over MSRP.  The connection can still be hijacked since
   the secret is sent in clear text to the other end of the TCP
   connection, but this risk is mitigated if all the MSRP content is
   encrypted and signed with S/MIME.  It is out of scope for this
   document but thre is nothing stopping the SIP negoatiation of MSRP
   session from negoatating symetric keying material that is used with
   S/MIME for intgrity and privacy.  Using TLS with a DH profile is also



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   possible.

   MSRP can not be used as an amplifier for DoS attacks, but it can be
   used to form a distributed attack to consume TCP connection resource
   on servers.  The attacker, Eve, sends an SIP INVITE with no offer to
   Alice.  Alice returns a 200 with an offer and Eve returns an answer
   with the SDP that indicates that her MSRP address is the address of
   Tom.  Since Alice sent the offer, Alice will initiate a connection to
   Tom using up resources on Tom's server.  Given the huge number of IM
   clients, and the relatively few TCP connections that most servers
   support, this is a fairly straightforward attack.

   SIP is attempting to address issues in dealing with spam.  The spam
   issue is probably best dealt with at the SIP level when an MSRP
   session is initiated and not at the MSRP level.

   TLS is used to authenticate devices and to provide integrity and
   confidentiality for the headers being transported.  MSRP elements
   MUST implement TLS and MUST also implement the TLS
   ClientExtendedHello extended hello information for server name
   indication as described in [10].  A TLS cipher-suite of
   TLS_RSA_WITH_AES_128_CBC_SHA [13] MUST be supported (other
   cipher-suites MAY also be supported).

   Since MSRP carries arbitrary MIME content, it can trivially carry
   S/MIME protected messages as well.  All MSRP implementations MUST
   support the multipart/signed MIME type even if they do not support
   S/MIME.  Since SIP can carry a session key, S/MIME messages in the
   context of a session could also be protected using a key-wrapped
   shared secret [25] provided in the session setup.  MSRP is a binary
   protocol and MIME bodies MUST be transfered with a transfer encoding
   of binary.  If a message is both signed and encrypted, it SHOULD be
   signed first, then encrypted.  If S/MIME is supported, SHA-1, RSA,
   and AES-128 MUST be supported.

   If a sender chooses to employ S/MIME to protect a message, all S/MIME
   operations MUST occur prior to breaking the message into chunks, if
   needed.

   The signaling will have set up the session to or from some specific
   URLs that will often have "im:" or "sip:" URI schemes.  When the
   signaling has been set up to a specific end users, and S/MIME is
   implemented, then the client needs to verify that the name in the
   SubjectAltName of the certificate contains an entry that matches the
   URI that was used for the other end in the signaling.  There are some
   cases, such as IM conferencing, where the S/MIME certificate name and
   the signaled identity will not match.  In these cases the client
   should ensure that the user is informed that the message came from



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   the user identified in the certificate and does not assume that the
   message came from the party they signaled.

   In some cases, a sending device may need to attribute a message to
   some other identity, and may use different identities for different
   messages in the same session.  For example, a conference server may
   send messages on behalf of multiple users on the same session.
   Rather than add additional headers to MSRP for this purpose, MSRP
   relies on the message/cpim format for this purpose.  The sender may
   envelope such a message in a message/cpim body, and place the actual
   sender identity in the From field.  The trustworthiness of such an
   attribution is affected by the security properties of the session in
   the same way that the trustworthiness of the identity of the actual
   peer is affected, with the additional issue of determining whether
   the recipient trusts the sender to assert the identity.

   This approach can result in nesting of message/cpim envelopes.  For
   example, a message originates from a CPIM gateway, and is then
   forwarded by a conference server onto a new session.  Both the
   gateway and the conference server introduce envelopes.  In this case,
   the recipient client SHOULD indicate the chain of identity assertions
   to the user, rather than allow the user to assume that either the
   gateway or the conference server originated the message.

   It is possible that a recipient might receive messages that are
   attributed to the same sender via different MSRP sessions.  For
   example, Alice might be in a conversation with Bob via an MSRP
   session over a TLS protected channel.  Alice might then receive a
   different message from Bob over a different session, perhaps with a
   conference server that asserts Bob's identity in a message/cpim
   envelope signed by the server.

   MSRP does not in any way prohibit multiple simultaneous sessions
   between the same pair of identities.  Nor does it prohibit an
   endpoint sending a message on behalf of another identity, such as may
   be the case for a conference server.  The recipient's endpoint should
   determine its level of trust of the authenticity of the sender
   independently for each session.  The fact that an endpoint trusts the
   authenticity of the sender on any given session should not affect the
   level of trust it assigns for apparently the same sender on a
   different session.

   When MSRP clients from or acquire a certificate, they SHOULD ensure
   that the subjectAltName has a GeneralName entry of type
   uniformResourceIdentifier for each URI corresponding to this client
   and should always include an "im:" URI as well as a "sip:" URI.  It
   is fine if the certificate contains other URIs such as an "xmpp:"
   URI.



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15.  IANA Considerations

15.1  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
   Section 6

15.2  MSRP URL Schemes

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

   Syntax See Section 6.
   Character Encoding See Section 6.
   Intended Usage See Section 6.
   Protocols The Message Session Relay Protocol (MSRP).
   Security Considerations See Section 14.
   Relevant Publications RFCXXXX
         [Note to RFC Editor: Please replace RFCXXXX in the above
         paragraph with the actual number assigned to this document.

15.3  SDP Parameters

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

15.3.1  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 8.1.

15.3.2  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 8.1.

15.3.3  Max Size

   Attribute-name:  max-size






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   Long-form Attribute Name Maximum message size.
   Type: Media level
   Subject to Charset Attribute No
   Purpose and Appropriate Values See Section 8.1.

15.3.4  Path

   Attribute-name:  path
   Long-form Attribute Name MSRP URL Path
   Type: Media level
   Subject to Charset Attribute No
   Purpose and Appropriate Values See Section 8.1.1.

16.  Change History

16.1  draft-ietf-simple-message-sessions-09

   o  Updated retransmission when receiving a failure report.
   o  Added applicability statement.
   o  Added CPIM application considerations.
   o  Added language to security considerations about receiving messages
      from the same sender over different sessions.
   o  Added 501 response code.
   o  Various scrubbing of the ABNF
   o  Change resource construction name to session-id in MSRP syntax.
   o  Added language to define the purpose of msrp URLs.
   o  Change RFC2396 reference to 2396bis
   o  Clarify that max-size is in octets.
   o  Clarify that userinfo is restricted to unreserved characters,
      which is an additional restriction over the RFC2396 version.
   o  Consolidated the ABNF for the MSRP URL into the formal syntax
      section.
   o  Clarified that if an MSRP endpoint receives and SDP offer and does
      not understand any of the media types, it SHOULD return a SIP 488
      response, or whatever is appropriate for the rendezvous protocol.
   o  Added more text around using message/cpim for identity
      attribution.

16.2  draft-ietf-simple-message-sessions-08

   o  Removed DSN section.  Removed statements that an error report
      SHOULD contain a body.  REPORT requests may now contain
      informational bodies no larger than 2K, but the recipient is free
      to ignore them.
   o  Added the "#" value for the continuation-flag to indicate the last
      chunk of an abandoned message.
   o  Added direction that s/mime and cpim envelops must be applied
      before chunking.



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   o  Added direction to set the last-byte field in byte-range to "*" if
      there is any chance of interrupting a SEND request.
   o  Changed to refer to entire message, instead of a particular MIME
      content-type
   o  Added requirent for the use of UTF-8, and reference to RFC3629
   o  Added requrement to ignore unknown headers.
   o  Several ABNF fixes
   o  Removed redundant material between normative sections.
   o  Numerous editorial fixes.

16.3  draft-ietf-simple-message-sessions-07

   o  Significant re-write to attempt to improve readability.
   o  Added maximum size parameter in accept-types
   o  Changed the Boundary field to be part of the start-line rather
      than a header field.
   o  Removed the TR-IDheader, and changed request-response matching to
      be based on the Boundary field value.  Responses still contain the
      TR-ID header, which must match the Boundary from the request.
   o  Removed transport selection from URL scheme and added the "tcp"
      parameter.
   o  Added description of the "simple" mode with no transaction
      responses, and made mode selection dependent on the reporting
      level requested for a give message.
   o  Changed the DSN section to reflect separate request of success and
      failure reports.  Enhanced REPORT method to be useful even without
      a payload.
   o  removed SRV usage for URL resolution.  This is only used for relay
      discovery, and therefore should be moved to the relay draft.
   o  Added discussion about late REPORT handling.  Asserted that REPORT
      requests are always sent in simple mode.
   o  Removed the dependency on multipart/byteranges for fragmentation.
      Incorporated the Byte-Range header into the base MSRP header set.
   o  Removed the VISIT method.  Change to use SEND to serve the purpose
      formerly reserved to VISIT.

16.4  draft-ietf-simple-message-sessions-06

   o  Changed To and From header names to To-Path and From-Path.  Added
      more clarification to path handling, and commentary on how it
      enables relay usage.
   o  Changed mechanism for signaling transport and TLS protection into
      the MSRP URL, rather than the SDP M-Line.
   o  Removed length field from start line and added Boundary header
      field and Closing field.
   o  Added recommendation to fragment any content over 2k.
   o  Added Rohan's proposal to make offerer connect to answerer.  (With
      open issue for more discussion.)



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   o  Changed To-Path and From-Path usage in responses to indicate the
      destination and source of the response, rather than merely copy
      from the associated request.
   o  Updated DSN section.  Added text on field usage.
   o  Fixed change TR-ID header from version 05 were erroneously
      attributed to 04.

16.5  draft-ietf-simple-message-sessions-05

   o  Changed the use of session URLs.  Instead of a single session URL,
      each endpoint is identified by a distinct URL.  MSRP requests will
      put the destination URL in a To header, and the sender URL in a
      From header.
   o  Changed the SDP exchange of MSRP URLs to handle the URL for each
      endpoint.  Further, changed the SDP attribute to support a list of
      URLs in each direction.  This may be used with relays to exchange
      paths, rather than single URLs.  MSRP endpoints must be able to
      intelligently process such a list if received.  This document does
      not, however, describe how to generate such a list.
   o  Added section for Delivery Status Notification handling, and added
      associated entries into the syntax definition.
   o  Added content fragmentation section.
   o  Removed recommendation to start separate session for large
      transfers.
   o  Corrected some mistakes in the syntax definitions.
   o  Added Chris Boulton as a co-author for his contribution of the DSN
      text.

16.6  draft-ietf-simple-message-sessions-04

   o  Removed the direction attribute.  Rather than using a comedia
      styled direction negotiation, we just state that the answerer
      opens any needed connection.

16.7  draft-ietf-simple-message-sessions-03

   o  Removed all specification of relays, and all features specific to
      the use of relays.  The working group has chosen to move relay
      work into a separate effort, in order to advance the base
      specification.  (The MSRP acronym is unchanged for the sake of
      convenience.) This included removal of the BIND method, all
      response codes specific to BIND, Digest Authentication, and the
      inactivity timeout.
   o  Removed text indicating that an endpoint could retry failed
      requests on the same connection.  Rather, the endpoint should
      consider the connection dead, and either signal a reconnection or
      end the session.




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   o  Added text describing subsequent SDP exchanges.  Added mandatory
      "count" parameter to the direction attribute to allow explicit
      signaling of the need to reconnect.
   o  Added text to describe the use of send and receive only indicators
      in SDP for one-way transfer of large content.
   o  Added text requiring unique port field values if multiple M-line's
      exist.
   o  Corrected a number of editorial mistakes.

16.8  draft-ietf-simple-message-sessions-02

   o  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.
   o  Added "other-method" construction to the message syntax to allow
      for extensible methods.
   o  Consolidated all syntax definitions into the same section.
      Cleaned up ABNF for digest challenge and response syntax.
   o  Changed the session inactivity timeout to 12 minutes.
   o  Required support for the SHA1 algorithm.
   o  Required support for the message/cpim format.
   o  Fixed lots of editorial issues.
   o  Documented a number of open issues from recent list discussions.

16.9  draft-ietf-simple-message-sessions-01

   o  Abstract rewritten.
   o  Added architectural considerations section.
   o  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.
   o  Added a standard dummy value for the m-line port field.  Clarified
      that a zero in this field has normal SDP meaning.
   o  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.
   o  Changed digest algorithm to SHA1.  Added TR-ID and S-URI to the
      hash for digest authentication.
   o  CMS usage replaced with S/MIME.
   o  TLS and msrps: usage clarified.
   o  Session state timeout is now based on SEND activity, rather than
      BIND and VISIT refreshes.
   o  Default port added.
   o  Added sequence diagrams to the example message flows.
   o  Added discussion of self-signed certificates in the security
      considerations section.




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16.10  draft-ietf-simple-message-sessions-00

   o  Name changed to reflect status as a work group item.
   o  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.
   o  BIND and VISIT now create soft state, eliminating the need for the
      RELEASE and LEAVE methods.
   o  The MSRP URL format was changed to better reflect generic URL
      standards.  URL comparison and resolution rules were added.  SRV
      usage added.
   o  Determination of host and visitor roles now uses a direction
      attribute much like the one used in COMEDIA.
   o  Format list negotiation expanded to allow a "prefer these formats
      but try anything" semantic
   o  Clarified handling of direction notification failures.
   o  Clarified signaling associated with session failure due to dropped
      connections.
   o  Clarified security related motivations for MSRP.
   o  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.

16.11  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.

17.  Contributors and Acknowledgments

   In addition to the editors, The following people contributed
   extensive work to this document: Chris Boulton, Paul Kyzivat, Orit
   Levin, Adam Roach, Jonathan Rosenberg, and Robert Sparks.

   The following people contributed substantial discussion and feedback
   to this ongoing effort: Eric Burger, Allison Mankin, Jon Peterson,
   Brian Rosen, Dean Willis, Aki Niemi, Hisham Khartabil, Pekka Pessi,
   Miguel Garcia, Peter Ridler, and Sam Hartman.





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18.  References

18.1  Normative References

   [1]   Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
         2246, January 1999.

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

   [3]   Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
         Session Description Protocol (SDP)", RFC 3264, June 2002.

   [4]   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.

   [5]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

   [6]   Crocker, D. and P. Overell, "Augmented BNF for Syntax
         Specifications: ABNF", RFC 2234, November 1997.

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

   [8]   Troost, R., Dorner, S. and K. Moore, "Communicating
         Presentation Information in Internet Messages: The
         Content-Disposition Header Field", RFC 2183, August 1997.

   [9]   Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
         Resource Identifiers (URI): Generic Syntax",
         draft-fielding-uri-rfc2396bis-07 (work in progress), September
         2004.

   [10]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J. and
         T. Wright, "Transport Layer Security (TLS) Extensions", RFC
         3546, June 2003.

   [11]  Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE
         Method", RFC 3311, October 2002.

   [12]  Klyne, G. and D. Atkins, "Common Presence and Instant Messaging
         (CPIM): Message Format", RFC 3862, August 2004.

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



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   [14]  Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
         3269, November 2003.

18.2  Informational References

   [15]  Johnston, A. and O. Levin, "Session Initiation Protocol Call
         Control - Conferencing for User Agents",
         draft-ietf-sipping-cc-conferencing-05 (work in progress),
         October 2004.

   [16]  Rosenberg, J., Peterson, J., Schulzrinne, H. and G. Camarillo,
         "Best Current Practices for Third Party Call Control in the
         Session  Initiation Protocol", rfc 3725, April 2004.

   [17]  Sparks, R. and A. Johnston, "Session Initiation Protocol Call
         Control - Transfer", draft-ietf-sipping-cc-transfer-03 (work in
         progress), October 2004.

   [18]  Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C. and
         D. Gurle, "Session Initiation Protocol (SIP) Extension for
         Instant Messaging", RFC 3428, December 2002.

   [19]  Mahy, R., "Benefits and Motivation for Session Mode Instant
         Messaging", draft-mahy-simple-why-session-mode-01 (work in
         progress), February 2004.

   [20]  Jennings, C. and R. Mahy, "Relay Extensions for Message
         Sessions Relay Protocol (MSRP)",
         draft-ietf-simple-msrp-relays-02 (work in progress), October
         2004.

   [21]  Peterson, J. and C. Jennings, "Enhancements for Authenticated
         Identity Management in the Session Initiation  Protocol (SIP)",
         draft-ietf-sip-identity-03 (work in progress), September 2004.

   [22]  Jennings, C. and J. Peterson, "Certificate Management Service
         for SIP", draft-ietf-sipping-certs-00 (work in progress),
         October 2004.

   [23]  Yon, D., "Connection-Oriented Media Transport in SDP",
         draft-ietf-mmusic-sdp-comedia-09 (work in progress), September
         2004.

   [24]  Peterson, J., "A Common Profile for Instant Messaging (CPIM)",
         rfc 3860, August 2004.

   [25]  Housley, R., "Triple-DES and RC2 Key Wrapping", RFC 3217,
         December 2001.



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

   [27]  Camarillo, G. and H. Schulzrinne, "Early Media and Ringing Tone
         Generation in the Session Initiation Protocol (SIP)",
         draft-ietf-sipping-early-media-02 (work in progress), June
         2004.

   [28]  Saint-Andre, P., "Extensible Messaging and Presence Protocol
         (XMPP): Instant Messaging and  Presence", rfc 3921, October
         2004.

   [29]  Rosenberg, J., "Indicating User Agent Capabilities in the
         Session Initiation Protocol  (SIP)", rfc 3840, August 2004.


Authors' Addresses

   Ben Campbell (editor)
   Estacado Systems

   EMail: ben@estacado.net


   Rohan Mahy (editor)
   Airespace
   110 Nortech Parkway
   San Jose, CA  95134
   USA

   EMail: rohan@ekabal.com


   Cullen Jennings (editor)
   Cisco Systems, Inc.
   170 West Tasman Dr.
   MS: SJC-21/2
   San Jose, CA  95134
   USA

   Phone: +1 408 421-9990
   EMail: fluffy@cisco.com









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