SIMPLE Working Group                                         B. Campbell
Internet-Draft                                              J. Rosenberg
Expires: July 27, 2004                                         R. Sparks
                                                              P. Kyzivat
                                                           Cisco Systems
                                                        January 27, 2004

                   The Message Session Relay Protocol

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   This Internet-Draft will expire on July 27, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.


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

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Table of Contents

   1.     Introduction . . . . . . . . . . . . . . . . . . . . . . .   4
   2.     Motivation for Session-mode Messaging  . . . . . . . . . .   4
   3.     Scope of this Document . . . . . . . . . . . . . . . . . .   5
   4.     Protocol Overview  . . . . . . . . . . . . . . . . . . . .   6
   5.     Architectural Considerations . . . . . . . . . . . . . . .   7
   5.1    Transferring Large Content . . . . . . . . . . . . . . . .   7
   6.     SDP Offer-Answer Exchanges for MSRP Sessions . . . . . . .   7
   6.1    Use of the SDP M-line  . . . . . . . . . . . . . . . . . .   8
   6.2    The Direction Attribute  . . . . . . . . . . . . . . . . .   8
   6.3    The Accept Types Attribute . . . . . . . . . . . . . . . .  10
   6.4    MIME Wrappers  . . . . . . . . . . . . . . . . . . . . . .  10
   6.5    URL Negotiations . . . . . . . . . . . . . . . . . . . . .  11
   6.6    Updated SDP Offers . . . . . . . . . . . . . . . . . . . .  12
   6.7    Example SDP Exchange . . . . . . . . . . . . . . . . . . .  13
   7.     The Message Session Relay Protocol . . . . . . . . . . . .  13
   7.1    MSRP URLs  . . . . . . . . . . . . . . . . . . . . . . . .  14
   7.1.1  MSRP URL Comparison  . . . . . . . . . . . . . . . . . . .  14
   7.1.2  Resolving MSRP Host Device . . . . . . . . . . . . . . . .  15
   7.1.3  The msrps URL Scheme . . . . . . . . . . . . . . . . . . .  16
   7.2    MSRP messages  . . . . . . . . . . . . . . . . . . . . . .  16
   7.3    MSRP Transactions  . . . . . . . . . . . . . . . . . . . .  17
   7.4    MSRP Sessions  . . . . . . . . . . . . . . . . . . . . . .  17
   7.4.1  Initiating an MSRP session . . . . . . . . . . . . . . . .  17
   7.4.2  Handling VISIT requests  . . . . . . . . . . . . . . . . .  21
   7.4.3  Sending Instant Messages on a Session  . . . . . . . . . .  21
   7.4.4  Ending a Session . . . . . . . . . . . . . . . . . . . . .  22
   7.4.5  Managing Session State and Connections . . . . . . . . . .  23
   7.5    Method Descriptions  . . . . . . . . . . . . . . . . . . .  24
   7.5.1  SEND . . . . . . . . . . . . . . . . . . . . . . . . . . .  24
   7.5.2  VISIT  . . . . . . . . . . . . . . . . . . . . . . . . . .  24
   7.6    Response Code Descriptions . . . . . . . . . . . . . . . .  24
   7.6.1  200  . . . . . . . . . . . . . . . . . . . . . . . . . . .  24
   7.6.2  400  . . . . . . . . . . . . . . . . . . . . . . . . . . .  25
   7.6.3  415  . . . . . . . . . . . . . . . . . . . . . . . . . . .  25
   7.6.4  426  . . . . . . . . . . . . . . . . . . . . . . . . . . .  25
   7.6.5  481  . . . . . . . . . . . . . . . . . . . . . . . . . . .  25
   7.6.6  506  . . . . . . . . . . . . . . . . . . . . . . . . . . .  25
   7.7    Header Field Descriptions  . . . . . . . . . . . . . . . .  25
   7.7.1  TR-ID  . . . . . . . . . . . . . . . . . . . . . . . . . .  25
   7.7.2  Content-Type . . . . . . . . . . . . . . . . . . . . . . .  25
   7.7.3  S-URL  . . . . . . . . . . . . . . . . . . . . . . . . . .  26
   8.     Example  . . . . . . . . . . . . . . . . . . . . . . . . .  26
   9.     IANA Considerations  . . . . . . . . . . . . . . . . . . .  28
   9.1    MSRP Port  . . . . . . . . . . . . . . . . . . . . . . . .  28
   9.2    MSRP URL Schemes . . . . . . . . . . . . . . . . . . . . .  28
   9.2.1  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . .  28

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   9.2.2  Character Encoding . . . . . . . . . . . . . . . . . . . .  28
   9.2.3  Intended Usage . . . . . . . . . . . . . . . . . . . . . .  28
   9.2.4  Protocols  . . . . . . . . . . . . . . . . . . . . . . . .  28
   9.2.5  Security Considerations  . . . . . . . . . . . . . . . . .  29
   9.2.6  Relevant Publications  . . . . . . . . . . . . . . . . . .  29
   9.3    SDP Parameters . . . . . . . . . . . . . . . . . . . . . .  29
   9.3.1  Direction  . . . . . . . . . . . . . . . . . . . . . . . .  29
   9.3.2  Accept Types . . . . . . . . . . . . . . . . . . . . . . .  29
   9.3.3  Wrapped Types  . . . . . . . . . . . . . . . . . . . . . .  29
   10.    Security Considerations  . . . . . . . . . . . . . . . . .  29
   10.1   TLS and the MSRPS Scheme . . . . . . . . . . . . . . . . .  30
   10.1.1 Sensitivity of the Session URL . . . . . . . . . . . . . .  30
   10.1.2 End to End Protection of IMs . . . . . . . . . . . . . . .  31
   10.1.3 CPIM compatibility . . . . . . . . . . . . . . . . . . . .  31
   10.1.4 PKI Considerations . . . . . . . . . . . . . . . . . . . .  32
   11.    Changes from Previous Draft Versions . . . . . . . . . . .  32
   11.1   draft-ietf-simple-message-sessions-03  . . . . . . . . . .  32
   11.2   draft-ietf-simple-message-sessions-02  . . . . . . . . . .  33
   11.3   draft-ietf-simple-message-sessions-01  . . . . . . . . . .  33
   11.4   draft-ietf-simple-message-sessions-00  . . . . . . . . . .  34
   11.5   draft-campbell-simple-im-sessions-01 . . . . . . . . . . .  34
   12.    Contributors . . . . . . . . . . . . . . . . . . . . . . .  34
          Normative References . . . . . . . . . . . . . . . . . . .  35
          Informational References . . . . . . . . . . . . . . . . .  35
          Authors' Addresses . . . . . . . . . . . . . . . . . . . .  36
          Intellectual Property and Copyright Statements . . . . . .  38

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1. Introduction

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

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

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

2. Motivation for Session-mode Messaging

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

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

   Due to network congestion concerns, the MESSAGE method has

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

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

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

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

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

3. Scope of this Document

   This document describes the use of MSRP between endpoints. It does
   not specify the use of intermediaries, nor does it prohibit such use.
   We expect an extension to this specification to define MSRP
   intermediaries and their use.

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

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4. Protocol Overview

   The Message Session Relay Protocol (MSRP) provides a mechanism for
   transporting session-mode messages between endpoints. MSRP uses
   connection oriented, reliable network transport protocols only. It
   can operate in the presence of many NAT and firewall environments, as
   it allows participants to positively associate message sessions with
   specific connections, and does not depend upon connection source
   address, which may be obscured by NATs.

   MSRP uses the following primitives:

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

   VISIT: Used by an endpoint to establish a session association to the
      host endpoint.

   Assume A is an endpoint that wishes to establish a message session,
   and B is the endpoint invited by A. A invites B to participate in a
   message session by sending a URL that represents the session. This
   URL is temporary, and must not duplicate the URL used for any other
   active sessions.

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

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

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

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

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   A->B (MSRP): 200 OK

5. Architectural Considerations

   There are a number of considerations that, if handled in a reasonable
   fashion, will allow more effective use of the protocols described in
   this document.

5.1 Transferring Large Content

   MSRP endpoints may attempt to send very long messages in a session.
   For example, most commercial instant messaging systems have a file
   transfer feature. Since MSRP does not impose message size limits,
   there is nothing to prevent endpoints from transferring files over

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

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

   These issues can be mitigated greatly if the endpoint simply
   establishes a separate session for the transfer. This allows the
   transfer to be sent without interfering with any instant messages
   being sent on other sessions. Further, the endpoint can abort the
   transfer by simply tearing down the transfer session. Therefore, if a
   peer wishes to send very large content, it SHOULD establish a
   dedicated session for that purpose. It should also indicate that the
   dedicated session is send only, so that the receiving endpoint does
   not attempt to send content back along the same session.

6. SDP Offer-Answer Exchanges for MSRP Sessions

   MSRP sessions will typically be initiated using the Session
   Description Protocol (SDP) [1] offer-answer mechanism, carried in the
   Session Initiation Protocol (SIP) [2] or any other protocol
   supporting it. MSRP borrows the idea of the direction attributes from

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   COMEDIA [18], but does not depend on that specification.

6.1 Use of the SDP M-line

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

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

   For non-RTP media sessions, The media field specifies the top level
   MIME media type for the session. For MSRP sessions, the media field
   MUST have the value of "message". The port field is normally not
   used, and MAY be set to any value chosen by the endpoint. A port
   field value of zero has the standard SDP meaning. Non-zero values
   MUST not be repeated in other MSRP m-lines in the same SDP document.

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

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

   The port field in the M-line is not used to determine the port to
   which to connect. Rather, the actual port is determined by the
   contents of the session URL (Section 7.1). However, a port value of
   zero has the normal SDP meaning.

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

      m=message 9999 msrp/tcp *

6.2 The Direction Attribute

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

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   The endpoint that accepts the connection is said to "host" the
   session, and is known as the hosting endpoint. The endpoint that
   initiates the connection is said to "visit" the session, and is known
   as the visiting endpoint.

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

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

   The values for the role field are as follows:

   passive: The endpoint wishes to host the session

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

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

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

   The count is used to determine if a new connection is required in
   future SDP exchanges for a given stream. For the initial SDP
   exchange, the count pamameter MUST be set to zero. Endpoints sending
   a new offer related to an existing stream MUST increment this count
   from the value in the most recent successful exchange for the stream.

   The SDP answer also MUST contain a direction attribute, but its value
   choices are limited based on the value in the offer. If the offer
   contained "active", then the answerer MUST either select "passive" or
   reject the offer. Likewise, if the offer contained  "passive", then
   the answerer MUST select "active" or reject the offer. If the offer

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   contained "both", the answerer SHOULD select "active", but MAY select
   "passive" if it is unable to reach the host device, or if local
   policy requires it to act as host. The answerer MUST set the count
   parameter to the same value as that in the offer.

6.3 The Accept Types Attribute

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

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

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

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

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

6.4 MIME Wrappers

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

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

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

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

   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

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

6.5 URL Negotiations

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


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

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

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   The following example shows an SDP offer with a session URL of

           o=someuser 2890844526 2890844527 IN IP4
           c=IN IP4
           m=message 9999 msrp/tcp *
           a=direction:both 0

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

6.6 Updated SDP Offers

   MSRP endpoints may sometimes need to send additional SDP exchanges
   for an existing session. For example, they may need to negotiate a
   new connection because of a TCP failure or some other reason. They
   may need to send periodic exchanges with no change to refresh state
   in the network, for example, SIP 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

   Once MSRP endpoints have completed an intitial negotiation, further
   exchanges do not change their roles as the active or passive party
   for that particular stream. This means that if the visitor sends a
   new SDP offer, it MUST remain the visitor, i.e. it MUST offer a
   direction of "active" and it MUST NOT include an MSRP URL. Likewise,
   if the host sends a new offer, it MUST include a direction of
   "passive" and it MUST include a URL. Updated offers MUST NOT include
   a direction of "both."

   If offering party wishes to establish a new connection as a result of
   the updated exchange,  it MUST increment the count parameter in the
   direction attribute from that of the most recent successful exchange.
   If the passive endpoint wishes the the visitor to re-connect, it the
   included URL MUST be different than the URL from previous offers.
   This new URL MAY be completely different from the original and MAY
   even resolve to a different location. If the active party sends a new
   offer with an incremented count parameter, the passive party MUST
   supply a new URL, or reject the offer. If either party sends a new

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   offer with the same count value as the previous exchange, the session
   URI MUST NOT change.

   If this negotiation results in a new session URL, the active party
   MUST close the existing connection, open a new connection, and send a
   VISIT request as described below.

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

6.7 Example SDP Exchange

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

     o=usera 2890844526 2890844527 IN IP4
     c=IN IP4
     t=0 0
     m=message 9999 msrp/tcp *
     a=accept-types: message/cpim text/plain text/html
     a=direction:both 0

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

     o=userb 2890844530 2890844532 IN IP4
     c=IN IP4 dontlookhere
     t=0 0
     m=message 9999 msrp/tcp *
     a=accept-types:message/cpim text/plain
     a=direction:active 0

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

7. The Message Session Relay Protocol

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

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


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

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

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

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

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

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

   The following is an example of a typical MSRP URL:


7.1.1 MSRP URL Comparison

   MSRP URL comparisons MUST be performed according to the following

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   1.  The host part is compared as case insensitive.

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

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

   4.  Userinfo parts are not considered for URL comparison.

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

7.1.2 Resolving MSRP Host Device

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

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

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

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

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

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

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

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

   5.  If the connection attempt fails, the device SHOULD attempt to
       connect to the addresses returned in any additional A or AAAA
       records, in the order the records were presented. If all of these

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       fail, the device SHOULD attempt to use any additional SRV records
       that may have been returned, following the normal rules for SRV
       record selection.

   In most cases, the transport protocol will be determined separately
   from the resolution process. For example, if the MSRP URL was
   communicated in an SDP offer or answer, the SDP M-line will contain
   the transport protocol. When an MSRP URL is communicated outside of
   SDP, the protocol SHOULD also be communicated. If a device needs to
   resolve an MSRP URL and does not know the protocol, it SHOULD assume

7.1.3 The msrps URL Scheme

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

7.2 MSRP messages

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

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

       length       = 1*DIGIT  ; the length of the message,
                               ;  exclusive of the start line.
       Method       = SEND / VISIT / other-method
       other-method = token
       header       = Transaction-ID / Session-URL / Content-Types
       Status-Code  = 200    ;Success
                    / 400    ;Bad Request
                    / 403    ;Forbidden
                    / 415    ;Unsupported Content Type
                    / 426    ;Upgrade Required
                    / 481    ;No session
                    / 506    ;duplicate session

       Reason              = token ; Human readable text describing status
       Transaction-ID      = "Tr-ID" ":" token

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       Content-Type        = "Content-Type" ":" quoted-string
       Session-URL         = "S-URL" ":" msrp_url

   All requests and responses MUST contain at least a TR-ID header
   field. Messages MAY contain other fields, depending on the method or
   response code.

7.3 MSRP Transactions

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

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

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

7.4 MSRP Sessions

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

7.4.1 Initiating an MSRP session

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

   The offerer SHOULD volunteer to act as the hosting endpoint if
   allowed by policy and network topology. The peer that is not the host
   is designated as the visitor. The offerer MAY request the answerer to
   act as host if it is prevented from accepting connections by network
   topology or policy.

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   If the offerer wishes to host the session, it MUST perform the
   following steps:

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

   2.  Listen for a connection from the peer.

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

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

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

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

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

   2.  Insert a direction attribute with a value of "active", with an
       appropriate count parameter value.

   3.  Send the offer using normal processing for the signaling

   When the answerer receives the SDP offer and chooses to participate
   in the session, it must choose whether to act as the host or the
   visitor. A direction attribute value of "both" in the offer indicates
   that the offerer prefers to host, but will allow the answerer to
   host.  In this case the answerer SHOULD act as the visitor, but MAY
   choose to host. A value of "passive" means the offerer insists upon

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   hosting, in which case the answerer MUST act as visitor or decline
   the offer.

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

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

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

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

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

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

       3.  A size field containing size of the message subsequent to the

   4.  Send the request and wait for a response

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

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

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

       3.  A direction attribute containing the value "active", and the
           count value copied from the offer.

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

   Some TCP connection failure conditions may ordinarily take some time
   to notice. For example, if the offerer is unable to open a TCP
   connection to the host device, this connection attempt may take a
   fairly large number of seconds to timeout. This length of time will

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   not be acceptable for many call flow scenarios. Therefore, the
   devices SHOULD limit the time they wait for the TCP connection to a
   shorter timeout value, which will default to 30 seconds. However, the
   offerer MAY supply a different time in the timeout parameter of the
   "both" direction value. If the offerer supplies a value, the answerer
   SHOULD use that value for the TCP connection timeout, interpreted as
   an integer number of seconds.

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

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

   2.  Listen for a connection from the peer.

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

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

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

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

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

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

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

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

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

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

       3.  A size field containing size of the message subsequent to the

   5.  Send the request and wait for a response

   6.  If either the connection attempt or the VISIT transaction fail,
       acknowledge the answer, then initiate the tear-down of the
       session using the signaling protocol.

7.4.2 Handling VISIT requests

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

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

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

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

7.4.3 Sending Instant Messages on a Session

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

   1.  Insert the message payload in the body, and the media type in the
       Content-Type header field. The media type MUST match one of the
       types in the format list negotiated in the SDP exchange. If a "*"
       was present in the accept-types attribute, then the media type
       SHOULD match one of the explicitly listed entries, but MAY be any
       other arbitrary value.

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

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

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

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

   6.  If any other response code is received, or if the transaction
       times out, the endpoint SHOULD assume the session has failed, and
       initiate tear-down, either ending the session, or by sending an
       updated SDP offer proposing a new connection. If a new connection
       is established, the endpoint MAY choose to resend the content on
       the new connection.

      Open Issue: Do we need to create a duplicate mechanism to suppress
      duplicate messages when a new connection is established in this
      fashion? mechanism? List consensus seems to indicate we do. We may
      need to specify that the tr-id space spans a sequence of
      connections if they are associated with same stream, and of
      course, specify what it means for a stream to span connections.

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

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

   2.  If it does, return a 200 response and render the message to the
       user. The method of rendering is a matter of local policy. If the
       message contained no body at all, the endpoint should quietly
       ingore it.

   3.  If it does not understand the media type, return a 415 response.
       The endpoint MUST NOT return a 415 response for any media type
       for which it indicated support in the SDP exchange.

7.4.4 Ending a Session

   When either endpoint in an MSRP session wishes to end the session, it
   first signals its intent using the normal processing for the
   signaling protocol. For example, in SIP, it would send a BYE request
   to the peer. After agreeing to end the session, the host endpoint
   MUST release any resources acquired as part of the session.

   The host MUST destroy local state for the session. This involves
   completely removing the state entry for this session and invalidating
   session URL.

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      Since these host actions completely destroy the session state at
      the hosting device, the visitor is not required to take further
      action beyond cleaning up any local state.

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

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

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

7.4.5 Managing Session State and Connections

   A MSRP session is represented by state at the host device. As mention
   previously, session state is identified by an MSRP URL. An active
   session also has an associated network connection.

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

   If the connection fails for any reason, the session hosting device
   MUST invalidate the session state. Once a connection is dropped, the
   associated session state MUST NOT be reused. If an endpoint wishes to
   continue to communicate after detecting a connection failure, it MAY
   initiate a new SDP exchange to negotiate a new session URL.
   Otherwise, it SHOULD attempt to tear down the session using the rules
   of the signaling protocol.

      It would be nice to allow sessions to be recovered after a
      connection failure, perhaps by allowing the active endpoint to
      reconnect, and send a new VISIT request. However, this approach
      creates a race condition between the time that the hosting device
      notices the failed connection, and the time that the endpoint

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      tries to recover the session. If the endpoint attempts to
      reconnect prior to the hosting device noticing the failure, the
      hosting device will interpret the recovery attempt as a conflict.
      The only way around this would be to force the hosting device to
      do a liveness check on the original connection, which would create
      a lot of complexity and overhead that do not seem to be worth the

7.5 Method Descriptions

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

7.5.1 SEND

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

      To Do: We plan to expand the use of MIME headers to allow any
      standard MIME header in a SEND request. This is not included in
      this version for the sake of getting the draft out as soon as
      possible, but will follow soon.

7.5.2 VISIT

   The visiting endpoint uses the VISIT method to associate a network
   connection with the session state at the hosting device. The request
   MUST contain a S-URL header matching the session URL.

7.6 Response Code Descriptions

   This section summarizes the various response codes. Except where
   noted, all responses MUST contain a TR-ID header field matching the
   TR-ID header field of the associated request.

7.6.1 200

   The 200 response code indicates a successful transaction.

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7.6.2 400

   A 400 response indicates a request was unintelligible.

7.6.3 415

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

7.6.4 426

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

7.6.5 481

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

7.6.6 506

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

7.7 Header Field Descriptions

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

7.7.1 TR-ID

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

7.7.2 Content-Type

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

      To Do: The work group has agreed to allow the use of any standard
      MIME header. This is not reflected in this version, but will be in
      a shortly forthcoming one.

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7.7.3 S-URL

   The S-URL header field is used to identify the session. The S-URI
   header field must be included in VISIT requests.

8. Example

   This section shows an example message 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 and the answerer is In any given MSRP message, an "xx" in the length
   field indicates the actual length of the rest of the message.

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

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

        Alice->Bob (SIP): INVITE

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        o=alice 2890844557 2890844559 IN IP4
        c=IN IP4 fillername
        t=0 0
        m=message 9999 msrp/tcp *
        a=direction:both 0

   2.   Bob opens a TCP connection to

        Bob->Alice (MSRP):

        MSRP xx VISIT
        Tr-ID: sie09s

   3.   Alice->Bob (MSRP):

        MSRP xx 200 OK
        Tr-ID: sie09s

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

        o=bob 2890844612 2890844616 IN IP4
        c=IN IP4 ignorefield
        t=0 0
        m=message 9999 msrp/tcp *
        a=direction:active 0

   5.   Alice->Bob (SIP): ACK

   6.   Alice->Bob (MSRP):

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

   7.   Bob->Alice (MSRP):

        MSRP xx 200 OK
        TR-ID: 123

   8.   Bob->Alice (MSRP):

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

        Hi, Alice! I'm Bob!

   9.   Alice->Bob (MSRP):

        MSRP xx 200 OK
        TR-ID: 456

   10.  Alice->Bob (SIP): BYE

        Alice invalidates session and drops connection.

   11.  Bob invalidates local state for the session.

        Bob->Alice (SIP): 200 OK

9. IANA Considerations

9.1 MSRP Port

   MSRP uses TCP port XYX, to be determined by IANA after this document
   is approved for publication. Usage of this value is described in
   Section 7.1

9.2 MSRP URL Schemes

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

9.2.1 Syntax

   See Section 7.1.

9.2.2 Character Encoding

   See Section 7.1.

9.2.3 Intended Usage

   See Section 7.1.

9.2.4 Protocols

   The Message Session Relay Protocol (MSRP).

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9.2.5 Security Considerations

   See Section 10.

9.2.6 Relevant Publications


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

9.3 SDP Parameters

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

9.3.1 Direction

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

9.3.2 Accept Types

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

9.3.3 Wrapped Types

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

10. Security Considerations

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

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10.1 TLS and the MSRPS Scheme

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

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

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

   MSRP devices acting in the role of TCP client MAY perform a TLS
   handshake at any time, as long as the request occurs between MSRP
   messages. The endpoint MUST NOT send a start TLS request in the
   middle of an MSRP message.

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

   The MSRPS URI scheme indicates that all hops in an MSRP session MUST
   be protected with TLS. Since this document does not specify the use
   of intermidiary devices, then MSRPS support is trivially equivilant
   to TLS support. However, if intermediaries do exist, either as
   described in an MSRP extension document, or as sort of proprietary
   devices, they MUST ensure protection at all hops for an MSRPS URL.

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

10.1.1 Sensitivity of the Session URL

   The URL of a MSRP session is used by the visiting endpoint to

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

   Therefore an MSRP device MUST support the use of TLS for at least the
   VISIT request, which by extension indicates the endpoint MUST support
   the use of TLS for all MSRP messages. Further, MSRP connections
   SHOULD actually be protected with TLS. Further, an MSRP endpoint MUST
   be capable of using the security features of the signaling protocol
   in order to protect the SDP exchange and SHOULD actually use them on
   all such exchanges. End-to-end protection schemes SHOULD be preferred
   over hop-by-hop schemes for protection of the SDP exchange.

10.1.2 End to End Protection of IMs

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

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

10.1.3 CPIM compatibility

   MSRP sessions may be gatewayed to other CPIM [17]compatible
   protocols. If this occurs, the gateway MUST maintain session state,
   and MUST translate between the MSRP session semantics and CPIM
   semantics that do not include a concept of sessions. Furthermore,
   when one endpoint of the session is a CPIM gateway, instant messages
   SHOULD be wrapped in "message/cpim" [5] bodies. Such a gateway MUST

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   include "message/cpim" as the first entry in its SDP accept-types
   attribute. MSRP endpoints sending instant messages to a peer that has
   included 'message/cpim" as the first entry in the accept-types
   attribute SHOULD encapsulate all instant message bodies in "message/
   cpim" wrappers. All MSRP endpoints MUST support the message/cpim
   type, and SHOULD support the S/MIME features of that format.

10.1.4 PKI Considerations

   Several aspects of MSRP will benefit from being used in the context
   of a public key infrastructure. For example, the MSRPS scheme allows,
   and even encourages, TLS connections between endpoint devices.  And
   while MSRP allows for a symmetric session key to protect all messages
   in a session, it is most likely that session key itself would be
   exchanged in a signaling protocol such as SIP. Since that key is
   extremely sensitive, its exchange would also need to be protected. In
   SIP, the preferred mechanism for this would be S/MIME, which would
   also benefit from a PKI.

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

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

      To Do: Add text discussion the use of TLS certificates in more

11. Changes from Previous Draft Versions

   This section to be deleted prior to publication as an RFC

11.1 draft-ietf-simple-message-sessions-03

      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.

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

11.2 draft-ietf-simple-message-sessions-02

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

11.3 draft-ietf-simple-message-sessions-01

      Abstract rewritten.
      Added architectural considerations section.
      The m-line format list now only describes the root body part for a
      request. Contained body part types may be described in the
      "accept-wrapped-types" a-line attribute.
      Added a standard dummy value for the m-line port field. Clarified
      that a zero in this field has normal SDP meaning.
      Clarified that an endpoint is globally configured as to whether or
      not to use a relay. There is no relay discovery mechanism
      intrinsic to MSRP.
      Changed digest algorithm to SHA1. Added TR-ID and S-URI to the
      hash for digest authentication.
      CMS usage replaced with S/MIME.
      TLS and MSRPS usage clarified.
      Session state timeout is now based on SEND activity, rather than
      BIND and VISIT refreshes.

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      Default port added.
      Added sequence diagrams to the example message flows.
      Added discussion of self-signed certificates in the security
      considerations section.

11.4 draft-ietf-simple-message-sessions-00

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

11.5 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.

12. Contributors

   The following people contributed substantially to this ongoing

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

Normative References

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

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

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

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

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

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

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

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

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

Informational References

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

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

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

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

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

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

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

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

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

Authors' Addresses

   Ben Campbell
   5100 Tennyson Parkway
   Suite 1200
   Plano, TX  75024


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   Jonathan Rosenberg
   600 Lanidex Plaza
   Parsippany, NJ  07054


   Robert Sparks
   5100 Tennyson Parkway
   Suite 1200
   Plano, TX  75024


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


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