SIMPLE Working Group                                         B. Campbell
Internet-Draft                                              J. Rosenberg
Expires: October 11, 2004                                      R. Sparks
                                                              P. Kyzivat
                                                           Cisco Systems
                                                              C. Boulton
                                           Ubiquity Software Corporation
                                                          April 12, 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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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   The list of current Internet-Drafts can be accessed at http://

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on October 11, 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
   6.     SDP Offer-Answer Exchanges for MSRP Sessions . . . . . . .   7
   6.1    Use of the SDP M-line  . . . . . . . . . . . . . . . . . .   7
   6.2    The Accept Types Attribute . . . . . . . . . . . . . . . .   8
   6.3    MIME Wrappers  . . . . . . . . . . . . . . . . . . . . . .   8
   6.4    URL Negotiations . . . . . . . . . . . . . . . . . . . . .   9
   6.5    Path Attributes with Multiple URLs . . . . . . . . . . . .  10
   6.6    Updated SDP Offers . . . . . . . . . . . . . . . . . . . .  11
   6.7    Example SDP Exchange . . . . . . . . . . . . . . . . . . .  11
   6.8    Connection Negotiation . . . . . . . . . . . . . . . . . .  12
   7.     The Message Session Relay Protocol . . . . . . . . . . . .  12
   7.1    MSRP URLs  . . . . . . . . . . . . . . . . . . . . . . . .  12
   7.1.1  MSRP URL Comparison  . . . . . . . . . . . . . . . . . . .  13
   7.1.2  Resolving MSRP Host Device . . . . . . . . . . . . . . . .  13
   7.1.3  The msrps URL Scheme . . . . . . . . . . . . . . . . . . .  14
   7.2    Connection Managment . . . . . . . . . . . . . . . . . . .  14
   7.3    MSRP messages  . . . . . . . . . . . . . . . . . . . . . .  15
   7.4    MSRP Transactions  . . . . . . . . . . . . . . . . . . . .  16
   7.5    MSRP Sessions  . . . . . . . . . . . . . . . . . . . . . .  17
   7.5.1  Initiating an MSRP session . . . . . . . . . . . . . . . .  17
   7.5.2  Handling VISIT requests  . . . . . . . . . . . . . . . . .  19
   7.5.3  Sending Instant Messages on a Session  . . . . . . . . . .  19
   7.5.4  Ending a Session . . . . . . . . . . . . . . . . . . . . .  20
   7.5.5  Managing Session State and Connections . . . . . . . . . .  21
   7.6    Delivery Status Notification . . . . . . . . . . . . . . .  22
   7.6.1  Endpoint DSN Request . . . . . . . . . . . . . . . . . . .  22
   7.6.2  DSN generation . . . . . . . . . . . . . . . . . . . . . .  23
   7.6.3  Receiving positive DSN . . . . . . . . . . . . . . . . . .  24
   7.6.4  Receiving negative DSN . . . . . . . . . . . . . . . . . .  24
   7.6.5  DSN headers in MSRP  . . . . . . . . . . . . . . . . . . .  24
   7.7    Message Fragmentation  . . . . . . . . . . . . . . . . . .  24
   7.7.1  MSRP Usage of message/byteranges . . . . . . . . . . . . .  24
   7.8    Method Descriptions  . . . . . . . . . . . . . . . . . . .  25
   7.8.1  SEND . . . . . . . . . . . . . . . . . . . . . . . . . . .  25
   7.8.2  VISIT  . . . . . . . . . . . . . . . . . . . . . . . . . .  25
   7.8.3  REPORT . . . . . . . . . . . . . . . . . . . . . . . . . .  26
   7.9    Response Code Descriptions . . . . . . . . . . . . . . . .  26
   7.9.1  200  . . . . . . . . . . . . . . . . . . . . . . . . . . .  26
   7.9.2  400  . . . . . . . . . . . . . . . . . . . . . . . . . . .  26
   7.9.3  415  . . . . . . . . . . . . . . . . . . . . . . . . . . .  26
   7.9.4  426  . . . . . . . . . . . . . . . . . . . . . . . . . . .  26
   7.9.5  481  . . . . . . . . . . . . . . . . . . . . . . . . . . .  26

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   7.9.6  506  . . . . . . . . . . . . . . . . . . . . . . . . . . .  26
   7.10   Header Field Descriptions  . . . . . . . . . . . . . . . .  26
   7.10.1 TR-ID  . . . . . . . . . . . . . . . . . . . . . . . . . .  27
   7.10.2 To . . . . . . . . . . . . . . . . . . . . . . . . . . . .  27
   7.10.3 From . . . . . . . . . . . . . . . . . . . . . . . . . . .  27
   7.10.4 Content-Type . . . . . . . . . . . . . . . . . . . . . . .  27
   8.     Example  . . . . . . . . . . . . . . . . . . . . . . . . .  27
   9.     IANA Considerations  . . . . . . . . . . . . . . . . . . .  30
   9.1    MSRP Port  . . . . . . . . . . . . . . . . . . . . . . . .  30
   9.2    MSRP URL Schemes . . . . . . . . . . . . . . . . . . . . .  30
   9.2.1  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . .  30
   9.2.2  Character Encoding . . . . . . . . . . . . . . . . . . . .  30
   9.2.3  Intended Usage . . . . . . . . . . . . . . . . . . . . . .  30
   9.2.4  Protocols  . . . . . . . . . . . . . . . . . . . . . . . .  30
   9.2.5  Security Considerations  . . . . . . . . . . . . . . . . .  30
   9.2.6  Relevant Publications  . . . . . . . . . . . . . . . . . .  30
   9.3    SDP Parameters . . . . . . . . . . . . . . . . . . . . . .  31
   9.3.1  Accept Types . . . . . . . . . . . . . . . . . . . . . . .  31
   9.3.2  Wrapped Types  . . . . . . . . . . . . . . . . . . . . . .  31
   9.3.3  Path . . . . . . . . . . . . . . . . . . . . . . . . . . .  31
   10.    Security Considerations  . . . . . . . . . . . . . . . . .  31
   10.1   TLS and the MSRPS Scheme . . . . . . . . . . . . . . . . .  31
   10.1.1 Sensitivity of the Session URL . . . . . . . . . . . . . .  32
   10.1.2 End to End Protection of IMs . . . . . . . . . . . . . . .  33
   10.1.3 CPIM compatibility . . . . . . . . . . . . . . . . . . . .  33
   10.1.4 PKI Considerations . . . . . . . . . . . . . . . . . . . .  34
   11.    Changes from Previous Draft Versions . . . . . . . . . . .  34
   11.1   draft-ietf-simple-message-sessions-04  . . . . . . . . . .  34
   11.2   draft-ietf-simple-message-sessions-03  . . . . . . . . . .  35
   11.3   draft-ietf-simple-message-sessions-02  . . . . . . . . . .  35
   11.4   draft-ietf-simple-message-sessions-01  . . . . . . . . . .  35
   11.5   draft-ietf-simple-message-sessions-00  . . . . . . . . . .  36
   11.6   draft-campbell-simple-im-sessions-01 . . . . . . . . . . .  37
   12.    Contributors . . . . . . . . . . . . . . . . . . . . . . .  37
          Normative References . . . . . . . . . . . . . . . . . . .  37
          Informational References . . . . . . . . . . . . . . . . .  38
          Authors' Addresses . . . . . . . . . . . . . . . . . . . .  39
          Intellectual Property and Copyright Statements . . . . . .  40

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1. Introduction

   The MESSAGE [12] 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 [14], third party call control [15], call transfer [16],
   QoS integration [17], and privacy [18] 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. This URL is temporary, and must not
   duplicate any URL that A has offered for 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 with a URL
   of its own. This informs A that B has accepted the session, and will
   accept messages at that URL. A and B may now exchange messages using
   SEND requests on the connection. Each party targets such requests to
   the peer's URL.

   When either party wishes to end the session, it informs its peer
   using the appropriate mechanism of the chosen signaling protocol,
   such as a SIP BYE request.

   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, msrp://B/456)
   A->B (MSRP): 200 OK
   B->A (SDP): answer(msrp://B/456)
   A->B (MSRP): SEND (msrp://B/456)
   B->A (MSRP): 200 OK

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   B->A (MSRP): SEND (msrp://A/123)
   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.

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.

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 the
   MSRP URL (Section 7.1) in the path attribute. However, a port value
   of zero has the normal SDP meaning.

   The following example illustrates an m-line for a message session,

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   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 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

6.3 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

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   inside compound types using the "accept-wrapped-types" attribute in
   an SDP a-line. This attribute has the following syntax:

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

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

   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.

6.4 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 7.1.

   The answerer will use the offered URL(s) to resolve the host address
   and port when connecting, and to identify the target when sending
   messages. 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 usual meaning for SDP.

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   Both offerer and answerer store the path values received from the
   peer. For a given endpoint, the local URL is the URL that the
   endpoint put into a path attribute value to send to its peer. The
   peer URL is the URL received from the peer. If the path attribute
   received from the peer contains more than one URL, then the peer URL
   is the last entry, while the first entry is the connection URL. If
   only one entry is present, then it is both the peer and connection
   URL. The remote path is the entire path attribute value received from
   the peer.

   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 *

   The first URI in the path attribute MUST identify the endpoint that
   generated the SDP document, or some other location where that
   endpoint wishes to receive messages associated with the session. If
   the URL identifies the endpoint, it MUST MUST be a temporary URL
   assigned just for this particular session, and MUST NOT duplicate any
   URL in use for any other session in which the endpoint is currently
   participating. Further, it SHOULD be hard to guess, and protected
   from eavesdroppers. This will be discussed in more detail in Section

6.5 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 the near
   future. The path attribute supports lists of URLs in order to
   facilitate that work. For peer-to-peer session, a path attribute will
   contain exactly one URL, describing an endpoint. This means that
   endpoints that only implement this specification will never send more
   than one URL in a path attribute, but MUST be prepared to receive
   more than one. 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, thus
   the term connection URL.

   If an endpoint puts more than one URL in a path attribute, final URL
   in the path (the peer URL) attribute MUST exhibit the uniqueness

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   properties described above. Uniqueness requirements for other entries
   in the attribute are out of scope for this document.

6.6 Updated SDP Offers

   To do: Revisit this section based on new connection management rules

   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
   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.

   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:

                     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

   Endpoint B performs a VISIT transaction passing the URL of msrp:// 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 may now send IMs to B by executing SEND transactions.

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6.8 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 COMEDIA [20]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 accpet 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 for
   a number of reasons:

      It added a great deal of complexity to session creation.
      The work in MSRP had begun to diverge from the work in MMUSIC.
      There was a lack of successful implementation experience of the
      COMEDIA work.

7. The Message Session Relay Protocol

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

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


   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 a participant in 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 the
   participant. 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.

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   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

   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,

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   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
       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 Connection Managment

   When an MSRP endpoint receives an SDP offer, and intends to accept
   it, it MUST establish a connection device described by the connection
   URL, if a connection does not already exist. If it already has a
   connection associated with another session for which the connection

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   URL host part matches the host part of the connection URL for this
   session, it SHOULD use the that connection, instead. Once connected,
   the answerer MUST send a VISIT request to associate the new session
   with the connection, prior to sending the SDP answer.

   Either endpoint MAY tear down a connection when it no longer has any
   active or proposed sessions associated with the connection.

7.3 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       = Tran-ID / Session-URL / Content-Types /
                      From / To / Message-Receipt / Receipt-ID /

       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
       Tran-ID      = "Tr-ID" ":" token

       Content-Type = "Content-Type" ":" media-type
       media-type   = type "/" subtype *( ";" parameter )
       type         = token
       subtype      = token
       parameter    = attribute "=" value
       attribute    = token

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       value        = token | quoted-string

       To                 = "To" ":" msrp_url *(SP msrp_url)
       From               = "From" ":" msrp_url

       Message-Receipt  = "Message-Receipt" ":" message-receipt-spec
                          ( SEMI receipt-type )
       message-receipt-spec     = "negative" / "none" / "all"
       receipt-type                = "receipt-type" "=" alt-receipt-type
       alt-receipt-type = r-type SLASH r-subtype *(SEMI r-parameter)
       r-type                   = discrete-type / composite-type
       discrete-type            = "text" / "image" / "audio" / "video"
                          / "application" / extension-token
       composite-type           = "message" / "multipart" / extension-token
       extension-token          =  ietf-token / x-token
       ietf-token               =  token
       x-token                  =  "x-" token
       r-subtype                =  extension-token / iana-token
       iana-token               =  token
       r-parameter              =  r-attribute "=" r-value
       r-attribute              =  token
       r-value          =  token / quoted-string

       Receipt-ID               = "Receipt-ID" ":" token

       Byte-Range         = "Byte-Range" ":" byte-range-spec
       byte-range-spec          = first-byte "-" last-byte
       first-byte               = 1*DIGIT
       last-byte                = 1*DIGIT

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

7.4 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

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   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.

7.5 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,
   identified by MSRP URLs.

7.5.1 Initiating an MSRP session

   When an endpoint wishes to engage a peer 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 MUST be prepared to accept a connection from the

   The offerer MUST perform the following steps:

   1.  Construct a MSRP URL to serve as the local URL. This URL MUST
       resolve to the location that the offerer wishes to host 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 puts its local URL into the path attribute, as
       described in Section 6.4. This URL becomes the offerer's local

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

   If the answerer chooses to participate, it MUST perform the following

   1.  Parse the first URL from the offered path attribute, to be the
       connection URL. The full path attribute value will be the

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       answerer's remote path. If the path only contained a single URL
       entry, then the connection URL and the remote path are identical.

   2.  Determine if it has any existing connection that is associated
       with a connection URL host part that matches that of the
       connection URL for this session, and with a transport protocol
       matching that from the M-line. If one exists, the answerer SHOULD
       use it for the new session rather than establishing a new

       [Open Issue: Should we discuss situations when an endpoint may
       want to intentially not share a connection?]

   3.  If no appropriate connection already exists, determine the host
       address and port from the peer URL, following the procedures in
       section Section 7.1, and connect using the transport protocol
       from the M-line.

   4.  Construct a MSRP URL . This URL MUST resolve to the the answerer.
       This URL becomes the answerer's local URL.

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

       1.  An To header field containing the remote URL.

       2.  A From containing the answerer's local URL.

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

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

   6.  Send the request and wait for a response

   7.  If the VISIT 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.

       3.  The accept-types attribute contains the SEND payload media
           types that the answerer is willing to accept. The
           accept-types attribute in the answer MUST be either the same
           as that of the offer, or it MUST be a subset.

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       4.  The path attribute contains the answerer's local URL.
   8.  If the VISIT transaction fails, the answerer MUST reject the

7.5.2 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 local URL that matches
       the To header field value of the VISIT request. If so, and if no
       previous VISIT request has been received for that URL, then
       return a 200 response, and save state associating the URL in the
       From header field with the connection on which the request was
       received .

   2.  If the state exists, and a matching VISIT transaction has already
       occured, return a 506 response and do not change session state in
       any way.

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

7.5.3 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 a To header field containing the remote path. Note that
       this is the full remote path, not just the connection or peer

   2.  Insert a From header field containing the local URL.

   3.  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.

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

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

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

   7.  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.

   8.  If any other response code is received, or if the transaction
       times out, the endpoint SHOULD assume the session has failed,
       either tear down the session, or attempt to re-establish the
       session 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.  Check that it has state for a session with a local URL matching
       the To value. If no matching session exists, return a 481
   2.  Determine that it understands the media type in the body, if any

   3.  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.

   4.  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.

7.5.4 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.

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   Each peer MUST destroy all local state for the session. This involves
   completely removing the state entry for the session and invalidating
   the session URL.

   If no other sessions are using the connection, the endpoint that
   opened it SHOULD tear it down. However, the passive party MAY tear
   down an unused connection after a locally configured timeout period.

   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.

7.5.5 Managing Session State and Connections

   A MSRP session is represented by state at each endpoint, identified
   by the local URL and remote path. An active session also has an
   associated network connection.

   If the connection fails for any reason, the session hosting device
   MUST invalidate the session state for all sessions using the
   connection. Once a connection is dropped, any 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

      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

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

   Open Issue: Is this still an issue with shared connections?

7.6 Delivery Status Notification

   Delivery Status Notification (DSN)[10] provides an extensible MIME
   content-type that is used to convey both positive and negative status
   of messages in the network.  This functionality is extremely useful
   for MSRP sessions that traverse a relay device.  Relay support is
   considered out of scope for this specification and will be included
   in a separate specification.  This section will only cover
   functionality required by non-relay aware endpoints for basic MSRP
   operation.  An MSRP endpoint MUST be capable of performing the DSN
   operations described in this specification and SHOULD support the DSN
   MIME type outlined.  An MSRP endpoint MAY use an alternative payload
   for reporting message status using the procedures outlined in this
   specification which MUST be negotiated during the SDP offer/answer

7.6.1 Endpoint DSN Request

   An endpoint that wishes to be informed of message delivery/failure
   needs to request such information. This is achieved by including an
   MSRP Receipt-Request header in the request.  The header can equal one
   of three values:

   negative: Indicates the client only requires failure delivery report.

   none: Indicates the client requires no delivery reports.

   all: Indicates the client requires both positive and negative
   delivery reports.

   Within the scope of this specification the Receipt-Request header is
   only used in MSRP SEND requests.  Future extensions to this
   specification MAY use the mechanism described in this document for
   delivery/failure status notification of other MSRP requests.

   The default value for this header if not present in a request is

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   'negative'.  An example of this header would be:

   Message-Receipt: negative

   The default DSN MIME type is detailed in RFC 1894[10].  It is
   possible for MSRP endpoints to use a different format if required.
   This can be achieved by including a 'receipt-type' parameter in the
   Message-Receipt header.  This parameter contains the alternative MIME
   type that SHOULD be used for this particular receipt transaction.
   The value included in this header MUST equal a value negotiated
   during the SDP offer/answer exchange.

   Open Issue: If we negotiate this in the SDP, that also means the
   format would be legal for normal messages. Is this okay? Also, I
   assume that if we negotiated "*" in the sdp, then any format would be
   legal? Do we even need this to be extensible?

   Open Issue: Is the RFC1894 the right thing to use? Do we need to add
   further verbiage on the format beyond the reference to the RFC?

7.6.2 DSN generation

   An MSRP endpoint implementing this specification SHOULD be able to
   generate positive delivery status of MSRP requests.  On receiving an
   MSRP request containing a Message-Receipt header with a value of
   TRUE, the endpoint will carry out normal MSRP response generation and
   MUST generate an MSRP REPORT request using the following procedures:

   1.  Insert a To header containing the From value from the original
   2.  Insert a From header containing the To value from the original
   3.  Insert the message status payload in the body of the request.  If
       the default DSN MIME type from DSN[10] is used then the MSRP
       Content-Type header MUST use the value multipart/report.  The
       relevance of DSN headers in MSRP can be found in section 7.6.5.
       An alternative MIME type MAY be used but MUST be specified in the
       Content-Type header.  Negative DSN generation is considered out
       of the scope of this document and will be covered in a separate
       MSRP relay document.
   4.  Insert a new transaction ID (TR-ID).
   5.  Insert the TR-ID value that appeared in the original MSRP request
       into the Receipt-ID header.  This allows a requesting client to
       explicitly correlate a REPORT request with the original request.
       This correlation is implementation specific and makes no
       requirements on clients to hold state for transactions ID's.
       Information regarding the original request can be obtained from
       the DSN MIME type outlined in [10].

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   6.  If the associated SEND request contained a chunk, that is, used
       the "message/byteranges" fromat, insert an MSRP Byte-Range header
       containing the value from the Content-range header field.  It is
       possible that an intermediary device may have broken the MSRP
       SEND request into chunks without the knowledge of the sending

7.6.3 Receiving positive DSN

   An MSRP endpoint implementing this specification MUST be able to
   receive positive delivery status of MSRP requests.

7.6.4 Receiving negative DSN

   An MSRP endpoint implementing this specification MUST be able to
   receive negative delivery status of MSRP requests.

7.6.5 DSN headers in MSRP

   To Do - Define meaning + relevance of DSN headers.

7.7 Message Fragmentation

   MSRP devices MAY break large content into fragments, and send each
   fragment in a separate SEND request. Each fragment is encapsulated
   using the "message/byteranges" MIME type, defined in RFC2616 [11], to
   correlate parts of the message.  The definition of large is
   determined by local policy. MSRP endpoints MUST be capable of
   receiving and processing fragmented messages.

   Open Issue: Do we want to negotiate the use of message/byterange like
   any other MIME type? I assume no, as we want to allow relays to
   fragment messages, and relays are not privy to the content-types
   negotiated for a session.

   Although relays are not in scope for this document, we expect that
   relays will be able to introduce fragmentation, as well as change the
   fragmentation of previously fragemented messages. Therefore, all MSRP
   endpoints MUST be able to receive fragmented messages.

7.7.1 MSRP Usage of message/byteranges

   The "message/byteranges" type allows multiple ranges in a single
   document. However, MSRP devices MUST NOT include more than one byte
   range in a single request. Although the HTTP usage for a document
   containing a single byte range indicates putting the "Content-Range"
   header in a header field, rather  than in the body itself,
   "Content-Range" MUST NOT appear as an MSRP header field.

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   [Open Issue: How much of the message/byteranges specification should
   we explain or copy forward? Copying too much obscures the fact that
   rfc2616 is the normative definition, but it may be helpful to have
   more context here.]

   If the MSRP device has a priori knowledge of the overall content
   length, it SHOULD put that length into instance-length. The device
   MAY place a "*" in instance-length if it does not have such

   Similarly, if the device has a priori knowledge of the number of
   bytes in a byte range, it SHOULD place the last bype position in
   last-byte-pos. Otherwise, it MAY use a "*". If "*" is present, the
   recipient MUST determine the last-byte-position through normal
   request framing and body processing. An MSRP device MUST put the
   initial byte position in first-byte-pos.

7.8 Method Descriptions

   This section summarizes the purpose of each MSRP method. All MSRP
   messages MUST contain the TR-ID, From, and To 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.

7.8.1 SEND

   The SEND method is used by both the host and visitor endpoints to
   send instant messages to its peer endpoint. A SEND request MUST
   contain a To header field containing the sender's remote path, and a
   From header field containing the sender's local URL. 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.

7.8.2 VISIT

   The visiting endpoint uses the VISIT method to associate a network
   connection with the session state at the hosting device. A VISIT
   request MUST include a To header including the sender's connection
   URL, and a From header field containing the sender's local URL.

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7.8.3 REPORT

   Report is used by an endpoint/relay to convey message delivery status

7.9 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 original request, and To and From headers
   matching those of the original request.

7.9.1 200

   The 200 response code indicates a successful transaction.

7.9.2 400

   A 400 response indicates a request was unintelligible.

7.9.3 415

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

7.9.4 426

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

7.9.5 481

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

7.9.6 506

   A 506 response indicates that a VISIT request occurred in which the
   To header indicates a local path that is already associated with
   another connection. A 506 response MUST NOT be returned in response
   to any method other than VISIT.

7.10 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.

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7.10.1 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.

7.10.2 To

   The To header field is used to indicate the sender's remote path. All
   MSRP requests MUST contain a To header field.

7.10.3 From

   The From header field is used to indicate the sender's local URL. All
   MSRP requests MUST contain a From header field.

7.10.4 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.

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.

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           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 local URL of msrp://
        iau39 and listens for a connection on TCP port 7777.

        Alice->Bob (SIP): INVITE

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

   2.   Bob opens a TCP connection to

        Bob->Alice (MSRP):

        MSRP xx VISIT

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   3.   Alice->Bob (MSRP):

        MSRP xx 200 OK

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

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

   5.   Alice->Bob (SIP): ACK

   6.   Alice->Bob (MSRP):

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

   7.   Bob->Alice (MSRP):

        MSRP xx 200 OK
        TR-ID: 123

   8.   Bob->Alice (MSRP):

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

        Hi, Alice! I'm Bob!

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   9.   Alice->Bob (MSRP):

        MSRP xx 200 OK
        TR-ID: 456

   10.  Alice->Bob (SIP): BYE

        Alice invalidates local session state.

   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).

9.2.5 Security Considerations

   See Section 10.

9.2.6 Relevant Publications


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   [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 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 6.2.

9.3.2 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 6.3.

9.3.3 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 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.

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

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   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.

   Any 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 some sort of
   proprietary devices, they MUST ensure protection at all hops for an

   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 sent in the SDP offer for a MSRP session is used by the
   answerer to 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 this URL
   to the answerer SHOULD be protected from eavesdroppers and

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   man-in-the-middle attacks.

   Therefore a 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 [19]compatible
   protocols. If this occurs, the gateway MUST maintain session state,
   and MUST translate between the MSRP session semantics and CPIM
   semantics that do not include a concept of sessions. Furthermore,
   when one endpoint of the session is a CPIM gateway, instant messages
   SHOULD be wrapped in "message/cpim" [5] bodies. Such a gateway MUST
   include "message/cpim" as the first entry in its SDP accept-types
   attribute. MSRP endpoints sending instant messages to a peer that has
   included 'message/cpim" as the first entry in the accept-types
   attribute SHOULD encapsulate all instant message bodies in "message/
   cpim" wrappers. All MSRP endpoints MUST support the message/cpim
   type, and SHOULD support the S/MIME features of that format.

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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-04

      Removed the direction attribute. Rather than using a comedia
      styled direction negotiation, we just state that the answerer
      opens any needed connection.
      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.
      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.
      Added section for Delivery Status Notification handling, and added
      associated entries into the syntax definition.

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      Added content fragmentation section.
      Removed recommendation to start separate session for large
      Corrected some mistakes in the syntax definitions.
      Added Chris Boulton as a co-author for his contribution of the DSN

11.2 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.
      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.

11.3 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.

11.4 draft-ietf-simple-message-sessions-01

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

11.5 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.

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11.6 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
    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

   [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

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         (SHA1)", RFC 3174, September 2001.

   [10]  Moore, K. and G. Vaudreuil, "An Extensible Message Format for
         Delivery Status Notifications", RFC 1894, January 1996.

   [11]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
         Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
         HTTP/1.1", RFC 2616, June 1999.

Informational References

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

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

   [14]  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

   [15]  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.

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

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

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

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

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

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Authors' Addresses

   Ben Campbell
   5100 Tennyson Parkway
   Suite 1200
   Plano, TX  75024


   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


   Chris Boulton
   Ubiquity Software Corporation
   Langstone Park
   Newport, South Wales  NP18 2LH


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