path: root/doc/rfc/rfc4975.txt

Network Working Group                                   B. Campbell, Ed.
Request for Comments: 4975                              Estacado Systems
Category: Standards Track                                   R. Mahy, Ed.
                                                             Plantronics
                                                        C. Jennings, Ed.
                                                     Cisco Systems, Inc.
                                                          September 2007


               The Message Session Relay Protocol (MSRP)

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   This document describes the Message Session Relay Protocol, a
   protocol for transmitting a series of related instant messages in the
   context of a session.  Message sessions are treated like any other
   media stream when set up via a rendezvous or session creation
   protocol such as the Session Initiation Protocol.

























Campbell, et al.            Standards Track                     [Page 1]
^L
RFC 4975                          MSRP                    September 2007


Table of Contents

   1. Introduction ....................................................4
   2. Conventions .....................................................5
   3. Applicability of MSRP ...........................................5
   4. Protocol Overview ...............................................6
   5. Key Concepts ....................................................9
      5.1. MSRP Framing and Message Chunking ..........................9
      5.2. MSRP Addressing ...........................................10
      5.3. MSRP Transaction and Report Model .........................11
      5.4. MSRP Connection Model .....................................12
   6. MSRP URIs ......................................................14
      6.1. MSRP URI Comparison .......................................15
      6.2. Resolving MSRP Host Device ................................16
   7. Method-Specific Behavior .......................................17
      7.1. Constructing Requests .....................................17
           7.1.1. Sending SEND Requests ..............................18
           7.1.2. Sending REPORT Requests ............................21
           7.1.3. Generating Success Reports .........................22
           7.1.4. Generating Failure Reports .........................23
      7.2. Constructing Responses ....................................24
      7.3. Receiving Requests ........................................25
           7.3.1. Receiving SEND Requests ............................25
           7.3.2. Receiving REPORT Requests ..........................27
   8. Using MSRP with SIP and SDP ....................................27
      8.1. SDP Connection and Media-Lines ............................28
      8.2. URI Negotiations ..........................................29
      8.3. Path Attributes with Multiple URIs ........................30
      8.4. Updated SDP Offers ........................................31
      8.5. Connection Negotiation ....................................31
      8.6. Content Type Negotiation ..................................32
      8.7. Example SDP Exchange ......................................34
      8.8. MSRP User Experience with SIP .............................35
      8.9. SDP Direction Attribute and MSRP ..........................35
   9. Formal Syntax ..................................................36
   10. Response Code Descriptions ....................................38
      10.1. 200 ......................................................38
      10.2. 400 ......................................................38
      10.3. 403 ......................................................38
      10.4. 408 ......................................................39
      10.5. 413 ......................................................39
      10.6. 415 ......................................................39
      10.7. 423 ......................................................39
      10.8. 481 ......................................................39
      10.9. 501 ......................................................39
      10.10. 506 .....................................................40





Campbell, et al.            Standards Track                     [Page 2]
^L
RFC 4975                          MSRP                    September 2007


   11. Examples ......................................................40
      11.1. Basic IM Session .........................................40
      11.2. Message with XHTML Content ...............................42
      11.3. Chunked Message ..........................................43
      11.4. Chunked Message with Message/CPIM Payload ................43
      11.5. System Message ...........................................44
      11.6. Positive Report ..........................................44
      11.7. Forked IM ................................................45
   12. Extensibility .................................................48
   13. CPIM Compatibility ............................................48
   14. Security Considerations .......................................49
      14.1. Secrecy of the MSRP URI ..................................50
      14.2. Transport Level Protection ...............................50
      14.3. S/MIME ...................................................51
      14.4. Using TLS in Peer-to-Peer Mode ...........................52
      14.5. Other Security Concerns ..................................53
   15. IANA Considerations ...........................................55
      15.1. MSRP Method Names ........................................55
      15.2. MSRP Header Fields .......................................55
      15.3. MSRP Status Codes ........................................56
      15.4. MSRP Port ................................................56
      15.5. URI Schema ...............................................56
           15.5.1. MSRP Scheme .......................................56
           15.5.2. MSRPS Scheme ......................................57
      15.6. SDP Transport Protocol ...................................57
      15.7. SDP Attribute Names ......................................58
           15.7.1. Accept Types ......................................58
           15.7.2. Wrapped Types .....................................58
           15.7.3. Max Size ..........................................58
           15.7.4. Path ..............................................58
   16. Contributors and Acknowledgments ..............................59
   17. References ....................................................59
      17.1. Normative References .....................................59
      17.2. Informative References ...................................60

















Campbell, et al.            Standards Track                     [Page 3]
^L
RFC 4975                          MSRP                    September 2007


1.  Introduction

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

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

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

   When a user uses an Instant Messaging (IM) URL, RFC 3861 [32] defines
   how DNS can be used to map this to a particular protocol to establish
   the session such as SIP.  SIP can use an offer/answer model to
   transport the MSRP URIs for the media in SDP.  This document defines
   how the offer/answer exchange works to establish MSRP connections and
   how messages are sent across the MSRP, but it does not deal with the
   issues of mapping an IM URL to a session establishment protocol.

   This session model allows message sessions to be integrated into
   advanced communications applications with little to no additional
   protocol development.  For example, during the above chat session,
   Bob decides Alice really needs to be talking to Carol.  Bob can
   transfer [21] Alice to Carol, introducing them into their own
   messaging session.  Messaging sessions can then be easily integrated
   into call-center and dispatch environments using third-party call
   control [20] and conferencing [19] applications.



Campbell, et al.            Standards Track                     [Page 4]
^L
RFC 4975                          MSRP                    September 2007


   This document specifies MSRP behavior only for peer-to-peer sessions,
   that is, sessions crossing only a single hop.  MSRP relay devices
   [23] (referred to herein as "relays") are specified in a separate
   document.  An endpoint that implements this specification, but not
   the relay specification, will be unable to introduce relays into the
   message path, but will still be able to interoperate with peers that
   do use relays.

2.  Conventions

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

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

3.  Applicability of MSRP

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

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

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

   o  The rendezvous mechanism MUST be able to natively transport im:
      URIs or automatically translate im: URIs [27] into the addressing
      identifiers of the rendezvous protocol.

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

   SIP meets these requirements for a rendezvous mechanism.  The MSRP
   URIs are exchanged using SDP in an offer/answer exchange via SIP.





Campbell, et al.            Standards Track                     [Page 5]
^L
RFC 4975                          MSRP                    September 2007


   The exchanged SDP can also be used to negotiate MSRP extensions.
   This SDP can be secured using any of the mechanisms available in SIP,
   including using the sips mechanism to ensure transport security
   across intermediaries and Secure/Multipurpose Internet Mail
   Extensions (S/MIME) for end-to-end protection of the SDP body.  SIP
   can carry arbitrary URIs (including im: URIs) in the Request-URI, and
   procedures are available to map im: URIs to sip: or sips: URIs.  It
   is expected that initial deployments of MSRP will use SIP as its
   rendezvous mechanism.

4.  Protocol Overview

   MSRP is a text-based, connection-oriented protocol for exchanging
   arbitrary (binary) MIME [8] content, especially instant messages.
   This section is a non-normative overview of how MSRP works and how it
   is used with SIP.

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

       Alice sends to Bob:

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

   c=IN IP4 atlanta.example.com
   m=message 7654 TCP/MSRP *
   a=accept-types:text/plain
   a=path:msrp://atlanta.example.com:7654/jshA7weztas;tcp












Campbell, et al.            Standards Track                     [Page 6]
^L
RFC 4975                          MSRP                    September 2007


       Bob sends to Alice:

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

   c=IN IP4 biloxi.example.com
   m=message 12763 TCP/MSRP *
   a=accept-types:text/plain
   a=path:msrp://biloxi.example.com:12763/kjhd37s2s20w2a;tcp

       Alice sends to Bob:

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

                          Figure 1: Session Setup

   MSRP defines two request types, or methods.  SEND requests are used
   to deliver a complete message or a chunk (a portion of a complete
   message), while REPORT requests report on the status of a previously
   sent message, or a range of bytes inside a message.  When Alice
   receives Bob's answer, she checks to see if she has an existing
   connection to Bob.  If not, she opens a new connection to Bob using
   the URI he provided in the SDP.  Alice then delivers a SEND request
   to Bob with her initial message, and Bob replies indicating that
   Alice's request was received successfully.




















Campbell, et al.            Standards Track                     [Page 7]
^L
RFC 4975                          MSRP                    September 2007


   MSRP a786hjs2 SEND
   To-Path: msrp://biloxi.example.com:12763/kjhd37s2s20w2a;tcp
   From-Path: msrp://atlanta.example.com:7654/jshA7weztas;tcp
   Message-ID: 87652491
   Byte-Range: 1-25/25
   Content-Type: text/plain

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

   MSRP a786hjs2 200 OK
   To-Path: msrp://atlanta.example.com:7654/jshA7weztas;tcp
   From-Path: msrp://biloxi.example.com:12763/kjhd37s2s20w2a;tcp
   -------a786hjs2$

                      Figure 2: Example MSRP Exchange

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

   If Alice wants to deliver a very large message, she can split the
   message into chunks and deliver each chunk in a separate SEND
   request.  The message ID corresponds to the whole message, so the
   receiver can also use it to reassemble the message and tell which
   chunks belong with which message.  Chunking is described in more
   detail in Section 5.1.  The Byte-Range header field identifies the
   portion of the message carried in this chunk and the total size of
   the message.

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

   Alice and Bob choose their MSRP URIs in such a way that it is
   difficult to guess the exact URI.  Alice and Bob can reject requests
   to URIs they are not expecting to service and can correlate the
   specific URI with the probable sender.  Alice and Bob can also use



Campbell, et al.            Standards Track                     [Page 8]
^L
RFC 4975                          MSRP                    September 2007


   TLS [1] to provide channel security over this hop.  To receive MSRP
   requests over a TLS protected connection, Alice or Bob could
   advertise URIs with the "msrps" scheme instead of "msrp".

   MSRP is designed with the expectation that MSRP can carry URIs for
   nodes on the far side of relays.  For this reason, a URI with the
   "msrps" scheme makes no assertion about the security properties of
   other hops, just the next hop.  The user agent knows the URI for each
   hop, so it can verify that each URI has the desired security
   properties.

   MSRP URIs are discussed in more detail in Section 6.

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

5.  Key Concepts

5.1.  MSRP Framing and Message Chunking

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

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










Campbell, et al.            Standards Track                     [Page 9]
^L
RFC 4975                          MSRP                    September 2007


    MSRP dkei38sd SEND
    Message-ID: 4564dpWd
    Byte-Range: 1-*/8
    Content-Type: text/plain

    abcd
    -------dkei38sd+

    MSRP dkei38ia SEND
    Message-ID: 4564dpWd
    Byte-Range: 5-8/8
    Content-Type: text/plain

    EFGH
    -------dkei38ia$

                  Figure 3: Breaking a Message into Chunks

   This chunking mechanism allows a sender to interrupt a chunk part of
   the way through sending it.  The ability to interrupt messages allows
   multiple sessions to share a TCP connection, and for large messages
   to be sent efficiently while not blocking other messages that share
   the same connection, or even the same MSRP session.  Any chunk that
   is larger than 2048 octets MUST be interruptible.  While MSRP would
   be simpler to implement if each MSRP session used its own TCP
   connection, there are compelling reasons to conserve connections.
   For example, the TCP peer may be a relay device that connects to many
   other peers.  Such a device will scale better if each peer does not
   create a large number of connections.  (Note that in the above
   example, the initial chunk was interruptible for the sake of example,
   even though its size is well below the limit for which
   interruptibility would be required.)

   The chunking mechanism only applies to the SEND method, as it is the
   only method used to transfer message content.

5.2.  MSRP Addressing

   MSRP entities are addressed using URIs.  The MSRP URI schemes are
   defined in Section 6.  The syntax of the To-Path and From-Path header
   fields each allows for a list of URIs.  This was done to allow the
   protocol to work with relays, which are defined in a separate
   document, to provide a complete path to the end recipient.  When two
   MSRP nodes communicate directly, they need only one URI in the To-
   Path list and one URI in the From-Path list.






Campbell, et al.            Standards Track                    [Page 10]
^L
RFC 4975                          MSRP                    September 2007


5.3.  MSRP Transaction and Report Model

   A sender sends MSRP requests to a receiver.  The receiver MUST
   quickly accept or reject the request.  If the receiver initially
   accepted the request, it still may then do things that take
   significant time to succeed or fail.  For example, if the receiver is
   an MSRP to Extensible Messaging and Presence Protocol (XMPP) [30]
   gateway, it may forward the message over XMPP.  The XMPP side may
   later indicate that the request did not work.  At this point, the
   MSRP receiver may need to indicate that the request did not succeed.
   There are two important concepts here: first, the hop-by-hop delivery
   of the request may succeed or fail; second, the end result of the
   request may or may not be successfully processed.  The first type of
   status is referred to as "transaction status" and may be returned in
   response to a request.  The second type of status is referred to as
   "delivery status" and may be returned in a REPORT transaction.

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

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

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

   The combinations of reporting are needed to meet the various
   scenarios of currently deployed IM systems.  Success-Report might be
   "no" in many public systems to reduce load, but might be "yes" in
   certain enterprise systems, such as systems used for securities
   trading.  A Failure-Report value of "no" is useful for sending system
   messages such as "the system is going down in 5 minutes" without
   causing a response explosion to the sender.  A Failure-Report of
   "yes" is used by many systems that wish to notify the user if the
   message failed.  A Failure-Report of "partial" is a way to report
   errors other than timeouts.  Timeout error reporting requires the
   sending hop to run a timer and the receiving hop to send an





Campbell, et al.            Standards Track                    [Page 11]
^L
RFC 4975                          MSRP                    September 2007


   acknowledgment to stop the timer.  Some systems don't want the
   overhead of doing this.  "Partial" allows them to choose not to do
   so, but still allows error responses to be sent in many cases.

      The term "partial" denotes that the hop-by-hop acknowledgment
      mechanism that would be required with a Failure-Report value of
      "yes" is not invoked.  Thus, each device uses only "part" of the
      set of error detection tools available to them.  This allows a
      compromise between no reporting of failures at all, and reporting
      every possible failure.  For example, with "partial", a sending
      device does not have to keep transaction state around waiting for
      a positive acknowledgment.  But it still allows devices to report
      other types of errors.  The receiving device could still report a
      policy violation such as an unacceptable content-type, or an ICMP
      error trying to connect to a downstream device.

5.4.  MSRP Connection Model

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

   When a new MSRP session is created, the initiating endpoint MUST act
   as the "active" endpoint, meaning that it is responsible for opening
   the transport connection to the answerer, if a new connection is
   required.  However, this requirement MAY be weakened if standardized
   mechanisms for negotiating the connection direction become available
   and are implemented by both parties to the connection.

   Likewise, the active endpoint MUST immediately issue a SEND request.
   This initial SEND request MAY have a body if the sender has content
   to send, or it MAY have no body at all.

      The first SEND request serves to bind a connection to an MSRP
      session from the perspective of the passive endpoint.  If the
      connection is not authenticated with TLS, and the active endpoint
      did not send an immediate request, the passive endpoint would have
      no way to determine who had connected, and would not be able to
      safely send any requests towards the active party until after the
      active party sends its first request.

   When an element needs to form a new connection, it looks at the URI
   to decide on the type of connection (TLS, TCP, etc.) then connects to
   the host indicated by the URI, following the URI resolution rules in
   Section 6.2.  Connections using the "msrps" scheme MUST use TLS.  The



Campbell, et al.            Standards Track                    [Page 12]
^L
RFC 4975                          MSRP                    September 2007


   SubjectAltName in the received certificate MUST match the hostname
   part of the URI and the certificate MUST be valid according to RFC
   3280 [16], including having a date that is valid and being signed by
   an acceptable certification authority.  At this point, the device
   that initiated the connection can assume that this connection is with
   the correct host.

   The rules on certificate name matching and CA signing MAY be relaxed
   when using TLS peer-to-peer.  In this case, a mechanism to ensure
   that the peer used a correct certificate MUST be used.  See Section
   14.4 for details.

   If the connection used mutual TLS authentication, and the TLS client
   presented a valid certificate, then the element accepting the
   connection can verify the identity of the connecting device by
   comparing the hostname part of the target URI in the SDP provided by
   the peer device against the SubjectAltName in the client certificate.

   When mutual TLS authentication is not used, the listening device MUST
   wait until it receives a request on the connection, at which time it
   infers the identity of the connecting device from the associated
   session description.

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

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

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



Campbell, et al.            Standards Track                    [Page 13]
^L
RFC 4975                          MSRP                    September 2007


   resent messages MUST match those used in the initial attempts.  If
   the receiving endpoint receives more than one message with the same
   Message-ID, it SHOULD assume that the messages are duplicates.  The
   specific action that an endpoint takes when it receives a duplicate
   message is a matter of local policy, except that it SHOULD NOT
   present the duplicate messages to the user without warning of the
   duplication.  Note that acknowledgments as needed based on the
   Failure-Report and Success-Report settings are still necessary even
   for requests containing duplicate content.

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

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

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

6.  MSRP URIs

   URIs using the "msrp" and "msrps" schemes are used to identify a
   session of instant messages at a particular MSRP device, as well as
   to identify an MSRP relay in general.  This document describes the
   former usage; the latter usage is described in the MSRP relay
   specification [23].  MSRP URIs that identify sessions are ephemeral;
   an MSRP device will use a different MSRP URI for each distinct
   session.  An MSRP URI that identifies a session has no meaning
   outside the scope of that session.

   An MSRP URI follows a subset of the URI syntax in Appendix A of RFC
   3986 [10], with a scheme of "msrp" or "msrps".  The syntax is
   described in Section 9.

   MSRP URIs are primarily expected to be generated and exchanged
   between systems, and are not intended for "human consumption".
   Therefore, they are encoded entirely in US-ASCII.

   The constructions for "authority", "userinfo", and "unreserved" are
   detailed in RFC 3986 [10].  URIs designating MSRP over TCP MUST
   include the "tcp" transport parameter.







Campbell, et al.            Standards Track                    [Page 14]
^L
RFC 4975                          MSRP                    September 2007


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

   The MSRP URI authority field identifies a participant in a particular
   MSRP session.  If the authority field contains a numeric IP address,
   it MUST also contain a port.  The session-id part identifies a
   particular session of the participant.  The absence of the session-id
   part indicates a reference to an MSRP host device, but does not refer
   to a particular session at that device.  A particular value of
   session-id is only meaningful in the context of the associated
   authority; thus, the authority component can be thought of as
   identifying the "authority" governing a namespace for the session-id.

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

   MSRP has an IANA-registered recommended port defined in Section 15.4.
   This value is not a default, as the URI negotiation process described
   herein will always include explicit port numbers.  However, the URIs
   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 authority component 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.  A userinfo part MUST NOT contain
   password information.

   The following is an example of a typical MSRP URI:

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

6.1.  MSRP URI Comparison

   In the context of the MSRP protocol, MSRP URI comparisons MUST be
   performed according to the following rules:

   1.  The scheme MUST match.  Scheme comparison is case insensitive.

   2.  If the authority component contains an explicit IP address and/or
       port, these are compared for address and port equivalence.
       Percent-encoding normalization [10] applies; that is, if any
       percent-encoded nonreserved characters exist in the authority
       component, they must be decoded prior to comparison.  Userinfo




Campbell, et al.            Standards Track                    [Page 15]
^L
RFC 4975                          MSRP                    September 2007


       parts are not considered for URI comparison.  Otherwise, the
       authority component is compared as a case-insensitive character
       string.

   3.  If the port exists explicitly in either URI, then it MUST match
       exactly.  A URI with an explicit port is never equivalent to
       another with no port specified.

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

   5.  URIs with different "transport" parameters never match.  Two URIs
       that are identical except for transport are not equivalent.  The
       transport parameter is case insensitive.

   Path normalization [10] is not relevant for MSRP URIs.

6.2.  Resolving MSRP Host Device

   An MSRP host device is identified by the authority component of an
   MSRP URI.

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

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

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

      This process assumes that the connection port is always known
      prior to resolution.  This is always true for the MSRP URI uses
      described in this document, that is, URIs exchanged in the SDP
      offer and answer.  The introduction of relays creates situations
      where this is not the case.  For example, when a user configures
      her client to use a relay, it is desirable that the relay's MSRP
      URI is easy to remember and communicate to humans.  Often this
      type of MSRP will omit the port number.  Therefore, the relay
      specification [23] describes additional steps to resolve the port
      number.

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



Campbell, et al.            Standards Track                    [Page 16]
^L
RFC 4975                          MSRP                    September 2007


7.  Method-Specific Behavior

7.1.  Constructing Requests

   To form a new request, the sender creates a transaction identifier
   and uses this and the method name to create an MSRP request start
   line.  The transaction identifier MUST NOT collide with that of other
   transactions that exist at the same time.  Therefore, it MUST contain
   at least 64 bits of randomness.

   Next, the sender places the target path in a To-Path header field,
   and the sender's URI in a From-Path header field.  If multiple URIs
   are present in the To-Path, the leftmost is the first URI visited;
   the rightmost URI is the last URI visited.  The processing then
   becomes method specific.  Additional method-specific header fields
   are added as described in the following sections.

   After any method-specific header fields are added, processing
   continues to handle a body, if present.  If the request has a body,
   it MUST contain a Content-Type header field.  It may contain other
   MIME-specific header fields.  The Content-Type header field MUST be
   the last field in the message header section.  The body MUST be
   separated from the header fields with an extra CRLF.

   Non-SEND requests are not intended to carry message content, and are
   therefore not interruptible.  Non-SEND request bodies MUST NOT be
   larger than 10240 octets.

      Although this document does not discuss any particular usage of
      bodies in non-SEND requests, they may be useful in the future for
      carrying security or identity information, information about a
      message in progress, etc.  The 10K size limit was chosen to be
      large enough for most of such applications, but small enough to
      avoid the fairness issues caused by sending arbitrarily large
      content in non-interruptible method bodies.

   A request with no body MUST NOT include a Content-Type or any other
   MIME-specific header fields.  A request without a body MUST contain
   an end-line after the final header field.  No extra CRLF will be
   present between the header section and the end-line.

      Requests with no bodies are useful when a client wishes to send
      "traffic", but does not wish to send content to be rendered to the
      peer user.  For example, the active endpoint sends a SEND request
      immediately upon establishing a connection.  If it has nothing to
      say at the moment, it can send a request with no body.  Bodiless
      requests may also be used in certain applications to keep Network
      Address Translation (NAT) bindings alive, etc.



Campbell, et al.            Standards Track                    [Page 17]
^L
RFC 4975                          MSRP                    September 2007


      Bodiless requests are distinct from requests with empty bodies.  A
      request with an empty body will have a Content-Type header field
      value and will generally be rendered to the recipient according to
      the rules for that type.

   The end-line that terminates the request MUST be composed of seven
   "-" (minus sign) characters, the transaction ID as used in the start
   line, and a flag character.  If a body is present, the end-line MUST
   be preceded by a CRLF that is not part of the body.  If the chunk
   represents the data that forms the end of the complete message, the
   flag value MUST be a "$".  If the sender is aborting an incomplete
   message, and intends to send no further chunks in that message, the
   flag MUST be a "#".  Otherwise, the flag MUST be a "+".

   If the request contains a body, the sender MUST ensure that the end-
   line (seven hyphens, the transaction identifier, and a continuation
   flag) is not present in the body.  If the end-line is present in the
   body, the sender MUST choose a new transaction identifier that is not
   present in the body, and add a CRLF if needed, and the end-line,
   including the "$", "#", or "+" character.

   Some implementations may choose to scan for the closing sequence as
   they send the body, and if it is encountered, simply interrupt the
   chunk at that point and start a new transaction with a different
   transaction identifier to carry the rest of the body.  Other
   implementations may choose to scan the data and ensure that the body
   does not contain the transaction identifier before they start sending
   the transaction.

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

7.1.1.  Sending SEND Requests

   When an endpoint has a message to deliver, it first generates a new
   Message-ID.  The value MUST be highly unlikely to be repeated by
   another endpoint instance, or by the same instance in the future.  If
   necessary, the endpoint breaks the message into chunks.  It then
   generates a SEND request for each chunk, following the procedures for
   constructing requests (Section 7.1).

      The Message-ID header field provides a unique message identifier
      that refers to a particular version of a particular message.  The
      term "Message" in this context refers to a unit of content that
      the sender wishes to convey to the recipient.  While such a
      message may be broken into chunks, the Message-ID refers to the
      entire message, not a chunk of the message.



Campbell, et al.            Standards Track                    [Page 18]
^L
RFC 4975                          MSRP                    September 2007


      The uniqueness of the message identifier is ensured by the host
      that generates it.  This message identifier is intended to be
      machine readable and not necessarily meaningful to humans.  A
      message identifier pertains to exactly one version of a particular
      message; subsequent revisions to the message each receive new
      message identifiers.  Endpoints can ensure sufficient uniqueness
      in any number of ways, the selection of which is an implementation
      choice.  For example, an endpoint could concatenate an instance
      identifier such as a MAC address, its idea of the number of
      seconds since the epoch, a process ID, and a monotonically
      increasing 16-bit integer, all base-64 encoded.  Alternately, an
      endpoint without an on-board clock could simply use a 64-bit
      random number.

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

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

   If the value of "Failure-Report" is set to "yes", then the sender of
   the request runs a timer.  If a 200 response to the transaction is
   not received within 30 seconds from the time the last byte of the
   transaction is sent, or submitted to the operating system for
   sending, the element MUST inform the user that the request probably
   failed.  If the value is set to "partial", then the element sending
   the transaction does not have to run a timer, but MUST inform the
   user if it receives a non-recoverable error response to the
   transaction.  Regardless of the Failure-Report value, there is no
   requirement to wait for a response prior to sending the next request.

      The treatment of timers for success reports and failure reports is
      intentionally inconsistent.  An explicit timeout value makes sense
      for failure reports since such reports will usually refer to a
      message "chunk" that is acknowledged on a hop-by-hop basis.  This





Campbell, et al.            Standards Track                    [Page 19]
^L
RFC 4975                          MSRP                    September 2007


      is not the case for success reports, which are end-to-end and may
      refer to the entire message content, which can be arbitrarily
      large.

   If no Success-Report header field is present in a SEND request, it
   MUST be treated the same as a Success-Report header field with a
   value of "no".  If no Failure-Report header field is present, it MUST
   be treated the same as a Failure-Report header field with a value of
   "yes".  If an MSRP endpoint receives a REPORT for a Message-ID it
   does not recognize, it SHOULD silently ignore the REPORT.

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

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

   There are some circumstances where an endpoint may choose to send an
   empty SEND request.  For the sake of consistency, a Byte-Range header
   field referring to nonexistent or zero-length content MUST still have
   a range-start value of 1.  For example, "1-0/0".

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



Campbell, et al.            Standards Track                    [Page 20]
^L
RFC 4975                          MSRP                    September 2007


   receive for a complete message.  Senders SHOULD NOT break messages
   into chunks smaller than 2048 octets, except for the final chunk of a
   complete message.

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

   SEND requests larger than 2048 octets MUST be interrupted if the
   sender needs to send pending responses or REPORT requests.  If
   multiple SEND requests from different sessions are concurrently being
   sent over the same connection, the device SHOULD implement some
   scheme to alternate between them such that each concurrent request
   gets a chance to send some fair portion of data at regular intervals
   suitable to the application.

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

   The default disposition of messages is to be rendered to the user.
   If the sender wants a different disposition, it MAY insert a Content-
   Disposition [9] header field.  Values MAY include any from RFC 2183
   [9] or the IANA registry it defines.  Since MSRP can carry unencoded
   binary payloads, transfer encoding is always "binary", and transfer-
   encoding parameters MUST NOT be present.

7.1.2.  Sending REPORT Requests

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

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

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

   REPORT requests will normally not include a body, as the REPORT
   request header fields can carry sufficient information in most cases.
   However, REPORT requests MAY include a body containing additional
   information about the status of the associated SEND request.  Such a



Campbell, et al.            Standards Track                    [Page 21]
^L
RFC 4975                          MSRP                    September 2007


   body is informational only, and the sender of the REPORT request
   SHOULD NOT assume that the recipient pays any attention to the body.
   REPORT requests are not interruptible.

   Success-Report and Failure-Report header fields MUST NOT be present
   in REPORT requests.  MSRP nodes MUST NOT send REPORT requests in
   response to REPORT requests.  MSRP nodes MUST NOT send MSRP responses
   to REPORT requests.

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

7.1.3.  Generating Success Reports

   When an endpoint receives a message in one or more chunks that
   contain a Success-Report value of "yes", it MUST send a success
   report or reports covering all bytes that are received successfully.
   The success reports are sent in the form of REPORT requests,
   following the normal procedures (Section 7.1), with a few additional
   requirements.

   The receiver MAY wait until it receives the last chunk of a message,
   and send a success report that covers the complete message.
   Alternately, it MAY generate incremental success REPORTs as the
   chunks are received.  These can be sent periodically and cover all
   the bytes that have been received so far, or they can be sent after a
   chunk arrives and cover just the part from that chunk.

      It is helpful to think of a success REPORT as reporting on a
      particular range of bytes, rather than on a particular chunk sent
      by a client.  The sending client cannot depend on the Byte-Range
      header field in a given success report matching that of a
      particular SEND request.  For example, an intervening MSRP relay
      may break chunks into smaller chunks, or aggregate multiple chunks
      into larger ones.  A side effect of this is, even if no relay is
      used, the receiving client may report on byte ranges that do not
      exactly match those in the original chunks sent by the sender.  It
      can wait until all bytes in a message are received and report on
      the whole, it can report as it receives each chunk, or it can
      report on any other received range.  Reporting on ranges smaller
      than the entire message contents allows certain improved user
      experiences for the sender.  For example, a sending client could
      display incremental status information showing which ranges of
      bytes have been acknowledged by the receiver.  However, the choice
      on whether to report incrementally is entirely up to the receiving
      client.  There is no mechanism for the sender to assert its desire
      to receive incremental reports or not.  Since the presence of a



Campbell, et al.            Standards Track                    [Page 22]
^L
RFC 4975                          MSRP                    September 2007


      relay can cause the receiver to see a very different chunk
      allocation than the sender, such a mechanism would be of
      questionable value.

   When generating a REPORT request, the endpoint inserts a To-Path
   header field containing the From-Path value from the original
   request, and a From-Path header field containing the URI identifying
   itself in the session.  The endpoint then inserts a Status header
   field with a namespace of "000", a status-code of "200", and an
   implementation-defined comment phrase.  It also inserts a Message-ID
   header field containing the value from the original request.

      The namespace field denotes the context of the status-code field.
      The namespace value of "000" means the status-code should be
      interpreted in the same way as the matching MSRP transaction
      response code.  If a future specification uses the status-code
      field for some other purpose, it MUST define a new namespace field
      value.

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

7.1.4.  Generating Failure Reports

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

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

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




Campbell, et al.            Standards Track                    [Page 23]
^L
RFC 4975                          MSRP                    September 2007


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

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

      This section only describes failure report generation behavior for
      MSRP endpoints.  Relay behavior is beyond the scope of this
      document, and will be considered in a separate document [23].  We
      expect failure reports to be more commonly generated by relays
      than by endpoints.

7.2.  Constructing Responses

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

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

   The endpoint then inserts an appropriate To-Path header field.  If
   the request triggering the response was a SEND request, the To-Path
   header field is formed by copying the first (leftmost) URI in the
   From-Path header field of the request. (Responses to SEND requests
   are returned only to the previous hop.) For responses to all other
   request methods, the To-Path header field contains the full path back
   to the original sender.  This full path is generated by copying the
   list of URIs from the From-Path of the original request into the To-
   Path of the response. (Legal REPORT requests do not request
   responses, so this specification doesn't exercise the behavior
   described above; however, we expect that extensions for gateways and
   relays will need such behavior.)

   Finally, the endpoint inserts a From-Path header field containing the
   URI that identifies it in the context of the session, followed by the
   end-line after the last header field.  Since a response is never



Campbell, et al.            Standards Track                    [Page 24]
^L
RFC 4975                          MSRP                    September 2007


   chunked, the continuation flag in the end-line will always contain a
   dollar sign ("$").  The response MUST be transmitted back on the same
   connection on which the original request arrived.

7.3.  Receiving Requests

   The receiving endpoint MUST first check the URI in the To-Path to
   make sure the request belongs to an existing session.  When the
   request is received, the To-Path will have exactly one URI, which
   MUST map to an existing session that is associated with the
   connection on which the request arrived.  If this is not true, then
   the receiver MUST generate a 481 error and ignore the request.  Note
   that if the Failure-Report header field had a value of "no", then no
   error report would be sent.

   Further request processing by the receiver is method specific.

7.3.1.  Receiving SEND Requests

   When the receiving endpoint receives a SEND request, it first
   determines if it contains a complete message or a chunk from a larger
   message.  If the request contains no Byte-Range header field, or
   contains one with a range-start value of "1", and the closing line
   continuation flag has a value of "$", then the request contained the
   entire message.  Otherwise, the receiver looks at the Message-ID
   value to associate chunks together into the original message.  The
   receiver forms a virtual buffer to receive the message, keeping track
   of which bytes have been received and which are missing.  The
   receiver takes the data from the request and places it in the
   appropriate place in the buffer.  The receiver SHOULD determine the
   actual length of each chunk by inspecting the payload itself; it is
   possible the body is shorter than the range-end field indicates.
   This can occur if the sender interrupted a SEND request unexpectedly.
   It is worth noting that the chunk that has a termination character of
   "$" defines the total length of the message.

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

   Receivers MUST not assume that the chunks will be delivered in order
   or that they will receive all the chunks with "+" flags before they
   receive the chunk with the "$" flag.  In certain cases of connection
   failure, it is possible for information to be duplicated.  If chunk
   data is received that overlaps already received data for the same



Campbell, et al.            Standards Track                    [Page 25]
^L
RFC 4975                          MSRP                    September 2007


   message, the last chunk received SHOULD take precedence (even though
   this may not have been the last chunk transmitted).  For example, if
   bytes 1 to 100 were received and a chunk arrives that contains bytes
   50 to 150, this second chunk will overwrite bytes 50 to 100 of the
   data that had already been received.  Although other schemes work,
   this is the easiest for the receiver and results in consistent
   behavior between clients.

      There are situations in which the receiver may not be able to give
      precedence to the last chunk received when chunks overlap.  For
      example, the recipient might incrementally render chunks as they
      arrive.  If a new chunk arrives that overlaps with a previously
      rendered chunk, it would be too late to "take back" any
      conflicting data from the first chunk.  Therefore, the requirement
      to give precedence to the most recent chunk is specified at a
      "SHOULD" strength.  This requirement is not intended to disallow
      applications where this behavior does not make sense.

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

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

   If the SEND request contained a Content-Type header field indicating
   an unsupported media-type, and the Failure-Report value is not "no",
   the receiver MUST generate a response with a status code of 415.  All
   MSRP endpoints MUST be able to receive the multipart/mixed [15] and
   multipart/alternative [15] media-types.

   If the Success-Report header field was set to "yes", the receiver
   must construct and send one or more success reports, as described in
   Section 7.1.3.










Campbell, et al.            Standards Track                    [Page 26]
^L
RFC 4975                          MSRP                    September 2007


7.3.2.  Receiving REPORT Requests

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

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

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

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

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

   MSRP nodes MUST NOT send MSRP REPORT requests in response to other
   REPORT requests.

8.  Using MSRP with SIP and SDP

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

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

   An MSRP media-line (that is, a media-line proposing MSRP) in the
   session description is accompanied by a mandatory "path" attribute.
   This attribute contains a space-separated list of URIs to be visited



Campbell, et al.            Standards Track                    [Page 27]
^L
RFC 4975                          MSRP                    September 2007


   to contact the user agent advertising this session description.  If
   more than one URI is present, the leftmost URI is the first URI to be
   visited to reach the target resource.  (The path list can contain
   multiple URIs to allow for the deployment of gateways or relays in
   the future.)  MSRP implementations that can accept incoming
   connections without the need for relays will typically only provide a
   single URI here.

   An MSRP media line is also accompanied by an "accept-types"
   attribute, and optionally an "accept-wrapped-types" attribute.  These
   attributes are used to specify the media-types that are acceptable to
   the endpoint.

8.1.  SDP Connection and Media-Lines

   An SDP connection-line takes the following format:

   c=<network type> <address type> <connection address>

                  Figure 4: Standard SDP Connection Line

   The network type and address type fields are used as normal for SDP.
   The connection address field MUST be set to the IP address or fully
   qualified domain name from the MSRP URI identifying the endpoint in
   its path attribute.

   The general format of an SDP media-line is:

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

                     Figure 5: Standard SDP Media Line

   An offered or accepted media-line for MSRP over TCP MUST include a
   protocol field value of "TCP/MSRP", or "TCP/TLS/MSRP" for TLS.  The
   media field value MUST be "message".  The format list field MUST be
   set to "*".

   The port field value MUST match the port value used in the endpoint's
   MSRP URI in the path attribute, except that, as described in [3], a
   user agent that wishes to accept an offer, but not a specific media-
   line, MUST set the port number of that media-line to zero (0) in the
   response.  Since MSRP allows multiple sessions to share the same TCP
   connection, multiple m-lines in a single SDP document may share the
   same port field value; MSRP devices MUST NOT assume any particular
   relationship between m-lines on the sole basis that they have
   matching port field values.





Campbell, et al.            Standards Track                    [Page 28]
^L
RFC 4975                          MSRP                    September 2007


      MSRP devices do not use the c-line address field, or the m-line
      port and format list fields to determine where to connect.
      Rather, they use the attributes defined in this specification.
      The connection information is copied to the c-line and m-line for
      purposes of backwards compatibility with conventional SDP usages.
      While MSRP could theoretically carry any media-type, "message" is
      appropriate.

8.2.  URI Negotiations

   Each endpoint in an MSRP session is identified by a URI.  These URIs
   are negotiated in the SDP exchange.  Each SDP offer or answer that
   proposes MSRP MUST contain a "path" attribute containing one or more
   MSRP URIs.  The path attribute is used in an SDP a-line, and has the
   following syntax:

        path = path-label ":" path-list
        path-label = "path"
        path-list= MSRP-URI *(SP MSRP-URI)

                            Figure 6: Path Attribute

   where MSRP-URI is an "msrp" or "msrps" URI as defined in Section 6.
   MSRP URIs included in an SDP offer or answer MUST include explicit
   port numbers.

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

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

   The following example shows an SDP offer with a session URI of
   "msrp://alice.example.com:7394/2s93i9ek2a;tcp"









Campbell, et al.            Standards Track                    [Page 29]
^L
RFC 4975                          MSRP                    September 2007


    v=0
    o=alice 2890844526 2890844527 IN IP4 alice.example.com
    s= -
    c=IN IP4 alice.example.com
    t=0 0
    m=message 7394 TCP/MSRP *
    a=accept-types:text/plain
    a=path:msrp://alice.example.com:7394/2s93i9ek2a;tcp

                 Figure 7: Example SDP with Path Attribute

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

8.3.  Path Attributes with Multiple URIs

   As mentioned previously, this document describes MSRP for peer-to-
   peer scenarios, that is, when no relays are used.  The use of relays
   is described in a separate document [23].  In order to allow an MSRP
   device that only implements the core specification to interoperate
   with devices that use relays, this document must include a few
   assumptions about how relays work.

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

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

   Even though endpoints that implement only this specification will
   never introduce a relay, they need to be able to interoperate with
   other endpoints that do use relays.  Therefore, they MUST be prepared
   to receive more than one URI in the SDP path attribute.  When an
   endpoint receives more than one URI in a path attribute, only the





Campbell, et al.            Standards Track                    [Page 30]
^L
RFC 4975                          MSRP                    September 2007


   first entry is relevant for purposes of resolving the address and
   port, and establishing the network connection, as it describes the
   first adjacent hop.

   If an endpoint puts more than one URI in a path attribute, the final
   URI in the path attribute (the peer URI) identifies the session, and
   MUST not duplicate the URI of any other session in which the endpoint
   is currently participating.  Uniqueness requirements for other
   entries in the path attribute are out of scope for this document.

8.4.  Updated SDP Offers

   MSRP endpoints may sometimes need to send additional SDP exchanges
   for an existing session.  They may need to send periodic exchanges
   with no change to refresh state in the network, for example, SIP
   session timers or the SIP UPDATE [24] request.  They may need to
   change some other stream in a session without affecting the MSRP
   stream, or they may need to change an MSRP stream without affecting
   some other stream.

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

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

8.5.  Connection Negotiation

   Previous versions of this document included a mechanism to negotiate
   the direction for any required TCP connection.  The mechanism was
   loosely based on the Connection-Oriented Media (COMEDIA) [26] work
   done by the MMUSIC working group.  The primary motivation was to
   allow MSRP sessions to succeed in situations where the offerer could
   not accept connections but the answerer could.  For example, the
   offerer might be behind a NAT, while the answerer might have a
   globally routable address.

   The SIMPLE working group chose to remove that mechanism from MSRP, as
   it added a great deal of complexity to connection management.
   Instead, MSRP now specifies a default connection direction.  The
   party that sent the original offer is responsible for connecting to
   its peer.




Campbell, et al.            Standards Track                    [Page 31]
^L
RFC 4975                          MSRP                    September 2007


8.6.  Content Type Negotiation

   An SDP media-line proposing MSRP MUST be accompanied by an accept-
   types attribute.

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

   This specification requires the support of certain data formats.
   Mandatory formats MUST be signaled like any other, either explicitly
   or by the use of a "*".

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

   Occasionally, an endpoint will need to specify a MIME media-type that
   can only be used if wrapped inside a listed container type.

   Endpoints MAY specify media-types that are only allowed when wrapped
   inside compound types using the "accept-wrapped-types" attribute in
   an SDP a-line.

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

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



Campbell, et al.            Standards Track                    [Page 32]
^L
RFC 4975                          MSRP                    September 2007


      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
      Common Presence and Instant Messaging (CPIM) [12] gateway device
      may require all messages to be wrapped inside message/cpim bodies,
      but may allow several content types inside the wrapper.  If the
      gateway were to specify the wrapped types in the accept-types
      attribute, its peer might attempt to use those types without the
      wrapper.

   If the recipient of an offer does not understand any of the payload
   types indicated in the offered SDP, it SHOULD indicate that using the
   appropriate mechanism of the rendezvous protocol.  For example, in
   SIP, it SHOULD return a SIP 488 response.

   An MSRP endpoint MUST NOT send content of a type not signaled by the
   peer in either an accept-types or an accept-wrapped-types attribute.
   Furthermore, it MUST NOT send a top-level (i.e., not wrapped) MIME
   document of a type not signaled in the accept-types attribute.  In
   either case, the signaling could be explicit, or implicit through the
   use of the "*" character.

   An endpoint MAY indicate the maximum size message it wishes to
   receive using the max-size a-line attribute.  Max-size refers to the
   complete message in octets, not the size of any one chunk.  Senders
   SHOULD NOT exceed the max-size limit for any message sent in the
   resulting session.  However, the receiver should consider max-size
   value as a hint.

   Media format entries may include parameters.  The interpretation of
   such parameters varies between media-types.  For the purposes of
   media-type negotiation, a format-entry with one or more parameters is
   assumed to match the same format-entry with no parameters.


















Campbell, et al.            Standards Track                    [Page 33]
^L
RFC 4975                          MSRP                    September 2007


   The formal syntax for these attributes is as follows:

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

        type = token
        subtype = token
        type-param = parm-attribute "=" parm-value
        parm-attribute = token
        parm-value = token / quoted-string

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

                           Figure 8: Attribute Syntax

8.7.  Example SDP Exchange

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

    v=0
    o=usera 2890844526 2890844527 IN IP4 alice.example.com
    s= -
    c=IN IP4 alice.example.com
    t=0 0
    m=message 7394 TCP/MSRP *
    a=accept-types:message/cpim text/plain text/html
    a=path:msrp://alice.example.com:7394/2s93i93idj;tcp

                       Figure 9: SDP from Endpoint A












Campbell, et al.            Standards Track                    [Page 34]
^L
RFC 4975                          MSRP                    September 2007


   B responds with its own URI:

    v=0
    o=userb 2890844530 2890844532 IN IP4 bob.example.com
    s= -
    c=IN IP4 bob.example.com
    t=0 0
    m=message 8493 TCP/MSRP *
    a=accept-types:message/cpim text/plain
    a=path:msrp://bob.example.com:8493/si438dsaodes;tcp

                       Figure 10: SDP from Endpoint B

8.8.  MSRP User Experience with SIP

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

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

      This guideline may not make sense for all situations, such as for
      mixed-media applications, where both MSRP and audio sessions are
      offered in the same INVITE.  In general, good application design
      should take precedence.

   SIP INVITE requests may be forked by a SIP proxy, resulting in more
   than one endpoint receiving the same INVITE.  SIP early media [29]
   techniques can be used to establish a preliminary session with each
   endpoint so the initial message(s) are displayed on each endpoint,
   and canceling the INVITE transaction for any endpoints that do not
   send MSRP traffic after some period of time, so that they cease
   receiving MSRP traffic from the inviter.

8.9.  SDP Direction Attribute and MSRP

   SDP defines a number of attributes that modify the direction of media
   flows.  These are the "sendonly", "recvonly", "inactive", and
   "sendrecv" attributes.





Campbell, et al.            Standards Track                    [Page 35]
^L
RFC 4975                          MSRP                    September 2007


   If a "sendonly" or "recvonly" attribute modifies an MSRP media
   description line, the attribute indicates the direction of MSRP SEND
   requests that contain regular message payloads.  Unless otherwise
   specified, these attributes do not affect the direction of other
   types of requests, such as REPORT.  SEND requests that contain some
   kind of control or reporting protocol rather than regular message
   payload (e.g., Instant Message Delivery Notification (IMDN) reports)
   should be generated according to the protocol rules as if no
   direction attribute were present.

9.  Formal Syntax

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

   The following syntax specification uses the augmented Backus-Naur
   Form (BNF) as described in RFC 4234 [6].

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

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

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

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

   MSRP-URI = msrp-scheme "://" authority
       ["/" session-id] ";" transport *( ";" URI-parameter)
                        ; authority as defined in RFC3986

   msrp-scheme = "msrp" / "msrps"
   session-id = 1*( unreserved / "+" / "=" / "/" )
                        ; unreserved as defined in RFC3986
   transport = "tcp" / 1*ALPHANUM
   URI-parameter = token ["=" token]

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




Campbell, et al.            Standards Track                    [Page 36]
^L
RFC 4975                          MSRP                    September 2007


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

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

   Status = "Status:" SP namespace SP status-code [SP comment]
   namespace = 3(DIGIT); "000" for all codes defined in this document.

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

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

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

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

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

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





Campbell, et al.            Standards Track                    [Page 37]
^L
RFC 4975                          MSRP                    September 2007


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

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

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

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

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

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

                           Figure 11: MSRP ABNF

10.  Response Code Descriptions

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

10.1.  200

   The 200 response code indicates a successful transaction.

10.2.  400

   A 400 response indicates that a request was unintelligible.  The
   sender may retry the request after correcting the error.

10.3.  403

   A 403 response indicates that the attempted action is not allowed.
   The sender should not try the request again.



Campbell, et al.            Standards Track                    [Page 38]
^L
RFC 4975                          MSRP                    September 2007


10.4.  408

   A 408 response indicates that a downstream transaction did not
   complete in the allotted time.  It is never sent by any elements
   described in this specification.  However, 408 is used in the MSRP
   relay extension; therefore, MSRP endpoints may receive it.  An
   endpoint MUST treat a 408 response in the same manner as it would
   treat a local timeout.

10.5.  413

   A 413 response indicates that the receiver wishes the sender to stop
   sending the particular message.  Typically, a 413 is sent in response
   to a chunk of an undesired message.

   If a message sender receives a 413 in a response, or in a REPORT
   request, it MUST NOT send any further chunks in the message, that is,
   any further chunks with the same Message-ID value.  If the sender
   receives the 413 while in the process of sending a chunk, and the
   chunk is interruptible, the sender MUST interrupt it.

10.6.  415

   A 415 response indicates that the SEND request contained a media type
   that is not understood by the receiver.  The sender should not send
   any further messages with the same content-type for the duration of
   the session.

10.7.  423

   A 423 response indicates that one of the requested parameters is out
   of bounds.  It is used by the relay extensions to this document.

10.8.  481

   A 481 response indicates that the indicated session does not exist.
   The sender should terminate the session.

10.9.  501

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

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




Campbell, et al.            Standards Track                    [Page 39]
^L
RFC 4975                          MSRP                    September 2007


10.10.  506

   A 506 response indicates that a request arrived on a session that is
   already bound to another network connection.  The sender should cease
   sending messages for that session on this connection.

11.  Examples

11.1.  Basic IM Session

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

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

                        Figure 12: Basic IM Session Example










Campbell, et al.            Standards Track                    [Page 40]
^L
RFC 4975                          MSRP                    September 2007


   1.  Alice constructs a local URI of
       msrp://alicepc.example.com:7777/iau39soe2843z;tcp .

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

       v=0
       o=alice 2890844557 2890844559 IN IP4 alicepc.example.com
       s= -
       c=IN IP4 alicepc.example.com
       t=0 0
       m=message 7777 TCP/MSRP *
       a=accept-types:text/plain
       a=path:msrp://alicepc.example.com:7777/iau39soe2843z;tcp

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

       Bob->Alice (SIP): 200 OK

       v=0
       o=bob 2890844612 2890844616 IN IP4 bob.example.com
       s= -
       c=IN IP4 bob.example.com
       t=0 0
       m=message 8888 TCP/MSRP *
       a=accept-types:text/plain
       a=path:msrp://bob.example.com:8888/9di4eae923wzd;tcp

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

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

       MSRP d93kswow SEND
       To-Path: msrp://bob.example.com:8888/9di4eae923wzd;tcp
       From-Path: msrp://alicepc.example.com:7777/iau39soe2843z;tcp
       Message-ID: 12339sdqwer
       Byte-Range: 1-16/16
       Content-Type: text/plain

       Hi, I'm Alice!
       -------d93kswow$

   5.  Bob->Alice (MSRP):

       MSRP d93kswow 200 OK
       To-Path: msrp://alicepc.example.com:7777/iau39soe2843z;tcp
       From-Path: msrp://bob.example.com:8888/9di4eae923wzd;tcp
       -------d93kswow$




Campbell, et al.            Standards Track                    [Page 41]
^L
RFC 4975                          MSRP                    September 2007


   6.  Bob->Alice (MSRP):

       MSRP dkei38sd SEND
       To-Path: msrp://alicepc.example.com:7777/iau39soe2843z;tcp
       From-Path: msrp://bob.example.com:8888/9di4eae923wzd;tcp
       Message-ID: 456s9wlk3
       Byte-Range: 1-21/21
       Content-Type: text/plain

       Hi, Alice!  I'm Bob!
       -------dkei38sd$

   7.  Alice->Bob (MSRP):

       MSRP dkei38sd 200 OK
       To-Path: msrp://bob.example.com:8888/9di4eae923wzd;tcp
       From-Path: msrp://alicepc.example.com:7777/iau39soe2843z;tcp
       -------dkei38sd$

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

       Alice invalidates local session state.

   9.  Bob invalidates local state for the session.

       Bob->Alice (SIP): 200 OK

11.2.  Message with XHTML Content

   MSRP dsdfoe38sd SEND
   To-Path: msrp://alice.example.com:7777/iau39soe2843z;tcp
   From-Path: msrp://bob.example.com:8888/9di4eae923wzd;tcp
   Message-ID: 456so39s
   Byte-Range: 1-374/374
   Content-Type: application/xhtml+xml
















Campbell, et al.            Standards Track                    [Page 42]
^L
RFC 4975                          MSRP                    September 2007


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

                   Figure 13: Example Message with XHTML

11.3.  Chunked Message

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

11.4.  Chunked Message with Message/CPIM Payload

   This example shows a chunked message containing a CPIM message that
   wraps a text/plain payload.  It is worth noting that MSRP considers
   the complete CPIM message before chunking the message; thus, the CPIM
   headers are included in only the first chunk.  The MSRP Content-Type
   and Byte-Range headers, present in both chunks, refer to the whole
   CPIM message.

      MSRP d93kswow SEND
      To-Path: msrp://bobpc.example.com:8888/9di4eae923wzd;tcp
      From-Path: msrp://alicepc.example.com:7654/iau39soe2843z;tcp
      Message-ID: 12339sdqwer
      Byte-Range: 1-137/148
      Content-Type: message/cpim

      To: Bob <sip:bob@example.com>
      From: Alice <sip:alice@example.com>
      DateTime: 2006-05-15T15:02:31-03:00
      Content-Type: text/plain

      ABCD
      -------d93kswow+

                            Figure 14: First Chunk





Campbell, et al.            Standards Track                    [Page 43]
^L
RFC 4975                          MSRP                    September 2007


   Alice sends the second and last chunk.

      MSRP op2nc9a SEND
      To-Path: msrp://bobpc.example.com:8888/9di4eae923wzd;tcp
      From-Path: msrp://alicepc.example.com:7654/iau39soe2843z;tcp
      Message-ID: 12339sdqwer
      Byte-Range: 138-148/148
      Content-Type: message/cpim

      1234567890
      -------op2nc9a$

                           Figure 15: Second Chunk

11.5.  System Message

   Sysadmin->Alice (MSRP):

   MSRP d93kswow SEND
   To-Path: msrp://alicepc.example.com:8888/9di4eae923wzd;tcp
   From-Path: msrp://example.com:7777/iau39soe2843z;tcp
   Message-ID: 12339sdqwer
   Byte-Range: 1-38/38
   Failure-Report: no
   Success-Report: no
   Content-Type: text/plain

   This conference will end in 5 minutes
   -------d93kswow$

11.6.  Positive Report

   Alice->Bob (MSRP):

   MSRP d93kswow SEND
   To-Path: msrp://bob.example.com:8888/9di4eae923wzd;tcp
   From-Path: msrp://alicepc.example.com:7777/iau39soe2843z;tcp
   Message-ID: 12339sdqwer
   Byte-Range: 1-106/106
   Success-Report: yes
   Failure-Report: no
   Content-Type: text/html









Campbell, et al.            Standards Track                    [Page 44]
^L
RFC 4975                          MSRP                    September 2007


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

                      Figure 16: Initial SEND Request

   Bob->Alice (MSRP):

   MSRP dkei38sd REPORT
   To-Path: msrp://alicepc.example.com:7777/iau39soe2843z;tcp
   From-Path: msrp://bob.example.com:8888/9di4eae923wzd;tcp
   Message-ID: 12339sdqwer
   Byte-Range: 1-106/106
   Status: 000 200 OK
   -------dkei38sd$

                         Figure 17: Success Report

11.7.  Forked IM

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

   Using SIP makes rendezvous decisions explicit, deterministic, and
   very flexible.  In contrast, "page-mode" IM systems use implicit
   implementation-specific decisions that IM clients cannot influence.
   With SIP session-mode messaging, rendezvous decisions can be under
   control of the client in a predictable, interoperable way for any
   host that implements callee capabilities [31].  As a result,
   rendezvous policy is managed consistently for each address of record.

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





Campbell, et al.            Standards Track                    [Page 45]
^L
RFC 4975                          MSRP                    September 2007


   When Romeo opens his IM program, he selects Juliet and types the
   message "art thou hither?" (instead of "you there?").  His client
   sends a SIP invitation to sip:juliet@thecapulets.example.com.  The
   proxy there tries first the balcony and the chamber simultaneously.
   A client is running on each of those systems, both of which set up
   early sessions of MSRP with Romeo's client.  The client automatically
   sends the message over MSRP to the two MSRP URIs involved.  After a
   delay of a several seconds with no reply or activity from Juliet, the
   proxy cancels the invitation at her first two contacts, and forwards
   the invitation on to Juliet's PDA.  Since her father is talking to
   her about her wedding, she selects "Do Not Disturb" on her PDA, which
   sends a "Busy Here" response.  The proxy then tries the nurse, who
   answers and tells Romeo what is going on.






































Campbell, et al.            Standards Track                    [Page 46]
^L
RFC 4975                          MSRP                    September 2007


    Romeo       Juliet's     Juliet/      Juliet/      Juliet/     Nurse
                 Proxy       balcony      chamber       PDA
      |            |            |            |           |           |
      |--INVITE--->|            |            |           |           |
      |            |--INVITE--->|            |           |           |
      |            |<----180----|            |           |           |
      |<----180----|            |            |           |           |
      |---PRACK---------------->|            |           |           |
      |<----200-----------------|            |           |           |
      |<===Early MSRP Session==>| art thou hither?       |           |
      |            |            |            |           |           |
      |            |--INVITE---------------->|           |           |
      |            |<----180-----------------|           |           |
      |<----180----|            |            |           |           |
      |---PRACK----------------------------->|           |           |
      |<----200------------------------------|           |           |
      |<========Early MSRP Session==========>| art thou hither?      |
      |            |            |            |           |           |
      |            |            |            |           |           |
      |            | .... Time Passes ....   |           |           |
      |            |            |            |           |           |
      |            |            |            |           |           |
      |            |--CANCEL--->|            |           |           |
      |            |<---200-----|            |           |           |
      |            |<---487-----|            |           |           |
      |            |----ACK---->|            |           |           |
      |            |--CANCEL---------------->|           |           |
      |            |<---200------------------|           |           |
      |            |<---487------------------|           |           |
      |            |----ACK----------------->|           |           |
      |            |--INVITE---------------------------->|  romeo wants
      |            |            |            |           |  to IM w/ you
      |            |<---486 Busy Here--------------------|           |
      |            |----ACK----------------------------->|           |
      |            |            |            |           |           |
      |            |--INVITE---------------------------------------->|
      |            |<---200 OK---------------------------------------|
      |<--200 OK---|            |            |           |           |
      |---ACK------------------------------------------------------->|
      |<================MSRP Session================================>|
      |            |            |            |           |           |
      |                                         Hi Romeo, Juliet is  |
      |                                         with her father now  |
      |                                         can I take a message?|
      |                                                              |
      |  Tell her to go to confession tomorrow....                   |

                        Figure 18: Forking Example



Campbell, et al.            Standards Track                    [Page 47]
^L
RFC 4975                          MSRP                    September 2007


12.  Extensibility

   MSRP was designed to be only minimally extensible.  New MSRP methods,
   header fields, and status codes can be defined in standards-track
   RFCs.  MSRP does not contain a version number or any negotiation
   mechanism to require or discover new features.  If an extension is
   specified in the future that requires negotiation, the specification
   will need to describe how the extension is to be negotiated in the
   encapsulating signaling protocol.  If a non-interoperable update or
   extension occurs in the future, it will be treated as a new protocol,
   and MUST describe how its use will be signaled.

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

   MSRP was designed to use lists of URIs instead of a single URI in the
   To-Path and From-Path header fields in anticipation of relay or
   gateway functionality being added.  In addition, "msrp" and "msrps"
   URIs can contain parameters that are extensible.

13.  CPIM Compatibility

   MSRP sessions may go to a gateway to other Common Profile for Instant
   Messaging (CPIM) [27] compatible protocols.  If this occurs, the
   gateway MUST maintain session state, and MUST translate between the
   MSRP session semantics and CPIM semantics, which do not include a
   concept of sessions.  Furthermore, when one endpoint of the session
   is a CPIM gateway, instant messages SHOULD be wrapped in
   "message/cpim" [12] bodies.  Such a gateway MUST include
   "message/cpim" as the first entry in its SDP accept-types attribute.
   MSRP endpoints sending instant messages to a peer that 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[7] features of that format.

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








Campbell, et al.            Standards Track                    [Page 48]
^L
RFC 4975                          MSRP                    September 2007


   All MSRP endpoints MUST recognize the From, To, DateTime, and Require
   header fields as defined in RFC 3862.  Such applications SHOULD
   recognize the CC header field, and MAY recognize the Subject header
   field.  Any MSRP application that recognizes any message/cpim header
   field MUST understand the NS (name space) header field.

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

   The NS, To, and CC header fields may occur multiple times.  Other
   header fields defined in RFC 3862 MUST NOT occur more than once in a
   given message/cpim body part in an MSRP message.  The Require header
   field MAY include multiple values.  The NS header field MAY occur
   zero or more times, depending on how many name spaces are being
   referenced.

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

      Using message/cpim envelopes is also useful if an MSRP device
      wishes to send a message on behalf of some other identity.  The
      device may add a message/cpim envelope with the appropriate From
      header field value.

14.  Security Considerations

   Instant messaging systems are used to exchange a variety of sensitive
   information ranging from personal conversations, to corporate
   confidential information, to account numbers and other financial
   trading information.  IM is used by individuals, corporations, and
   governments for communicating important information.  IM systems need
   to provide the properties of integrity and confidentiality for the
   exchanged information, and the knowledge that you are communicating
   with the correct party, and they need to allow the possibility of
   anonymous communication.  MSRP pushes many of the hard problems to
   SIP when SIP sets up the session, but some of the problems remain.
   Spam and Denial of Service (DoS) attacks are also very relevant to IM
   systems.

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






Campbell, et al.            Standards Track                    [Page 49]
^L
RFC 4975                          MSRP                    September 2007


14.1.  Secrecy of the MSRP URI

   When an endpoint sends an MSRP URI to its peer in a rendezvous
   protocol, that URI is effectively a secret shared between the peers.
   If an attacker learns or guesses the URI prior to the completion of
   session setup, it may be able to impersonate one of the peers.

   Assuming the URI exchange in the rendezvous protocol is sufficiently
   protected, it is critical that the URI remain difficult to "guess"
   via brute force methods.  Most components of the URI, such as the
   scheme and the authority components, are common knowledge.  The
   secrecy is entirely provided by the session-id component.

   Therefore, when an MSRP device generates an MSRP URI to be used in
   the initiation of an MSRP session, the session-id component MUST
   contain at least 80 bits of randomness.

14.2.  Transport Level Protection

   When using only TCP connections, MSRP security is fairly weak.  If
   host A is contacting host B, B passes its hostname and a secret to A
   using a rendezvous protocol.  Although MSRP requires the use of a
   rendezvous protocol with the ability to protect this exchange, there
   is no guarantee that the protection will be used all the time.  If
   such protection is not used, anyone can see this secret.  Host A then
   connects to the provided hostname and passes the secret in the clear
   across the connection to B.  Host A assumes that it is talking to B
   based on where it sent the SYN packet and then delivers the secret in
   plain text across the connections.  Host B assumes it is talking to A
   because the host on the other end of the connection delivered the
   secret.  An attacker that could ACK the SYN packet could insert
   itself as a man-in-the-middle in the connection.

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

   Realistically, using TLS with certificates from well-known
   certification authorities is difficult for peer-to-peer connections,
   as the types of hosts that end clients use for sending instant



Campbell, et al.            Standards Track                    [Page 50]
^L
RFC 4975                          MSRP                    September 2007


   messages are unlikely to have long-term stable IP addresses or DNS
   names that the certificates can bind to.  In addition, the cost of
   server certificates from well-known certification authorities is
   currently expensive enough to discourage their use for each client.
   Using TLS in a peer-to-peer mode without well-known certificates is
   discussed in Section 14.4.

   TLS becomes much more practical when some form of relay is
   introduced.  Clients can then form TLS connections to relays, which
   are much more likely to have TLS certificates.  While this
   specification does not address such relays, they are described by a
   companion document [23].  That document makes extensive use of TLS to
   protect traffic between clients and relays, and between one relay and
   another.

   TLS is used to authenticate devices and to provide integrity and
   confidentiality for the header fields being transported.  MSRP
   elements MUST implement TLS and MUST also implement the TLS
   ClientExtendedHello extended hello information for server name
   indication as described in [11].  A TLS cipher-suite of
   TLS_RSA_WITH_AES_128_CBC_SHA [13] MUST be supported (other cipher-
   suites MAY also be supported).

14.3.  S/MIME

   The only strong security for non-TLS connections is achieved using
   S/MIME.

   Since MSRP carries arbitrary MIME content, it can trivially carry
   S/MIME protected messages as well.  All MSRP implementations MUST
   support the multipart/signed media-type even if they do not support
   S/MIME.  Since SIP can carry a session key, S/MIME messages in the
   context of a session could also be protected using a key-wrapped
   shared secret [28] provided in the session setup.  MSRP can carry
   unencoded binary payloads.  Therefore, MIME bodies MUST be
   transferred with a transfer encoding of binary.  If a message is both
   signed and encrypted, it SHOULD be signed first, then encrypted.  If
   S/MIME is supported, SHA-1, SHA-256, RSA, and AES-128 MUST be
   supported.  For RSA, implementations MUST support key sizes of at
   least 1024 bits and SHOULD support key sizes of 2048 bits or more.

   This does not actually require the endpoint to have certificates from
   a well-known certification authority.  When MSRP is used with SIP,
   the Identity [17] and Certificates [25] mechanisms provide S/MIME-
   based delivery of a secret between A and B.  No SIP intermediary
   except the explicitly trusted authentication service (one per user)
   can see the secret.  The S/MIME encryption of the SDP can also be
   used by SIP to exchange keying material that can be used in MSRP.



Campbell, et al.            Standards Track                    [Page 51]
^L
RFC 4975                          MSRP                    September 2007


   The MSRP session can then use S/MIME with this keying material to
   sign and encrypt messages sent over MSRP.  The connection can still
   be hijacked since the secret is sent in clear text to the other end
   of the TCP connection, but the consequences are mitigated if all the
   MSRP content is signed and encrypted with S/MIME.  Although out of
   scope for this document, the SIP negotiation of an MSRP session can
   negotiate symmetric keying material to be used with S/MIME for
   integrity and privacy.

14.4.  Using TLS in Peer-to-Peer Mode

   TLS can be used with a self-signed certificate as long as there is a
   mechanism for both sides to ascertain that the other side used the
   correct certificate.  When used with SDP and SIP, the correct
   certificate can be verified by passing a fingerprint of the
   certificate in the SDP and ensuring that the SDP has suitable
   integrity protection.  When SIP is used to transport the SDP, the
   integrity can be provided by the SIP Identity mechanism [17].  The
   rest of this section describes the details of this approach.

   If self-signed certificates are used, the content of the
   subjectAltName attribute inside the certificate MAY use the URI of
   the user.  In SIP, this URI of the user is the User's Address of
   Record (AOR).  This is useful for debugging purposes only and is not
   required to bind the certificate to one of the communication
   endpoints.  Unlike normal TLS operations in this protocol, when doing
   peer-to-peer TLS, the subjectAltName is not an important component of
   the certificate verification.  If the endpoint is also able to make
   anonymous sessions, a distinct, unique certificate MUST be used for
   this purpose.  For a client that works with multiple users, each user
   SHOULD have its own certificate.  Because the generation of
   public/private key pairs is relatively expensive, endpoints are not
   required to generate certificates for each session.

   A certificate fingerprint is the output of a one-way hash function
   computed over the Distinguished Encoding Rules (DER) form of the
   certificate.  The endpoint MUST use the certificate fingerprint
   attribute as specified in [18] and MUST include this in the SDP.  The
   certificate presented during the TLS handshake needs to match the
   fingerprint exchanged via the SDP, and if the fingerprint does not
   match the hashed certificate then the endpoint MUST tear down the
   media session immediately.

   When using SIP, the integrity of the fingerprint can be ensured
   through the SIP Identity mechanism [17].  When a client wishes to use
   SIP to set up a secure MSRP session with another endpoint, it sends
   an SDP offer in a SIP message to the other endpoint.  This offer
   includes, as part of the SDP payload, the fingerprint of the



Campbell, et al.            Standards Track                    [Page 52]
^L
RFC 4975                          MSRP                    September 2007


   certificate that the endpoint wants to use.  The SIP message
   containing the offer is sent to the offerer's SIP proxy, which will
   add an Identity header according to the procedures outlined in [17].
   When the far endpoint receives the SIP message, it can verify the
   identity of the sender using the Identity header.  Since the Identity
   header is a digital signature across several SIP headers, in addition
   to the body or bodies of the SIP message, the receiver can also be
   certain that the message has not been tampered with after the digital
   signature was added to the SIP message.

   An example of SDP with a fingerprint attribute is shown in the
   following figure.  Note the fingerprint is shown spread over two
   lines due to formatting consideration but should all be on one line.

   c=IN IP4 atlanta.example.com
   m=message 7654 TCP/TLS/MSRP *
   a=accept-types:text/plain
   a=path:msrps://atlanta.example.com:7654/jshA7weso3ks;tcp
   a=fingerprint:SHA-1 \
         4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB

                 Figure 19: SDP with Fingerprint Attribute

14.5.  Other Security Concerns

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

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

   If a sender chooses to employ S/MIME to protect a message, all S/MIME
   operations apply to the complete message, prior to any breaking of
   the message into chunks.

   The signaling will have set up the session to or from some specific
   URIs that will often have "im:" or "sip:" URI schemes.  When the
   signaling has been set up to a specific end user, and S/MIME is
   implemented, then the client needs to verify that the name in the
   SubjectAltName of the certificate contains an entry that matches the



Campbell, et al.            Standards Track                    [Page 53]
^L
RFC 4975                          MSRP                    September 2007


   URI that was used for the other end in the signaling.  There are some
   cases, such as IM conferencing, where the S/MIME certificate name and
   the signaled identity will not match.  In these cases, the client
   should ensure that the user is informed that the message came from
   the user identified in the certificate and does not assume that the
   message came from the party they signaled.

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

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

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

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







Campbell, et al.            Standards Track                    [Page 54]
^L
RFC 4975                          MSRP                    September 2007


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

   MSRP implementors should be aware of a potential attack on MSRP
   devices that involves placing very large values in the byte-range
   header field, potentially causing the device to allocate very large
   memory buffers to hold the message.  Implementations SHOULD apply
   some degree of sanity checking on byte-range values before allocating
   such buffers.

15.  IANA Considerations

   This specification instructs IANA to create a new registry for MSRP
   parameters.  The MSRP Parameter registry is a container for sub-
   registries.  This section further introduces sub-registries for MSRP
   method names, status codes, and header field names.

   Additionally, Section 15.4 through Section 15.7 register new
   parameters in existing IANA registries.

15.1.  MSRP Method Names

   This specification establishes the Methods sub-registry under MSRP
   Parameters and initiates its population as follows.  New parameters
   in this sub-registry must be published in an RFC (either as an IETF
   submission or RFC Editor submission).

      SEND - [RFC4975]
      REPORT - [RFC4975]

   The following information MUST be provided in an RFC publication in
   order to register a new MSRP method:

   o  The method name.
   o  The RFC number in which the method is registered.

15.2.  MSRP Header Fields

   This specification establishes the header field-Field sub-registry
   under MSRP Parameters.  New parameters in this sub-registry must be
   published in an RFC (either as an IETF submission or RFC Editor
   submission).  Its initial population is defined as follows:






Campbell, et al.            Standards Track                    [Page 55]
^L
RFC 4975                          MSRP                    September 2007


      To-Path - [RFC4975]
      From-Path - [RFC4975]
      Message-ID - [RFC4975]
      Success-Report - [RFC4975]
      Failure-Report - [RFC4975]
      Byte-Range - [RFC4975]
      Status - [RFC4975]

   The following information MUST be provided in an RFC publication in
   order to register a new MSRP header field:

   o  The header field name.
   o  The RFC number in which the method is registered.

15.3.  MSRP Status Codes

   This specification establishes the Status-Code sub-registry under
   MSRP Parameters.  New parameters in this sub-registry must be
   published in an RFC (either as an IETF submission or RFC Editor
   submission).  Its initial population is defined in Section 10.  It
   takes the following format:

      Code [RFC Number]

   The following information MUST be provided in an RFC publication in
   order to register a new MSRP status code:

   o  The status code number.
   o  The RFC number in which the method is registered.

15.4.  MSRP Port

   MSRP uses TCP port 2855, from the "registered" port range.  Usage of
   this value is described in Section 6.

15.5.  URI Schema

   This document requests permanent registration the URI schemes of
   "msrp" and "msrps".

15.5.1.  MSRP Scheme

   URI Scheme Name:  "msrp"
   URI Scheme Syntax:  See the ABNF construction for "MSRP-URI" in
      Section 9 of RFC 4975.
   URI Scheme Semantics:  See Section 6 of RFC 4975.
   Encoding Considerations:  See Section 6 of RFC 4975.




Campbell, et al.            Standards Track                    [Page 56]
^L
RFC 4975                          MSRP                    September 2007


   Applications/Protocols that use this URI Scheme:  The Message Session
      Relay Protocol (MSRP).
   Interoperability Considerations:  MSRP URIs are expected to be used
      only by implementations of MSRP.  No additional interoperability
      issues are expected.
   Security Considerations:  See Section 14.1 of RFC 4975 for specific
      security considerations for MSRP URIs, and Section 14 of RFC 4975
      for security considerations for MSRP in general.
   Contact:  Ben Campbell (ben@estacado.net).
   Author/Change Controller:  This is a permanent registration request.
      Change control does not apply.

15.5.2.  MSRPS Scheme

   URI Scheme Name:  "msrps"
   URI Scheme Syntax:  See the ABNF construction for "MSRP-URI" in
      Section 9 of RFC 4975.
   URI Scheme Semantics:  See Section 6 of RFC 4975.
   Encoding Considerations:  See Section 6 of RFC 4975.
   Applications/Protocols that use this URI Scheme:  The Message Session
      Relay Protocol (MSRP).
   Interoperability Considerations:  MSRP URIs are expected to be used
      only by implementations of MSRP.  No additional interoperability
      issues are expected.
   Security Considerations:  See Section 14.1 of RFC 4975 for specific
      security considerations for MSRP URIs, and Section 14 of RFC 4975
      for security considerations for MSRP in general.
   Contact:  Ben Campbell (ben@estacado.net).
   Author/Change Controller:  This is a permanent registration request.
      Change control does not apply.

15.6.  SDP Transport Protocol

   MSRP defines the new SDP protocol field values "TCP/MSRP" and "TCP/
   TLS/MSRP", which should be registered in the sdp-parameters registry
   under "proto".  This first value indicates the MSRP protocol when TCP
   is used as an underlying transport.  The second indicates that TLS
   over TCP is used.

   Specifications defining new protocol values must define the rules for
   the associated media format namespace.  The "TCP/MSRP" and "TCP/TLS/
   MSRP" protocol values allow only one value in the format field (fmt),
   which is a single occurrence of "*".  Actual format determination is
   made using the "accept-types" and "accept-wrapped-types" attributes.







Campbell, et al.            Standards Track                    [Page 57]
^L
RFC 4975                          MSRP                    September 2007


15.7.  SDP Attribute Names

   This document registers the following SDP attribute parameter names
   in the sdp-parameters registry.  These names are to be used in the
   SDP att-name field.

15.7.1.  Accept Types

   Contact Information:  Ben Campbell (ben@estacado.net)
   Attribute-name:   accept-types
   Long-form Attribute Name:  Acceptable media types
   Type:  Media level
   Subject to Charset Attribute:  No
   Purpose and Appropriate Values:  The "accept-types" attribute
      contains a list of media types that the endpoint is willing to
      receive.  It may contain zero or more registered media-types, or
      "*" in a space-delimited string.

15.7.2.  Wrapped Types

   Contact Information:  Ben Campbell (ben@estacado.net)
   Attribute-name:   accept-wrapped-types
   Long-form Attribute Name:  Acceptable media types Inside Wrappers
   Type:  Media level
   Subject to Charset Attribute:  No
   Purpose and Appropriate Values:  The "accept-wrapped-types" attribute
      contains a list of media types that the endpoint is willing to
      receive in an MSRP message with multipart content, but may not be
      used as the outermost type of the message.  It may contain zero or
      more registered media-types, or "*" in a space-delimited string.

15.7.3.  Max Size

   Contact Information:  Ben Campbell (ben@estacado.net)
   Attribute-name:   max-size
   Long-form Attribute Name:  Maximum message size
   Type:  Media level
   Subject to Charset Attribute:  No
   Purpose and Appropriate Values:  The "max-size" attribute indicates
      the largest message an endpoint wishes to accept.  It may take any
      whole numeric value, specified in octets.

15.7.4.  Path

   Contact Information:  Ben Campbell (ben@estacado.net)
   Attribute-name:   path
   Long-form Attribute Name:  MSRP URI Path
   Type:  Media level



Campbell, et al.            Standards Track                    [Page 58]
^L
RFC 4975                          MSRP                    September 2007


   Subject to Charset Attribute:  No
   Purpose and Appropriate Values:  The "path" attribute indicates a
      series of MSRP devices that must be visited by messages sent in
      the session, including the final endpoint.  The attribute contains
      one or more MSRP URIs, delimited by the space character.

16.  Contributors and Acknowledgments

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

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

17.  References

17.1.  Normative References

   [1]   Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS)
         Protocol Version 1.1", RFC 4346, April 2006.

   [2]   Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
         Description Protocol", RFC 4566, July 2006.

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

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

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

   [6]   Crocker, D. and P. Overell, "Augmented BNF for Syntax
         Specifications: ABNF", RFC 4234, October 2005.

   [7]   Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
         (S/MIME) Version 3.1 Message Specification", RFC 3851, July
         2004.

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



Campbell, et al.            Standards Track                    [Page 59]
^L
RFC 4975                          MSRP                    September 2007


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

   [10]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
         Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986,
         January 2005.

   [11]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., and
         T. Wright, "Transport Layer Security (TLS) Extensions", RFC
         4366, April 2006.

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

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

   [14]  Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD
         63, RFC 3629, November 2003.

   [15]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
         Extensions (MIME) Part Two: Media Types", RFC 2046, November
         1996.

   [16]  Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509
         Public Key Infrastructure Certificate and Certificate
         Revocation List (CRL) Profile", RFC 3280, April 2002.

   [17]  Peterson, J. and  C. Jennings, "Enhancements for Authenticated
         Identity Management in the Session Initiation Protocol (SIP)",
         RFC 4474, August 2006.

   [18]  Lennox, J., "Connection-Oriented Media Transport over the
         Transport Layer Security (TLS) Protocol in the Session
         Description Protocol (SDP)", RFC 4572, July 2006.

17.2.  Informative References

   [19]  Johnston, A. and O. Levin, "Session Initiation Protocol (SIP)
         Call Control - Conferencing for User Agents", BCP 119, RFC
         4579, August 2006.

   [20]  Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo,
         "Best Current Practices for Third Party Call Control (3pcc) in
         the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, April
         2004.




Campbell, et al.            Standards Track                    [Page 60]
^L
RFC 4975                          MSRP                    September 2007


   [21]  Sparks, R., Johnston, A., and D. Petrie, "Session Initiation
         Protocol Call Control - Transfer", Work in Progress, October
         2006.

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

   [23]  Jennings, C., Mahy, R., and A. Roach, "Relay Extensions for the
         Message Session Relay Protocol (MSRP)", RFC 4976, September
         2007.

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

   [25]  Jennings, C., Peterson, J., and J. Fischl, "Certificate
         Management Service for SIP", Work in Progress, July 2007.

   [26]  Yon, D. and G. Camarillo, "TCP-Based Media Transport in the
         Session Description Protocol (SDP)", RFC 4145, September 2005.

   [27]  Peterson, J., "Common Profile for Instant Messaging (CPIM)",
         RFC 3860, August 2004.

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

   [29]  Camarillo, G. and H. Schulzrinne, "Early Media and Ringing Tone
         Generation in the Session Initiation Protocol (SIP)", RFC 3960,
         December 2004.

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

   [31]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating
         User Agent Capabilities in the Session Initiation Protocol
         (SIP)", RFC 3840, August 2004.

   [32]  Peterson, J., "Address Resolution for Instant Messaging and
         Presence", RFC 3861, August 2004.










Campbell, et al.            Standards Track                    [Page 61]
^L
RFC 4975                          MSRP                    September 2007


Authors' Addresses

   Ben Campbell (editor)
   Estacado Systems
   17210 Campbell Road
   Suite 250
   Dallas, TX  75252
   USA

   EMail: ben@estacado.net


   Rohan Mahy (editor)
   Plantronics
   345 Encincal Street
   Santa Cruz, CA  95060
   USA

   EMail: rohan@ekabal.com


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

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





















Campbell, et al.            Standards Track                    [Page 62]
^L
RFC 4975                          MSRP                    September 2007


Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.












Campbell, et al.            Standards Track                    [Page 63]
^L