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RFC 4975


The Message Session Relay Protocol (MSRP)

Part 3 of 4, p. 36 to 55
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9.  Formal Syntax

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

   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 )

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   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
   BACKSLASH = "\"
   UPALPHA  = %x41-5A

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   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)

                 / %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.

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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.

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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 and the answerer is

           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

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   1.  Alice constructs a local URI of
       msrp://;tcp .

       Alice->Bob (SIP): INVITE

       o=alice 2890844557 2890844559 IN IP4
       s= -
       c=IN IP4
       t=0 0
       m=message 7777 TCP/MSRP *

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

       Bob->Alice (SIP): 200 OK

       o=bob 2890844612 2890844616 IN IP4
       s= -
       c=IN IP4
       t=0 0
       m=message 8888 TCP/MSRP *

   3.  Alice->Bob (SIP): ACK

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

       MSRP d93kswow SEND
       To-Path: msrp://;tcp
       From-Path: msrp://;tcp
       Message-ID: 12339sdqwer
       Byte-Range: 1-16/16
       Content-Type: text/plain

       Hi, I'm Alice!

   5.  Bob->Alice (MSRP):

       MSRP d93kswow 200 OK
       To-Path: msrp://;tcp
       From-Path: msrp://;tcp

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   6.  Bob->Alice (MSRP):

       MSRP dkei38sd SEND
       To-Path: msrp://;tcp
       From-Path: msrp://;tcp
       Message-ID: 456s9wlk3
       Byte-Range: 1-21/21
       Content-Type: text/plain

       Hi, Alice!  I'm Bob!

   7.  Alice->Bob (MSRP):

       MSRP dkei38sd 200 OK
       To-Path: msrp://;tcp
       From-Path: msrp://;tcp

   8.  Alice->Bob (SIP): BYE

       Alice invalidates local session state.

   9.  Bob invalidates local state for the session.

       Bob->Alice (SIP): 200 OK

11.2.  Message with XHTML Content

   MSRP dsdfoe38sd SEND
   To-Path: msrp://;tcp
   From-Path: msrp://;tcp
   Message-ID: 456so39s
   Byte-Range: 1-374/374
   Content-Type: application/xhtml+xml

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   <?xml version="1.0" encoding="UTF-8"?>
   <!DOCTYPE html
   PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
   <html xmlns="" xml:lang="en" lang="en">
       <title>FY2005 Results</title>
      <p>See the results at <a

                   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://;tcp
      From-Path: msrp://;tcp
      Message-ID: 12339sdqwer
      Byte-Range: 1-137/148
      Content-Type: message/cpim

      To: Bob <>
      From: Alice <>
      DateTime: 2006-05-15T15:02:31-03:00
      Content-Type: text/plain


                            Figure 14: First Chunk

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   Alice sends the second and last chunk.

      MSRP op2nc9a SEND
      To-Path: msrp://;tcp
      From-Path: msrp://;tcp
      Message-ID: 12339sdqwer
      Byte-Range: 138-148/148
      Content-Type: message/cpim


                           Figure 15: Second Chunk

11.5.  System Message

   Sysadmin->Alice (MSRP):

   MSRP d93kswow SEND
   To-Path: msrp://;tcp
   From-Path: msrp://;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

11.6.  Positive Report

   Alice->Bob (MSRP):

   MSRP d93kswow SEND
   To-Path: msrp://;tcp
   From-Path: msrp://;tcp
   Message-ID: 12339sdqwer
   Byte-Range: 1-106/106
   Success-Report: yes
   Failure-Report: no
   Content-Type: text/html

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   <p>Here is that important link...
   <a href="">foobar</a>

                      Figure 16: Initial SEND Request

   Bob->Alice (MSRP):

   MSRP dkei38sd REPORT
   To-Path: msrp://;tcp
   From-Path: msrp://;tcp
   Message-ID: 12339sdqwer
   Byte-Range: 1-106/106
   Status: 000 200 OK

                         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).

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   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  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.

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    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---|            |            |           |           |
      |<================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

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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

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   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

   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

   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

   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.

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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

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   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

   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

   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.

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   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

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   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
   m=message 7654 TCP/TLS/MSRP *
   a=fingerprint:SHA-1 \

                 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

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   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.

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   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.

(page 55 continued on part 4)

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