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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 RFC 4975

SIMPLE Working Group                                         B. Campbell
Internet-Draft                                              J. Rosenberg
Expires: November 20, 2003                                     R. Sparks
                                                             dynamicsoft
                                                              P. Kyzivat
                                                           Cisco Systems
                                                            May 22, 2003


                   Instant Message Sessions in SIMPLE
                 draft-ietf-simple-message-sessions-00

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   Internet-Drafts are draft documents valid for a maximum of six months
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   The list of current Internet-Drafts can be accessed at http://
   www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on November 20, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

   The SIP MESSAGE method is used to send instant messages, where each
   message is independent of any other message. This is often called
   pager-mode messaging, due to the fact that this model is similar to
   that of most two-way pager devices. Another model is called
   session-mode. In session-mode, the instant messages are part of a
   media session that provides ordering, a security context, and other
   functions. This media session is established using a SIP INVITE, just
   as an audio or video session would be established.




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   This document describes the Message Session Relay Protocol (MSRP), a
   mechanism for transmitting session-mode messages with minimalist
   relay support. Additionally, this document describes using the SDP
   offer/answer model to initiate such sessions.

Table of Contents

   1.     Introduction . . . . . . . . . . . . . . . . . . . . . . .   4
   2.     Motivation for Session-mode Messaging  . . . . . . . . . .   4
   3.     Scope of this Document . . . . . . . . . . . . . . . . . .   5
   4.     Protocol Overview  . . . . . . . . . . . . . . . . . . . .   5
   5.     SDP Offer-Answer Exchanges for MSRP Sessions.  . . . . . .   8
   5.1    Use of the SDP M-line  . . . . . . . . . . . . . . . . . .   8
   5.2    The Direction Attribute  . . . . . . . . . . . . . . . . .   9
   5.3    URL Negotiations . . . . . . . . . . . . . . . . . . . . .  10
   5.4    Example SDP Exchange . . . . . . . . . . . . . . . . . . .  11
   6.     The Message Session Relay Protocol . . . . . . . . . . . .  11
   6.1    MSRP URLs  . . . . . . . . . . . . . . . . . . . . . . . .  11
   6.2    MSRP URL Comparison  . . . . . . . . . . . . . . . . . . .  12
   6.3    Resolving MSRP Host Device . . . . . . . . . . . . . . . .  13
   6.3.1  The msrps URL Scheme . . . . . . . . . . . . . . . . . . .  14
   6.4    MSRP messages  . . . . . . . . . . . . . . . . . . . . . .  14
   6.5    MSRP Transactions  . . . . . . . . . . . . . . . . . . . .  15
   6.6    MSRP Sessions  . . . . . . . . . . . . . . . . . . . . . .  16
   6.6.1  Initiating an MSRP session . . . . . . . . . . . . . . . .  16
   6.6.2  Handling VISIT requests  . . . . . . . . . . . . . . . . .  19
   6.6.3  Sending Instant Messages on a Session  . . . . . . . . . .  20
   6.6.4  Managing Session State and Connections . . . . . . . . . .  21
   6.7    MSRP Relays  . . . . . . . . . . . . . . . . . . . . . . .  22
   6.7.1  Establishing Session State at a Relay  . . . . . . . . . .  22
   6.7.2  Removing Session State from a relay  . . . . . . . . . . .  24
   6.7.3  Sending IMs across an MSRP relay . . . . . . . . . . . . .  24
   6.7.4  Relay Pairs  . . . . . . . . . . . . . . . . . . . . . . .  24
   6.8    Session State Expiration . . . . . . . . . . . . . . . . .  26
   6.9    Digest Authentication  . . . . . . . . . . . . . . . . . .  26
   6.9.1  The MD5 Algorithm  . . . . . . . . . . . . . . . . . . . .  27
   6.10   Method Descriptions  . . . . . . . . . . . . . . . . . . .  28
   6.10.1 BIND . . . . . . . . . . . . . . . . . . . . . . . . . . .  28
   6.10.2 SEND . . . . . . . . . . . . . . . . . . . . . . . . . . .  28
   6.10.3 VISIT  . . . . . . . . . . . . . . . . . . . . . . . . . .  29
   6.11   Response Code Descriptions . . . . . . . . . . . . . . . .  29
   6.11.1 200  . . . . . . . . . . . . . . . . . . . . . . . . . . .  29
   6.11.2 400  . . . . . . . . . . . . . . . . . . . . . . . . . . .  29
   6.11.3 401  . . . . . . . . . . . . . . . . . . . . . . . . . . .  29
   6.11.4 403  . . . . . . . . . . . . . . . . . . . . . . . . . . .  30
   6.11.5 415  . . . . . . . . . . . . . . . . . . . . . . . . . . .  30
   6.11.6 481  . . . . . . . . . . . . . . . . . . . . . . . . . . .  30
   6.11.7 500  . . . . . . . . . . . . . . . . . . . . . . . . . . .  30



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   6.11.8 506  . . . . . . . . . . . . . . . . . . . . . . . . . . .  30
   6.12   Header Field Descriptions  . . . . . . . . . . . . . . . .  30
   6.12.1 TR-ID  . . . . . . . . . . . . . . . . . . . . . . . . . .  30
   6.12.2 Exp  . . . . . . . . . . . . . . . . . . . . . . . . . . .  30
   6.12.3 CAuth  . . . . . . . . . . . . . . . . . . . . . . . . . .  31
   6.12.4 SChal  . . . . . . . . . . . . . . . . . . . . . . . . . .  32
   6.12.5 Content-Type . . . . . . . . . . . . . . . . . . . . . . .  32
   6.12.6 S-URL  . . . . . . . . . . . . . . . . . . . . . . . . . .  32
   7.     Examples . . . . . . . . . . . . . . . . . . . . . . . . .  32
   7.1    No Relay . . . . . . . . . . . . . . . . . . . . . . . . .  33
   7.2    Single Relay . . . . . . . . . . . . . . . . . . . . . . .  34
   7.3    Two Relays . . . . . . . . . . . . . . . . . . . . . . . .  37
   8.     IANA Considerations  . . . . . . . . . . . . . . . . . . .  40
   9.     Security Considerations  . . . . . . . . . . . . . . . . .  40
   9.1    The MSRPS Scheme . . . . . . . . . . . . . . . . . . . . .  40
   9.2    Sensitivity of the Session URL . . . . . . . . . . . . . .  41
   9.3    End to End Protection of IMs . . . . . . . . . . . . . . .  41
   9.4    CPIM compatibility . . . . . . . . . . . . . . . . . . . .  42
   10.    Changes introduced in
          draft-ietf-simple-message-sessions-00  . . . . . . . . . .  42
   11.    Changes introduced in
          draft-campbell-simple-im-sessions-01 . . . . . . . . . . .  43
   12.    Contributors . . . . . . . . . . . . . . . . . . . . . . .  43
          Normative References . . . . . . . . . . . . . . . . . . .  43
          Informational References . . . . . . . . . . . . . . . . .  44
          Authors' Addresses . . . . . . . . . . . . . . . . . . . .  45
          Intellectual Property and Copyright Statements . . . . . .  46
























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1. Introduction

   The MESSAGE [9] extension to SIP [2] allows SIP to be used to
   transmit instant messages. Instant messages sent using the MESSAGE
   method are normally independent of each other. This approach is often
   called pager-mode messaging, since it follows a model similar to that
   used by many two-way pager devices. Pager-mode messaging makes sense
   for instant message exchanges where a small number of messages occur.

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

   One of the primary purposes of SIP and SDP (Section 5) is the
   management of media sessions. Session-mode messaging can be thought
   of as a media session like any other.  This document describes the
   motivations for session-mode messaging, the Message Session Relay
   Protocol, and the use of the SDP offer/answer mechanism for managing
   MSRP session.

2. Motivation for Session-mode Messaging

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

   Each pager mode message involves a complete SIP transaction, that is,
   a request and a response. Any pager-mode message exchange that
   involves more than 2 or 3 MESSAGE requests will generate more SIP
   requests than a minimal session initiation sequence. Since MESSAGE is
   normally used outside of a SIP dialog, these requests will typically
   traverse the entire proxy network between the endpoints.

   Due to network congestion concerns, the MESSAGE method has
   significant limitations in message size, a prohibition against
   overlapping requests, etc. Much of this has been required because of



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   perceived limitations in the congestion-avoidance features of SIP
   itself. Work is in progress to mitigate these concerns.

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

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

   Finally, SIP devices can treat message sessions like any other media
   sessions. Any SIP feature that can be applied to other sorts of media
   sessions can equally apply to message sessions. For example,
   conferencing [11], third party call control [12], call transfer [13],
   QoS integration [14], and privacy [15] can all be applied to message
   sessions.

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

3. Scope of this Document

   This document describes the use of MSRP between endpoints, or via one
   or two relays, where endpoints have advance knowledge of the relays.
   It does not provide a mechanism for endpoints to determine whether a
   relay is needed, or for endpoints to discover the presence of relays.

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

4. Protocol Overview




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   The Message Session Relay Protocol (MSRP) provides a mechanism for
   transporting session-mode messages between endpoints. MSRP also
   contains primitives to allow the use of one or two relay devices.
   MSRP uses connection oriented, reliable network transport protocols
   only. It is intrinsically NAT and firewall friendly, as it allows
   participants to positively associate message sessions with specific
   connections, and does not depend upon connection source address,
   which may be obscured by NATs.

   MSRP uses the following primitives:

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

   VISIT: Used by an endpoint to establish a session association to the
      opposite endpoint, or to a relay that was selected by the opposite
      endpoint.

   BIND: Used by an endpoint to establish a session at a relay, and
      allow the opposite endpoint to visit that relay.

   The simplest use case for MSRP is a session that goes directly
   between endpoints, with no intermediaries involved. Assume A is an
   endpoint that wishes to establish a message session, and B is the
   endpoint invited by A. A invites B to participate in a message
   session by sending a URL that represents the session. This URL is
   temporary, and must not duplicate the URL used for any other active
   sessions.

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

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

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

   A->B (SDP): offer (msrp://A/123)

   B->A (MSRP): VISIT (msrp://A/123)





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   A->B (MSRP): 200 OK

   B->A (SDP): answer(msrp://A/123)

   A->B (MSRP): SEND

   B->A (MSRP): 200 OK

   B->A (MSRP): SEND

   A->B (MSRP): 200 OK

   The state associated with the session will expire over time, based on
   an expiration time specified in the VISIT request. If the lifetime of
   the session is to exceed that expiration time, the visitor must
   update the expiration with a new VISIT request prior to expiration.

   A slightly more complicated case involves a single relay, known about
   in advance by one of the parties. The endpoint that has the
   preexisting relationship with the relay uses the BIND method to
   establish session state in the relay. The relay returns a temporary
   URL, that identifies the session. For endpoints A and B, and relay R,
   the flow would look like the following:

   A->R: MSRP: BIND(msrp://r)

   R->A: MSRP: 200 OK (msrp://r/4uye)

   A->B (SDP): offer (msrp://r/4uye)

   B->R (MSRP): VISIT (msrp://r/4uye)

   R->B (MSRP): 200 OK

   B->A (SDP): answer(msrp://r/4uye)

   A->R (MSRP): SEND

   R->B (MSRP): SEND

   B->R (MSRP): 200 OK

   R->A (MSRP): 200 OK

   B->R (MSRP): SEND






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   R->A (MSRP): SEND

   A->R (MSRP): 200 OK

   R->B (MSRP): 200 OK

   The BIND request contains an expiration time much the same as in
   VISIT. If the life of a relay-hosted session is to exceed the
   expiration value in the BIND request, the host endpoint will refresh
   the expiration time with a new BIND request prior to expiration.
   Additionally, when tearing down a session, the host endpoint
   invalidates the session state by issuing a BIND request with an
   expiration value of zero.

5. SDP Offer-Answer Exchanges for MSRP Sessions.

   MSRP sessions will typically be initiated using the Session
   Description Protocol (SDP) [1] offer-answer mechanism, carried in SIP
   [2] or any other protocol supporting it. MSRP borrows the idea of the
   direction attributes from COMEDIA [17], but does not depend on that
   specification.

5.1 Use of the SDP M-line

   The SDP m-line takes the following form:

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

   For non-RTP media sessions, The media field specifies the top level
   MIME media type for the session. For MSRP sessions, the media field
   MUST have the value of "message". The proto field MUST designate the
   message session mechanism and transport protocol, separated by a "/"
   character. For MSRP, left part of this value MUST be "msrp". For MSRP
   over TCP, the right part of this field MUST take the value "tcp". For
   MSRP over other transport protocols, the field value MUST be defined
   by the specification for that protocol binding. The format list MUST
   indicate the MIME content-types that the endpoint is willing to
   accept in the payload of SEND requests. If any of the allowed types
   are compound in nature, that is, they allow one or more arbitrary
   MIME body parts to be embedded within them, then the format list MUST
   include the content-types allowed for the embedded parts. If the
   final entry in the format list is a "*", this indicates that the
   endpoint is may be willing to receive other types as well, but the
   types listed explicitly are preferred. The format list in the SDP
   answer MUST be the same as, or a subset of, the list provided in the
   offer.





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

   The port field in the M-line is not used to determine the port to
   which to connect. Rather, the actual port is determined by the
   contents of the session URL. (Section 6.1).

   The following example illustrates an m-line for a CPIM message
   session, where the endpoint is willing to accept payloads of plain
   text or HTML, which may appear at the top level of the payload, or
   may be embedded inside a message/cpim body part.

      m=message 49232 msrp/tcp message/cpim text/plain text/html


5.2 The Direction Attribute

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

   The endpoint that accepts the connection, or has a relay accept the
   connection on its behalf, is said to "host" the session, and is known
   as the hosting endpoint. The endpoint that initiates the connection
   is said to "visit" the session, and is known as the visiting
   endpoint.

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

               direction = direction-label ":" role
                   direction-label = "direction"
                   role = active / passive / both
                   active = "active"
                   passive = "passive"
                   both = "both"

   The values for the role field are as follows:






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   passive The endpoint wishes to host the session

   active The endpoint wishes the peer to host the session.

   both The endpoint is willing to act as either host or visitor.

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

   The SDP answer also MUST contain a direction attribute, but its value
   choices are limited based on the value in the offer. If the offer
   contained "active", then the answerer MUST either select "passive" or
   reject the offer. Likewise, if the offer contained  "passive", then
   the answerer MUST select"active" or reject the offer. If the offer
   contained "both", the answerer SHOULD select "active", but MAY select
   "passive" if local policy requires it to act as host.

5.3 URL Negotiations

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

   a=session:<MSRP_URL>

   where <MSRP_URL> is an MSRP or MSRPS URL as defined in Section 6.1.

   The visitor will use the session URL established by the host both to
   resolve the host address and port, and to identify the session when
   connecting. For MSRP sessions, the address field in the C-line and
   the port field in the M-line are not relevant, and MUST be ignored.

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

           c=IN IP4 useless.host.name
           m=message 7394 msrp/tcp text/plain
           a=direction:both
           a=session:msrp://example.com:7394/2s93i




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

5.4 Example SDP Exchange

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

     c=IN IP4 alice.example.com
     m=message 7394 msrp/tcp message/cpim text/plain text/html
     a=direction:both
     a=session:msrp://alice.example.com:7394/2s93i9

   Endpoint B chooses to participate in the role of visitor, opens a TCP
   connection to alice.example.com:7394, and successfully performs a
   VISIT transaction passing the URL of msrp://alice.example.com:7394/
   2s93i9;. B indicates that it has accomplished this by answering with:

     c=IN IP4 dontlookhere
     m=message 7394 msrp/tcp message/cpim text/plain
     a=direction:active

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

6. The Message Session Relay Protocol

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

   MSRP messages MUST be sent over reliable, congestion-controlled,
   connection-oriented transport protocols, such as TCP.

6.1 MSRP URLs

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

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

      resource = 1*unreserved



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

   An MSRP URL server part identifies the hosting device of an MSRP
   session. There is no default port for MSRP URLs. If the server part
   contains a numeric IP address, it MUST also contain a port. The
   resource part identifies a particular session at that host device.
   The absence of the resource part indicates a reference to an MSRP
   host device, but does not specifically refer to a particular session
   resource.

      Open Issue: Do we need a default port? Cullen points out it would
      at least be useful for firewall configuration.

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

   The following is an example of a typical MSRP URL:

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


6.2 MSRP URL Comparison

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

      The host part is compared as case insensitive.

      If the port exists explicitly in either URL, then it must match
      exactly. Since there is no default port for MSRP, a URL with an
      explicit port is never equivalent to another with no port
      specified.

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

      Userinfo parts are not considered for URL comparison.

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



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6.3 Resolving MSRP Host Device

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

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

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

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

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

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

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

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

   5.  If the connection attempt fails, the device SHOULD attempt to
       connect to the addresses returned in any additional A or AAAA
       records, in the order the records were presented. If all of these
       fail, the device SHOULD attempt to use any additional SRV records
       that may have been returned, following the normal rules for SRV
       record selection.

      Open Issue: We need to carefully consider the rules about A RR
      selection. I am sure there are others who understand this much
      better than I do. Ted pointed us to RFC1794, which if I understand
      correctly indicates that some systems may attempt to load balance
      by controlling the order in which A RRs are presented. Attempts to
      randomize selection by the client could distort any such control.

   Note that in most cases, the transport protocol will be determined
   separately from the resolution process. For example, if the MSRP URL
   was communicated in an SDP offer or answer, the SDP M-line will
   contain the transport protocol. When an MSRP URL is communicated
   outside of SDP, the protocol SHOULD also be communicated. For



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   example, a client may be configured to use a particular relay that is
   referenced with an MSRP URL. The client MUST also be told what
   protocol to use. If a device needs to resolve an MSRP URL and does
   not know the protocol, it SHOULD assume TCP.

      Open Issue: Do we need to do an NAPTR query to determine the
      protocol?


6.3.1 The msrps URL Scheme

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

6.4 MSRP messages

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




























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       msrp-message = request-start/response-start *(header CRLF)
                                  [CRLF body]
       request-start = "MSRP" SP length SP  Method CRLF
       response-start= "MSRP" SP length SP Status-Code SP
                                Reason CRLF
       length = 1*DIGIT  ; the length of the message, exclusive of the start line.
       Method = SEND / BIND / VISIT
       header = Client-Authenticate / Server-Challenge /
                 Transaction-ID / Session-URL/ Content-Type / Expires
       Status-Code = 200    ;Success
                   / 400    ;Bad Request
                   / 401    ;Authentication Required
                   / 403    ;Forbidden
                   / 415    ;Unsupported Content Type
                   / 481    ;No session
                   / 500    ;Cannot Deliver
                   / 506    ;duplicate session
       Reason = token ; Human readable text describing status
       Client-Authenticate = "CAuth" credentials
       Server-Challenge = "SChal" ":" challenge
       Transaction-ID = "Tr-ID" ":" token
       Content-Type = "Content-Type" ":" quoted-string
       Session-URL = "S-URL" ":" msrp_url
       Expires = "Exp"":" delta-seconds
       delta-seconds= 1*DIGIT ; Integer number of seconds


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

6.5 MSRP Transactions

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

   BIND is always hop by hop. VISIT transactions are usually hop-by-hop,
   but may be relayed in situations where the visiting endpoint uses a
   relay.  However, SEND transactions are end to end, meaning that under
   normal circumstances the response is sent by the peer endpoint, even
   if there are intervening relays.

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



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   above. In particular, TR-ID values are not required to follow any
   sequence.

   MSRP Transactions complete when a response is received, or after a
   timeout interval expires with no response. Endpoints MUST treat such
   timeouts in exactly the same way they would treat a 500 response. The
   size of the timeout interval is a matter of local policy.

6.6 MSRP Sessions

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

6.6.1 Initiating an MSRP session

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

   The offerer SHOULD volunteer to act as the hosting endpoint if
   allowed by policy and network topology. An endpoint is said to host a
   session if one of two conditions are true. The host either directly
   listens for a connection from the peer endpoint, and maintains
   session state itself, or it uses a BIND request to initialize session
   state at a relay that will listen for a connection from the peer. The
   peer that is not the host is designated as the visitor. The offerer
   MAY request the answerer to act as host if it is prevented from
   accepting connections by network topology or policy, and is not able
   to bind to a relay to act on its behalf.

   If the offerer wishes to host the session directly, that is without
   using a relay, it MUST perform the following steps:

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

   2.  Listen for a connection from the peer.

   3.  Construct an SDP offer as described in Section 5, including the
       list of allowed IM payload formats in the format list. The
       offerer maps the session URL to the session attribute, as



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       described in Section 5.3.

   4.  Insert a direction attribute. This value SHOULD be "both",
       indicating that the offerer will allow the answerer to override
       the offerer's decision to host. The value MAY be "passive" if the
       offerer is prevented from allowing the answerer override this
       choice.

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

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

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

   2.  Insert a direction attribute with a value of "active".

   3.  Send the offer using normal processing for the signaling
       protocol.

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

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

   1.  Determine the host address and port from the session URL,
       following the procedures in section Section 6.1

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

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

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

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

       3.  An Exp header field containing the expiration time for the
           VISIT request.



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       4.  A size field containing size of the message subsequent to the
           start-line.

   4.  Send the request and wait for a response

   5.  If the transaction succeeds, set the actual expiration time to
       the value in the Exp header field in the response, and send a SDP
       answer via the signaling protocol, according to the following
       rules:

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

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

       3.  A direction attribute containing the value "active".

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

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

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

   2.  Listen for a connection from the peer.

   3.  Construct an SDP answer as described in Section 5, mapping the
       new session URL to the session attribute, and inserting a
       direction attribute with the value of "passive".

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

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



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   session.

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

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

   2.  Determine the host address and port from the session URL of the
       answer, following the procedures in section Section 6.1

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

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

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

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

       3.  An Exp header field containing the expiration time for the
           VISIT request.

       4.  A size field containing size of the message subsequent to the
           start-line.

   5.  Send the request and wait for a response

   6.  If the transaction succeeds, set the actual expiration time to
       the value in the Exp header field in the response, and
       acknowledge the answer via the signaling protocol. If either the
       connection attempt or the VISIT transaction fail, acknowledge the
       answer, then initiate the tear-down of the session using the
       signaling protocol.


6.6.2 Handling VISIT requests

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

   1.  Check if state exists for a session with a URL that matches the
       S-URL of the VISIT request. If so, and if no visitor connection
       has been associated with the session, determine the expiration
       time according to the procedures in Section 6.8, then return a
       200 response, and save state designating the connection on which



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       the request was received as the visitor leg of the session.

   2.  If the session exists, and the visitor connection has already
       been established, and the request arrived on the existing visitor
       connection,  treat the request as a refresh, as described in
       Section 6.8. If the request arrived on a different connection,
       return a 506 response and do not change session state in any way.

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


6.6.3 Sending Instant Messages on a Session

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

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

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

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

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

   5.  If a 5xx response code is received, the transaction failed, but
       may possibly be successful if retried. The endpoint MAY retry the
       request as a new transaction, that is, with a new TR-ID value. If
       the endpoint receives 5xx responses more than some threshold
       number of times in a row, it SHOULD assume the session has
       failed, and initiate tear-down via the signaling protocol. The
       threshold value is a matter of local policy.

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

   7.  If any other response code is received, the endpoint SHOULD
       assume the session has failed, and initiate tear-down.



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

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

   2.  If it does, return a 200 response and render the message to the
       user. The method of rendering is a matter of local policy.

   3.  If it does not understand the media type, return a 415 response.


6.6.3.1 Ending a Session

   When either endpoint in an MSRP session wishes to end the session, it
   first signals its intent using the normal processing for the
   signaling protocol. For example, in SIP, it would send a BYE request
   to the peer. After agreeing to end the session, the host endpoint
   MUST release any resources acquired as part of the session. The
   process for this differs depending on whether the session is hosted
   directly by the host, or an a relay.

   The host MUST destroy local state for the session. This involves
   completely removing the state entry for this session and invalidating
   session URL. If the host is using an MSRP relay, it MUST send a BIND
   containing an expires value of zero. This request MUST be sent host
   connection established by the original BIND request. This BIND
   request MUST include the session URL in the S-URL header field.

      Since these host actions completely destroy the session state at
      the hosting device, the visitor is not required to take further
      action beyond cleaning up any local state. If for some reason the
      host fails to destroy session state, the state will be invalidated
      anyway when either of the original BIND or VISIT requests expire.


6.6.4 Managing Session State and Connections

   A MSRP session is represented by state at the host device. As mention
   previously, session state is identified by an MSRP URL. An active
   session also has two associated network connections. The connection
   hosting device and the host endpoint is known as the host connection.
   The connection with the visiting endpoint is the visiting connection.
   Note that when the session state is hosted directly by an endpoint,
   the host connection may not involve a physical network connection;
   rather it is a logical connection the device maintains with itself.

   When session state is destroyed for any reason, the hosting device



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   SHOULD drop the connection(s).

   If a connection fails for any reason, the session hosting device MUST
   invalidate the session state. This is true regardless of whether the
   dropped connection is the host or visiting connection. Once a
   connection is dropped, the associated session state MUST NOT be
   reused. If the endpoints wish to continue to communicate after a
   connection failure, they must initiate a new session. An endpoint
   detecting a connection failure SHOULD attempt to tear down the
   session using the rules of the signaling protocol.

      It would be nice to allow sessions to be recovered after a
      connection failure, perhaps by allowing the opposite endpoint to
      reconnect, and send a new VISIT or BIND request. However, this
      approach creates a race condition between the time that the
      hosting device notices the failed connection, and the time that
      the endpoint tries to recover the session. If the endpoint
      attempts to reconnect prior to the hosting device noticing the
      failure, the hosting device will interpret the recovery attempt as
      a conflict. The only way around this would be to force the hosting
      device to do a liveness check on the original connection, which
      would create a lot of complexity and overhead that do not seem to
      be worth the trouble.


6.7 MSRP Relays

6.7.1 Establishing Session State at a Relay

   An endpoint that wishes to host a MSRP session MAY do so by
   initiating session state at a MSRP relay, rather than hosting
   directly. An endpoint may wish to do this because network topology or
   local policy prevents a peer from connecting directly to the
   endpoint. The use of a relay should not be the default case, that is,
   a hosting endpoint that is not prevented from doing so by topology or
   policy SHOULD host the session directly. In order to use a relay, an
   MSRP endpoint MUST have knowledge of that relay's existence and
   location..

   We previously mentioned how an endpoint wishing to host a MSRP
   session constructs session URL. When using a relay, the endpoint
   delegates that responsibility to the relay.

   To establish session state at a relay, the endpoint MUST perform the
   following steps:

   1.  Open a network connection to the relay at the relays address and
       the well-known port for MSRP relays, or at another port if so



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       configured.

   2.  Construct a BIND request with a S-URL that refers to the relay.

   3.  Set the Expire header field to a desired value.

   4.  Send the BIND request on the connection.

   5.  Respond to any authentication request from the relay.

   6.  If the response has a 2xx status code, use the URL in the S-URL
       header field as the session URL. The endpoint uses this URL in
       exactly the same manner as it had constructed it itself.
       Additionally, accept the expires value in the response as session
       expiration time.

   A MSRP relay listens for connections to its well-known port at all
   times. When it receives a BIND request, it SHOULD authenticate the
   request, either using digest-authentication, TLS authentication, or
   some other authentication mechanism. If authentication succeeds, the
   relay performs the following steps:

   1.  Verify the client is authorized to BIND to this relay. If not,
       return a 403 response and make no state change.

   2.  If the client is authorized, construct a session MSRP URL. The
       URL MUST resolve to the relay. It SHOULD be temporary, and hard
       to guess. It MUST not duplicate URL used in any active sessions
       hosted by the relay. If the relay wishes the visiting endpoint to
       connect over a point other than the MSRP relay well-know port, it
       MUST explicitly add the port number to visitor URL.

   3.  Establish the expiration time for the session according to
       section Section 6.8.

   4.  Create state for the session. The relay MUST associate the
       connection on which the BIND request arrived as the host
       connection for the session.

   5.  Return a 200 response, with the session URL in the S-URL header
       field, and the session expiration time in the Exp header field..

   When an MSRP relay receives a VISIT request, it MUST perform the
   following steps:

   1.  Check the S-URL header field value to see it matches the URL for
       an existing session state entry.




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   2.  If not, return a 481 response and make no state changes

   3.  If it matches, but another connection has already been associated
       with the session URL, return a 506 response and make no state
       changes. If the session has been previously associated with this
       connection, treate the request as a refresh.

   4.  If it matches, and no visiting connection has been previously
       associated with the session, then the VISIT succeeds. The relay
       assigns the connection on which it received the VISIT request as
       the visiting connection for the session, and returns a 200
       response. The visit expiration time is established as described
       in Section 6.8 and returned in the response.


6.7.2 Removing Session State from a relay

   An MSRP relay SHOULD remove state for a session when any of the
   following conditions occur:

   o  The expiration time for either the BIND or VISIT is reached
      without a respective refresh request.

   o  The host sends a BIND refresh request matching with an expiration
      value of zero.

   o  Either the host or visitor network connection fails for any
      reason.


6.7.3 Sending IMs across an MSRP relay

   Once a session is established at a relay, the host and visitor may
   exchange IMs by sending SEND requests. Under normal circumstances,
   the relay does not respond to SEND requests in any way. Rather, the
   relay MUST  forward the request to the peer connection unchanged.
   Likewise, if the relay receives a response it MUST forward the
   request unchanged on the peer connection.

   If a SEND request arrives on a connection that is not associated with
   a session, the relay MUST return a 481 response.

6.7.4 Relay Pairs

   In rare circumstances, two relays may be required in a session. For
   example, two endpoints may exist in separate administrative domains,
   where each domain's policy insist that all sessions must cross that
   domain's relay. A relay operating on behalf of the visiting endpoint



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   is known as a visiting relay. An MSRP relay MAY be capable of acting
   as a visiting relay.

   In a two relay scenario, the visitor connects to a relay operating on
   its behalf, rather than connecting directly to the hosting device.
   The visitor sends a VISIT request as it would if it had connected
   directly to the hosting device. The visiting relay then connect to
   the hosting device and performs a VISIT request on behalf of the
   visitor.

   When a relay that is capable of acting as a visiting relay receives a
   VISIT request, it MUST check to see if the S-URL of the request
   matches a domain that the relay hosts. If the URL matches, then the
   visitor is not requesting the relay act as a visiting relay, and it
   SHOULD operate normally. If the URL does not match, then the relay
   SHOULD perform the following steps:

   1.  The relay SHOULD authenticate the VISIT request, using digest
       authentication or some other mechanism.

   2.  Determine that the visiting endpoint is authorized to use this
       device as a visiting relay. If not, return a 403 response and
       drop the connection.

   3.  Attempt to open a connection to the hosting device, determining
       the address and port from the S-URL exactly as if it were a
       visiting endpoint connecting directly. If this connection is
       successful, continue with the remaining steps. Otherwise, return
       a 500 response.

   4.  Create local state to associate the connection to the host device
       with the connection to the visiting device.

   5.  Relay the VISIT request unchanged to the hosting device.

   6.  Relay the response to the VISIT request unchanged to the visiting
       endpoint. If the response is a 200, set the expiration time for
       the local session state to the value in the Exp header in the
       response.

   7.  Relay all subsequent arriving on one of the associated
       connections to the peer connection.

   The preceding steps result in local session state that expires based
   on the expiration time negotiated between the visiting endpoint and
   the hosting device. The visiting endpoint will send VISIT requests on
   the same connection from time to time to refresh the session state
   expiration time. A visiting relay MUST refresh the local expiration



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   time based on the Exp header field value in a successful response to
   such a VISIT request. If the local expiration time passes without a
   refresh, the visiting relay SHOULD invalidate the session state and
   SHOULD drop the associated connections.

   If either associated connection fails for any reason, the visiting
   relay SHOULD invalidate the session state, and SHOULD drop the peer
   connection.

6.8 Session State Expiration

   State associated with MSRP sessions, either at the host endpoint or a
   host relay, is soft-state; that is, it expires over time unless
   refreshed. The expiration time is determined by the Expires header
   field in VISIT and BIND requests. All VISIT and BIND requests MUST
   contain an Expires header field. This field is defined as an integer
   number of seconds from the time that the request is received.

   When a hosting device (endpoint or relay) creates session state due
   to a successful VISIT request, it SHOULD accept the Expires value
   from the request, although it MAY choose a smaller value. It MUST NOT
   choose a larger value. The device MUST communicate the actual chosen
   value back to the opposite endpoint by placing the value in an
   expires header field in the response.

   Likewise, when a relay creates session state due to a successful BIND
   request, it SHOULD accept the expires value from the request,
   although it MAY choose a smaller value. It MUST NOT choose a larger
   value. The device MUST communicate the actual chosen value back to
   the opposite endpoint by placing the value in an Expires header field
   in the response.

   A visiting relay does not normally respond to a VISIT request.
   Rather, it relays the request to the hosting device, and relays the
   resulting response back to the visiting endpoint. This prevents it
   from being able to negotiate the expiration time in the same way as
   hosting devices. Therefore, a visiting relay MUST determine session
   expiration time from the Exp header field in any 200 response
   returned by the hosting device.

6.9 Digest Authentication

   MSRP relays may use the digest authentication scheme to authenticate
   users. MSRP digest authentication is a simplified version of HTTP
   digest authentication [18], but this specification does not
   normatively depend on that document. MSRP digest authentication does
   not support the concept of a protection domain, nor does it support
   integrity protection. Since a user of a relay is expected to have



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   credentials for that particular relay, it does not support the realm
   concept. Finally, since digest authentication is only expected for
   the initial BIND or VISIT request, MSRP does not support HTTP digest
   optimizations such as MD5-sess and preemptive credential loading by
   the client.

   Typically, a hosting user that uses a relay will have a preexisting
   relationship with that relay. This relationship SHOULD include
   authentication credentials. An MSRP relay SHOULD authenticate initial
   BIND requests.

   It is less likely that the visiting user will have an account at the
   hosting relay, so in many cases the authentication of VISIT requests
   is not useful. However a relay MAY authenticate initial VISIT
   requests. A visiting relay SHOULD authenticate initial VISIT
   requests, as it is much more likely to share credentials with the
   visiting user.

      There has been some discussion that a hosting relay SHOULD also
      authenticate VISIT requests. However, it will be very common for
      visiting users to have no preexisting relationship with the host
      relay. Using authentication here would require the host endpoint
      to send temporary credentials in the SDP exchange, perhaps as part
      of the session URL. However, these temporary credentials would
      necessarily be transferred via the same channels as the session
      URL itself. If the credentials are sufficiently protected in
      transfer, then so is the session URL. Further, since the session
      URL is intended for a one time use, and is expected to be hard to
      guess, that URL itself may be sufficient for this purpose.

   MSRP relays MUST NOT request authentication for any method other than
   BIND and VISIT.

   If a relay wishes to authenticate a request using digest
   authentication, it MAY challenge the request by responding with a
   401 response, which MUST include a SChal header field.

   If an endpoint wishes to respond to a digest authentication challenge
   received in a 401 response, it MAY do so by sending a new VISIT or
   BIND request, identical to the previous request, but with a CAuth
   header field containing the response to the challenge.

6.9.1 The MD5 Algorithm

   The only digest authentication algorithm defined in this
   specification is MD5. [8] Other algorithms can be added as
   extensions. MD5 is the default algorithm if no algorithm directive is
   present in the challenge.



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   The MD5 algorithm is defined as follows:

   Let KD(secret, data) denote the string obtained by  performing the
   digest algorithm to the data "data" with the secret "secret". Let
   H(data) denote the string obtained by performing the checksum
   algorithm on the data "data".

   For the "MD5" algorithm, H(data) = MD5(data), and KD(secret,data) =
   H(concat(secret, ":", data)

   The request-digest value in a CAuth header field takes the following
   format. Note that unq(quoted-string) denotes the value of the string
   with the quotes removed.

         request-digest  = <"> < KD ( H(A1), unq(nonce-value) ":" H(A2) ) > <">
         A1 = unq(username-value) ":" shared-secret
         A2 = Method

   When the relay receives a CAuth header, it SHOULD check its validity
   by looking up the shared secret, or H(A1), performing the same digest
   operation as performed by the client, and comparing the results to
   the request-digest value.

6.10 Method Descriptions

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

6.10.1 BIND

   The BIND method is used by a host endpoint to establish or refresh
   session state at a hosting relay. BIND requests SHOULD be
   authenticated. BIND requests MUST contain the S-URL and  Exp header
   fields and MAY contain the CAuth header fields.

   A successful response to a BIND request MUST contain the S-URL and
   Exp header fields.

6.10.2 SEND

   The SEND method is used by both the host and visitor endpoints to
   send instant messages to its peer endpoint. SEND requests SHOULD
   contain a MIME body part. The body MUST be of a media type included



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   in the format list negotiated in the SDP exchange. If a body is
   present, the request MUST contain a Content-Type header field
   identifying the media type of the body.

   Unlike other methods, SEND requests are end to end in nature. This
   means the request is consumed only by the opposite endpoint. Under
   normal conditions, any intervening relays merely forward the request
   on towards the peer endpoint.

6.10.3 VISIT

   The visiting endpoint uses the VISIT method to associate a network
   connection with the session state at the hosting device, which could
   be either the host endpoint or a relay operating on behalf of the
   host endpoint. VISIT is also used to refresh the expiration time for
   the visiting connection. The request MUST contain a S-URL header
   matching the session URL. A VISIT request MUST contain the Expires
   header field.

   Successful responses to a VISIT request MUST contain the Expires
   header.

      There is normally no authentication operation for the VISIT
      request. This is because the session URL acts as a shared secret
      between host and the visitor. This puts certain requirements on
      the handling of the session URLs that are discussed in Section 9.
      However, if a visiting relay is used, it SHOULD authenticate
      initial VISIT requests, and MAY authenticate subsequent VISIT
      refresh requests.


6.11 Response Code Descriptions

   This section summarizes the various response codes. Except where
   noted, all responses MUST contain a TR-ID header field matching the
   TR-ID header field of the associated request. Responses are never
   consumed by relays.

6.11.1 200

   The 200 response code indicates a successful transaction.

6.11.2 400

   A 400 response indicates a request was unintelligible.

6.11.3 401




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   A 401 response indicates authentication is required. 401 responses
   MUST NOT be used in response to any method other than BIND and VISIT.
   A 401 response MUST contain a SChal header field.

6.11.4 403

   A 403 response indicates the user is not authorized to perform the
   action.

6.11.5 415

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

6.11.6 481

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

6.11.7 500

   A 500 response indicates that a relay was unable to deliver a SEND
   request to the target.

6.11.8 506

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

6.12 Header Field Descriptions

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

6.12.1 TR-ID

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

6.12.2 Exp

   The Exp header field specifies when the state associated with a BIND
   or VISIT request will expire. The value is specified as an integer
   number of seconds from the time the request is received. BIND and



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   VISIT requests MUST contain this header field. Furthermore,
   successful responses to BIND or VISIT requests must also contain the
   Exp header.

   The maximum value for the Exp header field is (2**32)-1 seconds.

   Exp has no meaning if it occurs in MSRP messages other than BIND and
   VISIT requests, and responses to those requests. MSRP compliant
   devices SHOULD NOT use Exp in other requests or responses, unless
   that usage is defined in an extension to this specification.

6.12.3 CAuth

   The CAuth header field is used by a host endpoint to respond to offer
   digest authentication credentials to a relay, usually in response to
   a digest authentication challenge. CAuth SHOULD NOT be present in a
   request of any method other than BIND and VISIT.

   The CAuth credentials adhere to the following syntax:

          credentials = "Digest" digest-response
          digest-response = 1#( username | nonce |
                                       response | [ algorithm ] |
                                      [auth-param] )

          username = "username" "=" username-value
          username-value   = quoted-string
          response = "response" "=" request-digest
          request-digest = <"> 32LHEX <">
          LHEX =  "0" | "1" | "2" | "3" |
                       "4" | "5" | "6" | "7" |
                       "8" | "9" | "a" | "b" |
                       "c" | "d" | "e" | "f"

   The meaning of each value is as follows:

   username: The user's account name.

   nonce: The nonce value copied from the challenge.

   response: A 32 hex digit string that proves user knowledge of the
      shared secret.

   algorithm: The algorithm value copied from the challenge.

   auth-param: Additional parameters for the sake of extensibility.





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6.12.4 SChal

   The SChal header field is used by a relay to carry the challenge in a
   digest authentication attempt. Exactly one SChal header field MUST
   exist in a 401 response. The SChal header MUST NOT be used in any
   message except for a 401 response. The SChal header field value is
   made up of a challenge according to the following syntax:

          challenge= digest-scheme SP digest-challenge

         digest-scheme = "Digest"
         digest-challenge  = 1#( nonce | [ algorithm ] |
                                        [auth-param] )
         nonce = "nonce" "=" nonce-value
         nonce-value = quoted-string
         algorithm = "algorithm" "=" ( "MD5" | token )


   The meaning of each value is as follows:

   digest scheme: A token to identify the particular authentication
      scheme. For digest, the value MUST be "Digest." This token is
      present for the sake of extensibility.

   nonce: A server-specified string, which the relay SHOULD uniquely
      generate each time it sends a 401 response. This string SHOULD
      take the form of base64 or hexadecimal data, to avoid the presence
      of a double-quote character, which is not allowed.

   algorithm: A token indicating the algorithms to be used to generate
      the digest and checksum. This directive exists for the sake of
      extensibility; the only value defined by this document is "MD5".
      absence of this directive indicates a value of "MD5."


6.12.5 Content-Type

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

6.12.6 S-URL

   The S-URL header field is used to identify the session. The S-URI
   header field MUST be present in a BIND request, a successful response
   to a BIND request, or a VISIT request.

7. Examples




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   This section shows some example message flows for various common
   scenarios. The examples assume 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 examples, assume the
   offerer is sip:alice@atlanta.com and the answerer is
   sip:bob@biloxi.com. In any given MSRP message, an "xx" in the length
   field indicates the actual length of the rest of the message.

7.1 No Relay

   1.   Alice constructs a session URL of msrp://alicepc.atlanta.com/
        iau39 and listens for a connection on TCP port 7777.

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

        c=IN IP4 fillername
        m=message 7777 msrp/tcp text/plain
        a=direction:both a=session:msrp://alicepc.atlanta.com/iau39:7777


   3.   Bob->Alice: Open TCP connection to alicepc.atlanta.com:7777.

   4.   Bob->Alice (MSRP):

        MSRP xx VISIT
        S-URL: msrp://alicepc.atlanta.com/iau39:7777
        Tr-ID: sie09s
        Exp:600


   5.   Alice->Bob (MSRP):

        MSRP xx 200 OK
        Tr-ID: sie09s
        Exp:300


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

        c=IN IP4 ignorefield
        m=message 7777 msrp/tcp text/plain
        a=direction:active


   7.   Alice->Bob (SIP): ACK

   8.   Alice->Bob (MSRP):




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


   9.   Bob->Alice (MSRP):

        MSRP xx 200 OK
        TR-ID: 123


   10.  Bob->Alice (MSRP):

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

        Hi, Alice! I'm Bob!


   11.  Alice->Bob (MSRP):

        MSRP xx 200 OK
        TR-ID: 456


   12.  Alice->Bob (SIP): BYE

   13.  Alice invalidates session and drops connection.

   14.  Bob invalidates local state for the session.

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


7.2 Single Relay

   This scenario introduces an MSRP relay at relay.atlanta.com.

   1.   Alice->Relay (MSRP): Alice opens a connection to the relay, and
        sends the following:

        MSRP xx BIND
        S-URL: msrp://relay.atlanta.com
        TR-ID: 321
        Exp:600




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   2.   Relay->Alice (MSRP):

        MSRP xx 200 OK
        TR-ID: 321
        S-URL: msrp://relay.atlanta.com:7777/iau39
        Exp:300


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

        c=IN IP4 dummyvalue
        m=message 7777 msrp/tcp text/plain
        a=direction:passive
        a=session:msrp://relay.atlanta.com:7777/iau39


   4.   Bob->Alice: Open connection to relay.atlanta.com:7777.

   5.   Bob->Relay (MSRP):

        MSRP xx VISIT
        S-URL:msrp://relay.atlanta.com:7777/iau39
        TR-ID: msrp:sie09s
        Exp:500


   6.   Relay->Bob (MSRP):

        MSRP xx 200 OK
        TR-ID: sie09s
        Exp:300


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

        c=IN IP4 nobodybutchickens
        m=message 7777 msrp/tcp text/plain
        a=direction:active


   8.   Alice->Bob (SIP): ACK

   9.   Alice->Relay (MSRP):

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



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   10.  Relay->Bob (MSRP):

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


   11.  Bob->Relay (MSRP):

        MSRP xx 200 OK
        TR-ID: 123


   12.  Relay->Alice (MSRP):

        MSRP xx 200 OK
        TR-ID: 123


   13.  Bob->Relay (MSRP):

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

        Hi, Alice! I'm Bob!


   14.  Relay->Alice (MSRP):

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

        Hi, Alice! I'm Bob!


   15.  Alice->relay (MSRP):

        MSRP xx 200 OK
        TR-ID: 456


   16.  Relay->Bob (MSRP):

        MSRP xx 200 OK
        TR-ID: 456




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   17.  Alice->Bob (SIP): BYE

   18.  Alice->Relay (MSRP):

        MSRP xx  BIND
        S-URL: msrp://relay.atlanta.com:7777/iau39
        TR-ID: 42
        Exp:0


   19.  Relay->Alice (MSRP): (relay invalidates session state)

        MSRP xx 200 OK
        TR-ID: 42
        Exp:0


   20.  Bob invalidates local state for the session.

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


7.3 Two Relays

   In this scenario, both Alice and Bob are each required by local
   policy to route all sessions through a different local relay.

   1.   Alice->AtlantaRelay (MSRP): Alice opens a connection to the
        relay, and sends the following:

        MSRP xx BIND
        S-URL: msrp://relay.atlanta.com
        TR-ID: 321
        Exp:600


   2.   AtlantaRelay->Alice (MSRP):

        MSRP xx 200 OK
        TR-ID: 321
        S-URL: msrp://relay.atlanta.com:7777/iau39
        Exp:600


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

        c=IN IP4 blahblahblah
        m=message 7777 msrp/tcp text/plain



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        a=session:msrp://relay.atlanta.com:7777/iau39
        a=direction:passive


   4.   Bob determines that, due to local policy, he must connect
        through his own proxy.

   5.   Bob->BiloxiRelay (MSRP): Bob opens a connection to his relay,
        and sends the following:

        MSRP xx VISIT
        S-URL: msrp://relay.atlanta.com:7777/iau39
        TR-ID: 934
        Exp:600


   6.   BiloxiRelay->AtlantaRelay (MSRP): BiloxiRelay resolves the URL,
        opens a connection to relay.atlanta.com:7777, and sends the
        following:

        MSRP xx VISIT
        S-URL: msrp://relay.atlanta.com:7777/iau39
        TR-ID: 934
        Exp:600


   7.   AtlantaRelay->BiloxiRelay(MSRP):

        MSRP xx 200 OK
        TR-ID: 934
        Exp:600


   8.   BiloxiRelay->Bob(MSRP):

        MSRP xx 200 OK
        TR-ID: 934
        Exp:600


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

        c=IN IP4 stuff
        m=message 7777 msrp/tcp text/plain
        a=direction: active


   10.  Alice->Bob (SIP): ACK



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   11.  Alice->AtlantaRelay (MSRP):

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


   12.  AtlantaRelay ->BiloxiRelay (MSRP):

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


   13.  BiloxiRelay->Bob (MSRP):

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


   14.  Bob->BiloxiRelay (MSRP):

        MSRP xx 200 OK
        TR-ID: 123


   15.  BiloxiRelay->AtlantaRelay (MSRP):

        MSRP xx 200 OK
        TR-ID: 123


   16.  AtlantaRelay->Alice (MSRP):

        MSRP xx 200 OK
        TR-ID: 123


   17.  Alice->Bob (SIP): BYE

   18.  Alice->AtlantaRelay (MSRP):

        MSRP xx BIND
        S-URL: msrp://relay.atlanta.com:7777/iau39
        TR-ID: 42



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        Exp:0


   19.  Relay->Alice (MSRP): (relay invalidates session state)

        MSRP xx 200 OK
        TR-ID: 42
        Exp:0


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


8. IANA Considerations

   To Do.

      Do we need to register URL schemes or SDP a-line attributes?


9. Security Considerations

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

9.1 The MSRPS Scheme

   The MSRPS scheme indicates that all hops in an MSRP session MUST be
   protected with TLS. Ensuring this implies some additional rules. An
   MSRP relay MUST NOT return an MSRPS URL in the S-URL header field in
   a response to a BIND request unless the BIND request itself was
   received over a TLS protected session. A VISIT request for an MSRPS
   URL MUST be sent over a TLS protected connection. If a visiting relay
   receives a VISIT request for an MSRPS URL, the connection to the
   hosting device MUST also be protected.

      There has been controversy on whether an MSRPS scheme is really
      needed, since there is a small limit to the total number of hops
      in an MSRP session. However, a mechanism is required to inform the
      visitor to use TLS in the first place. We could have used an SDP
      a-line attribute for this. However, there also needs to be a
      mechanism for a hosting relay to tell the host endpoint to request
      the visitor use TLS. The MSRPS scheme seems to best fit all of
      these requirements.

      Open Issue: There is also some controversy over how TLS should be
      used with MSRP. The changes in this draft version make it possible



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      for relays to act as tunnels, where the TLS handshake is
      end-to-end. This is much the same way that TLS is handled by HTTPS
      proxies. However, this usage requires at least one endpoint to
      have a TLS server certificate, which may not be likely. Another
      approach is to make TLS usage hop-by-hop. When at least one relay
      is used, only the relays would need server certificates. Even if
      we support end-to-end TLS, we may still need a way to specify
      hop-by-hop TLS, because since end-to-end TLS would delay the TLS
      handshake until _after_ the BIND and VISIT requests, these
      requests would not be protected.


9.2 Sensitivity of the Session URL

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

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

9.3 End to End Protection of IMs

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

   Note that while each protected body could use separate keying
   material, this is inefficient in that it requires an independent
   public key operation for each message. Endpoints wishing to invoke



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   end-to-end protection of message sessions SHOULD exchange symmetric
   keys in SDP k-lines, and use secret key encryption on for each MSRP
   message. When symmetric keys are present in the SDP, the offer-answer
   exchange MUST be protected from eavesdropping and tampering using the
   appropriate facilities of the signaling protocol. For example, if the
   signaling protocol is SIP, the SDP exchange MUST be protected using
   S/MIME.

      Open Issue: This subsection needs very close scrutiny for accuracy
      and security. In particular, do we need to say more about using
      secret key operations in CMS?


9.4 CPIM compatibility

   MSRP sessions may be gatewayed to other CPIM [16]compatible
   protocols. If this occurs, the gateway MUST maintain session state,
   and MUST translate between the MSRP session semantics and CPIM
   semantics that do not include a concept of sessions. Furthermore,
   when one endpoint of the session is a CPIM gateway, instant messages
   SHOULD be wrapped in "message/cpim" [5] bodies. Such a gateway MUST
   include "message/cpim" as the first entry in its SDP format list.
   MSRP endpoints sending instant messages to a peer that has included
   'message/cpim" as the first entry in the format list SHOULD
   encapsulate all instant message bodies in "message/cpim" wrappers.
   All MSRP endpoints SHOULD support the S/MIME features of that format.

10. Changes introduced in draft-ietf-simple-message-sessions-00

      Name changed to reflect status as a work group item.

      This version no longer supports the use of multiple sessions
      across a single TCP session. This has several related changes:
      There is now a single session URI, rather than a separate one for
      each endpoint. The session URI is not required to be in requests
      other than BIND and VISIT, as the session can be determined based
      on the connection on which it arrives.

      BIND and VISIT now create soft state, eliminating the need for the
      RELEASE and LEAVE methods.

      The MSRP URL format was changed to better reflect generic URL
      standards. URL comparison and resolution rules were added. SRV
      usage added.

      Determination of host and visitor roles now uses a direction
      attribute much like the one used in COMEDIA.




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      Format list negotiation expanded to allow a "prefer these formats
      but try anything" semantic

      Clarified handling of direction notification failures.

      Clarified signaling associated with session failure due to dropped
      connections.

      Clarified security related motivations for MSRP.

      Removed MIKEY dependency for session key exchange. Simple usage of
      k-lines in SDP, where the SDP exchange is protected end-to-end
      seems sufficient.


11. Changes introduced in draft-campbell-simple-im-sessions-01

   Version 01 is a significant re-write. References to COMEDIA were
   removed, as it was determined that COMEDIA would not allow
   connections to be used bidirectionally in the presence of NATs.
   Significantly more discussion of a concrete mechanism has been added
   to make up for no longer using COMEDIA. Additionally, this draft and
   draft-campbell-cpimmsg-sessions (which would have also changed
   drastically) have now been combined into this single draft.

12. Contributors

   The following people contributed substantially to this ongoing
   effort:

                                Rohan Mahy
                                Allison Mankin
                                Jon Peterson
                                Brian Rosen
                                Dean Willis
                                Adam Roach
                                Cullen Jennings

Normative References

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

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

   [3]  Day, M., Aggarwal, S. and J. Vincent, "Instant Messaging /



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        Presence Protocol Requirements", RFC 2779, February 2000.

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

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

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

   [7]  Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3369,
        August 2002.

   [8]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April
        1992.

Informational References

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

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

   [11]  Mahy, R., Campbell, B., Sparks, R., Rosenberg, J., Petrie, D.
         and A. Johnston, "A Multi-party Application Framework for SIP",
         draft-ietf-sipping-cc-framework-02 (work in progress), May
         2003.

   [12]  Rosenberg, J., Peterson, J., Schulzrinne, H. and G. Camarillo,
         "Best Current Practices for Third Party Call Control in the
         Session Initiation Protocol", draft-ietf-sipping-3pcc-03 (work
         in progress), March 2003.

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

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

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



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   [16]  Peterson, J., "A Common Profile for Instant Messaging (CPIM)",
         draft-ietf-impp-im-03 (work in progress), May 2003.

   [17]  Yon, D., "Connection-Oriented Media Transport in SDP",
         draft-ietf-mmusic-sdp-comedia-05 (work in progress), March
         2003.

   [18]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
         Leach, P., Luotonen, A. and L. Stewart, "HTTP Authentication:
         Basic and Digest Access Authentication", RFC 2617, June 1999.


Authors' Addresses

   Ben Campbell
   dynamicsoft
   5100 Tennyson Parkway
   Suite 1200
   Plano, TX  75024

   EMail: bcampbell@dynamicsoft.com


   Jonathan Rosenberg
   dynamicsoft
   600 Lanidex Plaza
   Parsippany, NJ  07054

   EMail: jdrosen@dynamicsoft.com


   Robert Sparks
   dynamicsoft
   5100 Tennyson Parkway
   Suite 1200
   Plano, TX  75024

   EMail: rsparks@dynamicsoft.com


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

   EMail: pkyzivat@cisco.com




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