path: root/doc/rfc/rfc3428.txt

Network Working Group                                   B. Campbell, Ed.
Request for Comments: 3428                                  J. Rosenberg
Category: Standards Track                                    dynamicsoft
                                                          H. Schulzrinne
                                                     Columbia University
                                                              C. Huitema
                                                                D. Gurle
                                                   Microsoft Corporation
                                                           December 2002


   Session Initiation Protocol (SIP) Extension for Instant Messaging

Status of this Memo

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

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   Instant Messaging (IM) refers to the transfer of messages between
   users in near real-time.  These messages are usually, but not
   required to be, short.  IMs are often used in a conversational mode,
   that is, the transfer of messages back and forth is fast enough for
   participants to maintain an interactive conversation.

   This document proposes the MESSAGE method, an extension to the
   Session Initiation Protocol (SIP) that allows the transfer of Instant
   Messages.  Since the MESSAGE request is an extension to SIP, it
   inherits all the request routing and security features of that
   protocol.  MESSAGE requests carry the content in the form of MIME
   body parts.  MESSAGE requests do not themselves initiate a SIP
   dialog; under normal usage each Instant Message stands alone, much
   like pager messages.  MESSAGE requests may be sent in the context of
   a dialog initiated by some other SIP request.









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Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
   RECOMMENDED", "MAY" and "OPTIONAL" are to be interpreted as described
   in BCP 14, RFC 2119 [6] and indicate requirement levels for compliant
   SIP implementations.

Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.    Scope of Applicability . . . . . . . . . . . . . . . . . . .  3
   3.    Overview of Operation  . . . . . . . . . . . . . . . . . . .  4
   4.    UAC Processing . . . . . . . . . . . . . . . . . . . . . . .  5
   5.    Use of Instant Message URIs  . . . . . . . . . . . . . . . .  6
   6.    Proxy Processing . . . . . . . . . . . . . . . . . . . . . .  6
   7.    UAS Processing . . . . . . . . . . . . . . . . . . . . . . .  7
   8.    Congestion Control . . . . . . . . . . . . . . . . . . . . .  8
   9.    Method Definition  . . . . . . . . . . . . . . . . . . . . .  9
   10.   Example Messages . . . . . . . . . . . . . . . . . . . . . . 11
   11.   Security Considerations  . . . . . . . . . . . . . . . . . . 13
   11.1  Outbound authentication  . . . . . . . . . . . . . . . . . . 13
   11.2  SIPS URIs  . . . . . . . . . . . . . . . . . . . . . . . . . 14
   11.3  End-to-End Protection  . . . . . . . . . . . . . . . . . . . 14
   11.4  Replay Prevention  . . . . . . . . . . . . . . . . . . . . . 14
   11.5  Using message/cpim bodies  . . . . . . . . . . . . . . . . . 15
   12.   IANA Considerations  . . . . . . . . . . . . . . . . . . . . 15
   13.   Contributors . . . . . . . . . . . . . . . . . . . . . . . . 15
   14.   Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . 15
   15.   Normative References . . . . . . . . . . . . . . . . . . . . 16
   16.   Informational References . . . . . . . . . . . . . . . . . . 16
   17.   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 17
   18.   Full Copyright Statement . . . . . . . . . . . . . . . . . . 18

1. Introduction

   Instant Messaging (IM) is defined as the exchange of content between
   a set of participants in near real time.  Generally, the content is
   short text messages, although that need not be the case.  Generally,
   the messages that are exchanged are not stored, but this also need
   not be the case.  IM differs from email in common usage in that
   instant messages are usually grouped together into brief live
   conversations, consisting of numerous small messages sent back and
   forth.

   Instant messaging as a service has been in existence within intranets
   and IP networks for quite some time.  Early implementations include
   zephyr [11], the UNIX talk application, and IRC.  More recently, IM



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   has been used as a service coupled with presence and buddy lists;
   that is, when a friend comes online, a user can be made aware of this
   and have the option of sending the friend an instant message.  The
   protocols for accomplishing this are all proprietary, which has
   seriously hampered interoperability.

   The integration of instant messaging, presence, and session-oriented
   communications is very powerful.  The Session Initiation Protocol
   (SIP) [1] provides mechanisms that are useful for presence
   applications, and for session-oriented communication applications,
   but not for instant messages.

   This document proposes an extension method for SIP called the MESSAGE
   method.  MESSAGE requests normally carry the instant message content
   in the request body.

   RFC 2778 [3] and RFC 2779 [2] give a model and requirements for
   presence and instant messaging protocols.  Implementations of the
   MESSAGE method SHALL support all the instant message requirements in
   RFC 2779 relevant to its scope of applicability.

2. Scope of Applicability

   This document describes the use of the MESSAGE method for sending
   instant messages using a metaphor similar to that of a two-way pager
   or SMS enabled handset.  That is, there are no explicit association
   between messages.  Each IM stands alone--any sense of a
   "conversation" only exists in the client user interface, or perhaps
   in the user's own imagination.  We contrast this with a "session"
   model, where there is an explicit conversation with a clear beginning
   and end.  In the SIP environment, an IM session would be a media
   session initiated with an INVITE transaction and terminated with a
   BYE transaction.

   There is value in each model.  Most modern IM clients offer both user
   experiences.  The user can choose to send an IM to a contact, or he
   can choose to invite one or more contacts to join a conversation.
   The pager model makes sense when the user wishes to send a small
   number of short IMs to a single (or small number of) recipients.  The
   session model makes sense for extended conversations, joining chat
   groups, if there is a need to associate a conversation with some
   other SIP initiated session, etc.

   This document addresses the pager model only.  We recognize the value
   of the session model as well, but we do not define it here.  Such a
   solution will require additional work beyond that of this document.
   The SIMPLE work group currently plans to address IM sessions in a
   separate document.



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   There may be a temptation to simulate a session of IMs by initiating
   a dialog, then sending MESSAGE requests in the context of that
   dialog.  This is not an adequate solution for IM sessions, in that
   this approach forces the MESSAGE requests to follow the same network
   path as any other SIP requests, even though the MESSAGE requests
   arguably carry media rather than signaling.  IM applications are
   typically high volume, and we expect the IM volume in sessions to be
   even higher.  This will likely cause congestion problems if sent over
   a transport without congestion control, and there is no clear
   mechanism in SIP to prevent some hop from forwarding a MESSAGE
   request over UDP.

   Additionally, MESSAGE requests sent over an existing dialog must, by
   the nature of SIP, go to the same destination as any other request
   sent in that dialog.  This prevents any separation between the IM
   endpoint and the signaling endpoint.  This is not an acceptable
   limitation for the session-model of instant messaging.

   The authors recognize that there may be valid reasons to send MESSAGE
   requests in the context of a dialog.  For example, one participant in
   a voice session may wish to send an IM to another participant, and
   associate that IM with the session.  But implementations SHOULD NOT
   create dialogs for the primary purpose of associating MESSAGE
   requests with one another.

   Note that this statement does not prohibit using SIP to initiate a
   media session made up of IMs, just like any other session.  Indeed,
   we expect the solution for IM sessions to use that metaphor.  The
   reader should avoid confusing the concepts of a SIP dialog and a
   media session.

3. Overview of Operation

   When one user wishes to send an instant message to another, the
   sender formulates and issues a SIP request using the new MESSAGE
   method defined by this document.  The Request-URI of this request
   will normally be the "address of record" for the recipient of the
   instant message, but it may be a device address in situations where
   the client has current information about the recipient's location.
   For example, the client could be coupled with a presence system that
   supplies an up to date device contact for a given address of record.
   The body of the request will contain the message to be delivered.
   This body can be of any MIME type, including message/cpim [7].  Since
   the message/cpim format is expected to be supported by other instant
   message protocols, endpoints using different IM protocols, but
   otherwise supporting message/cpim body types, should be able to





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   exchange messages without modification of the content by a gateway or
   other intermediary.  This helps to enable end-to-end security between
   endpoints that use different IM protocols.

   The request may traverse a set of SIP proxies, using a variety of
   transports, before reaching its destination.  The destination for
   each hop is located using the address resolution rules detailed in
   the Common Profile for Instant Messaging (CPIM) [8] and SIP
   specifications.  During traversal, each proxy may rewrite the request
   URI based on available routing information.

   Provisional and final responses to the request will be returned to
   the sender as with any other SIP request.  Normally, a 200 OK
   response will be generated by the user agent of the request's final
   recipient.  Note that this indicates that the user agent accepted the
   message, not that the user has seen it.

   MESSAGE requests do not establish dialogs.

4. UAC Processing

   Unless stated otherwise in this document, MESSAGE requests and
   associated responses are constructed according to the rules in
   section 8.1 of the SIP specification [1].

   All UACs which support the MESSAGE method MUST be prepared to send
   MESSAGE requests with a body of type text/plain.  They may send
   bodies of type message/cpim [7].

   MESSAGE requests do not initiate dialogs.  User Agents MUST NOT
   insert Contact header fields into MESSAGE requests.

   A UAC MAY associate a MESSAGE request with an existing dialog.  If a
   MESSAGE request is sent within a dialog, it is "associated" with any
   media session or sessions associated with that dialog.

   If the UAC receives a 200 OK response to a MESSAGE request, it may
   assume the message has been delivered to the final destination.  It
   MUST NOT assume that the recipient has actually read the instant
   message.  If the UAC receives a 202 Accepted response, the message
   has been delivered to a gateway, store and forward server, or some
   other service that may eventually deliver the message.  In this case,
   the UAC MUST NOT assume the message has been delivered to the final
   destination.  If confirmation of delivery is required for a message
   that has been responded to with a 202 Accepted, that confirmation
   must be delivered via some other mechanism, which is beyond the scope
   of this specification.




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   Note that a downstream proxy could fork a MESSAGE request.  If this
   occurs, the forking proxy will forward one final response upstream,
   even though it may receive multiple final responses.  The UAC will
   have no way to detect whether or not a fork occurs.  Therefore the
   UAC MUST NOT assume that a given final response represents the only
   UAS that receives the request.  For example, multiple branches of a
   fork could have resulted in 2xx responses.  Even though the UAC only
   sees one of those responses, the request has in fact been received by
   the second device as well.

   The UAC MAY add an Expires header field to limit the validity of the
   message content.  If the UAC adds an Expires header field with a
   non-zero value, it SHOULD also add a Date header field containing the
   time the message is sent.

5. Use of Instant Message URIs

   An instant inbox may be most generally referenced by an Instant
   Message URI [8] in the form of "im:user@domain".  IM URIs are
   abstract, and will eventually be translated to concrete, protocol-
   dependent URI.

   If a UA is presented with an IM URI as the address for an instant
   message, it SHOULD resolve it to a SIP URI, and place the resulting
   URI in the Request-URI of the MESSAGE request before sending.  If the
   UA is unable to resolve the IM URI, it MAY place the IM URI in the
   Request-URI, thus delegating the resolution to a downstream device
   such as proxy or gateway.  Performing this translation as early as
   possible allows SIP proxies, which may be unaware of the im:
   namespace, to route the requests normally.

   MESSAGE requests also contain logical identifiers of the sender and
   intended recipient, in the form of the From and To header fields.
   These identifiers SHOULD contain SIP (or SIPS) URIs, but MAY include
   IM URIs if the SIP URIs are not known at the time of request
   construction.

   Record-Route and Route header fields MUST NOT contain IM URIs.  These
   header fields contain concrete SIP or SIPS URIs according to the
   rules of SIP [1].

6. Proxy Processing

   Proxies route MESSAGE requests according to the rules of SIP [1].
   Note that the MESSAGE request MAY fork; this allows for delivery of
   the message to several possible terminals where the user might be.  A
   proxy forking a MESSAGE request follows the normal SIP rules for
   forking a non-INVITE request.  In particular, even if the fork



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   results in multiple successful deliveries, the forking proxy will
   only forward one final response upstream.

7. UAS Processing

   A UAS that receives a MESSAGE request processes it following the
   rules of SIP [1].

   A UAS receiving a MESSAGE request SHOULD respond with a final
   response immediately.  Note, however, that the UAS is not obliged to
   display the message to the user either before or after responding
   with a 200 OK.  That is, a 200 OK response does not necessarily mean
   the user has read the message.

   A 2xx response to a MESSAGE request MUST NOT contain a body.  A UAS
   MUST NOT insert a Contact header field into a 2xx response.

   A UAS which is, in fact, a message relay, storing the message and
   forwarding it later on, or forwarding it into a non-SIP domain,
   SHOULD return a 202 (Accepted) [5] response indicating that the
   message was accepted, but end to end delivery has not been
   guaranteed.

   A 4xx or 5xx response indicates that the message was not delivered
   successfully.  A 6xx response means it was delivered successfully,
   but refused.

   A UAS that supports the MESSAGE method MUST be prepared to receive
   and render bodies of type "text/plain", and may support reception and
   rendering of bodies of type "message/cpim" [7].

   A MESSAGE request is said to be expired if its expiration time has
   passed. The expiration time is determined by examining the Expires
   header field, if present.  MESSAGE requests without an Expires header
   field do not expire.  If the MESSAGE request containing an Expires
   header field also contains a Date header field, the UAS SHOULD
   interpret the Expires header field value as delta time from the Date
   header field value.  If the request does not contain a Date header
   field, the UAS SHOULD interpret the Expires header value as delta
   time from the time the UAS received the request.

   If the MESSAGE expires before the UAS is able to present the message
   to the user, the UAS SHOULD handle the message based on local policy.
   This policy could mean: the message is deleted undisplayed,







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   the message is still displayed to the user, or some other policy may
   be invoked.  If the message is displayed, the UAS SHOULD clearly
   indicate to the user that the message has expired.

   If the UAS is acting as a message relay, and is unable to deliver the
   message before expiration, it chooses an action based on local
   policy.  This action could involve deleting the message undelivered,
   delivering it as is, logging the expired message, or any other local
   policy.

8. Congestion Control

   Existing IM services have a history of very high volume usage.
   Additionally, MESSAGE requests differ from other sorts of SIP
   requests in that they carry media, in the form of IMs, as payload.
   Conventional SIP payloads carry signaling information about media,
   but not media itself.  There is potential that when a SIP
   infrastructure is shared between call signaling and instant
   messaging, the IM traffic will interfere with call signaling traffic.
   Congestion control in general is an issue that should be addressed at
   the SIP specification level rather than for an individual method.
   But since the traffic patterns are likely to be different for MESSAGE
   than for most other methods, it makes sense to give MESSAGE special
   consideration.

   Whenever possible, MESSAGE requests SHOULD be sent over transports
   that implement end-to-end congestion control, such as TCP or SCTP.
   However, SIP does not provide a mechanism to prevent a downstream hop
   from sending a request over UDP.  Even the requirement to use TCP for
   requests over a certain size can be overridden by the receiver.
   Therefore use of a congestion-controlled transport by the UAC is not
   sufficient.

   The size of MESSAGE requests outside of a media session MUST NOT
   exceed 1300 bytes, unless the UAC has positive knowledge that the
   message will not traverse a congestion-unsafe link at any hop, or
   that the message size is at least 200 bytes less than the lowest MTU
   value found en route to the UAS.  Larger payloads may be sent as part
   of a media session, or using some type of content-indirection.

      At the time of this writing, there is no mechanism for a UAC to
      gain such knowledge outside of trivial network architectures, or
      networks that are wholly controlled by a single administrative
      domain.  But if a mechanism for ensuring congestion-control at
      each hop is created in the future, MESSAGE clients can relax the
      size limit without requiring a change to this specification.  The
      authors expect that such a mechanism or mechanism will be created
      in the near future.



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      There have been discussions on making the 1300 byte limit based on
      the path MTU to the next hop SIP device.  The SIP specification
      does exactly that, choosing the limit 200 bytes less than the path
      MTU, or 1300 bytes if the device does not know the path MTU.
      Transport decisions are made on a per-hop basis.  However, the
      point of this limit is to make sure that no upstream proxy chooses
      to send a MESSAGE request with large content over UDP.  Since,
      except in trivial circumstances, a MESSAGE client is very unlikely
      to know the MTU for upstream devices beyond the next hop, an MTU
      based limit is not very useful.

   A UAC MUST NOT initiate a new out-of-dialog MESSAGE transaction to a
   given URI if there is a previous out-of-dialog transaction pending
   for the same URI.  Similarly, A UAC SHOULD NOT initiate overlapping
   MESSAGE transactions inside a dialog, and MUST NOT do so unless the
   route set for that dialog uses a congestion-controlled transport at
   every hop.

      The prohibition against overlapping MESSAGE request provides some
      degree of congestion-safe behavior.  A request and its associated
      response must each cross the full path between the UAC and the
      UAS.  The time required for this will increase as networks become
      congested.  This provides an adaptive mechanism to slow the
      introduction of new MESSAGE requests to the same destination.

   It has been suggested that provisional responses should not be
   allowed for pager-model MESSAGE requests.  However, it is not
   possible to require special treatment for MESSAGE, since many proxy
   servers will not be aware of the MESSAGE method.  Therefore MESSAGE
   requests will receive the same provisional response treatment as any
   other non-INVITE method, as described in the SIP specification.

9. Method Definition

   This specification defines a new SIP method, MESSAGE.  The BNF for
   this method is:

      MESSAGEm = %x4D.45.53.53.41.47.45 ;MESSAGE in caps

   As with all other methods, the MESSAGE method name is case sensitive.

   Tables 1 and 2 extend Tables 2 and 3 of SIP [1] by adding an
   additional column, defining the header fields that can be used in
   MESSAGE requests and responses.







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                   Header Field       where  proxy  MESSAGE
                   __________________________________________
                   Accept               R              -
                   Accept              2xx             -
                   Accept              415             m*
                   Accept-Encoding      R              -
                   Accept-Encoding     2xx             -
                   Accept-Encoding     415             m*
                   Accept-Language      R              -
                   Accept-Language     2xx             -
                   Accept-Language     415             m*
                   Alert-Info           R              -
                   Alert-Info          180             -
                   Allow                R              o
                   Allow               2xx             o
                   Allow                r              o
                   Allow               405             m
                   Authentication-Info 2xx             o
                   Authorization        R              o
                   Call-ID              c      r       m
                   Call-Info                  ar       o
                   Contact              R              -
                   Contact             1xx             -
                   Contact             2xx             -
                   Contact             3xx             o
                   Contact             485             o
                   Content-Disposition                 o
                   Content-Encoding                    o
                   Content-Language                    o
                   Content-Length             ar       t
                   Content-Type                        *
                   CSeq                c       r       m
                   Date                        a       o
                   Error-Info       300-699    a       o
                   Expires                             o
                   From                c       r       m
                   In-Reply-To         R               o
                   Max-Forwards        R      amr      m
                   Organization               ar       o

                   Table 1: Summary of header fields, A--O










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                   Header Field       where  proxy  MESSAGE
                   __________________________________________
                   Priority             R     ar         o
                   Proxy-Authenticate  407    ar         m
                   Proxy-Authenticate  401    ar         o
                   Proxy-Authorization  R     dr         o
                   Proxy-Require        R     ar         o
                   Record-Route               ar         -
                   Reply-To                              o
                   Require                    ar         c
                   Retry-After   404,413,480,486         o
                                     500,503             o
                                     600,603             o
                   Route                R     adr        o
                   Server               r                o
                   Subject              R                o
                   Timestamp                             o
                   To                 c(1)     r         m
                   Unsupported         420               o
                   User-Agent                            o
                   Via                  R     amr        m
                   Via                 rc     dr         m
                   Warning              r                o
                   WWW-Authenticate    401    ar         m
                   WWW-Authenticate    407    ar         o

                 (1): copied  with  possible addition of tag

                   Table 2: Summary of header fields, P--Z

   A MESSAGE request MAY contain a body, using the standard MIME header
   fields to identify the content.

10. Example Messages

   An example message flow is shown in Figure 1.  The message flow shows
   an initial IM sent from User 1 to User 2, both users in the same
   domain, "domain", through a single proxy.













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           |  F1 MESSAGE          |                         |
           |--------------------> |  F2 MESSAGE             |
           |                      | ----------------------->|
           |                      |                         |
           |                      |  F3 200 OK              |
           |                      | <-----------------------|
           |  F4 200 OK           |                         |
           |<-------------------- |                         |
           |                      |                         |
           |                      |                         |
           |                      |                         |
        User 1                  Proxy                    User 2

                   Figure 1: Example Message Flow

   Message F1 looks like:

   MESSAGE sip:user2@domain.com SIP/2.0
   Via: SIP/2.0/TCP user1pc.domain.com;branch=z9hG4bK776sgdkse
   Max-Forwards: 70
   From: sip:user1@domain.com;tag=49583
   To: sip:user2@domain.com
   Call-ID: asd88asd77a@1.2.3.4
   CSeq: 1 MESSAGE
   Content-Type: text/plain
   Content-Length: 18

   Watson, come here.

   User1 forwards this message to the server for domain.com.  The proxy
   receives this request, and recognizes that it is the server for
   domain.com.  It looks up user2 in its database (built up through
   registrations), and finds a binding from sip:user2@domain.com to
   sip:user2@user2pc.domain.com.  It forwards the request to user2.  The
   resulting message, F2, looks like:

   MESSAGE sip:user2@domain.com SIP/2.0
   Via: SIP/2.0/TCP proxy.domain.com;branch=z9hG4bK123dsghds
   Via: SIP/2.0/TCP user1pc.domain.com;branch=z9hG4bK776sgdkse;
                                              received=1.2.3.4
   Max-Forwards: 69
   From: sip:user1@domain.com;tag=49394
   To: sip:user2@domain.com
   Call-ID: asd88asd77a@1.2.3.4
   CSeq: 1 MESSAGE
   Content-Type: text/plain
   Content-Length: 18




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   Watson, come here.

   The message is received by user2, displayed, and a response is
   generated, message F3, and sent to the proxy:

   SIP/2.0 200 OK
   Via: SIP/2.0/TCP proxy.domain.com;branch=z9hG4bK123dsghds;
                                            received=192.0.2.1
   Via: SIP/2.0/TCP user1pc.domain.com;;branch=z9hG4bK776sgdkse;
                                               received=1.2.3.4
   From: sip:user1@domain.com;tag=49394
   To: sip:user2@domain.com;tag=ab8asdasd9
   Call-ID: asd88asd77a@1.2.3.4
   CSeq: 1 MESSAGE
   Content-Length: 0

   Note that most of the header fields are simply reflected in the
   response.  The proxy receives this response, strips off the top Via,
   and forwards to the address in the next Via, user1pc.domain.com, the
   result being message F4:

   SIP/2.0 200 OK
   Via: SIP/2.0/TCP user1pc.domain.com;branch=z9hG4bK776sgdkse;
                                              received=1.2.3.4
   From: sip:user1@domain.com;;tag=49394
   To: sip:user2@domain.com;tag=ab8asdasd9
   Call-ID: asd88asd77a@1.2.3.4
   CSeq: 1 MESSAGE
   Content-Length: 0

11. Security Considerations

   In normal usage, most SIP requests are used to setup and modify
   communication sessions.  The actual communication between
   participants happens in the media sessions, not in the SIP requests
   themselves.  The MESSAGE method changes this assumption; MESSAGE
   requests normally carry the actual communication between participants
   as payload.  This implies that MESSAGE requests have a greater need
   for security than most other SIP requests.  In particular, UAs that
   support the MESSAGE request MUST implement end-to-end authentication,
   body integrity, and body confidentiality mechanisms.

11.1 Outbound Authentication

   When local proxies are used for transmission of outbound messages,
   proxy authentication, as specified in RFC 3261, is RECOMMENDED.  This
   is useful to verify the identity of the originator, and prevent
   spoofing and spamming at the originating network.



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11.2 SIPS URIs

   The SIPS URI mechanism [1] allows a UA to assert that every hop must
   occur over a secure connection.  This provides some level of
   integrity and privacy protection.  However, this requires the users
   to trust that each proxy in the request path is well-behaved, that
   is, they do not violate the rules for routing SIPS URIs.  Also, any
   unencrypted bodies are fully exposed to the proxies.

   Additionally, the possibility of a forking proxy allows a MESSAGE
   request to be delivered to additional endpoints without the knowledge
   of the UAC.  If only hop-by-hop protection is used, the users must
   trust all proxies in the chain to not fork requests to unauthorized
   destinations.

11.3 End-to-End Protection

   When the goal is to remedy the concerns stated above, the MESSAGE
   bodies MUST be secured with S/MIME.  If bodies specified in future to
   be carried by the MESSAGE method have different means of providing
   end-to-end security, their specification MUST describe the usage.
   SIP MESSAGE endpoints MUST support encryption (CMS EnvelopeData) and
   S/MIME signatures (CMS SignedData).

   The S/MIME algorithms are set by RFC 3369 [4].  The AES [10]
   algorithm should be preferred, as it is expected that AES best suits
   the capabilities of many platforms.  However, an IETF specification
   for this is still incomplete as of the time of this writing.

11.4 Replay Prevention

   To prevent the replay of old SIP requests, all signed MESSAGE
   requests and responses MUST contain a Date header field covered by
   the message signature.  Any message with a date older than several
   minutes in the past, or which is more than several minutes in the
   future, SHOULD be answered with a 400 (Incorrect Date or Time)
   message, unless such messages arrive repeatedly from the same source,
   in which case they MAY be discarded without sending a response.
   Obviously, this replay attack prevention mechanism does not work for
   devices without clocks.

   Note that there are situations where an stale Date header field is
   normal.  For example, the MESSAGE request may have been stored in a
   store and forward server while the recipient was offline.  When the
   recipient returns, that server might then forward the message.  Final
   receipt of the message would then occur some time after it was
   originally sent.




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   If a UAS receives a stale message that can be confirmed to have come
   from a known store and forward server (perhaps over a TLS
   connection), it makes sense for it to accept the message normally.
   Also, if one or more stale messages arrive shortly after an offline
   period, the UAS MAY accept the message, but SHOULD warn the user that
   there is a risk the message has been replayed.

11.5 Using message/cpim Bodies

   The message/cpim format [7] allows for the S/MIME protection of
   metadata in addition to the message payload itself.  In many cases,
   this metadata is redundant with SIP header fields.  Still,
   message/cpim adds value in that the protection of metadata can extend
   across protocol boundaries.  For example, a signed message/cpim body
   can provide sender authentication using the message/cpim From header
   field, even if the message crosses a gateway to another CPIM
   compliant instant message service that does not understand SIP header
   fields.

12. IANA Considerations

   This specification registers the MESSAGE method in the
   http://www.iana.org/assignments/sip-parameters/Method registry,
   according to the following information:

   MESSAGE        [RFC3428]

13. Contributors

   The following people made substantial contributions to this work:

   Bernard Aboba     Microsoft
   Steve Donovan     dynamicsoft
   Jonathan Lennox   Columbia University
   Dave Oran         Cisco
   Robert Sparks     dynamicsoft
   Dean Willis       dynamicsoft

14. Acknowledgments

   The authors would like to thank the following people for their
   support of the concept of SIP for IM, support for this work, and for
   their useful comments and insights:

      Jon Peterson     NeuStar
      Sean Olson       Microsoft
      Adam Roach       dynamicsoft
      Billy Biggs      University of Waterloo



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      Stuart Barkley   UUNet
      Mauricio Arango  SUN
      Richard Shockey  NeuStar
      Jorgen Bjorker   Hotsip
      Henry Sinnreich  MCI Worldcom
      Ronald Akers     Motorola
      Torrey Searle    Indigo Software
      Rohan Mahy       Cisco
      Christian Groves Ericsson
      Allison Mankin   ISI
      Tan Ya-Ching     Siemens

15.Normative References

   [1]  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.

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

   [3]  Day, M., Rosenberg, J. and H. Sugano, "A Model for Presence and
        Instant Messaging", RFC 2778, February 2000.

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

   [5]  Roach, A., "Session Initiation Protocol (SIP)-Specific Event
        Notification", RFC 3265, June 2002.

   [6]  Bradner, S., "Keywords for Use in RFC's to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

16. Informational References

   [7]  Atkins, D. and G. Klyne, "Common Presence and Instant Messaging
        Message Format", Work in Progress.

   [8]  Crocker, D., Diacakis, A., Mazzoldi, F., Huitema, C., Klyne, G.,
        Rose, M., Rosenberg, J., Sparks, R., Sugano, H. and J. Peterson,
        "Address Resolution for Instant Messaging and Presence", Work in
        Progress.

   [9]  Rosenberg, J. and H. Schulzrinne, "SIP Caller Preferences and
        Callee Capabilities", Work in Progress.

   [10] Schaad, J. and R. Housley, "Use of the AES Encryption Algorithm
        and RSA-OAEP Key Transport in CMS", Work in Progress.



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   [11] DellaFera, C., Eichin, M., French, R., Jedlinski, D., Kohl, J.
        and W. Sommerfeld, "The Zephyr notification service", in USENIX
        Winter Conference (Dallas, Texas), Feb. 1988.

17. Authors' Addresses

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

   EMail: bcampbell@dynamicsoft.com


   Jonathan Rosenberg
   dynamicsoft
   72 Eagle Rock Avenue
   First Floor
   East Hanover, NJ  07936

   EMail: jdrosen@dynamicsoft.com


   Henning Schulzrinne
   Columbia University
   M/S 0401
   1214 Amsterdam Ave.
   New York, NY  10027-7003

   EMail: schulzrinne@cs.columbia.edu


   Christian Huitema
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052-6399

   EMail: huitema@microsoft.com


   David Gurle
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052-6399

   EMail: dgurle@microsoft.com




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18. Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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