SIMPLE WG                                                    C. Jennings
Internet-Draft                                                   R. Mahy
Expires: January 14, 2005                            Cisco Systems, Inc.
                                                           July 16, 2004

      Relay Extensions for Message Sessions Relay Protocol (MSRP)

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
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   This Internet-Draft will expire on January 14, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.


   The SIMPLE Working Group uses two separate models for conveying
   instant messages.  Pager-mode messages stand alone, whereas
   Session-mode messages are setup as part of a session using the SIP
   protocol.  MSRP (Message Sessions Relay Protocol) is a protocol for
   near-real-time, peer-to-peer exchange of binary content without
   intermediaries, which is designed to be signaled using a separate
   rendezvous protocol such as SIP.  This document introduces the notion
   of message relay intermediaries to MSRP and describes the extensions
   necessary to use them.

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Table of Contents

   1.  Conventions and Definitions  . . . . . . . . . . . . . . . . .  3
   2.  Introduction and Requirements  . . . . . . . . . . . . . . . .  3
   3.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Overview of New Protocol Elements  . . . . . . . . . . . . . .  9
     4.1   The AUTH Method  . . . . . . . . . . . . . . . . . . . . .  9
     4.2   The Use-Path header  . . . . . . . . . . . . . . . . . . . 10
     4.3   Authentication headers . . . . . . . . . . . . . . . . . . 10
     4.4   Time-related headers . . . . . . . . . . . . . . . . . . . 10
   5.  Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     5.1   Client behavior  . . . . . . . . . . . . . . . . . . . . . 11
       5.1.1   Connecting to relays acting on your behalf . . . . . . 11
       5.1.2   Sending requests . . . . . . . . . . . . . . . . . . . 13
       5.1.3   Receiving Requests . . . . . . . . . . . . . . . . . . 14
       5.1.4   Managing Connections . . . . . . . . . . . . . . . . . 14
     5.2   Relay behavior . . . . . . . . . . . . . . . . . . . . . . 14
       5.2.1   Handling Incoming Connections  . . . . . . . . . . . . 14
       5.2.2   Generic request behavior . . . . . . . . . . . . . . . 14
       5.2.3   Receiving AUTH requests  . . . . . . . . . . . . . . . 14
       5.2.4   Forwarding SEND requests . . . . . . . . . . . . . . . 16
       5.2.5   Forwarding non-SEND requests . . . . . . . . . . . . . 17
       5.2.6   Forwarding Responses . . . . . . . . . . . . . . . . . 17
       5.2.7   Managing Connections . . . . . . . . . . . . . . . . . 18
       5.2.8   Forwarding unknown requests  . . . . . . . . . . . . . 18
   6.  Formal Syntax  . . . . . . . . . . . . . . . . . . . . . . . . 18
   7.  Finding MSRP Servers . . . . . . . . . . . . . . . . . . . . . 19
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
     8.1   Using HTTP Authentication  . . . . . . . . . . . . . . . . 20
     8.2   Using TLS  . . . . . . . . . . . . . . . . . . . . . . . . 21
     8.3   Threat Model . . . . . . . . . . . . . . . . . . . . . . . 21
     8.4   Security Mechanism . . . . . . . . . . . . . . . . . . . . 22
     8.5   Preventing Spam and Denial of Service Attacks  . . . . . . 23
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23
   10.   Example SDP with multiple hops . . . . . . . . . . . . . . . 23
   11.   Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . 24
   12.   References . . . . . . . . . . . . . . . . . . . . . . . . . 24
   12.1  Normative References . . . . . . . . . . . . . . . . . . . . 24
   12.2  Informative References . . . . . . . . . . . . . . . . . . . 25
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 26
       Intellectual Property and Copyright Statements . . . . . . . . 27

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1.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC-2119 [1].

   Below we list several definitions important to MSRP:
   MSRP node: a host that implements the MSRP protocols as a Client or a
   MSRP Client: an MSRP role which is the initial sender or final target
      of messages and delivery status.
   MSRP Relay: an MSRP role which forwards messages and delivery status
      and may provide policy enforcement.  Relays can fragment and
      reassemble portions of messages.
   Message: arbitrary MIME content which one client wishes to send to
      another.  For the purposes of this specification, a complete MIME
      body as opposed to a portion of a complete message.
   chunk: a portion of a complete message delivered in a SEND request.
   end-to-end: delivery of data from the initiating client to the final
      target client
   hop: delivery of data between one MSRP node and an adjacent node.

2.  Introduction and Requirements

   The IETF SIMPLE Working Group has identified a number of scenarios
   where using a separate protocol for bulk messaging is desirable.  In
   particular, the SIMPLE WG will use this facility to handle a sequence
   of messages as a session of media initiated using SIP [2], just like
   any other media type.  (The benefits of the session-mode approach are
   further discussed in [19].) The SIMPLE Working Group has also
   developed MSRP (the Message Sessions Relay Protocol) to convey
   sessions of messages directly between two end systems with no
   intermediaries.  With MSRP, messages can be arbitrarily large and all
   traffic is sent over reliable, congestion-safe transports.

   This document describes extensions to the core MSRP protocol to
   introduce intermediaries called Relays.  With these extensions MSRP
   clients can communicate directly, or through an arbitrary number of
   relays.  Each client is responsible for identifying any relays acting
   on its behalf and providing appropriate credentials.  Clients which
   can receive new TCP connections directly do not have to implement any
   new functionality to work with these relays.

   The Goals of the MSRP Relay extensions are listed below:
   o  convey arbitrary binary MIME data without modification or transfer
   o  continue to support client to client operation (no relay servers

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   o  operate through an arbitrary number of relays for policy
   o  allow each client to control which relays are traversed on its
   o  prevent unsolicited messages (spam), "open relays", and denial of
      service amplification
   o  allow relays to use one or a small number of TCP or TLS [3]
      connections to carry messages for multiple sessions, recipients,
      and senders
   o  allow large messages to be sent over a slow connection without
      causing head-of-line blocking problems
   o  allow transmission of a large message to be interrupted and
      resumed in place when network connectivity is lost and later
   o  offer notification of message failure at any intermediary
   o  provide notification of message storage (desirable)
   o  allow relays to delete state after a short amount of time

3.  Protocol Overview

   With the introduction of this extension, MSRP has the concept of both
   clients and relays.  Clients send messages to relays and/or other
   clients.  Relays forward messages and message delivery status to
   clients and other relays.  Clients which can open TCP connections to
   each other without intervening policy restrictions, can communicate
   directly with each other.  Clients who are behind a firewall or who
   need to use an intermediary for policy reasons can use the services
   of a relay.  Each client is responsible for enlisting the assistance
   of one or more relays for its half of the communication.

   Clients which use a relay operate by first opening a TLS connection
   with a relay, authenticating, and retreiving an msrps: URI (from the
   relay) that the client can provide to its peers to receive messages
   later.  When a client uses a relay, it first opens a TLS connection
   to its first relay and authenticates using an AUTH request which can
   contain HTTP Digest [4] Authentication credentials.  In a successful
   AUTH response, the relay provides an msrps: URI associated with the
   path back to the client that the client can give to other clients for
   end-to-end message delivery.

   When clients wish to send a short message, they issue a SEND request
   with the entire contents of the message.  If any relays are required,
   they are included in the To-Path header.  The leftmost URI in the
   To-Path header is the next hop to deliver a request or response.  The
   rightmost URI in the To-Path header is the final target.  (Note that
   MSRP does not permit line folding.  A "\" in the examples shows a
   line continuation due to limitations in line length of this document.
   Neither the backslash, nor the extra CRLF are included in the actual

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   request or response.)

    MSRP 6aef SEND
    To-Path: \ \
    Report-Success: yes
    Content-Type: text/plain

    Hi Bob, I'm about to send you LoTR.mpeg

    MSRP 6aef 200 OK

    MSRP juh76 SEND
    To-Path: \
    From-Path: \
    Report-Success: yes
    Content-Type: text/plain

    Hi Bob, I'm about to send you LoTR.mpeg

    MSRP juh76 200 OK

    MSRP xght6 SEND
    From-Path: \ \
    Report-Success: yes
    Content-Type: text/plain

    Hi Bob, I'm about to send you LoTR.mpeg

    MSRP xght6 200 OK

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    MSRP yh67 REPORT
    To-Path: \ \
    Status: 000 200 OK
    Report-Failure: no

    MSRP yh67 REPORT
    To-Path: \
    From-Path: \
    Status: 000 200 OK
    Report-Failure: no

    MSRP yh67 REPORT
    From-Path: \ \
    Status: 000 200 OK
    Report-Failure: no

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                       Typical flow involving two relays

   Alice                 Bob
     |                     |                    |                     |
     |::::::::::::::::::::>| connection opened  |<::::::::::::::::::::|
     |--- AUTH ----------->|                    |<-- AUTH ------------|
     |<-- 401 Auth---------|                    |--- 401 Auth-------->|
     |--- AUTH ----------->|                    |<-- AUTH ------------|
     |<-- 200 OK-----------|                    |--- 200 OK---------->|
     |                     |                    |                     |
           ....                time passes           ....
     |                     |                    |                     |
     |--- SEND ----------->|                    |                     |
     |<-- 200 OK ----------|:::::::::::::::::::>|  (slow link)        |
     |                     |--- SEND ---------->|                     |
     |                     |<-- 200 OK ---------|--- SEND ----------->|
     |                     |                    |                ....>|
     |                     |                    |                  ..>|
     |                     |                    |<-- 200 OK ----------|
     |                     |                    |<-- REPORT ----------|
     |                     |<-- REPORT ---------|                     |
     |<-- REPORT ----------|                    |                     |
     |                     |                    |                     |

   SEND requests are sent hop-by-hop.  (Each relay that receives a SEND
   request acknowledges receipt of the request before forwarding the
   content in other SEND requests.) All other requests are sent

   With the introduction of relays, the subtle semantics of the To-Path
   and From-Path header becomes more relevant.  The To-Path in both
   requests and responses is the list of URIs that need to be visited in
   order to reach the final target of the request.  The From-Path is the
   list of URIs that indicate how to get back to the original sender of
   the request or response .  This differs from the To and From headers
   in SIP, which do not "swap" from request to response.  (Note that
   sometimes a request is sent to or from an intermediary directly.)

   When a relay forwards a request, it removes its address from the
   To-Path header and inserts it at as the first URI in the From-Path
   header.  For example if the path from Alice to Bob is through relays
   A and B, when B receives the request it contains path headers that
   look like this:

   To-Path: msrps:B msrps:Bob
   From-Path: msrps:A msrps:Alice

   after forwarding the request, the path headers look like this:

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   To-Path: msrps:Bob
   From-Path: msrps:B msrps:A msrps:Alice

   When the Report-Failure header is set appropriately, MSRP Nodes
   respond to SEND requests by taking the last (rightmost) URI in the
   From-Path and placing that in a To-Path header in the response, and
   placing their URI in the From-Path of the response.  Likwise, when
   the Report-Failure header is set appropriately, MSRP Nodes respond to
   all other requests addressed to them by swapping the To-Path and
   From-Path headers and reversing the order of the resulting To-Path

   When sending large content the client may split up a messsage into
   smaller pieces; each SEND request might contain only a portion of the
   complete message.  For example, when Alice sends Bob a 4GB file
   called "LoTR.mpeg", she sends several SEND requests each with a
   portion of the complete message.  Relays can repack message fragments
   en-route.  As individual parts of the complete message arrive at the
   final destination client, the receiving client can optionally send
   REPORT requests indicating delivery status.

   MSRP nodes can send individual portions of a complete message in
   multiple SEND requests.  As relays receive chunks they can reassemble
   or re-fragment them as long as they resend the resulting chunks in
   order.  (Receivers still need to be prepared to receive out-of-order
   chunks however).  If the sender set the Report-Success header to yes,
   once a chunk or complete message arrives at the destination client,
   the destination sends a REPORT request indicating that a chunk
   arrived end-to-end.  This request travels back along the reverse path
   of the SEND request.  Unlike the SEND request which is acknowledged
   along every hop, REPORT responses are never acknowledged.

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                  Flow involving re-chunking through two relays

   Alice                 Bob
     |                     |                    |                     |
     |                     |                    |                     |
     |--- AUTH ----------->|                    |<-- AUTH ------------|
     |<-- 401 Auth---------|                    |--- 401 Auth-------->|
     |--- AUTH ----------->|                    |<-- AUTH ------------|
     |<-- 200 OK-----------|                    |--- 200 OK---------->|
     |                     |                    |                     |
           ....                time passes           ....
     |                     |                    |                     |
     |--- SEND 0-3 ------->|                    |                     |
     |<-- 200 OK ----------|                    |  (slow link)        |
     |--- SEND 4-7 ------->|--- SEND 0-5 ------>|                     |
     |<-- 200 OK ----------|<-- 200 OK ---------|--- SEND 0-3 ------->|
     |--- SEND 8-10 ------>|--- SEND 6-10 ----->|                ....>|
     |<-- 200 OK ----------|<-- 200 OK ---------|                  ..>|
     |                     |                    |<-- 200 OK ----------|
     |                     |                    |<-- REPORT 0-3 ------|
     |                     |<-- REPORT 0-3 -----|--- SEND 4-7 ------->|
     |<-- REPORT 0-3 ------|                    |                 ...>|
     |                     |                    |<-- REPORT 4-7 ----->|
     |                     |<-- REPORT 4-7 -----|--- SEND 8-10 ------>|
     |<-- REPORT 4-7 ------|                    |                  ..>|
     |                     |                    |<-- 200 OK ----------|
     |                     |<-- REPORT done-----|<-- REPORT done -----|
     |<-- REPORT done -----|                    |                     |
     |                     |                    |                     |

   Relays only keep transaction state for a short period of time for
   each chunk.  Delivery over each hop should take no more than 32
   seconds after the last byte of data is sent.  Clients applications
   define their own implementation-dependent timers for end-to-end
   message delivery.

   For client to client communication, the sender of a message typically
   opens a new TCP connection (with or without TLS) if one is needed.
   Relays reuse existing connections first, but can open new connections
   (typically to another relay) to deliver requests such as SEND or
   REPORT.  Responses can only be sent over existing connections.

4.  Overview of New Protocol Elements

4.1  The AUTH Method

   AUTH requests are used by clients with ephemeral addresses to create
   a handle they can use to receive incoming requests.  AUTH requests

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   can also contain credentials used to authenticate a client, and
   authorization policy used to block Denial of Service attacks.  AUTH
   requests are discussed in more detail in a later section.

   In response to an AUTH request, a successful response contains a
   Use-Path header with a list of URIs that the Client can give to its
   peers to route responses back to the Client.

4.2  The Use-Path header

   The Use-Path header is a list of URIs provided by an MSRP Relay in
   response to a successful AUTH request.  This list of URIs can be used
   by the MSRP Client that sent the AUTH request to receive MSRP
   requests, and to advertise this list of URIs, for example in a
   session description.

4.3  Authentication headers

   The Authentication-Info header provides optional information for HTTP
   Digest authentication.  This header MAY be included in the response
   to an AUTH request.  Semantics of the header are described in RFC

   The Authorization header contains authentication credentials for HTTP
   Digest authentication in an AUTH request.  Section [x.y] .  Note that
   the parameters of this header are separated by commas instead of
   semicolons.  The presence of commas in this header does not imply
   that there is more than one header field value for this header field
   (only one header field value is allowed).  Semantics of the header
   are described in RFC 2617.  This header MUST NOT appear in any parcel
   other than an AUTH request.

   The WWW-Authenticate header contains an HTTP Digest challenge carried
   in a 401 Response.

   NOTE - only auth, not auth-int is needed because TLS provides

4.4  Time-related headers

   The Expires header in a provides a relative time after which the
   action implied by the method of the request is no longer of interest.
   In a request, the Expires header indicates how long the sender would
   like the request to remain valid.  In a response, the Expires header
   indicates how long the responder considers this information relevant.
   Specifically an Expires header in an AUTH request indicates how long
   the provided URIs will be valid.

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   The Min-Expires header contains the minimum duration a server will
   permit in an Expires header.  It is sent only in 423 "Interval
   Out-of-Bounds" responses.  Likewise the Max-Expires header contains
   the maximum duration a server will permit in an Expires header.

   423 Interval Out-of-bounds.  Max-Expires header

5.  Procedures

5.1  Client behavior

5.1.1  Connecting to relays acting on your behalf

   Clients which want to use the services of a relay or list of relays,
   need to send an AUTH request to each relay which will act on their
   behalf.  (For example, some organizations could deploy an "intra-org"
   relay and an "extra-org" relay.)

   Clients can be configured (typically through discovery or manual
   provisioning) with a list of relays they need to use.  They MUST be
   able to form a connection to each relay and send an AUTH command to
   get a URI that can be used in route headers.  The client can
   authenticate its first relay by looking at the relay's TLS
   certificate.  Each relay MUST authenticate the client using digest

   The relay will return a URI, or list of URIs, in the Use-Path header
   of the response.  These URIs are used by the client in the path
   attribute that is sent in the SDP to setup the session.  The same URI
   can be used for multiple sessions to send to the client.

   When sending an AUTH request, the client MAY add an Expires header to
   request a MSRP URI that is valid for no longer that the provided
   interval.  If an AUTH request returns a 401 Unauthorized request, the
   client SHOULD fetch the Digest challenge from the WWW-Authenticate
   header in the response and retry the AUTH request, including an
   Authorization header with the Digest response.  Unlike in HTTP and
   SIP, Digest authentication in MSRP is only permitted for AUTH

   When a client wishes to use more than one relay, they must AUTH to
   each relay they wish to use.  Consider a client A, that whishes
   messages to flow from A to the first relays, R1, then on to a second
   relays, R2.  This client with do a normal AUTH with R1.  It will then
   do an AUTH transaction with R2 that is routed through R1.  The client
   will form this AUTH messages by setting the To-Path to msrps:R1
   msrps:R2.  R1 will forward this (just like a REPORT request) on to

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   (Alice opens a TLS connection to

    MSRP 676sd AUTH

    MSRP 676sd 401 Authenticate

    MSRP 49fh AUTH

    MSRP 49fh 200 OK

   (Alice now sends an AUTH request to her "external" relay
    through her "internal" relay, using the URI she just obtained)

    MSRP quiyd2 AUTH
    To-Path: \

    MSRP quiyd2 AUTH
    From-Path: \

    MSRP quiyd2 401 Authenticate
    To-Path: \

    MSRP quiyd2 401 Authenticate

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    From-Path: \

    MSRP mnbvw AUTH
    To-Path: \

    MSRP mnbvw AUTH
    From-Path: \

    MSRP mnbvw 200 OK
    To-Path: \
    Use-Path: \

    MSRP mnbvw 200 OK
    From-Path: \
    Use-Path: \

5.1.2  Sending requests

   The procedure for forming SEND and REPORT requests is identical for
   clients whether relays are involved or not.  The specific procedures
   are described in section 6 of the core MSRP protocol.

   As usual, once the next-hop URI is determined, the client MUST find
   the appropriate address, port, and transport to use and then check if
   there is already an existing suitable connection to the next-hop
   target.  If so, the client MUST send the request over the most

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   suitable connection.  Suitability MAY be determined by a variety of
   factors such as measured load and local policy, however in most
   simple implementations a connection will be suitable if it exists and
   is in an active state.

5.1.3  Receiving Requests

   The procedure for receiving requests is identical for clients whether
   relays are involved or not.

5.1.4  Managing Connections

   Clients should open connection whenever they wish to deliver a
   request and no suitable connection exists.  For client to client
   connections, a client should close a connection when there are no
   longer any sessions associated with the connection.  For connections
   to relays, the client should leave a connection up until no sessions
   are using the connection for a locally defined period of time, which
   defaults to 5 minutes for foreign relays and one hour for the
   client's relays.

5.2  Relay behavior

5.2.1  Handling Incoming Connections

5.2.2  Generic request behavior

   Upon receiving a new request, relays first verify the validity of the
   request.  Relays then examine the first URI in the To-Path header and
   remove this URI if it matches a URI corresponding to the relay.  The
   relay performs Authorization to determine if the final target is a
   URI under its control or from a URI under its control.

5.2.3  Receiving AUTH requests

   When a relay receives an AUTH request, it must digest challenge the
   request.  Once the challenge is complete, it MUST provide a URI that
   can be used in future route headers.  When the route URI is received
   in future messages.  It MUST verify that this URI was issued by this
   relay.  It MUST ensure that the message is either being forwarded
   from an entity that did the AUTH request that resulted in this URI or
   it is being forwarded to the the entity that did the AUTH request
   that resulted in this URI.

   If there are additional URIs in the To-Path header, the relay
   attempts to forward the AUTH request to the remaining relays.  The
   relay MUST verify that the sender is authorized to route through this

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   relay to the target.

   The relay does not necessarily needs to save state to meet these
   requirements.  One way that a relay could implement this is the
   following.  When an AUTH request arrives, the relay concatenates the
   current time, the identity of the sender of the AUTH request, the
   identity of the previous hop the request came from.  It then takes
   the concatenates string and encrypts it with a key only the relay
   knows and uses this for form the user portion of the sims URI that it
   returns.  Later when it receives a URI, it can decrypt this
   information and use it to decide if the request should be forwarded
   or not.  If the relay is actually several servers that share a DNS
   name, the URI may also encrypt which server actually has the
   connection to the client.

   When a relay receive an AUTH request, it must authenticate the client
   that sent it with digest, it must also authenticate the previous hop
   that send the message to it.  When previous hop was a relay this is
   done with the mutual TLS while when the previous hop was a client
   mutual TLS MAY be used it is available or the client authorization
   from the digest is used.  The relay will generate the base URI of a
   family of URIs, each of which allows messages to be forwarded to and
   from this client.  If the previous hop was authenticated by mutual
   TLS, then the URI MUST be valid to route across any connection the
   relay has to the previous hop relay.  If the previous hop was not
   authenticated by mutual TLS, then the URI MUST only be valid to route
   across the same connection that the AUTH was received on.  If this
   connection is closed then reopened, the URI MUST NOT be valid.  Valid
   to route means that when the relay receives a messages that contains
   this URI, if the message it going to element that was the previous
   hop in the AUTH, then the relay can forward it and if the messages is
   coming from previous hop in the AUTH, then the relay can forward it
   to any location, otherwise the RELAY must discard the message and MAY
   send a REPORT indicating the auth URI was bad.  If the AUTH request
   contains an Expires header, then the relay MUST ensure that the URI
   is not valid to route after the expiry time.

   [*** NOTE: Consider moving to another section ***]

   It is possible to implement all of the above requirements without the
   relay saving any state.  When a relay starts up it could pick a
   crypto random 128 bit password (K) and 128 bit initialization vector
   (IV).  If the relay was actually a NDS farm, all the machines in the
   farm would need to share the same K.  When an ATUH request was
   received the relay form a string that contains: the expiry time of
   the URI, an indication if the previous hop was mutual TLS
   authenticated or not and it it was, the name of the previous hop, if
   it was not the identifier for the connection which received the AUTH

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   request.  This string would be padded by appending a byte with the
   value 0x80 then adding zero or more bytes with the value of 0x00
   until the string length is a multiple of 16 bytes long.  A new random
   IV vector would be selected (it needs to change because it forms the
   salt) and the padded string would be encrypted using AES-CBC with a
   key of K.  The IV and encrypted data and an SPI (security parameter
   index) that changed each time K changed would be base 64 encoded and
   form the user portion of the request URI.  The SPI allows the key to
   be changed and for the system to know which K should be used.  Later
   when the relay received this URI, it could decrypt it and check the
   current time was before the expiry time and check that the messages
   was coming from or going to the connection or location specified in
   the URI.  Integrity protection is not required because it is
   extremely unlikely that random data that was decrypted would result
   in a valid location that was the same as the messages was routing to
   or from.  When implementing something like this, implementers should
   be careful not to use a scheme like EBE that would allows portion of
   encrypted tokens to be cut and paste into others.

   Note: A successful AUTH response returns a Route header which
   contains a base MSRP URI that the client can use to create a number
   of different URIs which are all associated with the current

5.2.4  Forwarding SEND requests

   If an incoming SEND request Report-Success header is "yes", a MSRP
   relay that receives that SEND request MUST respond with a final
   response immediately.  A 200-class response indicates the successful
   receipt of a message fragment, but does not mean that the message has
   been forwarded on to its next hop.  The final response to the SEND
   MUST be sent to the previous hop, which could be a MSRP relay or the
   original sender of the SEND request.

   If there is a problem further processing the SEND request and the
   Report-Sucess header is "yes" or "partial", the relay MUST respond
   with an appropriate error response back to the previous hop.

   [Add more reporting requirements here].

   The MSRP relay MAY further break up the message fragment received in
   the SEND request into smaller fragments and forward them to the next
   hop in separate SEND requests.  It MAY also combine message fragments
   received before or after this SEND request, and forward them out in a
   single SEND request to the next hop identified in the Hops header.
   The MSRP relay MUST NOT combine message fragments from SEND requests
   with different values in the Message-ID header.

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   The MSRP relay MAY choose whether to further fragment the message, or
   combine message fragments, or send the message as is, based on some
   policy which is administered, or based on the network speed to the
   next hop, or any other mechanism.

   If the MSRP relay has knowledge of the byte range that it will
   transmit to the next hop, it SHOULD update the Byte-Range header in
   the SEND request appropriately.

   Before forwarding the SEND request to the next hop, the MSRP relay
   MUST inspect the first URI in the To-Path header.  If it indicates
   this relay, the relay removes this URI from the To-Path header and
   inserts this URI in the From-Path header before any other URIs.

5.2.5  Forwarding non-SEND requests

   An MSRP relay that receives any request other than a SEND request
   (including new methods unknown to the relay), first follows the
   validation and authorization rules for all requests in Section x.y.
   Then the relay moves its URI from the beginning of the To-Path
   header, to the beginning of the From-Path header and forwards the
   request on to the next hop.  It MUST use the most suitable conection,
   etc, etc..  If no suitable connection exists, the relay opens a new

5.2.6  Forwarding Responses

   Relays receiving a response, first check the transaction of the
   response.  If the response is a positive response, and the relay is
   unaware of this transaction, the response MUST be dropped.  Likewise
   if the message is unparsable, the relay MUST drop the response.  If
   the response matches an existing transaction, the transaction state
   MUST be deleted.  The relay MUST verify that the first URI in the
   To-Path corresponds to it.  If not, the response SHOULD be dropped.
   If there are additional URIs in the To-Path header, the relay can
   then move its URI from the list To-Path header, insert its URI in
   front of any other URIs in the From-Path header, and forward the
   response to the next URI in the To-Path header.  The relay sends the
   request over the best connection which corresponds to the next URI in
   the To-Path header.  If this connection has closed, then the response
   is silently discarded.

   [Need to add text on failure report handling here]  If the response
   is a negative response, the relay may need to send a REPORT
   indicating the nature of the failure.

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5.2.7  Managing Connections

   Relays should keep connection open as long as possible.  If a
   connection has not been used in a significant time (many minutes) it
   could be closed.  If the relay runs out of resource and must close
   connections, it should first stop accepting new connections from
   clients then start closing connections on a least recently used

5.2.8  Forwarding unknown requests

   Requests with an unknown method are forwarded as if they were REPORT

6.  Formal Syntax

   The following syntax specification uses the augmented Backus-Naur
   Form (BNF) as described in RFC-2234 [5].

   AUTHm           = %x41.55.54.48           ; AUTH in caps
   Method          = SENDm / REPORTm / AUTHm
                        / ext-method

   Authentication-Info  =  "Authentication-Info" HCOLON ainfo
                           *(COMMA ainfo)
   ainfo                =  nextnonce / message-qop
                            / response-auth / cnonce
                            / nonce-count
   nextnonce            =  "nextnonce" EQUAL nonce-value
   response-auth        =  "rspauth" EQUAL response-digest
   response-digest      =  LDQUOT *LHEX RDQUOT

   Authorization     =  "Authorization" HCOLON credentials
   credentials       =  ("Digest" LWS digest-response)
                        / other-response
   digest-response   =  dig-resp *(COMMA dig-resp)
   dig-resp          =  username / realm / nonce / digest-uri
                         / dresponse / algorithm / cnonce
                         / opaque / message-qop
                         / nonce-count / auth-param
   username          =  "username" EQUAL username-value
   username-value    =  quoted-string
   digest-uri        =  "uri" EQUAL LDQUOT digest-uri-value RDQUOT
   digest-uri-value  =  rquest-uri ; Equal to request-uri as specified
                        by HTTP/1.1
   message-qop       =  "qop" EQUAL qop-value

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   cnonce            =  "cnonce" EQUAL cnonce-value
   cnonce-value      =  nonce-value
   nonce-count       =  "nc" EQUAL nc-value
   nc-value          =  8LHEX
   dresponse         =  "response" EQUAL request-digest
   request-digest    =  LDQUOT 32LHEX RDQUOT
   auth-param        =  auth-param-name EQUAL
                        ( token / quoted-string )
   auth-param-name   =  token
   other-response    =  auth-scheme LWS auth-param
                        *(COMMA auth-param)
   auth-scheme       =  token
   LHEX              =  DIGIT / %x61-66 ;lowercase a-f
   ;   Some elements (authentication) force hex alphas to be lower case.

   WWW-Authenticate  =  "WWW-Authenticate" HCOLON challenge
   challenge           =  ("Digest" LWS digest-cln *(COMMA digest-cln))
                          / other-challenge
   other-challenge     =  auth-scheme LWS auth-param
                          *(COMMA auth-param)
   digest-cln          =  realm / domain / nonce
                           / opaque / stale / algorithm
                           / qop-options / auth-param
   realm               =  "realm" EQUAL realm-value
   realm-value         =  quoted-string
   domain              =  "domain" EQUAL LDQUOT URI
                          *( 1*SP URI ) RDQUOT
   URI                 =  MSRP-URI / anyURI
   nonce               =  "nonce" EQUAL nonce-value
   nonce-value         =  quoted-string
   opaque              =  "opaque" EQUAL quoted-string
   stale               =  "stale" EQUAL ( "true" / "false" )
   algorithm           =  "algorithm" EQUAL ( "MD5" / "MD5-sess"
                          / token )
   qop-options         =  "qop" EQUAL LDQUOT qop-value
                          *("," qop-value) RDQUOT
   qop-value           =  "auth" / token

   Expires  = "Expires" ":" SP 1*DIGIT

   Use-Path = "Use-Path" ":" SP URI *(SP URI)

7.  Finding MSRP Servers

   When resolving a an MSRP URI which contains an explicit port number,
   an MSRP node follows the rules in section 4.4 of MSRP.  MSRP URIs

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   exchanged in SDP and in To-Path and From-Path headers in non-AUTH
   requests MUST have an explicit port number.  The following rules
   allow MSRP clients to discover MSRP relays more easily in AUTH

   If the hostport of an msrps: URI is an IPv4 address or an IPv6
   reference and no port number is provided, use the default port number
   assigned by IANA.  If the hostport is a domain name and an explicit
   port number is provided, attempt to lookup a valid address record (A
   or AAAA) for the domain name.  Connect using the TLS over the default
   transport (TCP) with the default port number.

   If a domain name is provided, but no port number, perform a DNS SRV
   [6] lookup for and select the entry with the highest weight.  If no
   SRV records are found, try an address lookup (A or AAAA) using the
   default port number procedures described in the previous paragraph.
   Note that AUTH requests MUST only be sent over a TLS-protected
   channel.  An SRV lookup in the domain might return:

   ;; in      Pri Wght Port Target
   _msrps._tcp   IN SRV 0   1    9000
   _msrps._tcp   IN SRV 0   2    9000

   If implementing a relay farm, it is RECOMMENDED that each member of
   the relay farm have an SRV entry.  If any members of the farm have
   multiple IP addresses (for example an IPv4 and an IPv6 address), each
   of these addresses SHOULD be registered in DNS as separate A, AAAA,
   or A6 records corresponding to a single target.

8.  Security Considerations

   This section first describes the security mechanisms available for
   use in MSRP.  Then the threat model is presented.  Finally we list
   implementation requirements related to security.

8.1  Using HTTP Authentication

   AUTH requests SHOULD be authenticated using HTTP authentication.
   HTTP authentication is done as described in [RFC 2617], with the
   following exceptions.  Basic authentication MUST NOT be used.  A qop
   value of auth-int MUST NOT be used as the AUTH requests are integrity
   protected by TLS and there is no body to protect.  Note that unlike
   in some usages of HTTP Authentication (for example, SIP), the uri
   parameter in the Authorize header is the same as the Request-URI in
   the request line of the MSRP parcel of the AUTH request.  Note the
   BNF in RFC-2617 has an error--the value of the uri parameter MUST be
   in quotes.  The BNF in this document is correct, as are the examples
   in RFC 2617.

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8.2  Using TLS

   TLS is used to authenticate relays to senders and to provide
   integrity and confidentiality for the headers being transported.
   MSRP client and relays MUST support TLS.  Clients and relays MUST
   support the TLS ClientExtendedHello extended hello information for
   server name indication as described in RFC 3546 [7].  A TLS
   cipher-suite of TLS_RSA_WITH_AES_128_CBC_SHA [8] MUST be supported
   (other cipher-suites MAY also be suported).  Relays must act as TLS
   servers and present a certificate with their identity in the
   SubjectAltName using the choice type of dnsName.  Relay to relay
   connections MUST use TLS and client to relay communications MUST use
   TLS for AUTH requests and responses.
      Note that when relays are involved in a session, TCP without TLS
      is only used when a relay of one user connects directly to an MSRP
      endpoint of another user who does not use relays.

8.3  Threat Model

   This section discuses the threat model and the broad mechanism that
   must come into place to secure the protocol.  The next section
   describes the details of how the protocol mechanism meet the broad

   MSRP allows two peer to peer clients to exchange messages.  Each peer
   can select a set of relays to perform certain policy operation for
   them.  This combined set of relays is referred to as the route set.
   There often exists a channel outside of MSRP, such as out-of-band
   provisioning or an explicit rendezvous protocol such as SIP, that can
   securely negotiate setting up the MSRP session and communicate the
   route set to both clients.  A client may trust a relay with certain
   types of routing and policy decisions but it might or might not trust
   the relay with all the contents of the session.  For example, a relay
   being trusted to look for viruses would probably need to be allowed
   to see all the contents of the session.  A relay that helped deal
   with firewall traversal of the ISPs firewall would likely not be
   trusted with the contents of the session but would be trusted to
   correctly forward information.

   Clients need to be able to authenticate that the relay they are
   communicating with is the one they trust.  Likewise, relays need to
   be able to authenticate the client is the authorized client for them
   to forward information to.  Clients need the option of ensuring
   information between the relay and the client is integrity protected
   and confidential to elements other than the relays and clients.  To
   simplify the number of options, traffic between relays must always be
   integrity protected and encrypted regardless of if the client request
   it or not.  There is no way for the clients to tell the relays what

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   strength of crypto to use between relays other than the clients to
   choose to use relays that are operated by people requiring an
   adequate level of security.

   The system also need to stop the messages from being directed to
   relays that are not supposed to see them.  To keep the relays from
   being used in DDoS attacks, the relays must not forward messages
   unless they have a trust relationship with either the client sending
   or receiving the message and that they only forward that message if
   it is coming from or going to the client they have the trust
   relationship with.  If a relay has a trust relationship with the
   client that is the destination of the message, it should not send the
   message anywhere except the client that is the destination.

   Some terminology used in this discussion is SClient is the client
   sending a message and RClient is the client receiving a message.
   SRelay is a relay the sender trusts and RRelay is a relay the
   receiver trusts.  The message will go from SClient to SRelay1 to
   SRelay2 to RRelay2 to RRelay1 to RClient.

8.4  Security Mechanism

   Confidentiality and Privacy from elements not in the route set is
   provided by using TLS on all the transports.  If a client decided to
   not use TLS that is it's choice but relays must use TLS.  Clients
   must implement TLS.

   The relays authenticate to the clients using TLS (but don't have to
   do mutual TLS).  The clients authenticate to the relays using HTTP
   Digest inside of TLS.  Relays authenticate to each other using mutual

   The clients can protect the contents so that the relays can not see
   them by using S/MIME encryption.  End to end signing is also possible
   with S/MIME.

   The complex part is making sure that relays do not send messages
   place where they should not.  This is done by having the client
   authenticate to the relay and having the relay return a token.
   Messages that contain this token can be relayed if they come from the
   client that got the token or if they are being forwarded towards the
   client that got the token.  The tokens must only ever be seen by
   things in the route set or other elements that at least one of the
   parties trusts.  If some 3rd party discovers the token that RRelay2
   uses to forward messages to RClient, then that 3rd party can send as
   many messages as they want to RRelay2 and it will forward them to
   RClient.  The 3rd party can not cause them to be forwarded anywhere
   except to RClient eliminating the open relay problems.  SRelay1 will

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   not forward the message unless it contains a valid token.

   When SClient goes to get a token from SRelay2, this request is
   relayed through SRelay1.  SRelay authenticates that it really is
   SClient requesting the token but it generates a token that is only
   valid for forwarding messages to or from SRelay1.  SRelay two knows
   it is connected to SRelay1 because of the mutual TLS.

   The tokens are carried in the user portion of the MSRP URLs.

   Issues: How to tokens expire - rekeying.  Will probably use Expire
   header on AUTH response.  Token MAY be valid for between 10 minutes
   and 24 hours with 1 hour recommended.  Both sides need to do a SIP
   re-invite to set up new tokens before the old one expires.

   Issues: Token good for single session or for all session

   Note: tokens are only required for relays, not clients or note

   TODO talk about example from client to client and from Client A, then
   to a relay that A uses, RA, then on to client B.

8.5  Preventing Spam and Denial of Service Attacks

   While this specification already implements a number of significant
   improvements to prevent unsolicited messaging and Denial of Service,
   additional mechanisms are envisioned being useful in the future.  The
   402 Payment Required and 409 Puzzle Required response codes are
   reserved for future use and may be useful to further discourage
   unsolicited messages.

9.  IANA Considerations

   This document introduces no requirements for IANA.

10.  Example SDP with multiple hops

   A sample SDP offer for a MSRP session could look like:

   c=IN IP4 invalid.none
    m=message 9 msrp *
    a=accept-types: message/cpim text/plain text/html;tcp

   In this offer Alice wishes to receive MSRP messages at
   She wants to use TCP as the transport for the MSRP session.  She can

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   accept message/cpim, text/plain and text/html message bodies in SEND
   requests.  She does not need a relay to setup the MSRP session.

   To this offer, Bob's answer could look like:

   c=IN IP4 invalid.none
    m=message 9 msrp *
    a=accept-types: message/cpim text/plain;tcp  \;tcp

   Here Bob wishes to receive the MSRP messages at
   He can accept only message/cpim and text/plain message bodies in SEND
   requests and has rejected text/html offer made by Alice.  He wishes
   to use a relays for the MSRP session -

11.  Acknowledgments

   Many thanks to the following members of the SIMPLE WG for spirited
   discussions on session mode:  Ben Campbell, Jonathan Rosenberg,
   Robert Sparks, Paul Kyzivat, Allison Mankin, Jon Peterson,  Brian
   Rosen, Dean Willis, Adam Roach, Aki Niemi, Hisham Khartabil, Juhee
   Garg, Pekka Pessi, and Chris Boulton

12.  References

12.1  Normative References

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

   [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]   Dierks, T., Allen, C., Treese, W., Karlton, P., Freier, A. and
         P. Kocher, "The TLS Protocol Version 1.0", RFC 2246, January

   [4]   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.

   [5]   Crocker, D. and P. Overell, "Augmented BNF for Syntax
         Specifications: ABNF", RFC 2234, November 1997.

   [6]   Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for

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         specifying the location of services (DNS SRV)", RFC 2782,
         February 2000.

   [7]   Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J. and
         T. Wright, "Transport Layer Security (TLS) Extensions", RFC
         3546, June 2003.

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

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

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

   [11]  Ramsdell, B., "S/MIME Version 3 Message Specification", RFC
         2633, June 1999.

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

   [13]  Braden, R., "Requirements for Internet Hosts - Application and
         Support", STD 3, RFC 1123, October 1989.

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

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

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

   [17]  Burger, E., Candell, E., Eliot, C. and G. Klyne, "Message
         Context for Internet Mail", RFC 3458, January 2003.

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

12.2  Informative References

   [19]  Mahy, R., "Benefits of Session-Mode Instant Messaging",

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         draft-mahy-simple-why-session-mode-00.txt (work in progress),
         February 2004.

   [20]  Campbell, B., "Instant Message Sessions in SIMPLE",
         draft-ietf-simple-message-sessions-02 (work in progress), Oct

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

   [22]  Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming
         Protocol (RTSP)", RFC 2326, April 1998.

   [23]  Levinson, E., "Content-ID and Message-ID Uniform Resource
         Locators", RFC 2392, August 1998.

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

   [25]  Resnick, P., "Internet Message Format", RFC 2822, April 2001.

   [26]  Mahy, R., "Relay Requirements for Session-Mode Instant
         Messaging", draft-mahy-simple-session-relay-reqs-00.txt (work
         in progress), February 2004.

Authors' Addresses

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

   Phone: +1 408 527-9132

   Rohan Mahy
   Cisco Systems, Inc.
   5617 Scotts Valley Drive, Suite 200
   Scotts Valley, CA  95066


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