


Network Working Group                                           A. Niemi
Internet-Draft
Intended status: Standards Track                        M. Garcia-Martin
Expires: July 15, 2013                                          Ericsson
                                                           G. Sandbakken
                                                           Cisco Systems
                                                        January 11, 2013


    Multi-party Chat Using the Message Session Relay Protocol (MSRP)
                       draft-ietf-simple-chat-18

Abstract

   The Message Session Relay Protocol (MSRP) defines a mechanism for
   sending instant messages within a peer-to-peer session, negotiated
   using the Session Initiation Protocol (SIP) and the Session
   Description Protocol (SDP).  This document defines the necessary
   tools for establishing multi-party chat sessions, or chat rooms,
   using MSRP.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 15, 2013.

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   Copyright (c) 2013 IETF Trust and the persons identified as the
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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect



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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
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   it for publication as an RFC or to translate it into languages other
   than English.



































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Motivations and Requirements . . . . . . . . . . . . . . . . .  6
   4.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Policy Attributes of the Chat Room . . . . . . . . . . . . 10
   5.  Creating, Joining, and Deleting a Chat Room  . . . . . . . . . 11
     5.1.  Creating a Chat Room . . . . . . . . . . . . . . . . . . . 11
     5.2.  Joining a Chat Room  . . . . . . . . . . . . . . . . . . . 12
     5.3.  Deleting a Chat Room . . . . . . . . . . . . . . . . . . . 14
   6.  Sending and Receiving Instant Messages . . . . . . . . . . . . 14
     6.1.  Regular Messages . . . . . . . . . . . . . . . . . . . . . 14
     6.2.  Private Messages . . . . . . . . . . . . . . . . . . . . . 17
     6.3.  MSRP reports and responses . . . . . . . . . . . . . . . . 19
     6.4.  Congestion Avoidance . . . . . . . . . . . . . . . . . . . 20
   7.  Nicknames  . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     7.1.  Using Nicknames within a Chat Room . . . . . . . . . . . . 22
     7.2.  Modifying a Nickname . . . . . . . . . . . . . . . . . . . 24
     7.3.  Removing a Nickname  . . . . . . . . . . . . . . . . . . . 24
     7.4.  Nicknames in Conference Event Packages . . . . . . . . . . 25
   8.  The SDP 'chatroom' attribute . . . . . . . . . . . . . . . . . 25
   9.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
     9.1.  Joining a chat room  . . . . . . . . . . . . . . . . . . . 27
     9.2.  Setting up a nickname  . . . . . . . . . . . . . . . . . . 29
     9.3.  Sending a regular message to the chat room . . . . . . . . 30
     9.4.  Sending a private message to a participant . . . . . . . . 32
     9.5.  Chunked private message  . . . . . . . . . . . . . . . . . 34
     9.6.  Nickname in a conference information document  . . . . . . 34
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 36
     10.1. New MSRP Method  . . . . . . . . . . . . . . . . . . . . . 36
     10.2. New MSRP Header  . . . . . . . . . . . . . . . . . . . . . 36
     10.3. New MSRP Status Codes  . . . . . . . . . . . . . . . . . . 36
     10.4. New SDP Attribute  . . . . . . . . . . . . . . . . . . . . 37
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 37
   12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 39
   13. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 39
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 40
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 40
     14.2. Informative References . . . . . . . . . . . . . . . . . . 41
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 42










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1.  Introduction

   The Message Session Relay Protocol (MSRP) [RFC4975] defines a
   mechanism for sending a series of instant messages within a session.
   The Session Initiation Protocol (SIP) [RFC3261] in combination with
   the Session Description Protocol (SDP) [RFC4566] allows for two peers
   to establish and manage such sessions.

   In another application of SIP, a user agent can join in a multi-party
   conversation called a conference that is hosted by a specialized user
   agent called a focus [RFC4353].  Such a conference can naturally
   involve MSRP sessions.  It is the responsibility of an entity
   handling the media to relay instant messages received from one
   participant to the rest of the participants in the conference.

   Several such systems already exist in the Internet.  Participants in
   a chat room can be identified with a pseudonym or nickname, and
   decide whether their real identifier is disclosed to other
   participants.  Participants can also use a rich set of features such
   as the ability to send private instant messages to other
   participants.

   Similar conferences supporting chat rooms are already available
   today.  For example, Internet Relay Chat (IRC) [RFC2810], Extensible
   Messaging and Presence Protocol (XMPP): Core [RFC6120] based chat
   rooms, and many other proprietary systems provide chat room
   functionality.  Specifying equivalent functionality for MSRP-based
   systems eases interworking between these systems.

   This document defines requirements, conventions, and extensions for
   providing private messages and nickname management in centralized
   chat rooms with MSRP.  Participants in a chat room can be identified
   by a pseudonym, and decide if their real identifier is disclosed to
   other participants.  This memo uses the SIP Conferencing Framework
   [RFC4353] as a design basis.  It also aims to be compatible with the
   A Framework for Centralized Conferencing [RFC5239].  Should
   requirements arise, future mechanisms for providing similar
   functionality in generic conferences might be developed, for example,
   where the media is not only restricted to MSRP.  The mechanisms
   described in this document provide a future compatible short-term
   solution for MSRP centralized chat rooms.


2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119, BCP 14



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   [RFC2119], and indicate requirement levels for compliant
   implementations.

   This memo deals with tightly coupled SIP conferences defined in SIP
   Conferencing Framework [RFC4353] and adopts the terminology from that
   document.  In addition to that terminology, we introduce some new
   terms:

   Nickname:   a pseudonym or descriptive name associated to a
      participant.  See Section 7 for details.

   Multi-party chat:   an instance of a tightly coupled conference, in
      which the media exchanged between the participants consist of MSRP
      based instant messages.  Also known as a chat room.

   Chat Room:   a synonym for a multi-party chat.

   Chat Room URI:   a URI that identifies a particular chat room, and is
      a synonym of a Conference URI defined in RFC 4353 [RFC4353].

   Sender:   the chat room participant who originally created an instant
      message and sent it to the chat room server for further delivery.

   Recipient:   the destination chat room participant(s).  This defaults
      to the full conference participant list minus the Instant Message
      (IM) Sender.

   MSRP switch:   a media level entity that is a MSRP endpoint.  It is a
      special MSRP endpoint that receives MSRP messages and delivers
      them to the other chat room participants.  The MSRP switch has a
      similar role to a conference mixer with the exception that the
      MSRP switch does not actually "mix" together different input media
      streams; it merely relays the messages between chat room
      participants.

   Private Instant Message:   an instant message sent in a chat room
      intended for a single participant.  Generally speaking, a private
      IM is seen by the MSRP switch, in addition to the sender and
      recipient.  A private IM is usually rendered distinctly from the
      rest of the IMs, indicating that the message was a private
      communication.

   Anonymous URI:  a URI concealing the participant's SIP AOR from the
      other participants in the chat room.  The allocation of such a URI
      is out of scope of this specification.  An anonymous URI must be
      valid for the length of the chat room session and will be utilized
      by the MSRP switch to forward messages to and from anonymous
      participants.  Privacy and anonymity are discussed in greater



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      detail in RFC 3323 [RFC3323] and RFC 3325 [RFC3325].

   Conference Event Package:  a notification mechanism that allows
      conference participants to learn conference information including
      roster and state changes in a conference.  This would typically be
      A Session Initiation Protocol (SIP) Event Package for Conference
      State [RFC4575] or Conference Event Package Data Format Extension
      for Centralized Conferencing [RFC6502].

   Identifier:  a string used to recognize or establish as being a
      particular user.

   To log in:  to enter identifying data, as a name or password, into a
      chat room, so as to be able to do work with the chat room.


3.  Motivations and Requirements

   Although conference frameworks describing many types of conferencing
   applications already exist, such as the Framework for Centralized
   Conferencing [RFC5239] and the SIP Conferencing Framework [RFC4353],
   the exact details of session-based instant messaging conferences
   (chat rooms) are not well-defined at the moment.

   To allow interoperable chat implementations, for both conference-
   aware, and conference-unaware user agents, certain conventions for
   MSRP chat rooms need to be defined.  It also seems beneficial to
   provide a set of features that enhance the baseline multi-party MSRP
   in order to be able to create systems that have functionality on par
   with existing chat systems, as well as enable building interworking
   gateways to these existing chat systems.

   We define the following requirements:

   REQ-1:  A basic requirement is the existence of a chat room, where
           participants can join and leave the chat room and get instant
           messages exchanged to the rest of the participants.

   REQ-2:  A recipient of an instant message in a chat room must be able
           to determine the identifier of the sender of the message.
           Note that the actual identifier depends on the one which was
           used by the sender when they joined the chat room.

   REQ-3:  A recipient of an instant message in a chat room must be able
           to determine the identifier of the recipient of received
           messages.  For instance, the recipient of the message might
           be the entire chat room or a single participant (i.e., a
           private message).  Note that the actual identifier may depend



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           on the one which was used by the recipient when he or she
           joined the chat room.

   REQ-4:  It must be possible to send a message to a single participant
           within the chat room (i.e., a private instant message).

   REQ-5:  A chat room participant may have a nickname or pseudonym
           associated with their real identifier.

   REQ-6:  It must be possible for a participant to change their
           nickname during the progress of the chat room session.

   REQ-7:  It must be possible that a participant is only known by an
           anonymous identifier and not their real identifier to the
           rest of the chat room.

   REQ-8:  It must be possible for chat room participants to learn the
           chat room capabilities described in this document.


4.  Overview of Operation

   Before a chat room can be entered, it must be created.  Users wishing
   to host a chat room themselves can of course do just that; their User
   Agent (UA) simply morphs from an ordinary UA into a special purpose
   one called a Focus UA.  Another, commonly used setup is one where a
   dedicated node in the network functions as a Focus UA.

   Each chat room has an identifier of its own: a SIP URI that
   participants use to join the chat room, e.g. by sending an INVITE
   request to it.  The conference focus processes the invitations, and
   as such, maintains SIP dialogs with each participant.  In a multi-
   party chat, or chat room, MSRP is one of the established media
   streams.  Each chat room participant establishes an MSRP session with
   the MSRP switch, which is a special purpose MSRP application.  The
   MSRP sessions can be relayed by one or more MSRP relays, which are
   specified in RFC 4976 [RFC4976].  This is illustrated in Figure 1.














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                               MSRP Sessions
                       +--------------------------+
                       |                          |
                   +---+--+    +---+--+           |
                   | SIP  |    | SIP  |           |
                   | MSRP |    | MSRP |     +-----+-----+
                   |Client|    |Client|     |   MSRP    |
                   +---+--+    ++--+--+     |   Relay   |
                       |        |   \       +-----+-----+
           SIP Dialogs |       /     +----+       |
                       |      |            \      | MSRP Sessions
                  +----+------+--+          |     |
                  |              |        +-+-----+-----+
                  |  Conference  |        |    MSRP     |
                  |  Focus UA    |........|    Switch   |
                  |              |        |             |
                  +----+-------+-+        +-+-----+-----+
                       |        \           |     |
           SIP Dialogs |        |    +------+     | MSRP Sessions
                       |         \  /             |
                   +---+--+    +-+--+-+     +-----+-----+
                   | SIP  |    | SIP  |     |   MSRP    |
                   | MSRP |    | MSRP |     |   Relay   |
                   |Client|    |Client|     +-----+-----+
                   +---+--+    +------+           |
                       |                          |
                       +--------------------------+
                               MSRP sessions


     Figure 1: Multi-party chat overview shown with MSRP Relays and a
                            conference Focus UA

   The MSRP switch is similar to a conference mixer in that it handles
   media sessions with each of the participants and bridges these
   streams together.  However, unlike a conference mixer, the MSRP
   switch merely forwards messages between participants but doesn't
   actually mix the streams in any way.  The system is illustrated in
   Figure 2.












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                                 +------+
                                 | MSRP |
                                 |Client|
               +------+          +--.---+          +------+
               | MSRP |             |              | MSRP |
               |Client|             |             _|Client|
               +------._            |           ,' +------+
                        `._         |         ,'
                           `.. +----------+ ,'
                              `|          |'
                               |   MSRP   |
                               |  Switch  |
                              ,|          |_
                         _,-'' +----------+ ``-._
               +------.-'            |           `--+------+
               | MSRP |              |              | MSRP |
               |Client|              |              |Client|
               +------+              |              +------+
                                 +---'--+
                                 | MSRP |
                                 |Client|
                                 +------+

           Figure 2: Multi-party chat in a Centralized Chat Room

   Typically chat room participants also subscribe to a conference event
   package to gather information about the conference roster in the form
   of conference state notifications.  For example, participants can
   learn about other participants' identifiers, including their
   nicknames.

   All messages in the chat room use the 'Message/CPIM' wrapper content
   type [RFC3862], so that it is possible to distinguish between private
   and regular messages.  When a participant wants to send an instant
   message to the chat room, it constructs an MSRP SEND request and
   submits it to the MSRP switch including a regular payload (e.g. a
   Message/CPIM message that contains text, HTML, an image, etc.).  The
   Message/CPIM To header is set to the chat room URI.  The switch then
   fans out the SEND request to all of the other participants using
   their existing MSRP sessions.

   A participant can also send a private instant message addressed to a
   participant whose identifier has been learned, e.g. via a conference
   event package.  In this case the sender creates an MSRP SEND request
   with a Message/CPIM wrapper whose To header contains not the chat
   room URI but the recipient's URI.  The MSRP switch then forwards the
   SEND request to that recipient.  This specification supports the
   sending of private messages to one and only one recipient.  However,



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   if the recipient is logged in from different endpoints, the MSRP
   switch will distribute the private message to each endpoint the
   recipient is logged in.

   We extend the current MSRP negotiation that takes place in SDP
   [RFC4566] to allow participants to learn whether the chat room
   supports and is willing to accept (e.g. due to local policy
   restrictions) certain MSRP functions defined in this memo, such as
   nicknames or private messaging.  This is achieved by a new 'chatroom'
   attribute in SDP (please refer to Section 8 for a detailed
   description).

   Naturally, when a participant wishes to leave a chat room, it sends a
   SIP BYE request to the Focus UA and terminates the SIP dialog with
   the focus and MSRP sessions with the MSRP switch.

   This document assumes that each chat room is allocated its own SIP
   URI.  A user joining a chat room sends an INVITE request to that SIP
   URI, and as a result, a new MSRP session is established between the
   user and the MSRP switch.  It is assumed that an MSRP session is
   mapped to a chat room.  If a user wants to join a second chat room,
   he creates a different INVITE request, through a different SIP
   dialog, which leads to the creation of a second MSRP session between
   the user and the MSRP switch.  Notice that these two MSRP sessions
   can still be multiplexed over the same TCP connection as per regular
   MSRP procedures.  However, each chat room is associated to a unique
   MSRP session and a unique SIP dialog.

4.1.  Policy Attributes of the Chat Room

   The Conference Framework with SIP [RFC4353] introduces the notion of
   a Conference Policy as a complete set of rules governing a particular
   conference.  In the case of chat rooms, since they are a specialized
   type of conferences, the notion of a Conference Policy exists and it
   is sometimes extended with new chat-specific rules.  This section
   lists all the Conference Policy attributes used by the present
   document and refers to sections in the document where the usage of
   these attributes are described in greater detail.

   Nicknames:   Whether the chat room accepts users to be recognized
      with a nickname.  See Section 7, Section 7.1, and Section 8 for
      details.  Also, the scope of uniqueness of the nickname: the chat
      room (conference instance), a realm or domain, a server, etc.

   Nickname quarantine:   The quarantine to be imposed to a nickname
      once it is not currently in use (e.g., because the participant
      holding this nickname abandons the chat room), prior to the wide
      availability of this nickname to other users.  This allows the



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      initial holder of the nickname to join the chat room during the
      quarantine period and claim the same nickname they were previously
      using.  See Section 11 for details.

   Private messaging:   Whether the chat room accepts users to send
      private messages to other users of the chat room through the MSRP
      switch.  See Section 6.2 and Section 8 for details.

   Deletion of the chat room:   Whether the chat room can be deleted
      when the creator leaves the chat room or through an out of band
      mechanism.  See Section 5.3 for details.

   Simultaneous access:   Whether a user can log from different
      endpoints using the same identity.  See Section 6.1 and
      Section 6.2 for details.

   Force TLS transport:   Whether the MSRP switch accepts only TLS as an
      MSRP transport, in an effort to guarantee confidentiality and
      privacy.  See Section 11 for details.

   Maximum message size in congested MSRP sessions:   The maximum size
      of messages that can be distributed to a user over a congested
      MSRP session.  See Section 6.4 for details.

   Chunk reception timer:   The value of a time that controls the
      maximum time that the MSRP switch is waiting for the reception of
      different chunks belonging to the same message.  If the timer
      expires, the MSRP switch will discard the associated message
      state.  See Section 6.1 for details.

   Supported wrapped media types:   The list of media types that the
      MSRP switch accepts in Message/CPIM wrappers sent from
      participants.  This list is included in the 'accept-wrapped-types'
      attribute of the MSRP message media line in SDP.  If the MSRP
      switch accepts additional media types than those explicitly
      listed, a "*" is added to the list.  A single "*" indicates that
      the chat room accepts any wrapped media type.


5.  Creating, Joining, and Deleting a Chat Room

5.1.  Creating a Chat Room

   Since we consider a chat room a particular type of conference having
   MSRP media, the methods defined by the SIP Conference Framework
   [RFC4353] for creating conferences are directly applicable to a chat
   room.




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   Once a chat room is created, it is identified by a SIP URI, like any
   other conference.

5.2.  Joining a Chat Room

   Participants usually join the chat room by sending an INVITE request
   to the chat room URI.  The chat room then uses regular SIP mechanisms
   to authenticate the participant.  This may include, e.g., client
   certificates, SIP Digest authentication [RFC3261], asserted network
   identity [RFC3325], SIP Identity header field [RFC4474], etc.  As
   long as the user is authenticated, the INVITE request is accepted by
   the focus and the user is brought into the actual chat room.

   This specification requires all instant messages to be wrapped in a
   Message/CPIM wrapper [RFC3862].  Therefore, the 'accept-types'
   attribute for the MSRP message media in both the SDP offer and answer
   need to include at least the value 'Message/CPIM' (Notice that RFC
   4575 [RFC4575] mandates this 'accept-types' attribute in SDP).  If
   the 'accept-types' attribute does not contain the value 'Message/
   CPIM', the conference focus will reject the request.  The actual
   instant message payload type is negotiated in the 'accept-wrapped-
   types' attribute in SDP (see RFC 4975 [RFC4975] for details).  There
   is no default wrapped type.  Typical wrapped type values can include:
   text/plain, text/html, image/jpeg, image/png, audio/mp3, etc.  It is
   RECOMMENDED that participant endpoints add an 'accept-wrapped-types'
   attribute to the MSRP 'message' media line in SDP, where the
   supported wrapped types are declared, as per RFC 4975 procedures
   [RFC4975].

   The MSRP switch needs to be aware of the URIs of the participant
   (SIP, Tel, or IM URIs) in order to validate messages sent from this
   participant prior to their forwarding.  This information is known to
   the focus of the conference.  Therefore an interface between the
   focus and the MSRP switch is assumed.  However, the interface between
   the focus and the MSRP switch is outside the scope of this document.

   Conference-aware participants will detect that the peer is a focus
   due to the presence of the "isfocus" feature tag [RFC3840] in the
   Contact header field of the 200-class response to the INVITE request.
   Conference-unaware participants will not notice it is a focus, and
   can not apply the additional mechanisms defined in this document.
   Participants are also aware that the mixer is an MSRP switch due to
   the presence of a 'message' media type and either TCP/MSRP or TCP/
   TLS/MSRP as the protocol field in the media line of SDP [RFC4566].

   The conference focus of a chat room MUST only use a Message/CPIM
   [RFC3862] top-level wrapper as a payload of MSRP messages, and the
   focus MUST declare it in the SDP offer or answer as per regular RFC



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   4975 procedures [RFC4975].  This implies that if the conference focus
   receives from a participant's endpoint an SDP offer that does not
   include the value 'Message/CPIM' in the 'accept-types' attribute for
   the MSRP message media line, the conference focus SHOULD either
   reject the MSRP message media stream or the complete SDP offer by
   using regular SIP or SDP procedures (e.g., creating an SDP answer
   that sets to zero the port of the MSRP message media line, responding
   the INVITE with a 488 response, etc.).

   If the conference focus accepts the participant's SDP offer, when the
   conference focus generates the SDP answer, it MUST set the 'accept-
   types' attribute for the MSRP message media line to a value of
   'Message/CPIM'.  This specification requires all instant messages to
   be wrapped in a Message/CPIM wrapper, therefore, the 'accept-types'
   attribute in this SDP body contains a single value of 'Message/CPIM'.
   The actual instant message payload type is negotiated in the 'accept-
   wrapped-types' attribute in SDP (see RFC 4975 [RFC4975] for details).
   The conference focus MAY also add an 'accept-wrapped-types' attribute
   to the MSRP message media line in SDP containing the supported
   wrapped types, according to the supported wrapped media types policy.

      Note that the 'Message/CPIM' wrapper is used to carry the sender
      information that, otherwise, it will not be available to the
      recipient.  Additionally, 'Message/CPIM' wrapper carries the
      recipient information (e.g.  To and Cc: headers).

   If the user agent supports anonymous participation and the user
   chooses to use it, the participant's UA SHOULD do at least one of
   these options:

   (a)  provide an anonymous URI in SIP headers that otherwise reveal
        identifiers.  Please refer to RFC 3323 [RFC3323] for a detailed
        description of which headers are subject to reveal identifiers
        and how to populate them; or

   (b)  trust the conference focus and request privacy of their URI,
        e.g, by means of the SIP Privacy header field [RFC3323], Network
        asserted identity [RFC3325], or similar privacy mechanism.

   If the participant has requested privacy, the conference focus MUST
   expose a participant's anonymous URI through the conference event
   package [RFC4575].

   The conference focus of a chat room learns the supported chat room
   capabilities in the endpoint by means of the 'chatroom' attribute
   exchanged in the SDP offer/answer (please refer to Section 8 for a
   detailed description).  The conference focus MUST inform the MSRP
   switch of the chat room capabilities of each participant that joins



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   the chat room (note that the interface defined between the conference
   focus and the MSRP switch is outside the scope of this
   specification).  This information allows the MSRP switch, e.g., to
   avoid the distribution of private messages to participants whose
   endpoints do not support private messaging.

5.3.  Deleting a Chat Room

   As with creating a conference, the methods defined by the SIP
   Conference Framework [RFC4353] for deleting a conference are directly
   applicable to a chat room.  The MSRP switch will terminate the MSRP
   sessions with all the participants.

   Deleting a chat room is an action that heavily depends on the policy
   of the chat room.  The policy can determine that the chat room is
   deleted when the creator leaves the chat room, or with any out of
   band mechanism.


6.  Sending and Receiving Instant Messages

6.1.  Regular Messages

   This section describes the conventions used to send and receive
   instant messages that are addressed to all the participants in the
   chat room.  These are sent over a regular MSRP SEND request that
   contains a Message/CPIM wrapper [RFC3862] that in turn contains the
   desired payload (e.g. text, image, video-clip, etc.).

   When a chat room participant wishes to send an instant message to all
   the other participants in the chat room, it constructs an MSRP SEND
   request according to the procedures specified in RFC 4975 [RFC4975].
   The sender MAY choose the desired MSRP report model (e.g., populate
   the Success-Report and Failure-Report MSRP header fields).

   On sending a regular message the sender MUST populate the To header
   of the Message/CPIM wrapper with the URI of the chat room.  The
   sender MUST also populate the From header of the Message/CPIM wrapper
   with a proper identifier by which the user is recognized in the chat
   room.  Identifiers that can be used (among others) are:

   o  A SIP URI [RFC3261] representing the participant's address-of-
      record

   o  A tel URI [RFC3966] representing the participant's telephone
      number





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   o  An IM URI [RFC3860] representing the participant's instant
      messaging address

   o  An Anonymous URI representing the participant's anonymous address

   If the participant wants to remain anonymous, the participant's
   endpoint MUST populate an anonymous URI in the From header of the
   'Message/CPIM' wrapper.  Other participants of the chat room will use
   this anonymous URI in the To header of the 'Message/CPIM' wrapper
   when sending private messages.  Notice that in order for the
   anonymity mechanism to work, the anonymous URI MUST NOT reveal the
   participant's SIP AOR.  The mechanism for acquiring an anonymous URI
   is outside the scope of this specification.

   An MSRP switch that receives a SEND request from a participant SHOULD
   first verify that the From header field of the Message/CPIM wrapper
   is correctly populated with a valid URI of a participant.  This
   imposes a requirement for the focus of the conference to inform the
   MSRP switch of the URIs by which the participant is known, in order
   for the MSRP switch to validate messages.  Section 6.3 provides
   further information with the actions to be taken in case this
   validation fails.

   Then the MSRP switch should inspect the To header field of the
   Message/CPIM wrapper.  If the MSRP switch receives a message
   containing several To header fields in the Message/CPIM wrapper the
   MSRP switch MUST reject the MSRP SEND request with a 403 response, as
   per procedures in RFC 4975 [RFC4975].  Then, if the To header field
   of the Message/CPIM wrapper contains the chat room URI and there are
   no other To header fields, the MSRP switch can generate a copy of the
   SEND request to each of the participants in the chat room except the
   sender.  The MSRP switch MUST NOT modify the content received in the
   SEND request.  However, the MSRP switch MAY re-chunk any of the
   outbound MSRP SEND requests.

   When generating a copy of the SEND request to each participant in the
   chat room, the MSRP switch MUST evaluate the wrapped media types that
   the recipient is able to accept.  This was learned through the
   'accept-wrapped-types' attribute of the MSRP message media line in
   SDP.  If the MSRP switch is aware that the media type of the wrapped
   content is not acceptable to the recipient, the MSRP switch SHOULD
   NOT forward this message to that endpoint.  Note that this version of
   the specification does not require the MSRP switch to notify the
   sender about this failure.  Extensions to this specification may
   improve handling of unknown media types.

   Note that the MSRP switch does not need to wait for the reception of
   the complete MSRP chunk or MSRP message before it starts the



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   distribution to the rest of the participants.  Instead, once the MSRP
   switch has received the headers of the Message/CPIM wrapper it SHOULD
   start the distribution process.  But bear in mind that still the MSRP
   switch SHOULD implement some sanity checking.  Please refer to the
   security considerations in Section 11 for further details.

   When forwarding chunked messages as soon as they are received, the
   Message/CPIM wrapper is only present at the beginning of the message,
   typically within the first chunk.  Subsequent chunks will contain the
   rest of the message, but not the Message/CPIM headers.  Therefore, an
   MSRP switch that receives a subsequent message may face challenges in
   determining the correct list of recipients of the message.  An MSRP
   switch that uses this fast forwarding procedure MUST temporarily
   store the Message-Id of the MSRP message to correlate the different
   chunks, as well as it MUST temporarily store the list of recipients
   to which the initial chunks were delivered.  The MSRP switch SHOULD
   forward subsequent chunks only to those recipients who were sent the
   initial chunks, except if the MSRP switch has knowledge that one of
   the recipients of the initial chunks has dropped from the chat room.
   This behavior also avoids new participants who joined the chat room
   when the first chunk has been distributed to receive subsequent
   chunks that would otherwise need to be discarded.

   Once the MSRP switch receives the last chunk of a message, and that
   chunk is successfully sent to each of the recipients, the MSRP switch
   discards the temporary storage of MSRP Message-ID and the associated
   list of recipients.

   In some occasions, a sender might suffer a transport error condition
   (such as loss of connectivity or depletion of battery) that makes the
   sending of a message incomplete, e.g., some chunks were received by
   the MSRP switch, but not all of them.  This is a behavior already
   considered in the core MSRP specification (see RFC 4975 [RFC4975]
   Section 5.4).  The problem in the context of a chat room lies with
   the usage of temporary storage for fast forwarding.  In order to
   prevent attacks related to the exhaustion of temporary storage of
   chunked messages, on receiving a first chunk of a message, where the
   MSRP switch is using the fast forward method, the MSRP switch MUST
   set a chunk reception timer for controlling the reception of the
   remaining chunks.

   This chunk reception timer can be re-set every time a new chunk of
   the same message is received.  When this timer fires, the MSRP switch
   MUST consider that the sending of the message was aborted, and MAY
   discard all the message state associated to it, including the
   Message-ID and the list of recipients.  Additionally, if this chunk
   reception timer fires, the MSRP switch MAY choose to send an abort
   chunk (i.e., one with the "#" flag set) to each to the recipients.



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   This is just an optimization, since MSRP endpoints need to be able to
   handle incomplete messages as per regular MSRP.

   The specific value of this chunk reception timer is not standardized;
   it is subject of local policy.  However, it is recommended not to be
   a short value.  For example a time interval on the order of a normal
   TCP timeout (i.e., around 540 seconds) would be reasonable.  A value
   on the order of a few seconds would not.

   An MSRP endpoint that receives a SEND request from the MSRP switch
   containing a Message/CPIM wrapper SHOULD first inspect the To header
   field of the Message/CPIM wrapper.  If the To header field is set to
   the chat room URI, it should render it as a regular message that has
   been distributed to all the participants in the chat room.  Then the
   MSRP endpoint SHOULD inspect the From header field of the Message/
   CPIM wrapper to identify the sender.  The From header field will
   include a URI that identifies the sender.  The endpoint might have
   also received further identifier information through a subscription
   to a conference event package.

   It is possible that a participant, identified by a SIP Address of
   Record or other valid URI, joins a chat room simultaneously from two
   or more different SIP UAs.  It is recommended that the MSRP switch
   implements means to map a URI to two or more MSRP sessions.  If the
   policy of the chat room allows simultaneous access, the MSRP switch
   MUST copy all regular messages intended to the recipient through each
   MSRP session mapped to the recipient's URI.

6.2.  Private Messages

   This section describes the conventions used to send and receive
   private instant messages, i.e., instant messages that are addressed
   to one participant of the chat room rather to all of them.  The chat
   room has local policy that determines whether private messages are
   supported or not.  A chat room can signal support for private
   messages using the 'chatroom' attribute in SDP (please refer to
   Section 8 for a detailed description).

   When a chat room participant wishes to send a private instant message
   to a participant in the chat room, it follows the same procedures for
   creating a SEND request as for regular messages (Section 6.1).  The
   only difference is that the MSRP endpoint MUST populate a single To
   header of the Message/CPIM wrapper with the identifier of the
   intended recipient.  The identifier can be SIP, TEL, and IM URIs
   typically learned from the information received in notifications of a
   conference event package.





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      This version of the specification does not support sending a
      private message to multiple recipients, i.e., the presence of
      multiple To headers in the Message/CPIM wrapper of the MSRP SEND
      request.  This is due to added complexity, for example, with the
      need to determine whether a message was not deliver to some of the
      intended recipients.  Implementations that still want to recreate
      this function can send a series of single private messages, one
      private message per intended recipient.  The endpoint can
      correlate this series of messages and create the effect of a
      private message addressed to multiple recipients.

   As for regular messages, an MSRP switch that receives a SEND request
   from a participant SHOULD first verify that the From header field of
   the Message/CPIM wrapper is correctly populated with a valid URI
   (i.e., the URI is a participant of this chat room).  Section 6.3
   provides further information with the actions to be taken in case
   this validation fails.

   Then the MSRP switch inspects the To header field of the Message/CPIM
   wrapper.  If the MSRP switch receives a message containing several To
   header fields in the Message/CPIM wrapper the MSRP switch MUST reject
   the MSRP SEND request with a 403 response, as per procedures in RFC
   4975 [RFC4975].  Then the MSRP switch verifies that the To header of
   the Message/CPIM wrapper matches the URI of a participant of the chat
   room.  If this To header field does not contain the URI of a
   participant of the chat room or if the To header field cannot be
   resolved (e.g., caused by a mistyped URI), the MSRP switch MUST
   reject the request with a 404 response.  This new 404 status code
   indicates a failure to resolve the recipient URI in the To header
   field of the Message/CPIM wrapper.

      Notice the importance of the From and To headers in the Message/
      CPIM wrapper.  If an intermediary modifies these values, the MSRP
      switch might not be able to identify the source or intended
      destination of the message, resulting in a rejection of the
      message.

   Finally, the MSRP switch verifies that the recipient supports private
   messages.  If the recipient does not support private messages, the
   MSRP switch MUST reject the request with a 428 response.  This new
   response 428 indicates that the recipient does not support private
   messages.  Any potential REPORT request that the MSRP switch sends to
   the sender MUST include a Message/CPIM wrapper containing the
   original From header field included in the SEND request and the To
   header field of the original Message/CPIM wrapper.  The MSRP switch
   MUST NOT forward private messages to a recipient that does not
   support private messaging.




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   If successful, the MSRP switch should search its mapping table to
   find the MSRP sessions established toward the recipient.  If a match
   is found the MSRP switch MUST create a SEND request and MUST copy the
   contents of the sender's message to it.

   An MSRP endpoint that receives a SEND request from the MSRP switch
   does the same validations as for regular messages (Section 6.1).  If
   the To header field is different from the chat room URI, the MSRP
   endpoints knows that this is a private message.  The endpoint should
   render who it is from based on the value of the From header of the
   Message/CPIM wrapper.  The endpoint can also use the sender's
   nickname, possibly learned via a conference event package, to render
   such nickname rather than the sender's actual URI.

   As with regular messages, if the policy of the chat room allows
   simultaneous access, the MSRP switch MUST copy all private messages
   intended to the recipient through each MSRP session mapped to the
   recipient's URI.

6.3.  MSRP reports and responses

   This section discusses the common procedures for regular and private
   messages with respect to MSRP reports and responses.  Any particular
   procedure affecting only regular messages or only private messages is
   discussed in the previous Section 6.1 or Section 6.2, respectively.

   MSRP switches MUST follow the success report and failure report
   handling described in section 7 of RFC 4975 [RFC4975], complemented
   with the procedures described in this section.  The MSRP switch MUST
   act as an MSRP endpoint receiver of the request according to section
   5.3 of RFC 4975 [RFC4975].

   If the MSRP switch receives an MSRP SEND request that does not
   contain a Message/CPIM wrapper, the MSRP switch MUST reject the
   request with a 415 response (specified in RFC 4975 [RFC4975]).

   If the MSRP switch receives an MSRP SEND request where the URI
   included in the From header field of the Message/CPIM wrapper is not
   valid, (e.g, because it does not "belong" to the sender of the
   message or is not a valid participant of the chat room), the MSRP
   switch MUST reject the request with a 403 response.  In non-error
   cases, the MSRP switch MUST construct responses according to section
   7.2 of RFC 4975 [RFC4975].

   When the MSRP switch forwards a SEND request, it MAY use any report
   model in the copies intended for the recipients.  The receiver
   reports from the recipients MUST NOT be forwarded to the originator
   of the original SEND request.  This could lead to having the sender



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   receiving multiple reports for a single MSRP request.

6.4.  Congestion Avoidance

   Congestion can occur when multiple heterogeneous interfaces are used
   by a diversity of users who are participating in a chat room, and, in
   particular, when paths become overloaded by any application.  Some of
   these users might have fast path capable of high throughputs while
   other users might be slow paths with constrained throughputs.  Some
   paths might become congested only by the chat application; other
   paths gets congested by other applications different than the chat
   one.  It is therefore possible that a subset of the participants of
   the chat room are able to send and receive a large number of messages
   in a short time or with large contents (e.g., pictures), whereas
   others are not able to keep the pace.

   Additionally, since MSRP uses a connection-oriented transport
   protocol such as TCP, it is expected that TCP congestion control
   mechanisms are activated if congestion occurs.  Details on congestion
   control are specified in RFC 5681 [RFC5681].

   While this document does not mandate a particular MSRP-specific
   mechanism to avoid congestion in any of the paths, something that is
   deemed outside the scope of this document, this document provides
   some recommendations for implementors to consider.

   It is RECOMMENDED that MSRP switches implement one or more MSRP-
   specific strategies to detect and avoid congestion.  Possible
   strategies (but definitely not a comprehensive list) include:

   o  If the MSRP switch is writing data to a send buffer and detects
      that the send buffer associated to that TCP connection is getting
      full (e.g., close to 80% of its capacity), the MSRP switch marks
      the associated MSRP sessions making use of that TCP connection as
      "congested".

   o  Prior to sending a new MSRP message to a user, the MSRP switch
      verifies the congested flag associated to that MSRP session.  If
      the MSRP session is marked as congested, the MSRP switch can apply
      a congestion avoidance mechanism, such as:

      *  The MSRP switch MAY discard regular MSRP messages sent to that
         user while the congestion flag is raised for the user's TCP
         connection.  In order to inform the user of the congestion, the
         MSRP switch MAY send a regular MSRP message to the user whose
         congestion flag is raised.  This message indicates that some
         other messages are being discarded due to network congestion.
         However, it should be noted that this message can get stuck at



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         MSRP switch, if the path is fully congested, i.e., it may not
         be delivered anyhow.

      *  The MSRP can implement a temporary policy to disallow the
         distribution of messages larger than a certain size to MSRP
         sessions marked as congested.  Similarly, the user should be
         informed of this fact by the MSRP switch sending a regular MSRP
         message indicating this condition.

   o  If the MSRP switch determines that the congestion flag associated
      to a given TCP connection has been raised for quite some time (on
      the order of a few minutes), or if the interface is down, this may
      be considered as an indication that the TCP connection has not
      been able to recover from a congestion state.  The MSRP switch MAY
      close this congested TCP connection, as well as the MSRP session
      and SIP session.


7.  Nicknames

   A common characteristic of existing chat room services is that
   participants have the ability to present themselves with a nickname
   to the rest of the participants of the chat room.  It is used for
   easy reference of participants in the chat room, and can also provide
   anonymous participants with a meaningful descriptive name.

   A nickname is a useful construct in many use cases, of which MSRP
   chat is but one example.  It is associated with a URI of which the
   participant is known to the focus.  Therefore, if a user joins the
   chat room under the same URI from multiple devices, he or she may
   request the same nickname across all these devices.

   A nickname is a user selectable appearance of which the participant
   wants to be known to the other participants.  It is not a 'display-
   name', but it is used somewhat like a display name.  A main
   difference is that a nickname is unique inside a chat room to allow
   an unambiguous reference to a participant in the chat.  Nicknames may
   be long lived, or may be temporary.  Users also need to reserve a
   nickname prior to its utilization.

   This memo specifies the nickname as a string.  The nickname string
   MUST unambiguously be associated to a single user in the scope of the
   chat room (conference instance).  This scope is similar to having a
   nickname unique per user inside a chat room from Extensible Messaging
   and Presence Protocol [RFC6120].  The chat room may have policies
   associated with nicknames.  It may not accept nickname strings at
   all, or a it may provide a wider unambiguous scope like a domain or
   server, similar to Internet Relay Chat (IRC) [RFC2810].



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7.1.  Using Nicknames within a Chat Room

   This memo provides a mechanism to reserve a nickname for a
   participant for as long as the participant is logged into the chat
   room.  The mechanism is based on a NICKNAME MSRP method (see below)
   and a new "Use-Nickname" header.  Note that other mechanisms may
   exist (for example, a web page reservation system), although they are
   outside the scope of this document.

   A chat room participant who has established an MSRP session with the
   MSRP switch, where the MSRP switch has indicated the support and
   availability of nicknames with the 'nicknames' token in the
   'chatroom' SDP attribute, MAY send a NICKNAME request to the MSRP
   switch.  The NICKNAME request MUST include a new Use-Nickname header
   that contains the nickname string that the participant wants to
   reserve.  This nickname string MUST NOT be zero octets in length and
   MUST NOT be more than 1023 octets in length.  Last, MSRP NICKNAME
   requests MUST NOT include Success-Report or Failure-Report header
   fields.

      Bear in mind that nickname strings, like the rest of the MSRP
      message, use the UTF-8 transformation format [RFC3629].
      Therefore, a character may result encoded in more than one octet.

   An MSRP switch that receives a NICKNAME request containing a
   Use-Nickname header field SHOULD first verify whether the policy of
   the chat room allows the nickname functionality.  If not allowed, the
   MSRP switch MUST reject the request with a 403 response, as per RFC
   4975 [RFC4975].

   If the policy of the chat room allows the usage of nicknames, any new
   nickname requested MUST be prepared and compared with nicknames
   already in use or reserved following the rules defined in Preparation
   and Comparison of Nicknames [I-D.ietf-precis-nickname].

   This mitigates the problem of nickname duplication, but it does not
   solve a problem whereby users can choose similar (but different)
   characters to represent two different nicknames.  For example, "BOY"
   and "B0Y" are different nicknames which can mislead users.  The
   former uses the capital letter "O" while the latter uses the number
   zero "0".  In many fonts the letter "O" and the number zero "0" might
   be quite similar, and difficult to be perceived as different
   characters.  Chat rooms MAY provide a mechanism to mitigate
   confusable nicknames.

   In addition to preparing and comparing following the rules above, the
   MSRP switch SHOULD only allow the reservation of an already used
   nickname, if the same user (e.g., identified by the SIP AOR) that is



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   currently using the nickname is making this subsequent request.  This
   may include, e.g., allowing that the participant's URI may use the
   same nickname when the participant has joined the chat room from
   different devices under the same URI.  The participant's
   authenticated identifier can be derived after a successful SIP Digest
   Authentication [RFC3261], be included in a trusted SIP P-Asserted-
   Identity header field [RFC3325], be included in a valid SIP Identity
   header field [RFC4474], or be derived from any other present or
   future SIP authentication mechanism.  Once the MSRP switch has
   validated that the participant is entitled to reserve the requested
   nickname, the MSRP switch verifies if the suggested nickname can be
   accepted (see below).

   The reservation of a nickname can fail in several cases.  If the
   NICKNAME request contains a malformed value in the Use-Nickname
   header field, the MSRP switch MUST answer the NICKNAME request with a
   424 response code.  This can be the case when the value of the
   Use-Nickname header field does not conform to the syntax.

   The reservation of a nickname can also fail if the value of the
   Use-Nickname header field of the NICKNAME request is a reserved word
   (not to be used as a nickname by any user) or that particular value
   is already in use by another user.  In this case the MSRP switch MUST
   answer the NICKNAME request with a 425 response code.

   In both error conditions (receiving a 424 or 425 response code), the
   nickname usage is considered failed; the nickname is not allocated to
   this user.  The user can select a different nickname and retry
   another NICKNAME request.

   If the MSRP switch is able to accept the suggested nickname to be
   used by this user, the MSRP switch MUST answer the NICKNAME request
   with a 200 response as per regular MSRP procedures.

   As indicated earlier, this specification defines a new MSRP header
   field: "Use-Nickname".  The Use-Nickname header field carries a
   nickname string.  This specification defines the usage of the
   Use-Nickname header field in NICKNAME requests.  If need arises,
   usages of the Use-Nickname header field in other MSRP methods should
   be specified separately.

   According to RFC 4975 [RFC4975], MSRP uses the UTF-8 transformation
   format [RFC3629].  The syntax of the MSRP NICKNAME method and the
   "Use-Nickname" header field is built upon the MSRP formal syntax
   [RFC4975] using the Augmented Backus-Naur Form (ABNF) [RFC5234].






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               ext-method =/ NICKNAMEm
               NICKNAMEm = %x4E.49.43.4B.4E.41.4D.45 ; NICKNAME in caps
               ext-header =/ Use-Nickname
               ; ext-header defined in RFC 4975
               Use-Nickname = "Use-Nickname:" SP nickname
               nickname = DQUOTE 1*1023(qdtext / qd-esc) DQUOTE
                        ; qdtext and qd-esc defined in RFC 4975

   Note that, according to RFC 4975 [RFC4975], "quoted-string" admits a
   subset of UTF-8 characters [RFC3629].  Please refer to Section 9 of
   RFC 4975 [RFC4975] for more details.

   Once the MSRP switch has reserved a nickname and has bound it to a
   URI (e.g., a SIP Address-of-Record), the MSRP server MAY allow the
   usage of the same nickname by the same user (identified by the same
   URI, such as a SIP AoR) over a second MSRP session.  This might be
   the case if the user joins the same chat room from a different SIP
   User Agent.  In this case, the user MAY request the same or a
   different nickname than that used in conjunction with the first MSRP
   session; the MSRP server MAY accept the usage of the same nickname by
   the same user.  The MSRP switch MUST NOT automatically assign the
   same nickname to more than one MSRP session established from the same
   URI, because this can create confusion to the user as whether the
   same nickname is bound to the second MSRP session.

7.2.  Modifying a Nickname

   Typically a participant will reserve a nickname as soon as the
   participant joins the chat room.  But it is also possible for a
   participant to modify his/her own nickname and replace it with a new
   one at any time during the duration of the MSRP session.
   Modification of the nickname is not different from the initial
   reservation and usage of a nickname, thus the NICKNAME method is used
   as described in Section 7.1.

   If a NICKNAME request that attempts to modify the current nickname of
   the user for some reason fails, the current nickname stays in effect.
   A new nickname comes into effect and the old one is released only
   after a NICKNAME request is accepted with a 200 response.

7.3.  Removing a Nickname

   If the participant no longer wants to be known by a nickname in the
   chat room, the participant can follow the method described in
   Section 7.2.  The nickname element of the Use-Nickname header MUST be
   set to an empty quoted string.





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7.4.  Nicknames in Conference Event Packages

   Typically the conference focus acts as a notifier of the conference
   event package, RFC 4575 [RFC4575].  It is RECOMMENDED that conference
   foci and endpoints support RFC 6502 [RFC6502] for providing
   information regarding the conference, and in particular, supplying
   information of the roaster of the conference.  It is also RECOMMENDED
   that conference foci and endpoints support RFC 6501 [RFC6501], which
   extends the <user> element originally specified in RFC 4575 [RFC4575]
   with a new 'nickname' attribute.  This allows endpoints to learn the
   nicknames of participants of the chat room.


8.  The SDP 'chatroom' attribute

   There are a handful of use cases where a participant would like to
   learn the chat room capabilities supported by the local policy of the
   MSRP switch and the chat room.  For example, a participant would like
   to learn if the MSRP switch supports private messaging, otherwise,
   the participant may send what he believes is a private instant
   message addressed to a participant, but since the MSRP switch does
   not support the functions specified in this memo, the message gets
   eventually distributed to all the participants of the chat room.

   The reverse case also exists.  A participant, say Alice, whose user
   agent does not support the extensions defined by this document joins
   the chat room.  The MSRP switch learns that Alice's application does
   not support private messaging nor nicknames.  If another participant,
   say Bob, sends a private message to Alice, the MSRP switch does not
   distribute it to Alice, because Alice is not able to differentiate it
   from a regular message sent to the whole roster.  Furthermore, if
   Alice replied to this message, she would do it to the whole roster.
   Because of this, the MSRP switch also keeps track of users who do not
   support the extensions defined in this document.

   In another scenario, the policy of a chat room may indicate that
   certain functions are not allowed.  For example, the policy may
   indicate that nicknames or private messages are not allowed.

   In order to provide the user with a good chat room experience, we
   define a new 'chatroom' SDP attribute.  The 'chatroom' attribute is a
   media-level value attribute [RFC4566] that MAY be included in
   conjunction with an MSRP media stream (i.e., when an m= line in SDP
   indicates "TCP/MSRP" or "TCP/TLS/MSRP").  The 'chatroom' attribute
   without further modifiers (e.g., chat-tokens) indicates that the
   endpoint supports the procedures described in this document for
   transferring MSRP messages to/from a chat room.  The 'chatroom'
   attribute can be complemented with additional modifiers that further



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   indicate the intersection of support and local policy allowance for a
   number of functions specified in this document.  Specifically, we
   provide the means for indicating support to use nicknames and private
   messaging.

   The 'chatroom' attribute merely indicates the capabilities supported
   and allowed by the local policy.  This attribute is not a negotiation
   subject to the SDP offer/answer model [RFC3264], but instead a
   declaration.  Therefore, a 'chatroom' attribute included in an SDP
   answer does not need to be a subset of the values included in the
   'chatroom' attribute of its corresponding SDP offer.  Consequently,
   an SDP answer MAY contain a 'chatroom' attribute even if its
   corresponding SDP offer did not include it.

   On doing subsequent SDP offer/answer [RFC3264] exchanges pertaining
   to the same session, the 'chatroom' attribute MAY be modified with
   respect an earlier SDP offer/answer exchange.  The new value of this
   attribute indicate the current support and local policy, meaning that
   some restrictions can apply now or might have been removed.  If the
   'chatroom' attribute is not included in a subsequent SDP offer/
   answer, but is corresponding MSRP stream is still in place, it
   indicates that support for the procedures indicated in this document
   are disabled.

   The 'chatroom' SDP attribute has the following Augmented BNF (ABNF)
   [RFC5234] syntax:

             attribute         =/ chatroom-attr
                                       ; attribute defined in RFC 4566
             chatroom-attr     = chatroom-label [":" chat-token
                                 *(SP chat-token)]
             chatroom-label    = "chatroom"
             chat-token        = (nicknames-token / private-msg-token /
                                  ext-token)
             nicknames-token   = "nickname"
             private-msg-token = "private-messages"
             ext-token         = private-token / standard-token
             private-token     = toplabel "." *(domainlabel ".") token
                                       ; toplabel defined in RFC 3261
                                       ; domainlabel defined in RFC 3261
                                       ; token defined in RFC 3261
             standard-token    = token

   A given 'chat-token' value MUST NOT appear more than once in a
   'chatroom' attribute.

   A conference focus that includes the 'nicknames' token in the session
   description is signaling that the MSRP switch supports and the chat



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   room allows to use the procedures specified in Section 7.  A
   conference focus that includes the 'private-messages' in the SDP
   description is signaling that the MSRP switch supports and the chat
   room allows to use the procedures specified in Section 6.2.

   Example of the 'chatroom' attribute for an MSRP media stream that
   indicates the acceptance of nicknames and private messages:

             a=chatroom:nickname private-messages

   An example of a 'chatroom' attribute for an MSRP media stream where
   the endpoint, e.g., an MSRP switch, does not allow either nicknames
   nor private messages.

             a=chatroom

   The 'chatroom' attribute allows extensibility with the addition of
   new tokens.  No IANA registry is provided at this time, since no
   extensions are expected at the time of this writing.  Extensions to
   the 'chatroom' attribute can be defined in IETF documents or as
   private vendor extensions.

   Extensions defined in IETF document MUST follow the 'standard-token'
   ABNF previously defined.  In this type of extensions, care must be
   taken in the selection of the token to avoid a clash with any of the
   tokens previously defined.

   Private extensions MUST follow the 'private-token' ABNF previously
   defined.  The 'private-token' MUST include the DNS name of the
   vendor.  Then the token is reversed in order to avoid clashes of
   tokens.  The following is an example of a extension named "foo.chat"
   by a vendor "example.com"

           a=chatroom:nickname private-messages com.example.chat.foo

   Note that feature names created by different organizations are not
   intended to have the same semantics or even interoperate.


9.  Examples

9.1.  Joining a chat room

   Figure 3 presents a flow diagram where Alice joins a chat room by
   sending an INVITE request.  This INVITE request contains a session
   description that includes the chatroom extensions defined in this
   document.




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                    Alice               Conference focus
                      |                        |
                      |F1: (SIP) INVITE        |
                      |----------------------->|
                      |F2: (SIP) 200 OK        |
                      |<-----------------------|
                      |F3: (SIP) ACK           |
                      |----------------------->|
                      |                        |

           Figure 3: Flow diagram of a user joining a chat room

   F1: Alice constructs an SDP description that includes an MSRP media
   stream.  She also indicates her support for the chatroom extensions
   defined in this document.  She sends the INVITE request to the chat
   room server.

   INVITE sip:chatroom22@chat.example.com SIP/2.0
   Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   To: Chatroom 22 <sip:chatroom22@chat.example.com>
   Call-ID: 3848276298220188511@atlanta.example.com
   CSeq: 1 INVITE
   Contact: <sip:alice@client.atlanta.example.com;transport=tcp>
   Content-Type: application/sdp
   Content-Length: 290

   v=0
   o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
   s=-
   c=IN IP4 client.atlanta.example.com
   m=message 7654 TCP/MSRP *
   a=accept-types:message/cpim text/plain text/html
   a=path:msrp://client.atlanta.example.com:7654/jshA7weztas;tcp
   a=chatroom:nickname private-messages

   F2: The chat room server accepts the session establishment.  It
   includes the 'isfocus' and other relevant feature tags in the Contact
   header field of the response.  The chat room server also builds an
   SDP answer that forces the reception of messages wrapped in Message/
   CPIM wrappers.  It also includes the 'chatroom' attribute with the
   allowed extensions.








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   SIP/2.0 200 OK
   Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
    ;received=192.0.2.101
   From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   To: Chatroom 22 <sip:chatroom22@chat.example.com>;tag=8321234356
   Call-ID: 3848276298220188511@atlanta.example.com
   CSeq: 1 INVITE
   Contact: <sip:chatroom22@chat.example.com;transport=tcp> \
             ;methods="INVITE,BYE,OPTIONS,ACK,CANCEL,SUBSCRIBE,NOTIFY" \
             ;automata;isfocus;message;event="conference"
   Content-Type: application/sdp
   Content-Length: 290

   v=0
   o=chat 2890844527 2890844527 IN IP4 chat.example.com
   s=-
   c=IN IP4 chat.example.com
   m=message 12763 TCP/MSRP *
   a=accept-types:message/cpim
   a=accept-wrapped-types:text/plain text/html *
   a=path:msrp://chat.example.com:12763/kjhd37s2s20w2a;tcp
   a=chatroom:nickname private-messages

   F3: The session established is acknowledged (details not shown).

9.2.  Setting up a nickname

   Figure 4 shows an example of Alice setting up a nickname using the
   chat room as provider.  Her first proposal is not accepted because
   that proposed nickname is already in use.  Then, she makes a second
   proposal with a new nickname.  This second proposal is accepted.

                     Alice                  MSRP switch
                       |                        |
                       |F1: (MSRP) NICKNAME     |
                       |----------------------->|
                       |F2: (MSRP) 425          |
                       |<-----------------------|
                       |F3: (MSRP) NICKNAME     |
                       |----------------------->|
                       |F4: (MSRP) 200          |
                       |<-----------------------|
                       |                        |

         Figure 4: Flow diagram of a user setting up her nickname

   F1: Alice sends an MSRP NICKNAME request that contains her proposed
   nicknames in the Use-Nickname header field.



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   MSRP d93kswow NICKNAME
   To-Path: msrp://chat.example.com:12763/kjhd37s2s20w2a;tcp
   From-Path: msrp://client.atlanta.example.com:7654/jshA7weztas;tcp
   Use-Nickname: "Alice the great"
   -------d93kswow$

   F2: The MSRP switch analyzes the existing allocation of nicknames and
   detects that the nickname "Alice the great" is already provided to
   another participant in the chat room.  The MSRP switch answers with a
   425 response.

   MSRP d93kswow 425 Nickname reserved or already in use
   To-Path: msrp://client.atlanta.example.com:7654/jshA7weztas;tcp
   From-Path: msrp://chat.example.com:12763/kjhd37s2s20w2a;tcp
   -------d93kswow$

   F3: Alice receives the response.  She proposes a new nickname in a
   second NICKNAME request.

   MSRP 09swk2d NICKNAME
   To-Path: msrp://chat.example.com:12763/kjhd37s2s20w2a;tcp
   From-Path: msrp://client.atlanta.example.com:7654/jshA7weztas;tcp
   Use-Nickname: "Alice in Wonderland"
   -------09swk2d$

   F4: The MSRP switch accepts the nickname proposal and answers with a
   200 response.

   MSRP 09swk2d 200 OK
   To-Path: msrp://client.atlanta.example.com:7654/jshA7weztas;tcp
   From-Path: msrp://chat.example.com:12763/kjhd37s2s20w2a;tcp
   -------09swk2d$

9.3.  Sending a regular message to the chat room

   Figure 5 depicts a flow diagram where Alice is sending a regular
   message addressed to the chat room.  The MSRP switch distributes the
   message to the rest of the participants.













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      Alice               MSRP switch                   Bob   Charlie
        |                      |                        |       |
        | F1: (MSRP) SEND      |                        |       |
        |--------------------->|  F3: (MSRP) SEND       |       |
        | F2: (MSRP) 200       |----------------------->|       |
        |<---------------------|  F4: (MSRP) SEND       |       |
        |                      |------------------------------->|
        |                      |  F5: (MSRP) 200 OK     |       |
        |                      |<-----------------------|       |
        |                      |  F6: (MSRP) 200 OK     |       |
        |                      |<------------------------------ |
        |                      |                        |       |
        |                      |                        |       |

           Figure 5: Sending a regular message to the chat room

   F1: Alice builds a text message and wraps it in a Message/CPIM
   wrapper.  She addresses the message to the chat room.  She encloses
   the resulting Message/CPIM wrapper in an MSRP SEND request and sends
   it to the MSRP switch via the existing TCP connection.

   MSRP 3490visdm SEND
   To-Path: msrp://chat.example.com:12763/kjhd37s2s20w2a;tcp
   From-Path: msrp://client.atlanta.example.com:7654/jshA7weztas;tcp
   Message-ID: 99s9s2
   Byte-Range: 1-*/*
   Content-Type: message/cpim

   To: <sip:chatroom22@chat.example.com;transport=tcp>
   From: <sip:alice@atlanta.example.com>
   DateTime: 2009-03-02T15:02:31-03:00
   Content-Type: text/plain

   Hello guys, how are you today?
   -------3490visdm$

   F2: The MSRP switch acknowledges the reception of the SEND request
   with a 200 (OK) response.

   MSRP 3490visdm 200 OK
   To-Path: msrp://client.atlanta.example.com:7654/jshA7weztas;tcp
   From-Path: msrp://chat.example.com:12763/kjhd37s2s20w2a;tcp
   Message-ID: 99s9s2
   -------3490visdm$

   F3: The MSRP switch creates a new MSRP SEND request that contains the
   received Message/CPIM wrapper and sends it to Bob.




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   MSRP 490ej23 SEND
   To-Path: msrp://client.biloxi.example.com:4923/49dufdje2;tcp
   From-Path: msrp://chat.example.com:5678/jofofo3;tcp
   Message-ID: 304sse2
   Byte-Range: 1-*/*
   Content-Type: message/cpim

   To: <sip:chatroom22@chat.example.com;transport=tcp>
   From: <sip:alice@atlanta.example.com>
   DateTime: 2009-03-02T15:02:31-03:00
   Content-Type: text/plain

   Hello guys, how are you today?
   -------490ej23$

   Since the received message is addressed to the chat room URI in the
   From header of the Message/CPIM header, Bob knows that this is a
   regular message distributed all participants in the chat room, rather
   that a private message addressed to him.

   The rest of the message flows are analogous to the previous.  They
   are not shown here.

9.4.  Sending a private message to a participant

   Figure 6 depicts a flow diagram where Alice is sending a private
   message addressed to Bob's SIP AOR.  The MSRP switch distributes the
   message only to Bob.

           Alice               MSRP switch                   Bob
             |                      |                        |
             | F1: (MSRP) SEND      |                        |
             |--------------------->|  F3: (MSRP) SEND       |
             | F2: (MSRP) 200       |----------------------->|
             |<---------------------|  F4: (MSRP) 200        |
             |                      |<-----------------------|
             |                      |                        |

                Figure 6: Sending a private message to Bob

   F1: Alice builds a text message and wraps it in a Message/CPIM
   wrapper.  She addresses the message to Bob's URI, which she learned
   from a notification in the conference event package.  She encloses
   the resulting Message/CPIM wrapper in an MSRP SEND request and sends
   it to the MSRP switch via the existing TCP connection.






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   MSRP 6959ssdf SEND
   To-Path: msrp://chat.example.com:12763/kjhd37s2s20w2a;tcp
   From-Path: msrp://client.atlanta.example.com:7654/jshA7weztas;tcp
   Message-ID: okj3kw
   Byte-Range: 1-*/*
   Content-Type: message/cpim

   To: <sip:bob@example.com>
   From: <sip:alice@example.com>
   DateTime: 2009-03-02T15:02:31-03:00
   Content-Type: text/plain

   Hello Bob.
   -------6959ssdf$

   F2: The MSRP switch acknowledges the reception of the SEND request
   with a 200 (OK) response.

   MSRP 6959ssdfm 200 OK
   To-Path: msrp://client.atlanta.example.com:7654/jshA7weztas;tcp
   From-Path: msrp://chat.example.com:12763/kjhd37s2s20w2a;tcp
   Message-ID: okj3kw
   -------6959ssdfm$

   F3: The MSRP switch creates a new MSRP SEND request that contains the
   received Message/CPIM wrapper and sends it only to Bob. Bob can
   distinguish the sender in the From header of the Message/CPIM
   wrapper.  He also identifies this as a private message due to the
   presence of his own SIP AOR in the To header field of the Message/
   CPIM wrapper.

   MSRP 9v9s2 SEND
   To-Path: msrp://client.biloxi.example.com:4923/49dufdje2;tcp
   From-Path: msrp://chat.example.com:5678/jofofo3;tcp
   Message-ID: d9fghe982
   Byte-Range: 1-*/*
   Content-Type: message/cpim

   To: <sip:bob@example.com>
   From: <sip:alice@atlanta.example.com>
   DateTime: 2009-03-02T15:02:31-03:00
   Content-Type: text/plain

   Hello Bob.
   -------9v9s2$

   F4: Bob acknowledges the reception of the SEND request with a 200
   (OK) response.



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   MSRP 9v9s2 200 OK
   To-Path: msrp://chat.example.com:5678/jofofo3;tcp
   From-Path: msrp://client.biloxi.example.com:4923/49dufdje2;tcp
   Message-ID: d9fghe982
   -------9v9s2$

9.5.  Chunked private message

   The MSRP message below depicts the example of the same private
   message described in Section 9.4, but now the message is split in two
   chunks.  The MSRP switch must wait for the complete set of Message/
   CPIM headers before distributing the messages.

   MSRP 7443ruls SEND
   To-Path: msrp://chat.example.com:12763/kjhd37s2s20w2a;tcp
   From-Path: msrp://client.atlanta.example.com:7654/jshA7weztas;tcp
   Message-ID: aft4to
   Byte-Range: 1-*/174
   Content-Type: message/cpim

   To: <sip:bob@example.com>
   From: <sip:alice@example.com>
   -------7443ruls$

   MSRP 7443ruls SEND
   To-Path: msrp://chat.example.com:12763/kjhd37s2s20w2a;tcp
   From-Path: msrp://client.atlanta.example.com:7654/jshA7weztas;tcp
   Message-ID: aft4to
   Byte-Range: 68-174/174
   Content-Type: message/cpim

   DateTime: 2009-03-02T15:02:31-03:00
   Content-Type: text/plain

   Hello Bob
   -------7443ruls$

9.6.  Nickname in a conference information document

   Figure 7 depicts an XML Conference Information Document received in a
   SIP NOTIFY request as a notification to the XCON Conference Event
   Package, RFC 6502 [RFC6502].  The Conference Information Document
   follows the XCON Data Model specified in RFC 6501 [RFC6501].

   The Conference Information Document of Figure 7 presents information
   of two users who are participating in the conference (see each of the
   <user> elements).  Each participant is bound to a nickname, shown in
   the 'nickname' attribute of the <user> element.



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      NOTE: The purpose of Figure 7 is to show the user-to-nickname
      relation.  It is believed that the example is correct, according
      to RFC 6501 [RFC6501].  In case of contradictions between this
      specification and RFC 6501 [RFC6501], the latter has precedence
      over this one.



         <?xml version="1.0" encoding="UTF-8"?>
           <conference-info
            xmlns="urn:ietf:params:xml:ns:conference-info"
            xmlns:xcon="urn:ietf:params:xml:ns:xcon-conference-info"
            entity="sip:chatroom22@chat.example.com"
            state="full" version="1">
           <!--
             CONFERENCE INFO
           -->
            <conference-description>
              <subject>MSRP nickname example</subject>
             </conference-description>
           <!--
              CONFERENCE STATE
           -->
            <conference-state>
             <user-count>2</user-count>
            </conference-state>
           <!--
             USERS
           -->
            <users>
              <user entity="sip:bob@example.com"
                    state="full"
                    xcon:nickname="Dopey Donkey">
                 <display-text>Bob Hoskins</display-text>
              </user>
           <!--
             USER
           -->
              <user entity="sip:alice@atlanta.example.com"
                   state="full"
                   xcon:nickname="Alice the great">
                 <display-text>Alice Kay</display-text>
              </user>
            </users>

           </conference-info>

          Figure 7: Nickname in a conference information document



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10.  IANA Considerations

10.1.  New MSRP Method

   This specification defines a new MSRP method to be added to the
   Methods sub-registry of the Message Session Relay Protocol (MSRP)
   Parameters registry:

      NICKNAME

   See section Section 7 for details.

10.2.  New MSRP Header

   This specification defines a new MSRP header to be added to the
   Header Field sub-registry of the Message Session Relay Protocol
   (MSRP) Parameters registry:

      Use-Nickname

   See Section 7 for details.

10.3.  New MSRP Status Codes

   This specification defines three new MSRP status codes to be added to
   the Status-Code sub-registry of the Message Session Relay Protocol
   (MSRP) parameters registry.

   The 404 status code indicates the failure to resolve the recipient's
   URI in the To header field of the Message/CPIM wrapper in the SEND
   request, e.g, due to an unknown recipient.  See Section 6.2 for
   details.

   The 424 status code indicates a failure in allocating the requested
   nickname due to a malformed syntax in the Use-Nickname header field.
   See Section 7 for details.

   The 425 status code indicates a failure in allocating the requested
   nickname because the requested nickname in the Use-Nickname header
   field is reserved or is already in use by another user.  See
   Section 7 for details.

   The 428 status code indicates that the recipient of a SEND request
   does not support private messages.  See Section 6.2 for details.

   Table 1 summarizes the IANA registration data with respect to new
   MSRP status codes:




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        +-------+-------------------------------------+-----------+
        | Value | Description                         | Reference |
        +-------+-------------------------------------+-----------+
        |  404  | Failure to resolve recipient's URI  |  RFC XXXX |
        |  424  | Malformed nickname                  |  RFC XXXX |
        |  425  | Nickname reserved or already in use |  RFC XXXX |
        |  428  | Private messages not supported      |  RFC XXXX |
        +-------+-------------------------------------+-----------+

                         Table 1: New status codes

10.4.  New SDP Attribute

   This specification defines a new media-level attribute in the Session
   Description Protocol (SDP) Parameters registry.  The registration
   data is as follows:

      Contact: Miguel Garcia <miguel.a.garcia@ericsson.com>

      Phone: +34 91 339 1000

      Attribute name: chatroom

      Long-form attribute name: Chat Room

      Type of attribute: media level only

      This attribute is not subject to the charset attribute

      Description: This attribute identifies support and local policy
      allowance for a number of chat room related functions

      Specification: RFC XXXX

   See section Section 8 for details.


11.  Security Considerations

   This document proposes extensions to the Message Session Relay
   Protocol [RFC4975].  Therefore, the security considerations of that
   document apply to this document as well.

   A chat room is by its nature a potential Denial-of-Service (DoS)
   accellerator as it takes a message from one entity and sends that to
   many.  Implementers of both UAs and switches need to carefully
   consider the set of anti-DoS measures that are appropriate for this
   application and switch implementations in particular ought to include



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   appropriate anti-DoS features.  The details of what is appropriate
   will vary over time and will also depend on the specific needs of the
   implementation and so cannot be specified here.

   If the participant's SIP user agent does not understand the "isfocus"
   feature tag [RFC3840], it will not know that it is connected to a
   conference instance.  The participant might not be notified that the
   participant's MSRP client will try to send messages to the MSRP
   switch having potentially multiple recipients.  If the participant's
   MSRP client does not support the extensions of this specification, it
   is unlikely that it will try to send a message using 'Message/CPIM'
   wrapper content type [RFC3862], and the MSRP switch will reject the
   request with a 415 response [RFC4975].  Still if a participant's MSRP
   client does create a message with a valid 'Message/CPIM' wrapper
   content type [RFC3862] having the To header set to the URI of the
   chat room and the From header set to the URI of which the participant
   is known to the chat room, the participant might be unaware that the
   message can be forwarded to multiple recipients.  Equally if the To
   header is set to a valid URI of a recipient known to the chat room,
   the message can be forwarded as a private message without the
   participant knowing.

   To mitigate these problems, when the chat room detects that a user
   agent does not support the procedures of this document (i.e., when
   the SIP User Agent is not chat room aware), the MSRP switch SHOULD
   send a regular MSRP message indicating that the SIP User Agent is
   actually part of a chat room, and that all the messages that the user
   sends correctly formated will be distributed to a number of
   participants.  Additionally, the MSRP switch SHOULD also send a
   regular MSRP text message including the list of participants in the
   chat room, so that the user becomes aware of the roster.

   If a participant wants to avoid security concerns on the path between
   himself and the MSRP switch (e.g., being eavesdropped, faked packet
   injection, or packet corruption), the participant's user agent can
   force the usage of MSRP over a TLS [RFC5246] transport connection.
   This is negotiated in the SDP offer/answer exchange as per regular
   RFC 4975 [RFC4975] procedures.  This negotiation will result in both
   endpoints establishing a TLS [RFC5246] transport connection that is
   used to exchange MSRP messages.  The MSRP switch may also have local
   policy that forces the usage of TLS transport for all MSRP sessions,
   something that is also negotiated in SDP as per regular RFC 4975
   [RFC4975] procedures.

   Nicknames are used to show the appearance of the participants of the
   chat room.  A successful take over of a nickname from a participant
   might lead to private messages to be sent to the wrong destination.
   The recipient's URI will be different from the URI associated to the



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   original owner of the nickname, but the sender might not notice this.
   To avoid takeovers the MSRP switch MUST make sure that a nickname is
   unique inside a chat room.  Also the security consideration for any
   authenticated identity mechanisms used to validate the SIP AOR will
   apply to this document as well.  The chat room has a policy that
   determines the time that a nickname is still reserved to its holder,
   once it is no longer in used.  This allows, e.g., a user that
   accidentally looses its connectivity, to re-connect to the chat room
   and keep on using the same nickname.  It is up to the policy of the
   chat room to determine if a nickname that has been previously used by
   another participant of the chat room can be reserved or not.

   Section 7.1 discusses the problem of similar but different nicknames
   (e.g., thanks to the use of similar characters), and chat rooms MAY
   provide a mechanism to mitigate confusable nicknames.

   Recipients of instant messages should be cautious with the rendering
   of content, which can be malicious in nature.  This includes, but it
   is not only restricted to, the reception of HTML and Javascript
   scripts, executable code, phishing attempts, etc.  Endpoints SHOULD
   always request permission from the user before executing one of these
   actions.

   It must be noted that endpoints using TLS client side certificate
   with real names in the certificates will not be anonymous to the MSRP
   switch they connect to.  While the name in the certificate might not
   be used by MSRP, the server will have a certificate with the actual
   name in it.


12.  Contributors

   This work would have never been possible without the fruitful
   discussions in the SIMPLE WG mailing list, specially with Brian Rosen
   (Neustar) and Paul Kyzivat (Huawei), who provided extensive review
   and improvements throughout the document.


13.  Acknowledgments

   The authors want to thank Eva Leppanen, Adamu Haruna, Adam Roach,
   Matt Lepinski, Mary Barnes, Ben Campbell, Paul Kyzivat, Adrian
   Georgescu, Nancy Greene, Cullen Jennings, Flemming Andreasen, Suresh
   Krishnan, Christer Holmberg, Saul Ibarra, Enrico Marocco, Alexey
   Melnikov, Peter Saint-Andre, Stephen Farrel, and Martin Stiemerling
   for providing comments.





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14.  References

14.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3261]  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.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              June 2002.

   [RFC3323]  Peterson, J., "A Privacy Mechanism for the Session
              Initiation Protocol (SIP)", RFC 3323, November 2002.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC3840]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
              "Indicating User Agent Capabilities in the Session
              Initiation Protocol (SIP)", RFC 3840, August 2004.

   [RFC3860]  Peterson, J., "Common Profile for Instant Messaging
              (CPIM)", RFC 3860, August 2004.

   [RFC3862]  Klyne, G. and D. Atkins, "Common Presence and Instant
              Messaging (CPIM): Message Format", RFC 3862, August 2004.

   [RFC4353]  Rosenberg, J., "A Framework for Conferencing with the
              Session Initiation Protocol (SIP)", RFC 4353,
              February 2006.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC4575]  Rosenberg, J., Schulzrinne, H., and O. Levin, "A Session
              Initiation Protocol (SIP) Event Package for Conference
              State", RFC 4575, August 2006.

   [RFC4975]  Campbell, B., Mahy, R., and C. Jennings, "The Message
              Session Relay Protocol (MSRP)", RFC 4975, September 2007.

   [RFC4976]  Jennings, C., Mahy, R., and A. Roach, "Relay Extensions
              for the Message Sessions Relay Protocol (MSRP)", RFC 4976,



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              September 2007.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5239]  Barnes, M., Boulton, C., and O. Levin, "A Framework for
              Centralized Conferencing", RFC 5239, June 2008.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, September 2009.

   [RFC6501]  Novo, O., Camarillo, G., Morgan, D., and J. Urpalainen,
              "Conference Information Data Model for Centralized
              Conferencing (XCON)", RFC 6501, March 2012.

   [RFC6502]  Camarillo, G., Srinivasan, S., Even, R., and J.
              Urpalainen, "Conference Event Package Data Format
              Extension for Centralized Conferencing (XCON)", RFC 6502,
              March 2012.

   [I-D.ietf-precis-nickname]
              Saint-Andre, P., "Preparation and Comparison of
              Nicknames", draft-ietf-precis-nickname-05 (work in
              progress), November 2012.

14.2.  Informative References

   [RFC2810]  Kalt, C., "Internet Relay Chat: Architecture", RFC 2810,
              April 2000.

   [RFC3325]  Jennings, C., Peterson, J., and M. Watson, "Private
              Extensions to the Session Initiation Protocol (SIP) for
              Asserted Identity within Trusted Networks", RFC 3325,
              November 2002.

   [RFC3966]  Schulzrinne, H., "The tel URI for Telephone Numbers",
              RFC 3966, December 2004.

   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474, August 2006.

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, March 2011.




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Authors' Addresses

   Aki Niemi

   Email: aki.niemi@iki.fi


   Miguel A. Garcia-Martin
   Ericsson
   Calle Via de los Poblados 13
   Madrid, ES  28033
   Spain

   Email: miguel.a.garcia@ericsson.com


   Geir A. Sandbakken
   Cisco Systems
   Philip Pedersens vei 20
   N-1366 Lysaker
   Norway

   Phone: +47 67 125 125
   Email: geirsand@cisco.com
   URI:   http://www.cisco.com


























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