path: root/doc/rfc/rfc7701.txt

Internet Engineering Task Force (IETF)                          A. Niemi
Request for Comments: 7701
Category: Standards Track                               M. Garcia-Martin
ISSN: 2070-1721                                                 Ericsson
                                                           G. Sandbakken
                                                           Cisco Systems
                                                           December 2015


    Multi-party Chat Using the Message Session Relay Protocol (MSRP)

Abstract

   The Message Session Relay Protocol (MSRP) defines a mechanism for
   sending instant messages (IMs) 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 is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7701.


















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Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   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
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   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 .....................................................5
   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 ....................12
      5.1. Creating a Chat Room ......................................12
      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 .......................................25
      7.4. Nicknames in Conference Event Packages ....................25
   8. The SDP 'chatroom' Attribute ...................................25
   9. Examples .......................................................28
      9.1. Joining a Chat Room .......................................28
      9.2. Setting Up a Nickname .....................................30
      9.3. Sending a Regular Message to the Chat Room ................31
      9.4. Sending a Private Message to a Participant ................33
      9.5. Chunked Private Message ...................................35
      9.6. Nickname in a Conference Information Document .............35
   10. IANA Considerations ...........................................37
      10.1. New MSRP Method ..........................................37
      10.2. New MSRP Header ..........................................37
      10.3. New MSRP Status Codes ....................................37
      10.4. New SDP Attribute ........................................38
   11. Security Considerations .......................................38
   12. References ....................................................40
      12.1. Normative References .....................................40
      12.2. Informative References ...................................43
   Acknowledgments ...................................................43
   Contributors ......................................................43
   Authors' Addresses ................................................44










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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 (UA) can join in a multi-
   party conversation called a "conference" that is hosted by a
   specialized UA called a "focus" [RFC4353].  Such a conference can
   naturally involve MSRP sessions.  It is the responsibility of an
   entity handling the media to relay IMs 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 can
   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 room functionality are already
   available today.  For example, Internet Relay Chat (IRC) [RFC2810],
   Extensible Messaging and Presence Protocol (XMPP): Core [RFC6120], as
   well as many other proprietary systems.  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 should be
   disclosed to other participants.  This memo uses the SIP Conferencing
   Framework [RFC4353] as a design basis.  It also aims to be compatible
   with "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.










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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 BCP 14 [RFC2119] and
   indicate requirement levels for compliant implementations.

   This memo deals with "Tightly Coupled SIP Conferences" as defined in
   the SIP Conferencing Framework [RFC4353] and adopts the terminology
   from that document.  In addition, we introduce some new terms:

   Nickname:   a pseudonym or descriptive name associated with 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 IMs.  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
      that is a synonym of a "Conference URI" as defined in RFC 4353
      [RFC4353].

   Sender:   the chat room participant who originally created an IM 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 IM Sender.

   MSRP Switch:   a media-level entity that is an 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 IM:   an IM 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 address of
      record (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



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      session and will be utilized by the MSRP switch to forward
      messages to and from anonymous participants.  Privacy and
      anonymity are discussed in greater 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
      the mechanisms defined in "A Session Initiation Protocol (SIP)
      Event Package for Conference State" [RFC4575] or "Conference Event
      Package Data Format Extension for Centralized Conferencing (XCON)"
      [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 one in "A 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 UAs, 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 to enable the building of 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 exchange
           IMs with the rest of the participants.

   REQ-2:  A recipient of an IM 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 that was used by the
           sender when joining the chat room.





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   REQ-3:  A recipient of an IM 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 on the one that
           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 IM).

   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 for a participant to be known only by an
           anonymous identifier and not their real identifier by 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 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 both
   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: it 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], 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 IM 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, if the




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   recipient is logged in from different endpoints, the MSRP switch will
   distribute the private message to each endpoint at which 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 with 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 "The complete set of rules governing a
   particular conference."  A chat room is a specialized type of
   conference, and the conference policy 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 Sections 7, 7.1, and 8 for details.  Also, the
      scope of uniqueness of the nickname: the chat room (conference
      instance), a realm or domain, a server, etc.








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   Nickname quarantine:  The quarantine to be imposed on 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
      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 allows users to send
      private messages to other users of the chat room through the MSRP
      switch.  See Sections 6.2 and 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 in from different
      endpoints using the same identity.  See Sections 6.1 and 6.2 for
      details.

   Force TLS transport:  Whether the MSRP switch accepts only Transport
      Layer Security (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 to those explicitly listed, a "*" is added
      to the list.  A single "*" indicates that the chat room accepts
      any wrapped media type.









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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.

   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 IMs 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 4975 [RFC4975]
   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



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   cannot 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
   procedures in RFC 4975 [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 reject
   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 IMs 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
   IM 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, the Message/CPIM wrapper carries the
      recipient information (e.g., To and Cc headers).

   If the UA 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 a similar privacy
        mechanism.




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   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
   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.  For example, the policy can determine whether the
   chat room is deleted when the creator leaves the room or whether an
   out-of-band mechanism is responsible for the deletion.

6.  Sending and Receiving Instant Messages

6.1.  Regular Messages

   This section describes the conventions used to send and receive IMs
   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:



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

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

   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



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   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
   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 the
   MSRP switch SHOULD still 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; it MUST also 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 had joined the chat
   room when the first chunk was distributed from receiving 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 use 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.




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   This chunk reception timer can be reset every time a new chunk of the
   same message is received.  When this timer expires, the MSRP switch
   MUST consider that the sending of the message was aborted, and it MAY
   discard all the message state associated with it, including the
   Message-ID and the list of recipients.  Additionally, if this chunk
   reception timer expires, the MSRP switch MAY choose to send an abort
   chunk (i.e., one with the "#" flag set) to each to the recipients.
   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 AoR 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 IMs, i.e., IMs that are addressed to one participant of the
   chat room rather than to all of them.  The chat room has a local
   policy that determines whether or not private messages are supported.
   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 IM to a
   participant in the chat room, it follows the same procedures to
   create a SEND request as for regular messages (Section 6.1).  The



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   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.

      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 delivered 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 regarding 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
   428 response indicates that the recipient does not support private
   messages.  Any potential REPORT request that the MSRP switch sends to



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   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.

   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 know 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
   the sender of the message, instead of using 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 sections (Sections 6.1 or 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




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   switch MUST reject the request with a 403 response.  In cases without
   error, 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
   receiving multiple reports for a single MSRP request.

6.4.  Congestion Avoidance

   Congestion can occur when multiple heterogeneous interfaces are used
   by a number 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 paths 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.  Therefore, it is
   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 up the pace.

   Additionally, since MSRP uses a connection-oriented transport
   protocol such as TCP, it is expected that TCP congestion control
   mechanisms be 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 with 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:



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      *  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
         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
      with 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 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.  A nickname is associated with a URI; the
   focus knows the participant by its association to this URI.
   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 moniker by 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 they may be temporary.  Users also need to reserve
   a nickname prior to its utilization.





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   This memo specifies the nickname as a string.  The nickname string
   MUST unambiguously be associated with 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 (XMPP): Core" [RFC6120].  The chat
   room may have policies associated with nicknames.  It may not accept
   nickname strings at all, or it may provide a wider unambiguous scope
   like a domain or server, similar to IRC [RFC2810].

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.  Finally, 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 be 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, Enforcement, and Comparison of Internationalized
   Strings Representing Nicknames" [RFC7700].

   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"



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   and "B0Y" are different nicknames that 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 perceive 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
   currently using the nickname is making this subsequent request.  This
   may include, e.g., allowing the participant's URI to 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], included in a trusted SIP P-Asserted-
   Identity header field [RFC3325], included in a valid SIP Identity
   header field [RFC4474], or 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 these cases, 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.









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   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].

            other-method =/ NICKNAMEm
                     ; other-method defined in RFC 4975
            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 AoR), 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 UA.  In this case,
   the user MAY request a nickname that is the same or different 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.




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   If a NICKNAME request that attempts to modify the current nickname of
   the user fails for some reason, 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.

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 roster 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 IM addressed
   to a participant, but since the MSRP switch does not support the
   functions specified in this memo, the message would eventually be
   distributed to all the participants of the chat room.

   The reverse case also exists.  A participant, say Alice, whose UA
   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.






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   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 forbidden.

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

   In subsequent SDP offer/answer [RFC3264] exchanges pertaining to the
   same session, the 'chatroom' attribute MAY be modified with respect
   to an earlier SDP offer/answer exchange.  The new value of this
   attribute indicates 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 a corresponding MSRP stream is still in place, it
   indicates that support for the procedures indicated in this document
   are disabled.















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   The 'chatroom' SDP attribute has the following 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
   room allows the use of 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 the use of the procedures specified in Section 6.2.

   An 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 nicknames or
   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.







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   Extensions defined in an IETF document MUST follow the 'standard-
   token' ABNF previously defined.  In this type of extension, 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 be included in the DNS name of the
   vendor.  Then, the token is reversed in order to avoid clashes of
   tokens.  The following is an example of an 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 chat room extensions defined in this
   document.

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

   Figure 3: Flow Diagram of a User Joining a Chat Room















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   F1: Alice constructs an SDP description that includes an MSRP media
   stream.  She also indicates her support for the chat room 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.

   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







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   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.

   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.






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   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 is 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.

   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








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   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.

   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$





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   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 to all participants in the chat room
   rather than 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 is 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.

   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.



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   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.

   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$















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9.5.  Chunked Private Message

   The MSRP message below is a depiction 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 is a depiction of 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
      relationship.  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.

    <?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 that has been added to
   the "Methods" subregistry of the "Message Session Relay Protocol
   (MSRP) Parameters" registry:

      NICKNAME

   See Section 7 for details.

10.2.  New MSRP Header

   This specification defines a new MSRP header that has been added to
   the "Header Fields" subregistry 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 four new MSRP status codes that have been
   added to the "Status Codes" subregistry 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 7701 |
        |  424  | Malformed nickname                  |  RFC 7701 |
        |  425  | Nickname reserved or already in use |  RFC 7701 |
        |  428  | Private messages not supported      |  RFC 7701 |
        +-------+-------------------------------------+-----------+

                         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 7701 (this document)

   See 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)
   accelerator as it takes a message from one entity and sends it 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




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   include 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; thus, they cannot be specified here.

   If the participant's SIP UA 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 its MSRP client
   will try to send messages having potential multiple recipients to the
   MSRP switch.  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 the 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 that 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 UA does
   not support the procedures of this document (i.e., when the SIP UA is
   not chat room aware), the MSRP switch SHOULD send a regular MSRP
   message indicating that the SIP UA is actually part of a chat room
   and that all the messages that the user sends correctly formatted
   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., eavesdropping, faked packet
   injection, or packet corruption), the participant's UA can force the
   usage of MSRP over a TLS [RFC5246] transport connection.  This is
   negotiated in the SDP offer/answer exchange as per the regular
   procedures of RFC 4975 [RFC4975].  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 the regular
   procedures of RFC 4975 [RFC4975].

   Nicknames are used to show the appearance of the participants of the
   chat room.  A successful takeover of a nickname from a participant
   might lead to private messages being sent to the wrong destination.
   The recipient's URI will be different from the URI associated with
   the original owner of the nickname, but the sender might not notice



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   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 for its holder, once it is no longer being used.  This
   allows, e.g., a user that accidentally loses its connectivity, to
   reconnect to the chat room and keep on using the same nickname.  It
   depends on the policy of the chat room 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 IMs should be cautious with the rendering of content,
   which can be malicious in nature.  This includes, but is not limited
   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 a TLS client side certificate
   with real names in the certificates will not be anonymous to the MSRP
   switch to which they connect.  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.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,
              <http://www.rfc-editor.org/info/rfc3261>.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              DOI 10.17487/RFC3264, June 2002,
              <http://www.rfc-editor.org/info/rfc3264>.




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   [RFC3323]  Peterson, J., "A Privacy Mechanism for the Session
              Initiation Protocol (SIP)", RFC 3323,
              DOI 10.17487/RFC3323, November 2002,
              <http://www.rfc-editor.org/info/rfc3323>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <http://www.rfc-editor.org/info/rfc3629>.

   [RFC3840]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
              "Indicating User Agent Capabilities in the Session
              Initiation Protocol (SIP)", RFC 3840,
              DOI 10.17487/RFC3840, August 2004,
              <http://www.rfc-editor.org/info/rfc3840>.

   [RFC3860]  Peterson, J., "Common Profile for Instant Messaging
              (CPIM)", RFC 3860, DOI 10.17487/RFC3860, August 2004,
              <http://www.rfc-editor.org/info/rfc3860>.

   [RFC3862]  Klyne, G. and D. Atkins, "Common Presence and Instant
              Messaging (CPIM): Message Format", RFC 3862,
              DOI 10.17487/RFC3862, August 2004,
              <http://www.rfc-editor.org/info/rfc3862>.

   [RFC4353]  Rosenberg, J., "A Framework for Conferencing with the
              Session Initiation Protocol (SIP)", RFC 4353,
              DOI 10.17487/RFC4353, February 2006,
              <http://www.rfc-editor.org/info/rfc4353>.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <http://www.rfc-editor.org/info/rfc4566>.

   [RFC4575]  Rosenberg, J., Schulzrinne, H., and O. Levin, Ed., "A
              Session Initiation Protocol (SIP) Event Package for
              Conference State", RFC 4575, DOI 10.17487/RFC4575, August
              2006, <http://www.rfc-editor.org/info/rfc4575>.

   [RFC4975]  Campbell, B., Ed., Mahy, R., Ed., and C. Jennings, Ed.,
              "The Message Session Relay Protocol (MSRP)", RFC 4975,
              DOI 10.17487/RFC4975, September 2007,
              <http://www.rfc-editor.org/info/rfc4975>.

   [RFC4976]  Jennings, C., Mahy, R., and A. Roach, "Relay Extensions
              for the Message Sessions Relay Protocol (MSRP)", RFC 4976,
              DOI 10.17487/RFC4976, September 2007,
              <http://www.rfc-editor.org/info/rfc4976>.




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   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

   [RFC5239]  Barnes, M., Boulton, C., and O. Levin, "A Framework for
              Centralized Conferencing", RFC 5239, DOI 10.17487/RFC5239,
              June 2008, <http://www.rfc-editor.org/info/rfc5239>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
              <http://www.rfc-editor.org/info/rfc5681>.

   [RFC6501]  Novo, O., Camarillo, G., Morgan, D., and J. Urpalainen,
              "Conference Information Data Model for Centralized
              Conferencing (XCON)", RFC 6501, DOI 10.17487/RFC6501,
              March 2012, <http://www.rfc-editor.org/info/rfc6501>.

   [RFC6502]  Camarillo, G., Srinivasan, S., Even, R., and J.
              Urpalainen, "Conference Event Package Data Format
              Extension for Centralized Conferencing (XCON)", RFC 6502,
              DOI 10.17487/RFC6502, March 2012,
              <http://www.rfc-editor.org/info/rfc6502>.

   [RFC7700]  Saint-Andre, P., "Preparation, Enforcement, and Comparison
              of Internationalized Strings Representing Nicknames",
              RFC 7700, DOI 10.17487/RFC7700, December 2015,
              <http://www.rfc-editor.org/info/rfc7700>.


















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12.2.  Informative References

   [RFC2810]  Kalt, C., "Internet Relay Chat: Architecture", RFC 2810,
              DOI 10.17487/RFC2810, April 2000,
              <http://www.rfc-editor.org/info/rfc2810>.

   [RFC3325]  Jennings, C., Peterson, J., and M. Watson, "Private
              Extensions to the Session Initiation Protocol (SIP) for
              Asserted Identity within Trusted Networks", RFC 3325,
              DOI 10.17487/RFC3325, November 2002,
              <http://www.rfc-editor.org/info/rfc3325>.

   [RFC3966]  Schulzrinne, H., "The tel URI for Telephone Numbers",
              RFC 3966, DOI 10.17487/RFC3966, December 2004,
              <http://www.rfc-editor.org/info/rfc3966>.

   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474,
              DOI 10.17487/RFC4474, August 2006,
              <http://www.rfc-editor.org/info/rfc4474>.

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
              March 2011, <http://www.rfc-editor.org/info/rfc6120>.

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 Farrell, and Martin Stiemerling
   for providing comments.

Contributors

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










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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 Pedersensvei 1
   1366 Lysaker
   Norway

   Email: geirsand@cisco.com




























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