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Versions: (draft-ejzak-mmusic-msrp-usage-data-channel) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 24

MMUSIC                                                     J. Recio, Ed.
Internet-Draft                                              Unaffiliated
Updates: 4975 (if approved)                                  C. Holmberg
Intended status: Standards Track                                Ericsson
Expires: February 19, 2021                               August 18, 2020


                        MSRP over Data Channels
              draft-ietf-mmusic-msrp-usage-data-channel-24

Abstract

   This document specifies how a Web Real-Time Communication (WebRTC)
   data channel can be used as a transport mechanism for the Message
   Session Relay Protocol (MSRP) and how the Session Description
   Protocol (SDP) offer/answer mechanism can be used to negotiate such a
   data channel, referred to as an MSRP data channel.  Two network
   configurations are supported: connecting two MSRP data channel
   endpoints; and a gateway configuration, connecting an MSRP data
   channel endpoint with an MSRP over TCP or TLS endpoint.  This
   document updates RFC4975.  This document updates RFC4975.

Status of This Memo

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

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 19, 2021.

Copyright Notice

   Copyright (c) 2020 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
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  WebRTC Data Channel Considerations  . . . . . . . . . . . . .   4
     3.1.  MSRP Data Channel . . . . . . . . . . . . . . . . . . . .   4
   4.  SDP Considerations  . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  MSRP URI  . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  msrp-scheme . . . . . . . . . . . . . . . . . . . . . . .   4
     4.3.  Use of the dcmap Attribute  . . . . . . . . . . . . . . .   5
     4.4.  Use of the dcsa Attribute . . . . . . . . . . . . . . . .   5
     4.5.  Use of the dcsa embedded setup Attribute  . . . . . . . .   6
     4.6.  Session Closing . . . . . . . . . . . . . . . . . . . . .   7
     4.7.  Support for MSRP File Transfer Function . . . . . . . . .   7
     4.8.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  MSRP Considerations . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Session Mapping . . . . . . . . . . . . . . . . . . . . .   9
     5.2.  Session Opening . . . . . . . . . . . . . . . . . . . . .   9
     5.3.  Session Closing . . . . . . . . . . . . . . . . . . . . .   9
     5.4.  Data Framing  . . . . . . . . . . . . . . . . . . . . . .   9
     5.5.  Data Sending, Receiving and Reporting . . . . . . . . . .   9
     5.6.  Support for MSRP File Transfer Function . . . . . . . . .  10
   6.  Gateway Considerations  . . . . . . . . . . . . . . . . . . .  10
   7.  Updates to RFC4975  . . . . . . . . . . . . . . . . . . . . .  11
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
     9.1.  msrps URI scheme  . . . . . . . . . . . . . . . . . . . .  11
     9.2.  Subprotocol Identifier MSRP . . . . . . . . . . . . . . .  12
     9.3.  SDP Attributes  . . . . . . . . . . . . . . . . . . . . .  12
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  18
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     11.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   The Message Session Relay Protocol (MSRP) [RFC4975] is a protocol for
   transmitting a series of related instant messages in the context of a
   session.  In addition to instant messaging, MSRP can also be used for
   image sharing or file transfer.  MSRP was initially defined in




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   [RFC4975] to work over TCP and TLS connections, and over a WebSocket
   subprotocol specified by [RFC7977].

   This document specifies how a Web Real-Time Communication (WebRTC)
   data channel [I-D.ietf-rtcweb-data-channel] can be used as a
   transport mechanism for MSRP, without the TCP and TLS layers, and how
   the Session Description Protocol (SDP) offer/answer mechanism for
   data channels [I-D.ietf-mmusic-data-channel-sdpneg] can be used to
   negotiate such a data channel.

   In this document, an MSRP data channel refers to a WebRTC data
   channel for which the instantiated subprotocol is "msrp" and where
   the data channel is negotiated using the SDP offer/answer mechanism
   [I-D.ietf-mmusic-data-channel-sdpneg].

   Defining MSRP as a data channel subprotocol has many benefits:

   o  provides to applications a proven protocol enabling instant
      messaging, file transfer, image sharing

   o  integrates those features with other WebRTC voice, video and data
      features

   o  leverages the SDP-based negotiation already defined for MSRP

   o  allows the interworking with MSRP endpoints running on a TCP or
      TLS connection

   Compared to WebSockets, which provide a message passing protocol to
   applications with no direct access to TCP or TLS sockets, data
   channels provide a low latency transport, leverage NAT-aware
   connectivity and security features of WebRTC.

   Considering an MSRP endpoint as an MSRP application that uses a
   WebRTC data channel, this document describes two configurations where
   the other endpoint is respectively either another MSRP over data
   channel endpoint (e.g., a WebRTC application) or an MSRP endpoint
   using either TCP or TLS transport.

   This document updates [RFC4975] as described in Section 7.

2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119][RFC8174] when, and only when, they appear in all
   capitals, as shown here.



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3.  WebRTC Data Channel Considerations

3.1.  MSRP Data Channel

   The following WebRTC data channel property values
   [I-D.ietf-rtcweb-data-channel] apply to an MSRP data channel:

              +--------------------------+------------------+
              | Property                 | Value            |
              +--------------------------+------------------+
              | Subprotocol Identifier   | msrp             |
              | Transmission reliability | reliable         |
              | Transmission order       | in-order         |
              | Label                    | See Section 4.3  |
              +--------------------------+------------------+

4.  SDP Considerations

   The generic SDP considerations, including the SDP offer/answer
   procedures [RFC3264], for negotiating a WebRTC data channel are
   defined in [I-D.ietf-mmusic-data-channel-sdpneg].  This section, and
   its subsections, define the SDP considerations that are specific to
   an MSRP data channel, identified by the 'subprotocol' attribute
   parameter, with a "msrp" parameter value, in the 'dcmap' attribute.

4.1.  MSRP URI

   This document extends the MSRP URI syntax [RFC4975] by defining the
   new transport parameter value "dc" (an abbreviation of data channel):

       transport  /= "dc"
       ; Add "dc" to existing transports per Section 9 of [RFC4975]

   MSRP design provides for new transport bindings (see Section 6 of
   [RFC4975]).  MSRP implementations are expected to allow unrecognized
   transports for which there is no need to establish a connection to
   the resource described by the URI, as it's the case of data channels
   (Section 4.4).

4.2.  msrp-scheme

   The msrp-scheme portion of the MSRP-URI that represents an MSRP data
   channel endpoint (used in the SDP path attribute and in the MSRP
   message headers) is always "msrps", which indicates that the MSRP
   data channel is always secured using DTLS as described in
   [I-D.ietf-rtcweb-data-channel].





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4.3.  Use of the dcmap Attribute

   An offerer and answerer SHALL, in each offer and answer, include a
   'dcmap' attribute [I-D.ietf-mmusic-data-channel-sdpneg] in the SDP
   media description (m= section) [RFC4566] describing the SCTP
   association [RFC4960] used to realize the MSRP data channel.

   The attribute includes the following data channel parameters:

   o  "label=" labelstring

   o  "subprotocol=" "msrp"

   The labelstring is set by the MSRP application according to
   [I-D.ietf-mmusic-data-channel-sdpneg].

   The offerer and answerer SHALL NOT include the 'max-retr' and the
   'max-time' attribute parameters in the 'dcmap' attribute.

   The offerer and answerer MAY include the 'ordered' attribute
   parameter in the 'dcmap' attribute.  If included, the attribute
   parameter value SHALL be set to "true".

   Below is an example of a 'dcmap' attribute for an MSRP session to be
   negotiated with stream-id=2 and label="chat":

   a=dcmap:2 label="chat";subprotocol="msrp"

4.4.  Use of the dcsa Attribute

   An offerer and answerer can, in each offer and answer, include one or
   more 'dcsa' attributes [I-D.ietf-mmusic-data-channel-sdpneg] in the
   m= section describing the SCTP association used to realize the MSPR
   data channel.  An SDP attribute included in a 'dcsa' attribute is
   referred to as a dcsa embedded attribute.

   If an offerer or answerer receives a 'dcsa' attribute that contains
   an SDP attribute which usage has not been defined for an MSRP data
   channel, the offerer or answerer should ignore the 'dcsa' attribute,
   following the rules in Section 6.7 of
   [I-D.ietf-mmusic-data-channel-sdpneg].

   An offerer and answerer SHALL include a 'dcsa' attribute for each of
   the following MSRP-specific SDP attributes:

   o  defined in [RFC4975]: "path".

   o  defined in [RFC6714]: "msrp-cema".



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   o  defined in [RFC6135]: "setup".  See Section 4.5

   It is considered a protocol error if one or more of the dcsa embedded
   attributes listed above are not included in an offer or answer.

   An offerer and answerer MAY include a 'dcsa' attribute for any of the
   following MSRP-specific SDP attributes, following the procedures
   defined for each attribute:

   o  defined in [RFC4975]: "accept-types", "accept-wrapped-types" and
      "max-size"

   o  defined in [RFC4566]: "sendonly", "recvonly", "inactive" and
      "sendrecv"

   o  defined in [RFC5547]: all the parameters related to MSRP file
      transfer.  See Section 4.7.

   A subsequent offer or answer MAY update the previously negotiated
   MSRP subprotocol attributes while keeping the 'dcmap' attribute
   associated with the MSRP data channel unchanged.  The semantics for
   newly negotiated MSRP subprotocol attributes are per [RFC4975].

   When MSRP messages are transported on a data channel, the 'path'
   attribute is not used for routing of the messages.  The MSRP data
   channel is established using the SDP offer/answer procedures defined
   in [I-D.ietf-mmusic-data-channel-sdpneg], and the MSRP messages are
   then transported on that data channel.  This is different from legacy
   MSRP [RFC4975] but similar to MSRP CEMA [RFC6714].  Because of this,
   a dcsa embedded 'msrp-cema' attribute is mandated for MSRP sessions
   over data channels.  However, when an endpoint receives an MSRP
   message over a data channel, it MUST still perform the MSRP-URI
   comparison procedures defined in [RFC4975].

4.5.  Use of the dcsa embedded setup Attribute

   As described in Section 4.4, the usage of a dsca embedded 'setup'
   attribute is mandated for MSRP sessions over data channels.  It is
   used to negotiate which MSRP session endpoint assumes the active role
   as per Section 4.2.2 of [RFC6135] and Section 5.4 of [RFC4975].  It
   has no relationship with the DTLS connection establishment roles
   [I-D.ietf-mmusic-sctp-sdp].

   The dcsa embedded setup attribute is of the form "a=dcsa:x
   setup:<role>", with x being the data channel's SCTP stream
   identifier, so that such attribute is explicitly associated with an
   MSRP session over a specific data channel.




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4.6.  Session Closing

   An MSRP session is closed by closing the associated data channel,
   following the procedures in [I-D.ietf-mmusic-data-channel-sdpneg].

   The port value for the "m=" line SHOULD NOT be changed (e.g. to zero)
   when closing an MSRP session (unless all data channels are being
   closed and the SCTP association is no longer needed), since this
   would close the SCTP association and impact all of the data channels.
   In all cases in [RFC4975] where the procedure calls for setting the
   port to zero for the MSRP "m=" line in an SDP offer for TCP
   transport, the SDP offerer of an MSRP session with data channel
   transport SHALL remove the corresponding dcmap and dcsa attributes.

4.7.  Support for MSRP File Transfer Function

   SDP attributes specified in [RFC5547] for a file transfer "m=" line
   are embedded as subprotocol-specific attributes using the syntax
   defined in [I-D.ietf-mmusic-data-channel-sdpneg].

4.8.  Example

   Below is an example of an offer and an answer that include the
   attributes needed to establish two MSRP sessions: one for chat and
   one for file transfer.  The example is derived from a combination of
   examples in [RFC4975] and [RFC5547].

 Offer:

    m=application 54111 UDP/DTLS/SCTP webrtc-datachannel
    c=IN IP6 2001:db8::3
    a=max-message-size:100000
    a=sctp-port:5000
    a=setup:actpass
    a=fingerprint:SHA-256 12:DF:3E:5D:49:6B:19:E5:7C:AB:4A:AD:B9:B1:3F:\
       82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB:4A:AD
    a=tls-id:4a756565cddef001be82
    a=dcmap:0 label="chat";subprotocol="msrp"
    a=dcsa:0 msrp-cema
    a=dcsa:0 setup:active
    a=dcsa:0 accept-types:message/cpim text/plain
    a=dcsa:0 path:msrps://2001:db8::3:54111/si438dsaodes;dc
    a=dcmap:2 label="file transfer";subprotocol="msrp"
    a=dcsa:2 sendonly
    a=dcsa:2 msrp-cema
    a=dcsa:2 setup:active
    a=dcsa:2 accept-types:message/cpim
    a=dcsa:2 accept-wrapped-types:*



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    a=dcsa:2 path:msrps://2001:db8::3:54111/jshA7we;dc
    a=dcsa:2 file-selector:name:"picture1.jpg" type:image/jpeg \
       size:1463440 hash:sha-256:7C:DF:3E:5D:49:6B:19:E5:12:AB:4A:AD: \
       4A:B1:3F:82:3E:3B:54:12:02:5D:18:DF:49:6B:19:E5:7C:AB:B9:AD
    a=dcsa:2 file-transfer-id:rjEtHAcYVZ7xKwGYpGGwyn5gqsSaU7Ep
    a=dcsa:2 file-disposition:attachment
    a=dcsa:2 file-date:creation:"Tue, 11 Aug 2020 19:05:30 +0200"
    a=dcsa:2 file-icon:cid:id2@bob.example.com
    a=dcsa:2 file-range:1-1463440

 Answer:

    m=application 51444 UDP/DTLS/SCTP webrtc-datachannel
    c=IN IP6 IP6 2001:db8::1
    a=max-message-size:100000
    a=sctp-port:6000
    a=setup:passive
    a=fingerprint:SHA-256 5D:02:3E:AD:49:6B:19:E5:7C:AB:4A:AD:B9:B1: \
       3F:82:18:3B:54:DF:12:6B:3E:5D:49:DF:19:E5:7C:AB:4A:5D
    a=tls-id:65cd4a7565debe82f100
    a=dcmap:0 label="chat";subprotocol="msrp"
    a=dcsa:0 msrp-cema
    a=dcsa:0 setup:passive
    a=dcsa:0 accept-types:message/cpim text/plain
    a=dcsa:0 path:msrps://2001:db8::1:51444/di551fsaodes;dc
    a=dcmap:2 label="file transfer";subprotocol="msrp"
    a=dcsa:2 recvonly
    a=dcsa:2 msrp-cema
    a=dcsa:2 setup:passive
    a=dcsa:2 accept-types:message/cpim
    a=dcsa:2 accept-wrapped-types:*
    a=dcsa:2 path:msrps://2001:db8::1:51444/jksh7Bwc;dc
    a=dcsa:2 file-selector:name:"picture1.jpg" type:image/jpeg \
       size:1463440
    a=dcsa:2 file-transfer-id:rjEtHAcYVZ7xKwGYpGGwyn5gqsSaU7Ep
    a=dcsa:2 file-range:1-1463440


   Note that due to RFC formatting conventions, this document splits SDP
   across lines whose content would exceed 72 characters.  A backslash
   character marks where this line folding has taken place.  This
   backslash and its trailing CRLF and whitespace would not appear in
   actual SDP content.








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5.  MSRP Considerations

   The procedures specified in [RFC4975] apply except when this document
   specifies otherwise.  This section describes the MSRP considerations
   specific to an MSRP data channel.

5.1.  Session Mapping

   In this document, each MSRP session maps to one data channel exactly.

5.2.  Session Opening

   Section 4.5 describes how the active MSRP session endpoint role is
   negotiated.  The active MSRP session endpoint uses the data channel
   established for this MSRP session by the generic data channel opening
   procedure defined in [I-D.ietf-mmusic-data-channel-sdpneg].

   As soon as the WebRTC data channel is opened, the MSRP session is
   actually opened by the active MSRP session endpoint.  In order to do
   this the active MSRP endpoint sends an MSRP SEND message (empty or
   not) to the peer (passive) MSRP endpoint.

5.3.  Session Closing

   The closure of an MSRP session SHALL be signaled via SDP following
   the requirements in Section 4.6

   If the data channel used to transport the MSRP session fails and gets
   torn down, the endpoints SHALL consider the MSRP session failed.  An
   MSRP endpoint MAY, based on local policy, try to negotiate a new MSRP
   data channel.

5.4.  Data Framing

   Each text-based MSRP message is sent on the corresponding data
   channel using standard MSRP framing and chunking procedures, as
   defined in [RFC4975], with each MSRP chunk delivered in a single SCTP
   user message.  Therefore all sent MSRP chunks SHALL have lengths of
   less than or equal to the value of the peer's "max-message-size"
   attribute [I-D.ietf-mmusic-sctp-sdp] associated with the SCTP
   association.

5.5.  Data Sending, Receiving and Reporting

   Data sending, receiving and reporting procedures SHALL conform to
   [RFC4975].





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5.6.  Support for MSRP File Transfer Function

   [RFC5547] defines an end-to-end file transfer method based on MSRP
   and the SDP offer/answer mechanism.  This file transfer method is
   also usable by MSRP endpoints using data channels, with the following
   considerations:

   o  As an MSRP session maps to one data channel, a file transfer
      session maps also to one data channel.

   o  SDP attributes are negotiated as specified in Section 4.7.

   o  Once the file transfer is complete, the same data channel MAY be
      reused for another file transfer.

6.  Gateway Considerations

   This section describes the network configuration where one MSRP
   endpoint uses an MSRP data channel as MSRP transport, the other MSRP
   endpoint uses TLS/TCP connections as MSRP transport, and the two MSRP
   endpoints interwork via a gateway.

   Specifically, a gateway can be configured to interwork an MSRP
   session over a data channel with a peer that does not support data
   channel transport in one of two ways.

   In one model, the gateway performs as an MSRP Back-to-Back User Agent
   (B2BUA) to interwork all the procedures as necessary between the
   endpoints.  No further specification is needed for this model.

   Alternately, the gateway can provide transport level interworking
   between MSRP endpoints using different transport protocols.  In
   accordance with Section 4.4, path attributes SHALL NOT be used for
   transport level interworking.

   When the gateway performs transport level interworking between MSRP
   endpoints, all of the procedures in Section 5 and Section 4 apply to
   each peer, with the following additions:

   o  The gateway SHALL use the MSRP CEMA mechanism [RFC6714] towards
      the non-data channel endpoint.

   o  If the non-data channel endpoint does not support MSRP CEMA,
      transport level interworking mode is not possible, the gateway
      needs to act as an MSRP B2BUA.

   o  The gateway SHALL NOT modify the path attribute received from data
      channel or from non-data channel endpoints.



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   o  The gateway SHALL NOT modify the setup value received from data
      channel or from non-data channel endpoints.

   o  The endpoint establishing an MSRP session using data channel
      transport SHALL NOT request inclusion of any relays, although it
      MAY interoperate with a peer that signals the use of relays.

7.  Updates to RFC4975

   This document updates [RFC4975], by allowing the usage of the "msrps"
   scheme when the underlying connection is protected with DTLS.

8.  Security Considerations

   MSRP traffic over data channels is secured, including
   confidentiality, integrity and source authentication, as specified by
   by [I-D.ietf-rtcweb-data-channel].  However, [RFC4975] allows
   transport of MSRP traffic over non-secured TCP connections, and does
   not provide a mechanism to guarantee usage of TLS end-to-end.  As
   described in [RFC4975], even if TLS is used between some hops TCP
   might still be used between other hops.  Operators need to ensure
   that proper policies are established in order to ensure that the MSRP
   traffic is protected between endpoints.

   [RFC5547] specifies security considerations related to the usage of
   MSRP for file transfer.

   [RFC7092] specifies security considerations related to B2BUAs.

   Note that discussion in Section 14.5 of [RFC4975] on MSRP message
   attribution to remote identities applies to data channel transport.

   If the Session Initiation Protocol (SIP) [RFC3261] is used to
   implement the offer/answer transactions for establishing the MSRP
   data channel, the SIP security considerations specified in [RFC3261]
   apply.

9.  IANA Considerations

   NOTE to RFC Editor: Please replace all instances of all instances of
   RFCXXXX with the number of this RFC.

9.1.  msrps URI scheme

   This document modifies the usage of the msrps URI scheme, registered
   by [RFC4975], adding DTLS as a protected transport indicated by the
   URI scheme.




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   A reference to RFCXXXX is added to the URI scheme "msrps" in the
   Uniform Resource Identifier (URI) Schemes Registry.

9.2.  Subprotocol Identifier MSRP

   A reference to RFCXXXX is added to the subprotocol identifier "msrp"
   in the "WebSocket Subprotocol Name Registry"

9.3.  SDP Attributes

   This document modifies the usage of a set of SDP attributes, if any
   of those attributes is included in an SDP 'dsca' attribute associated
   with an MSRP data channel.  The modified usage of the SDP 'setup'
   attribute is described in Section 4.5.  The usage of the other SDP
   attributes is described in Section 4.4.

   o  "path"

   o  "msrp-cema"

   o  "accept-types"

   o  "accept-wrapped-types"

   o  "max-size"

   o  "sendonly"

   o  "recvonly"

   o  "inactive"

   o  "sendrecv"

   o  "file-selector"

   o  "file-transfer-id"

   o  "file-disposition"

   o  "file-date"

   o  "file-icon"

   o  "file-range"






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   The usage level "dcsa(msrp)" is added to the registration of the SDP
   'setup' attribute in the Session Description Protocol (SDP)
   Parameters "att-field" sub-registry as follows:

   +-----------------------+-------------------------------------------+
   | Contact name:         | IESG                                      |
   | Contact email:        | iesg@ietf.org                             |
   | Attribute name:       | setup                                     |
   | Usage level:          | dcsa(msrp)                                |
   | Purpose:              | Negotiate the active role of an MSRP      |
   |                       | session over a data channel as per        |
   |                       | Section 4.5                               |
   | Reference:            | RFCXXXX                                   |
   +-----------------------+-------------------------------------------+

   The usage level "dcsa(msrp)" is added to the registration of the SDP
   'path' attribute in the Session Description Protocol (SDP) Parameters
   "att-field" sub-registry as follows:

   +-----------------------+-------------------------------------------+
   | Contact name:         | IESG                                      |
   | Contact email:        | iesg@ietf.org                             |
   | Attribute name:       | path                                      |
   | Usage level:          | dcsa(msrp)                                |
   | Purpose:              | Indicate an endpoint, but not used for    |
   |                       | routing, as described in Section 4.4      |
   | Reference:            | RFCXXXX                                   |
   +-----------------------+-------------------------------------------+

   The usage level "dcsa(msrp)" is added to the registration of the SDP
   'msrp-cema' attribute in the Session Description Protocol (SDP)
   Parameters "att-field" sub-registry as follows:

   +-----------------------+-------------------------------------------+
   | Contact name:         | IESG                                      |
   | Contact email:        | iesg@ietf.org                             |
   | Attribute name:       | msrp-cema                                 |
   | Usage level:          | dcsa(msrp)                                |
   | Purpose:              | Indicate that the routing of MSRP         |
   |                       | messages transported on a data channel is |
   |                       | more similar to the MSRP CEMA mechanism   |
   |                       | than the legacy MSRP routing mechanism.   |
   | Reference:            | RFCXXXX                                   |
   +-----------------------+-------------------------------------------+

   The usage level "dcsa(msrp)" is added to the registration of the SDP
   'accept-types' attribute in the Session Description Protocol (SDP)
   Parameters "att-field" sub-registry as follows:



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   +-----------------------+-------------------------------------------+
   | Contact name:         | IESG                                      |
   | Contact email:        | iesg@ietf.org                             |
   | Attribute name:       | accept-types                              |
   | Usage level:          | dcsa(msrp)                                |
   | Purpose:              | Contain the list of media types that the  |
   |                       | endpoint is willing to receive.           |
   | Reference:            | RFCXXXX                                   |
   +-----------------------+-------------------------------------------+

   The usage level "dcsa(msrp)" is added to the registration of the SDP
   'accept-wrapped-types' attribute in the Session Description Protocol
   (SDP) Parameters "att-field" sub-registry as follows:

   +-----------------------+-------------------------------------------+
   | Contact name:         | IESG                                      |
   | Contact email:        | iesg@ietf.org                             |
   | Attribute name:       | accept-wrapped-types                      |
   | Usage level:          | dcsa(msrp)                                |
   | Purpose:              | Contain the list of media types that the  |
   |                       | endpoint is willing to receive in an MSRP |
   |                       | message with multipart content.           |
   | Reference:            | RFCXXXX                                   |
   +-----------------------+-------------------------------------------+

   The usage level "dcsa(msrp)" is added to the registration of the SDP
   'max-size' attribute in the Session Description Protocol (SDP)
   Parameters "att-field" sub-registry as follows:

   +-----------------------+-------------------------------------------+
   | Contact name:         | IESG                                      |
   | Contact email:        | iesg@ietf.org                             |
   | Attribute name:       | max-size                                  |
   | Usage level:          | dcsa(msrp)                                |
   | Purpose:              | Indicate the largest message an MSRP      |
   |                       | endpoint wishes to accept.                |
   | Reference:            | RFCXXXX                                   |
   +-----------------------+-------------------------------------------+

   The usage level "dcsa(msrp)" is added to the registration of the SDP
   'sendonly' attribute in the Session Description Protocol (SDP)
   Parameters "att-field" sub-registry as follows:









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   +-----------------------+-------------------------------------------+
   | Contact name:         | IESG                                      |
   | Contact email:        | iesg@ietf.org                             |
   | Attribute name:       | sendonly                                  |
   | Usage level:          | dcsa(msrp)                                |
   | Purpose:              | Negotiate the direction of the media flow |
   |                       | on an MSRP data channel.                  |
   | Reference:            | RFCXXXX                                   |
   +-----------------------+-------------------------------------------+

   The usage level "dcsa(msrp)" is added to the registration of the SDP
   'recvonly' attribute in the Session Description Protocol (SDP)
   Parameters "att-field" sub-registry as follows:

   +-----------------------+-------------------------------------------+
   | Contact name:         | IESG                                      |
   | Contact email:        | iesg@ietf.org                             |
   | Attribute name:       | recvonly                                  |
   | Usage level:          | dcsa(msrp)                                |
   | Purpose:              | Negotiate the direction of the media flow |
   |                       | on an MSRP data channel.                  |
   | Reference:            | RFCXXXX                                   |
   +-----------------------+-------------------------------------------+

   The usage level "dcsa(msrp)" is added to the registration of the SDP
   'inactive' attribute in the Session Description Protocol (SDP)
   Parameters "att-field" sub-registry as follows:

   +-----------------------+-------------------------------------------+
   | Contact name:         | IESG                                      |
   | Contact email:        | iesg@ietf.org                             |
   | Attribute name:       | inactive                                  |
   | Usage level:          | dcsa(msrp)                                |
   | Purpose:              | Negotiate the direction of the media flow |
   |                       | on an MSRP data channel.                  |
   | Reference:            | RFCXXXX                                   |
   +-----------------------+-------------------------------------------+

   The usage level "dcsa(msrp)" is added to the registration of the SDP
   'sendrecv' attribute in the Session Description Protocol (SDP)
   Parameters "att-field" sub-registry as follows:










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   +-----------------------+-------------------------------------------+
   | Contact name:         | IESG                                      |
   | Contact email:        | iesg@ietf.org                             |
   | Attribute name:       | sendrecv                                  |
   | Usage level:          | dcsa(msrp)                                |
   | Purpose:              | Negotiate the direction of the media flow |
   |                       | on an MSRP data channel.                  |
   | Reference:            | RFCXXXX                                   |
   +-----------------------+-------------------------------------------+

   The usage level "dcsa(msrp)" is added to the registration of the SDP
   'file-selector' attribute in the Session Description Protocol (SDP)
   Parameters "att-field" sub-registry as follows:

   +-----------------------+-------------------------------------------+
   | Contact name:         | IESG                                      |
   | Contact email:        | iesg@ietf.org                             |
   | Attribute name:       | file-selector                             |
   | Usage level:          | dcsa(msrp)                                |
   | Purpose:              | Indicate a file in an MSRP file transfer  |
   |                       | negotiation.                              |
   | Reference:            | RFCXXXX                                   |
   +-----------------------+-------------------------------------------+

   The usage level "dcsa(msrp)" is added to the registration of the SDP
   'file-transfer-id' attribute in the Session Description Protocol
   (SDP) Parameters "att-field" sub-registry as follows:

   +-----------------------+-------------------------------------------+
   | Contact name:         | IESG                                      |
   | Contact email:        | iesg@ietf.org                             |
   | Attribute name:       | file-transfer-id                          |
   | Usage level:          | dcsa(msrp)                                |
   | Purpose:              | Indicate a unique identifier of the file  |
   |                       | transfer operation in an MSRP file        |
   |                       | transfer negotiation.                     |
   | Reference:            | RFCXXXX                                   |
   +-----------------------+-------------------------------------------+

   The usage level "dcsa(msrp)" is added to the registration of the SDP
   'file-disposition' attribute in the Session Description Protocol
   (SDP) Parameters "att-field" sub-registry as follows:









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   +-----------------------+-------------------------------------------+
   | Contact name:         | IESG                                      |
   | Contact email:        | iesg@ietf.org                             |
   | Attribute name:       | file-disposition                          |
   | Usage level:          | dcsa(msrp)                                |
   | Purpose:              | Provide a suggestion to the other         |
   |                       | endpoint about the intended disposition   |
   |                       | of the file in an MSRP file transfer      |
   |                       | negotiation.                              |
   | Reference:            | RFCXXXX                                   |
   +-----------------------+-------------------------------------------+

   The usage level "dcsa(msrp)" is added to the registration of the SDP
   'file-date' attribute in the Session Description Protocol (SDP)
   Parameters "att-field" sub-registry as follows:

   +-----------------------+-------------------------------------------+
   | Contact name:         | IESG                                      |
   | Contact email:        | iesg@ietf.org                             |
   | Attribute name:       | file-date                                 |
   | Usage level:          | dcsa(msrp)                                |
   | Purpose:              | Indicate one or more dates related to the |
   |                       | file in an MSRP file transfer             |
   |                       | negotiation.                              |
   | Reference:            | RFCXXXX                                   |
   +-----------------------+-------------------------------------------+

   The usage level "dcsa(msrp)" is added to the registration of the SDP
   'file-icon' attribute in the Session Description Protocol (SDP)
   Parameters "att-field" sub-registry as follows:

   +-----------------------+-------------------------------------------+
   | Contact name:         | IESG                                      |
   | Contact email:        | iesg@ietf.org                             |
   | Attribute name:       | file-icon                                 |
   | Usage level:          | dcsa(msrp)                                |
   | Purpose:              | Contain a pointer to a small preview icon |
   |                       | representing the contents of the file in  |
   |                       | an MSRP file transfer negotiation.        |
   | Reference:            | RFCXXXX                                   |
   +-----------------------+-------------------------------------------+

   The usage level "dcsa(msrp)" is added to the registration of the SDP
   'file-range' attribute in the Session Description Protocol (SDP)
   Parameters "att-field" sub-registry as follows:






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   +-----------------------+-------------------------------------------+
   | Contact name:         | IESG                                      |
   | Contact email:        | iesg@ietf.org                             |
   | Attribute name:       | file-range                                |
   | Usage level:          | dcsa(msrp)                                |
   | Purpose:              | Contain the range of transferred octets   |
   |                       | of the file in an MSRP file transfer      |
   |                       | negotiation.                              |
   | Reference:            | RFCXXXX                                   |
   +-----------------------+-------------------------------------------+

10.  Acknowledgments

   The authors wish to acknowledge the borrowing of ideas from another
   internet draft by Peter Dunkley and Gavin Llewellyn, and to thank
   Flemming Andreasen, Christian Groves, Paul Kyzivat, Jonathan Lennox,
   Uwe Rauschenbach, Albrecht Schwarz, and Keith Drage for their
   invaluable comments.

   Richard Ejzak, Keith Drage and Juergen Stoetzer-Bradler contributed
   an earlier version, before the draft was re-adopted.

   Julien Maisonneuve helped with the re-adoption of the draft, and
   Maridi R.  Makaraju (Raju) contributed valuable comments after the
   draft was re-adopted.

11.  References

11.1.  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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [I-D.ietf-rtcweb-data-channel]
              Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data
              Channels", draft-ietf-rtcweb-data-channel-13 (work in
              progress), January 2015.

   [I-D.ietf-mmusic-data-channel-sdpneg]
              Drage, K., Makaraju, M., Ejzak, R., Marcon, J., and R.
              Even, "SDP-based Data Channel Negotiation", draft-ietf-
              mmusic-data-channel-sdpneg-28 (work in progress), May
              2019.






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   [I-D.ietf-mmusic-sctp-sdp]
              Holmberg, C., Shpount, R., Loreto, S., and G. Camarillo,
              "Session Description Protocol (SDP) Offer/Answer
              Procedures For Stream Control Transmission Protocol (SCTP)
              over Datagram Transport Layer Security (DTLS) Transport.",
              draft-ietf-mmusic-sctp-sdp-26 (work in progress), April
              2017.

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

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

   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, DOI 10.17487/RFC4960, September 2007,
              <https://www.rfc-editor.org/info/rfc4960>.

   [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,
              <https://www.rfc-editor.org/info/rfc4975>.

   [RFC5547]  Garcia-Martin, M., Isomaki, M., Camarillo, G., Loreto, S.,
              and P. Kyzivat, "A Session Description Protocol (SDP)
              Offer/Answer Mechanism to Enable File Transfer", RFC 5547,
              DOI 10.17487/RFC5547, May 2009,
              <https://www.rfc-editor.org/info/rfc5547>.

   [RFC6135]  Holmberg, C. and S. Blau, "An Alternative Connection Model
              for the Message Session Relay Protocol (MSRP)", RFC 6135,
              DOI 10.17487/RFC6135, February 2011,
              <https://www.rfc-editor.org/info/rfc6135>.

   [RFC6714]  Holmberg, C., Blau, S., and E. Burger, "Connection
              Establishment for Media Anchoring (CEMA) for the Message
              Session Relay Protocol (MSRP)", RFC 6714,
              DOI 10.17487/RFC6714, August 2012,
              <https://www.rfc-editor.org/info/rfc6714>.

   [RFC7977]  Dunkley, P., Llewellyn, G., Pascual, V., Salgueiro, G.,
              and R. Ravindranath, "The WebSocket Protocol as a
              Transport for the Message Session Relay Protocol (MSRP)",
              RFC 7977, DOI 10.17487/RFC7977, September 2016,
              <https://www.rfc-editor.org/info/rfc7977>.



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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

11.2.  Informative References

   [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,
              <https://www.rfc-editor.org/info/rfc3261>.

   [RFC7092]  Kaplan, H. and V. Pascual, "A Taxonomy of Session
              Initiation Protocol (SIP) Back-to-Back User Agents",
              RFC 7092, DOI 10.17487/RFC7092, December 2013,
              <https://www.rfc-editor.org/info/rfc7092>.

Authors' Addresses

   Jose M. Recio (editor)
   Unaffiliated

   Email: jose@ch3m4.com


   Christer Holmberg
   Ericsson
   Hirsalantie 11
   Jorvas 02420
   Finland

   Email: christer.holmberg@ericsson.com



















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