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MMUSIC                                                     K. Drage, Ed.
Internet-Draft                                               M. Makaraju
Intended status: Standards Track                     J. Stoetzer-Bradler
Expires: April 7, 2016                                    Alcatel-Lucent
                                                                R. Ejzak
                                                               J. Marcon
                                                            Unaffiliated
                                                         October 5, 2015


                   SDP-based Data Channel Negotiation
                draft-ietf-mmusic-data-channel-sdpneg-05

Abstract

   The Real-Time Communication in WEB-browsers (RTCWeb) working group is
   charged to provide protocols to support direct interactive rich
   communications using audio, video, and data between two peers' web-
   browsers.  For the support of data communication, the RTCWeb working
   group has in particular defined the concept of bi-directional data
   channels over SCTP, where each data channel might be used to
   transport other protocols, called sub-protocols.  Data channel setup
   can be done using either the in-band Data Channel Establishment
   Protocol (DCEP) or using some out-of-band non-DCEP protocol.  This
   document specifies how the SDP offer/answer exchange can be used to
   achieve such an out-of-band non-DCEP negotiation.  Even though data
   channels are designed for RTCWeb use initially they may be used by
   other protocols like, but not limited to, the CLUE protocol.  This
   document is intended to be used wherever data channels are used.

Status of This Memo

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

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

   Internet-Drafts are draft documents valid for a maximum of six months
   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 April 7, 2016.





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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
   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Applicability Statement . . . . . . . . . . . . . . . . . . .   5
   5.  SDP Offer/Answer Negotiation  . . . . . . . . . . . . . . . .   5
     5.1.  SDP Syntax  . . . . . . . . . . . . . . . . . . . . . . .   5
       5.1.1.  SDP Attribute for Data Channel Parameter Negotiation    5
         5.1.1.1.  dcmap Attribute . . . . . . . . . . . . . . . . .   5
         5.1.1.2.  dcmap Multiplexing Category . . . . . . . . . . .   7
         5.1.1.3.  dcmap-stream-id Parameter . . . . . . . . . . . .   7
         5.1.1.4.  label Parameter . . . . . . . . . . . . . . . . .   7
         5.1.1.5.  subprotocol Parameter . . . . . . . . . . . . . .   8
         5.1.1.6.  max-retr Parameter  . . . . . . . . . . . . . . .   8
         5.1.1.7.  max-time Parameter  . . . . . . . . . . . . . . .   8
         5.1.1.8.  ordered Parameter . . . . . . . . . . . . . . . .   8
       5.1.2.  Sub-Protocol Specific Attributes  . . . . . . . . . .   9
         5.1.2.1.  dcsa Attribute  . . . . . . . . . . . . . . . . .   9
         5.1.2.2.  dcsa Multiplexing Category  . . . . . . . . . . .  10
     5.2.  Procedures  . . . . . . . . . . . . . . . . . . . . . . .  10
       5.2.1.  Managing Stream Identifiers . . . . . . . . . . . . .  10
       5.2.2.  Negotiating Data Channel Parameters . . . . . . . . .  11
       5.2.3.  Opening a Data Channel  . . . . . . . . . . . . . . .  12
       5.2.4.  Closing a Data Channel  . . . . . . . . . . . . . . .  14
       5.2.5.  Various SDP Offer/Answer Scenarios and Considerations  15
   6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  16
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
     8.1.  Subprotocol Identifiers . . . . . . . . . . . . . . . . .  18
     8.2.  New SDP Attributes  . . . . . . . . . . . . . . . . . . .  19
       8.2.1.  dcmap . . . . . . . . . . . . . . . . . . . . . . . .  19
       8.2.2.  dcsa  . . . . . . . . . . . . . . . . . . . . . . . .  19



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   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  20
   10. CHANGE LOG  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     10.1.  Changes against 'draft-ietf-mmusic-data-channel-
            sdpneg-04' . . . . . . . . . . . . . . . . . . . . . . .  20
     10.2.  Changes against 'draft-ietf-mmusic-data-channel-
            sdpneg-03' . . . . . . . . . . . . . . . . . . . . . . .  21
     10.3.  Changes against 'draft-ietf-mmusic-data-channel-
            sdpneg-02' . . . . . . . . . . . . . . . . . . . . . . .  22
     10.4.  Changes against 'draft-ietf-mmusic-data-channel-
            sdpneg-01' . . . . . . . . . . . . . . . . . . . . . . .  23
     10.5.  Changes against 'draft-ietf-mmusic-data-channel-
            sdpneg-00' . . . . . . . . . . . . . . . . . . . . . . .  25
     10.6.  Changes against 'draft-ejzak-mmusic-data-channel-
            sdpneg-02' . . . . . . . . . . . . . . . . . . . . . . .  28
     10.7.  Changes against '-01'  . . . . . . . . . . . . . . . . .  29
     10.8.  Changes against '-00'  . . . . . . . . . . . . . . . . .  29
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  30
     11.2.  Informative References . . . . . . . . . . . . . . . . .  30
   Appendix A.  Generic Data Channel Negotiation Aspects When Not
                Using DCEP . . . . . . . . . . . . . . . . . . . . .  32
     A.1.  Stream Identifier Numbering . . . . . . . . . . . . . . .  32
     A.2.  Generic Data Channel Negotiation Not Using DCEP . . . . .  33
       A.2.1.  Overview  . . . . . . . . . . . . . . . . . . . . . .  33
       A.2.2.  Opening a Data Channel  . . . . . . . . . . . . . . .  33
       A.2.3.  Closing a Data Channel  . . . . . . . . . . . . . . .  34
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  34

1.  Introduction

   The RTCWeb working group has defined the concept of bi-directional
   data channels running on top of SCTP/DTLS.  RTCWeb leaves it open for
   other applications to use data channels and its in-band DCEP or other
   in-band or out-of-band protocols for creating them.  Each data
   channel consists of paired SCTP streams sharing the same SCTP Stream
   Identifier.  Data channels are created by endpoint applications
   through the WebRTC API, or other users of data channel like CLUE, and
   can be used to transport proprietary or well-defined protocols, which
   in the latter case can be signaled by the data channel "sub-protocol"
   parameter, conceptually similar to the WebSocket "sub-protocol".
   However, apart from the "sub-protocol" value transmitted to the peer,
   RTCWeb leaves it open how endpoint applications can agree on how to
   instantiate a given sub-protocol on a data channel, and whether it is
   signaled in-band using DCEP or out-of-band using a non-DCEP protocol
   (or both).  In particular, the SDP offer generated by the RTCweb data
   channel stack includes no channel-specific information.





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   This document defines SDP offer/answer negotiation procedures to
   establish data channels for transport of well-defined sub-protocols,
   to enable out-of-band negotiation.

2.  Conventions

   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 [RFC2119].

3.  Terminology

   This document uses the following terms:

      Data channel: A WebRTC data channel as specified in
      [I-D.ietf-rtcweb-data-channel].

      Data channel stack: An entity which, upon application request,
      runs the data channel protocol to keep track of states, sending
      and receive data.  If the application is a browser based
      JavaScript application then this stack resides in the browser.  If
      the application is a native application then this stack resides in
      the application and is accessible via some sort of APIs.

      Data channel properties: Fixed properties assigned to a data
      channel at the time of its creation.  Some of these properties
      determine the way the data channel stack transmits data on this
      channel (e.g., stream identifier, reliability, order of
      delivery...).

      DCEP: Data Channel Establishment Protocol defined in
      [I-D.ietf-rtcweb-data-protocol].

      In-band: Transmission through the peer-to-peer SCTP association.

      Out-of-band: Transmission through the application signaling path.

      Peer: From the perspective of one of the agents in a session, its
      peer is the other agent.  Specifically, from the perspective of
      the SDP offerer, the peer is the SDP answerer.  From the
      perspective of the SDP answerer, the peer is the SDP offerer.

      SCTP Stream Sequence Number (SSN): the SCTP stream sequence number
      as specified in [RFC4960].

      Stream identifier: The identifier of the outbound and inbound SCTP
      streams composing a data channel.




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4.  Applicability Statement

   The mechanism in this specification only applies to the Session
   Description Protocol (SDP) [RFC4566], when used together with the SDP
   offer/answer mechanism [RFC3264].  Declarative usage of SDP is out of
   scope of this document, and is thus undefined.

5.  SDP Offer/Answer Negotiation

   This section defines an SDP extension by which two clients can
   negotiate data channel-specific and sub-protocol-specific parameters
   without using DCEP [I-D.ietf-rtcweb-data-protocol].  This SDP
   extension only defines usage in the context of SDP offer/answer.

   Appendix A provides information how data channels work in general and
   especially summarizes some key aspects, which should be considered
   for the negotiation of data channels if DCEP is not used.

5.1.  SDP Syntax

   Two new SDP attributes are defined to support SDP offer/answer
   negotiation of data channels.  The first attribute provides for
   negotiation of channel-specific parameters.  The second attribute
   provides for negotiation of sub-protocol-specific parameters.

5.1.1.  SDP Attribute for Data Channel Parameter Negotiation

   Associated with the SDP "m" line that defines the SCTP association
   for data channels (defined in Section 4), each SDP offer and answer
   includes one "a=dcmap:" attribute that defines the data channel
   parameters for each data channel to be negotiated.  Each such
   attribute line specifies the following parameters for a data channel:
   SCTP stream identifier, sub-protocol, label, reliability, order of
   delivery, and priority.

   The intention in exchanging these attributes is to create, on two
   peers, without use of DCEP [I-D.ietf-rtcweb-data-protocol], matched
   pairs of oppositely directed data channels having the same set of
   attributes.  It is assumed that the data channel properties
   (reliable/partially reliable, ordered/unordered) are suitable per the
   sub-protocol transport requirements.

5.1.1.1.  dcmap Attribute

   "a=dcmap:" is a media level attribute having following ABNF syntax.






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   Formal Syntax:

   Name: dcmap

   Value: dcmap-value

   Usage Level: media

   Charset Dependent: no

   Syntax:

   dcmap-value     = dcmap-stream-id
                     [ SP dcmap-opt *(";" dcmap-opt) ]
   dcmap-opt       = ordering-opt / subprotocol-opt / label-opt
                     / maxretr-opt / maxtime-opt
                     ; Either only maxretr-opt or maxtime-opt
                     ; is present.

   dcmap-stream-id = 1*DIGIT
   ordering-opt    = "ordered=" ordering-value
   ordering-value  = "true" / "false"
   subprotocol-opt = "subprotocol=" quoted-string
   label-opt       = "label=" quoted-string
   maxretr-opt     = "max-retr=" maxretr-value
   maxretr-value   = <from-Reliability-Parameter of
                      I-D.ietf-rtcweb-data-protocol>
                     ; number of retransmissions
   maxtime-opt     = "max-time=" maxtime-value
   maxtime-value   = <from-Reliability-Parameter of
                     I-D.ietf-rtcweb-data-protocol>
                     ; milliseconds

   quoted-string   = DQUOTE *(quoted-char / escaped-char) DQUOTE
   quoted-char     = SP / quoted-visible
   quoted-visible  = %21 / %23-24 / %26-7E ; VCHAR without " or %
   escaped-char    = "%" HEXDIG HEXDIG
   DQUOTE          = <from-RFC5234>
   integer         = <from-RFC5234>

   Examples:

   a=dcmap:0
   a=dcmap:1 subprotocol="BFCP";max-time=60000
   a=dcmap:2 subprotocol="MSRP";ordered=true;label="MSRP"
   a=dcmap:3 label="Label 1";ordered=false;max-retr=5
   a=dcmap:4 label="foo%09bar";ordered=true;max-time=15000;max-retr=3




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      Note: The last example (a=dcmap:4) shows a 'label' parameter value
      which contains one non-printable 'escaped-char' character (the
      tabulator character).

   Within an 'a=dcmap:' attribute line's 'dcmap-opt' value either only
   one 'maxretr-opt' parameter or one 'maxtime-opt' parameter MAY be
   present.  Both MUST NOT be present.

5.1.1.2.  dcmap Multiplexing Category

   Multiplexing characteristics of SDP attributes are described in
   [I-D.ietf-mmusic-sdp-mux-attributes].  Various SDP attribute
   multiplexing categories are introduced there.

   The multiplexing category of the "a=dcmap:" attribute is SPECIAL.
   Usage of this attribute is only applicable when associated with
   UDP/DTLS/SCTP or TCP/DTLS/SCTP proto SDP "m" lines.

   As the usage of multiple SCTP associations on top of a single DTLS
   connection is outside the scope of [I-D.ietf-mmusic-sctp-sdp], no
   multiplexing rules are specified for the UDP/DTLS/SCTP and TCP/DTLS/
   SCTP proto values.  If future extensions of
   [I-D.ietf-mmusic-sctp-sdp] define how to negotiate multiplexing of
   multiple SCTP associations of top of a single DTLS connection, then
   multiplexing rules for the "a=dcmap:" attribute need to be defined as
   well, for instance in an extension of this SDP based data channel
   negotiation specification.

5.1.1.3.  dcmap-stream-id Parameter

   The 'dcmap-stream-id' parameter indicates the SCTP stream identifier
   within the SCTP association used to form the data channel.

5.1.1.4.  label Parameter

   The 'label' parameter indicates the name of the channel.  It
   represents a label that can be used to distinguish, in the context of
   the WebRTC API [WebRtcAPI], an RTCDataChannel object from other
   RTCDataChannel objects.  This parameter maps to the 'Label' parameter
   defined in [I-D.ietf-rtcweb-data-protocol].  The 'label' parameter is
   optional.  If it is not present, then its value defaults to the empty
   string.

   Note: The empty string MAY also be explicitly used as 'label' value,
   such that 'label=""' is equivalent to the 'label' parameter not being
   present at all.  [I-D.ietf-rtcweb-data-protocol] allows the
   DATA_CHANNEL_OPEN message's 'Label' value to be an empty string.




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5.1.1.5.  subprotocol Parameter

   The 'subprotocol' parameter indicates which protocol the client
   expects to exchange via the channel.  This parameter maps to the
   'Protocol' parameter defined in [I-D.ietf-rtcweb-data-protocol].
   Section 8.1 specifies how new subprotocol parameter values are
   registered.  'Subprotocol' is an optional parameter.  If the
   'subprotocol' parameter is not present, then its value defaults to
   the empty string.

   Note: The empty string MAY also be explicitly used as 'subprotocol'
   value, such that 'subprotocol=""' is equivalent to the 'subprotocol'
   parameter not being present at all.  [I-D.ietf-rtcweb-data-protocol]
   allows the DATA_CHANNEL_OPEN message's 'Subprotocol' value to be an
   empty string.

5.1.1.6.  max-retr Parameter

   This parameter indicates that the data channel is partially reliable.
   The 'max-retr' parameter indicates the maximal number of times a user
   message will be retransmitted.  The max-retr parameter is optional.
   If the max-retr parameter is not present, then the maximal number of
   retransmissions is determined as per the generic SCTP retransmission
   rules as specified in [RFC4960].  This parameter maps to the 'Number
   of RTX' parameter defined in [I-D.ietf-rtcweb-data-protocol].

5.1.1.7.  max-time Parameter

   This parameter indicates that the data channel is partially reliable.
   A user message will no longer be transmitted or retransmitted after a
   specified life-time given in milliseconds in the 'max-time'
   parameter.  The max-time parameter is optional.  If the max-time
   parameter is not present, then the generic SCTP retransmission timing
   rules apply as specified in [RFC4960].  This parameter maps to the
   'Lifetime in ms' parameter defined in
   [I-D.ietf-rtcweb-data-protocol].

5.1.1.8.  ordered Parameter

   The 'ordered' parameter with value "true" indicates that the receiver
   MUST dispatch DATA chunks in the data channel to the upper layer
   while preserving the order.  The ordered parameter is optional and
   takes two values: "true" for ordered and "false" for unordered
   delivery with "true" as the default value.  Any other value is
   ignored and default "ordered=true" is assumed.  In the absence of
   this parameter "ordered=true" is assumed.  This parameter maps to the
   ordered or unordered data channel types as defined in
   [I-D.ietf-rtcweb-data-protocol].



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5.1.2.  Sub-Protocol Specific Attributes

   In the SDP, each data channel declaration MAY also be followed by
   other SDP attributes specific to the sub-protocol in use.  Each of
   these attributes is represented by one new attribute line, and it
   includes the contents of a media-level SDP attribute already defined
   for use with this (sub)protocol in another IETF specification.  Sub-
   protocol-specific attributes might also be defined for exclusive use
   with data channel transport, but should use the same syntax described
   here for other sub-protocol-specific attributes.

5.1.2.1.  dcsa Attribute

   Each sub-protocol specific SDP attribute that would normally be used
   to negotiate the subprotocol using SDP is replaced with an attribute
   of the form "a=dcsa:stream-id original-attribute", where dcsa stands
   for "data channel sub-protocol attribute", stream-id is the SCTP
   stream identifier assigned to this sub-protocol instance, and
   original-attribute represents the contents of the sub-protocol
   related attribute to be included.

   Formal Syntax:

   Name: dcsa

   Value: dcsa-value

   Usage Level: media

   Charset Dependent: no

   Syntax:

   dcsa-value      = stream-id SP attribute
   attribute       = <from-RFC4566>

   Example:

   a=dcsa:2 accept-types:text/plain

   Note that the above reference to RFC 4566 defines where the attribute
   definition can be found; it does not provide any limitation on
   support of attributes defined in other documents in accordance with
   this attribute definition.  Note however that not all SDP attributes
   are suitable as "a=dcsa:" parameter.  [IANA-SDP-Parameters] contains
   the lists of IANA registered session and media level or media level
   only SDP attributes.




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   Thus in the example above, the original attribute line "a=accept-
   types:text/plain" is represented by the attribute line "a=dcsa:2
   accept-types:text/plain", which specifies that this instance of MSRP
   being transported on the SCTP association using the data channel with
   stream id 2 accepts plain text files.

   As opposed to the data channel "a=dcmap:" attribute parameters, these
   parameters are subject to offer/answer negotiation following the
   procedures defined in the sub-protocol specific documents.

   The same syntax applies to any other SDP attribute required for
   negotiation of this instance of the sub-protocol.

   Note: This document does not provide a complete specification of how
   to negotiate the use of a data channel to transport MSRP.  Procedures
   specific to each sub-protocol such as MSRP will be documented
   elsewhere.  The use of MSRP is only an example of how the generic
   procedures described herein might apply to a specific sub-protocol.

5.1.2.2.  dcsa Multiplexing Category

   The multiplexing category of the "a=dcsa:" attribute is SPECIAL.
   Usage of this attribute is only applicable when associated with
   UDP/DTLS/SCTP or TCP/DTLS/SCTP proto SDP "m" lines.

   As the usage of multiple SCTP associations on top of a single DTLS
   connection is outside the scope of [I-D.ietf-mmusic-sctp-sdp], no
   multiplexing rules are specified for the UDP/DTLS/SCTP and TCP/DTLS/
   SCTP proto values.  If future extensions of
   [I-D.ietf-mmusic-sctp-sdp] define how to negotiate multiplexing of
   multiple SCTP associations of top of a single DTLS connection, then
   multiplexing rules for the "a=dcsa:" attribute need to be defined as
   well, for instance in an extension of this SDP based data channel
   negotiation specification.

5.2.  Procedures

5.2.1.  Managing Stream Identifiers

   If an SDP offer/answer exchange (could be the initial or a subsequent
   one) results in a UDP/DTLS/SCTP or TCP/DTLS/SCTP based media
   description being accepted, and if this SDP offer/answer exchange
   results in the establishment of a new SCTP association, then the SDP
   offerer owns the even SCTP stream ids of this new SCTP association
   and the answerer owns the odd SCTP stream identifiers.  If this "m"
   line is removed from the signaling session (its port number set to
   zero), and if usage of this or of a new UDP/DTLS/SCTP or TCP/DTLS/
   SCTP based "m" line is renegotiated later on, then the even and odd



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   SCTP stream identifier ownership is redetermined as well as described
   above.

   This specification allows simultaneous use of SDP offer/answer and
   DCEP negotiation.  However, an SCTP stream MUST NOT be referenced in
   a "a=dcmap:" or "a=dcsa:" attribute of an SDP offer/answer exchange
   if the associated SCTP stream has already been negotiated via DCEP.
   Stream ids that are not currently used in SDP can be used for DCEP
   negotiation.  Stream id allocation per SDP offer/answer negotiation
   may not align with DTLS role based allocation.  This could cause
   glare conditions when one side trying to do SDP offer/answer
   negotiation on a stream id while the other end trying to open a data
   channel on the same stream id using DCEP negotiation.  To avoid these
   glare conditions this specification recommends that the data channel
   stack user always selects stream ids per above described SDP offer/
   answer rule even when DCEP negotiation is used.  To avoid glare
   conditions, it is possible to come up with a different stream id
   allocation scheme, but such schemes are outside the scope of this
   specification.

5.2.2.  Negotiating Data Channel Parameters

   Conveying a reliable data channel is achieved by including neither
   'max-retr' nor 'max-time' in corresponding SDP offer's or answer's
   "a=dcmap:" attribute line.  Conveying a partially reliable data
   channel is achieved by including only one of 'max-retr' or 'max-
   time'.  By definition max-retr and max-time are mutually exclusive,
   so at most one of them MAY be present in the "a=dcmap:" attribute
   line.  If an SDP offer contains both of these parameters then the
   receiver of such an SDP offer MUST reject the SDP offer.  If an SDP
   answer contains both of these parameters then the offerer MAY treat
   it as an error and MAY assume the associated SDP offer/answer failed
   and MAY take appropriate recovery actions.  These recovery options
   are outside the scope of this specification.

   The SDP answer SHALL echo the same subprotocol, max-retr, max-time,
   ordered parameters, if those were present in the offer, and MAY
   include a label parameter.  They MAY appear in any order, which could
   be different from the SDP offer, in the SDP answer.

   When sending a subsequent offer or an answer, and for as long as the
   data channel is still open, the sender MUST replicate the same
   information.

   Data channel types defined in [I-D.ietf-rtcweb-data-protocol] are
   mapped to SDP in the following manner, where "ordered=true" is the
   default and may be omitted:




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   DATA_CHANNEL_RELIABLE
         ordered=true

   DATA_CHANNEL_RELIABLE_UNORDERED
         ordered=false

   DATA_CHANNEL_PARTIAL_RELIABLE_REXMIT
         ordered=true;max-retr=<number of retransmissions>

   DATA_CHANNEL_PARTIAL_RELIABLE_REXMIT_UNORDERED
         ordered=false;max-retr=<number of retransmissions>

   DATA_CHANNEL_PARTIAL_RELIABLE_TIMED
         ordered=true;max-time=<lifetime in milliseconds>

   DATA_CHANNEL_PARTIAL_RELIABLE_TIMED_UNORDERED
         ordered=false;max-time=<lifetime in milliseconds>

5.2.3.  Opening a Data Channel

   The procedure for opening a data channel using SDP offer/answer
   negotiation starts with the agent preparing to send an SDP offer.  If
   a peer receives an SDP offer before starting to send a new SDP offer
   with data channels that are to be SDP offer/answer negotiated, or
   loses an SDP offer glare resolution procedure in this case, it MUST
   wait until the ongoing SDP offer/answer completes before resuming the
   SDP offer/answer negotiation procedure.

   The agent that intends to send an SDP offer to create data channels
   through SDP offer/answer negotiation performs the following:

   o  Creates data channels using stream identifiers from the owned set
      (see Section 5.2.1).

   o  Generates a new SDP offer.

   o  Determines the list of stream identifiers assigned to data
      channels opened through SDP offer/answer negotiation.

   o  Completes the SDP offer with the "a=dcmap:" and "a=dcsa:"
      attributes needed, if any, for each SDP offer/answer negotiated
      data channel, as described in Section 5.1 and in Section 5.2.2.

   o  Sends the SDP offer.

   The peer receiving such an SDP offer performs the following:





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   o  Parses and applies the SDP offer.  Note that the typical parser
      normally ignores unknown SDP attributes, which includes data
      channel related attributes.

   o  Analyzes the channel parameters and sub-protocol attributes to
      determine whether to accept each offered data channel.

   o  For accepted data channels, it creates peer instances for the data
      channels with the agent using the channel parameters described in
      the SDP offer.  Note that the agent is asked to create data
      channels with SCTP stream identifiers contained in the SDP offer
      if the SDP offer is accepted.

   o  Generates an SDP answer.

   o  Completes the SDP answer with the "a=dcmap:" and optional
      "a=dcsa:" attributes needed for each SDP offer/answer negotiated
      data channel, as described in Section 5.1 and in Section 5.2.2.

   o  Sends the SDP answer.

   The agent receiving such an SDP answer performs the following:

   o  Closes any created data channels for which the expected "a=dcmap:"
      and "a=dcsa:" attributes are not present in the SDP answer.

   o  Applies the SDP answer.

   Each agent application MUST wait to send data until it has
   confirmation that the data channel at the peer is instantiated.  For
   WebRTC, this is when both data channel stacks have channel parameters
   instantiated.  This occurs:

   o  At both peers when a data channel is created without an
      established SCTP association, as soon as the SCTP association is
      successfully established.

   o  At the agent receiving an SDP offer for which there is an
      established SCTP association, as soon as it creates an SDP offer/
      answer negotiated data channel based on information signaled in
      the SDP offer.

   o  At the agent sending an SDP offer to create a new SDP offer/answer
      negotiated data channel for which there is an established SCTP
      association, when it receives the SDP answer confirming acceptance
      of the data channel or when it begins to receive data on the data
      channel from the peer, whichever occurs first.




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   Note: DCEP is not used, that is neither the SDP offerer nor the SDP
   answerer send an in-band DCEP DATA_CHANNEL_OPEN message.

5.2.4.  Closing a Data Channel

   When the application requests the closing of a data channel that was
   negotiated via SDP offer/answer, the data channel stack always
   performs an SCTP SSN reset for this channel.

   It is specific to the sub-protocol whether this closing MUST in
   addition be signaled to the peer via a new SDP offer/answer exchange.

   The intention to close a data channel can be signaled by sending a
   new SDP offer which excludes the "a=dcmap:" and "a=dcsa:" attribute
   lines for the data channel.  The offerer SHOULD NOT change the port
   value for the "m" line (e.g. to zero) when closing a data channel
   (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.  If the answerer accepts the SDP
   offer then the answerer MUST close those data channels whose
   "a=dcmap:" and "a=dcsa:" attribute lines were excluded from the
   received SDP offer, unless those data channels were already closed,
   and the answerer MUST also exclude the corresponding attribute lines
   in the answer.  In addition to that, the SDP answerer MAY exclude
   other data channels which were closed but not yet communicated to the
   peer.  So, the offerer MUST inspect the answer to see if it has to
   close other data channels which are now not included in the answer.

   If a new SDP offer/answer is used to close data channels then the
   data channel(s) SHOULD only be closed by the answerer/offerer after a
   successful SDP answer is sent/received.

      This delayed closure is RECOMMENDED in order to handle cases where
      a successful SDP answer is not received, in which case the state
      of the session SHOULD be kept per the last successful SDP offer/
      answer.

   If a client receives a data channel close indication (due to inband
   SCTP SSN reset or some other reason) without associated SDP offer
   then the client SHOULD generate an SDP offer which excludes this
   closed data channel.

   The application MUST also close any data channel that was negotiated
   via SDP offer/answer, for which the stream identifiers are not listed
   in an incoming SDP offer.

   A closed data channel using local close (SCTP SSN reset), without an
   additional SDP offer/answer to close it, may be reused for a new data



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   channel.  This can only be done via new SDP offer/answer, describing
   the new sub-protocol and its attributes, only after the corresponding
   data channel close acknowledgement is received from the peer (i.e.
   SCTP SSN reset of both incoming and outgoing streams is completed).
   This restriction is to avoid the race conditions between arrival of
   "SDP offer which reuses stream" with "SCTP SSN reset which closes
   outgoing stream" at the peer.

5.2.5.  Various SDP Offer/Answer Scenarios and Considerations

      SDP offer has no "a=dcmap:" attributes

      *  Initial SDP offer: No data channel is negotiated yet.  The DTLS
         connection and SCTP association is negotiated and, if agreed,
         established as per [I-D.ietf-mmusic-sctp-sdp].

      *  Subsequent SDP offer: All the SDP offer/answer negotiated data
         channels are expected be closed now.  The DTLS/SCTP association
         remains open for SDP offer/answer or DCEP negotiation of data
         channels.

      SDP answer has no "a=dcmap:" attributes

      *  Initial SDP answer: Either the peer does not support "a=dcmap:"
         attributes or it rejected all the data channels.  In either
         case the offerer closes all the SDP offer/answer negotiated
         data channels that were open at the time of initial offer.  The
         DTLS connection and SCTP association will still be setup.

      *  Subsequent SDP answer: All the SDP offer/answer negotiated data
         channels are expected be closed now.  The DTLS/SCTP association
         remains open for future SDP offer/answer or DCEP negotiation of
         data channels.

      SDP offer has no "a=dcsa:" attributes for a data channel.

      *  This is allowed and indicates there are no sub-protocol
         parameters to convey.

      SDP answer has no "a=dcsa:" attributes for a data channel.

      *  This is allowed and indicates there are no sub-protocol
         parameters to convey in the SDP answer.  The number of
         "a=dcsa:" attributes in the SDP answer does not have to match
         the number of "a=dcsa:" attributes in the SDP offer.






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6.  Examples

   SDP offer:
     m=application 10001 UDP/DTLS/SCTP webrtc-datachannel
     c=IN IP4 10.10.10.1
     a=max-message-size:100000
     a=sctp-port:5000
     a=setup:actpass
     a=connection:new
     a=fingerprint:SHA-1 \
         4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
     a=dcmap:0 subprotocol="BFCP";label="BFCP"

   SDP answer:
     m=application 10002 UDP/DTLS/SCTP webrtc-datachannel
     c=IN IP4 10.10.10.2
     a=max-message-size:100000
     a=sctp-port:5002
     a=setup:passive
     a=connection:new
     a=fingerprint:SHA-1 \
         5B:AD:67:B1:3E:82:AC:3B:90:02:B1:DF:12:5D:CA:6B:3F:E5:54:FA

                            Figure 1: Example 1

   In the above example the SDP answerer rejected the data channel with
   stream id 0 either for explicit reasons or because it does not
   understand the "a=dcmap:" attribute.  As a result the offerer will
   close the data channel created with the SDP offer/answer negotiation
   option.  The SCTP association will still be setup over DTLS.  At this
   point the offerer or the answerer may use DCEP negotiation to open
   data channels.



















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   SDP offer:
     m=application 10001 UDP/DTLS/SCTP webrtc-datachannel
     c=IN IP4 10.10.10.1
     a=max-message-size:100000
     a=sctp-port:5000
     a=setup:actpass
     a=connection:new
     a=fingerprint:SHA-1 \
         4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
     a=dcmap:0 subprotocol="BFCP";label="BFCP"
     a=dcmap:2 subprotocol="MSRP";label="MSRP"
     a=dcsa:2 accept-types:message/cpim text/plain
     a=dcsa:2 path:msrp://alice.example.com:10001/2s93i93idj;dc

   SDP answer:
     m=application 10002 UDP/DTLS/SCTP webrtc-datachannel
     c=IN IP4 10.10.10.2
     a=max-message-size:100000
     a=sctp-port:5002
     a=setup:passive
     a=connection:new
     a=fingerprint:SHA-1 \
         5B:AD:67:B1:3E:82:AC:3B:90:02:B1:DF:12:5D:CA:6B:3F:E5:54:FA
     a=dcmap:2 subprotocol="MSRP";label="MSRP"
     a=dcsa:2 accept-types:message/cpim text/plain
     a=dcsa:2 path:msrp://bob.example.com:10002/si438dsaodes;dc

                            Figure 2: Example 2

   In the above example the SDP offer contains data channels for BFCP
   and MSRP sub-protocols.  The SDP answer rejected BFCP and accepted
   MSRP.  So, the offerer should close the data channel for BFCP and
   both offerer and answerer may start using the MSRP data channel
   (after SCTP/DTLS association is setup).  The data channel with stream
   id 0 is free and can be used for future DCEP or SDP offer/answer
   negotiation.

   Continuing on the earlier example in Figure 1.













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   Subsequent SDP offer:
     m=application 10001 UDP/DTLS/SCTP webrtc-datachannel
     c=IN IP4 10.10.10.1
     a=max-message-size:100000
     a=sctp-port:5000
     a=setup:actpass
     a=connection:existing
     a=fingerprint:SHA-1 \
         4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
     a=dcmap:4 subprotocol="MSRP";label="MSRP"
     a=dcsa:4 accept-types:message/cpim text/plain
     a=dcsa:4 path:msrp://alice.example.com:10001/2s93i93idj;dc

   Subsequent SDP answer:
     m=application 10002 UDP/DTLS/SCTP webrtc-datachannel
     c=IN IP4 10.10.10.2
     a=max-message-size:100000
     a=sctp-port:5002
     a=setup:passive
     a=connection:existing
     a=fingerprint:SHA-1 \
         5B:AD:67:B1:3E:82:AC:3B:90:02:B1:DF:12:5D:CA:6B:3F:E5:54:FA
     a=dcmap:4 subprotocol="MSRP";label="MSRP"
     a=dcsa:4 accept-types:message/cpim text/plain
     a=dcsa:4 path:msrp://bob.example.com:10002/si438dsaodes;dc

                            Figure 3: Example 3

   The above example is a continuation of the example in Figure 1.  The
   SDP offer now removes the MSRP data channel with stream id 2, but
   opens a new MSRP data channel with stream id 4.  The answerer accepts
   the entire offer.  As a result the offerer closes the earlier
   negotiated MSRP related data channel and both offerer and answerer
   may start using new the MSRP related data channel.

7.  Security Considerations

   No security considerations are envisaged beyond those already
   documented in [RFC4566].

8.  IANA Considerations

8.1.  Subprotocol Identifiers

   Registration of new subprotocol identifiers is performed using the
   existing IANA table "WebSocket Subprotocol Name Registry".

   The following text should be added following the title of the table.



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   "This table also includes subprotocol identifiers specified for usage
   within a WebRTC data channel."

   The following reference should be added to under the heading
   reference: "RFC XXXX".

   This document assigns no new values to this table.

   NOTE to RFC Editor: Please replace "XXXX" with the number of this
   RFC.

8.2.  New SDP Attributes

8.2.1.  dcmap

   NOTE to RFC Editor: Please replace "XXXX" with the number of this
   RFC.

   This document defines a new SDP media-level attribute "a=dcmap:" as
   follows:

     +---------------------+-----------------------------------------+
     | Attribute name:     | dcmap                                   |
     | Type of attribute:  | media                                   |
     | Mux category:       | SPECIAL                                 |
     | Charset Dependent:  | No                                      |
     | Purpose:            | Define data channel specific parameters |
     | Appropriate values: | As defined in Section 5.1.1             |
     | Contact name:       | Maridi R. Makaraju (Raju)               |
     | Contact e-mail:     | Raju.Makaraju@alcatel-lucent.com        |
     | Reference:          | RFCXXXX                                 |
     +---------------------+-----------------------------------------+

8.2.2.  dcsa

   NOTE to RFC Editor: Please replace "XXXX" with the number of this
   RFC.

   This document defines a new SDP media-level attribute "a=dcsa:" as
   follows:











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   +---------------------+---------------------------------------------+
   | Attribute name:     | dcsa                                        |
   | Type of attribute:  | media                                       |
   | Mux category:       | SPECIAL                                     |
   | Charset Dependent:  | No                                          |
   | Purpose:            | Define data channel sub-protocol specific   |
   |                     | attributes                                  |
   | Appropriate values: | As defined in Section 5.1.2                 |
   | Contact name:       | Maridi R. Makaraju (Raju)                   |
   | Contact e-mail:     | Raju.Makaraju@alcatel-lucent.com            |
   | Reference:          | RFCXXXX                                     |
   +---------------------+---------------------------------------------+

9.  Acknowledgments

   The authors wish to acknowledge the borrowing of ideas from other
   internet drafts by Salvatore Loreto, Gonzalo Camarillo, Peter Dunkley
   and Gavin Llewellyn, and to thank Roni Even, Christian Groves,
   Christer Holmberg, Paul Kyzivat, Jonathan Lennox, and Uwe
   Rauschenbach for their invaluable comments.

10.  CHANGE LOG

10.1.  Changes against 'draft-ietf-mmusic-data-channel-sdpneg-04'

   o  In Section 5.1.1.5 the first (and only) paragraph was "The
      'subprotocol' parameter indicates which protocol the client
      expects to exchange via the channel.  'Subprotocol' is an optional
      parameter.  If the 'subprotocol' parameter is not present, then
      its value defaults to the empty string."  Replacement of this
      paragraph with following two paragraphs:

      *  The 'subprotocol' parameter indicates which protocol the client
         expects to exchange via the channel.  This parameter maps to
         the 'Protocol' parameter defined in
         [I-D.ietf-rtcweb-data-protocol].  Section 8.1 specifies how new
         subprotocol parameter values are registered.  'Subprotocol' is
         an optional parameter.  If the 'subprotocol' parameter is not
         present, then its value defaults to the empty string.

      *  Note: The empty string MAY also be explicitly used as
         'subprotocol' value, such that 'subprotocol=""' is equivalent
         to the 'subprotocol' parameter not being present at all.
         [I-D.ietf-rtcweb-data-protocol] allows the DATA_CHANNEL_OPEN
         message's 'Subprotocol' value to be an empty string.

   o  Addition of [I-D.ietf-mmusic-sdp-mux-attributes] to list the of
      normative references.



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   o  Addition of dcmap attribute specific IANA registration
      Section 8.2.1.

   o  Addition of dcsa attribute specific IANA registration
      Section 8.2.2.

   o  Addition of new Section 5.1.1.2 describing the mux category of the
      dcmap SDP attribute.  This section and the new "a=dcsa:" attribute
      related mux category section are similar to the "Mux Category"
      sections of the "a=sctp-port:" and "a=max-message-size:"
      attributes in [I-D.ietf-mmusic-sctp-sdp].

   o  Addition of new Section 5.1.2.2 describing the mux category of the
      dcsa SDP attribute.

10.2.  Changes against 'draft-ietf-mmusic-data-channel-sdpneg-03'

   o  In Section 1 replacement of "RTCWeb leaves it open for other
      applications to use data channels and its in-band DCEP or out-of-
      band non-DCEP protocols for creating them" with "... to use data
      channels and its in-band DCEP or other in-band or out-of-band
      protocols for creating them".  Additionally replacement of "In
      particular, the SDP offer generated by the application includes no
      channel-specific information" with "... generated by the RTCweb
      data channel stack includes no channel-specific information".

   o  Move of former section 5 ("Data Channels") to new Appendix A and
      removal of JavaScript API specific discussions from this moved
      text (like mentioning of data channel stack specific states).
      Therefore former section 6 ("SDP Offer/Answer Negotiation") is now
      Section 5.

   o  In Section 5:

      *  Relacement of Section 5's first paragraph "This section defines
         a method of non-DCEP negotiation by which two clients can
         negotiate data channel-specific and sub-protocol-specific
         parameters, using the out-of-band SDP offer/answer exchange.
         This SDP extension can only be used with the SDP offer/answer
         model." with "This section defines an SDP extension by which
         two clients can negotiate data channel-specific and sub-
         protocol-specific parameters without using DCEP
         [I-D.ietf-rtcweb-data-protocol].  This SDP extension only
         defines usage in the context of SDP offer/answer."

      *  Addition of new paragraph: "Appendix A provides information how
         data channels work in general and especially summarizes some




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         key aspects, which should be considered for the negotiation of
         data channels if DCEP is not used."

   o  In Section 5.1.1 replacement of "The intention of exchanging these
      attributes is to create data channels on both the peers with the
      same set of attributes without actually using the DCEP
      [I-D.ietf-rtcweb-data-protocol]" with "The intention in exchanging
      these attributes is to create, on two peers, without use of DCEP
      [I-D.ietf-rtcweb-data-protocol], matched pairs of oppositely
      directed data channels having the same set of attributes".

   o  In Section 5.1.1.6 replacement of "The 'max-retr' parameter
      indicates the maximal number a user message will be retransmitted"
      with "The 'max-retr' parameter indicates the maximal number of
      times a user message will be retransmitted".

   o  In Section 5.2.1 replacement of "However, an SDP offer/answer
      exchange MUST NOT be initiated if the associated SCTP stream is
      already negotiated via DCEP" with "However, an SCTP stream MUST
      NOT be referenced in a dcmap or dcsa attribute of an SDP offer/
      answer exchange if the associated SCTP stream has already been
      negotiated via DCEP".

   o  In the examples in Section 6 addition of the previously missing
      colons to the "a=sctp-port" attribute lines.

10.3.  Changes against 'draft-ietf-mmusic-data-channel-sdpneg-02'

   o  Move of reference [I-D.ietf-rtcweb-jsep] from the list of
      normative references to the list of informative references.

   o  Addition of [IANA-SDP-Parameters] to the list of informative
      references and addition of following two sentences to the first
      paragraph after the ABNF definition: "Note however that not all
      SDP attributes are suitable as "a=dcsa:" parameter.
      [IANA-SDP-Parameters] contains the lists of IANA registered
      session and media level or media level only SDP attributes."

   o  In the introduction replacement of last sentence "This document
      defines SDP-based out-of-band negotiation procedures to establish
      data channels for transport of well-defined sub-protocols" with
      "This document defines SDP offer/answer negotiation procedures to
      establish data channels for transport of well-defined sub-
      protocols, to enable out-of-band negotiation".

   o  Throughout the document replacement of "external negotiation" with
      "SDP offer/answer negotiation" and removal of term "external
      negotiation" from the terminology list in Section 3.



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   o  Throughout the document replacement of "internal negotiation" with
      "DCEP" and removal of terms "internal negotiation" and "in-band
      negotiation" from the terminology list in Section 3.

   o  Addition of "SCTP Stream Sequence Number (SSN)" to the list of
      terms.

   o  In Section 5.2.1 replacement of sentence "However, a single stream
      is managed using one method at a time." with "However, an SDP
      offer/answer exchange MUST NOT be initiated if the associated SCTP
      stream is already negotiated via DCEP".

   o  In Section 5.2.2 replacement of sentence "By definition max-retr
      and max-time are mutually exclusive, so only one of them can be
      present in a=dcmap" with "By definition max-retr and max-time are
      mutually exclusive, so at most one of them MAY be present in
      a=dcmap".

   o  Move of reference [WebRtcAPI] from list of normative references to
      list of informative references.

   o  Removal of almost all text parts, which discussed JavaScript or
      other API specific aspects.  Such API specific aspects were mainly
      discussed in sub-sections of Section 5 and Section 5 of draft-
      ietf-mmusic-data-channel-sdpneg-02.

10.4.  Changes against 'draft-ietf-mmusic-data-channel-sdpneg-01'

   o  New Section 4 regarding applicability to SDP offer/answer only.

   o  Addition of new Section 8.1 "Subprotocol identifiers" as
      subsection of the "IANA Considerations" related Section 8.  Also
      removal of the temporary note "To be completed.  As [I-D.ietf-
      rtcweb-data-protocol] this document should refer to IANA's
      WebSocket Subprotocol Name Registry defined in [RFC6455]."

   o  In Section 5.2.2:

      *  In the first paragraph replacement of the sentence "If an SDP
         offer contains both of these parameters then such an SDP offer
         will be rejected." with "If an SDP offer contains both of these
         parameters then the receiver of such an SDP offer MUST reject
         the SDP offer."

      *  In the second paragraph capitalization of "shall" and "may"
         such that both sentences now read: "The SDP answer SHALL echo
         the same subprotocol, max-retr, max-time, ordered parameters,
         if those were present in the offer, and MAY include a label



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         parameter.  They MAY appear in any order, which could be
         different from the SDP offer, in the SDP answer."

      *  In the third paragraph replacement of the sentence "The same
         information MUST be replicated without changes in any
         subsequent offer or answer, as long as the data channel is
         still opened at the time of offer or answer generation." with
         "When sending a subsequent offer or an answer, and for as long
         as the data channel is still open, the sender MUST replicate
         the same information.".

   o  In Section 5.2.2 the mapping of data channel types defined in
      [I-D.ietf-rtcweb-data-protocol] to the SDP "a=dcmap" attribute
      parameters were illustrated using example "a=dcmap" attribute
      lines.  Replacement of these example "a=dcmap" attribute lines
      with just the "a=dcmap" attribute parameters being relevant for
      the channel type.

   o  In Section 5.2.5 the description of bullet point "SDP offer has no
      a=dcmap attributes - Initial SDP offer:" was "Initial SDP offer:
      No data channel negotiated yet."  Replacement of this description
      with "Initial SDP offer: No data channel is negotiated yet.  The
      DTLS connection and SCTP association is negotiated and, if agreed,
      established as per [I-D.ietf-mmusic-sctp-sdp]."

   o  In Section 5.2.5 in both bullet points related to "Subsequent SDP
      offer" and "Subsequent SDP answer" replacement of "All the
      externally negotiated data channels must be closed now." with "All
      the externally negotiated data channels are expected be closed
      now.".

   o  In Appendix A.2.2's sixth paragraph beginning with "[ASSUMPTION]"
      replacement of the two occurrences of "must" with "MUST".

   o  In Section 5.1.1.1 in the definition of the ABNF rule "dcmap-opt"
      there was a comment saying that "Either only maxretr-opt or
      maxtime-opt is present.  Both MUST not be present."  Removal of
      the second normative sentence and instead addition of following
      new paragraph to the end of this section: "Within an 'a=dcmap'
      attribute line's 'dcmap-opt' value either only one 'maxretr-opt'
      parameter or one 'maxtime-opt' parameter is present.  Both MUST
      NOT be present."

   o  In Section 5.1.1.8 replacement of the first sentence "The
      'ordered' parameter with value "true" indicates that DATA chunks
      in the channel MUST be dispatched to the upper layer by the
      receiver while preserving the order." with "The 'ordered'
      parameter with value "true" indicates that the receiver MUST



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      dispatch DATA chunks in the data channel to the upper layer while
      preserving the order.".

   o  In Section 5.2.3's first paragraph replacement of the one
      occurrence of "must" with "..., it MUST wait until ...".

   o  In Section 5.2.4:

      *  In the second paragraph replacement of "must" with "... whether
         this closing MUST in addition ..."

      *  In the third paragraph replacement of the sentence "The port
         value for the "m" line SHOULD not be changed (e.g., to zero)
         when closing a data channel ..." with "The offerer SHOULD NOT
         change the port value for the "m" line (e.g., to zero) when
         closing a data channel ...".

      *  In the last but two paragraph replacement of the sentence "...
         then an SDP offer which excludes this closed data channel
         SHOULD be generated." with "... then the client SHOULD generate
         an SDP offer which excludes this closed data channel.".

      *  In the last but one paragraph replacement of "must" with "The
         application MUST also close...".

   o  In Section 5.1.2 addition of following note after the formal
      definition of the 'a=dcsa' attribute: "Note that the above
      reference to RFC 4566 defines were the attribute definition can be
      found; it does not provide any limitation on support of attributes
      defined in other documents in accordance with this attribute
      definition."

10.5.  Changes against 'draft-ietf-mmusic-data-channel-sdpneg-00'

   o  In Section 3 "WebRTC data channel" was defined as "A bidirectional
      channel consisting of paired SCTP outbound and inbound streams."
      Replacement of this definition with "Data channel: A WebRTC data
      channel as specified in [I-D.ietf-rtcweb-data-channel]", and
      consistent usage of "data channel" in the remainder of the
      document including the document's headline."

   o  In Section 5 removal of following note: 'OPEN ISSUE: The syntax in
      [I-D.ietf-mmusic-sctp-sdp] may change as that document progresses.
      In particular we expect "webrtc-datachannel" to become a more
      general term.'

   o  Consistent usage of '"m" line' in whole document as per [RFC4566].




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   o  In Section 5.1.1 removal of the example dcmap attribute line
      'a=dcmap:2 subprotocol="BFCP";label="channel 2' as there are
      already four examples right after the ABNF rules in
      Section 5.1.1.1.  Corresponding removal of following related note:
      "Note: This document does not provide a complete specification of
      how to negotiate the use of a WebRTC data channel to transport
      BFCP.  Procedures specific to each sub-protocol such as BFCP will
      be documented elsewhere.  The use of BFCP is only an example of
      how the generic procedures described herein might apply to a
      specific sub-protocol."

   o  In Section 5.1.1 removal of following note: "Note: This attribute
      is derived from attribute "webrtc-DataChannel", which was defined
      in old version 03 of the following draft, but which was removed
      along with any support for SDP external negotiation in subsequent
      versions: [I-D.ietf-mmusic-sctp-sdp]."

   o  Insertion of following new sentence to the beginning of
      Section 5.1.1.1: "dcmap is a media level attribute having
      following ABNF syntax:"

   o  Insertion of new Section 5.1.1.3 containing the dcmap-stream-id
      specifying sentence, which previously was placed right before the
      formal ABNF rules.  Removal of the sentence 'Stream is a mandatory
      parameter and is noted directly after the "a=dcmap:" attribute's
      colon' as this information is part of the ABNF specification.

   o  In Section 5.1.1.1 modification of the 'ordering-value' values
      from "0" or "1" to "true" or "false".  Corresponding text
      modifications in Section 5.1.1.8.

   o  In Section 5.1.1.1 the ABNF definition of "quoted-string" referred
      to rule name "escaped-char", which was not defined.  Instead a
      rule with name "escaped" was defined.  Renamed that rule's name to
      "escaped-char".

   o  Insertion of a dedicated note right after the "a=dcmap:4"
      attribute example in Section 5.1.1.1 regarding the non-printable
      "escaped-char" character within the "label" value.

   o  In Section 5.1.2's second paragraph replacement of "sctp stream
      identifier" with "SCTP stream identifier".

   o  In first paragraph of Section 5.2.1 replacement of first two
      sentences 'For the SDP-based external negotiation described in
      this document, the initial offerer based "SCTP over DTLS" owns by
      convention the even stream identifiers whereas the initial
      answerer owns the odd stream identifiers.  This ownership is



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      invariant for the whole lifetime of the signaling session, e.g. it
      does not change if the initial answerer sends a new offer to the
      initial offerer.' with 'If an SDP offer/answer exchange (could be
      the initial or a subsequent one) results in a UDP/DTLS/SCTP or
      TCP/DTLS/SCTP based media description being accepted, and if this
      SDP offer/answer exchange results in the establishment of a new
      SCTP association, then the SDP offerer owns the even SCTP stream
      ids of this new SCTP association and the answerer owns the odd
      SCTP stream identifiers.  If this "m" line is removed from the
      signaling session (its port number set to zero), and if usage of
      this or of a new UDP/DTLS/SCTP or TCP/DTLS/SCTP based "m" line is
      renegotiated later on, then the even and odd SCTP stream
      identifier ownership is redetermined as well as described above.'

   o  In Section 5.2.3 the first action of an SDP answerer, when
      receiving an SDP offer, was described as "Applies the SDP offer.
      Note that the browser ignores data channel specific attributes in
      the SDP."  Replacement of these two sentences with "Parses and
      applies the SDP offer.  Note that the typical parser normally
      ignores unknown SDP attributes, which includes data channel
      related attributes."

   o  In Section 5.2.3 the second sentence of the third SDP answerer
      action was "Note that the browser is asked to create data channels
      with stream identifiers not "owned" by the agent.".  Replacement
      of this sentence with "Note that the agent is asked to create data
      channels with SCTP stream identifiers contained in the SDP offer
      if the SDP offer is accepted."

   o  In Section 5.2.4 the third paragraph began with "A data channel
      can be closed by sending a new SDP offer which excludes the dcmap
      and dcsa attribute lines for the data channel.  The port value for
      the m line should not be changed (e.g., to zero) when closing a
      data channel (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.  If the
      answerer accepts the SDP offer then it MUST also exclude the
      corresponding attribute lines in the answer. ..." Replacement of
      this part with "The intention to close a data channel can be
      signaled by sending a new SDP offer which excludes the "a=dcmap:"
      and "a=dcsa:" attribute lines for the data channel.  The port
      value for the "m" line SHOULD not be changed (e.g., to zero) when
      closing a data channel (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.
      If the answerer accepts the SDP offer then it MUST close those
      data channels whose "a=dcmap:" and "a=dcsa:" attribute lines were
      excluded from the received SDP offer, unless those data channels



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      were already closed, and it MUST also exclude the corresponding
      attribute lines in the answer."

   o  In Section 5.2.4 the hanging text after the third paragraph was
      "This delayed close is to handle cases where a successful SDP
      answer is not received, in which case the state of session should
      be kept per the last successful SDP offer/answer."  Replacement of
      this sentence with "This delayed closure is RECOMMENDED in order
      to handle cases where a successful SDP answer is not received, in
      which case the state of the session SHOULD be kept per the last
      successful SDP offer/answer."

   o  Although dedicated to "a=dcmap" and "a=dcsa" SDP syntax aspects
      Section 5.1.1 contained already procedural descriptions related to
      data channel reliability negotiation.  Creation of new
      Section 5.2.2 and moval of reliability negotiation related text to
      this new section.

10.6.  Changes against 'draft-ejzak-mmusic-data-channel-sdpneg-02'

   o  Removal of note "[ACTION ITEM]" from section "subprotocol
      parameter".  As [I-D.ietf-rtcweb-data-protocol] this document
      should refer to IANA's WebSocket Subprotocol Name Registry defined
      in [RFC6455].

   o  In whole document, replacement of "unreliable" with "partially
      reliable", which is used in [I-D.ietf-rtcweb-data-channel] and in
      [I-D.ietf-rtcweb-data-protocol] in most places.

   o  Clarification of the semantic if the "max-retr" parameter is not
      present in an a=dcmap attribute line.  In section "max-retr
      parameter" the sentence "The max-retr parameter is optional with
      default value unbounded" was replaced with "The max-retr parameter
      is optional.  If the max-retr parameter is not present, then the
      maximal number of retransmissions is determined as per the generic
      SCTP retransmission rules as specified in [RFC4960]".

   o  Clarification of the semantic if the "max-time" parameter is not
      present in an a=dcmap attribute line.  In section "max-time
      parameter" the sentence "The max-time parameter is optional with
      default value unbounded" was replaced with "The max-time parameter
      is optional.  If the max-time parameter is not present, then the
      generic SCTP retransmission timing rules apply as specified in
      [RFC4960]".

   o  In section "label parameter" the sentence "Label is a mandatory
      parameter." was removed and following new sentences (including the
      note) were added: "The 'label' parameter is optional.  If it is



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      not present, then its value defaults to the empty string.  Note:
      The empty string may also be explicitly used as 'label' value,
      such that 'label=""' is equivalent to the 'label' parameter not
      being present at all.  [I-D.ietf-rtcweb-data-protocol] allows the
      DATA_CHANNEL_OPEN message's 'Label' value to be an empty string."

   o  In section "subprotocol parameter" the sentence "Subprotocol is a
      mandatory parameter." was replaced with "'Subprotocol' is an
      optional parameter.  If the 'subprotocol' parameter is not
      present, then its value defaults to the empty string."

   o  In the "Examples" section, in the first two SDP offer examples in
      the a=dcmap attribute lines 'label="BGCP"' was replaced with
      'label="BFCP"'.

   o  In all examples, the "m" line proto value "DTLS/SCTP" was replaced
      with "UDP/DTLS/SCTP" and the "a=fmtp" attribute lines were
      replaced with "a=max-message-size" attribute lines, as per draft-
      ietf-mmusic-sctp-sdp-12.

10.7.  Changes against '-01'

   o  Formal syntax for dcmap and dcsa attribute lines.

   o  Making subprotocol as an optional parameter in dcmap.

   o  Specifying disallowed parameter combinations for max-time and max-
      retr.

   o  Clarifications on WebRTC data channel close procedures.

10.8.  Changes against '-00'

   o  Revisions to identify difference between internal and external
      negotiation and their usage.

   o  Introduction of more generic terminology, e.g. "application"
      instead of "browser".

   o  Clarification of how "max-retr and max-time affect the usage of
      unreliable and reliable WebRTC data channels.

   o  Updates of examples to take into account the SDP syntax changes
      introduced with draft-ietf-mmusic-sctp-sdp-07.

   o  Removal of the SCTP port number from the a=dcmap and a=dcsa
      attributes as this is now contained in the a=sctp-port attribute,




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      and as draft-ietf-mmusic-sctp-sdp-07 supports only one SCTP
      association on top of the DTLS connection.

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

   [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>.

   [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>.

   [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>.

   [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-sctp-sdp]
              Holmberg, C., Loreto, S., and G. Camarillo, "Stream
              Control Transmission Protocol (SCTP)-Based Media Transport
              in the Session Description Protocol (SDP)", draft-ietf-
              mmusic-sctp-sdp-15 (work in progress), September 2015.

   [I-D.ietf-mmusic-sdp-mux-attributes]
              Nandakumar, S., "A Framework for SDP Attributes when
              Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-10
              (work in progress), July 2015.

11.2.  Informative References

   [I-D.ietf-rtcweb-data-protocol]
              Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data Channel
              Establishment Protocol", draft-ietf-rtcweb-data-
              protocol-09 (work in progress), January 2015.



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   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, DOI 10.17487/RFC4960, September 2007,
              <http://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,
              <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>.

   [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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc6135>.

   [RFC6455]  Fette, I. and A. Melnikov, "The WebSocket Protocol",
              RFC 6455, DOI 10.17487/RFC6455, December 2011,
              <http://www.rfc-editor.org/info/rfc6455>.

   [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,
              <http://www.rfc-editor.org/info/rfc6714>.

   [I-D.ietf-rtcweb-jsep]
              Uberti, J., Jennings, C., and E. Rescorla, "Javascript
              Session Establishment Protocol", draft-ietf-rtcweb-jsep-11
              (work in progress), July 2015.

   [IANA-SDP-Parameters]
              "Session Description Protocol (SDP) Parameters", Internet
              Assigned Numbers Authority Protocol Assignments Session
              Description Protocol (SDP) Parameters,
              <http://www.iana.org/assignments/sdp-parameters/
              sdp-parameters.xhtml>.





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   [WebRtcAPI]
              Bergkvist, A., Burnett, D., Jennings, C., and A.
              Narayanan, "WebRTC 1.0: Real-time Communication Between
              Browsers", World Wide Web Consortium WD-webrtc-20150210,
              February 2015,
              <http://www.w3.org/TR/2015/WD-webrtc-20150210/>.

Appendix A.  Generic Data Channel Negotiation Aspects When Not Using
             DCEP

   This appendix summarizes how data channels work in general and
   discusses some key aspects, which should be considered for the out-
   of-band negotiation of data channels if DCEP is not used.

   A WebRTC application creates a data channel by providing a number of
   setup parameters (sub-protocol, label, reliability, order of
   delivery, priority).  The application also specifies if it wants to
   make use of the negotiation using the DCEP
   [I-D.ietf-rtcweb-data-protocol], or if the application intends to
   negotiate data channels using the SDP offer/answer protocol.

   In any case, the SDP offer generated by the application is per
   [I-D.ietf-mmusic-sctp-sdp].  In brief, it contains one "m" line for
   the SCTP association on top of which data channels will run:

   m=application 54111 UDP/DTLS/SCTP webrtc-datachannel
   c=IN IP4 79.97.215.79
   a=max-message-size:100000
   a=sctp-port:5000
   a=setup:actpass
   a=connection:new
   a=fingerprint:SHA-1 \
       4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB

   Note: A WebRTC application will only use "m" line format "webrtc-
   datachannel", and will not use other formats in the "m" line for
   other protocols such as t38.  [I-D.ietf-mmusic-sctp-sdp] supports
   only one SCTP association to be established on top of a DTLS
   association.

   Note: Above SDP media description does not contain any channel-
   specific information.

A.1.  Stream Identifier Numbering

   Independently from the requested type of negotiation, the application
   creating a data channel can either pass to the data channel stack the




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   stream identifier to assign to the data channel or else let the data
   channel stack pick one identifier from the ones unused.

   To avoid glare situations, each endpoint can moreover own an
   exclusive set of stream identifiers, in which case an endpoint can
   only create a data channel with a stream identifier it owns.

   Which set of stream identifiers is owned by which endpoint is
   determined by convention or other means.

      For data channels negotiated with the DCEP, one endpoint owns by
      convention the even stream identifiers, whereas the other owns the
      odd stream identifiers, as defined in
      [I-D.ietf-rtcweb-data-protocol].

      For data channels negotiated via some protocol different from
      DCEP, no convention is defined by default.

A.2.  Generic Data Channel Negotiation Not Using DCEP

A.2.1.  Overview

   DCEP negotiation only provides for negotiation of data channel
   transport parameters and does not provide for negotiation of sub-
   protocol specific parameters.  DCEP-less data channel negotiation can
   be defined to allow negotiation of parameters beyond those handled by
   DCEP, e.g., parameters specific to the sub-protocol instantiated on a
   particular data channel.

   The following procedures are common to all methods of data channel
   negotiation not using DCEP, whether in-band (communicated using
   proprietary means on an already established data channel) or out-of-
   band (using SDP offer/answer or some other protocol associated with
   the signaling channel).

A.2.2.  Opening a Data Channel

   In the case of DCEP-less negotiation, the endpoint application has
   the option to fully control the stream identifier assignments.
   However these assignments have to coexist with the assignments
   controlled by the data channel stack for the DCEP negotiated data
   channels (if any).  It is the responsibility of the application to
   ensure consistent assignment of stream identifiers.

   When the application requests the creation of a new data channel to
   be set up via DCEP-less negotiation, the data channel stack creates
   the data channel locally without sending any DATA_CHANNEL_OPEN
   message in-band.  However, even if the ICE, DTLS and SCTP procedures



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   were already successfully completed, the application can't send data
   on this data channel until the negotiation is complete with the peer.
   This is because the peer needs to be aware of and accept the usage of
   this data channel.  The peer, after accepting the data channel offer,
   can start sending data immediately.  This implies that the offerer
   may receive data channel sub-protocol messages before the negotiation
   is complete and the application should be ready to handle it.

   If the peer rejects the data channel part of the offer then it
   doesn't have to do anything as the data channel was not created using
   the stack.  The offerer on the other hand needs to close the data
   channel that was opened by invoking relevant data channel stack API
   procedures.

   It is also worth noting that a data channel stack implementation may
   not provide any API to create and close data channels; instead the
   data channels may be used on the fly as needed just by communicating
   via non-DCEP means or by even having some local configuration/
   assumptions on both the peers.

   The application then negotiates the data channel properties and sub-
   protocol properties with the peer's application using a mechanism
   different from DCEP.

   The peer then symmetrically creates a data channel with these
   negotiated data channel properties.  This is the only way for the
   peer's data channel stack to know which properties to apply when
   transmitting data on this channel.  The data channel stack must allow
   data channel creation with any non-conflicting stream identifier so
   that both peers can create the data channel with the same stream
   identifier.

A.2.3.  Closing a Data Channel

   When the application requests the closing of a data channel
   negotiated without DCEP, the data channel stack always performs an
   SCTP SSN reset for this channel.

   Depending upon the method used for DCEP-less negotiation and the sub-
   protocol associated with the data channel, the closing might in
   addition be signaled to the peer via SDP offer/answer negotiation.

Authors' Addresses








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   Keith Drage (editor)
   Alcatel-Lucent
   Quadrant, Stonehill Green, Westlea
   Swindon
   UK

   Email: keith.drage@alcatel-lucent.com


   Maridi R. Makaraju (Raju)
   Alcatel-Lucent
   2000 Lucent Lane
   Naperville, Illinois
   US

   Email: Raju.Makaraju@alcatel-lucent.com


   Juergen Stoetzer-Bradler
   Alcatel-Lucent
   Lorenzstrasse 10
   D-70435 Stuttgart
   Germany

   Email: Juergen.Stoetzer-Bradler@alcatel-lucent.com


   Richard Ejzak
   Unaffiliated

   Email: richard.ejzak@gmail.com


   Jerome Marcon
   Unaffiliated
















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