MMUSIC K. Drage
Internet-Draft Unaffiliated
Intended status: Standards Track M. Makaraju
Expires: November 12, 2019 Nokia
R. Ejzak
J. Marcon
Unaffiliated
R. Even, Ed.
Huawei
May 11, 2019

SDP-based Data Channel Negotiation
draft-ietf-mmusic-data-channel-sdpneg-28

Abstract

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 (Session Description Protocol) offer/answer exchange can be used to achieve an out-of-band non-DCEP negotiation for establishing a data channel.

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 November 12, 2019.

Copyright Notice

Copyright (c) 2019 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 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

The concept of establishing a bi-directional data channel running on top of the Stream Control Transmission Protocol (SCTP) is in [I-D.ietf-rtcweb-data-channel] allowing applications to use data channels. An in-band Data Channel Establishment Protocol (DCEP) is in [I-D.ietf-rtcweb-data-protocol], however other in-band or out-of-band protocols may be used for establishing data channels. Each data channel consists of paired SCTP streams sharing the same SCTP Stream Identifier. Data channels are created by endpoint applications using the WebRTC API (Application Programming Interface), or other protocols like CLUE [I-D.ietf-clue-datachannel]. The protocols can be signaled by the data channel "subprotocol" parameter, conceptually similar to the WebSocket [RFC5234] "subprotocol". However, apart from the "subprotocol" value transmitted to the peer, an endpoint application can agree on how to instantiate a given subprotocol on a data channel, and whether it is signaled in-band using DCEP or out-of-band using a non-DCEP protocol (or both).

This document defines SDP offer/answer [RFC3264] procedures that enable out-of-band negotiation for establishing data channels for transport of well-defined subprotocols. These procedures are based on generic SDP offer/answer negotiation rules for SCTP based media transport as specified in [I-D.ietf-mmusic-sctp-sdp] for the SDP "m" line proto values UDP/DTLS/SCTP and TCP/DTLS/SCTP.

This document uses MSRP (Message Session Relay Protocol) [RFC4975] and BFCP (Binary Floor Control Protocol) [RFC4582] in many of the examples. It does not provide a complete specification of how to negotiate the use of a data channel to transport MSRP. Procedures specific to each subprotocol would have to be documented elsewhere. For MSRP they are documented in [I-D.ietf-mmusic-msrp-usage-data-channel] . The use of MSRP in some examples is only to show how the generic procedures described herein might apply to a specific subprotocol.

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 BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

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 receiving 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, etc.).
Data channel subprotocol: The application protocol which is transported over a single data channel. Data channel subprotocol messages are sent as data channel payload over an established data channel. SDP offer/answer exchange can be used as specified in this document to negotiate the establishment of data channels, corresponding data channel properties, associated data channel subprotocols and data channel subprotocol properties. In this case the data channel subprotocols may be identified by the values of the "subprotocol" parameters of the SDP "a=dcmap" attribute as described in Section 5.1.4. Within this document the term "data channel subprotocol" is often abbreviated as just "subprotocol".
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.

4. Applicability Statement

The mechanism in this document only applies to the Session Description Protocol (SDP) [I-D.ietf-mmusic-rfc4566bis] when used together with the SDP offer/answer mechanism [RFC3264]. Declarative usage of SDP is out of scope for this document, and is thus undefined.

5. SDP Data Channel Attributes

This section defines two new SDP media-level attributes that can be used together with the SDP Offer/Answer mechanism to negotiate data channel-specific and subprotocol-specific parameters without the usage of DCEP [I-D.ietf-rtcweb-data-protocol]. The first attribute provides for negotiation of channel-specific parameters. The second attribute provides for negotiation of subprotocol-specific parameters.

Note: 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 DCMAP Attribute

This section defines a new media level attribute "a=dcmap:" that defines the data channel parameters for each data channel to be negotiated.

The attribute is used to create bi-directional SCTP data channels having the same set of attributes. The data channel properties (reliable/partially reliable, ordered/unordered) need to be suitable per the subprotocol transport requirements.

5.1.1. DCMAP Attribute Syntax 

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 / priority-opt
                  ; maxretr-opt and maxtime-opt are mutually exclusive 
                  ;

dcmap-stream-id = 1*5DIGIT
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   = "0" / integer
                  ; number of retransmissions,
                  ; less than 2^32,
                  ; derived from 'Reliability Parameter' of
                  ; [I-D.ietf-rtcweb-data-protocol]
maxtime-opt     = "max-time=" maxtime-value
maxtime-value   = "0" / integer
                  ; milliseconds,
                  ; less than 2^32,
                  ; derived from 'Reliability Parameter' of
                  ; [I-D.ietf-rtcweb-data-protocol]
priority-opt    = "priority=" priority-value
priority-value  = "0" / integer
                  ; unsigned integer value indicating the priority of 
                  ; the data channel,
                  ; less than 2^16,
                  ; derived from 'Priority' of
                  ; [I-D.ietf-rtcweb-data-protocol]

quoted-string   = DQUOTE *(quoted-char / escaped-char) DQUOTE
quoted-char     = SP / quoted-visible
quoted-visible  = %x21 / %x23-24 / %x26-7E ; VCHAR without " or %
escaped-char    = "%" HEXDIG HEXDIG
DQUOTE          = <from-RFC5234>
integer         = <from-RFC4566>

Examples:

a=dcmap:0
a=dcmap:1 subprotocol="bfcp";max-time=60000;priority=512
a=dcmap:2 subprotocol="msrp";ordered=true;label="msrp"
a=dcmap:3 label="Label 1";ordered=false;max-retr=5;priority=128
a=dcmap:4 label="foo%09bar";ordered=true;max-time=15000

"a=dcmap:" is a media level attribute having the following ABNF (Augmented Backus-Naur Form, [RFC5234]) syntax.

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 only one 'maxretr-opt' parameter or one 'maxtime-opt' parameter may be present. Both MUST NOT be present.

5.1.2. Dcmap-stream-id Parameter

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

5.1.3. 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.

In order to communicate with WEbRTC API the label attribute should:

Note: The empty string MAY also be explicitly used as a '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.

5.1.4. 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 9.1 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 an 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.

5.1.5. 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 and the max-time parameter are not present, then reliable transmission is performed as specified in [RFC4960]. This parameter maps to the 'Number of RTX' parameter defined in [I-D.ietf-rtcweb-data-protocol].

5.1.6. 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 life-time starts when providing the user message to the protocol stack. The max-time parameter is optional. If the max-retr parameter and the max-time parameter are not present, then reliable transmission is performed as specified in [RFC4960]. This parameter maps to the 'Lifetime in ms' parameter defined in [I-D.ietf-rtcweb-data-protocol].

5.1.7. Ordered Parameter

The 'ordered' parameter with value "true" indicates that the receiver will 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].

5.1.8. Priority Parameter

The 'priority' parameter indicates the data channel's priority relative to the priorities of other data channels, which may additionally exist over the same SCTP association. The 'priority' parameter maps to the 'Priority' parameter defined in [I-D.ietf-rtcweb-data-protocol]. The 'priority' parameter is optional. In the absence of this parameter "priority=256" is assumed.

5.1.9. DCMAP Multiplexing Category

The multiplexing category [I-D.ietf-mmusic-sdp-mux-attributes] of the "a=dcmap:" attribute is SPECIAL.

As the usage of multiple SCTP associations on top of a single DTLS association is outside the scope of [I-D.ietf-mmusic-sctp-sdp], no "a=dcmap:" attribute 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 on top of a single DTLS association, or how to add multiple SCTP associations to one BUNDLE group, then multiplexing rules for the "a=dcmap:" attribute need to be defined as well, for instance in an extension of this SDP offer/answer based data channel negotiation specification.

5.2. SDP DCSA Attribute

In the SDP media description, each data channel declaration MAY also be followed by other media level SDP attributes, which are either specifically defined for or applied to the subprotocol 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 document. Subprotocol specific attributes MAY also be defined for exclusive use with data channel transport, but MUST use the same syntax described here for other subprotocol related attributes.

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

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

The detailed offer/answer procedures for the dcsa attribute are dependent on the associated sub-protocol. If no offer/answer procedures exist for the sub-protocol when used outside of the dcsa attribute, no specification is needed for use with dcsa. The IANA registration procedures for the WebSocket Subprotocol Name Registry (Section 9.1) do not strictly require a specification of the offer/answer procedures for the sub-protocol when used with dcsa. If the sub-protocol has defined offer/answer procedures when used outside of dcsa, such a specification is encouraged to ensure interoperability. If the sub-protocol has defined offer/answer procedures when used outside of dcsa, but no specification exists for the offer/answer procedures for the sub-protocol when used with dcsa, implementations SHOULD assume the use of the default values for all otherwise-negotiable and applicable sub-protocol parameters.

5.2.1. DCSA Syntax 

Formal Syntax:

Name: dcsa

Value: dcsa-value

Usage Level: media

Charset Dependent: no

Syntax:

dcsa-value      = stream-id SP attribute
stream-id 		= 1*5DIGIT
attribute       = <from-RFC4566>

Example:

a=dcmap:2 subprotocol="msrp";ordered=true;label="msrp"

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

Note that the reference to [I-D.ietf-mmusic-rfc4566bis] 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 "a=dcsa:" parameter. IANA SDP parameters contains the lists of IANA (Internet Assigned Numbers Authority) registered session and media level or media level only SDP attributes.

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 the MSRP subprotocol 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 subprotocol specific documents.

It is assumed that in general the usages of subprotocol related media level attributes are independent from the subprotocol's transport protocol. Such transport protocol independent subprotocol related attributes are used in the same way as defined in the original subprotocol specification, also if the subprotocol is transported over a data channel and if the attribute is correspondingly embedded in a "a=dcsa" attribute.

There may be cases, where the usage of a subprotocol related media level attribute depends on the subprotocol's transport protocol. In such cases the subprotocol related usage of the attribute is expected to be described for the data channel transport. A data channel specific usage of a subprotocol attribute is expected to be specified in the same document that registers the subprotocol's identifier for data channel usage as described in Section 9.1.

SDP attributes that are only defined for use at the dcsa usage level, SHALL use the dcsa usage level when registering the attribute. If existing media attributes are used in a datachannel subprotocol specific way, then a new dcsa usage level MUST be defined for the existing media attribute. Where the SDP attribute is applicable to a particular subprotocol/s this SHALL also be registered by indicating the applicable subprotocol identifiers (see /[I-D.ietf-mmusic-rfc4566bis] section-8.5) along with the dcsa usage level.

5.2.2. DCSA Multiplexing Category

The multiplexing category of the "a=dcsa:" attribute is SPECIAL.

As the usage of multiple SCTP associations on top of a single DTLS association is outside the scope of [I-D.ietf-mmusic-sctp-sdp], no "a=dcsa:" attribute 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 on top of a single DTLS association, or how to add multiple SCTP associations to one BUNDLE group, 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.

6. SDP Offer/Answer Procedures

This section defines how data channels can be negotiated using the SDP offer/answer mechanism. A given media description can describe multiple data channels (each represented by a separate SDP dcmap attribute) that can be created, modified and closed using different offer/answer exchanges. The procedures in this section apply for a given data channel.

The generic offer/answer procedures for negotiating the SCTP association used to realize data channels are defined in [I-D.ietf-mmusic-sctp-sdp]. This section only defines the data channel specific procedures.

“Initial offer” refers to the offer in which a data channel is opened. It can be the initial offer, or a subsequent offer, of the associated SDP session.

The detailed offer/answer procedures for the dcsa attribute are dependent on the associated sub-protocol see Section 5.2.

6.1. Managing Stream Identifiers

In order to avoid SCTP Stream identifier collisions, in alignment with [I-D.ietf-rtcweb-data-protocol], the endpoint acting as DTLS client (for the SCTP association used to realize data channels) MUST use even identifier values, and the endpoint acting as DTLS server MUST use odd identifier values.

SCTP stream identifiers associated with data channels that have been negotiated using DCEP MUST NOT be included in SDP offers and answers.

6.2. Negotiating Data Channel Parameters

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

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>

By definition max-retr and max-time are mutually exclusive, so both MUST NOT 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 MUST treat the associated SDP offer/answer as failed.

6.3. Generating the Initial Offer for A Data Channel

When an offerer sends an initial offer, in order to negotiate an SCTP stream for a data channel, the offerer:

6.4. Generating SDP Answer

When an answerer receives an offer that includes an “m=" section for an SCTP association, that describes an SCTP stream for a data channel, if the answerer accepts the data channel it:

6.5. Offerer Processing of the SDP Answer

An offerer receiving an SDP answer performs the following:

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:

6.6. Modifying the Session

When an offer sends a subsequent offer, that includes information for a previously negotiated data channel, unless the offerer intends to close the data channel (Section 6.6.1), the offerer SHALL include the previously negotiated SDP attributes and attribute values associated with the data channel. The answerer may reject the offer. The means for rejecting an offer are dependent on the higher layer protocol. The offer/answer exchange is atomic; if the answer is rejected, the session reverts to the state prior to the offer [RFC3264].

6.6.1. Closing a Data Channel

In order to close a data channel, the endpoint that wants to close SHALL send the SCTP SSN reset message [RFC6525], following the procedures in section 6.7 of [I-D.ietf-rtcweb-data-channel]. In addition, if the closed data channel was negotiated using the offer/answer mechanism Section 6.3, the endpoint that closed the data channel SHALL send a subsequent offer in which it either:

6.7. Various SDP Offer/Answer Considerations

7. Examples 

SDP offer:

  m=application 10001 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-1 \
      4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
  a=tls-id:abc3de65cddef001be82
  a=dcmap:0 subprotocol="bfcp";label="bfcp"
    
SDP answer:

  m=application 10002 UDP/DTLS/SCTP webrtc-datachannel
  c=IN IP6 2001:db8::1
  a=max-message-size:100000
  a=sctp-port:5002
  a=setup:passive
  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=tls-id:dcb3ae65cddef0532d42

Figure 1: Example 1

In the example in Figure 1 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.

SDP offer:

  m=application 10001 UDP/DTLS/SCTP webrtc-datachannel
  c=IN IP4 192.0.2.1
  a=max-message-size:100000
  a=sctp-port:5000
  a=setup:actpass
  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=tls-id:abc3de65cddef001be82
  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 192.0.2.2
  a=max-message-size:100000
  a=sctp-port:5002
  a=setup:passive
  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=tls-id:dcb3ae65cddef0532d42
  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 example in Figure 2 the SDP offer contains data channels for BFCP (Binary Floor Control Protocol) and MSRP subprotocols. The SDP answer rejected BFCP and accepted MSRP. So, the offerer closes the data channel for BFCP and both offerer and answerer may start using the MSRP data channel (after the SCTP association is set up). The data channel with stream id 0 is free and can be used for future DCEP or SDP offer/answer negotiation.

Continuing the example in Figure 2.

Subsequent SDP offer:

  m=application 10001 UDP/DTLS/SCTP webrtc-datachannel
  c=IN IP4 192.0.2.1
  a=max-message-size:100000
  a=sctp-port:5000
  a=setup:actpass
  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=tls-id:abc3de65cddef001be82
  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 192.0.2.2
  a=max-message-size:100000
  a=sctp-port:5002
  a=setup:passive
  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=tls-id:dcb3ae65cddef0532d42
  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 example in Figure 3 is a continuation of the example in Figure 2. The SDP offerer 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.

8. Security Considerations

This document specifies new SDP attributes used in the negotiation of the DATA channel parameters.

These parameter are negotiated as part of opening a SCTP channel over DTLS as specified in [I-D.ietf-mmusic-sctp-sdp]. Each subprotocol may come with it’s own security considerations that need to be documented as part of the subprotocol definition. Otherwise this document does not add any security considerations to the ones specified in [I-D.ietf-mmusic-sctp-sdp]

Error cases like the use of unknown parameter values or violation the odd/even rule MUST be handled by closing the corresponding Data Channel.

9. IANA Considerations

9.1. Subprotocol Identifiers

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

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

"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.

A subprotocol may simultaneously be defined for data channel transport and for Websocket transport. In such a case the "Subprotocol Definition" and "Reference" cells in the subprotocol's row of the IANA "WebSocket Subprotocol Name Registry" table should contain two entries. One entry in each of these cells should refer to the Websocket related subprotocol specification, and the other entry should refer to the data channel related subprotocol specification.

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

9.2. New SDP Attributes

9.2.1. 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:

Contact name: IESG
Contact email: iesg@ietf.org
Attribute name: dcmap
Attribute syntax: As per Section 5.1.1
Attribute semantics: As per Section 5.1.1
Usage level: media
Charset dependent: No
Purpose: Define data channel specific parameters
Appropriate values: As per Section 5.1.1
O/A procedures: As per Section 6
Mux category: SPECIAL. See Section 5.1.9
Reference: RFCXXXX

9.2.2. 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:

Contact name: IESG
Contact email: iesg@ietf.org
Attribute name: dcsa
Attribute syntax: As per Section 5.2.1
Attribute semantics: As per Section 5.2.1
Usage level: media
Charset dependent: No
Purpose: Define data channel subprotocol specific attributes
Appropriate values: As per Section 5.2.1
O/A procedures: As per Section 6
Mux category: SPECIAL. See Section 5.2.2
Reference: RFCXXXX

10. Contributors

Juergen Stoetzer-Bradler co-authored this document.

11. 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 Flemming Andreasen, Christian Groves, Gunnar Hellstrom, Paul Kyzivat, Jonathan Lennox, Uwe Rauschenbach and Roman Shpount for their invaluable comments.

Special thanks to Christer Holmberg for helping finish the document and cleaning the SDP offer/answer section.

12. CHANGE LOG

12.1. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-15'

12.2. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-14'

12.3. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-12'

12.4. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-11'

12.5. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-10'

12.6. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-09'

12.7. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-08'

12.8. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-07'

12.9. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-06'

12.10. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-05'

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

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

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

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

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

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

12.17. Changes against '-01'

12.18. Changes against '-00'

13. References

13.1. Normative References

[I-D.ietf-mmusic-rfc4566bis] Begen, A., Kyzivat, P., Perkins, C. and M. Handley, "SDP: Session Description Protocol", Internet-Draft draft-ietf-mmusic-rfc4566bis-32, December 2018.
[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.", Internet-Draft draft-ietf-mmusic-sctp-sdp-26, April 2017.
[I-D.ietf-mmusic-sdp-mux-attributes] Nandakumar, S., "A Framework for SDP Attributes when Multiplexing", Internet-Draft draft-ietf-mmusic-sdp-mux-attributes-17, February 2018.
[I-D.ietf-rtcweb-data-channel] Jesup, R., Loreto, S. and M. Tuexen, "WebRTC Data Channels", Internet-Draft draft-ietf-rtcweb-data-channel-13, January 2015.
[I-D.ietf-rtcweb-data-protocol] Jesup, R., Loreto, S. and M. Tuexen, "WebRTC Data Channel Establishment Protocol", Internet-Draft draft-ietf-rtcweb-data-protocol-09, January 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, DOI 10.17487/RFC3264, June 2002.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2003.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol", RFC 4960, DOI 10.17487/RFC4960, September 2007.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008.
[RFC6525] Stewart, R., Tuexen, M. and P. Lei, "Stream Control Transmission Protocol (SCTP) Stream Reconfiguration", RFC 6525, DOI 10.17487/RFC6525, February 2012.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.

13.2. Informative References

[I-D.ietf-clue-datachannel] Holmberg, C., "CLUE Protocol data channel", Internet-Draft draft-ietf-clue-datachannel-18, April 2019.
[I-D.ietf-mmusic-msrp-usage-data-channel] Drage, K., Makaraju, M., Stoetzer-Bradler, J., Ejzak, R., Marcon, J. and J. Recio, "MSRP over Data Channels", Internet-Draft draft-ietf-mmusic-msrp-usage-data-channel-10, April 2019.
[RFC4582] Camarillo, G., Ott, J. and K. Drage, "The Binary Floor Control Protocol (BFCP)", RFC 4582, DOI 10.17487/RFC4582, November 2006.
[RFC4975] Campbell, B., Mahy, R. and C. Jennings, "The Message Session Relay Protocol (MSRP)", RFC 4975, DOI 10.17487/RFC4975, September 2007.
[RFC6455] Fette, I. and A. Melnikov, "The WebSocket Protocol", RFC 6455, DOI 10.17487/RFC6455, December 2011.
[WebRtcAPI] Bergkvist, A., Burnett, D., Jennings, C., Narayanan, A., Aboba, B., Brandstetter, T. and J. Bruaroey, "WebRTC 1.0: Real-time Communication Between Browsers", World Wide Web Consortium CR CR-webrtc-20180927, September 2018.

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 (subprotocol, label, maximal number of retransmissions, maximal retransmission time, 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 192.0.2.1
a=max-message-size:100000
a=sctp-port:5000
a=tls-id:abc3de65cddef001be82
a=setup:actpass
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: The 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 the stream identifier to the data channel stack to assign to the data channel or else let the data channel stack pick one identifier from the unused ones.

To avoid glare situations [RFC3264], 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.

Note: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].
Note:For data channels negotiated via different protocol 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 subprotocol 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 subprotocol 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 (Interactive Connectivity Establishment), DTLS and SCTP procedures 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 subprotocol 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 subprotocol 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 subprotocol associated with the data channel, the closing might in addition be signaled to the peer via SDP offer/answer negotiation.

Authors' Addresses

Keith Drage Unaffiliated EMail: drageke@ntlworld.com
Maridi R. Makaraju (Raju) Nokia 2000 Lucent Lane Naperville, Illinois US EMail: Raju.Makaraju@nokia.com
Richard Ejzak Unaffiliated EMail: richard.ejzak@gmail.com
Jerome Marcon Unaffiliated EMail: jeromee.marcon@free.fr
Roni Even (editor) Huawei EMail: roni.even@huawei.com