wdiff rfc8864.original rfc8864.txt

MMUSIC

Internet Engineering Task Force (IETF) K. Drage
Internet-Draft Unaffiliated
Intended status: Standards Track
Request for Comments: 8864 M. Makaraju
Expires: November 12, 2019 Nokia
Category: Standards Track R. Ejzak
ISSN: 2070-1721 J. Marcon
Unaffiliated
R. Even, Ed.
Huawei
May 11, 2019

SDP-based Data Channel
January 2021

Negotiation
draft-ietf-mmusic-data-channel-sdpneg-28 Data Channels Using the Session Description Protocol (SDP)

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 an Internet Standards Track document.

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

Internet-Drafts are draft documents valid the IETF community. It has
received public review and has been approved for a maximum publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of six months this document, any errata,
and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained 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.
https://www.rfc-editor.org/info/rfc8864.

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Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Applicability Statement . . . . . . . . . . . . . . . . . . . 6
5. SDP Data Channel Attributes . . . . . . . . . . . . . . . . . 6
5.1. SDP DCMAP Attribute . . . . . . . . . . . . . . . . . . . 6
5.1.1. DCMAP Attribute Syntax . . . . . . . . . . . . . . . 6
5.1.2. Dcmap-stream-id 'dcmap-stream-id' Parameter . . . . . . . . . . . . . . 8
5.1.3. Label 'label' Parameter . . . . . . . . . . . . . . . . . . . 8
5.1.4. Subprotocol 'subprotocol' Parameter . . . . . . . . . . . . . . . . 8
5.1.5. Max-retr 'max-retr' Parameter . . . . . . . . . . . . . . . . . 9
5.1.6. Max-time 'max-time' Parameter . . . . . . . . . . . . . . . . . 9
5.1.7. Ordered 'ordered' Parameter . . . . . . . . . . . . . . . . . . 9
5.1.8. Priority 'priority' Parameter . . . . . . . . . . . . . . . . . 10
5.1.9. DCMAP Multiplexing Category . . . . . . . . . . . . . 10
5.2. SDP DCSA Attribute . . . . . . . . . . . . . . . . . . . 10
5.2.1. DCSA Attribute Syntax . . . . . . . . . . . . . . . . . . . . . 11
5.2.2. DCSA Multiplexing Category . . . . . . . . . . . . . 12
6. SDP Offer/Answer Procedures . . . . . . . . . . . . . . . . . 13
6.1. Managing Stream Identifiers . . . . . . . . . . . . . . . 13
6.2. Negotiating Data Channel Parameters . . . . . . . . . . . 13
6.3. Generating the Initial Offer for A a Data Channel . . . . . 14
6.4. Generating the SDP Answer . . . . . . . . . . . . . . . . . . 14
6.5. Offerer Processing of the SDP Answer . . . . . . . . . . 15
6.6. Modifying the Session . . . . . . . . . . . . . . . . . . 15
6.6.1. Closing a Data Channel . . . . . . . . . . . . . . . 16
6.7. Various SDP Offer/Answer Considerations . . . . . . . . . 16
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 17
8. Security Considerations . . . . . . . . . . . . . . . . . . . 19
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
9.1. Subprotocol Identifiers . . . . . . . . . . . . . . . . . 20
9.2. New SDP Attributes . . . . . . . . . . . . . . . . . . . 20
9.2.1. dcmap . . . . . . . . . . . . . . . . . . . . . . . . 20
9.2.2. dcsa . . . . . . . . . . . . . . . . . . . . . . . . 21
9.3. Registering Attributes for Use with Data Channels
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 21
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
12. CHANGE LOG . . . . . . . . . . . . . . . . . . . . . . . . . 22
12.1. Changes against 'draft-ietf-mmusic-data-channel-
sdpneg-15' . . . . . . . . . . . . . . . . . . . . . . . 22
12.2. Changes against 'draft-ietf-mmusic-data-channel-
sdpneg-14' . . . . . . . . . . . . . . . . . . . . . . . 22
12.3. Changes against 'draft-ietf-mmusic-data-channel-
sdpneg-12' . . . . . . . . . . . . . . . . . . . . . . . 22
12.4. Changes against 'draft-ietf-mmusic-data-channel-
sdpneg-11' . . . . . . . . . . . . . . . . . . . . . . . 22
12.5. Changes against 'draft-ietf-mmusic-data-channel-
sdpneg-10' . . . . . . . . . . . . . . . . . . . . . . . 22
12.6. Changes against 'draft-ietf-mmusic-data-channel-
sdpneg-09' . . . . . . . . . . . . . . . . . . . . . . . 22
12.7. Changes against 'draft-ietf-mmusic-data-channel-
sdpneg-08' . . . . . . . . . . . . . . . . . . . . . . . 23
12.8. Changes against 'draft-ietf-mmusic-data-channel-
sdpneg-07' . . . . . . . . . . . . . . . . . . . . . . . 24
12.9. Changes against 'draft-ietf-mmusic-data-channel-
sdpneg-06' . . . . . . . . . . . . . . . . . . . . . . . 24
12.10. Changes against 'draft-ietf-mmusic-data-channel-
sdpneg-05' . . . . . . . . . . . . . . . . . . . . . . . 24
12.11. Changes against 'draft-ietf-mmusic-data-channel-
sdpneg-04' . . . . . . . . . . . . . . . . . . . . . . . 25
12.12. Changes against 'draft-ietf-mmusic-data-channel-
sdpneg-03' . . . . . . . . . . . . . . . . . . . . . . . 26
12.13. Changes against 'draft-ietf-mmusic-data-channel-
sdpneg-02' . . . . . . . . . . . . . . . . . . . . . . . 27
12.14. Changes against 'draft-ietf-mmusic-data-channel-
sdpneg-01' . . . . . . . . . . . . . . . . . . . . . . . 28
12.15. Changes against 'draft-ietf-mmusic-data-channel-
sdpneg-00' . . . . . . . . . . . . . . . . . . . . . . . 30
12.16. Changes against 'draft-ejzak-mmusic-data-channel-
sdpneg-02' . . . . . . . . . . . . . . . . . . . . . . . 33
12.17. Changes against '-01' . . . . . . . . . . . . . . . . . 34
12.18. Changes against '-00' . . . . . . . . . . . . . . . . . 34
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 34
13.1.
10.1. Normative References . . . . . . . . . . . . . . . . . . 34
13.2.
10.2. Informative References . . . . . . . . . . . . . . . . . 36
Appendix A. Generic Data Channel Negotiation Aspects When when Not
Using DCEP . . . . . . . . . . . . . . . . . . . . . 36
A.1. Stream Identifier Numbering . . . . . . . . . . . . . . . 37
A.2. Generic Data Channel Negotiation Not Using DCEP . . . . . 38
A.2.1. Overview . . . . . . . . . . . . . . . . . . . . . . 38
A.2.2. Opening a Data Channel . . . . . . . . . . . . . . . 38
A.2.3. Closing a Data Channel . . . . . . . . . . . . . . . 39
Acknowledgements
Contributors
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39

1. Introduction

The concept of establishing a bi-directional bidirectional data channel running on
top of the Stream Control Transmission Protocol (SCTP) is discussed
in
[I-D.ietf-rtcweb-data-channel] [RFC8831], allowing applications to use data channels. An in-band
Data Channel Establishment Protocol (DCEP) is described in [I-D.ietf-rtcweb-data-protocol], however [RFC8832];
however, other in-band or out-of-
band 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 (1) the WebRTC API
(Application Programming Interface), Interface) [WebRtcAPI] or (2) other
protocols like CLUE [I-D.ietf-clue-datachannel]. (e.g., Controlling Multiple Streams for Telepresence (CLUE)
[RFC8850]). The protocols can be signaled by the data channel "subprotocol"
'subprotocol' parameter, conceptually similar to the a WebSocket [RFC5234] "subprotocol".
subprotocol as described in [RFC6455]. 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 out-of-band
using a non-DCEP protocol (or both).

This document defines SDP Session Description Protocol (SDP) offer/answer [RFC3264]
procedures [RFC3264] 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 SCTP-based media transport as specified in [I-D.ietf-mmusic-sctp-sdp]
[RFC8841] for the SDP "m" "m=" line proto values UDP/DTLS/SCTP and
TCP/DTLS/SCTP.

This document uses MSRP (Message (the Message Session Relay Protocol)
[RFC4975] and BFCP (Binary (the Binary Floor Control Protocol) [RFC4582] [RFC8855] in many of the
several 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 MSRP, they are documented in
[I-D.ietf-mmusic-msrp-usage-data-channel] . [RFC8873]. 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", RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP14
BCP 14 [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]. [RFC8831].

Data channel stack: An entity which, that, upon application request, runs
the data channel protocol to keep track of states, states as well as the
sending and receiving of data. If the application is a browser browser-
based JavaScript application application, then this stack resides in the
browser. If the application is a native application application, then this
stack resides in the application and is accessible via some sort
of API or 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.). delivery).

Data channel subprotocol: The application protocol which that is
transported over a single data channel. Data channel subprotocol
messages are sent as data channel payload over an established data
channel. An 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
subprotocols, and data channel subprotocol properties. In this
case
case, the data channel subprotocols may be identified by the
values of the "subprotocol" 'subprotocol' parameters of the SDP "a=dcmap" "a=dcmap:"
attribute as described in Section 5.1.4. Within this document document,
the term "data channel subprotocol" is often abbreviated as just
"subprotocol".

DCEP: Data Channel Establishment Protocol Protocol, as defined in
[I-D.ietf-rtcweb-data-protocol]. [RFC8832].

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 The SCTP stream sequence number 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 described in this document only applies to the Session
Description Protocol (SDP) [I-D.ietf-mmusic-rfc4566bis] SDP
[RFC8866] when used together with the SDP offer/answer mechanism
[RFC3264]. Declarative usage of SDP is out of scope for this document,
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 data-
channel-specific and subprotocol-specific parameters without the
usage of DCEP [I-D.ietf-rtcweb-data-protocol]. [RFC8832]. The first attribute (Section 5.1) provides
for negotiation of channel-specific parameters. The second attribute
(Section 5.2) provides for negotiation of subprotocol-specific
parameters.

| Note: Appendix A provides information regarding how data
| channels work in
general and especially general. In particular, it summarizes some
| key aspects, which aspects that 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:" media-level attribute, "a=dcmap:", that
defines the data channel parameters for each data channel to be
negotiated.

The

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

5.1.1. DCMAP Attribute Syntax

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

Formal Syntax:

Name: dcmap

Value: dcmap-value

Usage Level: Form) syntax [RFC5234].

Syntax: Dependent | No |
+-------------------+-------------+

Table 1: "a=dcmap:" Attribute
Definition

Formal 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] [RFC8832]
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] [RFC8832]
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] [RFC8832]

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> <from RFC 5234>
integer = <from-RFC4566> <from RFC 8866>

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

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

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

5.1.2. Dcmap-stream-id '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 '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]. [RFC8832]. 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:

o WebRTC API, the 'label' parameter
should

* Serialize the WebRTC label as a UTF-8 string [RFC3629].

* Treat the UTF-8 serialization as a series of bytes

o bytes.

* For each byte in the serialization:

* serialization,

- If the byte can be expressed as a `quoted-char`, 'quoted-char', do so

* so.

- Otherwise, express the byte as an `escaped-char`. 'escaped-char'.

| Note: The empty string MAY can 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] [RFC8832] allows the
| DATA_CHANNEL_OPEN message's 'Label' value to be an empty
| string.

5.1.4. Subprotocol '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]. [RFC8832]. Section 9.1 specifies how
values for new subprotocol parameter values parameters 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 can also be explicitly used as a
| 'subprotocol' value, such that 'subprotocol=""' is equivalent
| to the 'subprotocol' parameter not being present at all. [I-D.ietf-rtcweb-data-protocol]
| [RFC8832] allows the DATA_CHANNEL_OPEN message's 'Subprotocol' 'Protocol'
| value to be an empty string.

5.1.5. Max-retr '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 'max-retr' parameter is optional.
If the max-retr 'max-retr' parameter and the max-time '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]. [RFC8832].

5.1.6. Max-time '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 lifetime, given in milliseconds milliseconds, in the 'max-time'
parameter. The life-time lifetime starts when providing the user message to
the protocol stack. The max-time 'max-time' parameter is optional. If the max-
retr
'max-retr' parameter and the max-time '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].
[RFC8832].

5.1.7. Ordered '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 'ordered' parameter is optional and
takes two values: values -- "true" for ordered delivery and "false" for
unordered delivery -- with "true" as the default value. Any other
value is
ignored ignored, and the default "ordered=true" is assumed. In the
absence of this parameter 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].
[RFC8832].

5.1.8. Priority '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]. [RFC8832]. The
'priority' parameter is optional. In the absence of this parameter parameter,
"priority=256" is assumed.

5.1.9. DCMAP Multiplexing Category

The multiplexing category [I-D.ietf-mmusic-sdp-mux-attributes] [RFC8859] 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], [RFC8841], 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] [RFC8841] define
how to negotiate multiplexing of multiple SCTP associations on top of
a single DTLS association, 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, well -- for instance 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 apply to the subprotocol in use. corresponding
data channel and its subprotocol. Each of these attributes is
represented by one new "a=dcsa:" attribute line, and it
includes the contents of a media-level line that references
another SDP attribute already defined for use with this (sub)protocol in another IETF document.
Subprotocol specific data channel's
subprotocol. Instructions for registering attributes MAY also be defined for exclusive use with a
data channel transport, but MUST use the same syntax described
here for other subprotocol related attributes. are given in Section 9.3.

Each SDP attribute, attribute that is related to the subprotocol, subprotocol and that would
normally be used to negotiate the subprotocol using the SDP offer/answer offer/
answer mechanism is replaced with an attribute of the form
"a=dcsa:stream-id original-
attribute", original-attribute", where dcsa "dcsa" stands for "data
channel subprotocol attribute", stream-id "stream-id" is the SCTP stream
identifier assigned to this subprotocol instance, and original-attribute "original-
attribute" represents the contents of the subprotocol related 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. subprotocol. If no offer/answer
procedures exist for the sub-protocol subprotocol when used outside of the dcsa
attribute, no specification is needed for use with dcsa. The IANA
(Internet Assigned Numbers Authority) registration procedures for the WebSocket
"WebSocket Subprotocol Name Registry Registry" (Section 9.1) do not strictly
require a specification of the offer/
answer offer/answer procedures for the sub-protocol
subprotocol when used with dcsa. If the
sub-protocol subprotocol has defined
offer/answer procedures when used outside of dcsa, such a
specification is encouraged to ensure interoperability. If the sub-protocol
subprotocol has defined offer/answer procedures when used outside of dcsa,
dcsa but no specification exists for the offer/answer procedures for
the sub-protocol subprotocol when used with dcsa, implementations SHOULD assume
the use of the default values for all otherwise-
negotiable otherwise-negotiable and
applicable sub-protocol subprotocol parameters.

5.2.1. DCSA Attribute Syntax

Formal Syntax:

Name:

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

+================================+
| "a=dcsa:" Attribute |
+===================+============+
| Name | dcsa

Syntax:

dcsa-value = Dependent | No |
+-------------------+------------+

Table 2: "a=dcsa:" Attribute
Definition

Formal syntax:

dcsa-value = stream-id SP attribute
stream-id = 1*5DIGIT
attribute = <from-RFC4566> <from RFC 8866>

Example:

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

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

Note that the

The reference to [I-D.ietf-mmusic-rfc4566bis] [RFC8866] defines where the attribute definition can
be found; it does not provide any
limitation limitations on support of
attributes defined in other documents in accordance with this
attribute definition. Note however that However, not all SDP attributes are suitable
as a an "a=dcsa:" parameter. The registry of IANA SDP parameters
contains the lists of IANA (Internet Assigned Numbers
Authority) registered session IANA-registered session-level and media level media-level
or media level only media-level-only SDP attributes.

Thus

Thus, in the example above, the original attribute line "a=accept-
types:text/plain"
"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 plaintext
files.

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

It is assumed that in general the usages of subprotocol related media
level subprotocol-related
media-level attributes are independent from the subprotocol's
transport protocol. Such transport protocol independent subprotocol 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" an "a=dcsa:" attribute.

There may be cases, cases where the usage of a subprotocol related media subprotocol-related media-
level attribute depends on the subprotocol's transport protocol. In
such cases cases, the subprotocol related subprotocol-related usage of the attribute is
expected to be described for the data channel transport. A data channel
specific 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], [RFC8841], 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] [RFC8841] define
how to negotiate multiplexing of multiple SCTP associations on top of
a single DTLS association, 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, well -- for instance 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 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]. [RFC8841].
This section only defines the data
channel specific data-channel-specific procedures.

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

The detailed offer/answer procedures for the dcsa attribute are
dependent on the associated sub-protocol subprotocol; 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], [RFC8832], the endpoint acting as a DTLS client (for the SCTP
association used to realize data channels) MUST use even identifier
values, and the endpoint acting as a 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] [RFC8832] are mapped to the dcmap
SDP attribute parameters in the following manner 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 definition, 'max-retr' and max-time '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 parameters, then the receiver of such an
SDP offer MUST reject the SDP offer. If an SDP answer contains both
of these parameters parameters, then the offerer MUST treat the associated SDP
offer/answer as failed.

6.3. Generating the Initial Offer for A a Data Channel

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

o offerer

* MAY include one or more SDP dcsa attributes (Section 5.2)
associated with the data channel. The value of the stream-id 'stream-id'
part of each attribute SHALL match the dcmap-stream-id 'dcmap-stream-id' value of
the dcmap attribute.

6.4. Generating the SDP Answer

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

o channel, it

* SHALL include an SDP dcmap attribute (Section 5 (Sections 5.1 and Section 6.2)
associated with the data channel in the "m=" section representing
the SCTP association used to realize the data channel. The value
of the dcmap-stream-id, max-retr 'dcmap-stream-id', 'max-retr', and max-time 'max-time' values of the
dcmap attribute SHALL be identical to the value used for the data
channel in the offer; offer, and

* MAY include one or more SDP dcsa attributes (Section 5.2)
associated with the data channel.

6.5. Offerer Processing of the SDP Answer

An offerer receiving an SDP answer performs the following:

* It SHALL close any created data channels as described in
Section 6.6.1 for which the expected "a=dcmap:" attributes are not
present in the SDP answer. If the SDP answer has no "a=dcmap"
attribute "a=dcmap:"
attributes, either the peer does not support "a=dcmap:" attributes
or it rejected all the data channels. In either case case, the offerer
closes all the SDP offered data channels offered by SDP that were open at the
time of the offer. The DTLS association and SCTP association will
still be setup. set up. At this point point, the offerer may use DCEP
negotiation [I-D.ietf-rtcweb-data-protocol] [RFC8832] to open data channels channels.

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
instantiated and occurs as follows:

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

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

* At the agent sending an SDP offer to create a new 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.

6.6. Modifying the Session

When an offer offerer sends a subsequent offer, 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 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
the data channel SHALL send the an SCTP SSN reset Reset message [RFC6525],
following the
procedures procedure in section Section 6.7 of [I-D.ietf-rtcweb-data-channel]. [RFC8831]. In addition, if
the closed data channel was negotiated using the offer/
answer offer/answer
mechanism Section 6.3, (Section 6.3), the endpoint that closed the data channel
SHALL send a subsequent offer in which it either:

o removes does one of the following:

* Removes the SDP dcmap attribute and SDP dcsa attributes associated
with the closed data channel. Once the endpoint receives a
successful answer, the SCTP stream identifier value can later be
used for a new data channel (negotiated using DCTP either SCTP or using the
offer/answer mechanism); mechanism), or

o after

* After a reset has been performed re-uses performed, reuses the SCTP stream used for
the closed data channel for a new data channel, using following the
procedures
procedure in Section 6.3. The offerer SHALL use a different SDP
dcmap attribute value for the data channel using the same SCTP
stream.

6.7. Various SDP Offer/Answer Considerations

An SDP offer or answer has no "a=dcmap:" attributes but has "a=dcsa:" attributes.
attributes:

* This is considered an error case. In this case case, the receiver of
such an SDP offer or answer MUST discard this the "a=dcsa:" attributes.

An SDP offer or answer has an "a=dcsa" attribute, "a=dcsa:" attribute whose subprotocol
attribute is unknown. unknown:

* The receiver of such an SDP offer or answer SHOULD ignore this
entire "a=dcsa" "a=dcsa:" attribute line.

An SDP offer or answer has an "a=dcsa" attribute, "a=dcsa:" attribute whose subprotocol
attribute is known, known but whose subprotocol attribute semantic is not
known for the data channel transport case. case:

* The receiver of such an SDP offer or answer SHOULD ignore this
entire "a=dcsa" "a=dcsa:" attribute line.

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 shows an example in Figure 1 of an SDP offer and answer where the SDP
answerer rejected rejects the data channel with stream id 0 either for
explicit reasons or because it does not understand the "a=dcmap:"
attribute. As a result result, the offerer will close the data channel
created with the SDP offer/answer negotiation option. The SCTP
association will still be setup set up over DTLS. At this
point 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 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"
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 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
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: 1: Example 2

In the example in 1

Figure 2 shows an example of an SDP offer and answer where the SDP
offer contains data channels for BFCP (Binary Floor Control Protocol) and MSRP subprotocols. The SDP
answer rejected rejects BFCP and accepted accepts MSRP. So, the offerer closes the
data channel for BFCP BFCP, and both the offerer and the 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:

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
a=dcmap:2 subprotocol="msrp";label="msrp"
a=dcsa:4
a=dcsa:2 accept-types:message/cpim text/plain
a=dcsa:4
a=dcsa:2 path:msrp://bob.example.com:10002/si438dsaodes;dc

Figure 3: 2: Example 3 2

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, 2
but opens a new MSRP data channel with stream id 4. The answerer
accepts the entire offer. As a result result, the offerer closes the earlier
previously negotiated MSRP related MSRP-related data channel channel, and both the offerer
and the answerer may start using new the MSRP related new MSRP-related data channel.

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
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

8. Security Considerations

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

These parameter parameters are negotiated as part of opening a an SCTP channel
over DTLS as specified in [I-D.ietf-mmusic-sctp-sdp]. [RFC8841]. Each subprotocol may come with it's
its own security considerations that need to be documented as part of
the subprotocol definition. Otherwise Otherwise, this document does not add
any security considerations to the ones those specified in [I-D.ietf-mmusic-sctp-sdp] [RFC8841].

Error cases like such as the use of unknown parameter values or violation violations
of the odd/even rule (Section 6.1) MUST be handled by closing the
corresponding Data
Channel. 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 has been added following below the title of the table.

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

The following reference should be

This document (RFC 8864) has been added to under the heading
reference: "RFC XXXX". "Reference" list for
the registry.

This document assigns no new values to this table.

A subprotocol may simultaneously be defined for data channel
transport and for Websocket WebSocket transport. In such a case 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 WebSocket-related subprotocol specification, and the other
entry should refer to the data channel related 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:" attribute, "a=dcmap:", as
follows:

+-----------------------+-------------------------------------------+

Table 3: New "a=dcmap:" Attribute

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:" attribute, "a=dcsa:", as
follows:

+-----------------------+-------------------------------------------+

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
+---------------------+---------------------------------------+

Table 4: New "a=dcsa:" Attribute

9.3. Registering Attributes for their invaluable comments.

Special thanks to Christer Holmberg Use with Data Channels

When a subprotocol is defined for helping finish the document
and cleaning use over data channels with the SDP
offer/answer section.

12. CHANGE LOG

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

o Editorial changes separate sections for offer/answer procedures.

o Update security section.

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

o Change "dtls-id" to "tls-id" and assign 20 octet long values

o Remove of RFC4566bis draft from list of normative references.

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

o Modification of Keith's address information.

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

o dcmap-stream-id syntax change to limit size to 5 digits.

o Add missing 'x' prefix to quoted-visible syntax.

o Align mechanism, any SDP offerer and answerer handling when both max-retr and
max-time are present.

o Use of TEST-NET-1 ip addresses in examples.

o Add missing a=dtls-id in one example.

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

o Removal of attributes that may be negotiated
using the "a=connection" "a=dcsa:" attribute lines from all SDP
examples.

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

o In MUST be added to the introduction:

* Replacement IANA "attribute-
name registry (formerly "att-field")", as specified in [RFC8866],
Section 8.2.4. This document specifies that new Usage Levels of the sentence "The RTCWeb working group has
defined
form "dcsa (foo)" (where "foo" is a placeholder for the concept of bi-directional data channels running on
top subprotocol
name) should be registered by documents that specify negotiation of SCTP/DTLS (SCTP over the Datagram Transport Layer
Security protocol)" with "The RTCWeb working group
particular subprotocols.

IANA has defined
the concept of bi-directional data channels running on top of
the Stream Control Transmission Protocol (SCTP)".

* Addition of following sentences to the second paragraph: "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. In the future, data channels
could be defined over other SCTP based protocols, such as SCTP
over IP. However, corresponding potential other "m" line proto
values are not considered in this document."

o Replacement of "DTLS connection" with "DTLS association"
throughout the document.

o In sections Section 5.1.9 and Section 5.2.2 removal of the
sentences "This document also does not specify multiplexing rules
for this attribute for SCTP or SCTP/DTLS proto values".

o In the text related to "Subsequent SDP answer" in section
Section 6.7 replacement of "The DTLS/SCTP association remains open
..." with "The SCTP association remains open ...".

o In the text after the second SDP answer in section Section 7
replacement of "... (after SCTP/DTLS association is setup)" with
"... (after the SCTP association is set up)".

o Addition of draft-ietf-mmusic-dtls-sdp to the list of informative
references.

o Addition of "a=dtls-id" attribute lines to the SDP offer/answer
examples in Section 7.

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

o Addition of definition of "data channel subprotocol" to Section 3
as proposed on the MMUSIC list, https://www.ietf.org/mail-
archive/web/mmusic/current/msg15827.html.

o Addition of RFC4566bis draft to list of normative references.

o Updates of tables in Section 9.2.1 and Section 9.2.2 as per
section 8.2.4 of RFC4566bis draft.

o Addition of new "new-usage-level">.

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

o Addition of two new paragraphs to Section 5.2.1 regarding
subprotocol attribute relationship with transport protocol.

o Addition of a note to Section 9.1 regarding subprotocols
simultaneously defined for data channel and Websocket usage.

o Addition of two further SDP offer/answer considerations to
Section 6.7 regarding unknown subprotocol attributes and known
subprotocol attributes with unknown data channel transport related
semantic.

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

o Changes addressing Christian Groves's WGLC review comments raised
in http://www.ietf.org/mail-archive/web/mmusic/current/
msg15357.html and http://www.ietf.org/mail-
archive/web/mmusic/current/msg15359.html.

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

o In IANA registration Section 9.2.1 and Section 9.2.2 replacement
of contact name and e-mail address with "MMUSIC Chairs" and
"mmusic-chairs@ietf.org".

o In Section 5.2.1 replacement of "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." with "... which specifies
that this instance of the MSRP subprotocol being transported ...".

o The last paragraph of Section 5.2.1 started with "Note: This
document does not provide a complete specification ...". Removal
of "Note:" and move of this paragraph to the introduction in
Section 1 as last paragraph.

o Section 5.2's headline was "Subprotocol Specific Attributes".
Change of this headline to "Other Media Level Attributes" and
adaptations of the first paragraph of this section and the first
paragraph of Section 5.2.1 in order to clarify that not only those
attributes may be encapsulated in a "dcsa" attribute, which are
specifically defined for the subprotocol, but that also other
attributes may be encapsulated if they are related to the specific
subprotocol instance.

o Move of the last but one paragraph of Section 5.2.1 starting with
"The same syntax applies to ..." right in front of the formal
syntax definition of the "dcsa" attribute.

o Modifications of the text in Section 5.1.9 and Section 5.2.2 in
order not to explicitly restrict usage of the "a=dcmap:" and
"a=dcsa:" attributes to "m" lines with proto values "UDP/DTLS/
SCTP" or "TCP/DTLS/SCTP".

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

o In Section 5.1.4 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 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 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.

o Addition of dcmap attribute specific IANA registration
Section 9.2.1.

o Addition of dcsa attribute specific IANA registration
Section 9.2.2.

o Addition of new Section 5.1.9 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 5.2.2 describing the mux category of the
dcsa SDP attribute.

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

o In Section 5.1 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 5.1.5 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 6.1 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 7 addition of the previously missing
colons to the "a=sctp-port" attribute lines.

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

o Move of reference draft-ietf-rtcweb-jsep from the list of
normative references to the list of informative references.
Remover in -07 since not referenced

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 subprotocols" with
"This document defines SDP offer/answer negotiation procedures to
establish data channels for transport of well-defined
subprotocols, 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.

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 6.1 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 6.2 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 aBoth MUST NOT 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.

12.14. 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 9.1 "Subprotocol identifiers" as
subsection of the "IANA Considerations" related Section 9. 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 6.2:

* 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
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 6.2 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 6.7 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 6.7 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 to be closed
now.".

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

o In Section 5.1.1 in the definition of the ABNF rule "dcmap-opt"
there was a comment saying that "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 only one 'maxretr-opt' parameter or one
'maxtime-opt' parameter is present. Both MUST NOT be present."

o In Section 5.1.7 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 dispatch DATA chunks
in the data channel to the upper layer while preserving the
order.".

o In Section 6.3's first paragraph replacement of the one occurrence
of "must" with "..., it MUST wait until ...".

o In Section 6.6.1:

* 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 5.2 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."

12.15. 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.

o In Section 5.1 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.
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 subprotocol 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
subprotocol."

o In Section 5.1 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: "dcmap is a media level attribute having following
ABNF syntax:"

o Insertion of new Section 5.1.2 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 modification of the 'ordering-value' values from
"0" or "1" to "true" or "false". Corresponding text modifications
in Section 5.1.7.

o In Section 5.1.1 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 regarding the non-printable
"escaped-char" character within the "label" value.

o In Section 5.2's second paragraph replacement of "sctp stream
identifier" with "SCTP stream identifier".

o In first paragraph of Section 6.1 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
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 6.3 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 6.3 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 6.6.1 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
were already closed, and it MUST also exclude the corresponding
attribute lines in the answer."

o In Section 6.6.1 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 contained already procedural descriptions related to
data channel reliability negotiation. Creation of new Section 6.2
and moval of reliability negotiation related text to this new
section.

12.16. 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
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.

12.17. 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.

12.18. 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 updated the usage of
unreliable and reliable WebRTC data channels.

o Updates of examples "attribute-name (formerly "att-field")" registry
to point 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,
and as draft-ietf-mmusic-sctp-sdp-07 supports only one SCTP
association on top of the DTLS connection.

13. document.

10. References

13.1.

10.1. Normative References

[I-D.ietf-mmusic-rfc4566bis]
Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP:
Session Description Protocol", draft-ietf-mmusic-
rfc4566bis-32 (work in progress), 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.",
draft-ietf-mmusic-sctp-sdp-26 (work in progress), April
2017.

[I-D.ietf-mmusic-sdp-mux-attributes]
Nandakumar, S., "A Framework for SDP Attributes when
Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-17
(work in progress), February 2018.

[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-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.

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

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

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

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

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

[RFC6525] Stewart, R., Tuexen, M., and P. Lei, "Stream Control
Transmission Protocol (SCTP) Stream Reconfiguration",
RFC 6525, DOI 10.17487/RFC6525, February 2012,
<https://www.rfc-editor.org/info/rfc6525>.

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

13.2. Informative References

[I-D.ietf-clue-datachannel]
Holmberg, C., "CLUE Protocol data channel", draft-ietf-
clue-datachannel-18 (work in progress), April 2019.

[I-D.ietf-mmusic-msrp-usage-data-channel]
Drage, K., Makaraju, M., Stoetzer-Bradler, J., Ejzak,

[RFC8831] Jesup, R.,
Marcon, J., Loreto, S., and J. Recio, "MSRP over M. Tüxen, "WebRTC Data
Channels",
draft-ietf-mmusic-msrp-usage-data-channel-10 (work in
progress), April 2019.

[RFC4582] Camarillo, G., Ott, J., RFC 8831, DOI 10.17487/RFC8831, January 2021,
<https://www.rfc-editor.org/info/rfc8831>.

[RFC8832] Jesup, R., Loreto, S., and K. Drage, "The Binary Floor M. Tüxen, "WebRTC Data Channel
Establishment Protocol", RFC 8832, DOI 10.17487/RFC8832,
January 2021, <https://www.rfc-editor.org/info/rfc8832>.

[RFC8841] Holmberg, C., Shpount, R., Loreto, S., and G. Camarillo,
"Session Description Protocol (SDP) Offer/Answer
Procedures for Stream Control Transmission Protocol (BFCP)", (SCTP)
over Datagram Transport Layer Security (DTLS) Transport",
RFC 4582, 8841, DOI 10.17487/RFC4582,
November 2006, <https://www.rfc-editor.org/info/rfc4582>. 10.17487/RFC8841, January 2021,
<https://www.rfc-editor.org/info/rfc8841>.

[RFC8859] Nandakumar, S., "A Framework for Session Description
Protocol (SDP) Attributes When Multiplexing", RFC 8859,
DOI 10.17487/RFC8859, January 2021,
<https://www.rfc-editor.org/info/rfc8859>.

[RFC8866] Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP:
Session Description Protocol", RFC 8866,
DOI 10.17487/RFC8866, January 2021,
<https://www.rfc-editor.org/info/rfc8866>.

10.2. Informative References

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

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

[RFC8850] Holmberg, C., "Controlling Multiple Streams for
Telepresence (CLUE) Protocol Data Channel", RFC 8850,
DOI 10.17487/RFC8850, January 2021,
<https://www.rfc-editor.org/info/rfc8850>.

[RFC8855] Camarillo, G., Drage, K., Kristensen, T., Ott, J., and C.
Eckel, "The Binary Floor Control Protocol (BFCP)",
RFC 8855, DOI 10.17487/RFC8855, January 2021,
<https://www.rfc-editor.org/info/rfc8855>.

[RFC8873] Recio, JM., Ed. and C. Holmberg, "Message Session Relay
Protocol (MSRP) over Data Channels", RFC 8873,
DOI 10.17487/RFC8873, January 2021,
<https://www.rfc-editor.org/info/rfc8873>.

[T38] International Telecommunication Union, "Procedures for
real-time Group 3 facsimile communication over IP
networks", ITU-T Recommendation T.38, November 2015,
<https://www.itu.int/rec/T-REC-T.38-201511-I/en>.

[WebRtcAPI]
Bergkvist, A., Burnett, D.,
Jennings, C., Narayanan, A.,
Aboba, B., Brandstetter, T., Boström, H., and J. J-I. Bruaroey, "WebRTC 1.0:
Real-time Communication Between Browsers", World Wide Web
Consortium CR CR-webrtc-20180927, September 2018,
<https://www.w3.org/TR/2018/CR-webrtc-20180927/>. W3C Proposed
Recommendation, <https://www.w3.org/TR/webrtc/>.

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

This appendix summarizes how data channels work in general and
discusses some key aspects, which aspects that should be considered for the out-
of-band 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 whether it wants to make
use of the negotiation using the DCEP [I-D.ietf-rtcweb-data-protocol], [RFC8832] 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].
[RFC8841]. In brief, it contains one "m" "m=" line for the SCTP
association on top of which the 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" the "m=" line format "webrtc-
datachannel",
| "webrtc-datachannel" and will not use other formats in the "m" "m="
| line for other protocols such as t38. [I-D.ietf-mmusic-sctp-sdp] T.38 [T38]. [RFC8841]
| 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
| channel-specific information.

A.1. Stream Identifier Numbering

Independently from the requested type of negotiation, the application
creating a data channel can either (1) pass the stream identifier to
the data channel stack to assign to the data channel or else (2) let the
data channel stack pick one identifier from the unused ones.

Moreover, 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

| 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 [RFC8832].

| Note: For data channels negotiated via different a protocol from other than
| 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
subprotocol-specific parameters. DCEP-less Non-DCEP 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 already-established data channel) or out-of-
band (using the SDP offer/answer mechanism or some other protocol
associated with the signaling channel).

A.2.2. Opening a Data Channel

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

When the application requests that the creation of a new data channel to
be set up via DCEP-less non-DCEP negotiation, the data channel stack creates
the data channel locally without sending any DATA_CHANNEL_OPEN
message
messages in-band. However, even if the ICE (Interactive Connectivity
Establishment), DTLS 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. peer is complete. 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 complete, and the
application should be ready to handle it.

If the peer rejects the data channel part of the offer offer, then it
doesn't have to do anything anything, as the data channel was not created
using the stack. The offerer offerer, on the other hand 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 APIs to create and close data channels; instead instead, the
data channels may be used on the fly as needed needed, just by communicating
via non-DCEP means or by even by having some local configuration/
assumptions on both of 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 nonconflicting 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 Reset for this channel.

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

Acknowledgements

The authors wish to acknowledge the borrowing of ideas from other
draft documents by Salvatore Loreto, Gonzalo Camarillo, Peter
Dunkley, and Gavin Llewellyn. The authors also wish to thank
Flemming Andreasen, Christian Groves, Gunnar Hellström, 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 up Section 6.

Contributors

Juergen Stoetzer-Bradler made significant contributions to this
document and should be considered a coauthor.

Authors' Addresses

Keith Drage
Unaffiliated

Email: drageke@ntlworld.com

Maridi R. Makaraju (Raju)
Nokia
2000 Lucent Lane
Naperville, Illinois
US
Unaffiliated

Email: Raju.Makaraju@nokia.com mmraju@gmail.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 ron.even.tlv@gmail.com