SDP-based Data Channel Negotiation
draft-ietf-mmusic-data-channel-sdpneg-07
MMUSIC K. Drage, Ed.
Internet-Draft M. Makaraju
Intended status: Standards Track J. Stoetzer-Bradler
Expires: June 2, 2016 Alcatel-Lucent
R. Ejzak
J. Marcon
Unaffiliated
November 30, 2015
SDP-based Data Channel Negotiation
draft-ietf-mmusic-data-channel-sdpneg-07
Abstract
The Real-Time Communication in WEB-browsers (RTCWeb) working group is
charged to provide protocols to support direct interactive rich
communications using audio, video, and data between two peers' web-
browsers. For the support of data communication, the RTCWeb working
group has in particular defined the concept of bi-directional data
channels over SCTP (Stream Control Transmission Protocol), where each
data channel might be used to transport other protocols, called
subprotocols. 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
such an out-of-band non-DCEP negotiation. Even though data channels
are designed for RTCWeb use initially, they may be used by other
protocols like, but not limited to, the CLUE protocol (which is
defined by the IETF "ControLling mUltiple streams for tElepresence"
working group). This document is intended to be used wherever data
channels are used.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on June 2, 2016.
Copyright Notice
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Applicability Statement . . . . . . . . . . . . . . . . . . . 5
5. SDP Offer/Answer Negotiation . . . . . . . . . . . . . . . . 5
5.1. SDP Syntax . . . . . . . . . . . . . . . . . . . . . . . 5
5.1.1. SDP Attribute for Data Channel Parameter Negotiation 5
5.1.1.1. dcmap Attribute . . . . . . . . . . . . . . . . . 6
5.1.1.2. dcmap Multiplexing Category . . . . . . . . . . . 7
5.1.1.3. dcmap-stream-id Parameter . . . . . . . . . . . . 8
5.1.1.4. label Parameter . . . . . . . . . . . . . . . . . 8
5.1.1.5. subprotocol Parameter . . . . . . . . . . . . . . 8
5.1.1.6. max-retr Parameter . . . . . . . . . . . . . . . 8
5.1.1.7. max-time Parameter . . . . . . . . . . . . . . . 9
5.1.1.8. ordered Parameter . . . . . . . . . . . . . . . . 9
5.1.1.9. priority Parameter . . . . . . . . . . . . . . . 9
5.1.2. Other Media Level Attributes . . . . . . . . . . . . 9
5.1.2.1. dcsa Attribute . . . . . . . . . . . . . . . . . 10
5.1.2.2. dcsa Multiplexing Category . . . . . . . . . . . 11
5.2. Procedures . . . . . . . . . . . . . . . . . . . . . . . 11
5.2.1. Managing Stream Identifiers . . . . . . . . . . . . . 11
5.2.2. Negotiating Data Channel Parameters . . . . . . . . . 12
5.2.3. Opening a Data Channel . . . . . . . . . . . . . . . 13
5.2.4. Closing a Data Channel . . . . . . . . . . . . . . . 15
5.2.5. Various SDP Offer/Answer Scenarios and Considerations 16
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . . 19
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
8.1. Subprotocol Identifiers . . . . . . . . . . . . . . . . . 19
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8.2. New SDP Attributes . . . . . . . . . . . . . . . . . . . 20
8.2.1. dcmap . . . . . . . . . . . . . . . . . . . . . . . . 20
8.2.2. dcsa . . . . . . . . . . . . . . . . . . . . . . . . 20
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
10. CHANGE LOG . . . . . . . . . . . . . . . . . . . . . . . . . 21
10.1. Changes against 'draft-ietf-mmusic-data-channel-
sdpneg-06' . . . . . . . . . . . . . . . . . . . . . . . 21
10.2. Changes against 'draft-ietf-mmusic-data-channel-
sdpneg-05' . . . . . . . . . . . . . . . . . . . . . . . 21
10.3. Changes against 'draft-ietf-mmusic-data-channel-
sdpneg-04' . . . . . . . . . . . . . . . . . . . . . . . 22
10.4. Changes against 'draft-ietf-mmusic-data-channel-
sdpneg-03' . . . . . . . . . . . . . . . . . . . . . . . 23
10.5. Changes against 'draft-ietf-mmusic-data-channel-
sdpneg-02' . . . . . . . . . . . . . . . . . . . . . . . 24
10.6. Changes against 'draft-ietf-mmusic-data-channel-
sdpneg-01' . . . . . . . . . . . . . . . . . . . . . . . 25
10.7. Changes against 'draft-ietf-mmusic-data-channel-
sdpneg-00' . . . . . . . . . . . . . . . . . . . . . . . 27
10.8. Changes against 'draft-ejzak-mmusic-data-channel-
sdpneg-02' . . . . . . . . . . . . . . . . . . . . . . . 30
10.9. Changes against '-01' . . . . . . . . . . . . . . . . . 31
10.10. Changes against '-00' . . . . . . . . . . . . . . . . . 31
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
11.1. Normative References . . . . . . . . . . . . . . . . . . 32
11.2. Informative References . . . . . . . . . . . . . . . . . 32
Appendix A. Generic Data Channel Negotiation Aspects When Not
Using DCEP . . . . . . . . . . . . . . . . . . . . . 33
A.1. Stream Identifier Numbering . . . . . . . . . . . . . . . 34
A.2. Generic Data Channel Negotiation Not Using DCEP . . . . . 34
A.2.1. Overview . . . . . . . . . . . . . . . . . . . . . . 34
A.2.2. Opening a Data Channel . . . . . . . . . . . . . . . 35
A.2.3. Closing a Data Channel . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 36
1. Introduction
The RTCWeb working group has defined the concept of bi-directional
data channels running on top of SCTP/DTLS (SCTP over the Datagram
Transport Layer Security protocol). RTCWeb allows applications to
use data channels. RTCWeb defines an in-band DCEP, 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 through the WebRTC API (Application Programming
Interface), or other users of a data channel like CLUE. They can be
used to transport proprietary or well-defined protocols, which in the
latter case can be signaled by the data channel "subprotocol"
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parameter, conceptually similar to the WebSocket "subprotocol".
However, apart from the "subprotocol" value transmitted to the peer,
RTCWeb leaves it open how endpoint applications 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). In particular, the SDP offer generated by the RTCweb data
channel stack includes no channel-specific information.
This document defines SDP offer/answer negotiation procedures to
establish data channels for transport of well-defined subprotocols,
to enable out-of-band negotiation.
This document makes use of MSRP (Message Session Relay Protocol) 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 such as MSRP are documented
elsewhere. 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", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Terminology
This document uses the following terms:
Data channel: A WebRTC data channel as specified in
[I-D.ietf-rtcweb-data-channel].
Data channel stack: An entity which, upon application request,
runs the data channel protocol to keep track of states, sending
and receive data. If the application is a browser based
JavaScript application then this stack resides in the browser. If
the application is a native application then this stack resides in
the application and is accessible via some sort of APIs.
Data channel properties: Fixed properties assigned to a data
channel at the time of its creation. Some of these properties
determine the way the data channel stack transmits data on this
channel (e.g., stream identifier, reliability, order of
delivery...).
DCEP: Data Channel Establishment Protocol defined in
[I-D.ietf-rtcweb-data-protocol].
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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) [RFC4566], when used together with the SDP
offer/answer mechanism [RFC3264]. Declarative usage of SDP is out of
scope of this document, and is thus undefined.
5. SDP Offer/Answer Negotiation
This section defines an SDP extension by which two clients can
negotiate data channel-specific and 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.
Appendix A provides information how data channels work in general and
especially summarizes some key aspects, which should be considered
for the negotiation of data channels if DCEP is not used.
5.1. SDP Syntax
Two new SDP attributes are defined to support SDP offer/answer
negotiation of data channels. The first attribute provides for
negotiation of channel-specific parameters. The second attribute
provides for negotiation of subprotocol-specific parameters.
5.1.1. SDP Attribute for Data Channel Parameter Negotiation
Associated with the SDP "m" line that defines the SCTP association
for data channels (defined in Section 3), each SDP offer and answer
includes one "a=dcmap:" attribute that defines the data channel
parameters for each data channel to be negotiated. Each such
attribute line specifies the following parameters for a data channel:
SCTP stream identifier, subprotocol, label, maximal number of
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retransmissions, maximal retransmission time, order of delivery, and
priority.
The intention in exchanging these attributes is to create, on two
peers, without use of DCEP [I-D.ietf-rtcweb-data-protocol], matched
pairs of oppositely directed data channels having the same set of
attributes. It is assumed that the data channel properties
(reliable/partially reliable, ordered/unordered) are suitable per the
subprotocol transport requirements.
5.1.1.1. dcmap Attribute
"a=dcmap:" is a media level attribute having following ABNF
(Augmented Backus-Naur Form, [RFC5234]) 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
; Either only maxretr-opt or maxtime-opt
; is present.
dcmap-stream-id = 1*DIGIT
ordering-opt = "ordered=" ordering-value
ordering-value = "true" / "false"
subprotocol-opt = "subprotocol=" quoted-string
label-opt = "label=" quoted-string
maxretr-opt = "max-retr=" maxretr-value
maxretr-value = "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,
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; 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 = %21 / %23-24 / %26-7E ; 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
Note: The last example (a=dcmap:4) shows a 'label' parameter value
which contains one non-printable 'escaped-char' character (the
tabulator character).
Within an 'a=dcmap:' attribute line's 'dcmap-opt' value either only
one 'maxretr-opt' parameter or one 'maxtime-opt' parameter MAY be
present. Both MUST NOT be present.
5.1.1.2. dcmap Multiplexing Category
Multiplexing characteristics of SDP attributes are described in
[I-D.ietf-mmusic-sdp-mux-attributes]. Various SDP attribute
multiplexing categories are introduced there.
The multiplexing category of the "a=dcmap:" attribute is SPECIAL.
As the usage of multiple SCTP associations on top of a single DTLS
connection is outside the scope of [I-D.ietf-mmusic-sctp-sdp], no
"a=dcmap:" attribute multiplexing rules are specified for the
UDP/DTLS/SCTP and TCP/DTLS/SCTP proto values. This document also
does not specify multiplexing rules for this attribute for SCTP or
SCTP/DTLS proto values. If future extensions of
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[I-D.ietf-mmusic-sctp-sdp] define how to negotiate multiplexing of
multiple SCTP associations on top of a single DTLS connection, 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 based data channel
negotiation specification.
5.1.1.3. dcmap-stream-id Parameter
The 'dcmap-stream-id' parameter indicates the SCTP stream identifier
within the SCTP association used to form the data channel.
5.1.1.4. label Parameter
The 'label' parameter indicates the name of the channel. It
represents a label that can be used to distinguish, in the context of
the WebRTC API [WebRtcAPI], an RTCDataChannel object from other
RTCDataChannel objects. This parameter maps to the 'Label' parameter
defined in [I-D.ietf-rtcweb-data-protocol]. The 'label' parameter is
optional. If it is not present, then its value defaults to the empty
string.
Note: The empty string MAY also be explicitly used as 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.
5.1.1.5. subprotocol Parameter
The 'subprotocol' parameter indicates which protocol the client
expects to exchange via the channel. This parameter maps to the
'Protocol' parameter defined in [I-D.ietf-rtcweb-data-protocol].
Section 8.1 specifies how new subprotocol parameter values are
registered. 'Subprotocol' is an optional parameter. If the
'subprotocol' parameter is not present, then its value defaults to 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.
5.1.1.6. max-retr Parameter
This parameter indicates that the data channel is partially reliable.
The 'max-retr' parameter indicates the maximal number of times a user
message will be retransmitted. The max-retr parameter is optional.
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If the max-retr parameter is not present, then the maximal number of
retransmissions is determined as per the generic SCTP retransmission
rules as specified in [RFC4960]. This parameter maps to the 'Number
of RTX' parameter defined in [I-D.ietf-rtcweb-data-protocol].
5.1.1.7. max-time Parameter
This parameter indicates that the data channel is partially reliable.
A user message will no longer be transmitted or retransmitted after a
specified life-time given in milliseconds in the 'max-time'
parameter. The max-time parameter is optional. If the max-time
parameter is not present, then the generic SCTP retransmission timing
rules apply as specified in [RFC4960]. This parameter maps to the
'Lifetime in ms' parameter defined in
[I-D.ietf-rtcweb-data-protocol].
5.1.1.8. ordered Parameter
The 'ordered' parameter with value "true" indicates that the receiver
MUST dispatch DATA chunks in the data channel to the upper layer
while preserving the order. The ordered parameter is optional and
takes two values: "true" for ordered and "false" for unordered
delivery with "true" as the default value. Any other value is
ignored and default "ordered=true" is assumed. In the absence of
this parameter "ordered=true" is assumed. This parameter maps to the
ordered or unordered data channel types as defined in
[I-D.ietf-rtcweb-data-protocol].
5.1.1.9. 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.
5.1.2. Other Media Level Attributes
In the SDP, 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 (Internet Engineering Task
Force) document. Subprotocol specific attributes MAY also be defined
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for exclusive use with data channel transport, but SHOULD use the
same syntax described here for other subprotocol related attributes.
5.1.2.1. dcsa Attribute
Each subprotocol related SDP attribute that would normally be used to
negotiate the subprotocol using SDP is replaced with an attribute of
the form "a=dcsa:stream-id original-attribute", where dcsa stands for
"data channel 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.
Formal Syntax:
Name: dcsa
Value: dcsa-value
Usage Level: media
Charset Dependent: no
Syntax:
dcsa-value = stream-id SP attribute
attribute = <from-RFC4566>
Example:
a=dcsa:2 accept-types:text/plain
Note that the above reference to [RFC4566] 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
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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.
5.1.2.2. 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
connection 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. This document does
also not specify multiplexing rules for this attribute for SCTP or
SCTP/DTLS 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 connection, 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.
5.2. Procedures
5.2.1. Managing Stream Identifiers
If a 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 re-determined as described above.
This document allows simultaneous use of SDP offer/answer and DCEP
negotiation. However, an SCTP stream MUST NOT be referenced in a
"a=dcmap:" or "a=dcsa:" attribute of an SDP offer/answer exchange if
the associated SCTP stream has already been negotiated via DCEP.
Stream ids that are not currently used in SDP can be used for DCEP
negotiation. Stream id allocation per SDP offer/answer negotiation
may not align with DTLS role based allocation. This could cause
glare conditions when one side tries to do SDP offer/answer
negotiation on a stream id while the other end tries to open a data
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channel on the same stream id using DCEP negotiation. To avoid these
glare conditions this document recommends that the data channel stack
user always selects stream ids per the above described SDP offer/
answer rule even when DCEP negotiation is used. To avoid glare
conditions, it is possible to come up with a different stream id
allocation scheme, but such schemes are outside the scope of this
document.
5.2.2. Negotiating Data Channel Parameters
Conveying a reliable data channel is achieved by including neither
'max-retr' nor 'max-time' in corresponding SDP offer's or answer's
"a=dcmap:" attribute line. Conveying a partially reliable data
channel is achieved by including only one of 'max-retr' or 'max-
time'. By definition max-retr and max-time are mutually exclusive,
so at most one of them MAY be present in the "a=dcmap:" attribute
line. If a SDP offer contains both of these parameters then the
receiver of such an SDP offer MUST reject the SDP offer. If a SDP
answer contains both of these parameters then the offerer MAY treat
it as an error and MAY assume the associated SDP offer/answer failed
and MAY take appropriate recovery actions. These recovery options
are outside the scope of this document.
The SDP answer SHALL echo the same subprotocol, max-retr, max-time,
ordered parameters, if those were present in the offer, and MAY
include a label parameter. They MAY appear in any order, which could
be different from the SDP offer, in the SDP answer.
When sending a subsequent offer or an answer, and for as long as the
data channel is still open, the sender MUST replicate the same
information.
Data channel types defined in [I-D.ietf-rtcweb-data-protocol] are
mapped to SDP in the following manner, where "ordered=true" is the
default and may be omitted:
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DATA_CHANNEL_RELIABLE
ordered=true
DATA_CHANNEL_RELIABLE_UNORDERED
ordered=false
DATA_CHANNEL_PARTIAL_RELIABLE_REXMIT
ordered=true;max-retr=<number of retransmissions>
DATA_CHANNEL_PARTIAL_RELIABLE_REXMIT_UNORDERED
ordered=false;max-retr=<number of retransmissions>
DATA_CHANNEL_PARTIAL_RELIABLE_TIMED
ordered=true;max-time=<lifetime in milliseconds>
DATA_CHANNEL_PARTIAL_RELIABLE_TIMED_UNORDERED
ordered=false;max-time=<lifetime in milliseconds>
5.2.3. Opening a Data Channel
The procedure for opening a data channel using SDP offer/answer
negotiation starts with the agent preparing to send an SDP offer. If
a peer receives an SDP offer before starting to send a new SDP offer
with data channels that are to be SDP offer/answer negotiated, or
loses an SDP offer glare resolution procedure in this case, it MUST
wait until the ongoing SDP offer/answer completes before resuming the
SDP offer/answer negotiation procedure.
The agent that intends to send an SDP offer to create data channels
through SDP offer/answer negotiation performs the following:
o Creates data channels using stream identifiers from the owned set
(see Section 5.2.1).
o Generates a new SDP offer.
o Determines the list of stream identifiers assigned to data
channels opened through SDP offer/answer negotiation.
o Completes the SDP offer with the "a=dcmap:" and "a=dcsa:"
attributes needed, if any, for each SDP offer/answer negotiated
data channel, as described in Section 5.1 and in Section 5.2.2.
o Sends the SDP offer.
The peer receiving such an SDP offer performs the following:
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o Parses and applies the SDP offer. Note that the typical parser
normally ignores unknown SDP attributes, which includes data
channel related attributes.
o Analyzes the channel parameters and subprotocol attributes to
determine whether to accept each offered data channel.
o For accepted data channels, it creates peer instances for the data
channels with the agent using the channel parameters described in
the SDP offer. Note that the agent is asked to create data
channels with SCTP stream identifiers contained in the SDP offer
if the SDP offer is accepted.
o Generates an SDP answer.
o Completes the SDP answer with the "a=dcmap:" and optional
"a=dcsa:" attributes needed for each SDP offer/answer negotiated
data channel, as described in Section 5.1 and in Section 5.2.2.
o Sends the SDP answer.
The agent receiving such an SDP answer performs the following:
o Closes any created data channels as described in Section 5.2.4 for
which the expected "a=dcmap:" and "a=dcsa:" attributes are not
present in the SDP answer.
o Applies the SDP answer.
Each agent application MUST wait to send data until it has
confirmation that the data channel at the peer is instantiated. For
WebRTC, this is when both data channel stacks have channel parameters
instantiated. This occurs:
o At both peers when a data channel is created without an
established SCTP association, as soon as the SCTP association is
successfully established.
o At the agent receiving an SDP offer for which there is an
established SCTP association, as soon as it creates an SDP offer/
answer negotiated data channel based on information signaled in
the SDP offer.
o At the agent sending an SDP offer to create a new SDP offer/answer
negotiated data channel for which there is an established SCTP
association, when it receives the SDP answer confirming acceptance
of the data channel or when it begins to receive data on the data
channel from the peer, whichever occurs first.
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Note: DCEP is not used, that is neither the SDP offerer nor the SDP
answerer send an in-band DCEP DATA_CHANNEL_OPEN message.
5.2.4. Closing a Data Channel
When the application requests the closing of a data channel that was
negotiated via SDP offer/answer, the data channel stack always
performs an SCTP SSN reset for this channel.
It is specific to the subprotocol whether this closing MUST in
addition be signaled to the peer via a new SDP offer/answer exchange.
The intention to close a data channel can be signaled by sending a
new SDP offer which excludes the "a=dcmap:" and "a=dcsa:" attribute
lines for the data channel. The offerer SHOULD NOT change the port
value for the "m" line (e.g. to zero) when closing a data channel
(unless all data channels are being closed and the SCTP association
is no longer needed), since this would close the SCTP association and
impact all of the data channels. If the answerer accepts the SDP
offer then the answerer MUST close those data channels whose
"a=dcmap:" and "a=dcsa:" attribute lines were excluded from the
received SDP offer, unless those data channels were already closed,
and the answerer MUST also exclude the corresponding attribute lines
in the answer. In addition to that, the SDP answerer MAY exclude
other data channels which were closed but not yet communicated to the
peer. So, the offerer MUST inspect the answer to see if it has to
close other data channels which are now not included in the answer.
If a new SDP offer/answer is used to close data channels then the
data channel(s) SHOULD only be closed by the answerer/offerer after a
successful SDP answer is sent/received.
This delayed closure is RECOMMENDED in order to handle cases where
a successful SDP answer is not received, in which case the state
of the session SHOULD be kept per the last successful SDP offer/
answer.
If a client receives a data channel close indication (due to inband
SCTP SSN reset or some other reason) without associated SDP offer
then the client SHOULD generate an SDP offer which excludes this
closed data channel.
The application MUST also close any data channel that was negotiated
via SDP offer/answer, for which the stream identifiers are not listed
in an incoming SDP offer.
A closed data channel using local close (SCTP SSN reset), without an
additional SDP offer/answer to close it, may be reused for a new data
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channel. This can only be done via new SDP offer/answer, describing
the new subprotocol and its attributes, only after the corresponding
data channel close acknowledgement is received from the peer (i.e.
SCTP SSN reset of both incoming and outgoing streams is completed).
This restriction is to avoid the race conditions between arrival of
"SDP offer which reuses stream" with "SCTP SSN reset which closes
outgoing stream" at the peer.
5.2.5. Various SDP Offer/Answer Scenarios and Considerations
SDP offer has no "a=dcmap:" attributes.
* Initial SDP offer: No data channel is negotiated yet. The DTLS
connection and SCTP association is negotiated and, if agreed,
established as per [I-D.ietf-mmusic-sctp-sdp].
* Subsequent SDP offer: All the SDP offer/answer negotiated data
channels are expected be closed now. The DTLS/SCTP association
remains open for SDP offer/answer or DCEP negotiation of data
channels.
SDP answer has no "a=dcmap:" attributes.
* Initial SDP answer: Either the peer does not support "a=dcmap:"
attributes or it rejected all the data channels. In either
case the offerer closes all the SDP offer/answer negotiated
data channels that were open at the time of initial offer. The
DTLS connection and SCTP association will still be setup.
* Subsequent SDP answer: All the SDP offer/answer negotiated data
channels are expected be closed now. The DTLS/SCTP association
remains open for future SDP offer/answer or DCEP negotiation of
data channels.
SDP offer or answer has no "a=dcmap:" attributes but has "a=dcsa:"
attributes.
* This is considered an error case. In this case the receiver of
such an SDP offer or answer SHOULD ignore the "a=dcsa:"
attributes and SHOULD process the SDP offer or answer as per
above case 'SDP offer has no "a=dcmap:" attributes' or case
'SDP answer has no "a=dcmap:" attributes'.
SDP offer has no "a=dcsa:" attributes for a data channel.
* This is allowed and indicates there are no subprotocol
parameters to convey.
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SDP answer has no "a=dcsa:" attributes for a data channel.
* This is allowed and indicates there are no subprotocol
parameters to convey in the SDP answer. The number of
"a=dcsa:" attributes in the SDP answer does not have to match
the number of "a=dcsa:" attributes in the SDP offer.
6. Examples
SDP offer:
m=application 10001 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 10.10.10.1
a=max-message-size:100000
a=sctp-port:5000
a=setup:actpass
a=connection:new
a=fingerprint:SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
a=dcmap:0 subprotocol="BFCP";label="BFCP"
SDP answer:
m=application 10002 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 10.10.10.2
a=max-message-size:100000
a=sctp-port:5002
a=setup:passive
a=connection:new
a=fingerprint:SHA-1 \
5B:AD:67:B1:3E:82:AC:3B:90:02:B1:DF:12:5D:CA:6B:3F:E5:54:FA
Figure 1: Example 1
In the above example the SDP answerer rejected the data channel with
stream id 0 either for explicit reasons or because it does not
understand the "a=dcmap:" attribute. As a result the offerer will
close the data channel created with the SDP offer/answer negotiation
option. The SCTP association will still be setup over DTLS. At this
point the offerer or the answerer may use DCEP negotiation to open
data channels.
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SDP offer:
m=application 10001 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 10.10.10.1
a=max-message-size:100000
a=sctp-port:5000
a=setup:actpass
a=connection:new
a=fingerprint:SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
a=dcmap:0 subprotocol="BFCP";label="BFCP"
a=dcmap:2 subprotocol="MSRP";label="MSRP"
a=dcsa:2 accept-types:message/cpim text/plain
a=dcsa:2 path:msrp://alice.example.com:10001/2s93i93idj;dc
SDP answer:
m=application 10002 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 10.10.10.2
a=max-message-size:100000
a=sctp-port:5002
a=setup:passive
a=connection:new
a=fingerprint:SHA-1 \
5B:AD:67:B1:3E:82:AC:3B:90:02:B1:DF:12:5D:CA:6B:3F:E5:54:FA
a=dcmap:2 subprotocol="MSRP";label="MSRP"
a=dcsa:2 accept-types:message/cpim text/plain
a=dcsa:2 path:msrp://bob.example.com:10002/si438dsaodes;dc
Figure 2: Example 2
In the above example the SDP offer contains data channels for BFCP
(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 SCTP/DTLS association is setup). The
data channel with stream id 0 is free and can be used for future DCEP
or SDP offer/answer negotiation.
Continuing the example in Figure 2.
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Subsequent SDP offer:
m=application 10001 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 10.10.10.1
a=max-message-size:100000
a=sctp-port:5000
a=setup:actpass
a=connection:existing
a=fingerprint:SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
a=dcmap:4 subprotocol="MSRP";label="MSRP"
a=dcsa:4 accept-types:message/cpim text/plain
a=dcsa:4 path:msrp://alice.example.com:10001/2s93i93idj;dc
Subsequent SDP answer:
m=application 10002 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 10.10.10.2
a=max-message-size:100000
a=sctp-port:5002
a=setup:passive
a=connection:existing
a=fingerprint:SHA-1 \
5B:AD:67:B1:3E:82:AC:3B:90:02:B1:DF:12:5D:CA:6B:3F:E5:54:FA
a=dcmap:4 subprotocol="MSRP";label="MSRP"
a=dcsa:4 accept-types:message/cpim text/plain
a=dcsa:4 path:msrp://bob.example.com:10002/si438dsaodes;dc
Figure 3: Example 3
The above example is a continuation of the example in Figure 2. The
SDP offer now removes the MSRP data channel with stream id 2, but
opens a new MSRP data channel with stream id 4. The answerer accepts
the entire offer. As a result the offerer closes the earlier
negotiated MSRP related data channel and both offerer and answerer
may start using new the MSRP related data channel.
7. Security Considerations
No security considerations are envisaged beyond those already
documented in [RFC4566].
8. IANA Considerations
8.1. Subprotocol Identifiers
Registration of new subprotocol identifiers is performed using the
existing IANA "WebSocket Subprotocol Name Registry" table.
The following text should be added following the title of the table.
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"This table also includes subprotocol identifiers specified for usage
within a WebRTC data channel."
The following reference should be added to under the heading
reference: "RFC XXXX".
This document assigns no new values to this table.
NOTE to RFC Editor: Please replace "XXXX" with the number of this
RFC.
8.2. New SDP Attributes
8.2.1. dcmap
NOTE to RFC Editor: Please replace "XXXX" with the number of this
RFC.
This document defines a new SDP media-level attribute "a=dcmap:" as
follows:
+---------------------+-----------------------------------------+
| Attribute name: | dcmap |
| Type of attribute: | media |
| Mux category: | SPECIAL |
| Charset Dependent: | No |
| Purpose: | Define data channel specific parameters |
| Appropriate values: | As defined in Section 5.1.1 |
| Contact name: | MMUSIC Chairs |
| Contact e-mail: | mmusic-chairs@ietf.org |
| Reference: | RFCXXXX |
+---------------------+-----------------------------------------+
8.2.2. dcsa
NOTE to RFC Editor: Please replace "XXXX" with the number of this
RFC.
This document defines a new SDP media-level attribute "a=dcsa:" as
follows:
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+---------------------+---------------------------------------------+
| Attribute name: | dcsa |
| Type of attribute: | media |
| Mux category: | SPECIAL |
| Charset Dependent: | No |
| Purpose: | Define data channel subprotocol specific |
| | attributes |
| Appropriate values: | As defined in Section 5.1.2.1 |
| Contact name: | MMUSIC Chairs |
| Contact e-mail: | mmusic-chairs@ietf.org |
| Reference: | RFCXXXX |
+---------------------+---------------------------------------------+
9. Acknowledgments
The authors wish to acknowledge the borrowing of ideas from other
internet drafts by Salvatore Loreto, Gonzalo Camarillo, Peter Dunkley
and Gavin Llewellyn, and to thank Roni Even, Christian Groves, Gunnar
Hellstrom, Christer Holmberg, Paul Kyzivat, Jonathan Lennox, and Uwe
Rauschenbach for their invaluable comments.
10. CHANGE LOG
10.1. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-06'
o Changes addressing Christian Grove'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.
10.2. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-05'
o In IANA registration Section 8.2.1 and Section 8.2.2 replacement
of contact name and e-mail address with "MMUSIC Chairs" and
"mmusic-chairs@ietf.org".
o In Section 5.1.2.1 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 5.1.2.1 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.
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o Section 5.1.2'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 5.1.2.1 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 5.1.2.1 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 5.1.1.2 and Section 5.1.2.2
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".
10.3. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-04'
o In Section 5.1.1.5 the first (and only) paragraph was "The
'subprotocol' parameter indicates which protocol the client
expects to exchange via the channel. 'Subprotocol' is an optional
parameter. If the 'subprotocol' parameter is not present, then
its value defaults to the empty string." Replacement of this
paragraph with following two paragraphs:
* The 'subprotocol' parameter indicates which protocol the client
expects to exchange via the channel. This parameter maps to
the 'Protocol' parameter defined in
[I-D.ietf-rtcweb-data-protocol]. Section 8.1 specifies how new
subprotocol parameter values are registered. 'Subprotocol' is
an optional parameter. If the 'subprotocol' parameter is not
present, then its value defaults to the empty string.
* Note: The empty string MAY also be explicitly used as
'subprotocol' value, such that 'subprotocol=""' is equivalent
to the 'subprotocol' parameter not being present at all.
[I-D.ietf-rtcweb-data-protocol] allows the DATA_CHANNEL_OPEN
message's 'Subprotocol' value to be an empty string.
o Addition of [I-D.ietf-mmusic-sdp-mux-attributes] to list the of
normative references.
o Addition of dcmap attribute specific IANA registration
Section 8.2.1.
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o Addition of dcsa attribute specific IANA registration
Section 8.2.2.
o Addition of new Section 5.1.1.2 describing the mux category of the
dcmap SDP attribute. This section and the new "a=dcsa:" attribute
related mux category section are similar to the "Mux Category"
sections of the "a=sctp-port:" and "a=max-message-size:"
attributes in [I-D.ietf-mmusic-sctp-sdp].
o Addition of new Section 5.1.2.2 describing the mux category of the
dcsa SDP attribute.
10.4. 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."
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o In Section 5.1.1 replacement of "The intention of exchanging these
attributes is to create data channels on both the peers with the
same set of attributes without actually using the DCEP
[I-D.ietf-rtcweb-data-protocol]" with "The intention in exchanging
these attributes is to create, on two peers, without use of DCEP
[I-D.ietf-rtcweb-data-protocol], matched pairs of oppositely
directed data channels having the same set of attributes".
o In Section 5.1.1.6 replacement of "The 'max-retr' parameter
indicates the maximal number a user message will be retransmitted"
with "The 'max-retr' parameter indicates the maximal number of
times a user message will be retransmitted".
o In Section 5.2.1 replacement of "However, an SDP offer/answer
exchange MUST NOT be initiated if the associated SCTP stream is
already negotiated via DCEP" with "However, an SCTP stream MUST
NOT be referenced in a dcmap or dcsa attribute of an SDP offer/
answer exchange if the associated SCTP stream has already been
negotiated via DCEP".
o In the examples in Section 6 addition of the previously missing
colons to the "a=sctp-port" attribute lines.
10.5. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-02'
o Move of reference [I-D.ietf-rtcweb-jsep] from the list of
normative references to the list of informative references.
o Addition of [IANA-SDP-Parameters] to the list of informative
references and addition of following two sentences to the first
paragraph after the ABNF definition: "Note however that not all
SDP attributes are suitable as "a=dcsa:" parameter.
[IANA-SDP-Parameters] contains the lists of IANA registered
session and media level or media level only SDP attributes."
o In the introduction replacement of last sentence "This document
defines SDP-based out-of-band negotiation procedures to establish
data channels for transport of well-defined 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.
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o Throughout the document replacement of "internal negotiation" with
"DCEP" and removal of terms "internal negotiation" and "in-band
negotiation" from the terminology list in Section 3.
o Addition of "SCTP Stream Sequence Number (SSN)" to the list of
terms.
o In Section 5.2.1 replacement of sentence "However, a single stream
is managed using one method at a time." with "However, an SDP
offer/answer exchange MUST NOT be initiated if the associated SCTP
stream is already negotiated via DCEP".
o In Section 5.2.2 replacement of sentence "By definition max-retr
and max-time are mutually exclusive, so only one of them can be
present in a=dcmap" with "By definition max-retr and max-time are
mutually exclusive, so at most one of them MAY be present in
a=dcmap".
o Move of reference [WebRtcAPI] from list of normative references to
list of informative references.
o Removal of almost all text parts, which discussed JavaScript or
other API specific aspects. Such API specific aspects were mainly
discussed in sub-sections of Section 5 and Section 5 of draft-
ietf-mmusic-data-channel-sdpneg-02.
10.6. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-01'
o New Section 4 regarding applicability to SDP offer/answer only.
o Addition of new Section 8.1 "Subprotocol identifiers" as
subsection of the "IANA Considerations" related Section 8. Also
removal of the temporary note "To be completed. As [I-D.ietf-
rtcweb-data-protocol] this document should refer to IANA's
WebSocket Subprotocol Name Registry defined in [RFC6455]."
o In Section 5.2.2:
* In the first paragraph replacement of the sentence "If an SDP
offer contains both of these parameters then such an SDP offer
will be rejected." with "If an SDP offer contains both of these
parameters then the receiver of such an SDP offer MUST reject
the SDP offer."
* In the second paragraph capitalization of "shall" and "may"
such that both sentences now read: "The SDP answer SHALL echo
the same subprotocol, max-retr, max-time, ordered parameters,
if those were present in the offer, and MAY include a label
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parameter. They MAY appear in any order, which could be
different from the SDP offer, in the SDP answer."
* In the third paragraph replacement of the sentence "The same
information MUST be replicated without changes in any
subsequent offer or answer, as long as the data channel is
still opened at the time of offer or answer generation." with
"When sending a subsequent offer or an answer, and for as long
as the data channel is still open, the sender MUST replicate
the same information.".
o In Section 5.2.2 the mapping of data channel types defined in
[I-D.ietf-rtcweb-data-protocol] to the SDP "a=dcmap" attribute
parameters were illustrated using example "a=dcmap" attribute
lines. Replacement of these example "a=dcmap" attribute lines
with just the "a=dcmap" attribute parameters being relevant for
the channel type.
o In Section 5.2.5 the description of bullet point "SDP offer has no
a=dcmap attributes - Initial SDP offer:" was "Initial SDP offer:
No data channel negotiated yet." Replacement of this description
with "Initial SDP offer: No data channel is negotiated yet. The
DTLS connection and SCTP association is negotiated and, if agreed,
established as per [I-D.ietf-mmusic-sctp-sdp]."
o In Section 5.2.5 in both bullet points related to "Subsequent SDP
offer" and "Subsequent SDP answer" replacement of "All the
externally negotiated data channels must be closed now." with "All
the externally negotiated data channels are expected be closed
now.".
o In Appendix A.2.2's sixth paragraph beginning with "[ASSUMPTION]"
replacement of the two occurrences of "must" with "MUST".
o In Section 5.1.1.1 in the definition of the ABNF rule "dcmap-opt"
there was a comment saying that "Either only maxretr-opt or
maxtime-opt is present. Both MUST not be present." Removal of
the second normative sentence and instead addition of following
new paragraph to the end of this section: "Within an 'a=dcmap'
attribute line's 'dcmap-opt' value either only one 'maxretr-opt'
parameter or one 'maxtime-opt' parameter is present. Both MUST
NOT be present."
o In Section 5.1.1.8 replacement of the first sentence "The
'ordered' parameter with value "true" indicates that DATA chunks
in the channel MUST be dispatched to the upper layer by the
receiver while preserving the order." with "The 'ordered'
parameter with value "true" indicates that the receiver MUST
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dispatch DATA chunks in the data channel to the upper layer while
preserving the order.".
o In Section 5.2.3's first paragraph replacement of the one
occurrence of "must" with "..., it MUST wait until ...".
o In Section 5.2.4:
* In the second paragraph replacement of "must" with "... whether
this closing MUST in addition ..."
* In the third paragraph replacement of the sentence "The port
value for the "m" line SHOULD not be changed (e.g., to zero)
when closing a data channel ..." with "The offerer SHOULD NOT
change the port value for the "m" line (e.g., to zero) when
closing a data channel ...".
* In the last but two paragraph replacement of the sentence "...
then an SDP offer which excludes this closed data channel
SHOULD be generated." with "... then the client SHOULD generate
an SDP offer which excludes this closed data channel.".
* In the last but one paragraph replacement of "must" with "The
application MUST also close...".
o In Section 5.1.2 addition of following note after the formal
definition of the 'a=dcsa' attribute: "Note that the above
reference to RFC 4566 defines were the attribute definition can be
found; it does not provide any limitation on support of attributes
defined in other documents in accordance with this attribute
definition."
10.7. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-00'
o In Section 3 "WebRTC data channel" was defined as "A bidirectional
channel consisting of paired SCTP outbound and inbound streams."
Replacement of this definition with "Data channel: A WebRTC data
channel as specified in [I-D.ietf-rtcweb-data-channel]", and
consistent usage of "data channel" in the remainder of the
document including the document's headline."
o In Section 5 removal of following note: 'OPEN ISSUE: The syntax in
[I-D.ietf-mmusic-sctp-sdp] may change as that document progresses.
In particular we expect "webrtc-datachannel" to become a more
general term.'
o Consistent usage of '"m" line' in whole document as per [RFC4566].
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o In Section 5.1.1 removal of the example dcmap attribute line
'a=dcmap:2 subprotocol="BFCP";label="channel 2' as there are
already four examples right after the ABNF rules in
Section 5.1.1.1. Corresponding removal of following related note:
"Note: This document does not provide a complete specification of
how to negotiate the use of a WebRTC data channel to transport
BFCP. Procedures specific to each 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 5.1.1 removal of following note: "Note: This attribute
is derived from attribute "webrtc-DataChannel", which was defined
in old version 03 of the following draft, but which was removed
along with any support for SDP external negotiation in subsequent
versions: [I-D.ietf-mmusic-sctp-sdp]."
o Insertion of following new sentence to the beginning of
Section 5.1.1.1: "dcmap is a media level attribute having
following ABNF syntax:"
o Insertion of new Section 5.1.1.3 containing the dcmap-stream-id
specifying sentence, which previously was placed right before the
formal ABNF rules. Removal of the sentence 'Stream is a mandatory
parameter and is noted directly after the "a=dcmap:" attribute's
colon' as this information is part of the ABNF specification.
o In Section 5.1.1.1 modification of the 'ordering-value' values
from "0" or "1" to "true" or "false". Corresponding text
modifications in Section 5.1.1.8.
o In Section 5.1.1.1 the ABNF definition of "quoted-string" referred
to rule name "escaped-char", which was not defined. Instead a
rule with name "escaped" was defined. Renamed that rule's name to
"escaped-char".
o Insertion of a dedicated note right after the "a=dcmap:4"
attribute example in Section 5.1.1.1 regarding the non-printable
"escaped-char" character within the "label" value.
o In Section 5.1.2's second paragraph replacement of "sctp stream
identifier" with "SCTP stream identifier".
o In first paragraph of Section 5.2.1 replacement of first two
sentences 'For the SDP-based external negotiation described in
this document, the initial offerer based "SCTP over DTLS" owns by
convention the even stream identifiers whereas the initial
answerer owns the odd stream identifiers. This ownership is
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invariant for the whole lifetime of the signaling session, e.g. it
does not change if the initial answerer sends a new offer to the
initial offerer.' with 'If an SDP offer/answer exchange (could be
the initial or a subsequent one) results in a UDP/DTLS/SCTP or
TCP/DTLS/SCTP based media description being accepted, and if this
SDP offer/answer exchange results in the establishment of a new
SCTP association, then the SDP offerer owns the even SCTP stream
ids of this new SCTP association and the answerer owns the odd
SCTP stream identifiers. If this "m" line is removed from the
signaling session (its port number set to zero), and if usage of
this or of a new UDP/DTLS/SCTP or TCP/DTLS/SCTP based "m" line is
renegotiated later on, then the even and odd SCTP stream
identifier ownership is redetermined as well as described above.'
o In Section 5.2.3 the first action of an SDP answerer, when
receiving an SDP offer, was described as "Applies the SDP offer.
Note that the browser ignores data channel specific attributes in
the SDP." Replacement of these two sentences with "Parses and
applies the SDP offer. Note that the typical parser normally
ignores unknown SDP attributes, which includes data channel
related attributes."
o In Section 5.2.3 the second sentence of the third SDP answerer
action was "Note that the browser is asked to create data channels
with stream identifiers not "owned" by the agent.". Replacement
of this sentence with "Note that the agent is asked to create data
channels with SCTP stream identifiers contained in the SDP offer
if the SDP offer is accepted."
o In Section 5.2.4 the third paragraph began with "A data channel
can be closed by sending a new SDP offer which excludes the dcmap
and dcsa attribute lines for the data channel. The port value for
the m line should not be changed (e.g., to zero) when closing a
data channel (unless all data channels are being closed and the
SCTP association is no longer needed), since this would close the
SCTP association and impact all of the data channels. If the
answerer accepts the SDP offer then it MUST also exclude the
corresponding attribute lines in the answer. ..." Replacement of
this part with "The intention to close a data channel can be
signaled by sending a new SDP offer which excludes the "a=dcmap:"
and "a=dcsa:" attribute lines for the data channel. The port
value for the "m" line SHOULD not be changed (e.g., to zero) when
closing a data channel (unless all data channels are being closed
and the SCTP association is no longer needed), since this would
close the SCTP association and impact all of the data channels.
If the answerer accepts the SDP offer then it MUST close those
data channels whose "a=dcmap:" and "a=dcsa:" attribute lines were
excluded from the received SDP offer, unless those data channels
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were already closed, and it MUST also exclude the corresponding
attribute lines in the answer."
o In Section 5.2.4 the hanging text after the third paragraph was
"This delayed close is to handle cases where a successful SDP
answer is not received, in which case the state of session should
be kept per the last successful SDP offer/answer." Replacement of
this sentence with "This delayed closure is RECOMMENDED in order
to handle cases where a successful SDP answer is not received, in
which case the state of the session SHOULD be kept per the last
successful SDP offer/answer."
o Although dedicated to "a=dcmap" and "a=dcsa" SDP syntax aspects
Section 5.1.1 contained already procedural descriptions related to
data channel reliability negotiation. Creation of new
Section 5.2.2 and moval of reliability negotiation related text to
this new section.
10.8. Changes against 'draft-ejzak-mmusic-data-channel-sdpneg-02'
o Removal of note "[ACTION ITEM]" from section "subprotocol
parameter". As [I-D.ietf-rtcweb-data-protocol] this document
should refer to IANA's WebSocket Subprotocol Name Registry defined
in [RFC6455].
o In whole document, replacement of "unreliable" with "partially
reliable", which is used in [I-D.ietf-rtcweb-data-channel] and in
[I-D.ietf-rtcweb-data-protocol] in most places.
o Clarification of the semantic if the "max-retr" parameter is not
present in an a=dcmap attribute line. In section "max-retr
parameter" the sentence "The max-retr parameter is optional with
default value unbounded" was replaced with "The max-retr parameter
is optional. If the max-retr parameter is not present, then the
maximal number of retransmissions is determined as per the generic
SCTP retransmission rules as specified in [RFC4960]".
o Clarification of the semantic if the "max-time" parameter is not
present in an a=dcmap attribute line. In section "max-time
parameter" the sentence "The max-time parameter is optional with
default value unbounded" was replaced with "The max-time parameter
is optional. If the max-time parameter is not present, then the
generic SCTP retransmission timing rules apply as specified in
[RFC4960]".
o In section "label parameter" the sentence "Label is a mandatory
parameter." was removed and following new sentences (including the
note) were added: "The 'label' parameter is optional. If it is
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not present, then its value defaults to the empty string. Note:
The empty string may also be explicitly used as 'label' value,
such that 'label=""' is equivalent to the 'label' parameter not
being present at all. [I-D.ietf-rtcweb-data-protocol] allows the
DATA_CHANNEL_OPEN message's 'Label' value to be an empty string."
o In section "subprotocol parameter" the sentence "Subprotocol is a
mandatory parameter." was replaced with "'Subprotocol' is an
optional parameter. If the 'subprotocol' parameter is not
present, then its value defaults to the empty string."
o In the "Examples" section, in the first two SDP offer examples in
the a=dcmap attribute lines 'label="BGCP"' was replaced with
'label="BFCP"'.
o In all examples, the "m" line proto value "DTLS/SCTP" was replaced
with "UDP/DTLS/SCTP" and the "a=fmtp" attribute lines were
replaced with "a=max-message-size" attribute lines, as per draft-
ietf-mmusic-sctp-sdp-12.
10.9. Changes against '-01'
o Formal syntax for dcmap and dcsa attribute lines.
o Making subprotocol as an optional parameter in dcmap.
o Specifying disallowed parameter combinations for max-time and max-
retr.
o Clarifications on WebRTC data channel close procedures.
10.10. Changes against '-00'
o Revisions to identify difference between internal and external
negotiation and their usage.
o Introduction of more generic terminology, e.g. "application"
instead of "browser".
o Clarification of how "max-retr and max-time affect the usage of
unreliable and reliable WebRTC data channels.
o Updates of examples to take into account the SDP syntax changes
introduced with draft-ietf-mmusic-sctp-sdp-07.
o Removal of the SCTP port number from the a=dcmap and a=dcsa
attributes as this is now contained in the a=sctp-port attribute,
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and as draft-ietf-mmusic-sctp-sdp-07 supports only one SCTP
association on top of the DTLS connection.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <http://www.rfc-editor.org/info/rfc4566>.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002,
<http://www.rfc-editor.org/info/rfc3264>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
[I-D.ietf-rtcweb-data-channel]
Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data
Channels", draft-ietf-rtcweb-data-channel-13 (work in
progress), January 2015.
[I-D.ietf-mmusic-sctp-sdp]
Holmberg, C., Loreto, S., and G. Camarillo, "Stream
Control Transmission Protocol (SCTP)-Based Media Transport
in the Session Description Protocol (SDP)", draft-ietf-
mmusic-sctp-sdp-15 (work in progress), September 2015.
[I-D.ietf-mmusic-sdp-mux-attributes]
Nandakumar, S., "A Framework for SDP Attributes when
Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-10
(work in progress), July 2015.
11.2. Informative References
[I-D.ietf-rtcweb-data-protocol]
Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data Channel
Establishment Protocol", draft-ietf-rtcweb-data-
protocol-09 (work in progress), January 2015.
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Internet-Draft SDP-based Data Channel Negotiation November 2015
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<http://www.rfc-editor.org/info/rfc4960>.
[IANA-SDP-Parameters]
"Session Description Protocol (SDP) Parameters", Internet
Assigned Numbers Authority Protocol Assignments Session
Description Protocol (SDP) Parameters,
<http://www.iana.org/assignments/sdp-parameters/
sdp-parameters.xhtml>.
[WebRtcAPI]
Bergkvist, A., Burnett, D., Jennings, C., and A.
Narayanan, "WebRTC 1.0: Real-time Communication Between
Browsers", World Wide Web Consortium WD-webrtc-20150210,
February 2015,
<http://www.w3.org/TR/2015/WD-webrtc-20150210/>.
Appendix A. Generic Data Channel Negotiation Aspects When Not Using
DCEP
This appendix summarizes how data channels work in general and
discusses some key aspects, which should be considered for the out-
of-band negotiation of data channels if DCEP is not used.
A WebRTC application creates a data channel by providing a number of
setup parameters (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 79.97.215.79
a=max-message-size:100000
a=sctp-port:5000
a=setup:actpass
a=connection:new
a=fingerprint:SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
Note: A WebRTC application will only use "m" line format "webrtc-
datachannel", and will not use other formats in the "m" line for
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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 to the data channel stack the
stream identifier to assign to the data channel or else let the data
channel stack pick one identifier from the unused ones.
To avoid glare situations, each endpoint can moreover own an
exclusive set of stream identifiers, in which case an endpoint can
only create a data channel with a stream identifier it owns.
Which set of stream identifiers is owned by which endpoint is
determined by convention or other means.
For data channels negotiated with the DCEP, one endpoint owns by
convention the even stream identifiers, whereas the other owns the
odd stream identifiers, as defined in
[I-D.ietf-rtcweb-data-protocol].
For data channels negotiated via some protocol different from
DCEP, no convention is defined by default.
A.2. Generic Data Channel Negotiation Not Using DCEP
A.2.1. Overview
DCEP negotiation only provides for negotiation of data channel
transport parameters and does not provide for negotiation of
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).
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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.
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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 (editor)
Alcatel-Lucent
Quadrant, Stonehill Green, Westlea
Swindon
UK
Email: keith.drage@alcatel-lucent.com
Maridi R. Makaraju (Raju)
Alcatel-Lucent
2000 Lucent Lane
Naperville, Illinois
US
Email: Raju.Makaraju@alcatel-lucent.com
Juergen Stoetzer-Bradler
Alcatel-Lucent
Lorenzstrasse 10
D-70435 Stuttgart
Germany
Email: Juergen.Stoetzer-Bradler@alcatel-lucent.com
Richard Ejzak
Unaffiliated
Email: richard.ejzak@gmail.com
Jerome Marcon
Unaffiliated
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