Internet DRAFT - draft-ejzak-mmusic-data-channel-sdpneg
draft-ejzak-mmusic-data-channel-sdpneg
MMUSIC K. Drage, Ed.
Internet-Draft M. Makaraju
Intended status: Standards Track J. Stoetzer-Bradler
Expires: April 30, 2015 Alcatel-Lucent
R. Ejzak
J. Marcon
Unaffiliated
October 27, 2014
SDP-based "SCTP over DTLS" data channel negotiation
draft-ejzak-mmusic-data-channel-sdpneg-02
Abstract
The Real-Time Communication in WEB-browsers (RTCWeb) working group is
charged to provide protocols to support direct interactive rich
communications using audio, video, and data between two peers' web-
browsers. For the support of data communication, the RTCWeb working
group has in particular defined the concept of bi-directional data
channels over SCTP, where each data channel might be used to
transport other protocols, called sub-protocols. Data channel setup
can be done using either the internal in-band band (also referred to
as 'internal' for the rest of the document) WebRTC Data Channel
Establishment Protocol or some external out-of-band simply referred
to as 'external negotiation' in the rest of the document . This
document specifies how the SDP offer/answer exchange can be used to
achieve such an external negotiation. Even though data channels are
designed for RTCWeb use initially they may be used by other protocols
like, but not limited to, the CLUE protocol. This document is
intended to be used wherever data channels are used.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 30, 2015.
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Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Data Channels . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Stream identifier numbering . . . . . . . . . . . . . . . 5
4.2. Generic external negotiation . . . . . . . . . . . . . . 6
4.2.1. Overview . . . . . . . . . . . . . . . . . . . . . . 6
4.2.2. Opening a data channel . . . . . . . . . . . . . . . 6
4.2.3. Closing a data channel . . . . . . . . . . . . . . . 7
5. SDP-based external negotiation . . . . . . . . . . . . . . . 7
5.1. SDP syntax . . . . . . . . . . . . . . . . . . . . . . . 8
5.1.1. SDP attribute for data channel parameter negotiation 8
5.1.1.1. dcmap attribute . . . . . . . . . . . . . . . . . 9
5.1.1.2. label parameter . . . . . . . . . . . . . . . . . 10
5.1.1.3. subprotocol parameter . . . . . . . . . . . . . . 11
5.1.1.4. max-retr parameter . . . . . . . . . . . . . . . 11
5.1.1.5. max-time parameter . . . . . . . . . . . . . . . 11
5.1.1.6. ordered parameter . . . . . . . . . . . . . . . . 11
5.1.2. Sub-protocol specific attributes . . . . . . . . . . 11
5.2. Procedures . . . . . . . . . . . . . . . . . . . . . . . 13
5.2.1. Managing stream identifiers . . . . . . . . . . . . . 13
5.2.2. Opening a data channel . . . . . . . . . . . . . . . 13
5.2.3. Closing a data channel . . . . . . . . . . . . . . . 15
5.2.4. Various SDP offer/answer scenarios and considerations 16
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . . 19
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
10. CHANGE LOG . . . . . . . . . . . . . . . . . . . . . . . . . 20
10.1. Changes against '-01' . . . . . . . . . . . . . . . . . 20
10.2. Changes against '-00' . . . . . . . . . . . . . . . . . 20
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11. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
11.1. Normative References . . . . . . . . . . . . . . . . . . 20
11.2. Informative References . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
The RTCWeb working group has defined the concept of bi-directional
data channels running on top of SCTP/DTLS. RTCWeb leaves it open for
other applications to use data channels and its in-band or out-of-
band protocol for creating them. Each data channel consists of
paired SCTP streams sharing the same SCTP Stream Identifier. Data
channels are created by endpoint applications through the WebRTC API,
or other users of data channel like CLUE, and can be used to
transport proprietary or well-defined protocols, which in the latter
case can be signaled by the data channel "sub-protocol" parameter,
conceptually similar to the WebSocket "sub-protocol". However, apart
from the "sub-protocol" value transmitted to the peer, RTCWeb leaves
it open how endpoint applications can agree on how to instantiate a
given sub-protocol on a data channel, and whether it is signaled in-
band or out-of-band (or both). In particular, the SDP offer
generated by the application includes no channel-specific
information.
This document defines SDP-based out-of-band negotiation procedures to
establish data channels for transport of well-defined sub-protocols.
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 bidirectional channel consisting of paired SCTP
outbound and inbound streams.
Data channel stack: An entity which, upon application request,
runs data channel protocol to keep track of states, sending and
receive data. If the application is browser based Javascript
application then this stack resides in the browser. If the
application is a native application then this stack resides in
application and accessible to it via some sort of APIs.
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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].
External negotiation: Data channel negotiation based on SDP offer/
answer outlined in this specification.
Internal negotiation: Data channel negotiation based on Data
Channel Establishment Protocol defined in
[I-D.ietf-rtcweb-data-protocol].
In-band: transmission through the peer-to-peer SCTP association.
In-band negotiation: data channel negotiation based Data Channel
Establishment Protocol defined in [I-D.ietf-rtcweb-data-protocol].
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.
Stream identifier: the identifier of the outbound and inbound SCTP
streams composing a data channel.
4. Data Channels
This section summarizes how data channels work in general. Note that
the references to 'browser' here is intentional as in this specific
example the data channel user is a webrtc enabled browser.
A WebRTC application creates a data channel via the Data Channel API,
by providing a number of setup parameters (sub-protocol, label,
reliability, order of delivery, priority). The application also
specifies if it wants to make use of the in-band negotiation using
the DCEP [I-D.ietf-rtcweb-data-protocol], or if the application
intends to perform an "external negotiation" using some other in-band
or out-of-band mechanism.
In any case, the SDP offer generated by the browser is per
[I-D.ietf-mmusic-sctp-sdp]. In brief, it contains one m-line for the
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SCTP association on top of which data channels will run, and one
attribute per protocol assigned to the SCTP ports:
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.
m=application 54111 DTLS/SCTP webrtc-datachannel
c=IN IP4 79.97.215.79
a=fmtp:webrtc-datachannel 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 browser will only use m-line format "webrtc-
datachannel", and will not use other formats in the m-line for other
protocols such as t38. [I-D.ietf-mmusic-sctp-sdp] supports only one
SCTP association to be established on top of a DTLS session.
Note: This SDP syntax does not contain any channel-specific
information.
4.1. Stream identifier numbering
Independently from the requested type of negotiation, the application
creating a data channel can either pass to the browser the stream
identifier to assign to the data channel or else let the browser pick
one identifier from the ones unused.
To avoid glare situations, each endpoint can moreover own an
exclusive set of stream identifiers, in which case an endpoint can
only create a data channel with a stream identifier it owns.
Which set of stream identifiers is owned by which endpoint is
determined by convention or other means.
For data channels negotiated in-band, 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 externally negotiated, no convention is defined
by default.
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4.2. Generic external negotiation
4.2.1. Overview
In-band negotiation only provides for negotiation of data channel
transport parameters and does not provide for negotiation of sub-
protocol specific parameters. External negotiation can be defined to
allow negotiation of parameters beyond those handled by in-band
negotiation, e.g., parameters specific to the sub-protocol
instantiated on a particular data channel. See Section 5.1.2 for an
example of such a parameter.
The following procedures are common to all methods of external
negotiation, whether in-band (communicated using proprietary means on
an already established data channel) or out-of-band (using SDP or
some other protocol associated with the signaling channel).
4.2.2. Opening a data channel
In the case of external 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 in-band 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 external negotiation, the data channel stack creates
the data channel locally without sending any DATA CHANNEL OPEN
message in-band, and sets the data channel state to Connecting if the
SCTP association is not yet established, or sets the data channel
state to Open if the SCTP association is already established. The
side which starts external negotiation creates data channel using
underlying data channel stack API and the data channel is put into
open state immediately (assuming ICE, SCTP procedures were already
done). However, the application can't send data on this data channel
until external negotiation is complete with the peer. This is
because peer needs to be aware and accept the data channel via
external negotiation. The peer after accepting the data channel
offer can start sending data immediately. This implies that offerer
may get data channel message before external 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.
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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 are used on the fly as needed just by communicating via
external means or by even having some local configuration/assumptions
on both the peers.
The application then externally negotiates the data channel
properties and sub-protocol properties with the peer's application.
[ASSUMPTION] The peer must then symmetrically create 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.
In case the external negotiation is correlated with an SDP offer/
answer exchange that establishes the SCTP association, the SCTP
initialization completion triggers a callback from the data channel
stack to an application on both the ends to change the data channel
state from Connecting to Open. The details of this interface is
specific to the data channel user application. Browser based
applications (could include hybrid apps) will use [WebRtcAPI], while
native applications use a compatible API, which is yet to be
specified. See Section 5.2.2 for details on when the data channel
stack can assume the data channel is open, and on when the
application can assume the data channel is open.
4.2.3. Closing a data channel
When the application requests the closing of an externally negotiated
data channel, the data channel stack always performs an in-band SSN
reset for this channel.
Depending upon the method used for external negotiation and the sub-
protocol associated with the data channel, the closing might in
addition be signaled to the peer via external negotiation.
5. SDP-based external negotiation
This section defines a method of external negotiation by which two
clients can negotiate data channel-specific and sub-protocol-specific
parameters, using the out-of-band SDP offer/answer exchange. This
SDP extension can only be used with SDP offer/answer model.
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5.1. SDP syntax
Two new SDP attributes are defined to support external negotiation of
data channels. The first attribute provides for negotiation of
channel-specific parameters. The second attribute provides for
negotiation of sub-protocol-specific parameters.
5.1.1. SDP attribute for data channel parameter negotiation
Associated with the SDP "m" line that defines the SCTP association
for data channels (defined in Section 4), each SDP offer and answer
includes an attribute line that defines the data channel parameters
for each data channel to be negotiated. Each attribute line
specifies the following parameters for a data channel: Stream
Identifier, sub-protocol, label, reliability, order of delivery, and
priority. Conveying a reliable data channel is achieved by including
neither 'max-retr' nor 'max-time'. Conveying an unreliable 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 only one of them can be present in a=dcmap. If an SDP offer
contains both of these parameters then such an SDP offer will be
rejected. If an SDP answer contains both of these parameters then
the offerer may treat it as an error and may assume the associated
SDP offer/answer failed and may take appropriate recovery actions.
These recovery options are outside the scope of this specification.
Following is an example of the attribute line for sub-protocol "BFCP"
and stream id "2":
a=dcmap:2 subprotocol="BFCP";label="channel 2"
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.
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.
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].
Note: This document does not provide a complete specification of
how to negotiate the use of a data channel to transport BFCP.
Procedures specific to each sub-protocol such as BFCP will be
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documented elsewhere. The use of BFCP is only an example of how
the generic procedures described herein might apply to a specific
sub-protocol.
The intention of exchanging these attributes is to create data
channels on both the peers with the same set of attributes without
actually using [I-D.ietf-rtcweb-data-protocol]. It is assumed that
the data channel properties (reliable/unreliable, ordered/unordered)
are suitable per the sub-protocol transport requirements. Data
channel types defined in [I-D.ietf-rtcweb-data-protocol] are mapped
to SDP in the following manner:
DATA_CHANNEL_RELIABLE
a=dcmap:2 subprotocol="BFCP";label="channel 2"
DATA_CHANNEL_RELIABLE_UNORDERED
a=dcmap:2 subprotocol="BFCP";label="channel 2";\
ordered=0
DATA_CHANNEL_PARTIAL_RELIABLE_REXMIT
a=dcmap:2 subprotocol="BFCP";label="channel 2";\
max-retr=3
DATA_CHANNEL_PARTIAL_RELIABLE_REXMIT_UNORDERED
a=dcmap:2 subprotocol="BFCP";label="channel 2";\
max-retr=3;ordered=0;
DATA_CHANNEL_PARTIAL_RELIABLE_TIMED
a=dcmap:2 subprotocol="BFCP";label="channel 2";\
max-time=10000;
DATA_CHANNEL_PARTIAL_RELIABLE_TIMED_UNORDERED
a=dcmap:2 subprotocol="BFCP";label="channel 2";\
max-time=10000; ordered=0
5.1.1.1. dcmap attribute
The 'stream' parameter indicates the actual stream identifier within
the association used to form the channel. Stream is a mandatory
parameter and is noted directly after the "a=dcmap:" attribute's
colon.
Formal Syntax:
TBD: Should this be moved to SDP grammar section?
Name: dcmap
Value: dcmap-value
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Usage Level: media
Charset Dependent: no
Syntax:
dcmap-value = dcmap-stream-id
[ SP dcmap-opt *(";" dcmap-opt) ]
dcmap-opt = ordering-opt / subprotocol-opt / label-opt
/ maxretr-opt / maxtime-opt
; Either only maxretr-opt or maxtime-opt
; is present.
; Both MUST not be present.
dcmap-stream-id = 1*DIGIT
ordering-opt = "ordered=" ordering-value
ordering-value = "0"/"1"
subprotocol-opt = "subprotocol=" quoted-string
label-opt = "label=" quoted-string
maxretr-opt = "max-retr=" maxretr-value
maxretr-value = <from-Reliability-Parameter of
I-D.ietf-rtcweb-data-protocol>
; number of retransmissions
maxtime-opt = "max-time=" maxtime-value
maxtime-value = <from-Reliability-Parameter of
I-D.ietf-rtcweb-data-protocol>
; milliseconds
quoted-string = DQUOTE *(quoted-char / escaped-char) DQUOTE
quoted-char = SP / quoted-visible
quoted-visible = %21 / %23-24 / %26-7E ; VCHAR without " or %
escaped = "%" HEXDIG HEXDIG
DQUOTE = <from-RFC5234>
integer = <from-RFC5234>
Examples:
a=dcmap:0
a=dcmap:1 subprotocol="BFCP";max-time=60000
a=dcmap:2 subprotocol="MSRP";ordered;label="MSRP"
a=dcmap:3 label="Label 1";unordered;max-retr=5
a=dcmap:4 label="foo%09bar";ordered;max-time=15000;max-retr=3
5.1.1.2. label parameter
The optional '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, an RTCDataChannel object from other RTCDataChannel
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objects. Label is a mandatory parameter. This parameter maps to the
'label' parameter defined in [I-D.ietf-rtcweb-data-protocol]
5.1.1.3. subprotocol parameter
The 'subprotocol' parameter indicates which protocol the client
expects to exchange via the channel. Subprotocol is a mandatory
parameter.
[ACTION ITEM] The IANA registry to be used for the subprotocol
parameter is still to be determined. It also needs to be determined
what the relationship is to existing registries and how to reference
already-existing protocols.
5.1.1.4. max-retr parameter
This parameter indicates that the data channel is unreliable. The
'max-retr' parameter indicates the max times a user message will be
retransmitted. The max-retr parameter is optional with default value
unbounded. This parameter maps to the 'Number of RTX' parameter
defined in [I-D.ietf-rtcweb-data-protocol]
5.1.1.5. max-time parameter
This parameter indicates that the data channel is unreliable. A user
messages 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 with default value
unbounded. This parameter maps to the 'Lifetime in ms' parameter
defined in [I-D.ietf-rtcweb-data-protocol]
5.1.1.6. ordered parameter
The ordered' parameter indicates that DATA chunks in the channel MUST
be dispatched to the upper layer by the receiver while preserving the
order. The ordered parameter is optional and takes two values: "0"
for ordered and "1" for ordered delivery with "1" as the default
value. Any other value is ignored and default ordered is assumed.
If the ordered parameter is absent, the receiver is required to
deliver DATA chunks to the upper layer in proper order. This
parameter maps to the ordered or unorderd data channel types as
defined in [I-D.ietf-rtcweb-data-protocol]
5.1.2. Sub-protocol specific attributes
In the SDP, each data channel declaration MAY also be followed by
other SDP attributes specific to the sub-protocol in use. Each of
these attributes is represented by one new attribute line, and it
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includes the contents of a media-level SDP attribute already defined
for use with this (sub)protocol in another IETF specification. Sub-
protocol-specific attributes might also be defined for exclusive use
with data channel transport, but should use the same syntax described
here for other sub-protocol-specific attributes.
Each sub-protocol specific SDP attribute that would normally be used
to negotiate the subprotocol using SDP is replaced with an attribute
of the form "a=dcsa: stream-id original-attribute", where dcsa stands
for "data channel sub-protocol attribute", stream-id is the sctp
stream identifier assigned to this sub-protocol instance, and
original-attribute represents the contents of the sub-protocol
related attribute to be included.
Formal Syntax:
Name: dcsa
Value: dcsa-value
Usage Level: media
Charset Dependent: no
Syntax:
dcsa-value = stream-id SP attribute
attribute = <from-RFC4566>
Examples:
a=dcsa:2 accept-types:text/plain
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. The above example creates a
reliable, ordered data channel.
As opposed to the data channel setup parameters, these parameters are
subject to offer/answer negotiation following the procedures defined
in the sub-protocol specific documents.
The same syntax applies to any other SDP attribute required for
negotiation of this instance of the sub-protocol.
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Note: This document does not provide a complete specification of how
to negotiate the use of a data channel to transport MSRP. Procedures
specific to each sub-protocol such as MSRP will be documented
elsewhere. The use of MSRP is only an example of how the generic
procedures described herein might apply to a specific sub-protocol.
5.2. Procedures
5.2.1. Managing stream identifiers
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.
This specification allows simultaneous use of external and internal
negotiation. However, a single stream is managed using one method at
a time. Stream ids that are not currently used in SDP can be used
for internal negotiation. Stream id allocation per SDP based
external negotiation may not align with DTLS role based allocation.
This could cause glare conditions when one side trying to do external
negotiation on a stream id while the other end trying to open data
channel on the same stream id using internal negotiation. To avoid
these glare conditions this specification recommends that the data
channel stack user always selects stream ids per SDP offer/answer
rule even when internal negotiation is used. To avoid glare
conditions, it is possible to come up with a different stream id
allocation scheme, but such schemes are outside the scope of this
specification.
5.2.2. Opening a data channel
The procedure for opening a data channel using external negotiation
starts with the agent preparing to send an SDP offer. If a peer
receives an SDP offer before getting to send a new SDP offer with
data channels that are to be externally 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 external
negotiation procedure.
The agent that intends to send an SDP offer to create data channels
through SDP-based external negotiation performs the following:
o Creates data channels using stream identifiers from the owned set
(see Section 5.2.1).
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o As described in Section 4.2.2, if the SCTP association is not yet
established, then the newly created data channels are in the
Connecting state, else if the SCTP association is already
established, then the newly created data channels are in the Open
state.
o Generates a new SDP offer. In the case of the browser based
applications the browser generates the offer via the createOffer()
API call [I-D.ietf-rtcweb-jsep].
o Determines the list of stream identifiers assigned to data
channels opened through external negotiation.
o Completes the SDP offer with the dcmap and dcsa attributes needed,
if any, for each externally-negotiated data channel, as described
in Section 5.1.
o Sends the SDP offer.
The peer receiving such an SDP offer performs the following:
o Applies the SDP offer. Note that the browser ignores data channel
specific attributes in the SDP.
o Analyzes the channel parameters and sub-protocol attributes to
determine whether to accept each offered data channel.
o For accepted data channels, creates peer instances for the data
channels with the browser using the channel parameters described
in the SDP offer. Note that the browser is asked to create data
channels with stream identifiers not "owned" by the agent.
o As described in Section 4.2.2, if the SCTP association is not yet
established, then the newly created data channels are in the
Connecting state, else if the SCTP association is already
established, then the newly created data channels are in the Open
state.
o Generates an SDP answer.
o Completes the SDP answer with the dcmap and optional dcsa
attributes needed for each externally-negotiated data channel, as
described in Section 5.1.
o Sends the SDP answer.
The agent receiving such an SDP answer performs the following:
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o Closes any created data channels (whether in Connecting or Open
state) for which the expected dcmap and dcsa attributes are not
present in the SDP answer.
o Applies the SDP answer.
Any data channels in Connecting state are transitioned to the Open
state when the SCTP association is established.
Each agent application MUST wait to send data until it has
confirmation that the data channel at the peer is in the Open state.
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 data channel stacks
report that the data channel transitions to the Open state from
the Connecting state.
o At the agent receiving an SDP offer for which there is an
established SCTP association, as soon as it creates an externally
negotiated data channel in the Open state based on information
signaled in the SDP offer.
o At the agent sending an SDP offer to create a new externally
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.
5.2.3. Closing a data channel
When the application requests the closing of a data channel that was
externally negotiated, the data channel stack always performs an in-
band SSN reset for this channel.
It is specific to the sub-protocol whether this closing must in
addition be signaled to the peer via a new SDP offer/answer exchange.
A data channel can be closed by sending a new SDP offer which
excludes the dcmap and dcsa attributes 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 answerer accepts the SDP offer then it MUST also exclude the
corresponding attribute lines in the answer. In addition to that,
SDP answerer may exclude other data channels which were closed but
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not yet communicated to the peer. So, 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
successful SDP answer is sent/received.
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.
If a client receives a data channel close indication (due to inband
SSN reset or some other reason) without associated SDP offer then an
SDP offer which excludes this closed data channel SHOULD be
generated.
The application must also close any data channel that was externally
negotiated, for which the stream identifiers are not listed in an
incoming SDP offer.
A closed data channel using local close (SCTP reset), without an
additional SDP offer/answer to close it, may be reused for a new data
channel. This can only be done via new SDP offer/answer, describing
the new sub-protocol and its attributes, only after the corresponding
data channel close acknowledgement is received from the peer (i.e.
SCTP 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 reset which closes outgoing
stream" at the peer
5.2.4. Various SDP offer/answer scenarios and considerations
SDP offer has no a=dcmap attributes
* Initial SDP offer: No data channel negotiated yet.
* Subsequent SDP offer: All the externally negotiated data
channels must be closed now. The DTLS/SCTP association remains
open for external or internal negotiation of data channels.
SDP answer has no a=dcmap attributes
* Initial SDP answer: Either the peer does not support dcmap
attributes or it rejected all the data channels. In either
case offerer closes all the externally negotiated data channels
that were open at the time of initial offer. The DTLS/SCTP
association will still be setup.
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* Sub-sequent SDP answer: All the externally negotiated data
channels must be closed now. The DTLS/SCTP association remains
open for future external or internal negotiation of data
channels.
SDP offer has no a=dcsa attributes for a data channel.
* This is allowed and indicates there are no sub-protocol
parameters to convey.
SDP answer has no a=dcsa attributes for a data channel.
* This is allowed and indicates there are no sub-protocol
parameters to convey in the SDP answer. The number of dcsa
attributes in the SDP answer does not have to match the number
of dcsa attributes in the SDP offer.
6. Examples
SDP offer:
m=application 10001 DTLS/SCTP webrtc-datachannel
c=IN IP4 10.10.10.1
a=fmtp:webrtc-datachannel 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="BGCP"
SDP answer:
m=application 10002 DTLS/SCTP webrtc-datachannel
c=IN IP4 10.10.10.2
a=fmtp:webrtc-datachannel 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 external negotiation option. The
SCTP association will still be setup over DTLS. At this point
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offerer or answerer may use internal negotiation to open data
channels.
SDP offer:
m=application 10001 DTLS/SCTP webrtc-datachannel
c=IN IP4 10.10.10.1
a=fmtp:webrtc-datachannel 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="BGCP"
a=dcmap:2 subprotocol="MSRP";label="MSRP"
a=dcsa:2 accept-types:message/cpim text/plain text/
a=dcsa:2 path:msrp://alice.example.com:10001/2s93i93idj;dc
SDP answer:
m=application 10002 DTLS/SCTP webrtc-datachannel
c=IN IP4 10.10.10.2
a=fmtp:webrtc-datachannel 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 SDP offer contains data channels for BFCP and
MSRP sub-protocols. SDP answer rejected BFCP and accepted MSRP. So,
the offerer should close the data channel for BFCP and both offerer
and answerer may start using MSRP data channel (after SCTP/DTLS
association is setup). The data channel with stream id 0 is free and
can be used for future internal or external negotiation.
Continuing on the earlier example in Figure 1.
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Subsequent SDP offer:
m=application 10001 DTLS/SCTP webrtc-datachannel
c=IN IP4 10.10.10.1
a=fmtp:webrtc-datachannel 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 DTLS/SCTP webrtc-datachannel
c=IN IP4 10.10.10.2
a=fmtp:webrtc-datachannel max-message-size=100000
a=sctp-port 5002
a=setup:passive
a=connection:existing
a=fingerprint:SHA-1 \
5B:AD:67:B1:3E:82:AC:3B:90:02:B1:DF:12:5D:CA:6B:3F:E5:54:FA
a=dcmap:4 subprotocol="MSRP";label="MSRP"
a=dcsa:4 accept-types:message/cpim text/plain
a=dcsa:4 path:msrp://bob.example.com:10002/si438dsaodes;dc
Figure 3: Example 3
The above example is a continuation of the example in Figure 1. The
SDP offer now removes the MSRP data channel with stream id 2, but
opens a new MSRP data channel with stream id 4. The answerer
accepted 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
To be completed.
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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 Paul Kyzivat, Jonathan Lennox, and
Uwe Rauschenbach for their invaluable comments.
10. CHANGE LOG
10.1. 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 data channel close procedures.
10.2. 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 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,
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, March 1997.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
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[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, June
2002.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[I-D.ietf-rtcweb-jsep]
Uberti, J., Jennings, C., and E. Rescorla, "Javascript
Session Establishment Protocol", draft-ietf-rtcweb-jsep-07
(work in progress), July 2014.
[I-D.ietf-rtcweb-data-channel]
Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data
Channels", draft-ietf-rtcweb-data-channel-12 (work in
progress), September 2014.
[I-D.ietf-mmusic-sctp-sdp]
Loreto, S. and G. Camarillo, "Stream Control Transmission
Protocol (SCTP)-Based Media Transport in the Session
Description Protocol (SDP)", draft-ietf-mmusic-sctp-sdp-07
(work in progress), July 2014.
[WebRtcAPI]
Bergkvist, A., Burnett, D., Jennings, C., and A.
Narayanan, "WebRTC 1.0: Real-time Communication Between
Browsers", World Wide Web Consortium WD-webrtc-20130910,
September 2013,
<http://www.w3.org/TR/2013/WD-webrtc-20130910/>.
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-08 (work in progress), September 2014.
[RFC4975] Campbell, B., Mahy, R., and C. Jennings, "The Message
Session Relay Protocol (MSRP)", RFC 4975, September 2007.
[RFC4976] Jennings, C., Mahy, R., and A. Roach, "Relay Extensions
for the Message Sessions Relay Protocol (MSRP)", RFC 4976,
September 2007.
[RFC5547] Garcia-Martin, M., Isomaki, M., Camarillo, G., Loreto, S.,
and P. Kyzivat, "A Session Description Protocol (SDP)
Offer/Answer Mechanism to Enable File Transfer", RFC 5547,
May 2009.
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[RFC6135] Holmberg, C. and S. Blau, "An Alternative Connection Model
for the Message Session Relay Protocol (MSRP)", RFC 6135,
February 2011.
[RFC6714] Holmberg, C., Blau, S., and E. Burger, "Connection
Establishment for Media Anchoring (CEMA) for the Message
Session Relay Protocol (MSRP)", RFC 6714, August 2012.
Authors' Addresses
Keith Drage (editor)
Alcatel-Lucent
Quadrant, Stonehill Green, Westlea
Swindon
UK
Email: keith.drage@alcatel-lucent.com
Raju Makaraju
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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