One document matched: draft-ietf-mmusic-data-channel-sdpneg-01.xml
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<rfc category="std" docName="draft-ietf-mmusic-data-channel-sdpneg-01" ipr="trust200902">
<front>
<title abbrev="SDP-based Data Channel Negotiation">
SDP-based Data Channel Negotiation
</title>
<author initials="K. E." surname="Drage" fullname="Keith Drage" role="editor">
<organization>Alcatel-Lucent</organization>
<address>
<postal>
<street>Quadrant, Stonehill Green, Westlea</street>
<city>Swindon</city>
<country>UK</country>
</postal>
<email> keith.drage@alcatel-lucent.com</email>
</address>
</author>
<author initials="M. R." surname="Makaraju" fullname="Maridi R. Makaraju (Raju)">
<organization>Alcatel-Lucent</organization>
<address>
<postal>
<street>2000 Lucent Lane</street>
<city>Naperville</city>
<region>Illinois</region>
<country>US</country>
</postal>
<email> Raju.Makaraju@alcatel-lucent.com</email>
</address>
</author>
<author initials="J." surname="Stoetzer-Bradler" fullname="Juergen Stoetzer-Bradler">
<organization>Alcatel-Lucent</organization>
<address>
<postal>
<street>Lorenzstrasse 10</street>
<city>D-70435 Stuttgart</city>
<region></region>
<country>Germany</country>
</postal>
<email>Juergen.Stoetzer-Bradler@alcatel-lucent.com</email>
</address>
</author>
<author fullname="Richard Ejzak" initials="R.P." surname="Ejzak">
<organization>Unaffiliated</organization>
<address>
<email>richard.ejzak@gmail.com</email>
</address>
</author>
<author fullname="Jerome Marcon" initials="J.M." surname="Marcon">
<organization>Unaffiliated</organization>
</author>
<date year="2015"/>
<area>RAI</area>
<workgroup>MMUSIC</workgroup>
<abstract>
<t>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) 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.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>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.</t>
<t>This document defines SDP-based out-of-band negotiation procedures
to establish data channels for transport of well-defined sub-protocols.</t>
</section>
<section title="Conventions">
<t>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
<xref target="RFC2119"/>.</t>
</section>
<section title="Terminology" anchor="terminology">
<t>This document uses the following terms:
<list style="hanging">
<t>Data channel: A WebRTC data channel as specified in
<xref target="I-D.ietf-rtcweb-data-channel"/>.</t>
<t>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.</t>
<t>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...).</t>
<t>DCEP - Data Channel Establishment Protocol defined in
<xref target="I-D.ietf-rtcweb-data-protocol"/>.</t>
<t>External negotiation: Data channel negotiation based on SDP
offer/answer outlined in this specification.</t>
<t>Internal negotiation: Data channel negotiation based on the Data
Channel Establishment Protocol defined in
<xref target="I-D.ietf-rtcweb-data-protocol"/>.</t>
<t>In-band: transmission through the peer-to-peer SCTP association.</t>
<t>In-band negotiation: data channel negotiation based on the
Data Channel Establishment Protocol defined in
<xref target="I-D.ietf-rtcweb-data-protocol"/>.</t>
<t>Out-of-band: transmission through the application signaling path.</t>
<t>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.</t>
<t>Stream identifier: the identifier of the outbound and inbound
SCTP streams composing a data channel.</t>
</list>
</t>
</section>
<section title="Data Channels" anchor="defch">
<t>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.</t>
<t>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
<xref target="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.</t>
<t>In any case, the SDP offer generated by the browser is per
<xref target="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,
and one attribute per protocol assigned to the SCTP ports:</t>
<figure align="left" title="">
<artwork align="left"><![CDATA[
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
]]></artwork>
</figure>
<t>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.
<xref target="I-D.ietf-mmusic-sctp-sdp"/> supports only one SCTP
association to be established on top of a DTLS session.
</t>
<t>Note: This SDP syntax does not contain any channel-specific
information.</t>
<section title="Stream Identifier Numbering" anchor="si_num">
<t>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.</t>
<t>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.</t>
<t>Which set of stream identifiers is owned by which endpoint is
determined by convention or other means.
<list style="hanging">
<t>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
<xref target="I-D.ietf-rtcweb-data-protocol"/>.</t>
<t>For data channels externally negotiated, no convention is defined by
default.</t>
</list>
</t>
</section>
<section title="Generic External Negotiation" anchor="sec-gen-ext-neg">
<section title="Overview">
<t>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 <xref target="prot_att"/>
for an example of such a parameter.</t>
<t>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).</t>
</section>
<section title="Opening a Data Channel" anchor="ext_open">
<t>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.</t>
<t>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 the offerer may get data channel message
before external negotiation is complete and the application should be
ready to handle it.</t>
<t>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.</t>
<t>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.</t>
<t>The application then externally negotiates the data channel
properties and sub-protocol properties with the peer's application.</t>
<t>[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.</t>
<t>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 <xref target="WebRtcAPI"/>, while native
applications use a compatible API, which is yet to be specified.
See <xref target="opendc"/> 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.</t>
</section>
<section title="Closing a Data Channel">
<t>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.</t>
<t>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.</t>
</section>
</section> <!-- sec-gen-ext-neg -->
</section> <!-- defch -->
<section title="SDP-based External Negotiation" anchor="sec-sdp-ext-neg">
<t>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.</t>
<section title="SDP Syntax" anchor="sdp_synt">
<t>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.</t>
<section title="SDP Attribute for Data Channel Parameter Negotiation"
anchor="subsec-sdp-attr-for-dc-par-neg">
<t>Associated with the SDP "m" line that defines the SCTP association for
data channels (defined in Section 4), each SDP offer and answer
includes one "a=dcmap:" attribute that defines the data channel parameters for
each data channel to be negotiated. Each such attribute line specifies the
following parameters for a data channel: SCTP stream identifier,
sub-protocol, label, reliability, order of delivery, and priority.</t>
<t>The intention of exchanging these attributes is to create data
channels on both the peers with the same set of attributes without
actually using <xref target="I-D.ietf-rtcweb-data-protocol"/>. It
is assumed that the data channel properties (reliable/partially reliable,
ordered/unordered) are suitable per the sub-protocol transport
requirements.</t>
<section title="dcmap Attribute"
anchor="dcmap-attr-definition">
<t>"a=dcmap:" is a media level attribute having following ABNF syntax.
<figure align="left" title="">
<artwork align="left"><![CDATA[
Formal Syntax:
Name: dcmap
Value: dcmap-value
Usage Level: media
Charset Dependent: no
Syntax:
dcmap-value = dcmap-stream-id
[ SP dcmap-opt *(";" dcmap-opt) ]
dcmap-opt = ordering-opt / subprotocol-opt / label-opt
/ maxretr-opt / maxtime-opt
; Either only maxretr-opt or maxtime-opt
; is present.
; Both MUST not be present.
dcmap-stream-id = 1*DIGIT
ordering-opt = "ordered=" ordering-value
ordering-value = "true" / "false"
subprotocol-opt = "subprotocol=" quoted-string
label-opt = "label=" quoted-string
maxretr-opt = "max-retr=" maxretr-value
maxretr-value = <from-Reliability-Parameter of
I-D.ietf-rtcweb-data-protocol>
; number of retransmissions
maxtime-opt = "max-time=" maxtime-value
maxtime-value = <from-Reliability-Parameter of
I-D.ietf-rtcweb-data-protocol>
; milliseconds
quoted-string = DQUOTE *(quoted-char / escaped-char) DQUOTE
quoted-char = SP / quoted-visible
quoted-visible = %21 / %23-24 / %26-7E ; VCHAR without " or %
escaped-char = "%" HEXDIG HEXDIG
DQUOTE = <from-RFC5234>
integer = <from-RFC5234>
Examples:
a=dcmap:0
a=dcmap:1 subprotocol="BFCP";max-time=60000
a=dcmap:2 subprotocol="MSRP";ordered=true;label="MSRP"
a=dcmap:3 label="Label 1";ordered=false;max-retr=5
a=dcmap:4 label="foo%09bar";ordered=true;max-time=15000;max-retr=3
]]></artwork>
</figure>
</t>
<t>
<list style="hanging">
<t>Note: The last example (a=dcmap:4) shows a 'label' parameter value
which contains one non-printable 'escaped-char' character
(the tabulator character).</t>
</list>
</t>
</section>
<section title="dcmap-stream-id Parameter"
anchor="dcmap-stream-id-param-definition">
<t>The 'dcmap-stream-id' parameter indicates the SCTP stream identifier
within the SCTP association used to form the data channel.</t>
</section>
<section title="label Parameter">
<t>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, an RTCDataChannel object from
other RTCDataChannel objects.
This parameter maps to the 'Label' parameter defined in
<xref target="I-D.ietf-rtcweb-data-protocol"/>.
The 'label' parameter is optional. If it is not
present, then its value defaults to the empty string.
</t>
<t>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.
<xref target="I-D.ietf-rtcweb-data-protocol"/> allows the DATA_CHANNEL_OPEN
message's 'Label' value to be an empty string.</t>
</section>
<section title="subprotocol Parameter">
<t>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.</t>
</section>
<section title="max-retr Parameter">
<t>This parameter indicates that the data channel is partially reliable.
The 'max-retr' parameter indicates the max times a user message will
be retransmitted. The max-retr parameter is optional. If
the max-retr parameter is not present, then the maximal number of
retransmissions is determined as per the generic SCTP retransmission
rules as specified in <xref target="RFC4960"/>.
This parameter maps to the 'Number of RTX' parameter
defined in <xref target="I-D.ietf-rtcweb-data-protocol"/>.</t>
</section>
<section title="max-time Parameter">
<t> 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 <xref target="RFC4960"/>.
This parameter maps to the 'Lifetime in ms' parameter
defined in <xref target="I-D.ietf-rtcweb-data-protocol"/>.</t>
</section>
<section title="ordered Parameter"
anchor="ordered-param-description">
<t>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. 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
<xref target="I-D.ietf-rtcweb-data-protocol"/>.</t>
</section>
</section> <!-- subsec-sdp-attr-for-dc-par-neg -->
<section title="Sub-Protocol Specific Attributes" anchor="prot_att">
<t>In the SDP, each data channel declaration MAY also be followed
by other SDP attributes specific to the sub-protocol in use. Each
of these attributes is represented by one new attribute line, and
it includes the contents of a media-level SDP attribute already
defined for use with this (sub)protocol in another IETF specification.
Sub-protocol-specific attributes might also be defined for exclusive
use with data channel transport, but should use the same syntax
described here for other sub-protocol-specific attributes.</t>
<t>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.
<figure align="left" title="">
<artwork align="left"><![CDATA[
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
]]></artwork>
</figure>
</t>
<t>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.</t>
<t>As opposed to the data channel "a=dcmap:" attribute parameters,
these parameters
are subject to offer/answer negotiation following the procedures
defined in the sub-protocol specific documents.</t>
<t>The same syntax applies to any other SDP attribute required for
negotiation of this instance of the sub-protocol.</t>
<t>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.</t>
</section> <!-- prot_att -->
</section> <!-- sdp_sync -->
<section title="Procedures">
<section title="Managing Stream Identifiers" anchor="id_mngt">
<t>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.</t>
<t>
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 a 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 above described 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.</t>
</section> <!-- id_mngt -->
<section title="Negotiating Data Channel Parameters"
anchor="param_negotiation">
<t> 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 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.</t>
<t>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.</t>
<t>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.</t>
<t>Data channel types defined in
<xref target="I-D.ietf-rtcweb-data-protocol"/> are mapped to SDP
in the following manner:</t>
<t><figure align="left" title="">
<artwork align="left"><![CDATA[
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
]]></artwork>
</figure></t>
</section> <!-- ="Negotiating Data Channel Parameters" -->
<section title="Opening a Data Channel"
anchor="opendc">
<t>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.</t>
<t>The agent that intends to send an SDP offer to create data
channels through SDP-based external negotiation performs the following:
<list style="symbols">
<t>Creates data channels using stream identifiers from the owned
set (see <xref target="id_mngt"/>).</t>
<t>As described in <xref target="ext_open"/>, 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.</t>
<t>Generates a new SDP offer. In the case of the browser based
applications the browser generates the offer via the createOffer() API
call <xref target="I-D.ietf-rtcweb-jsep"/>.</t>
<t>Determines the list of stream identifiers assigned to data channels
opened through external negotiation.</t>
<t>Completes the SDP offer with the dcmap and dcsa attributes needed,
if any, for each externally-negotiated data channel, as described in
<xref target="sdp_synt"/> and in <xref target="param_negotiation"/>.</t>
<t>Sends the SDP offer.</t>
</list>
</t>
<t>The peer receiving such an SDP offer performs the following:
<list style="symbols">
<t>Parses and applies the SDP offer. Note that the typical parser
normally ignores unknown SDP attributes, which includes data
channel related attributes.</t>
<t>Analyzes the channel parameters and sub-protocol attributes to
determine whether to accept each offered data channel.</t>
<t>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.</t>
<t>As described in <xref target="ext_open"/>, 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.</t>
<t>Generates an SDP answer.</t>
<t>Completes the SDP answer with the dcmap and optional dcsa
attributes needed for each externally-negotiated data channel,
as described in <xref target="sdp_synt"/>
and in <xref target="param_negotiation"/>.</t>
<t>Sends the SDP answer.</t>
</list>
</t>
<t>The agent receiving such an SDP answer performs the following:
<list style="symbols">
<t>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.</t>
<t>Applies the SDP answer.</t>
</list>
</t>
<t>Any data channels in Connecting state are transitioned to the
Open state when the SCTP association is established.</t>
<t>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:
<list style="symbols">
<t>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.</t>
<t>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.</t>
<t>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.</t>
</list>
</t>
</section> <!-- opendc -->
<section title="Closing a Data Channel"
anchor="close-dc">
<t>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.</t>
<t>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.</t>
<t>The intention to close a data channel can be signaled
by sending a new SDP offer which
excludes the "a=dcmap:" and "a=dcsa:" attribute lines for the data channel.
The port value for the "m" line SHOULD not be changed (e.g., to zero)
when closing a data channel (unless all data channels are being
closed and the SCTP association is no longer needed), since this
would close the SCTP association and impact all of the data channels.
If the answerer accepts the SDP offer then it MUST close those data channels
whose "a=dcmap:" and "a=dcsa:" attribute lines were excluded from the received
SDP offer, unless those data channels were already closed,
and it MUST also exclude the
corresponding attribute lines in the answer. 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.</t>
<t>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.
<list style="hanging">
<t>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.</t>
</list>
</t>
<t>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.</t>
<t>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.</t>
<t> 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</t>
</section> <!-- Closing a Data Channel -->
<section title="Various SDP Offer/Answer Scenarios and Considerations">
<t>
<list>
<t>SDP offer has no a=dcmap attributes
<list style="symbols">
<t>Initial SDP offer:
No data channel negotiated yet.</t>
<t>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.</t>
</list>
</t>
<t>SDP answer has no a=dcmap attributes
<list style="symbols">
<t>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.</t><t>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.</t></list>
</t>
<t>SDP offer has no a=dcsa attributes for a data channel.
<list style="symbols">
<t>This is allowed and indicates there are no sub-protocol parameters
to convey.</t>
</list>
</t>
<t>SDP answer has no a=dcsa attributes for a data channel.
<list style="symbols">
<t>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.</t></list>
</t>
</list>
</t>
</section><!-- various SDP offer/answer ... -->
</section> <!-- Procedures -->
</section> <!-- sec-sdp-ext-neg -->
<section title="Examples">
<figure align="left" title="Example 1" anchor="example1">
<artwork align="left"><![CDATA[
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
]]></artwork>
</figure>
<t>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 the offerer or the answerer may use internal negotiation to open data
channels.</t>
<figure align="left" title="Example 2" anchor="example2">
<artwork align="left"><![CDATA[
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 text/
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
]]></artwork>
</figure>
<t>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.</t>
<t>Continuing on the earlier example in <xref target="example1"/>.</t>
<figure align="left" title="Example 3" anchor="example3">
<artwork align="left"><![CDATA[
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
]]></artwork>
</figure>
<t>The above example is a continuation of the example in <xref target="example1"/>.
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.</t>
</section> <!-- Examples -->
<section title="Security Considerations" anchor="sec-cons">
<t>No security considerations are envisaged beyond those already
documented in <xref target="RFC4566"/></t>
</section>
<section title="IANA Considerations" anchor="IANA">
<t>To be completed.
As <xref target="I-D.ietf-rtcweb-data-protocol"/> this document
should refer to IANA's WebSocket Subprotocol Name Registry defined in
<xref target="RFC6455"/>.</t>
</section>
<section title="Acknowledgments">
<t>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 Christian Groves, Christer Holmberg,
Paul Kyzivat, Jonathan Lennox, and
Uwe Rauschenbach for their invaluable comments.</t>
</section>
<section title="CHANGE LOG">
<section title="Changes against 'draft-ietf-mmusic-data-channel-sdpneg-00'">
<t>
<list style="symbols">
<t>In <xref target="terminology"/> "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
<xref target="I-D.ietf-rtcweb-data-channel"/>", and consistent usage
of "data channel" in the remainder of the document including the
document's headline."</t>
<t>In <xref target="defch"/> removal of following note:
'OPEN ISSUE: The syntax in <xref target="I-D.ietf-mmusic-sctp-sdp"/> may
change as that document progresses. In particular we expect
"webrtc-datachannel" to become a more general term.'</t>
<t>Consistent usage of '"m" line' in whole document as per
<xref target="RFC4566"/>.</t>
<t>In <xref target="subsec-sdp-attr-for-dc-par-neg"/>
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
<xref target="dcmap-attr-definition"/>.
Corresponding removal of following related note:
"Note: This document does not provide a complete specification
of how to negotiate the use of a WebRTC data channel to transport BFCP.
Procedures specific to each sub-protocol such as BFCP will be
documented elsewhere. The use of BFCP is only an example of how
the generic procedures described herein might apply to a specific
sub-protocol."</t>
<t>In <xref target="subsec-sdp-attr-for-dc-par-neg"/>
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:
<xref target="I-D.ietf-mmusic-sctp-sdp"/>."</t>
<t>Insertion of following new sentence to the beginning of
<xref target="dcmap-attr-definition"/>:
"dcmap is a media level attribute having following ABNF syntax:"</t>
<t>Insertion of new <xref target="dcmap-stream-id-param-definition"/>
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.</t>
<t>In <xref target="dcmap-attr-definition"/> modification of the
'ordering-value' values from "0" or "1" to "true" or "false".
Corresponding text modifications in <xref target="ordered-param-description"/>.</t>
<t>In <xref target="dcmap-attr-definition"/> 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".</t>
<t>Insertion of a dedicated note right after the "a=dcmap:4" attribute example
in <xref target="dcmap-attr-definition"/> regarding the non-printable
"escaped-char" character within the "label" value.</t>
<t>In <xref target="prot_att"/>'s second paragraph replacement of
"sctp stream identifier" with "SCTP stream identifier".</t>
<t>In first paragraph of <xref target="id_mngt"/> replacement of first two sentences
'For the SDP-based external negotiation described in this document,
the initial offerer based "SCTP over DTLS" owns by convention the
even stream identifiers whereas the initial answerer owns the odd stream
identifiers. This ownership is invariant for the whole
lifetime of the signaling session, e.g. it does not change if the
initial answerer sends a new offer to the initial offerer.'
with
'If an SDP offer / answer exchange
(could be the initial or a subsequent one)
results in a UDP/DTLS/SCTP or TCP/DTLS/SCTP based media description being
accepted, and if this SDP offer / answer exchange results in the
establishment of a new SCTP association, then the SDP offerer owns the
even SCTP stream ids of this new SCTP association and the answerer owns
the odd SCTP stream identifiers.
If this "m" line is removed from the signaling session (its port number
set to zero), and if usage of this or of a new UDP/DTLS/SCTP or
TCP/DTLS/SCTP based "m" line is renegotiated later on,
then the even and odd SCTP stream identifier
ownership is redetermined as well as described above.'</t>
<t>In <xref target="opendc"/> 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."</t>
<t>In <xref target="opendc"/> 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."</t>
<t>In <xref target="close-dc"/> the third paragraph began with
"A data channel can be closed by sending a new SDP offer which
excludes the dcmap and dcsa attribute lines for the data channel.
The port value for the m line should not be changed (e.g., to zero)
when closing a data channel (unless all data channels are being
closed and the SCTP association is no longer needed), since this
would close the SCTP association and impact all of the data channels.
If the answerer accepts the SDP offer then it MUST also exclude the
corresponding attribute lines in the answer. ..."
Replacement of this part with
"The intention to close a data channel can be signaled
by sending a new SDP offer which
excludes the "a=dcmap:" and "a=dcsa:" attribute lines for the data channel.
The port value for the "m" line SHOULD not be changed (e.g., to zero)
when closing a data channel (unless all data channels are being
closed and the SCTP association is no longer needed), since this
would close the SCTP association and impact all of the data channels.
If the answerer accepts the SDP offer then it MUST close those data channels
whose "a=dcmap:" and "a=dcsa:" attribute lines were excluded from the received
SDP offer, unless those data channels were already closed,
and it MUST also exclude the
corresponding attribute lines in the answer."</t>
<t>In <xref target="close-dc"/> 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."</t>
<t>Although dedicated to "a=dcmap" and "a=dcsa" SDP syntax aspects
<xref target="subsec-sdp-attr-for-dc-par-neg"/> contained already
procedural descriptions related to data channel reliability negotiation.
Creation of new <xref target="param_negotiation"/> and moval of
reliability negotiation related text to this new section.</t>
</list>
</t>
</section>
<section title="Changes against 'draft-ejzak-mmusic-data-channel-sdpneg-02'">
<t>
<list style="symbols">
<t>Removal of note "[ACTION ITEM]" from section "subprotocol parameter".
As <xref target="I-D.ietf-rtcweb-data-protocol"/> this document should refer to IANA's
WebSocket Subprotocol Name Registry defined in <xref target="RFC6455"/>.</t>
<t>In whole document, replacement of "unreliable" with "partially reliable",
which is used in <xref target="I-D.ietf-rtcweb-data-channel"/> and in
<xref target="I-D.ietf-rtcweb-data-protocol"/> in most places.</t>
<t>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 <xref target="RFC4960"/>".</t>
<t>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 <xref target="RFC4960"/>".</t>
<t>In section "label parameter" the sentence "Label is a mandatory parameter."
was removed and following new sentences (including the note) were added:
"The 'label' parameter is optional. If it is not
present, then its value defaults to the empty string.
Note: The empty string may also be explicitly used as 'label' value,
such that 'label=""' is equivalent to the 'label' parameter not being
present at all.
<xref target="I-D.ietf-rtcweb-data-protocol"/> allows the DATA_CHANNEL_OPEN
message's 'Label' value to be an empty string."</t>
<t>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."</t>
<t>In the "Examples" section, in the first two SDP offer examples in
the a=dcmap attribute lines 'label="BGCP"' was replaced with 'label="BFCP"'.</t>
<t>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.</t>
</list>
</t>
</section>
<section title="Changes against '-01'">
<t>
<list style="symbols">
<t>Formal syntax for dcmap and dcsa attribute lines. </t>
<t>Making subprotocol as an optional parameter in dcmap. </t>
<t>Specifying disallowed parameter combinations for max-time and max-retr. </t>
<t>Clarifications on WebRTC data channel close procedures. </t>
</list>
</t>
</section>
<section title="Changes against '-00'">
<t>
<list style="symbols">
<t> Revisions to identify difference between internal and external
negotiation and their usage.</t>
<t>Introduction of more generic terminology, e.g. "application" instead of
"browser".</t>
<t>Clarification of how "max-retr and max-time affect the usage of
unreliable and reliable WebRTC data channels.</t>
<t>Updates of examples to take into account the SDP syntax changes
introduced with draft-ietf-mmusic-sctp-sdp-07.</t>
<t>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.</t>
</list>
</t>
</section>
</section>
</middle>
<back>
<references title="Normative References">
&RFC2119;
&RFC4566;
&RFC3264;
&RFC5234;
&JSEP;
&DATAREQ;
&SDPSCTP;
<reference anchor="WebRtcAPI" target="http://www.w3.org/TR/2013/WD-webrtc-20130910/"><front><title>WebRTC 1.0: Real-time Communication Between Browsers</title><author initials="A." surname="Bergkvist" fullname="Adam Bergkvist"><organization/></author><author initials="D." surname="Burnett" fullname="Daniel C. Burnett"><organization/></author><author initials="C." surname="Jennings" fullname="Cullen Jennings"><organization/></author><author initials="A." surname="Narayanan" fullname="Anant Narayanan"><organization/></author><date month="September" day="10" year="2013"/></front><seriesInfo name="World Wide Web Consortium" value="WD-webrtc-20130910"/><format type="HTML" target="http://www.w3.org/TR/2013/WD-webrtc-20130910/"/></reference>
</references>
<references title="Informative References">
&DATAPROTO;
&RFC4960;
&RFC4975;
&RFC4976;
&RFC5547;
&RFC6135;
&RFC6455;
&RFC6714;
</references>
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-24 01:20:23 |