One document matched: draft-ietf-mmusic-data-channel-sdpneg-04.xml
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<rfc category="std" docName="draft-ietf-mmusic-data-channel-sdpneg-04" 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 in-band Data Channel Establishment
Protocol (DCEP) or using some out-of-band non-DCEP protocol.
This document specifies
how the SDP offer/answer exchange can be used to achieve such an out-of-band
non-DCEP negotiation. Even though data channels are designed for RTCWeb
use initially they may be used by other protocols like, but not limited
to, the CLUE protocol. This document is intended to be used wherever
data channels are used.</t>
</abstract>
</front>
<middle>
<section title="Introduction" anchor="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 DCEP or other in-band or out-of-band
protocols 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 using DCEP or out-of-band
using a non-DCEP protocol (or both).
In particular, the SDP offer generated by the RTCweb data channel stack includes
no channel-specific information.</t>
<t>This document defines SDP offer/answer negotiation procedures
to establish data channels for transport of well-defined sub-protocols,
to enable out-of-band negotiation.</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 the data channel protocol to keep track of states, sending and receive
data. If the application is a browser based JavaScript application
then this stack resides in the browser. If the application is a
native application then this stack resides in the application and is accessible
via some sort of APIs.</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>In-band: Transmission through the peer-to-peer SCTP association.</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>SCTP Stream Sequence Number (SSN): the SCTP stream sequence number as specified
in <xref target="RFC4960"/>.</t>
<t>Stream identifier: The identifier of the outbound and inbound
SCTP streams composing a data channel.</t>
</list>
</t>
</section>
<section title=" Applicability Statement" anchor="appl_statement">
<t> The mechanism in this specification only applies to the Session Description
Protocol (SDP) <xref target="RFC4566"/>, when used together with the SDP offer/answer
mechanism <xref target=" RFC3264"/>. Declarative usage of SDP is out of scope of this
document, and is thus undefined.</t>
</section>
<section title="SDP Offer/Answer Negotiation" anchor="sec-sdp-ext-neg">
<t>This section defines an SDP extension by which
two clients can negotiate data channel-specific and sub-protocol-specific
parameters without using DCEP <xref target="I-D.ietf-rtcweb-data-protocol"/>.
This SDP extension only defines usage in the context of SDP offer/answer.</t>
<t><xref target="generic-out-of-band-aspects"/> provides information how data channels
work in general and especially summarizes some key aspects, which should be
considered for the negotiation of data channels if DCEP is not used.
</t>
<section title="SDP Syntax" anchor="sdp_synt">
<t>Two new SDP attributes are defined to support SDP offer/answer negotiation of
data channels. The first attribute provides for negotiation of
channel-specific parameters. The second attribute provides for
negotiation of 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 in exchanging these attributes is to create, on two peers,
without use of DCEP <xref target="I-D.ietf-rtcweb-data-protocol"/>,
matched pairs of oppositely directed data channels having the same set of
attributes.
It is assumed that the data channel properties (reliable/partially reliable,
ordered/unordered) are suitable per the 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.
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>
<t>Within an 'a=dcmap' attribute line's 'dcmap-opt' value either only one
'maxretr-opt' parameter or one 'maxtime-opt' parameter MAY be present.
Both MUST NOT be present.</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 <xref target="WebRtcAPI"/>,
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" anchor="max-retr-param-definition">
<t>This parameter indicates that the data channel is partially reliable.
The 'max-retr' parameter indicates the maximal number of times a user message will
be retransmitted. The max-retr parameter is optional. If
the max-retr parameter 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 the receiver MUST
dispatch DATA chunks in the data channel to the upper layer
while preserving the order. The ordered parameter is optional and
takes two values: "true" for ordered and "false" for unordered delivery
with "true" as the default value.
Any other value is ignored and default "ordered=true" is assumed.
In the absence of this parameter "ordered=true" is assumed.
This parameter maps to
the ordered or unordered data channel types as defined in
<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>Note that the above reference to RFC 4566 defines where the
attribute definition can be found;
it does not provide any limitation on support of attributes
defined in other documents in accordance with this attribute
definition. Note however that not all SDP attributes are suitable
as "a=dcsa:" parameter. <xref target="IANA-SDP-Parameters" /> contains
the lists of IANA registered session and media level or
media level only SDP attributes.</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 SDP offer/answer and DCEP
negotiation. However, an SCTP stream MUST NOT be referenced in a dcmap or dcsa
attribute of an SDP offer/answer exchange if the associated SCTP stream has already
been negotiated via DCEP.
Stream ids that are not currently used in SDP can be used
for DCEP negotiation. Stream id allocation per SDP offer/answer
negotiation may not align with DTLS role based allocation. This could
cause glare conditions when one side trying to do SDP offer/answer
negotiation on a stream id while the other end trying to open a data
channel on the same stream id using DCEP 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 DCEP negotiation is used. To avoid glare
conditions, it is possible to come up with a different stream id
allocation scheme, but such schemes are outside the scope of this
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 at most one of them MAY be present in the "a=dcmap" attribute line.
If an SDP offer contains
both of these parameters then the receiver of such an SDP offer
MUST reject the SDP offer.
If an SDP answer contains both of these parameters then the offerer 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>When sending a subsequent offer or an answer, and for as long as the data
channel is still open, the sender MUST replicate the same information.</t>
<t>Data channel types defined in
<xref target="I-D.ietf-rtcweb-data-protocol"/> are mapped to SDP
in the following manner,
where "ordered=true" is the default and may be omitted:</t>
<t><figure align="left" title="">
<artwork align="left"><![CDATA[
DATA_CHANNEL_RELIABLE
ordered=true
DATA_CHANNEL_RELIABLE_UNORDERED
ordered=false
DATA_CHANNEL_PARTIAL_RELIABLE_REXMIT
ordered=true;max-retr=<number of retransmissions>
DATA_CHANNEL_PARTIAL_RELIABLE_REXMIT_UNORDERED
ordered=false;max-retr=<number of retransmissions>
DATA_CHANNEL_PARTIAL_RELIABLE_TIMED
ordered=true;max-time=<lifetime in milliseconds>
DATA_CHANNEL_PARTIAL_RELIABLE_TIMED_UNORDERED
ordered=false;max-time=<lifetime in milliseconds>
]]></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 SDP offer/answer negotiation
starts with the agent preparing to send an SDP offer. If a peer
receives an SDP offer before starting to send a new SDP offer with
data channels that are to be SDP offer/answer negotiated, or loses an SDP
offer glare resolution procedure in this case, it MUST wait until the
ongoing SDP offer/answer completes before resuming the SDP offer/answer negotiation
procedure.</t>
<t>The agent that intends to send an SDP offer to create data
channels through SDP offer/answer 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>Generates a new SDP offer.</t>
<t>Determines the list of stream identifiers assigned to data channels
opened through SDP offer/answer negotiation.</t>
<t>Completes the SDP offer with the dcmap and dcsa attributes needed,
if any, for each SDP offer/answer 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>Generates an SDP answer.</t>
<t>Completes the SDP answer with the dcmap and optional dcsa
attributes needed for each SDP offer/answer 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
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>Each agent application MUST wait to send data until it has
confirmation that the data channel at the peer is instantiated.
For WebRTC, this is when both data channel stacks have channel
parameters instantiated. This occurs:
<list style="symbols">
<t>At both peers when a data channel is created without an
established SCTP association, as soon as the SCTP association
is successfully established.</t>
<t>At the agent receiving an SDP offer for which there is an
established SCTP association, as soon as it creates an SDP offer/answer
negotiated data channel based on information
signaled in the SDP offer.</t>
<t>At the agent sending an SDP offer to create a new SDP offer/answer
negotiated data channel for which there is an established SCTP
association, when it receives the SDP answer confirming acceptance
of the data channel or when it begins to receive data on the
data channel from the peer, whichever occurs first.</t>
</list>
</t>
<t>Note: DCEP is not used, that is neither the SDP offerer nor the SDP answerer send
an in-band DCEP DATA_CHANNEL_OPEN message.
</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 negotiated via SDP offer/answer, the data channel stack always performs an
SCTP 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 offerer SHOULD NOT change the port value for the "m" line (e.g. to zero)
when closing a data channel (unless all data channels are being
closed and the SCTP association is no longer needed), since this
would close the SCTP association and impact all of the data channels.
If the answerer accepts the SDP offer then the answerer MUST close those
data channels
whose "a=dcmap:" and "a=dcsa:" attribute lines were excluded from the received
SDP offer, unless those data channels were already closed,
and the answerer MUST also exclude the
corresponding attribute lines in the answer.
In addition to that,
the SDP answerer MAY exclude other data channels
which were closed but not yet
communicated to the peer. So, the offerer MUST inspect the answer to see if it
has to close other data channels which are now not included
in the answer.</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
SCTP SSN reset or some other reason) without associated SDP offer then
the client SHOULD generate an SDP offer which excludes this closed data channel.</t>
<t>The application MUST also close any data channel that was
negotiated via SDP offer/answer, for which the stream identifiers are not listed in
an incoming SDP offer.</t>
<t> A closed data channel using local close (SCTP SSN reset),
without an additional SDP offer/answer to close it,
may be reused for a new data 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 SSN reset of both incoming and outgoing
streams is completed). This restriction is to avoid the race conditions
between arrival of "SDP offer which reuses stream" with "SCTP SSN reset which
closes outgoing stream" at the peer.</t>
</section> <!-- Closing a Data Channel -->
<section title="Various SDP Offer/Answer Scenarios and Considerations"
anchor="various_SDP_OA_considerations">
<t>
<list>
<t>SDP offer has no "a=dcmap" attributes
<list style="symbols">
<t>Initial SDP offer:
No data channel is negotiated yet. The DTLS connection and SCTP association
is negotiated and, if agreed, established as per
<xref target="I-D.ietf-mmusic-sctp-sdp"/>.</t>
<t>Subsequent SDP offer:
All the SDP offer/answer negotiated data channels are expected be closed now.
The DTLS/SCTP association remains open for SDP offer/answer or DCEP
negotiation of data channels.</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 the offerer closes all the SDP offer/answer negotiated
data channels that were open at the time of initial offer.
The DTLS connection and SCTP association will still be setup.</t>
<t>Subsequent SDP answer:
All the SDP offer/answer negotiated data channels are expected be closed now.
The DTLS/SCTP association remains open for future SDP offer/answer or
DCEP negotiation of data channels.</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" anchor="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
SDP offer/answer negotiation option.
The SCTP association will still be setup over DTLS.
At this point the offerer or the answerer may use DCEP negotiation to open data
channels.</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 the SDP offer contains data channels for BFCP and
MSRP sub-protocols. The 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 the MSRP data channel
(after SCTP/DTLS association is setup).
The data channel with stream id 0 is free and can be used for future
DCEP or SDP offer/answer negotiation.</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 accepts the entire offer.
As a result the offerer closes the earlier negotiated MSRP related data channel
and both offerer and answerer may start using new the MSRP related data channel.</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">
<section title="Subprotocol Identifiers" anchor="IANA_subproto_ids">
<t>Registration of new subprotocol identifiers is performed using the existing IANA
table "WebSocket Subprotocol Name Registry".</t>
<t>The following text should be added following the title of the table.</t>
<t>"This table also includes subprotocol identifiers specified for usage within a
WebRTC data channel."</t>
<t>The following reference should be added to under the heading reference:
"RFC XXXX".</t>
<t>This document assigns no new values to this table.</t>
<t>NOTE to RFC Editor: Please replace "XXXX" with the number of this RFC.</t>
</section>
<section title="New SDP Attributes">
<section title="dcmap">
<t>[Editor's note: This section still needs to be completed.]</t>
</section>
<section title="dcsa">
<t>[Editor's note: This section still needs to be completed.]</t>
</section>
</section>
</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 Roni Even, 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-03'">
<t>
<list style="symbols">
<t>In <xref target="introduction"/> replacement of
"RTCWeb leaves it open for other applications to use data channels and its in-band
DCEP or out-of-band non-DCEP protocols for creating them" with
"... to use data channels and its in-band DCEP or other in-band or out-of-band
protocols for creating them".
Additionally replacement of
"In particular, the SDP offer generated by the application
includes no channel-specific information" with
"... generated by the RTCweb data channel stack includes
no channel-specific information".
</t>
<t>Move of former section 5 ("Data Channels") to new
<xref target="generic-out-of-band-aspects"/> and removal of JavaScript API specific
discussions from this moved text (like mentioning of data channel stack specific
states). Therefore former section 6 ("SDP Offer/Answer Negotiation") is now
<xref target="sec-sdp-ext-neg"/>.
</t>
<t>In <xref target="sec-sdp-ext-neg"/>:
<list style="symbols">
<t>Relacement of <xref target="sec-sdp-ext-neg"/>'s
first paragraph
"This section defines a method of non-DCEP 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 the SDP offer/answer model." with
"This section defines an SDP extension by which
two clients can negotiate data channel-specific and sub-protocol-specific
parameters without using DCEP <xref target="I-D.ietf-rtcweb-data-protocol"/>.
This SDP extension only defines usage in the context of SDP offer/answer."
</t>
<t>Addition of new paragraph:
"<xref target="generic-out-of-band-aspects"/> provides information how data channels
work in general and especially summarizes some key aspects, which should be
considered for the negotiation of data channels if DCEP is not used."
</t>
</list>
</t>
<t>In <xref target="subsec-sdp-attr-for-dc-par-neg"/> replacement of "The intention of
exchanging these attributes is to create data channels on both the peers with the same
set of attributes without actually using the DCEP
<xref target="I-D.ietf-rtcweb-data-protocol"/>" with "The intention in exchanging
these attributes is to create, on two peers, without use of DCEP
<xref target="I-D.ietf-rtcweb-data-protocol"/>,
matched pairs of oppositely directed data channels having the same set of attributes".
</t>
<t>In <xref target="max-retr-param-definition"/> replacement of "The 'max-retr'
parameter indicates the maximal number a user message will be retransmitted" with "The
'max-retr' parameter indicates the maximal number of times a user message will
be retransmitted".
</t>
<t>In <xref target="id_mngt"/> replacement of "However, an SDP offer/answer exchange
MUST NOT be initiated if the associated SCTP stream is already negotiated via DCEP"
with "However, an SCTP stream MUST NOT be referenced in a dcmap or dcsa
attribute of an SDP offer/answer exchange if the associated SCTP stream has already
been negotiated via DCEP".
</t>
<t>In the examples in <xref target="examples"/> addition of the previously missing
colons to the "a=sctp-port" attribute lines.
</t>
</list>
</t>
</section>
<section title="Changes against 'draft-ietf-mmusic-data-channel-sdpneg-02'">
<t>
<list style="symbols">
<t>Move of reference <xref target="I-D.ietf-rtcweb-jsep" /> from the list of normative
references to the list of informative references.</t>
<t>Addition of <xref target="IANA-SDP-Parameters" /> to the list of informative
references
and addition of following two sentences to the first paragraph after the ABNF
definition: "Note however that not all SDP attributes are suitable
as "a=dcsa:" parameter. <xref target="IANA-SDP-Parameters" /> contains
the lists of IANA registered session and media level or
media level only SDP attributes."</t>
<t>In the introduction replacement of last sentence
"This document defines SDP-based out-of-band negotiation procedures
to establish data channels for transport of well-defined sub-protocols"
with
"This document defines SDP offer/answer negotiation procedures
to establish data channels for transport of well-defined sub-protocols,
to enable out-of-band negotiation".
</t>
<t>Throughout the document replacement of "external negotiation" with
"SDP offer/answer negotiation" and removal of term "external negotiation" from the
terminology list in <xref target="terminology"/>.</t>
<t> Throughout the document replacement of "internal negotiation" with
"DCEP" and removal of terms "internal negotiation" and "in-band negotiation"
from the terminology list in <xref target="terminology"/>.</t>
<t>Addition of "SCTP Stream Sequence Number (SSN)" to the list of terms.</t>
<t>In <xref target="id_mngt"/> replacement of sentence
"However, a single stream is managed using one method at a time." with
"However, an SDP offer/answer exchange MUST NOT be initiated
if the associated SCTP stream is already negotiated via DCEP".</t>
<t>In <xref target="param_negotiation"/> replacement of sentence
"By definition max-retr and max-time are mutually exclusive, so only one of
them can be present in a=dcmap" with
"By definition max-retr and max-time are mutually exclusive,
so at most one of them MAY be present in a=dcmap".</t>
<t>Move of reference <xref target="WebRtcAPI"/> from list of normative references
to list of informative references.</t>
<t>Removal of almost all text parts, which discussed JavaScript or other API specific
aspects. Such API specific aspects were mainly discussed in sub-sections of Section 5
and <xref target="sec-sdp-ext-neg"/>
of draft-ietf-mmusic-data-channel-sdpneg-02.</t>
</list>
</t>
</section>
<section title="Changes against 'draft-ietf-mmusic-data-channel-sdpneg-01'">
<t>
<list style="symbols">
<t>New <xref target="appl_statement"/> regarding applicability to
SDP offer/answer only.</t>
<t>Addition of new <xref target="IANA_subproto_ids"/> "Subprotocol identifiers" as
subsection of the "IANA Considerations" related <xref target="IANA"/>.
Also removal of the temporary note
"To be completed. As [I-D.ietf-rtcweb-data-protocol] this document should refer to
IANA's WebSocket Subprotocol Name Registry defined in [RFC6455]."</t>
<t> In <xref target="param_negotiation"/>:
<list style="symbols">
<t>In the first paragraph
replacement of the sentence "If an SDP offer
contains both of these parameters then such an SDP offer will be rejected." with "If
an SDP offer contains both of these parameters then the receiver of such an SDP
offer MUST reject the SDP offer." </t>
<t>In the second paragraph
capitalization of "shall" and "may" such that both sentences now read:
"The SDP answer SHALL echo the same subprotocol, max-retr, max-time,
ordered parameters, if those were present in the offer, and MAY include
a label parameter. They MAY appear in any order, which could be
different from the SDP offer, in the SDP answer."</t>
<t>In the third paragraph
replacement of the sentence
"The same information MUST be replicated without changes in any subsequent offer or
answer, as long as the data channel is still opened at the time of offer or answer
generation." with
"When sending a subsequent offer or an answer, and for as long as the data channel
is still open, the sender MUST replicate the same information.".</t>
</list>
</t>
<t>In <xref target="param_negotiation"/> the mapping of data channel types defined in
<xref target="I-D.ietf-rtcweb-data-protocol"/> to the SDP "a=dcmap" attribute
parameters were illustrated using example "a=dcmap" attribute lines.
Replacement of these example "a=dcmap" attribute lines with just the "a=dcmap"
attribute parameters being relevant for the channel type.</t>
<t>In <xref target="various_SDP_OA_considerations"/> the description of bullet
point "SDP offer has no a=dcmap attributes - Initial SDP offer:" was
"Initial SDP offer: No data channel negotiated yet."
Replacement of this description with
"Initial SDP offer: No data channel is negotiated yet.
The DTLS connection and SCTP association is
negotiated and, if agreed, established as per
<xref target="I-D.ietf-mmusic-sctp-sdp"/>."</t>
<t>In <xref target="various_SDP_OA_considerations"/> in both bullet points related to
"Subsequent SDP offer" and "Subsequent SDP answer" replacement of
"All the externally negotiated data channels must be closed now." with
"All the externally negotiated data channels are expected be closed now.".</t>
<t>In <xref target="ext_open"/>'s sixth paragraph beginning with "[ASSUMPTION]"
replacement of the two occurrences of "must" with "MUST".</t>
<t>In <xref target="dcmap-attr-definition"/> in the definition of the ABNF rule
"dcmap-opt" there was a comment saying that
"Either only maxretr-opt or maxtime-opt is present. Both MUST not be present."
Removal of the second normative sentence and instead addition of following new
paragraph to the end of this section:
"Within an 'a=dcmap' attribute line's 'dcmap-opt' value either only one
'maxretr-opt' parameter or one 'maxtime-opt' parameter is present.
Both MUST NOT be present."</t>
<t>In <xref target="ordered-param-description"/> replacement of the first sentence
"The 'ordered' parameter with value "true" indicates that DATA chunks in the channel
MUST be dispatched to the upper layer by the receiver while preserving the order."
with
"The 'ordered' parameter with value "true" indicates that the receiver MUST dispatch
DATA chunks in the data channel to the upper layer while preserving the order.".</t>
<t>In <xref target="opendc"/>'s first paragraph replacement of the one occurrence of
"must" with "..., it MUST wait until ...".</t>
<t>In <xref target="close-dc"/>:
<list style="symbols">
<t>In the second paragraph replacement of "must" with "... whether this closing MUST
in addition ..."</t>
<t>In the third paragraph replacement of the sentence
"The port value for the "m" line SHOULD not be changed (e.g., to zero)
when closing a data channel ..." with
"The offerer SHOULD NOT change the port value for the "m" line (e.g., to zero) when
closing a data channel ...".</t>
<t>In the last but two paragraph replacement of the sentence
"... then an SDP offer which excludes this closed data channel SHOULD be generated."
with
"... then the client SHOULD generate an SDP offer which excludes this closed data
channel.".</t>
<t>In the last but one paragraph replacement of "must" with
"The application MUST also close...".</t>
</list>
</t>
<t>In <xref target="prot_att"/> addition of following note after the formal definition
of the 'a=dcsa' attribute:
"Note that the above reference to RFC 4566 defines were the
attribute definition can be found;
it does not provide any limitation on support of attributes
defined in other documents in accordance with this attribute
definition."</t>
</list>
</t>
</section>
<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 Section 5 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;
&DATAREQ;
&SDPSCTP;
</references>
<references title="Informative References">
&DATAPROTO;
&RFC4960;
&RFC4975;
&RFC4976;
&RFC5547;
&RFC6135;
&RFC6455;
&RFC6714;
&JSEP;
<reference
anchor="IANA-SDP-Parameters"
target="http://www.iana.org/assignments/sdp-parameters/sdp-parameters.xhtml">
<front>
<title>
Session Description Protocol (SDP) Parameters
</title>
<author/>
<date/>
</front>
<seriesInfo name="Internet Assigned Numbers Authority Protocol Assignments"
value="Session Description Protocol (SDP) Parameters" />
</reference>
<reference anchor="WebRtcAPI"
target="http://www.w3.org/TR/2015/WD-webrtc-20150210/">
<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="February"
day="10"
year="2015"/>
</front>
<seriesInfo name="World Wide Web Consortium"
value="WD-webrtc-20150210" />
<format type="HTML"
target="http://www.w3.org/TR/2015/WD-webrtc-20150210/" />
</reference>
</references>
<section title="Generic Data Channel Negotiation Aspects When Not Using DCEP"
anchor="generic-out-of-band-aspects">
<t>This appendix summarizes how data channels work in general and discusses
some key aspects, which should be considered for the out-of-band negotiation of data
channels if DCEP is not used.</t>
<t>A WebRTC application creates a data channel
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 negotiation using the DCEP
<xref target="I-D.ietf-rtcweb-data-protocol"/>, or if the application
intends to negotiate data channels using the SDP offer/answer protocol.</t>
<t>In any case, the SDP offer generated by the application 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:</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 application will only use "m" line format "webrtc-datachannel",
and will not use other formats in the "m" line for other protocols
such as t38.
<xref target="I-D.ietf-mmusic-sctp-sdp"/> supports only one SCTP
association to be established on top of a DTLS association.
</t>
<t>Note: Above SDP media description 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 data channel stack the stream
identifier to assign to the data channel or else let the data channel stack 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 with the DCEP, 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 negotiated via some protocol different from DCEP,
no convention is defined by default.</t>
</list>
</t>
</section>
<section title="Generic Data Channel Negotiation Not Using DCEP"
anchor="sec-gen-ext-neg">
<section title="Overview">
<t>DCEP negotiation only provides for negotiation of data channel
transport parameters and does not provide for negotiation of sub-protocol
specific parameters. DCEP-less data channel negotiation can be defined to
allow negotiation of parameters beyond those handled by DCEP,
e.g., parameters specific to the sub-protocol instantiated on a
particular data channel.</t>
<t>The following procedures are common to all methods of data channel
negotiation not using DCEP, whether in-band (communicated using proprietary means on
an already established data channel) or out-of-band (using SDP offer/answer or
some other protocol associated with the signaling channel).</t>
</section>
<section title="Opening a Data Channel" anchor="ext_open">
<t>In the case of DCEP-less negotiation, the endpoint application has the
option to fully control the stream identifier assignments. However
these assignments have to coexist with the assignments controlled by
the data channel stack for the DCEP negotiated data channels (if
any). It is the responsibility of the application to ensure consistent
assignment of stream identifiers.</t>
<t>When the application requests the creation of a new data channel to
be set up via DCEP-less negotiation, the data channel stack creates
the data channel locally without sending any DATA_CHANNEL_OPEN message
in-band.
However, even if the ICE, DTLS and SCTP procedures were already successfully
completed, the application can't send data on this data channel until the
negotiation is complete with the peer. This is because the peer needs
to be aware of and accept the usage of this data channel. The
peer, after accepting the data channel offer, can start sending data
immediately. This implies that the offerer may receive data channel
sub-protocol messages
before the 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 may be used on the fly as needed just by communicating
via non-DCEP means or by even having some local configuration/assumptions
on both the peers.</t>
<t>The application then negotiates the data channel
properties and sub-protocol properties with the peer's application
using a mechanism different from DCEP.</t>
<t>The peer then symmetrically creates a data channel
with these negotiated data channel properties. This is the only way
for the peer's data channel stack to know which properties to apply
when transmitting data on this channel.
The data channel stack must
allow data channel creation with any non-conflicting stream identifier
so that both peers can create the data channel with the same
stream identifier.
</t>
</section>
<section title="Closing a Data Channel">
<t>When the application requests the closing of a
data channel negotiated without DCEP,
the data channel stack always performs an SCTP SSN
reset for this channel.</t>
<t>Depending upon the method used for DCEP-less negotiation and the
sub-protocol associated with the data channel, the closing might in
addition be signaled to the peer via SDP offer/answer negotiation.</t>
</section>
</section> <!-- sec-gen-ext-neg -->
</section> <!-- generic-out-of-band-aspects -->
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-24 01:20:49 |