Subsequent SDP offer:
  m=application 10001 UDP/DTLS/SCTP webrtc-datachannel
  c=IN IP4 10.10.10.1
  a=max-message-size:100000
  a=sctp-port:5000
  a=setup:actpass
  a=connection:existing
  a=fingerprint:SHA-1 \
      4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
  a=dcmap:4 subprotocol="MSRP";label="MSRP"
  a=dcsa:4 accept-types:message/cpim text/plain
  a=dcsa:4 path:msrp://alice.example.com:10001/2s93i93idj;dc

Subsequent SDP answer:
  m=application 10002 UDP/DTLS/SCTP webrtc-datachannel
  c=IN IP4 10.10.10.2
  a=max-message-size:100000
  a=sctp-port:5002
  a=setup:passive
  a=connection:existing
  a=fingerprint:SHA-1 \
      5B:AD:67:B1:3E:82:AC:3B:90:02:B1:DF:12:5D:CA:6B:3F:E5:54:FA
  a=dcmap:4 subprotocol="MSRP";label="MSRP"
  a=dcsa:4 accept-types:message/cpim text/plain
  a=dcsa:4 path:msrp://bob.example.com:10002/si438dsaodes;dc

Figure 3: Example 3

The above example is a continuation of the example in Figure 1. The SDP offer now removes the MSRP data channel with stream id 2, but opens a new MSRP data channel with stream id 4. The answerer accepts the entire offer. As a result the offerer closes the earlier negotiated MSRP related data channel and both offerer and answerer may start using new the MSRP related data channel.

7. Security Considerations

No security considerations are envisaged beyond those already documented in [RFC4566].

8. IANA Considerations

8.1. Subprotocol Identifiers

Registration of new sub-protocol identifiers is performed using the existing IANA table "WebSocket Subprotocol Name Registry".

The following text should be added following the title of the table.

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

The following reference should be added to under the heading reference: "RFC XXXX".

This document assigns no new values to this table.

NOTE to RFC Editor: Please replace "XXXX" with the number of this RFC.

8.2. New SDP Attributes

8.2.1. dcmap

NOTE to RFC Editor: Please replace "XXXX" with the number of this RFC.

This document defines a new SDP media-level attribute "a=dcmap:" as follows:

Attribute name:	dcmap
Type of attribute:	media
Mux category:	SPECIAL
Charset Dependent:	No
Purpose:	Define data channel specific parameters
Appropriate values:	As defined in Section 5.1.1
Contact name:	MMUSIC Chairs
Contact e-mail:	mmusic-chairs@ietf.org
Reference:	RFCXXXX

8.2.2. dcsa

NOTE to RFC Editor: Please replace "XXXX" with the number of this RFC.

This document defines a new SDP media-level attribute "a=dcsa:" as follows:

Attribute name:	dcsa
Type of attribute:	media
Mux category:	SPECIAL
Charset Dependent:	No
Purpose:	Define data channel sub-protocol specific attributes
Appropriate values:	As defined in Section 5.1.2
Contact name:	MMUSIC Chairs
Contact e-mail:	mmusic-chairs@ietf.org
Reference:	RFCXXXX

9. Acknowledgments

The authors wish to acknowledge the borrowing of ideas from other internet drafts by Salvatore Loreto, Gonzalo Camarillo, Peter Dunkley and Gavin Llewellyn, and to thank Roni Even, Christian Groves, Gunnar Hellstrom, Christer Holmberg, Paul Kyzivat, Jonathan Lennox, and Uwe Rauschenbach for their invaluable comments.

10. CHANGE LOG

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

• In IANA registration Section 8.2.1 and Section 8.2.2 replacement of contact name and e-mail address with "MMUSIC Chairs" and "mmusic-chairs@ietf.org".
• In Section 5.1.2.1 replacement of "Thus in the example above, the original attribute line "a=accept- types:text/plain" is represented by the attribute line "a=dcsa:2 accept-types:text/plain", which specifies that this instance of MSRP being transported on the SCTP association using the data channel with stream id 2 accepts plain text files." with "... which specifies that this instance of the MSRP sub-protocol being transported ...".
• The last paragraph of Section 5.1.2.1 started with "Note: This document does not provide a complete specification ...". Removal of "Note:" and move of this paragraph to the introduction in Section 1 as last paragraph.
• Section 5.1.2's headline was "Sub-Protocol Specific Attributes". Change of this headline to "Other Media Level Attributes" and adaptations of the first paragraph of this section and the first paragraph of Section 5.1.2.1 in order to clarify that not only those attributes may be encapsulated in a "dcsa" attribute, which are specifically defined for the sub-protocol, but that also other attributes may be encapsulated if they are related to the specific sub-protocol instance.
• Move of the last but one paragraph of Section 5.1.2.1 starting with "The same syntax applies to ..." right in front of the formal syntax definition of the "dcsa" attribute.
• Modifications of the text in Section 5.1.1.2 and Section 5.1.2.2 in order not to explicitly restrict usage of the "a=dcmap:" and "a=dcsa:" attributes to "m" lines with proto values "UDP/DTLS/SCTP" or "TCP/DTLS/SCTP".

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

• In Section 5.1.1.5 the first (and only) paragraph was "The 'subprotocol' parameter indicates which protocol the client expects to exchange via the channel. 'Subprotocol' is an optional parameter. If the 'subprotocol' parameter is not present, then its value defaults to the empty string." Replacement of this paragraph with following two paragraphs:
  • The 'subprotocol' parameter indicates which protocol the client expects to exchange via the channel. This parameter maps to the 'Protocol' parameter defined in [I-D.ietf-rtcweb-data-protocol]. Section 8.1 specifies how new sub-protocol parameter values are registered. 'Subprotocol' is an optional parameter. If the 'subprotocol' parameter is not present, then its value defaults to the empty string.
  • Note: The empty string MAY also be explicitly used as 'subprotocol' value, such that 'subprotocol=""' is equivalent to the 'subprotocol' parameter not being present at all. [I-D.ietf-rtcweb-data-protocol] allows the DATA_CHANNEL_OPEN message's 'Subprotocol' value to be an empty string.

• Addition of [I-D.ietf-mmusic-sdp-mux-attributes] to list the of normative references.
• Addition of dcmap attribute specific IANA registration Section 8.2.1.
• Addition of dcsa attribute specific IANA registration Section 8.2.2.
• Addition of new Section 5.1.1.2 describing the mux category of the dcmap SDP attribute. This section and the new "a=dcsa:" attribute related mux category section are similar to the "Mux Category" sections of the "a=sctp-port:" and "a=max-message-size:" attributes in [I-D.ietf-mmusic-sctp-sdp].
• Addition of new Section 5.1.2.2 describing the mux category of the dcsa SDP attribute.

10.3. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-03'

• In Section 1 replacement of "RTCWeb leaves it open for other applications to use data channels and its in-band DCEP or out-of-band non-DCEP protocols for creating them" with "... to use data channels and its in-band DCEP or other in-band or out-of-band protocols for creating them". Additionally replacement of "In particular, the SDP offer generated by the application includes no channel-specific information" with "... generated by the RTCweb data channel stack includes no channel-specific information".
• Move of former section 5 ("Data Channels") to new Appendix A and removal of JavaScript API specific discussions from this moved text (like mentioning of data channel stack specific states). Therefore former section 6 ("SDP Offer/Answer Negotiation") is now Section 5.
• In Section 5:
  • Relacement of Section 5's first paragraph "This section defines a method of non-DCEP negotiation by which two clients can negotiate data channel-specific and 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 [I-D.ietf-rtcweb-data-protocol]. This SDP extension only defines usage in the context of SDP offer/answer."
  • Addition of new paragraph: "Appendix A provides information how data channels work in general and especially summarizes some key aspects, which should be considered for the negotiation of data channels if DCEP is not used."

• In Section 5.1.1 replacement of "The intention of exchanging these attributes is to create data channels on both the peers with the same set of attributes without actually using the DCEP [I-D.ietf-rtcweb-data-protocol]" with "The intention in exchanging these attributes is to create, on two peers, without use of DCEP [I-D.ietf-rtcweb-data-protocol], matched pairs of oppositely directed data channels having the same set of attributes".
• In Section 5.1.1.6 replacement of "The 'max-retr' parameter indicates the maximal number a user message will be retransmitted" with "The 'max-retr' parameter indicates the maximal number of times a user message will be retransmitted".
• In Section 5.2.1 replacement of "However, an SDP offer/answer exchange MUST NOT be initiated if the associated SCTP stream is already negotiated via DCEP" with "However, an SCTP stream MUST NOT be referenced in a dcmap or dcsa attribute of an SDP offer/answer exchange if the associated SCTP stream has already been negotiated via DCEP".
• In the examples in Section 6 addition of the previously missing colons to the "a=sctp-port" attribute lines.

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

• Move of reference [I-D.ietf-rtcweb-jsep] from the list of normative references to the list of informative references.
• Addition of [IANA-SDP-Parameters] to the list of informative references and addition of following two sentences to the first paragraph after the ABNF definition: "Note however that not all SDP attributes are suitable as "a=dcsa:" parameter. [IANA-SDP-Parameters] contains the lists of IANA registered session and media level or media level only SDP attributes."
• 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".
• Throughout the document replacement of "external negotiation" with "SDP offer/answer negotiation" and removal of term "external negotiation" from the terminology list in Section 3.
• Throughout the document replacement of "internal negotiation" with "DCEP" and removal of terms "internal negotiation" and "in-band negotiation" from the terminology list in Section 3.
• Addition of "SCTP Stream Sequence Number (SSN)" to the list of terms.
• In Section 5.2.1 replacement of sentence "However, a single stream is managed using one method at a time." with "However, an SDP offer/answer exchange MUST NOT be initiated if the associated SCTP stream is already negotiated via DCEP".
• In Section 5.2.2 replacement of sentence "By definition max-retr and max-time are mutually exclusive, so only one of them can be present in a=dcmap" with "By definition max-retr and max-time are mutually exclusive, so at most one of them MAY be present in a=dcmap".
• Move of reference [WebRtcAPI] from list of normative references to list of informative references.
• Removal of almost all text parts, which discussed JavaScript or other API specific aspects. Such API specific aspects were mainly discussed in sub-sections of Section 5 and Section 5 of draft-ietf-mmusic-data-channel-sdpneg-02.

10.5. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-01'

• New Section 4 regarding applicability to SDP offer/answer only.
• Addition of new Section 8.1 "Subprotocol identifiers" as subsection of the "IANA Considerations" related Section 8. Also removal of the temporary note "To be completed. As [I-D.ietf-rtcweb-data-protocol] this document should refer to IANA's WebSocket Subprotocol Name Registry defined in [RFC6455]."
• In Section 5.2.2:
  • In the first paragraph replacement of the sentence "If an SDP offer contains both of these parameters then such an SDP offer will be rejected." with "If an SDP offer contains both of these parameters then the receiver of such an SDP offer MUST reject the SDP offer."
  • In the second paragraph capitalization of "shall" and "may" such that both sentences now read: "The SDP answer SHALL echo the same subprotocol, max-retr, max-time, ordered parameters, if those were present in the offer, and MAY include a label parameter. They MAY appear in any order, which could be different from the SDP offer, in the SDP answer."
  • In the third paragraph replacement of the sentence "The same information MUST be replicated without changes in any subsequent offer or answer, as long as the data channel is still opened at the time of offer or answer generation." with "When sending a subsequent offer or an answer, and for as long as the data channel is still open, the sender MUST replicate the same information.".

• In Section 5.2.2 the mapping of data channel types defined in [I-D.ietf-rtcweb-data-protocol] to the SDP "a=dcmap" attribute parameters were illustrated using example "a=dcmap" attribute lines. Replacement of these example "a=dcmap" attribute lines with just the "a=dcmap" attribute parameters being relevant for the channel type.
• In Section 5.2.5 the description of bullet point "SDP offer has no a=dcmap attributes - Initial SDP offer:" was "Initial SDP offer: No data channel negotiated yet." Replacement of this description with "Initial SDP offer: No data channel is negotiated yet. The DTLS connection and SCTP association is negotiated and, if agreed, established as per [I-D.ietf-mmusic-sctp-sdp]."
• In Section 5.2.5 in both bullet points related to "Subsequent SDP offer" and "Subsequent SDP answer" replacement of "All the externally negotiated data channels must be closed now." with "All the externally negotiated data channels are expected be closed now.".
• In Appendix A.2.2's sixth paragraph beginning with "[ASSUMPTION]" replacement of the two occurrences of "must" with "MUST".
• In Section 5.1.1.1 in the definition of the ABNF rule "dcmap-opt" there was a comment saying that "Either only maxretr-opt or maxtime-opt is present. Both MUST not be present." Removal of the second normative sentence and instead addition of following new paragraph to the end of this section: "Within an 'a=dcmap' attribute line's 'dcmap-opt' value either only one 'maxretr-opt' parameter or one 'maxtime-opt' parameter is present. Both MUST NOT be present."
• In Section 5.1.1.8 replacement of the first sentence "The 'ordered' parameter with value "true" indicates that DATA chunks in the channel MUST be dispatched to the upper layer by the receiver while preserving the order." with "The 'ordered' parameter with value "true" indicates that the receiver MUST dispatch DATA chunks in the data channel to the upper layer while preserving the order.".
• In Section 5.2.3's first paragraph replacement of the one occurrence of "must" with "..., it MUST wait until ...".
• In Section 5.2.4:
  • In the second paragraph replacement of "must" with "... whether this closing MUST in addition ..."
  • In the third paragraph replacement of the sentence "The port value for the "m" line SHOULD not be changed (e.g., to zero) when closing a data channel ..." with "The offerer SHOULD NOT change the port value for the "m" line (e.g., to zero) when closing a data channel ...".
  • In the last but two paragraph replacement of the sentence "... then an SDP offer which excludes this closed data channel SHOULD be generated." with "... then the client SHOULD generate an SDP offer which excludes this closed data channel.".
  • In the last but one paragraph replacement of "must" with "The application MUST also close...".

• In Section 5.1.2 addition of following note after the formal definition of the 'a=dcsa' attribute: "Note that the above reference to RFC 4566 defines were the attribute definition can be found; it does not provide any limitation on support of attributes defined in other documents in accordance with this attribute definition."

10.6. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-00'

• In Section 3 "WebRTC data channel" was defined as "A bidirectional channel consisting of paired SCTP outbound and inbound streams." Replacement of this definition with "Data channel: A WebRTC data channel as specified in [I-D.ietf-rtcweb-data-channel]", and consistent usage of "data channel" in the remainder of the document including the document's headline."
• In Section 5 removal of following note: 'OPEN ISSUE: The syntax in [I-D.ietf-mmusic-sctp-sdp] may change as that document progresses. In particular we expect "webrtc-datachannel" to become a more general term.'
• Consistent usage of '"m" line' in whole document as per [RFC4566].
• In Section 5.1.1 removal of the example dcmap attribute line 'a=dcmap:2 subprotocol="BFCP";label="channel 2' as there are already four examples right after the ABNF rules in Section 5.1.1.1. Corresponding removal of following related note: "Note: This document does not provide a complete specification of how to negotiate the use of a WebRTC data channel to transport BFCP. Procedures specific to each 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."
• In Section 5.1.1 removal of following note: "Note: This attribute is derived from attribute "webrtc-DataChannel", which was defined in old version 03 of the following draft, but which was removed along with any support for SDP external negotiation in subsequent versions: [I-D.ietf-mmusic-sctp-sdp]."
• Insertion of following new sentence to the beginning of Section 5.1.1.1: "dcmap is a media level attribute having following ABNF syntax:"
• Insertion of new Section 5.1.1.3 containing the dcmap-stream-id specifying sentence, which previously was placed right before the formal ABNF rules. Removal of the sentence 'Stream is a mandatory parameter and is noted directly after the "a=dcmap:" attribute's colon' as this information is part of the ABNF specification.
• In Section 5.1.1.1 modification of the 'ordering-value' values from "0" or "1" to "true" or "false". Corresponding text modifications in Section 5.1.1.8.
• In Section 5.1.1.1 the ABNF definition of "quoted-string" referred to rule name "escaped-char", which was not defined. Instead a rule with name "escaped" was defined. Renamed that rule's name to "escaped-char".
• Insertion of a dedicated note right after the "a=dcmap:4" attribute example in Section 5.1.1.1 regarding the non-printable "escaped-char" character within the "label" value.
• In Section 5.1.2's second paragraph replacement of "sctp stream identifier" with "SCTP stream identifier".
• In first paragraph of Section 5.2.1 replacement of first two sentences 'For the SDP-based external negotiation described in this document, the initial offerer based "SCTP over DTLS" owns by convention the even stream identifiers whereas the initial answerer owns the odd stream identifiers. This ownership is 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.'
• In Section 5.2.3 the first action of an SDP answerer, when receiving an SDP offer, was described as "Applies the SDP offer. Note that the browser ignores data channel specific attributes in the SDP." Replacement of these two sentences with "Parses and applies the SDP offer. Note that the typical parser normally ignores unknown SDP attributes, which includes data channel related attributes."
• In Section 5.2.3 the second sentence of the third SDP answerer action was "Note that the browser is asked to create data channels with stream identifiers not "owned" by the agent.". Replacement of this sentence with "Note that the agent is asked to create data channels with SCTP stream identifiers contained in the SDP offer if the SDP offer is accepted."
• In Section 5.2.4 the third paragraph began with "A data channel can be closed by sending a new SDP offer which excludes the dcmap and dcsa attribute lines for the data channel. The port value for the m line should not be changed (e.g., to zero) when closing a data channel (unless all data channels are being closed and the SCTP association is no longer needed), since this would close the SCTP association and impact all of the data channels. If the answerer accepts the SDP offer then it MUST also exclude the corresponding attribute lines in the answer. ..." Replacement of this part with "The intention to close a data channel can be signaled by sending a new SDP offer which excludes the "a=dcmap:" and "a=dcsa:" attribute lines for the data channel. The port value for the "m" line SHOULD not be changed (e.g., to zero) when closing a data channel (unless all data channels are being closed and the SCTP association is no longer needed), since this would close the SCTP association and impact all of the data channels. If the answerer accepts the SDP offer then it MUST close those data channels whose "a=dcmap:" and "a=dcsa:" attribute lines were excluded from the received SDP offer, unless those data channels were already closed, and it MUST also exclude the corresponding attribute lines in the answer."
• In Section 5.2.4 the hanging text after the third paragraph was "This delayed close is to handle cases where a successful SDP answer is not received, in which case the state of session should be kept per the last successful SDP offer/answer." Replacement of this sentence with "This delayed closure is RECOMMENDED in order to handle cases where a successful SDP answer is not received, in which case the state of the session SHOULD be kept per the last successful SDP offer/answer."
• Although dedicated to "a=dcmap" and "a=dcsa" SDP syntax aspects Section 5.1.1 contained already procedural descriptions related to data channel reliability negotiation. Creation of new Section 5.2.2 and moval of reliability negotiation related text to this new section.

10.7. Changes against 'draft-ejzak-mmusic-data-channel-sdpneg-02'

• Removal of note "[ACTION ITEM]" from section "subprotocol parameter". As [I-D.ietf-rtcweb-data-protocol] this document should refer to IANA's WebSocket Subprotocol Name Registry defined in [RFC6455].
• In whole document, replacement of "unreliable" with "partially reliable", which is used in [I-D.ietf-rtcweb-data-channel] and in [I-D.ietf-rtcweb-data-protocol] in most places.
• Clarification of the semantic if the "max-retr" parameter is not present in an a=dcmap attribute line. In section "max-retr parameter" the sentence "The max-retr parameter is optional with default value unbounded" was replaced with "The max-retr parameter is optional. If the max-retr parameter is not present, then the maximal number of retransmissions is determined as per the generic SCTP retransmission rules as specified in [RFC4960]".
• Clarification of the semantic if the "max-time" parameter is not present in an a=dcmap attribute line. In section "max-time parameter" the sentence "The max-time parameter is optional with default value unbounded" was replaced with "The max-time parameter is optional. If the max-time parameter is not present, then the generic SCTP retransmission timing rules apply as specified in [RFC4960]".
• In section "label parameter" the sentence "Label is a mandatory parameter." was removed and following new sentences (including the note) were added: "The 'label' parameter is optional. If it is not present, then its value defaults to the empty string. Note: The empty string may also be explicitly used as 'label' value, such that 'label=""' is equivalent to the 'label' parameter not being present at all. [I-D.ietf-rtcweb-data-protocol] allows the DATA_CHANNEL_OPEN message's 'Label' value to be an empty string."
• 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."
• In the "Examples" section, in the first two SDP offer examples in the a=dcmap attribute lines 'label="BGCP"' was replaced with 'label="BFCP"'.
• In all examples, the "m" line proto value "DTLS/SCTP" was replaced with "UDP/DTLS/SCTP" and the "a=fmtp" attribute lines were replaced with "a=max-message-size" attribute lines, as per draft-ietf-mmusic-sctp-sdp-12.

10.8. Changes against '-01'

• Formal syntax for dcmap and dcsa attribute lines.
• Making subprotocol as an optional parameter in dcmap.
• Specifying disallowed parameter combinations for max-time and max-retr.
• Clarifications on WebRTC data channel close procedures.

10.9. Changes against '-00'

• Revisions to identify difference between internal and external negotiation and their usage.
• Introduction of more generic terminology, e.g. "application" instead of "browser".
• Clarification of how "max-retr and max-time affect the usage of unreliable and reliable WebRTC data channels.
• Updates of examples to take into account the SDP syntax changes introduced with draft-ietf-mmusic-sctp-sdp-07.
• Removal of the SCTP port number from the a=dcmap and a=dcsa attributes as this is now contained in the a=sctp-port attribute, and as draft-ietf-mmusic-sctp-sdp-07 supports only one SCTP association on top of the DTLS connection.

11. References

11.1. Normative References

[RFC2119]	Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC4566]	Handley, M., Jacobson, V. and C. Perkins, "SDP: Session Description Protocol", RFC 4566, DOI 10.17487/RFC4566, July 2006.
[RFC3264]	Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, DOI 10.17487/RFC3264, June 2002.
[RFC5234]	Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008.
[I-D.ietf-rtcweb-data-channel]	Jesup, R., Loreto, S. and M. Tuexen, "WebRTC Data Channels", Internet-Draft draft-ietf-rtcweb-data-channel-13, January 2015.
[I-D.ietf-mmusic-sctp-sdp]	Holmberg, C., Loreto, S. and G. Camarillo, "Stream Control Transmission Protocol (SCTP)-Based Media Transport in the Session Description Protocol (SDP)", Internet-Draft draft-ietf-mmusic-sctp-sdp-15, September 2015.
[I-D.ietf-mmusic-sdp-mux-attributes]	Nandakumar, S., "A Framework for SDP Attributes when Multiplexing", Internet-Draft draft-ietf-mmusic-sdp-mux-attributes-10, July 2015.

11.2. Informative References

, "

[I-D.ietf-rtcweb-data-protocol]	Jesup, R., Loreto, S. and M. Tuexen, "WebRTC Data Channel Establishment Protocol", Internet-Draft draft-ietf-rtcweb-data-protocol-09, January 2015.
[RFC4960]	Stewart, R., "Stream Control Transmission Protocol", RFC 4960, DOI 10.17487/RFC4960, September 2007.
[RFC4975]	Campbell, B., Mahy, R. and C. Jennings, "The Message Session Relay Protocol (MSRP)", RFC 4975, DOI 10.17487/RFC4975, September 2007.
[RFC4976]	Jennings, C., Mahy, R. and A. Roach, "Relay Extensions for the Message Sessions Relay Protocol (MSRP)", RFC 4976, DOI 10.17487/RFC4976, September 2007.
[RFC5547]	Garcia-Martin, M., Isomaki, M., Camarillo, G., Loreto, S. and P. Kyzivat, "A Session Description Protocol (SDP) Offer/Answer Mechanism to Enable File Transfer", RFC 5547, DOI 10.17487/RFC5547, May 2009.
[RFC6135]	Holmberg, C. and S. Blau, "An Alternative Connection Model for the Message Session Relay Protocol (MSRP)", RFC 6135, DOI 10.17487/RFC6135, February 2011.
[RFC6455]	Fette, I. and A. Melnikov, "The WebSocket Protocol", RFC 6455, DOI 10.17487/RFC6455, December 2011.
[RFC6714]	Holmberg, C., Blau, S. and E. Burger, "Connection Establishment for Media Anchoring (CEMA) for the Message Session Relay Protocol (MSRP)", RFC 6714, DOI 10.17487/RFC6714, August 2012.
[I-D.ietf-rtcweb-jsep]	Uberti, J., Jennings, C. and E. Rescorla, Javascript Session Establishment Protocol", Internet-Draft draft-ietf-rtcweb-jsep-12, October 2015.
[IANA-SDP-Parameters]	Session Description Protocol (SDP) Parameters", Internet Assigned Numbers Authority Protocol Assignments Session Description Protocol (SDP) Parameters
[WebRtcAPI]	Bergkvist, A., Burnett, D., Jennings, C. and A. Narayanan, "WebRTC 1.0: Real-time Communication Between Browsers", World Wide Web Consortium WD-webrtc-20150210, February 2015.

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

This appendix summarizes how data channels work in general and discusses some key aspects, which should be considered for the out-of-band negotiation of data channels if DCEP is not used.

A WebRTC application creates a data channel by providing a number of setup parameters (sub-protocol, label, reliability, order of delivery, priority). The application also specifies if it wants to make use of the negotiation using the DCEP [I-D.ietf-rtcweb-data-protocol], or if the application intends to negotiate data channels using the SDP offer/answer protocol.

In any case, the SDP offer generated by the application is per [I-D.ietf-mmusic-sctp-sdp]. In brief, it contains one "m" line for the SCTP association on top of which data channels will run:

m=application 54111 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 79.97.215.79
a=max-message-size:100000
a=sctp-port:5000
a=setup:actpass
a=connection:new
a=fingerprint:SHA-1 \
    4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB

Note: A WebRTC application will only use "m" line format "webrtc-datachannel", and will not use other formats in the "m" line for other protocols such as t38. [I-D.ietf-mmusic-sctp-sdp] supports only one SCTP association to be established on top of a DTLS association.

Note: Above SDP media description does not contain any channel-specific information.

A.1. Stream Identifier Numbering

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

To avoid glare situations, each endpoint can moreover own an exclusive set of stream identifiers, in which case an endpoint can only create a data channel with a stream identifier it owns.

Which set of stream identifiers is owned by which endpoint is determined by convention or other means.

For data channels negotiated with the DCEP, one endpoint owns by convention the even stream identifiers, whereas the other owns the odd stream identifiers, as defined in [I-D.ietf-rtcweb-data-protocol].

For data channels negotiated via some protocol different from DCEP, no convention is defined by default.

A.2. Generic Data Channel Negotiation Not Using DCEP

A.2.1. Overview

DCEP negotiation only provides for negotiation of data channel transport parameters and does not provide for negotiation of 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.

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).

A.2.2. Opening a Data Channel

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

When the application requests the creation of a new data channel to be set up via DCEP-less negotiation, the data channel stack creates the data channel locally without sending any DATA_CHANNEL_OPEN message in-band. However, even if the ICE, 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.

If the peer rejects the data channel part of the offer then it doesn't have to do anything as the data channel was not created using the stack. The offerer on the other hand needs to close the data channel that was opened by invoking relevant data channel stack API procedures.

It is also worth noting that a data channel stack implementation may not provide any API to create and close data channels; instead the data channels may be used on the fly as needed just by communicating via non-DCEP means or by even having some local configuration/assumptions on both the peers.

The application then negotiates the data channel properties and sub-protocol properties with the peer's application using a mechanism different from DCEP.

The peer then symmetrically creates a data channel with these negotiated data channel properties. This is the only way for the peer's data channel stack to know which properties to apply when transmitting data on this channel. The data channel stack must allow data channel creation with any non-conflicting stream identifier so that both peers can create the data channel with the same stream identifier.

A.2.3. Closing a Data Channel

When the application requests the closing of a data channel negotiated without DCEP, the data channel stack always performs an SCTP SSN reset for this channel.

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

Authors' Addresses