MMUSIC K. Drage Internet-Draft Unaffiliated Intended status: Standards Track M. Makaraju Expires: October 25, 2019 Nokia R. Ejzak J. Marcon Unaffiliated R. Even, Ed. Huawei April 23, 2019 SDP-based Data Channel Negotiation draft-ietf-mmusic-data-channel-sdpneg-26 Abstract Data channel setup can be done using either the in-band Data Channel Establishment Protocol (DCEP) or using some out-of-band non-DCEP protocol. This document specifies how the SDP (Session Description Protocol) offer/answer exchange can be used to achieve an out-of-band non-DCEP negotiation for establishing a data channel. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on October 25, 2019. Copyright Notice Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of Drage, et al. Expires October 25, 2019 [Page 1] Internet-Draft SDP-based Data Channel Negotiation April 2019 publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 4. Applicability Statement . . . . . . . . . . . . . . . . . . . 5 5. SDP Data Channel Attributes . . . . . . . . . . . . . . . . . 6 5.1. SDP DCMAP Attribute . . . . . . . . . . . . . . . . . . . 6 5.1.1. DCMAP Attribute Syntax . . . . . . . . . . . . . . . 6 5.1.2. Dcmap-stream-id Parameter . . . . . . . . . . . . . . 8 5.1.3. Label Parameter . . . . . . . . . . . . . . . . . . . 8 5.1.4. Subprotocol Parameter . . . . . . . . . . . . . . . . 8 5.1.5. Max-retr Parameter . . . . . . . . . . . . . . . . . 9 5.1.6. Max-time Parameter . . . . . . . . . . . . . . . . . 9 5.1.7. Ordered Parameter . . . . . . . . . . . . . . . . . . 9 5.1.8. Priority Parameter . . . . . . . . . . . . . . . . . 9 5.1.9. DCMAP Multiplexing Category . . . . . . . . . . . . . 10 5.2. SDP DCSA Attribute . . . . . . . . . . . . . . . . . . . 10 5.2.1. DCSA Syntax . . . . . . . . . . . . . . . . . . . . . 11 5.2.2. DCSA Multiplexing Category . . . . . . . . . . . . . 12 6. SDP Offer/Answer Procedures . . . . . . . . . . . . . . . . . 12 6.1. Managing Stream Identifiers . . . . . . . . . . . . . . . 13 6.2. Negotiating Data Channel Parameters . . . . . . . . . . . 13 6.3. Generating the Initial Offer for A Data Channel . . . . . 14 6.4. Generating SDP Answer . . . . . . . . . . . . . . . . . . 14 6.5. Offerer Processing of the SDP Answer . . . . . . . . . . 15 6.6. Modifying the Session . . . . . . . . . . . . . . . . . . 15 6.6.1. Closing a Data Channel . . . . . . . . . . . . . . . 16 6.7. Various SDP Offer/Answer Considerations . . . . . . . . . 16 7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 17 8. Security Considerations . . . . . . . . . . . . . . . . . . . 19 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 9.1. Subprotocol Identifiers . . . . . . . . . . . . . . . . . 20 9.2. New SDP Attributes . . . . . . . . . . . . . . . . . . . 20 9.2.1. dcmap . . . . . . . . . . . . . . . . . . . . . . . . 20 9.2.2. dcsa . . . . . . . . . . . . . . . . . . . . . . . . 21 10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 21 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21 12. CHANGE LOG . . . . . . . . . . . . . . . . . . . . . . . . . 22 12.1. Changes against 'draft-ietf-mmusic-data-channel- sdpneg-15' . . . . . . . . . . . . . . . . . . . . . . . 22 Drage, et al. Expires October 25, 2019 [Page 2] Internet-Draft SDP-based Data Channel Negotiation April 2019 12.2. Changes against 'draft-ietf-mmusic-data-channel- sdpneg-14' . . . . . . . . . . . . . . . . . . . . . . . 22 12.3. Changes against 'draft-ietf-mmusic-data-channel- sdpneg-12' . . . . . . . . . . . . . . . . . . . . . . . 22 12.4. Changes against 'draft-ietf-mmusic-data-channel- sdpneg-11' . . . . . . . . . . . . . . . . . . . . . . . 22 12.5. Changes against 'draft-ietf-mmusic-data-channel- sdpneg-10' . . . . . . . . . . . . . . . . . . . . . . . 22 12.6. Changes against 'draft-ietf-mmusic-data-channel- sdpneg-09' . . . . . . . . . . . . . . . . . . . . . . . 22 12.7. Changes against 'draft-ietf-mmusic-data-channel- sdpneg-08' . . . . . . . . . . . . . . . . . . . . . . . 23 12.8. Changes against 'draft-ietf-mmusic-data-channel- sdpneg-07' . . . . . . . . . . . . . . . . . . . . . . . 24 12.9. Changes against 'draft-ietf-mmusic-data-channel- sdpneg-06' . . . . . . . . . . . . . . . . . . . . . . . 24 12.10. Changes against 'draft-ietf-mmusic-data-channel- sdpneg-05' . . . . . . . . . . . . . . . . . . . . . . . 24 12.11. Changes against 'draft-ietf-mmusic-data-channel- sdpneg-04' . . . . . . . . . . . . . . . . . . . . . . . 25 12.12. Changes against 'draft-ietf-mmusic-data-channel- sdpneg-03' . . . . . . . . . . . . . . . . . . . . . . . 26 12.13. Changes against 'draft-ietf-mmusic-data-channel- sdpneg-02' . . . . . . . . . . . . . . . . . . . . . . . 27 12.14. Changes against 'draft-ietf-mmusic-data-channel- sdpneg-01' . . . . . . . . . . . . . . . . . . . . . . . 28 12.15. Changes against 'draft-ietf-mmusic-data-channel- sdpneg-00' . . . . . . . . . . . . . . . . . . . . . . . 30 12.16. Changes against 'draft-ejzak-mmusic-data-channel- sdpneg-02' . . . . . . . . . . . . . . . . . . . . . . . 33 12.17. Changes against '-01' . . . . . . . . . . . . . . . . . 34 12.18. Changes against '-00' . . . . . . . . . . . . . . . . . 34 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 34 13.1. Normative References . . . . . . . . . . . . . . . . . . 34 13.2. Informative References . . . . . . . . . . . . . . . . . 36 Appendix A. Generic Data Channel Negotiation Aspects When Not Using DCEP . . . . . . . . . . . . . . . . . . . . . 36 A.1. Stream Identifier Numbering . . . . . . . . . . . . . . . 37 A.2. Generic Data Channel Negotiation Not Using DCEP . . . . . 38 A.2.1. Overview . . . . . . . . . . . . . . . . . . . . . . 38 A.2.2. Opening a Data Channel . . . . . . . . . . . . . . . 38 A.2.3. Closing a Data Channel . . . . . . . . . . . . . . . 39 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39 Drage, et al. Expires October 25, 2019 [Page 3] Internet-Draft SDP-based Data Channel Negotiation April 2019 1. Introduction The concept of establishing a bi-directional data channel running on top of the Stream Control Transmission Protocol (SCTP) is in [I-D.ietf-rtcweb-data-channel] allowing applications to use data channels. An in-band Data Channel Establishment Protocol (DCEP) is in [I-D.ietf-rtcweb-data-protocol], however other in-band or out-of- band protocols may be used for establishing data channels. Each data channel consists of paired SCTP streams sharing the same SCTP Stream Identifier. Data channels are created by endpoint applications using the WebRTC API (Application Programming Interface), or other protocols like CLUE [I-D.ietf-clue-datachannel]. The protocols can be signaled by the data channel "subprotocol" parameter, conceptually similar to the WebSocket [RFC5234] "subprotocol". However, apart from the "subprotocol" value transmitted to the peer, an endpoint application can agree on how to instantiate a given subprotocol on a data channel, and whether it is signaled in-band using DCEP or out- of-band using a non-DCEP protocol (or both). This document defines SDP offer/answer [RFC3264] procedures that enable out-of-band negotiation for establishing data channels for transport of well-defined subprotocols. These procedures are based on generic SDP offer/answer negotiation rules for SCTP based media transport as specified in [I-D.ietf-mmusic-sctp-sdp] for the SDP "m" line proto values UDP/DTLS/SCTP and TCP/DTLS/SCTP. This document uses MSRP (Message Session Relay Protocol) [RFC4975] and BFCP (Binary Floor Control Protocol) [RFC4582] in many of the examples. It does not provide a complete specification of how to negotiate the use of a data channel to transport MSRP. Procedures specific to each subprotocol would have to be documented elsewhere. For MSRP they are documented in [I-D.ietf-mmusic-msrp-usage-data-channel] . The use of MSRP in some examples is only to show how the generic procedures described herein might apply to a specific subprotocol. 2. Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119] [RFC8174] 3. Terminology This document uses the following terms: Data channel: A WebRTC data channel as specified in [I-D.ietf-rtcweb-data-channel]. Drage, et al. Expires October 25, 2019 [Page 4] Internet-Draft SDP-based Data Channel Negotiation April 2019 Data channel stack: An entity which, upon application request, runs the data channel protocol to keep track of states, sending and receiving data. If the application is a browser based JavaScript application then this stack resides in the browser. If the application is a native application then this stack resides in the application and is accessible via some sort of APIs. Data channel properties: Fixed properties assigned to a data channel at the time of its creation. Some of these properties determine the way the data channel stack transmits data on this channel (e.g., stream identifier, reliability, order of delivery, etc.). Data channel subprotocol: The application protocol which is transported over a single data channel. Data channel subprotocol messages are sent as data channel payload over an established data channel. SDP offer/answer exchange can be used as specified in this document to negotiate the establishment of data channels, corresponding data channel properties, associated data channel subprotocols and data channel subprotocol properties. In this case the data channel subprotocols may be identified by the values of the "subprotocol" parameters of the SDP "a=dcmap" attribute as described in Section 5.1.4. Within this document the term "data channel subprotocol" is often abbreviated as just "subprotocol". DCEP: Data Channel Establishment Protocol defined in [I-D.ietf-rtcweb-data-protocol]. In-band: Transmission through the peer-to-peer SCTP association. Out-of-band: Transmission through the application signaling path. Peer: From the perspective of one of the agents in a session, its peer is the other agent. Specifically, from the perspective of the SDP offerer, the peer is the SDP answerer. From the perspective of the SDP answerer, the peer is the SDP offerer. SCTP Stream Sequence Number (SSN): the SCTP stream sequence number as specified in [RFC4960]. Stream identifier: The identifier of the outbound and inbound SCTP streams composing a data channel. 4. Applicability Statement The mechanism in this document only applies to the Session Description Protocol (SDP) [I-D.ietf-mmusic-rfc4566bis] when used Drage, et al. Expires October 25, 2019 [Page 5] Internet-Draft SDP-based Data Channel Negotiation April 2019 together with the SDP offer/answer mechanism [RFC3264]. Declarative usage of SDP is out of scope of this document, and is thus undefined. 5. SDP Data Channel Attributes This section defines two new SDP media-level attributes that can be used together with the SDP Offer/Answer mechanism to negotiate data channel-specific and subprotocol-specific parameters without the usage of DCEP [I-D.ietf-rtcweb-data-protocol]. The first attribute provides for negotiation of channel-specific parameters. The second attribute provides for negotiation of subprotocol-specific parameters. Note: Appendix A provides information how data channels work in general and especially summarizes some key aspects, which should be considered for the negotiation of data channels if DCEP is not used. 5.1. SDP DCMAP Attribute This section defines a new media level attribute "a=dcmap:" that defines the data channel parameters for each data channel to be negotiated. The attribute is used to create bi-directional SCTP data channels having the same set of attributes. The data channel properties (reliable/partially reliable, ordered/unordered) need to be suitable per the subprotocol transport requirements. 5.1.1. DCMAP Attribute Syntax "a=dcmap:" is a media level attribute having the following ABNF (Augmented Backus-Naur Form, [RFC5234]) syntax. Formal Syntax: Name: dcmap Value: dcmap-value Usage Level: media Charset Dependent: no Syntax: dcmap-value = dcmap-stream-id [ SP dcmap-opt *(";" dcmap-opt) ] dcmap-opt = ordering-opt / subprotocol-opt / label-opt Drage, et al. Expires October 25, 2019 [Page 6] Internet-Draft SDP-based Data Channel Negotiation April 2019 / maxretr-opt / maxtime-opt / priority-opt ; maxretr-opt and maxtime-opt are mutually exclusive ; dcmap-stream-id = 1*5DIGIT ordering-opt = "ordered=" ordering-value ordering-value = "true" / "false" subprotocol-opt = "subprotocol=" quoted-string label-opt = "label=" quoted-string maxretr-opt = "max-retr=" maxretr-value maxretr-value = "0" / integer ; number of retransmissions, ; less than 2^32, ; derived from 'Reliability Parameter' of ; [I-D.ietf-rtcweb-data-protocol] maxtime-opt = "max-time=" maxtime-value maxtime-value = "0" / integer ; milliseconds, ; less than 2^32, ; derived from 'Reliability Parameter' of ; [I-D.ietf-rtcweb-data-protocol] priority-opt = "priority=" priority-value priority-value = "0" / integer ; unsigned integer value indicating the priority of ; the data channel, ; less than 2^16, ; derived from 'Priority' of ; [I-D.ietf-rtcweb-data-protocol] quoted-string = DQUOTE *(quoted-char / escaped-char) DQUOTE quoted-char = SP / quoted-visible quoted-visible = %x21 / %x23-24 / %x26-7E ; VCHAR without " or % escaped-char = "%" HEXDIG HEXDIG DQUOTE = integer = Examples: a=dcmap:0 a=dcmap:1 subprotocol="bfcp";max-time=60000;priority=512 a=dcmap:2 subprotocol="msrp";ordered=true;label="msrp" a=dcmap:3 label="Label 1";ordered=false;max-retr=5;priority=128 a=dcmap:4 label="foo%09bar";ordered=true;max-time=15000 Note: The last example (a=dcmap:4) shows a 'label' parameter value which contains one non-printable 'escaped-char' character (the tabulator character). Drage, et al. Expires October 25, 2019 [Page 7] Internet-Draft SDP-based Data Channel Negotiation April 2019 Within an 'a=dcmap:' attribute line's 'dcmap-opt' value only one 'maxretr-opt' parameter or one 'maxtime-opt' parameter may be present. Both MUST NOT be present. 5.1.2. Dcmap-stream-id Parameter The 'dcmap-stream-id' parameter indicates the SCTP stream identifier within the SCTP association used to form the data channel. 5.1.3. Label Parameter The 'label' parameter indicates the name of the channel. It represents a label that can be used to distinguish, in the context of the WebRTC API [WebRtcAPI], an RTCDataChannel object from other RTCDataChannel objects. This parameter maps to the 'Label' parameter defined in [I-D.ietf-rtcweb-data-protocol]. The 'label' parameter is optional. If it is not present, then its value defaults to the empty string. In order to communicate with WEbRTC API the label attribute should: o Serialize the WebRTC label as a UTF-8 string [RFC3629]. o Treat the UTF-8 serialization as a series of bytes o For each byte in the serialization: * If the byte can be expressed as a `quoted-char`, do so * Otherwise, express the byte as an `escaped-char`. Note: The empty string MAY also be explicitly used as a 'label' value, such that 'label=""' is equivalent to the 'label' parameter not being present at all. [I-D.ietf-rtcweb-data-protocol] allows the DATA_CHANNEL_OPEN message's 'Label' value to be an empty string. 5.1.4. Subprotocol Parameter The 'subprotocol' parameter indicates which protocol the client expects to exchange via the channel. This parameter maps to the 'Protocol' parameter defined in [I-D.ietf-rtcweb-data-protocol]. Section 9.1 specifies how new subprotocol parameter values are registered. 'subprotocol' is an optional parameter. If the 'subprotocol' parameter is not present, then its value defaults to an empty string. Note: The empty string MAY also be explicitly used as 'subprotocol' value, such that 'subprotocol=""' is equivalent to the 'subprotocol' Drage, et al. Expires October 25, 2019 [Page 8] Internet-Draft SDP-based Data Channel Negotiation April 2019 parameter not being present at all. [I-D.ietf-rtcweb-data-protocol] allows the DATA_CHANNEL_OPEN message's 'Subprotocol' value to be an empty string. 5.1.5. Max-retr Parameter This parameter indicates that the data channel is partially reliable. The 'max-retr' parameter indicates the maximal number of times a user message will be retransmitted. The max-retr parameter is optional. If the max-retr parameter and the max-time parameter are not present, then reliable transmission is performed as specified in [RFC4960]. This parameter maps to the 'Number of RTX' parameter defined in [I-D.ietf-rtcweb-data-protocol]. 5.1.6. Max-time Parameter This parameter indicates that the data channel is partially reliable. A user message will no longer be transmitted or retransmitted after a specified life-time given in milliseconds in the 'max-time' parameter. The life-time starts when providing the user message to the protocol stack. The max-time parameter is optional. If the max- retr parameter and the max-time parameter are not present, then reliable transmission is performed as specified in [RFC4960]. This parameter maps to the 'Lifetime in ms' parameter defined in [I-D.ietf-rtcweb-data-protocol]. 5.1.7. Ordered Parameter The 'ordered' parameter with value "true" indicates that the receiver will dispatch DATA chunks in the data channel to the upper layer while preserving the order. The ordered parameter is optional and takes two values: "true" for ordered and "false" for unordered delivery with "true" as the default value. Any other value is ignored and default "ordered=true" is assumed. In the absence of this parameter "ordered=true" is assumed. This parameter maps to the ordered or unordered data channel types as defined in [I-D.ietf-rtcweb-data-protocol]. 5.1.8. Priority Parameter The 'priority' parameter indicates the data channel's priority relative to the priorities of other data channels, which may additionally exist over the same SCTP association. The 'priority' parameter maps to the 'Priority' parameter defined in [I-D.ietf-rtcweb-data-protocol]. The 'priority' parameter is optional. In the absence of this parameter "priority=256" is assumed. Drage, et al. Expires October 25, 2019 [Page 9] Internet-Draft SDP-based Data Channel Negotiation April 2019 5.1.9. DCMAP Multiplexing Category The multiplexing category [I-D.ietf-mmusic-sdp-mux-attributes] of the "a=dcmap:" attribute is SPECIAL. As the usage of multiple SCTP associations on top of a single DTLS association is outside the scope of [I-D.ietf-mmusic-sctp-sdp], no "a=dcmap:" attribute multiplexing rules are specified for the UDP/DTLS/SCTP and TCP/DTLS/SCTP proto values. If future extensions of [I-D.ietf-mmusic-sctp-sdp] define how to negotiate multiplexing of multiple SCTP associations on top of a single DTLS association, or how to add multiple SCTP associations to one BUNDLE group, then multiplexing rules for the "a=dcmap:" attribute need to be defined as well, for instance in an extension of this SDP offer/answer based data channel negotiation specification. 5.2. SDP DCSA Attribute In the SDP media description, each data channel declaration MAY also be followed by other media level SDP attributes, which are either specifically defined for or applied to the subprotocol in use. Each of these attributes is represented by one new attribute line, and it includes the contents of a media-level SDP attribute already defined for use with this (sub)protocol in another IETF document. Subprotocol specific attributes MAY also be defined for exclusive use with data channel transport, but MUST use the same syntax described here for other subprotocol related attributes. Each SDP attribute, related to the subprotocol, that would normally be used to negotiate the subprotocol using SDP offer/answer is replaced with an attribute of the form "a=dcsa:stream-id original- attribute", where dcsa stands for "data channel subprotocol attribute", stream-id is the SCTP stream identifier assigned to this subprotocol instance, and original-attribute represents the contents of the subprotocol related attribute to be included. The same syntax applies to any other SDP attribute required for negotiation of this instance of the subprotocol. The detailed offer/answer procedures for the dcsa attribute are dependent on the associated sub-protocol. If no offer/answer procedures exist for the sub-protocol when used outside of the dcsa attribute, no specification is needed for use with dcsa. The IANA registration procedures for the WebSocket Subprotocol Name Registry (Section 9.1) do not strictly require a specification of the offer/ answer procedures for the sub-protocol when used with dcsa. If the sub-protocol has defined offer/answer procedures when used outside of dcsa, such a specification is encouraged to ensure interoperability. Drage, et al. Expires October 25, 2019 [Page 10] Internet-Draft SDP-based Data Channel Negotiation April 2019 If the sub-protocol has defined offer/answer procedures when used outside of dcsa, but no specification exists for the offer/answer procedures for the sub-protocol when used with dcsa, implementations SHOULD assume the use of the default values for all otherwise- negotiable and applicable sub-protocol parameters. 5.2.1. DCSA Syntax Formal Syntax: Name: dcsa Value: dcsa-value Usage Level: media Charset Dependent: no Syntax: dcsa-value = stream-id SP attribute stream-id = 1*5DIGIT attribute = Example: a=dcmap:2 subprotocol="msrp";ordered=true;label="msrp" a=dcsa:2 accept-types:text/plain Note that the reference to [I-D.ietf-mmusic-rfc4566bis] defines where the attribute definition can be found; it does not provide any limitation on support of attributes defined in other documents in accordance with this attribute definition. Note however that not all SDP attributes are suitable as a "a=dcsa:" parameter. IANA SDP parameters contains the lists of IANA (Internet Assigned Numbers Authority) registered session and media level or media level only SDP attributes. Thus in the example above, the original attribute line "a=accept- types:text/plain" is represented by the attribute line "a=dcsa:2 accept-types:text/plain", which specifies that this instance of the MSRP subprotocol being transported on the SCTP association using the data channel with stream id 2 accepts plain text files. As opposed to the data channel "a=dcmap:" attribute parameters, these parameters are subject to offer/answer negotiation following the procedures defined in the subprotocol specific documents. Drage, et al. Expires October 25, 2019 [Page 11] Internet-Draft SDP-based Data Channel Negotiation April 2019 It is assumed that in general the usages of subprotocol related media level attributes are independent from the subprotocol's transport protocol. Such transport protocol independent subprotocol related attributes are used in the same way as defined in the original subprotocol specification, also if the subprotocol is transported over a data channel and if the attribute is correspondingly embedded in a "a=dcsa" attribute. There may be cases, where the usage of a subprotocol related media level attribute depends on the subprotocol's transport protocol. In such cases the subprotocol related usage of the attribute is expected to be described for the data channel transport. A data channel specific usage of a subprotocol attribute is expected to be specified in the same document that registers the subprotocol's identifier for data channel usage as described in Section 9.1. SDP attributes that are only defined for use at the dcsa usage level, SHALL use the dcsa usage level when registering the attribute. If existing media attributes are used in a datachannel subprotocol specific way, then a new dcsa usage level MUST be defined for the existing media attribute. Where the SDP attribute is applicable to a particular subprotocol/s this SHALL also be registered by indicating the applicable subprotocol identifiers (see /[I-D.ietf-mmusic-rfc4566bis] section-8.5) along with the dcsa usage level. 5.2.2. DCSA Multiplexing Category The multiplexing category of the "a=dcsa:" attribute is SPECIAL. As the usage of multiple SCTP associations on top of a single DTLS association is outside the scope of [I-D.ietf-mmusic-sctp-sdp], no "a=dcsa:" attribute multiplexing rules are specified for the UDP/DTLS/SCTP and TCP/DTLS/SCTP proto values. If future extensions of [I-D.ietf-mmusic-sctp-sdp] define how to negotiate multiplexing of multiple SCTP associations on top of a single DTLS association, or how to add multiple SCTP associations to one BUNDLE group, then multiplexing rules for the "a=dcsa:" attribute need to be defined as well, for instance in an extension of this SDP based data channel negotiation specification. 6. SDP Offer/Answer Procedures This section defines how data channels can be negotiated using the SDP offer/answer mechanism. A given media description can describe multiple data channels (each represented by a separate SDP dcmap attribute) that can be created, modified and closed using different Drage, et al. Expires October 25, 2019 [Page 12] Internet-Draft SDP-based Data Channel Negotiation April 2019 offer/answer exchanges. The procedures in this section apply for a given data channel. The generic offer/answer procedures for negotiating the SCTP association used to realize data channels are defined in [I-D.ietf-mmusic-sctp-sdp]. This section only defines the data channel specific procedures. "Initial offer" refers to the offer in which a data channel is opened. It can be the initial offer, or a subsequent offer, of the associated SDP session. The detailed offer/answer procedures for the dcsa attribute are dependent on the associated sub-protocol see Section 5.2. 6.1. Managing Stream Identifiers In order to avoid SCTP Stream identifier collisions, in alignment with [I-D.ietf-rtcweb-data-protocol], the endpoint acting as DTLS client (for the SCTP association used to realize data channels) MUST use even identifier values, and the endpoint acting as DTLS server MUST use odd identifier values. SCTP stream identifiers associated with data channels that have been negotiated using DCEP MUST NOT be included in SDP offers and answers. 6.2. Negotiating Data Channel Parameters The data channel types defined in [I-D.ietf-rtcweb-data-protocol] are mapped to the dcmap SDP attribute parameters in the following manner where "ordered=true" is the default and may be omitted: Drage, et al. Expires October 25, 2019 [Page 13] Internet-Draft SDP-based Data Channel Negotiation April 2019 DATA_CHANNEL_RELIABLE ordered=true DATA_CHANNEL_RELIABLE_UNORDERED ordered=false DATA_CHANNEL_PARTIAL_RELIABLE_REXMIT ordered=true;max-retr= DATA_CHANNEL_PARTIAL_RELIABLE_REXMIT_UNORDERED ordered=false;max-retr= DATA_CHANNEL_PARTIAL_RELIABLE_TIMED ordered=true;max-time= DATA_CHANNEL_PARTIAL_RELIABLE_TIMED_UNORDERED ordered=false;max-time= By definition max-retr and max-time are mutually exclusive, so both MUST NOT 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 MUST treat the associated SDP offer/answer failed. 6.3. Generating the Initial Offer for A Data Channel When an offerer sends an initial offer, in order to negotiate an SCTP stream for a data channel, the offerer: o SHALL include an SDP dcmap attribute (Section 5 and Section 6.2) associated with the data channel in the "m=" section representing the SCTP association used to realize the data channel; and o MAY include one or more SDP dcsa attributes (Section 5.2) associated with the data channel. The value of the stream-id part of each attribute SHALL match the dcmap-stream-id value of the dcmap attribute. 6.4. Generating SDP Answer When an answerer receives an offer that includes an "m=" section for an SCTP association, that describes an SCTP stream for a data channel, if the answerer accepts the data channel it: o SHALL include an SDP dcmap attribute (Section 5 and Section 6.2) associated with the data channel in the "m=" section representing the SCTP association used to realize the data channel. The value Drage, et al. Expires October 25, 2019 [Page 14] Internet-Draft SDP-based Data Channel Negotiation April 2019 of the dcmap-stream-id, max-retr and max-time values of the dcmap attribute SHALL be identical to the value used for the data channel in the offer; and o MAY include one or more SDP dcsa attributes (Section 5.2) associated with the data channel. 6.5. Offerer Processing of the SDP Answer An offerer receiving an SDP answer performs the following: o SHALL close any created data channels as described in Section 6.6.1 for which the expected "a=dcmap:" attributes are not present in the SDP answer. If the SDP answer has no "a=dcmap" attribute either the peer does not support "a=dcmap:" attributes or it rejected all the data channels. In either case the offerer closes all the SDP offered data channels that were open at the time of offer. The DTLS association and SCTP association will still be setup. Each agent application MUST wait to send data until it has confirmation that the data channel at the peer is instantiated. For WebRTC, this is when both data channel stacks have channel parameters instantiated. This occurs: o At both peers when a data channel is created without a previously established SCTP association, as soon as the SCTP association is successfully established. o At the agent receiving an SDP offer for which there is an established SCTP association, as soon as it creates the negotiated data channel based on information signaled in the SDP offer. o At the agent sending an SDP offer to create a new 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. 6.6. Modifying the Session When an offer sends a subsequent offer, that includes information for a previously negotiated data channel, unless the offerer intends to close the data channel (Section 6.6.1), the offerer SHALL include the previously negotiated SDP attributes and attribute values associated with the data channel. The answerer may reject the offer. The means for rejecting an offer are dependent on the higher layer protocol. Drage, et al. Expires October 25, 2019 [Page 15] Internet-Draft SDP-based Data Channel Negotiation April 2019 The offer/answer exchange is atomic; if the answer is rejected, the session reverts to the state prior to the offer [RFC3264]. 6.6.1. Closing a Data Channel In order to close a data channel, the endpoint that wants to close SHALL send the SCTP SSN reset message [RFC6525], following the procedures in section 6.7 of [I-D.ietf-rtcweb-data-channel]. In addition, if the closed data channel was negotiated using the offer/ answer mechanism Section 6.3, the endpoint that closed the data channel SHALL send a subsequent offer in which it either: o removes the SDP dcmap attribute and SDP dcsa attributes associated with the closed data channel. Once the endpoint receives a successful answer, the SCTP stream identifier value can later be used for a new data channel (negotiated using DCTP or using the offer/answer mechanism); or o after a reset has been performed re-uses the SCTP stream used for the closed data channel for a new data channel, using the procedures in Section 6.3. The offerer SHALL use a different SDP dcmap attribute value for the data channel using the same SCTP stream. 6.7. Various SDP Offer/Answer Considerations An SDP offer or answer has no "a=dcmap:" attributes but has "a=dcsa:" attributes. * This is considered an error case. In this case the receiver of such an SDP offer or answer MUST discard this "a=dcsa:" attributes. SDP offer or answer has an "a=dcsa" attribute, whose subprotocol attribute is unknown. * The receiver of such an SDP offer or answer SHOULD ignore this entire "a=dcsa" attribute line. SDP offer or answer has an "a=dcsa" attribute, whose subprotocol attribute is known, but whose subprotocol attribute semantic is not known for the data channel transport case. * The receiver of such an SDP offer or answer SHOULD ignore this entire "a=dcsa" attribute line. Drage, et al. Expires October 25, 2019 [Page 16] Internet-Draft SDP-based Data Channel Negotiation April 2019 7. Examples SDP offer: m=application 10001 UDP/DTLS/SCTP webrtc-datachannel c=IN IP6 2001:db8::3 a=max-message-size:100000 a=sctp-port:5000 a=setup:actpass 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=tls-id:abc3de65cddef001be82 a=dcmap:0 subprotocol="bfcp";label="bfcp" SDP answer: m=application 10002 UDP/DTLS/SCTP webrtc-datachannel c=IN IP6 2001:db8::1 a=max-message-size:100000 a=sctp-port:5002 a=setup:passive 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=tls-id:dcb3ae65cddef0532d42 Figure 1: Example 1 In the example in Figure 1 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. Drage, et al. Expires October 25, 2019 [Page 17] Internet-Draft SDP-based Data Channel Negotiation April 2019 SDP offer: m=application 10001 UDP/DTLS/SCTP webrtc-datachannel c=IN IP4 192.0.2.1 a=max-message-size:100000 a=sctp-port:5000 a=setup:actpass 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=tls-id:abc3de65cddef001be82 a=dcmap:0 subprotocol="bfcp";label="bfcp" a=dcmap:2 subprotocol="msrp";label="msrp" a=dcsa:2 accept-types:message/cpim text/plain a=dcsa:2 path:msrp://alice.example.com:10001/2s93i93idj;dc SDP answer: m=application 10002 UDP/DTLS/SCTP webrtc-datachannel c=IN IP4 192.0.2.2 a=max-message-size:100000 a=sctp-port:5002 a=setup:passive 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=tls-id:dcb3ae65cddef0532d42 a=dcmap:2 subprotocol="msrp";label="msrp" a=dcsa:2 accept-types:message/cpim text/plain a=dcsa:2 path:msrp://bob.example.com:10002/si438dsaodes;dc Figure 2: Example 2 In the example in Figure 2 the SDP offer contains data channels for BFCP (Binary Floor Control Protocol) and MSRP subprotocols. The SDP answer rejected BFCP and accepted MSRP. So, the offerer closes the data channel for BFCP and both offerer and answerer may start using the MSRP data channel (after the SCTP association is set up). The data channel with stream id 0 is free and can be used for future DCEP or SDP offer/answer negotiation. Continuing the example in Figure 2. Drage, et al. Expires October 25, 2019 [Page 18] Internet-Draft SDP-based Data Channel Negotiation April 2019 Subsequent SDP offer: m=application 10001 UDP/DTLS/SCTP webrtc-datachannel c=IN IP4 192.0.2.1 a=max-message-size:100000 a=sctp-port:5000 a=setup:actpass 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=tls-id:abc3de65cddef001be82 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 192.0.2.2 a=max-message-size:100000 a=sctp-port:5002 a=setup:passive 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=tls-id:dcb3ae65cddef0532d42 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 example in Figure 3 is a continuation of the example in Figure 2. The SDP offerer 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. 8. Security Considerations This document specifies new SDP attributes used in the negotiation of the DATA channel parameters. These parameter are negotiated as part of opening a SCTP channel over DTLS as specified in [I-D.ietf-mmusic-sctp-sdp]. Each subprotocol may come with it's own security considerations that need to be documented as part of the subprotocol definition. Otherwise this document does not add any security considerations to the ones specified in [I-D.ietf-mmusic-sctp-sdp] Drage, et al. Expires October 25, 2019 [Page 19] Internet-Draft SDP-based Data Channel Negotiation April 2019 Error cases like the use of unknown parameter values or violation the odd/even rule MUST be handled by closing the corresponding Data Channel. 9. IANA Considerations 9.1. Subprotocol Identifiers Registration of new subprotocol identifiers is performed using the existing IANA "WebSocket Subprotocol Name Registry" table. The following text should be added following the title of the table. "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. A subprotocol may simultaneously be defined for data channel transport and for Websocket transport. In such a case the "Subprotocol Definition" and "Reference" cells in the subprotocol's row of the IANA "WebSocket Subprotocol Name Registry" table should contain two entries. One entry in each of these cells should refer to the Websocket related subprotocol specification, and the other entry should refer to the data channel related subprotocol specification. NOTE to RFC Editor: Please replace "XXXX" with the number of this RFC. 9.2. New SDP Attributes 9.2.1. 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: Drage, et al. Expires October 25, 2019 [Page 20] Internet-Draft SDP-based Data Channel Negotiation April 2019 +-----------------------+-------------------------------------------+ | Contact name: | IESG | | Contact email: | iesg@ietf.org | | Attribute name: | dcmap | | Attribute syntax: | As per Section 5.1.1 | | Attribute semantics: | As per Section 5.1.1 | | Usage level: | media | | Charset dependent: | No | | Purpose: | Define data channel specific parameters | | Appropriate values: | As per Section 5.1.1 | | O/A procedures: | As per Section 6 | | Mux category: | SPECIAL. See Section 5.1.9 | | Reference: | RFCXXXX | +-----------------------+-------------------------------------------+ 9.2.2. 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: +-----------------------+-------------------------------------------+ | Contact name: | IESG | | Contact email: | iesg@ietf.org | | Attribute name: | dcsa | | Attribute syntax: | As per Section 5.2.1 | | Attribute semantics: | As per Section 5.2.1 | | Usage level: | media | | Charset dependent: | No | | Purpose: | Define data channel subprotocol specific | | | attributes | | Appropriate values: | As per Section 5.2.1 | | O/A procedures: | As per Section 6 | | Mux category: | SPECIAL. See Section 5.2.2 | | Reference: | RFCXXXX | +-----------------------+-------------------------------------------+ 10. Contributors Juergen Stoetzer-Bradler co-authored this document. 11. 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 Flemming Andreasen, Christian Drage, et al. Expires October 25, 2019 [Page 21] Internet-Draft SDP-based Data Channel Negotiation April 2019 Groves, Gunnar Hellstrom, Paul Kyzivat, Jonathan Lennox, Uwe Rauschenbach and Roman Shpount for their invaluable comments. Special thanks to Christer Holmberg for helping finish the document and cleaning the SDP offer/answer section. 12. CHANGE LOG 12.1. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-15' o Editorial changes separate sections for offer/answer procedures. o Update security section. 12.2. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-14' o Change "dtls-id" to "tls-id" and assign 20 octet long values o Remove of RFC4566bis draft from list of normative references. 12.3. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-12' o Modification of Keith's address information. 12.4. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-11' o dcmap-stream-id syntax change to limit size to 5 digits. o Add missing 'x' prefix to quoted-visible syntax. o Align SDP offerer and answerer handling when both max-retr and max-time are present. o Use of TEST-NET-1 ip addresses in examples. o Add missing a=dtls-id in one example. 12.5. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-10' o Removal of the "a=connection" attribute lines from all SDP examples. 12.6. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-09' o In the introduction: * Replacement of the sentence "The RTCWeb working group has defined the concept of bi-directional data channels running on Drage, et al. Expires October 25, 2019 [Page 22] Internet-Draft SDP-based Data Channel Negotiation April 2019 top of SCTP/DTLS (SCTP over the Datagram Transport Layer Security protocol)" with "The RTCWeb working group has defined the concept of bi-directional data channels running on top of the Stream Control Transmission Protocol (SCTP)". * Addition of following sentences to the second paragraph: "These procedures are based on generic SDP offer/answer negotiation rules for SCTP based media transport as specified in [I-D.ietf-mmusic-sctp-sdp] for the SDP "m" line proto values UDP/DTLS/SCTP and TCP/DTLS/SCTP. In the future, data channels could be defined over other SCTP based protocols, such as SCTP over IP. However, corresponding potential other "m" line proto values are not considered in this document." o Replacement of "DTLS connection" with "DTLS association" throughout the document. o In sections Section 5.1.9 and Section 5.2.2 removal of the sentences "This document also does not specify multiplexing rules for this attribute for SCTP or SCTP/DTLS proto values". o In the text related to "Subsequent SDP answer" in section Section 6.7 replacement of "The DTLS/SCTP association remains open ..." with "The SCTP association remains open ...". o In the text after the second SDP answer in section Section 7 replacement of "... (after SCTP/DTLS association is setup)" with "... (after the SCTP association is set up)". o Addition of draft-ietf-mmusic-dtls-sdp to the list of informative references. o Addition of "a=dtls-id" attribute lines to the SDP offer/answer examples in Section 7. 12.7. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-08' o Addition of definition of "data channel subprotocol" to Section 3 as proposed on the MMUSIC list, https://www.ietf.org/mail- archive/web/mmusic/current/msg15827.html. o Addition of RFC4566bis draft to list of normative references. o Updates of tables in Section 9.2.1 and Section 9.2.2 as per section 8.2.4 of RFC4566bis draft. o Addition of new "new-usage-level">. Drage, et al. Expires October 25, 2019 [Page 23] Internet-Draft SDP-based Data Channel Negotiation April 2019 12.8. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-07' o Addition of two new paragraphs to Section 5.2.1 regarding subprotocol attribute relationship with transport protocol. o Addition of a note to Section 9.1 regarding subprotocols simultaneously defined for data channel and Websocket usage. o Addition of two further SDP offer/answer considerations to Section 6.7 regarding unknown subprotocol attributes and known subprotocol attributes with unknown data channel transport related semantic. 12.9. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-06' o Changes addressing Christian Groves's WGLC review comments raised in http://www.ietf.org/mail-archive/web/mmusic/current/ msg15357.html and http://www.ietf.org/mail- archive/web/mmusic/current/msg15359.html. 12.10. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-05' o In IANA registration Section 9.2.1 and Section 9.2.2 replacement of contact name and e-mail address with "MMUSIC Chairs" and "mmusic-chairs@ietf.org". o In Section 5.2.1 replacement of "Thus in the example above, the original attribute line "a=accept- types:text/plain" is represented by the attribute line "a=dcsa:2 accept-types:text/ plain", which specifies that this instance of MSRP being transported on the SCTP association using the data channel with stream id 2 accepts plain text files." with "... which specifies that this instance of the MSRP subprotocol being transported ...". o The last paragraph of Section 5.2.1 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. o Section 5.2's headline was "Subprotocol Specific Attributes". Change of this headline to "Other Media Level Attributes" and adaptations of the first paragraph of this section and the first paragraph of Section 5.2.1 in order to clarify that not only those attributes may be encapsulated in a "dcsa" attribute, which are specifically defined for the subprotocol, but that also other attributes may be encapsulated if they are related to the specific subprotocol instance. Drage, et al. Expires October 25, 2019 [Page 24] Internet-Draft SDP-based Data Channel Negotiation April 2019 o Move of the last but one paragraph of Section 5.2.1 starting with "The same syntax applies to ..." right in front of the formal syntax definition of the "dcsa" attribute. o Modifications of the text in Section 5.1.9 and Section 5.2.2 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". 12.11. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-04' o In Section 5.1.4 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 9.1 specifies how new subprotocol parameter values are registered. 'subprotocol' is an optional parameter. If the 'subprotocol' parameter is not present, then its value defaults to the empty string. * Note: The empty string MAY also be explicitly used as 'subprotocol' value, such that 'subprotocol=""' is equivalent to the 'subprotocol' parameter not being present at all. [I-D.ietf-rtcweb-data-protocol] allows the DATA_CHANNEL_OPEN message's 'subprotocol' value to be an empty string. o Addition of [I-D.ietf-mmusic-sdp-mux-attributes] to list the of normative references. o Addition of dcmap attribute specific IANA registration Section 9.2.1. o Addition of dcsa attribute specific IANA registration Section 9.2.2. o Addition of new Section 5.1.9 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]. Drage, et al. Expires October 25, 2019 [Page 25] Internet-Draft SDP-based Data Channel Negotiation April 2019 o Addition of new Section 5.2.2 describing the mux category of the dcsa SDP attribute. 12.12. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-03' o In Section 1 replacement of "RTCWeb leaves it open for other applications to use data channels and its in-band DCEP or out-of- band non-DCEP protocols for creating them" with "... to use data channels and its in-band DCEP or other in-band or out-of-band protocols for creating them". Additionally replacement of "In particular, the SDP offer generated by the application includes no channel-specific information" with "... generated by the RTCweb data channel stack includes no channel-specific information". o Move of former section 5 ("Data Channels") to new Appendix A and removal of JavaScript API specific discussions from this moved text (like mentioning of data channel stack specific states). Therefore former section 6 ("SDP Offer/Answer Negotiation") is now Section 5. o In Section 5: * Relacement of Section 5's first paragraph "This section defines a method of non-DCEP negotiation by which two clients can negotiate data channel-specific and subprotocol-specific parameters, using the out-of-band SDP offer/answer exchange. This SDP extension can only be used with the SDP offer/answer model." with "This section defines an SDP extension by which two clients can negotiate data channel-specific and subprotocol-specific parameters without using DCEP [I-D.ietf-rtcweb-data-protocol]. This SDP extension only defines usage in the context of SDP offer/answer." * Addition of new paragraph: "Appendix A provides information how data channels work in general and especially summarizes some key aspects, which should be considered for the negotiation of data channels if DCEP is not used." o In Section 5.1 replacement of "The intention of exchanging these attributes is to create data channels on both the peers with the same set of attributes without actually using the DCEP [I-D.ietf-rtcweb-data-protocol]" with "The intention in exchanging these attributes is to create, on two peers, without use of DCEP [I-D.ietf-rtcweb-data-protocol], matched pairs of oppositely directed data channels having the same set of attributes". o In Section 5.1.5 replacement of "The 'max-retr' parameter indicates the maximal number a user message will be retransmitted" Drage, et al. Expires October 25, 2019 [Page 26] Internet-Draft SDP-based Data Channel Negotiation April 2019 with "The 'max-retr' parameter indicates the maximal number of times a user message will be retransmitted". o In Section 6.1 replacement of "However, an SDP offer/answer exchange MUST NOT be initiated if the associated SCTP stream is already negotiated via DCEP" with "However, an SCTP stream MUST NOT be referenced in a dcmap or dcsa attribute of an SDP offer/ answer exchange if the associated SCTP stream has already been negotiated via DCEP". o In the examples in Section 7 addition of the previously missing colons to the "a=sctp-port" attribute lines. 12.13. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-02' o Move of reference draft-ietf-rtcweb-jsep from the list of normative references to the list of informative references. Remover in -07 since not referenced o Addition of IANA SDP parameters to the list of informative references and addition of following two sentences to the first paragraph after the ABNF definition: "Note however that not all SDP attributes are suitable as "a=dcsa:" parameter. IANA SDP parameters contains the lists of IANA registered session and media level or media level only SDP attributes." o In the introduction replacement of last sentence "This document defines SDP-based out-of-band negotiation procedures to establish data channels for transport of well-defined subprotocols" with "This document defines SDP offer/answer negotiation procedures to establish data channels for transport of well-defined subprotocols, to enable out-of-band negotiation". o Throughout the document replacement of "external negotiation" with "SDP offer/answer negotiation" and removal of term "external negotiation" from the terminology list in Section 3. o Throughout the document replacement of "internal negotiation" with "DCEP" and removal of terms "internal negotiation" and "in-band negotiation" from the terminology list in Section 3. o Addition of "SCTP Stream Sequence Number (SSN)" to the list of terms. o In Section 6.1 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". Drage, et al. Expires October 25, 2019 [Page 27] Internet-Draft SDP-based Data Channel Negotiation April 2019 o In Section 6.2 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 aBoth MUST NOT be present in a=dcmap". o Move of reference [WebRtcAPI] from list of normative references to list of informative references. o Removal of almost all text parts, which discussed JavaScript or other API specific aspects. Such API specific aspects were mainly discussed in sub-sections of Section 5 and Section 5 of draft- ietf-mmusic-data-channel-sdpneg-02. 12.14. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-01' o New Section 4 regarding applicability to SDP offer/answer only. o Addition of new Section 9.1 "Subprotocol identifiers" as subsection of the "IANA Considerations" related Section 9. Also removal of the temporary note "To be completed. As [I-D.ietf- rtcweb-data-protocol] this document should refer to IANA's WebSocket Subprotocol Name Registry defined in [RFC6455]" o In Section 6.2: * 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.". o In Section 6.2 the mapping of data channel types defined in [I-D.ietf-rtcweb-data-protocol] to the SDP "a=dcmap" attribute Drage, et al. Expires October 25, 2019 [Page 28] Internet-Draft SDP-based Data Channel Negotiation April 2019 parameters were illustrated using example "a=dcmap" attribute lines. Replacement of these example "a=dcmap" attribute lines with just the "a=dcmap" attribute parameters being relevant for the channel type. o In Section 6.7 the description of bullet point "SDP offer has no a=dcmap attributes - Initial SDP offer:" was "Initial SDP offer: No data channel negotiated yet." Replacement of this description with "Initial SDP offer: No data channel is negotiated yet. The DTLS connection and SCTP association is negotiated and, if agreed, established as per [I-D.ietf-mmusic-sctp-sdp]." o In Section 6.7 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 to be closed now.". o In Appendix A.2.2's sixth paragraph replacement of the two occurrences of "must" with "MUST". o In Section 5.1.1 in the definition of the ABNF rule "dcmap-opt" there was a comment saying that "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 only one 'maxretr-opt' parameter or one 'maxtime-opt' parameter is present. Both MUST NOT be present." o In Section 5.1.7 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.". o In Section 6.3's first paragraph replacement of the one occurrence of "must" with "..., it MUST wait until ...". o In Section 6.6.1: * 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 Drage, et al. Expires October 25, 2019 [Page 29] Internet-Draft SDP-based Data Channel Negotiation April 2019 change the port value for the "m" line (e.g., to zero) when closing a data channel ...". * In the last but two paragraph replacement of the sentence "... then an SDP offer which excludes this closed data channel SHOULD be generated." with "... then the client SHOULD generate an SDP offer which excludes this closed data channel.". * In the last but one paragraph replacement of "must" with "The application MUST also close...". o In Section 5.2 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." 12.15. Changes against 'draft-ietf-mmusic-data-channel-sdpneg-00' o In Section 3 "WebRTC data channel" was defined as "A bidirectional channel consisting of paired SCTP outbound and inbound streams." Replacement of this definition with "Data channel: A WebRTC data channel as specified in [I-D.ietf-rtcweb-data-channel]", and consistent usage of "data channel" in the remainder of the document including the document's headline." o In Section 5 removal of following note: 'OPEN ISSUE: The syntax in [I-D.ietf-mmusic-sctp-sdp] may change as that document progresses. In particular we expect "webrtc-datachannel" to become a more general term.' o Consistent usage of '"m" line' in whole document as per RFC4566. o In Section 5.1 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. Corresponding removal of following related note: "Note: This document does not provide a complete specification of how to negotiate the use of a WebRTC data channel to transport BFCP. Procedures specific to each subprotocol such as BFCP will be documented elsewhere. The use of BFCP is only an example of how the generic procedures described herein might apply to a specific subprotocol." o In Section 5.1 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 Drage, et al. Expires October 25, 2019 [Page 30] Internet-Draft SDP-based Data Channel Negotiation April 2019 with any support for SDP external negotiation in subsequent versions: [I-D.ietf-mmusic-sctp-sdp]." o Insertion of following new sentence to the beginning of Section 5.1.1: "dcmap is a media level attribute having following ABNF syntax:" o Insertion of new Section 5.1.2 containing the dcmap-stream-id specifying sentence, which previously was placed right before the formal ABNF rules. Removal of the sentence 'Stream is a mandatory parameter and is noted directly after the "a=dcmap:" attribute's colon' as this information is part of the ABNF specification. o In Section 5.1.1 modification of the 'ordering-value' values from "0" or "1" to "true" or "false". Corresponding text modifications in Section 5.1.7. o In Section 5.1.1 the ABNF definition of "quoted-string" referred to rule name "escaped-char", which was not defined. Instead a rule with name "escaped" was defined. Renamed that rule's name to "escaped-char". o Insertion of a dedicated note right after the "a=dcmap:4" attribute example in Section 5.1.1 regarding the non-printable "escaped-char" character within the "label" value. o In Section 5.2's second paragraph replacement of "sctp stream identifier" with "SCTP stream identifier". o In first paragraph of Section 6.1 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.' Drage, et al. Expires October 25, 2019 [Page 31] Internet-Draft SDP-based Data Channel Negotiation April 2019 o In Section 6.3 the first action of an SDP answerer, when receiving an SDP offer, was described as "Applies the SDP offer. Note that the browser ignores data channel specific attributes in the SDP." Replacement of these two sentences with "Parses and applies the SDP offer. Note that the typical parser normally ignores unknown SDP attributes, which includes data channel related attributes." o In Section 6.3 the second sentence of the third SDP answerer action was "Note that the browser is asked to create data channels with stream identifiers not "owned" by the agent.". Replacement of this sentence with "Note that the agent is asked to create data channels with SCTP stream identifiers contained in the SDP offer if the SDP offer is accepted." o In Section 6.6.1 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." o In Section 6.6.1 the hanging text after the third paragraph was "This delayed close is to handle cases where a successful SDP answer is not received, in which case the state of session should be kept per the last successful SDP offer/answer." Replacement of this sentence with "This delayed closure is RECOMMENDED in order to handle cases where a successful SDP answer is not received, in which case the state of the session SHOULD be kept per the last successful SDP offer/answer." o Although dedicated to "a=dcmap" and "a=dcsa" SDP syntax aspects Section 5.1 contained already procedural descriptions related to data channel reliability negotiation. Creation of new Section 6.2 Drage, et al. Expires October 25, 2019 [Page 32] Internet-Draft SDP-based Data Channel Negotiation April 2019 and moval of reliability negotiation related text to this new section. 12.16. Changes against 'draft-ejzak-mmusic-data-channel-sdpneg-02' o Removal of note "ACTION ITEM" from section "subprotocol parameter". As [I-D.ietf-rtcweb-data-protocol] this document should refer to IANA's WebSocket Subprotocol Name Registry defined in [RFC6455] o In whole document, replacement of "unreliable" with "partially reliable", which is used in [I-D.ietf-rtcweb-data-channel] and in [I-D.ietf-rtcweb-data-protocol] in most places. o Clarification of the semantic if the "max-retr" parameter is not present in an "a=dcmap" attribute line. In section "max-retr parameter" the sentence "The max-retr parameter is optional with default value unbounded" was replaced with "The max-retr parameter is optional. If the max-retr parameter is not present, then the maximal number of retransmissions is determined as per the generic SCTP retransmission rules as specified in [RFC4960]". o Clarification of the semantic if the "max-time" parameter is not present in an "a=dcmap" attribute line. In section "max-time parameter" the sentence "The max-time parameter is optional with default value unbounded" was replaced with "The max-time parameter is optional. If the max-time parameter is not present, then the generic SCTP retransmission timing rules apply as specified in [RFC4960]". o In section "label parameter" the sentence "Label is a mandatory parameter." was removed and following new sentences (including the note) were added: "The 'label' parameter is optional. If it is not present, then its value defaults to the empty string. Note: The empty string may also be explicitly used as 'label' value, such that 'label=""' is equivalent to the 'label' parameter not being present at all. [I-D.ietf-rtcweb-data-protocol] allows the DATA_CHANNEL_OPEN message's 'Label' value to be an empty string." o In section "subprotocol parameter" the sentence "subprotocol is a mandatory parameter." was replaced with "'subprotocol' is an optional parameter. If the 'subprotocol' parameter is not present, then its value defaults to the empty string." o In the "Examples" section, in the first two SDP offer examples in the "a=dcmap" attribute lines 'label="BGCP"' was replaced with 'label="bfcp"'. Drage, et al. Expires October 25, 2019 [Page 33] Internet-Draft SDP-based Data Channel Negotiation April 2019 o In all examples, the "m" line proto value "DTLS/SCTP" was replaced with "UDP/DTLS/SCTP" and the "a=fmtp" attribute lines were replaced with "a=max-message-size" attribute lines, as per draft- ietf-mmusic-sctp-sdp-12. 12.17. Changes against '-01' o Formal syntax for dcmap and dcsa attribute lines. o Making subprotocol as an optional parameter in dcmap. o Specifying disallowed parameter combinations for max-time and max- retr. o Clarifications on WebRTC data channel close procedures. 12.18. Changes against '-00' o Revisions to identify difference between internal and external negotiation and their usage. o Introduction of more generic terminology, e.g. "application" instead of "browser". o Clarification of how "max-retr and max-time affect the usage of unreliable and reliable WebRTC data channels. o Updates of examples to take into account the SDP syntax changes introduced with draft-ietf-mmusic-sctp-sdp-07. o Removal of the SCTP port number from the "a=dcmap" and "a=dcsa" attributes as this is now contained in the a=sctp-port attribute, and as draft-ietf-mmusic-sctp-sdp-07 supports only one SCTP association on top of the DTLS connection. 13. References 13.1. Normative References [I-D.ietf-mmusic-rfc4566bis] Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP: Session Description Protocol", draft-ietf-mmusic- rfc4566bis-32 (work in progress), December 2018. Drage, et al. Expires October 25, 2019 [Page 34] Internet-Draft SDP-based Data Channel Negotiation April 2019 [I-D.ietf-mmusic-sctp-sdp] Holmberg, C., Shpount, R., Loreto, S., and G. Camarillo, "Session Description Protocol (SDP) Offer/Answer Procedures For Stream Control Transmission Protocol (SCTP) over Datagram Transport Layer Security (DTLS) Transport.", draft-ietf-mmusic-sctp-sdp-26 (work in progress), April 2017. [I-D.ietf-mmusic-sdp-mux-attributes] Nandakumar, S., "A Framework for SDP Attributes when Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-17 (work in progress), February 2018. [I-D.ietf-rtcweb-data-channel] Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data Channels", draft-ietf-rtcweb-data-channel-13 (work in progress), January 2015. [I-D.ietf-rtcweb-data-protocol] Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data Channel Establishment Protocol", draft-ietf-rtcweb-data- protocol-09 (work in progress), January 2015. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, DOI 10.17487/RFC3264, June 2002, . [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2003, . [RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol", RFC 4960, DOI 10.17487/RFC4960, September 2007, . [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, . Drage, et al. Expires October 25, 2019 [Page 35] Internet-Draft SDP-based Data Channel Negotiation April 2019 [RFC6525] Stewart, R., Tuexen, M., and P. Lei, "Stream Control Transmission Protocol (SCTP) Stream Reconfiguration", RFC 6525, DOI 10.17487/RFC6525, February 2012, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . 13.2. Informative References [I-D.ietf-clue-datachannel] Holmberg, C., "CLUE Protocol data channel", draft-ietf- clue-datachannel-17 (work in progress), April 2019. [I-D.ietf-mmusic-msrp-usage-data-channel] Drage, K., Makaraju, M., Stoetzer-Bradler, J., Ejzak, R., Marcon, J., and J. Recio, "MSRP over Data Channels", draft-ietf-mmusic-msrp-usage-data-channel-10 (work in progress), April 2019. [RFC4582] Camarillo, G., Ott, J., and K. Drage, "The Binary Floor Control Protocol (BFCP)", RFC 4582, DOI 10.17487/RFC4582, November 2006, . [RFC4975] Campbell, B., Ed., Mahy, R., Ed., and C. Jennings, Ed., "The Message Session Relay Protocol (MSRP)", RFC 4975, DOI 10.17487/RFC4975, September 2007, . [RFC6455] Fette, I. and A. Melnikov, "The WebSocket Protocol", RFC 6455, DOI 10.17487/RFC6455, December 2011, . [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. Drage, et al. Expires October 25, 2019 [Page 36] Internet-Draft SDP-based Data Channel Negotiation April 2019 A WebRTC application creates a data channel by providing a number of setup parameters (subprotocol, label, maximal number of retransmissions, maximal retransmission time, order of delivery, priority). The application also specifies if it wants to make use of the negotiation using the DCEP [I-D.ietf-rtcweb-data-protocol], or if the application intends to negotiate data channels using the SDP offer/answer protocol. In any case, the SDP offer generated by the application is per [I-D.ietf-mmusic-sctp-sdp]. In brief, it contains one "m" line for the SCTP association on top of which data channels will run: m=application 54111 UDP/DTLS/SCTP webrtc-datachannel c=IN IP4 192.0.2.1 a=max-message-size:100000 a=sctp-port:5000 a=tls-id:abc3de65cddef001be82 a=setup:actpass 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: The above SDP media description does not contain any channel- specific information. A.1. Stream Identifier Numbering Independently from the requested type of negotiation, the application creating a data channel can either pass the stream identifier to the data channel stack to assign to the data channel or else let the data channel stack pick one identifier from the unused ones. To avoid glare situations, each endpoint can moreover own an exclusive set of stream identifiers, in which case an endpoint can only create a data channel with a stream identifier it owns. Which set of stream identifiers is owned by which endpoint is determined by convention or other means. Note: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]. Drage, et al. Expires October 25, 2019 [Page 37] Internet-Draft SDP-based Data Channel Negotiation April 2019 Note:For data channels negotiated via different protocol from DCEP, no convention is defined by default. A.2. Generic Data Channel Negotiation Not Using DCEP A.2.1. Overview DCEP negotiation only provides for negotiation of data channel transport parameters and does not provide for negotiation of subprotocol specific parameters. DCEP-less data channel negotiation can be defined to allow negotiation of parameters beyond those handled by DCEP, e.g., parameters specific to the subprotocol instantiated on a particular data channel. The following procedures are common to all methods of data channel negotiation not using DCEP, whether in-band (communicated using proprietary means on an already established data channel) or out-of- band (using SDP offer/answer or some other protocol associated with the signaling channel). A.2.2. Opening a Data Channel In the case of DCEP-less negotiation, the endpoint application has the option to fully control the stream identifier assignments. However these assignments have to coexist with the assignments controlled by the data channel stack for the DCEP negotiated data channels (if any). It is the responsibility of the application to ensure consistent assignment of stream identifiers. When the application requests the creation of a new data channel to be set up via DCEP-less negotiation, the data channel stack creates the data channel locally without sending any DATA_CHANNEL_OPEN message in-band. However, even if the ICE (Interactive Connectivity Establishment), DTLS and SCTP procedures were already successfully completed, the application can't send data on this data channel until the negotiation is complete with the peer. This is because the peer needs to be aware of and accept the usage of this data channel. The peer, after accepting the data channel offer, can start sending data immediately. This implies that the offerer may receive data channel subprotocol messages before the negotiation is complete and the application should be ready to handle it. If the peer rejects the data channel part of the offer then it doesn't have to do anything as the data channel was not created using the stack. The offerer on the other hand needs to close the data channel that was opened by invoking relevant data channel stack API procedures. Drage, et al. Expires October 25, 2019 [Page 38] Internet-Draft SDP-based Data Channel Negotiation April 2019 It is also worth noting that a data channel stack implementation may not provide any API to create and close data channels; instead the data channels may be used on the fly as needed just by communicating via non-DCEP means or by even having some local configuration/ assumptions on both the peers. The application then negotiates the data channel properties and subprotocol properties with the peer's application using a mechanism different from DCEP. The peer then symmetrically creates a data channel with these negotiated data channel properties. This is the only way for the peer's data channel stack to know which properties to apply when transmitting data on this channel. The data channel stack must allow data channel creation with any non-conflicting stream identifier so that both peers can create the data channel with the same stream identifier. A.2.3. Closing a Data Channel When the application requests the closing of a data channel negotiated without DCEP, the data channel stack always performs an SCTP SSN reset for this channel. Depending upon the method used for DCEP-less negotiation and the subprotocol associated with the data channel, the closing might in addition be signaled to the peer via SDP offer/answer negotiation. Authors' Addresses Keith Drage Unaffiliated Email: drageke@ntlworld.com Maridi R. Makaraju (Raju) Nokia 2000 Lucent Lane Naperville, Illinois US Email: Raju.Makaraju@nokia.com Drage, et al. Expires October 25, 2019 [Page 39] Internet-Draft SDP-based Data Channel Negotiation April 2019 Richard Ejzak Unaffiliated Email: richard.ejzak@gmail.com Jerome Marcon Unaffiliated Email: jeromee.marcon@free.fr Roni Even (editor) Huawei Email: roni.even@huawei.com Drage, et al. Expires October 25, 2019 [Page 40]