| RFC 8856 | SDP Format for BFCP Streams | January 2021 |
| Camarillo, et al. | Standards Track | [Page] |
This document defines the Session Description Protocol (SDP) offer/answer procedures for negotiating and establishing Binary Floor Control Protocol (BFCP) streams.¶
This document obsoletes RFC 4583.¶
This is an Internet Standards Track document.¶
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.¶
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8856.¶
Copyright (c) 2021 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 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.¶
As discussed in the BFCP (Binary Floor Control Protocol) specification [RFC8855], a given BFCP client needs a set of data in order to establish a BFCP connection to a floor control server. This data includes the transport address of the server, the conference identifier, and the user identifier.¶
One way for clients to obtain this information is to use a Session Description Protocol (SDP) offer/answer exchange [RFC3264]. This document specifies how to encode this information in the SDP session descriptions that are part of such an offer/answer exchange.¶
User agents typically use the offer/answer model to establish a number of media streams of different types. Following this model, a BFCP connection is described as any other media stream by using an SDP "m=" line, possibly followed by a number of SDP lines that also apply to the BFCP connection.¶
Section 4 defines how the field values in an "m=" line representing a BFCP connection are set.¶
Section 5 defines SDP attributes that are used when negotiating a BFCP connection.¶
Section 6 defines multiplexing considerations for a BFCP connection.¶
Section 7 defines procedures for managing a BFCP connection.¶
Section 8 defines TLS and DTLS considerations when negotiating a BFCP connection.¶
Section 9 defines considerations regarding Interactive Connectivity Establishment (ICE) [RFC8445] when negotiating a BFCP connection.¶
Section 10 defines the SDP offer/answer procedures for negotiating a BFCP connection.¶
This document obsoletes RFC 4583 [RFC4583]. Section 14 summarizes the changes from RFC 4583.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
When two endpoints establish a BFCP stream, they need to determine which of them acts as a floor control client and which acts as a floor control server.¶
Once the roles have been determined, the roles will apply to all BFCP-controlled streams associated with the BFCP stream.¶
According to the SDP specification [RFC8866], the "m=" line format is as follows:¶
m=<media> <port> <proto> <fmt> ...¶
This section describes how to generate an "m=" line of an SDP Media Description ("m=" section) describing a BFCP stream.¶
The media field MUST have a value of "application".¶
Depending on the value of the proto field, the port field is set as explained below. A port field value of zero has the standard SDP meaning (i.e., rejection of the media stream), regardless of the proto field.¶
This document defines five values for the proto field: 'TCP/BFCP', 'TCP/DTLS/BFCP', 'TCP/TLS/BFCP', 'UDP/BFCP', and 'UDP/TLS/BFCP'.¶
The proto values are used as described below:¶
The fmt (format) list is not applicable to BFCP. The fmt list of "m=" lines in the case of any proto field value related to BFCP MUST contain a single "*" character. If the fmt list contains any other value, it MUST be ignored.¶
The following is an example of an "m=" line for a BFCP connection:¶
m=application 50000 TCP/TLS/BFCP *¶
This section defines the SDP 'floorctrl' media-level attribute. The attribute is used to determine the floor control roles (client and server) for the endpoints associated with the BFCP stream.¶
The Augmented BNF syntax [RFC5234] for the attribute is:¶
floor-control = role *(SP role) role = "c-only" / "s-only" / "c-s"¶
An endpoint includes the attribute to indicate the role(s) it would be willing to perform for the BFCP-controlled media streams:¶
When inserted in an offer, the offerer MAY indicate multiple attribute values ("c-only" and "s-only"). When inserted in an answer, the answerer MUST indicate only one attribute value: "c-only" or "s-only". The answerer indicates the role taken by the answerer. The offerer will then take the opposite role.¶
In [RFC4583], there was a third attribute specified, "c-s", which meant that an endpoint was willing to act as both a floor control client and a floor control server at the same time for the BFCP stream, taking different roles for different BFCP-controlled media streams. The feature was underspecified and implemented in different ways; in particular, many implementations interpreted "c-s" to mean that the endpoint is willing to act as either a client or a server (equivalent to "c-only s-only"). An implementation compliant with this specification MUST NOT include the "c-s" 'floorctrl' attribute value in an offer or in an answer but MUST accept the attribute value in an offer and process it as equivalent to "c-only s-only" (or "s-only c-only"). Also, as an implementation compliant with this specification is only allowed to include one role -- either "c-only" or "s-only" -- in an answer, each endpoint will only take one role, and as a result the endpoint will take the same role for each BFCP-controlled media stream associated with the BFCP stream.¶
Table 1 shows the roles that the answerer is allowed to take, based on what roles the offerer has indicated that it is willing to take.¶
Endpoints compliant with [RFC4583] might not include the 'floorctrl' attribute in offers and answers. If the 'floorctrl' attribute is not present, in order to be interoperable with such endpoints, the offerer will act as a floor control client and the answerer will act as a floor control server.¶
The SDP Offer/Answer procedures for the 'floorctrl' attribute are defined in Section 10.¶
The following is an example of a 'floorctrl' attribute in an offer:¶
a=floorctrl:c-only s-only¶
This section defines the SDP 'confid' media-level attribute. The attribute is used by a floor control server to convey the conference ID value to the floor control client, using decimal integer representation.¶
The Augmented BNF syntax [RFC5234] for the attribute is:¶
conference-id = 1*DIGIT DIGIT = <DIGIT as defined in [RFC5234]>¶
The maximum value of the attribute is determined by the COMMON-HEADER format [RFC8855].¶
The SDP Offer/Answer procedures for the 'confid' attribute are defined in Section 10.¶
This section defines the SDP 'userid' media-level attribute. The attribute is used by a floor control server to convey the user ID value to the floor control client, using decimal integer representation.¶
The Augmented BNF syntax [RFC5234] for the attribute is:¶
user-id = 1*DIGIT DIGIT = <DIGIT as defined in [RFC5234]>¶
The maximum value of the attribute is determined by the COMMON-HEADER format [RFC8855].¶
The SDP Offer/Answer procedures for the 'userid' attribute are defined in Section 10.¶
This section defines the SDP 'floorid' media-level attribute. The attribute is used to convey a floor identifier, using decimal integer representation, and, optionally, pointers to one or more BFCP-controlled media streams.¶
The Augmented BNF syntax [RFC5234] for the attribute is:¶
floor-id = 1*DIGIT SP "mstrm:" token *(SP token) DIGIT = <DIGIT as defined in [RFC5234]> token = <token as defined in [RFC8866]>¶
The maximum value of the attribute is determined by the FLOOR-ID format [RFC8855].¶
The floor identifier value is the integer representation of the Floor ID field value [RFC8855] to be used in BFCP. Each media stream pointer value is associated with an SDP 'label' attribute [RFC4574] of a media stream.¶
The SDP Offer/Answer procedures for the 'floorid' attribute are defined in Section 10.¶
This section defines the SDP 'bfcpver' media-level attribute. The attribute is used to negotiate the BFCP version, using decimal integer representation.¶
The Augmented BNF syntax [RFC5234] for the attribute is:¶
bfcp-version = version *(SP version) version = 1*DIGIT DIGIT = <DIGIT as defined in [RFC5234]>¶
The maximum value of the attribute is determined by the COMMON-HEADER format [RFC8855].¶
An endpoint uses the 'bfcpver' attribute to convey the version(s) of BFCP supported by the endpoint, using integer values. For a given version, the attribute value representing the version MUST match the version ("Ver") field that would be presented in the BFCP COMMON‑HEADER [RFC8855]. The BFCP version that will eventually be used will be conveyed with a BFCP-level Hello/HelloAck.¶
Endpoints compliant with [RFC4583] might not always include the 'bfcpver' attribute in offers and answers. The attribute value, if present, MUST be in accordance with the definition of the version ("Ver") field in [RFC8855]. If the attribute is not present, endpoints MUST assume a default value in accordance with [RFC8855]: when used over a reliable transport, the default attribute value is "1", and when used over an unreliable transport, the default attribute value is "2". The value is inferred from the transport specified in the "m=" line (Section 4) of the "m=" section associated with the stream.¶
The SDP Offer/Answer procedures for the 'bfcpver' attribute are defined in Section 10.¶
[RFC8843] defines how multiplexing of multiple media streams can be negotiated. This specification does not define how BFCP streams can be multiplexed with other media streams. Therefore, a BFCP stream MUST NOT be associated with a BUNDLE group [RFC8843]. Note that BFCP-controlled media streams might be multiplexed with other media streams.¶
[RFC8859] defines the mux categories for the SDP attributes defined in this specification, except for the 'bfcpver' attribute. Table 2 defines the mux category for the 'bfcpver' attribute:¶
| Name | Notes | Level | Mux Category |
|---|---|---|---|
| bfcpver | Needs further analysis in a separate specification | M | TBD |
BFCP streams can use TCP or UDP as the underlying transport. Endpoints exchanging BFCP messages over UDP send the BFCP messages towards the peer using the connection address and port provided in the SDP "c=" and "m=" lines. TCP connection management is more complicated and is described in the following section.¶
The management of the TCP connection used to transport BFCP messages is performed using the SDP 'setup' and 'connection' attributes [RFC4145]. The 'setup' attribute indicates which of the endpoints initiates the TCP connection. The 'connection' attribute handles TCP connection re-establishment.¶
The BFCP specification [RFC8855] describes a number of situations when the TCP connection between a floor control client and the floor control server needs to be re-established. However, [RFC8855] does not describe the re-establishment process, because this process depends on how the connection was established in the first place. Endpoints using the offer/answer mechanism follow the following rules.¶
When the existing TCP connection is closed and re-established following the rules in [RFC8855], the floor control client MUST send an offer towards the floor control server in order to re-establish the connection. If a TCP connection cannot deliver a BFCP message and times out, the endpoint that attempted to send the message (i.e., the one that detected the TCP timeout) MUST send an offer in order to re-establish the TCP connection.¶
Endpoints that use the offer/answer mechanism to negotiate TCP connections MUST support the 'setup' and 'connection' attributes.¶
When DTLS is used with UDP, the generic procedures defined in Section 5 of [RFC8842] MUST be followed.¶
When TLS is used with TCP, once the underlying connection is established, the answerer always acts as the TLS server. If the TCP connection is lost, the active endpoint [RFC4583] is responsible for re-establishing the TCP connection. Unless a new TLS connection is negotiated, subsequent SDP offers and answers will not impact the previously negotiated TLS roles.¶
Generic SDP offer/answer procedures for ICE are defined in [RFC8839].¶
When BFCP is used with UDP-based ICE candidates [RFC8445], the procedures for UDP/TLS/BFCP are used.¶
When BFCP is used with TCP-based ICE candidates [RFC6544], the procedures for TCP/DTLS/BFCP are used.¶
Based on the procedures defined in [RFC8842], endpoints treat all ICE candidate pairs associated with a BFCP stream on top of a DTLS association as part of the same DTLS association. Thus, there will only be one BFCP handshake and one DTLS handshake even if there are multiple valid candidate pairs, and even if BFCP media is shifted between candidate pairs (including switching between UDP candidate pairs and TCP candidate pairs) prior to nomination. If new candidates are added, they will also be part of the same DTLS association.¶
In order to maximize the likelihood of interoperability between the endpoints, all ICE-enabled BFCP-over-DTLS endpoints SHOULD implement support for UDP/TLS/BFCP.¶
When an SDP offer or answer conveys multiple ICE candidates for a BFCP stream, UDP-based candidates SHOULD be included and the default candidate SHOULD be chosen from one of those UDP candidates. If UDP transport is used for the default candidate, then the "m=" line proto value MUST be 'UDP/TLS/BFCP'. If TCP transport is used for the default candidate, the "m=" line proto value MUST be 'TCP/DTLS/BFCP'.¶
This section defines the SDP offer/answer [RFC3264] procedures for negotiating and establishing a BFCP stream. Generic procedures for DTLS are defined in [RFC8842]. Generic procedures for TLS are defined in [RFC8122].¶
This section only defines the BFCP-specific procedures. Unless explicitly stated otherwise, the procedures apply to an "m=" section describing a BFCP stream. If an offer or answer contains multiple "m=" sections describing BFCP streams, the procedures are applied independently to each stream.¶
Within this document, 'initial offer' refers to the first offer within an SDP session (e.g., a Session Initiation Protocol (SIP) dialog when SIP [RFC3261] is used to carry SDP) in which the offerer indicates that it wants to negotiate the establishment of a BFCP stream.¶
If the "m=" line 'proto' value is 'TCP/TLS/BFCP', 'TCP/DTLS/BFCP', or 'UDP/TLS/BFCP', the offerer and answerer follow the generic procedures defined in [RFC8122].¶
If the "m=" line proto value is 'TCP/BFCP', 'TCP/TLS/BFCP', 'TCP/DTLS/TCP', or 'UDP/TLS/BFCP', the offerer and answerer use the SDP 'setup' attribute according to the procedures in [RFC4145].¶
If the "m=" line proto value is 'TCP/BFCP', 'TCP/TLS/BFCP', or 'TCP/DTLS/BFCP', the offerer and answerer use the SDP 'connection' attribute according to the procedures in [RFC4145].¶
When the offerer creates an initial offer, the offerer MUST include an SDP 'floorctrl' attribute (Section 5.1) and an SDP 'bfcpver' attribute (Section 5.5) in the "m=" section.¶
In addition, if the offerer includes an SDP 'floorctrl' attribute with "s-only" or "c-s" attribute values in the offer, the offerer¶
When the answerer receives an offer that contains an "m=" section describing a BFCP stream, the answerer MUST check whether it supports one or more of the BFCP versions supported by the offerer (Section 5.5). If the answerer does not support any of the BFCP versions, it MUST NOT accept the "m=" section. Otherwise, if the answerer accepts the "m=" section, the answerer¶
In addition, if the answerer includes an SDP 'floorctrl' attribute with an "s-only" attribute value in the answer, the answerer¶
Once the answerer has sent the answer, the answerer¶
If the answerer does not accept the "m=" section in the offer, it MUST assign a zero port value to the "m=" line of the corresponding "m=" section in the answer. In addition, the answerer MUST NOT establish a TCP connection or a TLS/DTLS connection associated with the "m=" section.¶
When the offerer receives an answer that contains an "m=" section describing a BFCP stream and with a non-zero port value in the "m=" line, the offerer¶
If the "m=" line in the answer contains a zero port value or if the offerer for some other reason does not accept the answer (e.g., if the answerer only indicates support of BFCP versions not supported by the offerer), the offerer MUST NOT establish a TCP connection or a TLS/DTLS connection associated with the "m=" section.¶
When an offerer sends an updated offer, in order to modify a previously established BFCP stream, it follows the procedures in Section 10.1, with the following exceptions:¶
For the purpose of brevity, the main portion of the session description is omitted in the examples, which only show "m=" sections and their "m=" lines and attributes.¶
The following is an example of an offer sent by a conference server to a client.¶
m=application 50000 TCP/TLS/BFCP *
a=setup:actpass
a=connection:new
a=fingerprint:sha-256 \
19:E2:1C:3B:4B:9F:81:E6:B8:5C:F4:A5:A8:D8:73:04: \
BB:05:2F:70:9F:04:A9:0E:05:E9:26:33:E8:70:88:A2
a=floorctrl:c-only s-only
a=confid:4321
a=userid:1234
a=floorid:1 mstrm:10
a=floorid:2 mstrm:11
a=bfcpver:1 2
m=audio 50002 RTP/AVP 0
a=label:10
m=video 50004 RTP/AVP 31
a=label:11¶
Note that due to RFC formatting conventions, this document splits the SDP entries across lines whose content would exceed the maximum line length. A backslash character ("\") marks where this line folding has taken place. This backslash and its trailing CRLF and whitespace would not appear in actual SDP content.¶
The following is the answer returned by the client.¶
m=application 9 TCP/TLS/BFCP *
a=setup:active
a=connection:new
a=fingerprint:sha-256 \
6B:8B:F0:65:5F:78:E2:51:3B:AC:6F:F3:3F:46:1B:35: \
DC:B8:5F:64:1A:24:C2:43:F0:A1:58:D0:A1:2C:19:08
a=floorctrl:c-only
a=bfcpver:1
m=audio 55000 RTP/AVP 0
m=video 55002 RTP/AVP 31¶
A similar example using an unreliable transport and DTLS is shown below, where the offer is sent from a client.¶
m=application 50000 UDP/TLS/BFCP *
a=setup:actpass
a=dtls-id:abc3dl
a=fingerprint:sha-256 \
19:E2:1C:3B:4B:9F:81:E6:B8:5C:F4:A5:A8:D8:73:04: \
BB:05:2F:70:9F:04:A9:0E:05:E9:26:33:E8:70:88:A2
a=floorctrl:c-only s-only
a=confid:4321
a=userid:1234
a=floorid:1 mstrm:10
a=floorid:2 mstrm:11
a=bfcpver:1 2
m=audio 50002 RTP/AVP 0
a=label:10
m=video 50004 RTP/AVP 31
a=label:11¶
The following is the answer returned by the server.¶
m=application 55000 UDP/TLS/BFCP *
a=setup:active
a=dtls-id:abc3dl
a=fingerprint:sha-256 \
6B:8B:F0:65:5F:78:E2:51:3B:AC:6F:F3:3F:46:1B:35: \
DC:B8:5F:64:1A:24:C2:43:F0:A1:58:D0:A1:2C:19:08
a=floorctrl:s-only
a=confid:4321
a=userid:1234
a=floorid:1 mstrm:10
a=floorid:2 mstrm:11
a=bfcpver:2
m=audio 55002 RTP/AVP 0
m=video 55004 RTP/AVP 31¶
The BFCP specification [RFC8855], SDP specification [RFC8866], and offer/answer specification [RFC3264] discuss security issues related to BFCP, SDP, and offer/answer, respectively. In addition, [RFC4145] and [RFC8122] discuss security issues related to the establishment of TCP and TLS connections using an offer/answer model. Furthermore, when using DTLS over UDP, the generic offer/answer considerations defined in [RFC8842] MUST be followed.¶
The usage of certain proto values in the SDP offer/answer negotiation will result in a BFCP stream that is not protected by TLS or DTLS. Operators will need to provide integrity protection and confidentiality protection of the BFCP stream using other means.¶
The generic security considerations associated with SDP attributes are defined in [RFC3264]. While the attributes defined in this specification do not reveal information about the content of individual BFCP-controlled media streams, they do reveal which media streams will be BFCP controlled.¶
This document registers three new values in the "proto" subregistry within the "Session Description Protocol (SDP) Parameters" registry: 'TCP/DTLS/BFCP', 'UDP/BFCP', and 'UDP/TLS/BFCP' (see Section 13.1).¶
This document also registers a new SDP attribute in the 'attribute-name (formerly "att-field")' subregistry within the "Session Description Protocol (SDP) Parameters" registry: 'bfcpver' (see Section 5.5).¶
The remaining values are unchanged from [RFC4582], except that the references have been updated to refer to this document.¶
The IANA has registered three new values in the SDP 'proto' field under the "Session Description Protocol (SDP) Parameters" registry.¶
| Value | Reference |
|---|---|
| TCP/BFCP | RFC 8856 |
| TCP/DTLS/BFCP | RFC 8856 |
| TCP/TLS/BFCP | RFC 8856 |
| UDP/BFCP | RFC 8856 |
| UDP/TLS/BFCP | RFC 8856 |
This document defines the SDP 'floorctrl' attribute. Details regarding this attribute are provided in Section 5.1.¶
This document defines the SDP 'confid' attribute. Details regarding this attribute are provided in Section 5.2.¶
This document defines the SDP 'userid' attribute. Details regarding this attribute are provided in Section 5.3.¶
This document defines the SDP 'floorid' attribute. Details regarding this attribute are provided in Section 5.4.¶
This document defines the SDP 'bfcpver' attribute. Details regarding this attribute are provided in Section 5.5.¶
The technical changes and other fixes from [RFC4583] are listed below.¶
The main purpose of this work was to add signaling support necessary to support BFCP over an unreliable transport, as described in [RFC8855], resulting in the following changes:¶
Fields in the "m=" Line (Section 4):¶
This section has been rewritten to remove reference to the exclusivity of TCP as a transport for BFCP streams. The proto field values 'TCP/DTLS/BFCP', 'UDP/BFCP', and 'UDP/TLS/BFCP' have been added.¶
Security Considerations (Section 12):¶
For the DTLS-over-UDP case, we direct the reader to existing considerations and requirements for the offer/answer exchange as provided in [RFC8842].¶
Registration of SDP 'proto' Values (Section 13.1):¶
This document registers the three new values 'TCP/DTLS/BFCP', 'UDP/BFCP', and 'UDP/TLS/BFCP' in the "Session Description Protocol (SDP) Parameters" registry.¶
SDP 'bfcpver' Attribute (Section 5.5):¶
A new 'bfcpver' SDP media-level attribute has been added, in order to signal the supported version number.¶
In addition to the changes associated with support of BFCP over an unreliable transport, the possibility that an endpoint can act as both a floor control client and a floor control server at the same time has been removed. An endpoint will now take the same role for all BFCP-controlled streams associated with the BFCP stream.¶
Clarifications and bug fixes:¶
Erratum ID 712 (Sections 3 and 10 of [RFC4583]; see [Err712] for details):¶
Do not use language such as 'used in an "m=" line' when discussing an SDP attribute; instead, make clear that the attribute is a media-level attribute.¶
Spelling corrected in the first SDP example in Section 9 of [RFC4583]:¶
Do not use 'm-stream' as listed in the first SDP example in [RFC4583]; instead, use the correct 'mstrm' as specified in Section 11 of this document. However, we recommend continuing to interpret 'm-stream', if received, because it is still present in some implementations.¶
Assorted clarifications (throughout the document):¶
Language clarifications were made as a result of reviews. Also, normative language was "tightened" where appropriate, i.e., changed from "SHOULD" strength to "MUST" in a number of places.¶
Jörg Ott, Keith Drage, Alan Johnston, Eric Rescorla, Roni Even, and Oscar Novo provided useful ideas for the original [RFC4583]. The authors also acknowledge contributions to the revision of BFCP for use over an unreliable transport from Geir Arne Sandbakken, Charles Eckel, Alan Ford, Eoin McLeod, and Mark Thompson. Useful and important final reviews were done by Ali C. Begen, Mary Barnes, and Charles Eckel. In the final stages, Roman Shpount made a considerable effort in adding proper ICE support and considerations.¶