| BFCPbis Working Group | C. Holmberg |
| Internet-Draft | G. Camarillo |
| Obsoletes: 4583 (if approved) | Ericsson |
| Intended status: Standards Track | T. Kristensen |
| Expires: September 25, 2018 | Cisco |
| March 24, 2018 |
Session Description Protocol (SDP) Format for Binary Floor Control Protocol (BFCP) Streams
draft-ietf-bfcpbis-rfc4583bis-20
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. Changes from RFC 4583 are summarized in Section 15.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
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As discussed in the BFCP (Binary Floor Control Protocol) specification [7], 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 an SDP offer/answer [4] exchange. 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 attributes encoded in 'a' lines.
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, RFC 2119 and indicate requirement levels for compliant implementations.
This section describes how to generate an 'm' line for a BFCP stream.
According to the SDP specification [10], the 'm' line format is the following:
The media field MUST have a value of "application".
The port field is set depending on the value of the proto field, 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.
TCP/BFCP is used when BFCP runs directly on top of TCP. TCP/TLS/BFCP is used when BFCP runs on top of TLS, which in turn runs on top of TCP. TCP/DTLS/BFCP is used when running BFCP on top of DTLS [11], as described in this specification, which in turn runs on top of TCP using the framing method defined in [12] with DTLS packets being sent and received instead of RTP/RTCP packets using the shim defined in RFC4571 such that the length field defined in RFC4571 precedes each DTLS message.
Similarly, UDP/BFCP is used when BFCP runs directly on top of UDP, and UDP/TLS/BFCP is used when BFCP runs on top of DTLS, which in turn runs on top of UDP.
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 the fmt list contains any other value it is ignored.
The following is an example of an 'm' line for a BFCP connection:
m=application 50000 TCP/TLS/BFCP *
When two endpoints establish a BFCP stream, they need to determine which of them acts as floor control client and which acts as floor control server. Typically, a client that establishes a BFCP stream with a conference server will act as floor control client, while the conference server will act as floor control server. However, there are scenarios where both endpoints would be able to act as floor control server. For example, in a two-party session that involves an audio stream and a shared whiteboard, the endpoints need to determine which party will be act as floor control server.
Furthermore, there are situations where both endpoints act as both floor control client and floor control server within the same session. For example, in a two-party session that involves an audio stream and a shared whiteboard, one endpoint acts as the floor control server for the audio stream and the other endpoint acts as the floor control server for the shared whiteboard. However, for a given BFCP-controlled media stream one endpoint MUST act as floor control client and one endpoint MUST act as floor control server.
This section defines the SDP 'floorctrl' media-level attribute. The attribute is used to determine the floor control role(s) that the endpoints can take for the BFCP-controlled media streams. As described in Section 5, an endpoint can take different roles for different media streams, but for a given media stream an endpoint can only take one role.
The Augmented BNF syntax [2] for the attribute is:
floor-control-attribute = "a=floorctrl:" 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. When inserted in an answer, the answerer MUST indicate only one attribute value. The offerer indicates which floor control role(s) that it is willing to take. The answerer indicates the role taken by the answerer. Based on this, the floor control role(s) of the offerer is determined, as shown in Table 1.
| Offerer | Answerer |
|---|---|
| c-only | s-only |
| s-only | c-only |
| c-s | c-s |
Endpoints compliant with [16] might not include the 'floorctrl' attribute in offers and answerer. If the 'floorctrl' attribute is not present the offerer will act as floor control client, and the answerer will act as floor control server, for each BFCP-controlled media stream.
The SDP Offer/Answer procedures for the 'floorctrl' attribute are defined in Section 13.
The following is an example of a 'floorctrl' attribute in an offer:
a=floorctrl:c-only s-only c-s
This section defines the SDP 'confid' and the 'userid' media-level attributes. The attributes are used by a floor control server to convey the conference ID value and user ID value to the floor control client, using decimal integer representation.
The Augmented BNF syntax [2] for the attributes is:
confid-attribute = "a=confid:" conference-id
conference-id = token
userid-attribute = "a=userid:" user-id
user-id = token
token-char = %x21 / %x23-27 / %x2A-2B / %x2D-2E / %x30-39
/ %x41-5A / %x5E-7E
token = 1*(token-char)
;token-char and token elements are defined in [RFC4566].
The SDP Offer/Answer procedures for the 'confid' and 'userid' attributes are defined in Section 13.
This section defines the SDP 'floorid' media-level attribute. The attribute conveys a floor identifier, and optionally pointers to one or more BFCP-controlled media streams.
The Augmented BNF syntax [2] for the attribute is:
floor-id-attribute = "a=floorid:" token SP "mstrm:" token *(SP token)
The floor identifier value is the integer representation of the Floor ID to be used in BFCP. Each media stream pointer value is associated with an SDP 'label' attribute [8] of a media stream.
The SDP Offer/Answer procedures for the 'floorid' attribute are defined in Section 13.
This section defines the SDP 'bfcpver' media-level attribute. The attribute is used to negotiate the BFCP version.
The Augmented BNF syntax [2] for the attributes is:
bfcp-version-attribute = "a=bfcpver:" bfcp-version *(SP bfcp-version)
bfcp-version = token
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" field that would be presented in the BFCP COMMON-HEADER [7]. The BFCP version that will eventually be used will be conveyed with a BFCP-level Hello/HelloAck.
Endpoints compliant with [16] might not always include the 'bfcpver' attribute in offers and answers. If the 'bfcpver' attribute is not present, the default values are inferred from the transport specified in the 'm' line (Section 3) associated with the stream. In accordance with definition of the Version field in [7], 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 SDP Offer/Answer procedures for the 'bfcpver' attribute are defined in Section 13.
[20] 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 [20]. Note that BFCP-controlled media streams might be multiplexed with other media streams.
[21] defines the mux categories for the SDP attributes defined in this specification, excluding the SDP 'bfcpver' attribute. . Table 2 defines the mux category for the 'bfcpver' attribute:
| Name | Notes | Level | Mux Category |
|---|---|---|---|
| bfcpver | Needs further analysis | 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 [6]. The 'setup' attribute indicates which of the endpoints initiates the TCP connection. The 'connection' attribute handles TCP connection re-establishment.
The BFCP specification [7] 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, that specification 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 [7], 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 a BFCP stream is negotiated using the SDP offer/answer mechanism, it is assumed that the offerer and the answerer authenticate each other using some mechanism. TLS/DTLS is the preferred mechanism. Other mechanisms are possible, but are outside the scope of this document. Once this mutual authentication takes place, all the offerer and the answerer need to ensure is that the entity they are receiving BFCP messages from is the same as the one that generated the previous offer or answer.
The initial mutual authentication SHOULD take place at the signaling level. Additionally, signaling can use S/MIME [5] to provide an integrity-protected channel with optional confidentiality for the offer/answer exchange. BFCP takes advantage of this integrity-protected offer/answer exchange to perform authentication. Within the offer/answer exchange, the offerer and answerer exchange the fingerprints of their self-signed certificates. These self-signed certificates are then used to establish the TLS/DTLS connection that will carry BFCP traffic between the offerer and the answerer.
Endpoints follow the rules in [9] regarding certificate choice and presentation. Endpoints that use the offer/answer model to establish BFCP streams MUST support the 'fingerprint' attribute and MUST include it in their offers and answers.
When TLS is used with TCP, once the underlying connection is established, the answerer, which can be the floor control client or the floor control server, acts as the TLS server regardless of its role (passive or active) in the TCP establishment procedure. If the TCP connection is lost, the active endpoint is responsible for re-establishing the TCP connection. Unless a new TLS session is negotiated, subsequent SDP offers and answers will not impact the previously negotiated TLS roles.
When DTLS is used with UDP, the requirements specified in Section 5 of [14] MUST be followed.
Generic SDP offer/answer procedures for Interactive Connectivity Establishment (ICE) are defined in [18].
When BFCP is used with UDP based ICE candidates [17] then the procedures for UDP/TLS/BFCP are used.
When BFCP is used with TCP based ICE candidates [13] then the procedures for TCP/DTLS/BFCP are used.
Based on the procedures defined in [14], 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 if BFCF media is shifted between candidate pairs (including switching between UDP to 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 [4] procedures for negotiating and establishing a BFCP stream. Generic procedures for DTLS are defined in [14]. Generic procedures for TLS are defined in [9].
This section only defines the BFCP-specific procedures. Unless explicitly stated otherwise, the procedures apply to an 'm' line describing a BFCP stream. If an offer or answer contains multiple 'm' lines describing BFCP streams, the procedures are applied independently to each 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 [9].
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 [6].
If the 'm' line proto value is 'TCP/BFCP', 'TCP/TLS/BFCP' or 'TCP/DTLS/BFCP', the offerer and anwerer use the SDP 'connection' attribute according to the procedures in [6].
When the offerer creates an initial offer, the offerer:
When the answerer receives an offer, which contains an 'm' line describing a BFCP stream, if the answerer accepts the 'm' line it:
Once the answerer has sent the answer, the answerer:
If the answerer does not accept the 'm' line in the offer, it MUST assign a zero port value to the corresponding 'm' line in the answer. In addition, the answerer MUST NOT establish a TCP connection or a TLS/DTLS connection associated with the 'm' line.
When the offerer receives an answer, which contains an 'm' line with a non-zero port value, describing a BFCP stream, 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, the offerer MUST NOT establish a TCP connection or a TLS/DTLS connection associated with the 'm' line.
When an offerer sends an updated offer, in order to modify a previously established BFCP stream, it follows the procedures in Section 13.1, 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' lines and their 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
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 SDP across lines whose content would exceed 72 characters. 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 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: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 [7], SDP [10], and offer/answer [4] specifications discuss security issues related to BFCP, SDP, and offer/answer, respectively. In addition, [6] and [9] discuss security issues related to the establishment of TCP and TLS connections using an offer/answer model. Furthermore, when using DTLS over UDP, considerations for its use with RTP and RTCP are presented in [14]. The requirements for the offer/answer exchange, as listed in Section 5 of [14], MUST be followed.
An initial integrity-protected channel is REQUIRED for BFCP to exchange self-signed certificates between a client and the floor control server. For session descriptions carried in SIP [3], S/MIME [5] is the natural choice to provide such a channel.
The IANA has registered the following values for the SDP 'proto' field under the Session Description Protocol (SDP) Parameters registry:
| Value | Reference |
|---|---|
| TCP/BFCP | [RFC XXXX] |
| TCP/DTLS/BFCP | [RFC XXXX] |
| TCP/TLS/BFCP | [RFC XXXX] |
| UDP/BFCP | [RFC XXXX] |
| UDP/TLS/BFCP | [RFC XXXX] |
The IANA has registered the following SDP att-field under the Session Description Protocol (SDP) Parameters registry:
The IANA has registered the following SDP att-field under the Session Description Protocol (SDP) Parameters registry:
The IANA has registered the following SDP att-field under the Session Description Protocol (SDP) Parameters registry:
The IANA has registered the following SDP att-field under the Session Description Protocol (SDP) Parameters registry:
The IANA has registered the following SDP att-field under the Session Description Protocol (SDP) Parameters registry:
Following is the list of technical changes and other fixes from [16].
Main purpose of this work was to add signaling support necessary to support BFCP over unreliable transport, as described in [7], resulting in the following changes:
Clarification and bug fixes:
Joerg Ott, Keith Drage, Alan Johnston, Eric Rescorla, Roni Even, and Oscar Novo provided useful ideas for the original [16]. 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.
| [19] | Lennox, J., Ott, J. and T. Schierl, "Source-Specific Media Attributes in the Session Description Protocol (SDP)", RFC 5576, DOI 10.17487/RFC5576, June 2009. |
| [20] | Holmberg, C., Alvestrand, H. and C. Jennings, "Negotiating Media Multiplexing Using the Session Description Protocol (SDP)", Internet-Draft draft-ietf-mmusic-sdp-bundle-negotiation-49, March 2018. |
| [21] | Nandakumar, S., "A Framework for SDP Attributes when Multiplexing", Internet-Draft draft-ietf-mmusic-sdp-mux-attributes-17, February 2018. |