BFCP 协议

Abstract	BFCP protocol
Authors	Walter Fan
Status	WIP as draft
Updated	2024-08-21

Overview

BFCP 就是 Binary Floor Control Protocol 的缩写，即二进制发言权控制协议。 例如在礼堂里做演讲，讲台上的座席通常只能有一个人在演讲，另外的人如果想要发言，得走上讲台，抢过发言权，抢过麦克风。

BFCP 这个协议在 RFC4582 中有详细的阐述。 实践中， 我们有 BFCP 控制者以及 BFCP 参与者的概念之分，好比辩论比赛的主持人(BFCP chair)和辩手(BFCP participant)。

RFC8855 后来废弃了 RFC4582, 在传输层可以使用 TCP 或者 UDP 来承载 BFCP, 在协议 RFC8856: Session Description Protocol (SDP) Format for Binary Floor Control Protocol (BFCP) Streams 中定义了 BFCP 的 SDP 格式

例如

m=application 50000 UDP/BFCP *

Role

• c-only: The endpoint is willing to act as a floor control client. - 好比主持人

• s-only: The endpoint is willing to act as a floor control server only - 好比辩手

Packet Format

BFCP packets consist of a 12-octet COMMON-HEADER followed by attributes. All the protocol values MUST be sent in network byte order.

COMMON-HEADER Format

The following is the format of the COMMON-HEADER.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Ver |R|F| Res |  Primitive    |        Payload Length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Conference ID                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Transaction ID        |            User ID            |
+> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  | Fragment Offset (if F is set) | Fragment Length (if F is set) |
+> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
+---- These fragment fields are never present
      when using reliable transports

BFCP Primitive

This 8-bit field identifies the main purpose of the message. The following primitive values are defined:

“BFCP Primitive”

Value	Primitive	Direction
1	FloorRequest	P -> S
2	FloorRelease	P -> S
3	FloorRequestQuery	P -> S ; Ch -> S
4	FloorRequestStatus	P <- S ; Ch <- S
5	UserQuery	P -> S ; Ch -> S
6	UserStatus	P <- S ; Ch <- S
7	FloorQuery	P -> S ; Ch -> S
8	FloorStatus	P <- S ; Ch <- S
9	ChairAction	Ch -> S
10	ChairActionAck	Ch <- S
11	Hello	P -> S ; Ch -> S
12	HelloAck	P <- S ; Ch <- S
13	Error	P <- S ; Ch <- S
14	FloorRequestStatusAck	P -> S ; Ch -> S
15	FloorStatusAck	P -> S ; Ch -> S
16	Goodbye	P -> S ; Ch -> S ; P <- S ; Ch <- S
17	GoodbyeAck	P -> S ; Ch -> S ; P <- S ; Ch <- S

• S: Floor Control Server

• P: Floor Participant

• Ch: Floor Chair

Attribute Format

BFCP attributes are encoded in TLV (Type-Length-Value) format. Attributes are 32-bit aligned.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    Type     |M|    Length     |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
|                                                               |
/                       Attribute Contents                      /
/                                                               /
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

BFCP Attributes

“BFCP Primitive”

Value	Attribute	Format
1	BENEFICIARY-ID	Unsigned16
2	FLOOR-ID	Unsigned16
3	FLOOR-REQUEST-ID	Unsigned16
4	PRIORITY	OctetString16
5	REQUEST-STATUS	OctetString16
6	ERROR-CODE	OctetString
7	ERROR-INFO	OctetString
8	PARTICIPANT-PROVIDED-INFO	OctetString
9	STATUS-INFO	OctetString
10	SUPPORTED-ATTRIBUTES	OctetString
11	SUPPORTED-PRIMITIVES	OctetString
12	USER-DISPLAY-NAME	OctetString
13	USER-URI	OctetString
14	BENEFICIARY-INFORMATION	Grouped
15	FLOOR-REQUEST-INFORMATION	Grouped
16	REQUESTED-BY-INFORMATION	Grouped
17	FLOOR-REQUEST-STATUS	Grouped
18	OVERALL-REQUEST-STATUS	Grouped

TBD…