BFCP 协议
Abstract |
BFCP protocol |
Authors |
Walter Fan |
Status |
v1.0 |
Updated |
2026-03-20 |
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:
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
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 |
典型交互流程
以一个简单的发言权请求场景为例：
Participant(P) Server(S) Chair(Ch)
| | |
|--- FloorRequest ----------------->| |
| |--- FloorRequestStatus -------->|
| | (Status: Pending) |
| | |
| |<--- ChairAction ---------------|
| | (accept/deny) |
| |--- ChairActionAck ------------>|
| | |
|<-- FloorRequestStatus ------------| |
| (Status: Granted/Denied) | |
| | |
发言权的状态可以是：
Pending：等待主席审批
Accepted：主席同意，等待发言权可用
Granted：已获得发言权
Denied：被拒绝
Cancelled：被取消
Released：已释放
Revoked：被撤销
在 WebRTC/SIP 中的应用
BFCP 主要用于视频会议系统中的内容共享（Content Sharing）场景。 在 SDP 中声明 BFCP 流：
m=application 50000 UDP/BFCP *
a=setup:actpass
a=connection:new
a=floorctrl:c-s
a=confid:4321
a=userid:1234
a=floorid:1 mstrm:2
m=video 50002 RTP/AVP 31
a=label:2
其中 a=floorid:1 mstrm:2 将 floor ID 1 关联到 label 为 2 的视频流（即内容共享流）。