When message type id yes 1,2,3,5,6 when , It means that we should message Is a protocol control message (Protocol Control Message).

These types of messages are used to control chunk Some behaviors of hierarchy , The specific type name is as follows ：

• 1：Set Chunk Size
• 2：Abort Message
• 3：Acknowledgement
• 5：Windon Acknowledgement Size
• 6：Set Peer Bandwidth

Protocol control message There are several important rules ：

• Chunk stream id Must be 2,message stream id Must be 0.
• The receiver is receiving protocol control message Must be dealt with immediately after , Only after processing can subsequent processing be received chunk.
• timestamp Fields are ignored .

Before the start , Let's agree on a few definitions ：

• client： launch handshake On the side of .
• server：client The opposite end of .
• The sender ： send out Protocol Control Message On the side of .
• The receiver ： receive Protocol Control Message On the side of .

client/server, The sender / The receiver is two sets of concepts .client/server It's at the link level , After the link is established, it will not change . The sender / The receiving end is message level , It refers to the sender and receiver of a message .client or server It can be the sending end or the receiving end .

Set Chunk Size (1)

As mentioned before ,maximum chunk size It's a single chunk Of payload size Upper limit , The default value is 128, Company byte. The sending end can use Set Chunk Size Type of message adjustment maximum chunk size Size .

Set Chunk Size The message carries a 32-bit The integer of —— chunk size, The value range of this field is [1, 0x7FFFFFFF]. actually ,chunk size The value of should not exceed 0xFFFFFF . because message length The fields are only 24-bit, Its value cannot exceed 0xFFFFFF .

Be careful , Here is the sender and the receiver , After I send this message , follow-up chunk Of payload size The upper limit has been adjusted , But you sent me chunk Of payload size The upper limit will not change .

for instance , Suppose you want to send the following two message：

	Message Stream ID	Message Type ID	Timestamp	Length
MSG # 1	10	8	1000	280
MSG # 2	10	8	1020	150

stay maximum chunk size yes 128 when , Will be encapsulated as 5 individual chunk, As follows ：

	Chunk Stream ID	Chunk Format	Timestamp(Delta)	Message Length	Message Type ID	Message Stream ID	Chunk Data Size
CHK # 1	3	0	1000	280	8	10	128
CHK # 2	3	3	\	\	\	\	128
CHK # 3	3	3	\	\	\	\	24
CHK # 4	3	1	20	150	8	10	128
CHK # 5	3	3	\	\	\	\	22

If , The sender is at CHK#1 Then add a Set Chunk Size, The upper limit of the opposite end of the notification is adjusted to 152, As mentioned above chunk Turn into ：

	Chunk Stream ID	Chunk Format	Timestamp(Delta)	Message Length	Message Type ID	Message Stream ID	Chunk Data Size
CHK # 1	3	0	1000	280	8	10	128
CHK # S	2	0	0	4	1	0	4 ( The values are as follows 0,0,0,152)
CHK # 2	3	3	\	\	\	\	152
CHK # 4	3	1	20	150	8	10	150

obviously , The receiver is receiving CHK#S after , According to maximum chunk size by 152 Read subsequent chunk La .

Abort Message (2)

The message carries a 32-bit The integer of —— chunk stream id, It is used to notify the receiving end to discard the message chunk stream Not yet received on message.

But the news seems to have been abandoned , There is also a delete line in the document .

FFmpeg The message type is also not supported , After receiving this type of message, it will return the error code AVERROR_UNKNOWN.

// libavformat/rtmpproto.c
 434         } else if (pkt.type == RTMP_CTRL_ABORT_MESSAGE) {
    
 435             av_log(s, AV_LOG_ERROR, "received abort message\n");
 436             ff_rtmp_packet_destroy(&pkt);
 437             return AVERROR_UNKNOWN;
 438         } else if (pkt.type == RTMP_PT_BYTES_READ) {
    

Acknowledgement (3)

The message carries a 32-bit The integer of —— sequence number, Used to notify the receiving end of me ( The sender ) Number of bytes received .

A few small details ：

• Statistics are in all chunk stream Number of bytes read on .
• chunk header Of size It will also be counted .
• Every time w Bytes , Just send it once .w From the opposite end through Window Acknowledgement Size Appoint . About w The default value of , The document does not mention ,FFmpeg take $2^{20}$ As default .

Window Acknowledgement Size (5)

The message carries a 32-bit The integer of —— window size, Used to set the receiver to send Acknowledgement The frequency of the message . Every time the receiving end receives window size Bytes , At least Send a Acknowledgement news .

send out - Waiting mechanism ： The message that is expected to be sent by each end window size You get a message in a byte Acknowledgement news , If it is not received, it will stop sending data , until Acknowledgement arrival .

stay FFmpeg In the implementation of , Every time $\frac{windowsize}{2}$ Bytes , Just send it once Acknowledgement news , The specific code is as follows , The reason for this is also easy to understand , See also 1596 -1598 Comment on line .

// libavformat/rtmpproto.c
1578 static int handle_window_ack_size(URLContext *s, RTMPPacket *pkt)
1579 {
    
1580     RTMPContext *rt = s->priv_data;
1581
1582     if (pkt->size < 4) {
    
1583         av_log(s, AV_LOG_ERROR,
1584                "Too short window acknowledgement size packet (%d)\n",
1585                pkt->size);
1586         return AVERROR_INVALIDDATA;
1587     }
1588
1589     rt->receive_report_size = AV_RB32(pkt->data);
1590     if (rt->receive_report_size <= 0) {
    
1591         av_log(s, AV_LOG_ERROR, "Incorrect window acknowledgement size %d\n",
1592                rt->receive_report_size);
1593         return AVERROR_INVALIDDATA;
1594     }
1595     av_log(s, AV_LOG_DEBUG, "Window acknowledgement size = %d\n", rt->receive_report_size);
1596     // Send an Acknowledgement packet after receiving half the maximum
1597     // size, to make sure the peer can keep on sending without waiting
1598     // for acknowledgements.
1599     rt->receive_report_size >>= 1;
1600
1601     return 0;
1602 }

Set Peer Bandwidth (6)

The message contains two fields ：

• Window size：32-bit The integer of .
• Limit type：8-bit Integers .

Limit type There are three values for ：

• 0 - Hard： The receiving end shall use window size As a parameter for controlling bandwidth .
• 1 - Sort： Use window size And in effect window size Minimum of .
• 2 - Dynamic： If the previous one Limit Type yes Hard, Then the message is sent according to Hard The rules take effect , On the contrary, the message is discarded .

Set Peer Bandwidth Need to cooperate Acknowledgement And Window Acknowledgement Size Working together . There are two endpoints with successful handshake A and B, The workflow is as follows ：

• A towards B send out Set Peer Bandwidth,window size = 1024.
• B After receipt of , towards A send out Window Acknowledgement Size,window size Need to be the same , It's also 1024.
• A received Window Acknowledgement Size after , Every time 1024 Bytes , Send at least once Acknowledgement.
• B Sending Window Acknowledgement Size after , Will trigger send out - Waiting mechanism To control the bandwidth .

3,5,6 Example

There are two endpoints A and B The handshake has been successful , Now? A To pass the Set Peer Bandwidth control B The transmission rate of , The process is as follows ：