SRS 代码分析【RTMP Chunck数据读取】
1.RTMP Chunck数据读取主要是调用SrsProtocol::recv_interlaced_message,该函数的定义如下：
int SrsProtocol::recv_interlaced_message(SrsCommonMessage** pmsg)
{
int ret = ERROR_SUCCESS;
// chunk stream basic header.
char fmt = 0;
int cid = 0;
if ((ret = read_basic_header(fmt, cid)) != ERROR_SUCCESS) {
if (ret != ERROR_SOCKET_TIMEOUT && !srs_is_client_gracefully_close(ret)) {
srs_error("read basic header failed. ret=%d", ret);
}
return ret;
}
srs_verbose("read basic header success. fmt=%d, cid=%d", fmt, cid);
// the cid must not negative.
srs_assert(cid >= 0);
// get the cached chunk stream.
SrsChunkStream* chunk = NULL;
// use chunk stream cache to get the chunk info.
// @see https://github.com/ossrs/srs/issues/249
if (cid < SRS_PERF_CHUNK_STREAM_CACHE) {
// chunk stream cache hit.
srs_verbose("cs-cache hit, cid=%d", cid);
// already init, use it direclty
chunk = cs_cache[cid];
srs_verbose("cached chunk stream: fmt=%d, cid=%d, size=%d, message(type=%d, size=%d, time=%" PRId64 ", sid=%d)",
chunk->fmt, chunk->cid, (chunk->msg? chunk->msg->size : 0), chunk->header.message_type, chunk->header.payload_length,
chunk->header.timestamp, chunk->header.stream_id);
} else {
// chunk stream cache miss, use map.
if (chunk_streams.find(cid) == chunk_streams.end()) {
chunk = chunk_streams[cid] = new SrsChunkStream(cid);
// set the perfer cid of chunk,
// which will copy to the message received.
chunk->header.perfer_cid = cid;
srs_verbose("cache new chunk stream: fmt=%d, cid=%d", fmt, cid);
} else {
chunk = chunk_streams[cid];
srs_verbose("cached chunk stream: fmt=%d, cid=%d, size=%d, message(type=%d, size=%d, time=%" PRId64 ", sid=%d)",
chunk->fmt, chunk->cid, (chunk->msg? chunk->msg->size : 0), chunk->header.message_type, chunk->header.payload_length,
chunk->header.timestamp, chunk->header.stream_id);
}
}
// chunk stream message header
if ((ret = read_message_header(chunk, fmt)) != ERROR_SUCCESS) {
if (ret != ERROR_SOCKET_TIMEOUT && !srs_is_client_gracefully_close(ret)) {
srs_error("read message header failed. ret=%d", ret);
}
return ret;
}
srs_verbose("read message header success. "
"fmt=%d, ext_time=%d, size=%d, message(type=%d, size=%d, time=%" PRId64 ", sid=%d)",
fmt, chunk->extended_timestamp, (chunk->msg? chunk->msg->size : 0), chunk->header.message_type,
chunk->header.payload_length, chunk->header.timestamp, chunk->header.stream_id);
// read msg payload from chunk stream.
SrsCommonMessage* msg = NULL;
if ((ret = read_message_payload(chunk, &msg)) != ERROR_SUCCESS) {
if (ret != ERROR_SOCKET_TIMEOUT && !srs_is_client_gracefully_close(ret)) {
srs_error("read message payload failed. ret=%d", ret);
}
return ret;
}
// not got an entire RTMP message, try next chunk.
if (!msg) {
srs_verbose("get partial message success. size=%d, message(type=%d, size=%d, time=%" PRId64 ", sid=%d)",
(msg? msg->size : (chunk->msg? chunk->msg->size : 0)), chunk->header.message_type, chunk->header.payload_length,
chunk->header.timestamp, chunk->header.stream_id);
return ret;
}
*pmsg = msg;
srs_info("get entire message success. size=%d, message(type=%d, size=%d, time=%" PRId64 ", sid=%d)",
(msg? msg->size : (chunk->msg? chunk->msg->size : 0)), chunk->header.message_type, chunk->header.payload_length,
chunk->header.timestamp, chunk->header.stream_id);
return ret;
}2).读取完基本头部信息后，根据读取到的cid（chunk stream id）在chunk stream cache中查找是该id对应的SrsChunkStream对象是否已经被创建，不存在就创建新的，存在就直接使用。
3).接着调用read_message_header读取Message Header（消息头信息）
4).接着调用read_message_payload读取负载数据
2.SrsProtocol::read_basic_header读取基本头信息,该函数的定义如下：
/**
* 6.1.1. Chunk Basic Header
* The Chunk Basic Header encodes the chunk stream ID and the chunk
* type(represented by fmt field in the figure below). Chunk type
* determines the format of the encoded message header. Chunk Basic
* Header field may be 1, 2, or 3 bytes, depending on the chunk stream
* ID.
*
* The bits 0-5 (least significant) in the chunk basic header represent
* the chunk stream ID.
*
* Chunk stream IDs 2-63 can be encoded in the 1-byte version of this
* field.
* 0 1 2 3 4 5 6 7
* +-+-+-+-+-+-+-+-+
* |fmt| cs id |
* +-+-+-+-+-+-+-+-+
* Figure 6 Chunk basic header 1
*
* Chunk stream IDs 64-319 can be encoded in the 2-byte version of this
* field. ID is computed as (the second byte + 64).
* 0 1
* 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
* +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* |fmt| 0 | cs id - 64 |
* +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* Figure 7 Chunk basic header 2
*
* Chunk stream IDs 64-65599 can be encoded in the 3-byte version of
* this field. ID is computed as ((the third byte)*256 + the second byte
* + 64).
* 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
* +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* |fmt| 1 | cs id - 64 |
* +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* Figure 8 Chunk basic header 3
*
* cs id: 6 bits
* fmt: 2 bits
* cs id - 64: 8 or 16 bits
*
* Chunk stream IDs with values 64-319 could be represented by both 2-
* byte version and 3-byte version of this field.
*/
int SrsProtocol::read_basic_header(char& fmt, int& cid)
{
int ret = ERROR_SUCCESS;
if ((ret = in_buffer->grow(skt, 1)) != ERROR_SUCCESS) {
if (ret != ERROR_SOCKET_TIMEOUT && !srs_is_client_gracefully_close(ret)) {
srs_error("read 1bytes basic header failed. required_size=%d, ret=%d", 1, ret);
}
return ret;
}
fmt = in_buffer->read_1byte();
cid = fmt & 0x3f;
fmt = (fmt >> 6) & 0x03;
// 2-63, 1B chunk header
if (cid > 1) {
srs_verbose("basic header parsed. fmt=%d, cid=%d", fmt, cid);
return ret;
}
// 64-319, 2B chunk header
if (cid == 0) {
if ((ret = in_buffer->grow(skt, 1)) != ERROR_SUCCESS) {
if (ret != ERROR_SOCKET_TIMEOUT && !srs_is_client_gracefully_close(ret)) {
srs_error("read 2bytes basic header failed. required_size=%d, ret=%d", 1, ret);
}
return ret;
}
cid = 64;
cid += (uint8_t)in_buffer->read_1byte();
srs_verbose("2bytes basic header parsed. fmt=%d, cid=%d", fmt, cid);
// 64-65599, 3B chunk header
} else if (cid == 1) {
if ((ret = in_buffer->grow(skt, 2)) != ERROR_SUCCESS) {
if (ret != ERROR_SOCKET_TIMEOUT && !srs_is_client_gracefully_close(ret)) {
srs_error("read 3bytes basic header failed. required_size=%d, ret=%d", 2, ret);
}
return ret;
}
cid = 64;
cid += (uint8_t)in_buffer->read_1byte();
cid += ((uint8_t)in_buffer->read_1byte()) * 256;
srs_verbose("3bytes basic header parsed. fmt=%d, cid=%d", fmt, cid);
} else {
srs_error("invalid path, impossible basic header.");
srs_assert(false);
}
return ret;
}包含了chunk stream ID（流通道Id）和chunk type（chunk的类型），chunk stream id一般被简写为CSID，用来唯一标识一个特定的流通道，chunk type决定了后面Message Header的格式。Basic Header的长度可能是1，2，或3个字节，其中chunk type的长度是固定的（占2位，注意单位是位，bit），Basic Header的长度取决于CSID的大小,在足够存储这两个字段的前提下最好用尽量少的字节从而减少由于引入Header增加的数据量。
RTMP协议支持用户自定义［3，65599］之间的CSID，0，1，2由协议保留表示特殊信息。0代表Basic Header总共要占用2个字节，CSID在［64，319］之间，1代表占用3个字节，CSID在［64，65599］之间，2代表该chunk是控制信息和一些命令信息，后面会有详细的介绍。
chunk type的长度固定为2位，因此CSID的长度是（6=8-2）、（14=16-2）、（22=24-2）中的一个。
当Basic Header为1个字节时，CSID占6位，6位最多可以表示64个数，因此这种情况下CSID在［0，63］之间，其中用户可自定义的范围为［3，63］。
当Basic Header为2个字节时，CSID占14位，此时协议将与chunk type所在字节的其他位都置为0，剩下的一个字节来表示CSID－64，这样共有8个字节来存储CSID，8位可以表示［0，255］共256个数，因此这种情况下CSID在［64，319］，其中319=255+64。
当Basic Header为3个字节时，CSID占22位，此时协议将［2，8］字节置为1，余下的16个字节表示CSID－64，这样共有16个位来存储CSID，16位可以表示［0，65535］共65536个数，因此这种情况下CSID在［64，65599］，其中65599=65535+64， 需要注意的是，Basic Header是采用小端存储的方式，越往后的字节数量级越高，因此通过这3个字节每一位的值来计算CSID时，应该是:<第三个字节的值>x256+<第二个字节的值>+64
可以看到2个字节和3个字节的Basic Header所能表示的CSID是有交集的［64，319］，但实际实现时还是应该秉着最少字节的原则使用2个字节的表示方式来表示［64，319］的CSID。
3.SrsProtocol::read_message_header读取消息头信息,该函数的定义如下：
/**
* parse the message header.
* 3bytes: timestamp delta, fmt=0,1,2
* 3bytes: payload length, fmt=0,1
* 1bytes: message type, fmt=0,1
* 4bytes: stream id, fmt=0
* where:
* fmt=0, 0x0X
* fmt=1, 0x4X
* fmt=2, 0x8X
* fmt=3, 0xCX
*/
int SrsProtocol::read_message_header(SrsChunkStream* chunk, char fmt)
{
int ret = ERROR_SUCCESS;
/**
* we should not assert anything about fmt, for the first packet.
* (when first packet, the chunk->msg is NULL).
* the fmt maybe 0/1/2/3, the FMLE will send a 0xC4 for some audio packet.
* the previous packet is:
* 04 // fmt=0, cid=4
* 00 00 1a // timestamp=26
* 00 00 9d // payload_length=157
* 08 // message_type=8(audio)
* 01 00 00 00 // stream_id=1
* the current packet maybe:
* c4 // fmt=3, cid=4
* it's ok, for the packet is audio, and timestamp delta is 26.
* the current packet must be parsed as:
* fmt=0, cid=4
* timestamp=26+26=52
* payload_length=157
* message_type=8(audio)
* stream_id=1
* so we must update the timestamp even fmt=3 for first packet.
*/
// fresh packet used to update the timestamp even fmt=3 for first packet.
// fresh packet always means the chunk is the first one of message.
bool is_first_chunk_of_msg = !chunk->msg;
// but, we can ensure that when a chunk stream is fresh,
// the fmt must be 0, a new stream.
if (chunk->msg_count == 0 && fmt != RTMP_FMT_TYPE0) {
// for librtmp, if ping, it will send a fresh stream with fmt=1,
// 0x42 where: fmt=1, cid=2, protocol contorl user-control message
// 0x00 0x00 0x00 where: timestamp=0
// 0x00 0x00 0x06 where: payload_length=6
// 0x04 where: message_type=4(protocol control user-control message)
// 0x00 0x06 where: event Ping(0x06)
// 0x00 0x00 0x0d 0x0f where: event data 4bytes ping timestamp.
// @see: https://github.com/ossrs/srs/issues/98
if (chunk->cid == RTMP_CID_ProtocolControl && fmt == RTMP_FMT_TYPE1) {
srs_warn("accept cid=2, fmt=1 to make librtmp happy.");
} else {
// must be a RTMP protocol level error.
ret = ERROR_RTMP_CHUNK_START;
srs_error("chunk stream is fresh, fmt must be %d, actual is %d. cid=%d, ret=%d",
RTMP_FMT_TYPE0, fmt, chunk->cid, ret);
return ret;
}
}
// when exists cache msg, means got an partial message,
// the fmt must not be type0 which means new message.
if (chunk->msg && fmt == RTMP_FMT_TYPE0) {
ret = ERROR_RTMP_CHUNK_START;
srs_error("chunk stream exists, "
"fmt must not be %d, actual is %d. ret=%d", RTMP_FMT_TYPE0, fmt, ret);
return ret;
}
// create msg when new chunk stream start
if (!chunk->msg) {
chunk->msg = new SrsCommonMessage();
srs_verbose("create message for new chunk, fmt=%d, cid=%d", fmt, chunk->cid);
}
// read message header from socket to buffer.
static char mh_sizes[] = {11, 7, 3, 0};
int mh_size = mh_sizes[(int)fmt];
srs_verbose("calc chunk message header size. fmt=%d, mh_size=%d", fmt, mh_size);
if (mh_size > 0 && (ret = in_buffer->grow(skt, mh_size)) != ERROR_SUCCESS) {
if (ret != ERROR_SOCKET_TIMEOUT && !srs_is_client_gracefully_close(ret)) {
srs_error("read %dbytes message header failed. ret=%d", mh_size, ret);
}
return ret;
}
/**
* parse the message header.
* 3bytes: timestamp delta, fmt=0,1,2
* 3bytes: payload length, fmt=0,1
* 1bytes: message type, fmt=0,1
* 4bytes: stream id, fmt=0
* where:
* fmt=0, 0x0X
* fmt=1, 0x4X
* fmt=2, 0x8X
* fmt=3, 0xCX
*/
// see also: ngx_rtmp_recv
if (fmt <= RTMP_FMT_TYPE2) {
char* p = in_buffer->read_slice(mh_size);
char* pp = (char*)&chunk->header.timestamp_delta;
pp[2] = *p++;
pp[1] = *p++;
pp[0] = *p++;
pp[3] = 0;
// fmt: 0
// timestamp: 3 bytes
// If the timestamp is greater than or equal to 16777215
// (hexadecimal 0x00ffffff), this value MUST be 16777215, and the
// 'extended timestamp header' MUST be present. Otherwise, this value
// SHOULD be the entire timestamp.
//
// fmt: 1 or 2
// timestamp delta: 3 bytes
// If the delta is greater than or equal to 16777215 (hexadecimal
// 0x00ffffff), this value MUST be 16777215, and the 'extended
// timestamp header' MUST be present. Otherwise, this value SHOULD be
// the entire delta.
chunk->extended_timestamp = (chunk->header.timestamp_delta >= RTMP_EXTENDED_TIMESTAMP);
if (!chunk->extended_timestamp) {
// Extended timestamp: 0 or 4 bytes
// This field MUST be sent when the normal timsestamp is set to
// 0xffffff, it MUST NOT be sent if the normal timestamp is set to
// anything else. So for values less than 0xffffff the normal
// timestamp field SHOULD be used in which case the extended timestamp
// MUST NOT be present. For values greater than or equal to 0xffffff
// the normal timestamp field MUST NOT be used and MUST be set to
// 0xffffff and the extended timestamp MUST be sent.
if (fmt == RTMP_FMT_TYPE0) {
// 6.1.2.1. Type 0
// For a type-0 chunk, the absolute timestamp of the message is sent
// here.
chunk->header.timestamp = chunk->header.timestamp_delta;
} else {
// 6.1.2.2. Type 1
// 6.1.2.3. Type 2
// For a type-1 or type-2 chunk, the difference between the previous
// chunk's timestamp and the current chunk's timestamp is sent here.
chunk->header.timestamp += chunk->header.timestamp_delta;
}
}
if (fmt <= RTMP_FMT_TYPE1) {
int32_t payload_length = 0;
pp = (char*)&payload_length;
pp[2] = *p++;
pp[1] = *p++;
pp[0] = *p++;
pp[3] = 0;
// for a message, if msg exists in cache, the size must not changed.
// always use the actual msg size to compare, for the cache payload length can changed,
// for the fmt type1(stream_id not changed), user can change the payload
// length(it's not allowed in the continue chunks).
if (!is_first_chunk_of_msg && chunk->header.payload_length != payload_length) {
ret = ERROR_RTMP_PACKET_SIZE;
srs_error("msg exists in chunk cache, "
"size=%d cannot change to %d, ret=%d",
chunk->header.payload_length, payload_length, ret);
return ret;
}
chunk->header.payload_length = payload_length;
chunk->header.message_type = *p++;
if (fmt == RTMP_FMT_TYPE0) {
pp = (char*)&chunk->header.stream_id;
pp[0] = *p++;
pp[1] = *p++;
pp[2] = *p++;
pp[3] = *p++;
srs_verbose("header read completed. fmt=%d, mh_size=%d, ext_time=%d, time=%" PRId64 ", payload=%d, type=%d, sid=%d",
fmt, mh_size, chunk->extended_timestamp, chunk->header.timestamp, chunk->header.payload_length,
chunk->header.message_type, chunk->header.stream_id);
} else {
srs_verbose("header read completed. fmt=%d, mh_size=%d, ext_time=%d, time=%" PRId64 ", payload=%d, type=%d",
fmt, mh_size, chunk->extended_timestamp, chunk->header.timestamp, chunk->header.payload_length,
chunk->header.message_type);
}
} else {
srs_verbose("header read completed. fmt=%d, mh_size=%d, ext_time=%d, time=%" PRId64 "",
fmt, mh_size, chunk->extended_timestamp, chunk->header.timestamp);
}
} else {
// update the timestamp even fmt=3 for first chunk packet
if (is_first_chunk_of_msg && !chunk->extended_timestamp) {
chunk->header.timestamp += chunk->header.timestamp_delta;
}
srs_verbose("header read completed. fmt=%d, size=%d, ext_time=%d",
fmt, mh_size, chunk->extended_timestamp);
}
// read extended-timestamp
if (chunk->extended_timestamp) {
mh_size += 4;
srs_verbose("read header ext time. fmt=%d, ext_time=%d, mh_size=%d", fmt, chunk->extended_timestamp, mh_size);
if ((ret = in_buffer->grow(skt, 4)) != ERROR_SUCCESS) {
if (ret != ERROR_SOCKET_TIMEOUT && !srs_is_client_gracefully_close(ret)) {
srs_error("read %dbytes message header failed. required_size=%d, ret=%d", mh_size, 4, ret);
}
return ret;
}
// the ptr to the slice maybe invalid when grow()
// reset the p to get 4bytes slice.
char* p = in_buffer->read_slice(4);
uint32_t timestamp = 0x00;
char* pp = (char*)×tamp;
pp[3] = *p++;
pp[2] = *p++;
pp[1] = *p++;
pp[0] = *p++;
// always use 31bits timestamp, for some server may use 32bits extended timestamp.
// @see https://github.com/ossrs/srs/issues/111
timestamp &= 0x7fffffff;
/**
* RTMP specification and ffmpeg/librtmp is false,
* but, adobe changed the specification, so flash/FMLE/FMS always true.
* default to true to support flash/FMLE/FMS.
*
* ffmpeg/librtmp may donot send this filed, need to detect the value.
* @see also: http://blog.csdn.net/win_lin/article/details/13363699
* compare to the chunk timestamp, which is set by chunk message header
* type 0,1 or 2.
*
* @remark, nginx send the extended-timestamp in sequence-header,
* and timestamp delta in continue C1 chunks, and so compatible with ffmpeg,
* that is, there is no continue chunks and extended-timestamp in nginx-rtmp.
*
* @remark, srs always send the extended-timestamp, to keep simple,
* and compatible with adobe products.
*/
uint32_t chunk_timestamp = (uint32_t)chunk->header.timestamp;
/**
* if chunk_timestamp<=0, the chunk previous packet has no extended-timestamp,
* always use the extended timestamp.
*/
/**
* about the is_first_chunk_of_msg.
* @remark, for the first chunk of message, always use the extended timestamp.
*/
if (!is_first_chunk_of_msg && chunk_timestamp > 0 && chunk_timestamp != timestamp) {
mh_size -= 4;
in_buffer->skip(-4);
srs_info("no 4bytes extended timestamp in the continued chunk");
} else {
chunk->header.timestamp = timestamp;
}
srs_verbose("header read ext_time completed. time=%" PRId64 "", chunk->header.timestamp);
}
// the extended-timestamp must be unsigned-int,
// 24bits timestamp: 0xffffff = 16777215ms = 16777.215s = 4.66h
// 32bits timestamp: 0xffffffff = 4294967295ms = 4294967.295s = 1193.046h = 49.71d
// because the rtmp protocol says the 32bits timestamp is about "50 days":
// 3. Byte Order, Alignment, and Time Format
// Because timestamps are generally only 32 bits long, they will roll
// over after fewer than 50 days.
//
// but, its sample says the timestamp is 31bits:
// An application could assume, for example, that all
// adjacent timestamps are within 2^31 milliseconds of each other, so
// 10000 comes after 4000000000, while 3000000000 comes before
// 4000000000.
// and flv specification says timestamp is 31bits:
// Extension of the Timestamp field to form a SI32 value. This
// field represents the upper 8 bits, while the previous
// Timestamp field represents the lower 24 bits of the time in
// milliseconds.
// in a word, 31bits timestamp is ok.
// convert extended timestamp to 31bits.
chunk->header.timestamp &= 0x7fffffff;
// valid message, the payload_length is 24bits,
// so it should never be negative.
srs_assert(chunk->header.payload_length >= 0);
// copy header to msg
chunk->msg->header = chunk->header;
// increase the msg count, the chunk stream can accept fmt=1/2/3 message now.
chunk->msg_count++;
return ret;
} Message Header（消息头信息）说明：
包含了要发送的实际信息（可能是完整的，也可能是一部分）的描述信息。Message Header的格式和长度取决于Basic Header的chunk type，共有4种不同的格式，由上面所提到的Basic Header中的fmt字段控制。其中第一种格式可以表示其他三种表示的所有数据，但由于其他三种格式是基于对之前chunk的差量化的表示，因此可以更简洁地表示相同的数据，实际使用的时候还是应该采用尽量少的字节表示相同意义的数据。以下按照字节数从多到少的顺序分别介绍这4种格式的Message Header。
Type＝0:
type=0时Message Header占用11个字节，其他三种能表示的数据它都能表示，但在chunk stream的开始的第一个chunk和头信息中的时间戳后退（即值与上一个chunk相比减小，通常在回退播放的时候会出现这种情况）的时候必须采用这种格式。
- timestamp（时间戳）：占用3个字节，因此它最多能表示到16777215=0xFFFFFF=2
24-1, 当它的值超过这个最大值时，这三个字节都置为1，这样实际的timestamp会转存到Extended Timestamp字段中，接受端在判断timestamp字段24个位都为1时就会去Extended timestamp中解析实际的时间戳。 - message length（消息数据的长度）：占用3个字节，表示实际发送的消息的数据如音频帧、视频帧等数据的长度，单位是字节。注意这里是Message的长度，也就是chunk属于的Message的总数据长度，而不是chunk本身Data的数据的长度。
- message type id(消息的类型id)：占用1个字节，表示实际发送的数据的类型，如8代表音频数据、9代表视频数据。
- msg stream id（消息的流id）：占用4个字节，表示该chunk所在的流的ID，和Basic Header的CSID一样，它采用小端存储的方式，
Type = 1:
type=1时Message Header占用7个字节，省去了表示msg stream id的4个字节，表示此chunk和上一次发的chunk所在的流相同，如果在发送端只和对端有一个流链接的时候可以尽量去采取这种格式。
timestamp delta：占用3个字节，注意这里和type＝0时不同，存储的是和上一个chunk的时间差。类似上面提到的timestamp，当它的值超过3个字节所能表示的最大值时，三个字节都置为1，实际的时间戳差值就会转存到Extended Timestamp字段中，接受端在判断timestamp delta字段24个位都为1时就会去Extended timestamp中解析时机的与上次时间戳的差值。
Type = 2:
type=2时Message Header占用3个字节，相对于type＝1格式又省去了表示消息长度的3个字节和表示消息类型的1个字节，表示此chunk和上一次发送的chunk所在的流、消息的长度和消息的类型都相同。余下的这三个字节表示timestamp delta，使用同type＝1
Type = 3
0字节！！！好吧，它表示这个chunk的Message Header和上一个是完全相同的，自然就不用再传输一遍了。当它跟在Type＝0的chunk后面时，表示和前一个chunk的时间戳都是相同的。什么时候连时间戳都相同呢？就是一个Message拆分成了多个chunk，这个chunk和上一个chunk同属于一个Message。而当它跟在Type＝1或者Type＝2的chunk后面时，表示和前一个chunk的时间戳的差是相同的。比如第一个chunk的Type＝0，timestamp＝100，第二个chunk的Type＝2，timestamp delta＝20，表示时间戳为100+20=120，第三个chunk的Type＝3，表示timestamp delta＝20，时间戳为120+20=140
3.3.3 Extended Timestamp（扩展时间戳）：
上面我们提到在chunk中会有时间戳timestamp和时间戳差timestamp delta，并且它们不会同时存在，只有这两者之一大于3个字节能表示的最大数值0xFFFFFF＝16777215时，才会用这个字段来表示真正的时间戳，否则这个字段为0。扩展时间戳占4个字节，能表示的最大数值就是0xFFFFFFFF＝4294967295。当扩展时间戳启用时，timestamp字段或者timestamp delta要全置为1，表示应该去扩展时间戳字段来提取真正的时间戳或者时间戳差。注意扩展时间戳存储的是完整值，而不是减去时间戳或者时间戳差的值。 -
四种格式的使用：第一个消息的第一个chunk 使用type0，携带全部的信息；后面的chunk使用type3；第二个消息，如果是视频消息，长度可变，则第一个chunk使用type1，值丢弃message stream id即可。第二个消息的其他chunk使用type3。如果是音频，且长度相同，则第一个消息用type0+type3，第二个消息用type2+type3。如果音频消息，长度，消息间时戳增量（delta）都相同，则第一个消息可以用type0+type3，后面的所有消息全部用type3即可。这样的话也要求第一个消息的时戳必须和消息间的时戳增量相同
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音频使用一个单独的chunk stream id，视频也使用一个单独的chunk stream id，它们是不同的，发给同一个客户端的message stream id 是相同的，就是createStream时返回的message stream id，一个message stream id 可以使用多个chunk stream id。
4.SrsProtocol::read_message_payload读取负载数据,该函数的定义如下：
int SrsProtocol::read_message_payload(SrsChunkStream* chunk, SrsCommonMessage** pmsg)
{
int ret = ERROR_SUCCESS;
// empty message
if (chunk->header.payload_length <= 0) {
srs_trace("get an empty RTMP "
"message(type=%d, size=%d, time=%" PRId64 ", sid=%d)", chunk->header.message_type,
chunk->header.payload_length, chunk->header.timestamp, chunk->header.stream_id);
*pmsg = chunk->msg;
chunk->msg = NULL;
return ret;
}
srs_assert(chunk->header.payload_length > 0);
// the chunk payload size.
int payload_size = chunk->header.payload_length - chunk->msg->size;
payload_size = srs_min(payload_size, in_chunk_size);
srs_verbose("chunk payload size is %d, message_size=%d, received_size=%d, in_chunk_size=%d",
payload_size, chunk->header.payload_length, chunk->msg->size, in_chunk_size);
// create msg payload if not initialized
if (!chunk->msg->payload) {
chunk->msg->create_payload(chunk->header.payload_length);
}
// read payload to buffer
if ((ret = in_buffer->grow(skt, payload_size)) != ERROR_SUCCESS) {
if (ret != ERROR_SOCKET_TIMEOUT && !srs_is_client_gracefully_close(ret)) {
srs_error("read payload failed. required_size=%d, ret=%d", payload_size, ret);
}
return ret;
}
memcpy(chunk->msg->payload + chunk->msg->size, in_buffer->read_slice(payload_size), payload_size);
chunk->msg->size += payload_size;
srs_verbose("chunk payload read completed. payload_size=%d", payload_size);
// got entire RTMP message?
if (chunk->header.payload_length == chunk->msg->size) {
*pmsg = chunk->msg;
chunk->msg = NULL;
srs_verbose("get entire RTMP message(type=%d, size=%d, time=%" PRId64 ", sid=%d)",
chunk->header.message_type, chunk->header.payload_length,
chunk->header.timestamp, chunk->header.stream_id);
return ret;
}
srs_verbose("get partial RTMP message(type=%d, size=%d, time=%" PRId64 ", sid=%d), partial size=%d",
chunk->header.message_type, chunk->header.payload_length,
chunk->header.timestamp, chunk->header.stream_id,
chunk->msg->size);
return ret;
}