1.FU-A的分片格式
数据比较大的H264视频包，被RTP分片发送。12字节的RTP头后面跟随的就是FU-A分片：
FU indicator有以下格式
+---------------+
|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+
|F|NRI| Type |
+---------------+
FU指示字节的类型域 Type=28表示FU-A。。NRI域的值必须根据分片NAL单元的NRI域的值设置
F: forbidden_zero_bit 在H.264规范中规定这一位必须为0
NRI： 2个bit，取00~11似乎指示这个NALU的重要性
type：5个bit 这个NALU单元的类型
0 没有定义
1-23 NAL单元 单个NAL单元包 （即一个RTP包仅由一个完整的NALU组成，）
24 STAP-A 单一时间的组合包 （多个NALU单元组成一个RTP包）
25 STAP-B 单一时间的组合包
26 MTAP16 多个时间的组合包
27 MTAP24 多个时间的组合包
28 FU-A 分片的单元 （一个NALU单元封装成多个RTP包）
29 FU-B 分片的单元
30-31 没有定义
FU header的格式如下：
+---------------+
|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+
|S|E|R| Type |
+---------------+
S: 1 bit
当设置成1,开始位指示分片NAL单元的开始。当跟随的FU荷载不是分片NAL单元荷载的开始，开始位设为0。
E: 1 bit
当设置成1, 结束位指示分片NAL单元的结束，即, 荷载的最后字节也是分片NAL单元的最后一个字节。当跟随的FU荷载不是分片NAL单元的最后分片,结束位设置为0。
R: 1 bit
保留位必须设置为0，接收者必须忽略该位。
Type: 5 bits
结合live555代码分析如下
void H264FUAFragmenter::doGetNextFrame() {
if (fNumValidDataBytes == 1) {
// We have no NAL unit data currently in the buffer. Read a new one:
fInputSource->getNextFrame(&fInputBuffer[1], fInputBufferSize - 1,
afterGettingFrame, this,
FramedSource::handleClosure, this);
} else {
// We have NAL unit data in the buffer. There are three cases to consider:
// 1. There is a new NAL unit in the buffer, and it's small enough to deliver
// to the RTP sink (as is).
// 2. There is a new NAL unit in the buffer, but it's too large to deliver to
// the RTP sink in its entirety. Deliver the first fragment of this data,
// as a FU-A packet, with one extra preceding header byte.
// 3. There is a NAL unit in the buffer, and we've already delivered some
// fragment(s) of this. Deliver the next fragment of this data,
// as a FU-A packet, with two extra preceding header bytes.
if (fMaxSize < fMaxOutputPacketSize) { // shouldn't happen
envir() << "H264FUAFragmenter::doGetNextFrame(): fMaxSize ("
<< fMaxSize << ") is smaller than expected\n";
} else {
fMaxSize = fMaxOutputPacketSize;
}
fLastFragmentCompletedNALUnit = True; // by default
if (fCurDataOffset == 1) { // case 1 or 2
if (fNumValidDataBytes - 1 <= fMaxSize) { // case 1
memmove(fTo, &fInputBuffer[1], fNumValidDataBytes - 1);
fFrameSize = fNumValidDataBytes - 1;
fCurDataOffset = fNumValidDataBytes;
} else { // case 2
// We need to send the NAL unit data as FU-A packets. Deliver the first
// packet now. Note that we add FU indicator and FU header bytes to the front
// of the packet (reusing the existing NAL header byte for the FU header).
fInputBuffer[0] = (fInputBuffer[1] & 0xE0) | 28; // FU indicator
fInputBuffer[1] = 0x80 | (fInputBuffer[1] & 0x1F); // FU header (with S bit)
memmove(fTo, fInputBuffer, fMaxSize);
fFrameSize = fMaxSize;
fCurDataOffset += fMaxSize - 1;
fLastFragmentCompletedNALUnit = False;
}
} else { // case 3
// We are sending this NAL unit data as FU-A packets. We've already sent the
// first packet (fragment). Now, send the next fragment. Note that we add
// FU indicator and FU header bytes to the front. (We reuse these bytes that
// we already sent for the first fragment, but clear the S bit, and add the E
// bit if this is the last fragment.)
fInputBuffer[fCurDataOffset-2] = fInputBuffer[0]; // FU indicator
fInputBuffer[fCurDataOffset-1] = fInputBuffer[1]&~0x80; // FU header (no S bit)
unsigned numBytesToSend = 2 + fNumValidDataBytes - fCurDataOffset;
if (numBytesToSend > fMaxSize) {
// We can't send all of the remaining data this time:
numBytesToSend = fMaxSize;
fLastFragmentCompletedNALUnit = False;
} else {
// This is the last fragment:
fInputBuffer[fCurDataOffset-1] |= 0x40; // set the E bit in the FU header
fNumTruncatedBytes = fSaveNumTruncatedBytes;
}
memmove(fTo, &fInputBuffer[fCurDataOffset-2], numBytesToSend);
fFrameSize = numBytesToSend;
fCurDataOffset += numBytesToSend - 2;
}
if (fCurDataOffset >= fNumValidDataBytes) {
// We're done with this data. Reset the pointers for receiving new data:
fNumValidDataBytes = fCurDataOffset = 1;
}
// Complete delivery to the client:
FramedSource::afterGetting(this);
}
}
原文：https://blog.csdn.net/occupy8/article/details/47067323