H.264 RTP Streaming

據RFC3984以RTP
封裝H.264 raw data來作video streaming.

1.H.264 raw data
以00 00 01 或 00 00 00 01作為開頭(Start Code),接著是8 bit NALU 
NALU的format

+---------------+
|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+
|F|NRI|  Type   |
+---------------+

F      :   forbidden zero bit, 一定為0
NRI :  nal_ref_idc, 表示資料的重要性, 00為最不重要.
Type :nal_unit_type, H.264只定義1~23的範圍

一個H.264 raw data看起來像這樣
00 00 00 01 09 30 ......

2.RTP header
因為一個H.264 video frame資料的大小往往會在數k bytes到數十K bytes,
在傳送封包時就會將資料切割分別封裝,也因此需要加入一些額外的參數讓
接收端可以正確組合被分割的video frame.這也是RFC3984最主要的目的.

RTP header 中有三個參數要注意
timestamp : 以90KHz作為基準,以30 fps為例,timestamp遞增 90000 / 30.
實務上是以payload實際間隔時間作計算.同一個video frame的
            分割資料timestamp是相同的
sequence : 每個RTP封包sequence number都遞增.
mark bit : RTP封包封裝的是最後一個分割的video frame時mark bit 為 1.

2.Payload format
1~23 : Single NAL unit packet.

RFC3984使用了H.264 NALU中未定義的type 24~29 (相當於增加H.264 nal_unit_type定義)

24 : STAP-A 單一時間組合
25 : STAP-B 單一時間組合
26 : MTAP16 多個時間組合
27 : MTAP32 多個時間組合
28 : FU-A 分割資料
29 : FU-B 分割資料

比較常見的是28,29.

3.Single NAL unit packet
當資料少於MTU的大小就用此方式封裝.
H.264 raw data foramt 為 [Start code][NALU][Raw Data]
封裝時去掉Start Code即可.Format 如下
[RTP Header][NALU][Raw Date]

4.FU-A (Fragmentation unit)
當資料大於MTU以此方式分割
H.264 raw data foramt 為 [Start code][NALU][Raw Data]
去掉[Start code],[NALU],以不超過MTU大小分割[Raw Data]
以NALU產生FU indicator, FU Header.

RFC 3984

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FU indicator  |   FU header   |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
|                                                               |
|                         FU payload                            |
|                                                               |
|                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                               :...OPTIONAL RTP padding        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 14.  RTP payload format for FU-A

The FU indicator octet has the following format:

+---------------+
|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+
|F|NRI|  Type   |
+---------------+

The FU header has the following format:

+---------------+
|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+
|S|E|R|  Type   |
+---------------+

[FU indicator] = (NALU & 0x60) | 28;
[FU Header] = (start << 7) | (end << 6) | (NALU & 0x1f);

format如下

[RTP Header][FU indicator][FU header][Raw Data Part 0]
[RTP Header][FU indicator][FU header][Raw Data Part 1]
[RTP Header][FU indicator][FU header][Raw Data Part 2]
...

5.FU-B (Fragmentation unit)

RFC 3984

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FU indicator  |   FU header   |               DON             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
|                                                               |
|                         FU payload                            |
|                                                               |
|                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                               :...OPTIONAL RTP padding        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 15.  RTP payload format for FU-B

format如下
[RTP Header][FU indicator][FU header][DON][Raw Data Part 0]
[RTP Header][FU indicator][FU header][DON][Raw Data Part 1]
[RTP Header][FU indicator][FU header][DON][Raw Data Part 2]
...

6.Encoding sample code with TI DSP

/* Write the encoded frame to the network */
if (Buffer_getNumBytesUsed(hOutBuf)) {
struct iovec data[3];
rtp_hdr_t rtp;
rtp.version = 2;
rtp.p = 0;
rtp.x = 0;
rtp.cc = 0;
rtp.m = 0;
rtp.pt = 96;

rtp.seq = htons( sequence );
rtp.ts = htonl( timestamp );

rtp.ssrc = 10;

data[0].iov_base = &rtp;
data[0].iov_len = sizeof(rtp_hdr_t);

unsigned char *ptr = (unsigned char *)Buffer_getUserPtr(hOutBuf);
size_t len = Buffer_getNumBytesUsed(hOutBuf);
size_t mtu = DEFAULT_MTU;

/* Skip NAL Start Code 00 00 00 01 */
unsigned char nalType = ptr[4] & 0x1f;
//printf("Processing Buffer with NAL TYPE=%d\n", nalType);

if(len < (mtu - sizeof(rtp_hdr_t))) {
/* Remove NAL Start Code 00 00 00 01 */
data[1].iov_base = (void *)ptr + 4;
data[1].iov_len = len - 4;

//printf("NAL Unit fit in one packet size=%d\n", len);
/* only set the marker bit on packets containing access units */
if (IS_ACCESS_UNIT (nalType))
rtp.m = 1;

writev(sfd, data, 2);

sequence++;
} else  {
int start = 1, end = 0;
/* We keep 2 bytes for FU indicator and FU Header */
unsigned payload_len = mtu - sizeof(rtp_hdr_t) - 2;
unsigned char nalHeader = ptr[4];

/* Remove NAL Start Code 00 00 00 01 and NAL Type*/
ptr += 5;
len -= 5;

//printf("Using FU-A fragmentation for data size=%d\n", len);

while(end == 0)  {
unsigned char fu[2];
payload_len = len < payload_len ? len : payload_len;
if (payload_len == len)
end = 1;

if (IS_ACCESS_UNIT (nalType))
rtp.m = end;

/* FU indicator */
fu[0] = (nalHeader & 0x60) | 28;
/* FU Header */
fu[1] = (start << 7) | (end << 6) | (nalHeader & 0x1f);

rtp.seq = htons( sequence );

data[1].iov_base = fu;
data[1].iov_len = 2;

data[2].iov_base = (void *)ptr;
data[2].iov_len = payload_len;

writev(sfd, data, 3);
start = 0;

ptr += payload_len;
len -= payload_len;

sequence++;
}
}
if (Time_delta(hTimestamp, &time) < 0) {
printf("Failed to get timer delta\n");
goto cleanup;
}

//printf("Read time: %uus\n", (Uns)time);
timestamp += (unsigned int)(0.09 * time);  /*15 fps : 90000 / 15*/
}