在安防行业，有个协议是无论如何都要适配的，因为公安监控网络用的就是它，它就是：GB28181。而这份协议主要由海康制定，所以除了海康其他厂商想要适配都会少许有点儿麻烦。
1. GB28181要求的RTP流格式
首先，我们来看看I帧的PS流格式，这里需要注意的是SPS、PPS之前要加上PES头部。如下图所示，其中绿色部分就是我们拿到的H.264裸流数据，须将它拆分成三段并在前面加上PES头部。这一点在GB28181标准中没有细说，需要通过分析海康IPC流才能看出。
一般情况下IDR帧很大，超过了RTP的负载长度限制（1400字节），所以上面这一个I帧要拆分成若干包RTP分多次发送。第一包的结构如上图所示，第二包以后RTP的结构就简单多了，它是这样的：
上面提到的是I帧的情况，相比它，P/B帧的帧格式真是太简单了，因为它既没有SYS、PSM，也没有SPS、PPS：
P/B帧大小一般不超过1400字节，如果超过1400字节，也需分成多包RTP数据进行传输，超出1400部分的第二包RTP结构：
2. 头部信息
首先是RTP包头信息，它一般长度为12个字节
#define RTP_HDR_LEN 12
static int gb28181_make_rtp_header(char *pData, int marker_flag, unsigned short cseq, long long curpts, unsigned int ssrc)
{
bits_buffer_s bitsBuffer;
if (pData == NULL)
return -1;
bitsBuffer.i_size = RTP_HDR_LEN;
bitsBuffer.i_data = 0;
bitsBuffer.i_mask = 0x80;
bitsBuffer.p_data = (unsigned char *)(pData);
memset(bitsBuffer.p_data, 0, RTP_HDR_SIZE);
bits_write(&bitsBuffer, 2, RTP_VERSION); /* rtp version 版本号，固定为2 */
bits_write(&bitsBuffer, 1, 0); /* rtp padding */
bits_write(&bitsBuffer, 1, 0); /* rtp extension */
bits_write(&bitsBuffer, 4, 0); /* rtp CSRC count */
bits_write(&bitsBuffer, 1, (marker_flag)); /* rtp marker 结束标志位，一帧图像的最后一包RTP置1*/
bits_write(&bitsBuffer, 7, 96); /* rtp payload type，96代表PS*/
bits_write(&bitsBuffer, 16, (cseq)); /* rtp sequence */
bits_write(&bitsBuffer, 32, (curpts)); /* rtp timestamp */
bits_write(&bitsBuffer, 32, (ssrc)); /* rtp SSRC */
return 0;
}
然后是PSH头部，它占了14个字节：
#define PS_HDR_LEN 14
static int gb28181_make_ps_header(char *pData, unsigned long long s64Scr)
{
unsigned long long lScrExt = (s64Scr) % 100;
s64Scr = s64Scr / 100;
bits_buffer_s bitsBuffer;
bitsBuffer.i_size = PS_HDR_LEN;
bitsBuffer.i_data = 0;
bitsBuffer.i_mask = 0x80;
bitsBuffer.p_data = (unsigned char *)(pData);
memset(bitsBuffer.p_data, 0, PS_HDR_LEN);
bits_write(&bitsBuffer, 32, 0x000001BA); /*start codes 起始码*/
bits_write(&bitsBuffer, 2, 1); /*marker bits '01b'*/
bits_write(&bitsBuffer, 3, (s64Scr>>30)&0x07); /*System clock [32..30]*/
bits_write(&bitsBuffer, 1, 1); /*marker bit*/
bits_write(&bitsBuffer, 15, (s64Scr>>15)&0x7FFF); /*System clock [29..15]*/
bits_write(&bitsBuffer, 1, 1); /*marker bit*/
bits_write(&bitsBuffer, 15, s64Scr & 0x7fff); /*System clock [14..0]*/
bits_write(&bitsBuffer, 1, 1); /*marker bit*/
bits_write(&bitsBuffer, 9, 0); /*SCR extension*/
bits_write(&bitsBuffer, 1, 1); /*marker bit*/
bits_write(&bitsBuffer, 22, (255)&0x3fffff); /*bit rate(n units of 50 bytes per second.)*/
bits_write(&bitsBuffer, 2, 3); /*marker bits '11'*/
bits_write(&bitsBuffer, 5, 0x1f); /*reserved(reserved for future use)*/
bits_write(&bitsBuffer, 3, 0); /*stuffing length*/
return 0;
}
后面是SYS，它包含了流类型信息，比如音频还是视频、视频编码格式
#define SYS_HDR_LEN 18
static int gb28181_make_sys_header(char *pData)
{
bits_buffer_s bitsBuffer;
bitsBuffer.i_size = SYS_HDR_LEN;
bitsBuffer.i_data = 0;
bitsBuffer.i_mask = 0x80;
bitsBuffer.p_data = (unsigned char *)(pData);
memset(bitsBuffer.p_data, 0, SYS_HDR_LEN);
/*system header*/
bits_write( &bitsBuffer, 32, 0x000001BB); /*start code*/
bits_write( &bitsBuffer, 16, SYS_HDR_LEN-6); /* 减6，是因为start code加上length这两位，占了6个字节*/
bits_write( &bitsBuffer, 1, 1); /*marker_bit*/
bits_write( &bitsBuffer, 22, 50000); /*rate_bound*/
bits_write( &bitsBuffer, 1, 1); /*marker_bit*/
bits_write( &bitsBuffer, 6, 1); /*audio_bound*/
bits_write( &bitsBuffer, 1, 0); /*fixed_flag */
bits_write( &bitsBuffer, 1, 1); /*CSPS_flag */
bits_write( &bitsBuffer, 1, 1); /*system_audio_lock_flag*/
bits_write( &bitsBuffer, 1, 1); /*system_video_lock_flag*/
bits_write( &bitsBuffer, 1, 1); /*marker_bit*/
bits_write( &bitsBuffer, 5, 1); /*video_bound*/
bits_write( &bitsBuffer, 1, 0); /*dif from mpeg1*/
bits_write( &bitsBuffer, 7, 0x7F); /*reserver*/
/*audio stream bound*/
bits_write( &bitsBuffer, 8, 0xC0); /*stream_id 音频的流id*/
bits_write( &bitsBuffer, 2, 3); /*marker_bit */
bits_write( &bitsBuffer, 1, 0); /*PSTD_buffer_bound_scale*/
bits_write( &bitsBuffer, 13, 512); /*PSTD_buffer_size_bound*/
/*video stream bound*/
bits_write( &bitsBuffer, 8, 0xE0); /*stream_id 视频的流id*/
bits_write( &bitsBuffer, 2, 3); /*marker_bit */
bits_write( &bitsBuffer, 1, 1); /*PSTD_buffer_bound_scale*/
bits_write( &bitsBuffer, 13, 2048); /*PSTD_buffer_size_bound*/
return 0;
}
接下来是PSM，这里面记录了媒体信息，比如音视频的编码格式：
static int gb28181_make_psm_header(char *pData)
{
bits_buffer_s bitsBuffer;
bitsBuffer.i_size = PSM_HDR_LEN;
bitsBuffer.i_data = 0;
bitsBuffer.i_mask = 0x80;
bitsBuffer.p_data = (unsigned char *)(pData);
memset(bitsBuffer.p_data, 0, PSM_HDR_LEN);//24Bytes
bits_write(&bitsBuffer, 24,0x000001); /*start code*/
bits_write(&bitsBuffer, 8, 0xBC); /*map stream id*/
bits_write(&bitsBuffer, 16,18); /*program stream map length*/
bits_write(&bitsBuffer, 1, 1); /*current next indicator */
bits_write(&bitsBuffer, 2, 3); /*reserved*/
bits_write(&bitsBuffer, 5, 0); /*program stream map version*/
bits_write(&bitsBuffer, 7, 0x7F); /*reserved */
bits_write(&bitsBuffer, 1, 1); /*marker bit */
bits_write(&bitsBuffer, 16,0); /*programe stream info length*/
bits_write(&bitsBuffer, 16, 8); /*elementary stream map length is*/
/*video*/
bits_write(&bitsBuffer, 8, 0x1B); /*stream_type 视频编码格式H.264*/
bits_write(&bitsBuffer, 8, 0xE0); /*elementary_stream_id*/
bits_write(&bitsBuffer, 16, 0); /*elementary_stream_info_length */
/*audio*/
bits_write(&bitsBuffer, 8, 0x90); /*stream_type 音频编码格式G711*/
bits_write(&bitsBuffer, 8, 0xC0); /*elementary_stream_id*/
bits_write(&bitsBuffer, 16, 0); /*elementary_stream_info_length is*/
/*crc (2e b9 0f 3d)*/
bits_write(&bitsBuffer, 8, 0x45); /*crc (24~31) bits*/
bits_write(&bitsBuffer, 8, 0xBD); /*crc (16~23) bits*/
bits_write(&bitsBuffer, 8, 0xDC); /*crc (8~15) bits*/
bits_write(&bitsBuffer, 8, 0xF4); /*crc (0~7) bits*/
return 0;
}
最后是PES头部，它记录了帧的时间戳，DTS可以不填，如果填写要和PTS保持一致，且同一帧数据的PTS要也要一样（即SPS、PPS、IDR的PES要一致）：
#define PES_HDR_LEN 19
static int gb28181_make_pes_header(char *pData, int stream_id, int payload_len, unsigned long long pts, unsigned long long dts)
{
bits_buffer_s bitsBuffer;
bitsBuffer.i_size = PES_HDR_LEN;
bitsBuffer.i_data = 0;
bitsBuffer.i_mask = 0x80;
bitsBuffer.p_data = (unsigned char *)(pData);
memset(bitsBuffer.p_data, 0, PES_HDR_LEN);
/*system header*/
bits_write( &bitsBuffer, 24,0x000001); /*start code*/
bits_write( &bitsBuffer, 8, (stream_id)); /*streamID*/
bits_write( &bitsBuffer, 16,(payload_len)+13); /*packet_len pes剩余头部以及后面的es长度之和，比如SPS长度+13*/
bits_write( &bitsBuffer, 2, 2 ); /*'10'*/
bits_write( &bitsBuffer, 2, 0 ); /*scrambling_control*/
bits_write( &bitsBuffer, 1, 1 ); /*priority*/
bits_write( &bitsBuffer, 1, 1 ); /*data_alignment_indicator*/
bits_write( &bitsBuffer, 1, 0 ); /*copyright*/
bits_write( &bitsBuffer, 1, 0 ); /*original_or_copy*/
bits_write( &bitsBuffer, 1, 1 ); /*PTS_flag 是否有PTS*/
bits_write( &bitsBuffer, 1, 1 ); /*DTS_flag 是否有DTS信息*/
bits_write( &bitsBuffer, 1, 0 ); /*ESCR_flag*/
bits_write( &bitsBuffer, 1, 0 ); /*ES_rate_flag*/
bits_write( &bitsBuffer, 1, 0 ); /*DSM_trick_mode_flag*/
bits_write( &bitsBuffer, 1, 0 ); /*additional_copy_info_flag*/
bits_write( &bitsBuffer, 1, 0 ); /*PES_CRC_flag*/
bits_write( &bitsBuffer, 1, 0 ); /*PES_extension_flag*/
bits_write( &bitsBuffer, 8, 10); /*header_data_length*/
/*PTS,DTS*/
bits_write( &bitsBuffer, 4, 3 ); /*'0011'*/
bits_write( &bitsBuffer, 3, ((pts)>>30)&0x07 ); /*PTS[32..30]*/
bits_write( &bitsBuffer, 1, 1 );
bits_write( &bitsBuffer, 15,((pts)>>15)&0x7FFF); /*PTS[29..15]*/
bits_write( &bitsBuffer, 1, 1 );
bits_write( &bitsBuffer, 15,(pts)&0x7FFF); /*PTS[14..0]*/
bits_write( &bitsBuffer, 1, 1 );
bits_write( &bitsBuffer, 4, 1 ); /*'0001'*/
bits_write( &bitsBuffer, 3, ((dts)>>30)&0x07 ); /*DTS[32..30]*/
bits_write( &bitsBuffer, 1, 1 );
bits_write( &bitsBuffer, 15,((dts)>>15)&0x7FFF); /*DTS[29..15]*/
bits_write( &bitsBuffer, 1, 1 );
bits_write( &bitsBuffer, 15,(dts)&0x7FFF); /*DTS[14..0]*/
bits_write( &bitsBuffer, 1, 1 );
return 0;
}
关于时间戳的比特位结构，下图表示的比较清晰：
![[MySQL]基本数据类型及表的基本操作](https://img-blog.csdnimg.cn/cc7dc7bfef564d5d9b9ad8369dfd30aa.png)
