PS封装格式：GB28181协议RTP传输

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：