H265 Rtp封包

H265 Rtp封包可以参考Ffmpeg，具体实现在文件rtpenc_h264_hevc.c(4.0.1版本)，核心的方法是nal_send
这个方法有些绕，下面帖子具体的代码及注释。
static void nal_send(AVFormatContext *s1, const uint8_t *buf, int size, int last)
{
    RTPMuxContext *s = s1->priv_data;
    enum AVCodecID codec = s1->streams[0]->codecpar->codec_id;

    av_log(s1, AV_LOG_DEBUG, "Sending NAL %x of len %d M=%d\n", buf[0] & 0x1F, size, last);
    if (size <= s->max_payload_size) {            //max_payload_size一般<= 1500
        int buffered_size = s->buf_ptr - s->buf;
        int header_size;
        int skip_aggregate = 0;

        if (codec == AV_CODEC_ID_H264) {
            header_size = 1;
            skip_aggregate = s->flags & FF_RTP_FLAG_H264_MODE0;
        } else {
            header_size = 2; //Hevc 头两个字节，第二个字节是0x01;第一个字节表示NAL Type，具体哪些类型可以参考Hevc.h
        }

        // Flush buffered NAL units if the current unit doesn't fit
        if (buffered_size + 2 + size > s->max_payload_size) {
            flush_buffered(s1, 0);
            buffered_size = 0;
        }
        // If we aren't using mode 0, and the NAL unit fits including the
        // framing (2 bytes length, plus 1/2 bytes for the STAP-A/AP marker),
        // write the unit to the buffer as a STAP-A/AP packet, otherwise flush
        // and send as single NAL.
        if (buffered_size + 2 + header_size + size <= s->max_payload_size &&
            !skip_aggregate) {
            if (buffered_size == 0) { //发送第一个数据包
                if (codec == AV_CODEC_ID_H264) {
                    *s->buf_ptr++ = 24;
                } else {
                    *s->buf_ptr++ = 48 << 1;   //hevc head flag  Header 为96 后面一般会跟VPS,PPS,SPS等信息，这些包字节少，可以放在一起一次发送VPS,PPS,SSP之间插入两个字节表示各类型包长度。
                    *s->buf_ptr++ = 1;
                }
            }
            AV_WB16(s->buf_ptr, size);
            s->buf_ptr += 2; //数据包长度（两个字节表示）
            memcpy(s->buf_ptr, buf, size);
            s->buf_ptr += size;
            s->buffered_nals++;
        } else {
            flush_buffered(s1, 0);
            ff_rtp_send_data(s1, buf, size, last);
        }
    } else {
        int flag_byte, header_size;
        flush_buffered(s1, 0);
        if (codec == AV_CODEC_ID_H264 && (s->flags & FF_RTP_FLAG_H264_MODE0)) {
            av_log(s1, AV_LOG_ERROR,
                   "NAL size %d > %d, try -slice-max-size %d\n", size,
                   s->max_payload_size, s->max_payload_size);
            return;
        }
        av_log(s1, AV_LOG_DEBUG, "NAL size %d > %d\n", size, s->max_payload_size);
        if (codec == AV_CODEC_ID_H264) {    //视频类型为H264
            uint8_t type = buf[0] & 0x1F;
            uint8_t nri = buf[0] & 0x60;

            s->buf[0] = 28;        /* FU Indicator; Type = 28 ---> FU-A */
            s->buf[0] |= nri;
            s->buf[1] = type;
            s->buf[1] |= 1 << 7;
            buf  += 1;
            size -= 1;

            flag_byte   = 1;
            header_size = 2;
        } else {
            uint8_t nal_type = (buf[0] >> 1) & 0x3F;  // For hevc 
            /*
             * create the HEVC payload header and transmit the buffer as fragmentation units (FU)
             *
             *    0                   1
             *    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
             *   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             *   |F|   Type    |  LayerId  | TID |
             *   +-------------+-----------------+
             *
             *      F       = 0
             *      Type    = 49 (fragmentation unit (FU))
             *      LayerId = 0
             *      TID     = 1
             */
            s->buf[0] = 49 << 1; //98只是Header，并不是真正意义上视频包类型。
            s->buf[1] = 1; //固定不变

            /*
             *     create the FU header
             *
             *     0 1 2 3 4 5 6 7
             *    +-+-+-+-+-+-+-+-+
             *    |S|E|  FuType   |
             *    +---------------+
             *
             *       S       = variable
             *       E       = variable
             *       FuType  = NAL unit type
             */
            s->buf[2]  = nal_type; //该字节才表示真正意义上的数据类型
            /* set the S bit: mark as start fragment */
            s->buf[2] |= 1 << 7;  //第一个数据包分片

            /* pass the original NAL header */
            buf  += 2; //去掉两个字节的头
            size -= 2;

            flag_byte   = 2;
            header_size = 3;
        }

        while (size + header_size > s->max_payload_size) {
　　　　　　//中间数据包分片
            memcpy(&s->buf[header_size], buf, s->max_payload_size - header_size);
            ff_rtp_send_data(s1, s->buf, s->max_payload_size, 0);
            buf  += s->max_payload_size - header_size;
            size -= s->max_payload_size - header_size;
            s->buf[flag_byte] &= ~(1 << 7);
        }
        s->buf[flag_byte] |= 1 << 6;//最后一个数据包分片
        memcpy(&s->buf[header_size], buf, size);
        ff_rtp_send_data(s1, s->buf, size + header_size, last);
    }
}
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