static void nal_send(AVFormatContext *s1, const uint8_t *buf, int size, int last)

{

    RTPMuxContext *s = s1->priv_data;

    enum AVCodecID codec = s1->streams[]->codecpar->codec_id;

    av_log(s1, AV_LOG_DEBUG, "Sending NAL %x of len %d M=%d\n", buf[] & 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 = ;

        if (codec == AV_CODEC_ID_H264) {

            header_size = ;

            skip_aggregate = s->flags & FF_RTP_FLAG_H264_MODE0;

        } else {

            header_size = ; //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 +  + size > s->max_payload_size) {

            flush_buffered(s1, );

            buffered_size = ;

        }

        // 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 +  + header_size + size <= s->max_payload_size &&

            !skip_aggregate) {

            if (buffered_size == ) { //发送第一个数据包

                if (codec == AV_CODEC_ID_H264) {

                    *s->buf_ptr++ = ;

                } else {

                    *s->buf_ptr++ =  << ;   //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++ = ;

                }

            }

            AV_WB16(s->buf_ptr, size);

            s->buf_ptr += ; //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, );

            ff_rtp_send_data(s1, buf, size, last);

        }

    } else {

        int flag_byte, header_size;

        flush_buffered(s1, );

        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[] & 0x1F;

            uint8_t nri = buf[] & 0x60;

            s->buf[] = ;        /* FU Indicator; Type = 28 ---> FU-A */

            s->buf[] |= nri;

            s->buf[] = type;

            s->buf[] |=  << ;

            buf  += ;

            size -= ;

            flag_byte   = ;

            header_size = ;

        } else {

            uint8_t nal_type = (buf[] >> ) & 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[] =  << ; //98只是Header，并不是真正意义上视频包类型。

            s->buf[] = ; //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[]  = 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[] |=  << ;  //第一个数据包分片

            /* pass the original NAL header */

            buf  += ; //去掉两个字节的头

            size -= ;

            flag_byte   = ;

            header_size = ;

        }

        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, );

            buf  += s->max_payload_size - header_size;

            size -= s->max_payload_size - header_size;

            s->buf[flag_byte] &= ~( << );

        }

        s->buf[flag_byte] |=  << ;//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);

    }

}