H265（HEVC） nal 单元头介绍及rtp发送中的fu分组发送详解
首先来介绍下h265（HEVC）nal单元头，与h264的nal层相比，h265的nal unit header有两个字节构成，如下图所示：
从图中可以看出hHEVC的nal包结构与h264有明显的不同，hevc加入了nal所在的时间层的ＩＤ，取去除了nal_ref_idc，此信息合并到了naltype中，通常情况下F为0，layerid为0,TID为1。
nal单元的类型有如下几种：
enum NalUnitType
{
NAL_UNIT_CODED_SLICE_TRAIL_N = 0, // 0
NAL_UNIT_CODED_SLICE_TRAIL_R, // 1
NAL_UNIT_CODED_SLICE_TSA_N, // 2
NAL_UNIT_CODED_SLICE_TLA, // 3 // Current name in the spec: TSA_R
NAL_UNIT_CODED_SLICE_STSA_N, // 4
NAL_UNIT_CODED_SLICE_STSA_R, // 5
NAL_UNIT_CODED_SLICE_RADL_N, // 6
NAL_UNIT_CODED_SLICE_DLP, // 7 // Current name in the spec: RADL_R
NAL_UNIT_CODED_SLICE_RASL_N, // 8
NAL_UNIT_CODED_SLICE_TFD, // 9 // Current name in the spec: RASL_R
NAL_UNIT_RESERVED_10,
NAL_UNIT_RESERVED_11,
NAL_UNIT_RESERVED_12,
NAL_UNIT_RESERVED_13,
NAL_UNIT_RESERVED_14,
NAL_UNIT_RESERVED_15, NAL_UNIT_CODED_SLICE_BLA, // 16 // Current name in the spec: BLA_W_LP
NAL_UNIT_CODED_SLICE_BLA, // 16 // Current name in the spec: BLA_W_LP
NAL_UNIT_CODED_SLICE_BLANT, // 17 // Current name in the spec: BLA_W_DLP
NAL_UNIT_CODED_SLICE_BLA_N_LP, // 18
NAL_UNIT_CODED_SLICE_IDR, // 19 // Current name in the spec: IDR_W_DLP
NAL_UNIT_CODED_SLICE_IDR_N_LP, // 20
NAL_UNIT_CODED_SLICE_CRA, // 21
NAL_UNIT_RESERVED_22,
NAL_UNIT_RESERVED_23,
NAL_UNIT_RESERVED_24,
NAL_UNIT_RESERVED_25,
NAL_UNIT_RESERVED_26,
NAL_UNIT_RESERVED_27,
NAL_UNIT_RESERVED_28,
NAL_UNIT_RESERVED_29,
NAL_UNIT_RESERVED_30,
NAL_UNIT_RESERVED_31,
NAL_UNIT_VPS, // 32
NAL_UNIT_SPS, // 33
NAL_UNIT_PPS, // 34
NAL_UNIT_ACCESS_UNIT_DELIMITER, // 35
NAL_UNIT_EOS, // 36
NAL_UNIT_EOB, // 37
NAL_UNIT_FILLER_DATA, // 38
NAL_UNIT_SEI, // 39 Prefix SEI
NAL_UNIT_SEI_SUFFIX, // 40 Suffix SEI
NAL_UNIT_RESERVED_41,
NAL_UNIT_RESERVED_42,
NAL_UNIT_RESERVED_43,
NAL_UNIT_RESERVED_44,
NAL_UNIT_RESERVED_45,
NAL_UNIT_RESERVED_46,
NAL_UNIT_RESERVED_47,
NAL_UNIT_UNSPECIFIED_48,
NAL_UNIT_UNSPECIFIED_49,
NAL_UNIT_UNSPECIFIED_50,
NAL_UNIT_UNSPECIFIED_51,
NAL_UNIT_UNSPECIFIED_52,
NAL_UNIT_UNSPECIFIED_53,
NAL_UNIT_UNSPECIFIED_54,
NAL_UNIT_UNSPECIFIED_55,
NAL_UNIT_UNSPECIFIED_56,
NAL_UNIT_UNSPECIFIED_57,
NAL_UNIT_UNSPECIFIED_58,
NAL_UNIT_UNSPECIFIED_59,
NAL_UNIT_UNSPECIFIED_60,
NAL_UNIT_UNSPECIFIED_61,
NAL_UNIT_UNSPECIFIED_62,
NAL_UNIT_UNSPECIFIED_63,
NAL_UNIT_INVALID,
};fus包头包含了两个字节的payloadhdr，一个字节的fu header，fu header与h264一样，结构如下图，包含开始位（1b）、停止位(1b)、futype(6b)
paylodhdr两个自己的赋值，其实就是把hevc帧数据的nal unit header的naltype替换为49即可，下面是从ffmpeg源码中截取出来的fu打包方式代码片段：
static void nal_send(AVFormatContext *ctx, const uint8_t *buf, int len, int last_packet_of_frame)
{
RTPMuxContext *rtp_ctx = ctx->priv_data;
int rtp_payload_size = rtp_ctx->max_payload_size - RTP_HEVC_HEADERS_SIZE;
int nal_type = (buf[0] >> 1) & 0x3F;
/* send it as one single NAL unit? */
if (len <= rtp_ctx->max_payload_size) //小于对定的最大值时，直接发送（最大值一般小于mtu）
{
/* use the original NAL unit buffer and transmit it as RTP payload */
ff_rtp_send_data(ctx, buf, len, last_packet_of_frame);
}
else //大于最大值时进行fu分组发送
{
/*
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
*/
rtp_ctx->buf[0] = 49 << 1;
rtp_ctx->buf[1] = 1;
//此处为paylaodhdr,规范赋值应该是替换hevc数据nal 的payloadhdr的type
//rtp_ctx->buf[0] = (buf[0] &0x81) | (49<<1);
//rtp_ctx->buf[1] = buf[1]
/*
create the FU header
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|S|E| FuType |
+---------------+
S = variable
E = variable
FuType = NAL unit type
*/
rtp_ctx->buf[2] = nal_type;
/* set the S bit: mark as start fragment */
rtp_ctx->buf[2] |= 1 << 7;
/* pass the original NAL header */
//此处要注意，当是分组的第一报数据时，应该覆盖掉前两个字节的数据，h264要覆盖前一个字节的数据，即是第一包要去除hevc帧数据的paylaodhdr
buf += 2;
len -= 2;
while (len > rtp_payload_size)
{
/* complete and send current RTP packet */
memcpy(&rtp_ctx->buf[RTP_HEVC_HEADERS_SIZE], buf, rtp_payload_size);
ff_rtp_send_data(ctx, rtp_ctx->buf, rtp_ctx->max_payload_size, 0);
buf += rtp_payload_size;
len -= rtp_payload_size;
/* reset the S bit */
rtp_ctx->buf[2] &= ~(1 << 7);
}
/* set the E bit: mark as last fragment */
rtp_ctx->buf[2] |= 1 << 6;
/* complete and send last RTP packet */
memcpy(&rtp_ctx->buf[RTP_HEVC_HEADERS_SIZE], buf, len);
ff_rtp_send_data(ctx, rtp_ctx->buf, len + 2, last_packet_of_frame);
}
}通过rtp发送hevc视频数据，当hevc帧数据大于mtu时，应该进行fu分组发送，从上面代码流程就是对超过max_payload_size数据进行fu分组的流程，这个h264 fu-A很类似，很容易理解。
参考规范：
https://tools.ietf.org/html/draft-ietf-payload-rtp-h265-14
ffmpeg相关代码
https://www.ffmpeg.org/doxygen/2.5/rtpenc__hevc_8c_source.html