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, // 1NAL_UNIT_CODED_SLICE_TSA_N,// 2 NAL_UNIT_CODED_SLICE_TLA,// 3 // Current name in the spec: TSA_RNAL_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_RNAL_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_LPNAL_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,};下面接收下fu分组打包方式，fu分组包头格式如下：
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)010 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|F| Type | LayerId | TID |+————-+—————–+F= 0Type = 49 (fragmentation unit (FU))LayerId = 0TID= 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 header0 1 2 3 4 5 6 7+-+-+-+-+-+-+-+-+|S|E| FuType |+—————+S= variableE= variableFuType = 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paylaodhdrbuf += 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。
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