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, };

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) 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