H265（HEVC） nal 單元頭介紹及rtp發送中的fu分組發送詳解

        首先來介紹下h265（HEVC）nal單元頭，與h264的nal層相比，h265的nal unit header有兩個字節構成，如下圖所示：

從圖中可以看出hHEVC的nal包結構與h264有明顯的不同，hevc加入了nal所在的時間層的ＩＤ，取去除了nal_ref_idc，此信息合併到了naltype中，通常情況下F爲0，layerid爲0,TID爲1。

        nal單元的類型有如下幾種：

     

 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分組打包方式，fu分組包頭格式如下：

fus包頭包含了兩個字節的payloadhdr，一個字節的fu header，fu header與h264一樣，結構如下圖，包含開始位（1b）、停止位(1b)、futype(6b)

paylodhdr兩個自己的賦值，其實就是把hevc幀數據的nal unit header的naltype替換爲49即可，下面是從ffmpeg源碼中截取出來的fu打包方式代碼片段：

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) //小於對定的最大值時，直接發送（最大值一般小於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 //大於最大值時進行fu分組發送
	{
		/*
					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;
           //此處爲paylaodhdr,規範賦值應該是替換hevc數據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 */
		//此處要注意，當是分組的第一報數據時，應該覆蓋掉前兩個字節的數據，h264要覆蓋前一個字節的數據，即是第一包要去除hevc幀數據的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);

	}
}

通過rtp發送hevc視頻數據，當hevc幀數據大於mtu時，應該進行fu分組發送，從上面代碼流程就是對超過max_payload_size數據進行fu分組的流程，這個h264 fu-A很類似，很容易理解。

參考規範：

https://tools.ietf.org/html/draft-ietf-payload-rtp-h265-14

ffmpeg相關代碼

https://www.ffmpeg.org/doxygen/2.5/rtpenc__hevc_8c_source.html