音视频同步！RTCP协议解析及代码实现

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项的SDES包必须包含在每个组合RTCP包中。SDES包可能包括其他源描述项，这要根据特别的应用需要，并同时考虑带宽限制。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Wireshark抓包：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/78/78cfb47577b028adb71043d03f0c52b7.png","alt":"在这里插入图片描述","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"SDES源描述包提供了直观的文本信息来描述会话的参加者，包括CNAME、NAME、EMAIL、PHONE、LOC等源描述项。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这些为接收方获取发送方的有关信息提供了方便。SDES 包由包头与数据块组成，数据块可以没有，也可有多个。包头由版本（V）、填充（P）、长度指示、包类型（PT）和源计数（SC）组成。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"PT占8位，用于识别RTCP的SDES包，SC占5位，指示包含在SDES包中的SSRC/CSRC块数量，零值有效，但没有意义。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"BYE: 通知离开","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"BYE分组用于表示一个或多个媒体源不再是处于激活状态。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/49/49fe19e19a6a1c01aed12468bf6386ce.png","alt":"在这里插入图片描述","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Wireshark抓包：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/fb/fb5a24478fa26dccdc3fc25d7d7f6e17.png","alt":"在这里插入图片描述","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"作为可选项，BYE包可包括一个8位八进制计数，后跟文本信息，表示离开原因。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"最后，组合包中每个RTCP包可独立处理，而不需要按照包组合的先后顺序处理。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在组合包中有以下几条强制约束。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":1,"normalizeStart":1},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"只要带宽允许，在SR包或RR包中的接收统计应该经常发送，因此每个周期发送的组合RTCP 包中应包含报告包。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"每个组合包中都应该包含SDES CNAME，因为新接收者需要通过接收CNAME来识别源，并与媒体联系进行同步。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"组合包前面是包类型数量，其增长应该受到限制。","attrs":{}}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"RTCP协议如何实现媒体流的同步","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"通过抓包分析RTCP发送端报告，RTP的同步其实就靠这三个域：","attrs":{}}]},{"type":"numberedlist","attrs":{"start":1,"normalizeStart":1},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"sender SSRC ：SR包发送的同步源标识符。与对应RTP包中的SSRC一样","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"NTP timestamp：SR包发送时的绝对时间。用于同步不同的流。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"RTP timestamp：与NTP时间戳对应，与RTP包中的时间戳具有相同的初始值。","attrs":{}}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/1b/1bc8a64fb2f82722461e8fbee7258593.png","alt":"在这里插入图片描述","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"那怎么计算NTP时间呢？在RTCP中NTP时间存放在8个字节中，分为：MSW和LSW，分别占用4个字节。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"cpp"},"content":[{"type":"text","text":"const char *tvb_ntp_fmt_ts_sec(u_char *rtcp_info, int offset)\n{\n\tuint32_t tempstmp = 0;\n\ttime_t temptime = 0;\n\tstruct tm *bd;\n\tchar *buff = NULL;\n\t\n\ttempstmp = ntohl(*(uint32_t*)(rtcp_info + offset));\n\tif (tempstmp == 0){\n\t\treturn \"NULL\";\n\t}\n\n\t/* We need a temporary variable here so the unsigned math\n\t* works correctly (for years > 2036 according to RFC 2030\n\t* chapter 3).\n\t*/\n\ttemptime = (time_t)(tempstmp - NTP_BASETIME);\n\tbd = gmtime(&temptime);\n\tif (!bd){\n\t\treturn \"Not representable\";\n\t}\n\n\tbuff = (char *)malloc(NTP_TS_SIZE);\n\tsnprintf(buff, NTP_TS_SIZE,\n\t\t\"%s %2d, %d %02d:%02d:%02d UTC\",\n\t\tmon_names[bd->tm_mon],\n\t\tbd->tm_mday,\n\t\tbd->tm_year + 1900,\n\t\tbd->tm_hour,\n\t\tbd->tm_min,\n\t\tbd->tm_sec);\n\treturn buff;\n}","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"NTP timestamp","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"c"},"content":[{"type":"text","text":"\n /* NTP timestamp */\n ts_msw = ntohl(*(uint32_t*)(rtcp_info + offset));\n\tprintf(\"ts_msw: 0x%x\\n\",ts_msw);\n ts_lsw = ntohl(*(uint32_t*)(rtcp_info + offset + 4));\n\tprintf(\"ts_lsw: 0x%x\\n\",ts_lsw);\n\tprintf(\"MSW: %s\\n\",tvb_ntp_fmt_ts_sec(rtcp_info,offset));\n offset += 8;","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"RTCP协议实现","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"下面我给出对RTCP协议解析实现的代码，根据回放报文，解析字段信息。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"cpp"},"content":[{"type":"text","text":"/* 接收者/发送者计数是最后5位 */\n#define RTCP_COUNT(octet) ((octet) & 0x1F)\n\n\n#define RTCP_PT_MIN 192\n/* Supplemental H.261 specific RTCP packet types according to Section C.3.5 */\n#define RTCP_FIR 192\n#define RTCP_NACK 193\n#define RTCP_SMPTETC 194\n#define RTCP_IJ 195\n/* RTCP packet types according to Section A.11.1 */\n/* And https://www.iana.org/assignments/rtp-parameters/rtp-parameters.xhtml */\n#define RTCP_SR 200\n#define RTCP_RR 201\n#define RTCP_SDES 202\n#define RTCP_BYE 203\n#define RTCP_APP 204\n#define RTCP_RTPFB 205\n#define RTCP_PSFB 206\n#define RTCP_XR 207\n#define RTCP_AVB 208\n#define RTCP_RSI 209\n#define RTCP_TOKEN 210\n\n#define RTCP_PT_MAX 210\n\n\nstatic const char mon_names[12][4] = {\n\t\"Jan\",\n\t\"Feb\",\n\t\"Mar\",\n\t\"Apr\",\n\t\"May\",\n\t\"Jun\",\n\t\"Jul\",\n\t\"Aug\",\n\t\"Sep\",\n\t\"Oct\",\n\t\"Nov\",\n\t\"Dec\"\n};\n\n/** data structure to hold time values with nanosecond resolution*/\ntypedef struct {\n\ttime_t\tsecs;\n\tint\tnsecs;\n} nstime_t;\n\n\n/*\n * 1900-01-01 00:00:00 (proleptic?) UTC.\n * Used by a number of time formats.\n */\n#define EPOCH_DELTA_1900_01_01_00_00_00_UTC 2208988800U\n\n/*\n * NTP_BASETIME is in fact epoch - ntp_start_time; ntp_start_time\n * is January 1, 2036, 00:00:00 UTC.\n */\n#define NTP_BASETIME EPOCH_DELTA_1900_01_01_00_00_00_UTC\n#define NTP_FLOAT_DENOM 4294967296.0\n#define NTP_TS_SIZE 100\n\n\n/* 解剖长度字段。附加到此字段的文字表示转换为的实际字节数 (即 (原始值 + 1) * 4) */\nstatic int dissect_rtcp_length_field(u_char *rtcp_info, int offset)\n{\n\n uint16_t raw_length = ntohs(*(uint16_t*)(rtcp_info + offset));\n printf(\"(%u bytes)\\n\", (raw_length+1)*4);\n offset += 2;\n return offset;\n}\nstatic int dissect_rtcp_rr(u_char *rtcp_info, int offset,int count, int packet_length )\n{\n int counter = 0;\n uint8_t rr_flt = 0;\n int rr_offset = offset;\n\t\n counter = 1;\n while ( counter <= count ) {\n uint32_t lsr = 0, dlsr = 0;\n\n /* Create a new subtree for a length of 24 bytes */\n\n /* SSRC_n source identifier, 32 bits */\n\n offset += 4;\n\n /* Fraction lost, 8bits */\n rr_flt = rtcp_info[offset];\n\n offset++;\n\n /* Cumulative number of packets lost, 24 bits */\n offset += 3;\n\n\n /* Sequence number cycles */\n\n offset += 2;\n /* highest sequence number received */\n\n offset += 2;\n\n /* Interarrival jitter */\n\n offset += 4;\n\n /* Last SR timestamp */\n lsr = ntohl(*(uint32_t*)(rtcp_info + offset));\n\t\tprintf(\"Last SR timestamp: 0x%x\\n\",lsr);\n offset += 4;\n\n /* Delay since last SR timestamp */\n dlsr = ntohl(*(uint32_t*)(rtcp_info + offset));\n\n printf(\"(%d milliseconds)\\n\",(int)(((double)dlsr/(double)65536) * 1000.0));\n offset += 4;\n\n counter++;\n }\n\n return offset;\n}\n\nconst char *tvb_ntp_fmt_ts_sec(u_char *rtcp_info, int offset)\n{\n\tuint32_t tempstmp = 0;\n\ttime_t temptime = 0;\n\tstruct tm *bd;\n\tchar *buff = NULL;\n\t\n\ttempstmp = ntohl(*(uint32_t*)(rtcp_info + offset));\n\tif (tempstmp == 0){\n\t\treturn \"NULL\";\n\t}\n\n\t/* We need a temporary variable here so the unsigned math\n\t* works correctly (for years > 2036 according to RFC 2030\n\t* chapter 3).\n\t*/\n\ttemptime = (time_t)(tempstmp - NTP_BASETIME);\n\tbd = gmtime(&temptime);\n\tif (!bd){\n\t\treturn \"Not representable\";\n\t}\n\n\tbuff = (char *)malloc(NTP_TS_SIZE);\n\tsnprintf(buff, NTP_TS_SIZE,\n\t\t\"%s %2d, %d %02d:%02d:%02d UTC\",\n\t\tmon_names[bd->tm_mon],\n\t\tbd->tm_mday,\n\t\tbd->tm_year + 1900,\n\t\tbd->tm_hour,\n\t\tbd->tm_min,\n\t\tbd->tm_sec);\n\treturn buff;\n}\n\nstatic int dissect_rtcp_sr(u_char *rtcp_info, int offset,int count, int packet_length)\n{\n\n uint32_t ts_msw = 0, ts_lsw = 0;\n int sr_offset = offset;\n\n /* NTP timestamp */\n ts_msw = ntohl(*(uint32_t*)(rtcp_info + offset));\n\tprintf(\"ts_msw: 0x%x\\n\",ts_msw);\n ts_lsw = ntohl(*(uint32_t*)(rtcp_info + offset + 4));\n\tprintf(\"ts_lsw: 0x%x\\n\",ts_lsw);\n\n\t//printf(\"offset: 0x%x 0x%x 0x%x 0x%x\\n\",rtcp_info[offset],rtcp_info[offset + 1],rtcp_info[offset + 2],rtcp_info[offset + 3]);\n\tprintf(\"MSW: %s\\n\",tvb_ntp_fmt_ts_sec(rtcp_info,offset));\n offset += 8;\n\n /* RTP timestamp, 32 bits */\n\t\n offset += 4;\n /* Sender's packet count, 32 bits */\n\n offset += 4;\n /* Sender's octet count, 32 bits */\n\n offset += 4;\n\n\n /* The rest of the packet is equal to the RR packet */\n if ( count != 0 )\n offset = dissect_rtcp_rr(rtcp_info, offset, count, packet_length-(offset-sr_offset));\n else\n {\n /* If length remaining, assume profile-specific extension bytes */\n if ((offset-sr_offset) < packet_length)\n {\n\n offset = sr_offset + packet_length;\n }\n }\n\n return offset;\n}\n\nstatic int dissect_rtcp_sdes(u_char *rtcp_info, int offset, int count)\n{\n int chunk = 0;\n\n int start_offset = 0;\n int items_start_offset = 0;\n uint32_t ssrc = 0;\n unsigned int item_len = 0;\n unsigned int sdes_type = 0;\n unsigned int prefix_len = 0;\n\n chunk = 1;\n while ( chunk <= count ) \n\t{\n\t\t/* Create a subtree for this chunk; we don't yet know\n\t\t the length. */\n\t\tstart_offset = offset;\n\n\t\tssrc = ntohl(*(uint32_t*)(rtcp_info + offset));\n\t\tprintf(\"Chunk %u, SSRC/CSRC 0x%X\\n\", chunk, ssrc);\n\n\t\t/* SSRC_n source identifier, 32 bits */\n\t\toffset += 4;\n\n\t\t/* Create a subtree for the SDES items; we don't yet know\n\t\t the length */\n\n\n\t\t/*\n\t\t * Not every message is ended with \"null\" bytes, so check for\n\t\t * end of frame as well.\n\t\t */\n\t\t \n\t\t /* ID, 8 bits */\n\t\tsdes_type = rtcp_info[offset];\n\t\tprintf(\"Type: %d\\n\",sdes_type);\n\t\tif (sdes_type == 0)\n\t\t\tbreak;\n\t\toffset++;\n\t\t/* Item length, 8 bits */\n\t\titem_len = rtcp_info[offset];\n\t\tprintf(\"Length: %d\\n\",item_len);\n\t\toffset++;\n\t\t\n\t\tchar *pszText = (char*)malloc(item_len);\n\t\tif (pszText != 0)\n\t\t{\n\t\t\tmemcpy(pszText, rtcp_info + offset,item_len);\n\t\t\tpszText[item_len] = '\\0';\n\t\t\tprintf(\"Text = %s\\n\",pszText);\n\t\t}\t\n\n\t\tchunk++;\n }\n\n return offset;\n}\n\nstatic void dissect_rtcp(u_char *rtcp_info,int packet_type, int offset,int PayloadLen)\n{\n\tunsigned int temp_byte = 0;\n\tint elem_count = 0;\n\tint packet_length = 0;\n\tint total_packet_length = 0;\n\tint loop = 2;\n\tbool flag_rtcp = false;\n\n\t\t/*检查是否为有效类型*/\n\t\tif ( ( packet_type < RTCP_PT_MIN ) || ( packet_type > RTCP_PT_MAX ) )\n\t\t\texit(-1);\n\n\t\t/*\n\t\t * 获取完整的RTCP数据包的长度\n\t\t */\n\t\t \n\t\tpacket_length = (ntohs(*(uint16_t*)(rtcp_info + offset + 1)) + 1) * 4 ;\n\t\t//printf(\"packet_length: %d\\n\",packet_length);\n\n\t\t\n\t\ttemp_byte = rtcp_info[offset-1];\n\t\telem_count = RTCP_COUNT( temp_byte );/* Source count, 5 bits */\n\t\tprintf(\"Reception report count: %d\\n\",elem_count); \n\n\t\tswitch ( packet_type ) \n\t\t{\n\t\t\t\n\t\t\tcase RTCP_SR:\n\t\t\tcase RTCP_RR:\n\t\t\t\t/*\n\t\t\t\t\tReal-time Transport Control Protocol (Receiver Report)\n\t\t\t\t\t10.. .... = Version: RFC 1889 Version (2)\n\t\t\t\t\t..0. .... = Padding: False\n\t\t\t\t\t...0 0001 = Reception report count: 1\n\t\t\t\t\tPacket type: Receiver Report (201)\n\t\t\t\t\tLength: 7 (32 bytes)\n\t\t\t\t\tSender SSRC: 0xb584b03e (3045371966)\n\t\t\t\t\tSource 1\n\t\t\t\t*/\n\n\t\t\t\t/* Packet type, 8 bits */\n\t\t\t\toffset++;\n\t\t\t\t/* Packet length in 32 bit words MINUS one, 16 bits */\n\t\t\t\toffset = dissect_rtcp_length_field(rtcp_info, offset);\n\t\t\t\t/* Sender Synchronization source, 32 bits */\n\t\t\t\toffset += 4;\n\n\t\t\t\tif ( packet_type == RTCP_SR )\n\t\t\t\t{\n\t\t\t\t\toffset = dissect_rtcp_sr(rtcp_info, offset, elem_count, packet_length-8 );\n\t\t\t\t\tprintf(\"dissect_rtcp_sr\\n\");\n\t\t\t\t}\n\t\t\t\telse\n\t\t\t\t{\t\n\t\t\t\t\toffset = dissect_rtcp_rr(rtcp_info, offset, elem_count, packet_length-8 );\t\t\t\t\t\t\t\n\t\t\t\t}\n\t\t\t\t\n\t\t\t\t//uint16_t second_packet_type = ntohs(*(uint16_t*)(rtcp_info + offset));\n\n\t\t\t\t//printf(\"111offset: 0x%x 0x%x 0x%x 0x%x\\n\",rtcp_info[offset],rtcp_info[offset + 1],rtcp_info[offset + 2],rtcp_info[offset + 3]);\n\t\t\t\t\n\t\t\t\tif (rtcp_info[offset + 1] == RTCP_SDES)\n\t\t\t\t{\n\t\t\t\t\t\n\t\t\t\t\t/* Source count, 5 bits */\n\t\t\t\t\toffset++;\n\t\t\t\t\t/* Packet type, 8 bits */\n\t\t\t\t\toffset++;\n\t\t\t\t\t/* Packet length in 32 bit words MINUS one, 16 bits */\n\t\t\t\t\toffset = dissect_rtcp_length_field(rtcp_info, offset);\n\t\t\t\t\toffset = dissect_rtcp_sdes(rtcp_info,offset,elem_count);\n\n\t\t\t\t}\n\n\t\t\tbreak;\n\n\t\t\tdefault:\n\t\t\t\t/*\n\t\t\t\t * To prevent endless loops in case of an unknown message type\n\t\t\t\t * increase offset. Some time the while will end :-)\n\t\t\t\t */\n\t\t\t\toffset++;\n\t\t\t\tbreak;\n\n\t\t}\n\t\t\n}\n\n\nstatic bool dissect_rtcp_heur(u_char *rtcp_info,int PayloadLen)\n{\n unsigned int offset = 0;\n unsigned int first_byte = 0;\n unsigned int packet_type = 0;\n\n\n /* 查看第一个字节 */\n first_byte = rtcp_info[offset];\n\n /* 版本位是否设置为2*/\n\t//printf(\"version: %d\\n\",((first_byte & 0xC0) >> 6));\n if (((first_byte & 0xC0) >> 6) != 2)\n {\n return false;\n }\n\n /* 看包类型 */\n\toffset += 1;\n packet_type = rtcp_info[offset];\n\t//printf(\"packet_type: %d\\n\",packet_type);\n /* 复合数据包中的第一个数据包应该是发送方或接收者报告 */\n if (!((packet_type == RTCP_SR) || (packet_type == RTCP_RR) ||\n (packet_type == RTCP_BYE) || (packet_type == RTCP_APP) ||\n (packet_type == RTCP_PSFB)))\n {\n return false;\n }\n\n /*总长度必须是4个字节的倍数*/\n\t//printf(\"PayloadLen: %d\\n\",PayloadLen);\n if (PayloadLen % 4)\n {\n return false;\n }\n\n /* OK, dissect as RTCP */\n dissect_rtcp(rtcp_info,packet_type,offset,PayloadLen);\n return true;\n}\n\n\nstatic void confirm_rtcp_packet(struct ip *pIp)\n{\n\tint iIpTotalLen = ntohs(pIp->ip_len);\n\tint offset = 0;\n\tint nFragSeq = 0;\n\tstruct udphdr* pUdpHdr = (struct udphdr*)((char*)pIp + (pIp->ip_hl<<2));\n\tif (pIp->ip_p == IPPROTO_UDP) \n\t{\n\t\tprintf(\"\\n\");\n\t\t\n\t\tint iPayloadLen = iIpTotalLen - (pIp->ip_hl<<2) - 8;\n\t\tprintf(\"UDP Payload Len %d\\n\", iPayloadLen);\n\t\t\n\t\tu_char *pDnsHdr = (u_char*)(pUdpHdr+1);\n\t\tdissect_rtcp_heur(pDnsHdr,iPayloadLen);\n\t\t\n\t}\t\n}","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"编译运行：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/88/8853ff58f287bc19b757a5977f9482bb.png","alt":"在这里插入图片描述","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"总结","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"RTCP协议是流媒体通信的基石。RTCP协议则负责可靠传输、流量控制和拥塞控制等服务质量保证。上面讲解了RTCP功能、RTCP数据包格式及代码实现。最后，学习一个新的协议，最好还是研究学习官方文档，因为这是最权威的资料。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}}]}