视音频数据处理入门：UDP-RTP协议解析

原作者雷霄骅，于前几日去世，可惜，愿一路走好。
本文介绍网络协议数据的处理程序。网络协议数据在视频播放器中的位置如下所示。

原理

MPEG-TS封装格式数据打包为RTP/UDP协议然后发送出去的流程如下图所示。图中首先每7个MPEG-TS Packet打包为一个RTP，然后每个RTP再打包为一个UDP。其中打包RTP的方法就是在MPEG-TS数据前面加上RTP Header，而打包UDP的方法就是在RTP数据前面加上UDP Header。

有关MPEG-TS、RTP、UDP的知识不再详细介绍，可以参考相关的文档了解其中的细节信息。本文记录的程序是一个收取流媒体的程序，因此本文程序的流程和上述发送MPEG-TS的流程正好是相反的。该程序可以通过Socket编程收取UDP包，解析其中的RTP包的信息，然后再解析RTP包中MPEG-TS Packet的信息。

代码

整个程序位于simplest_udp_parser()函数中，如下所示

#include <stdio.h>  
#include <winsock2.h>  
  
#pragma comment(lib, "ws2_32.lib")   
  
#pragma pack(1)  
  
/* 
 * [memo] FFmpeg stream Command: 
 * ffmpeg -re -i sintel.ts -f mpegts udp://127.0.0.1:8880 
 * ffmpeg -re -i sintel.ts -f rtp_mpegts udp://127.0.0.1:8880 
 */  
  
typedef struct RTP_FIXED_HEADER{  
    /* byte 0 */  
    unsigned char csrc_len:4;       /* expect 0 */  
    unsigned char extension:1;      /* expect 1 */  
    unsigned char padding:1;        /* expect 0 */  
    unsigned char version:2;        /* expect 2 */  
    /* byte 1 */  
    unsigned char payload:7;  
    unsigned char marker:1;        /* expect 1 */  
    /* bytes 2, 3 */  
    unsigned short seq_no;              
    /* bytes 4-7 */  
    unsigned  long timestamp;          
    /* bytes 8-11 */  
    unsigned long ssrc;            /* stream number is used here. */  
} RTP_FIXED_HEADER;  
  
typedef struct MPEGTS_FIXED_HEADER {  
    unsigned sync_byte: 8;   
    unsigned transport_error_indicator: 1;   
    unsigned payload_unit_start_indicator: 1;  
    unsigned transport_priority: 1;   
    unsigned PID: 13;  
    unsigned scrambling_control: 2;  
    unsigned adaptation_field_exist: 2;  
    unsigned continuity_counter: 4;  
} MPEGTS_FIXED_HEADER;  
  
  
  
int simplest_udp_parser(int port)  
{  
    WSADATA wsaData;  
    WORD sockVersion = MAKEWORD(2,2);  
    int cnt=0;  
  
    //FILE *myout=fopen("output_log.txt","wb+");  
    FILE *myout=stdout;  
  
    FILE *fp1=fopen("output_dump.ts","wb+");  
  
    if(WSAStartup(sockVersion, &wsaData) != 0){  
        return 0;  
    }  
  
    SOCKET serSocket = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);   
    if(serSocket == INVALID_SOCKET){  
        printf("socket error !");  
        return 0;  
    }  
  
    sockaddr_in serAddr;  
    serAddr.sin_family = AF_INET;  
    serAddr.sin_port = htons(port);  
    serAddr.sin_addr.S_un.S_addr = INADDR_ANY;  
    if(bind(serSocket, (sockaddr *)&serAddr, sizeof(serAddr)) == SOCKET_ERROR){  
        printf("bind error !");  
        closesocket(serSocket);  
        return 0;  
    }  
  
    sockaddr_in remoteAddr;  
    int nAddrLen = sizeof(remoteAddr);   
  
    //How to parse?  
    int parse_rtp=1;  
    int parse_mpegts=1;  
  
    printf("Listening on port %d\n",port);  
  
    char recvData[10000];    
    while (1){  
  
        int pktsize = recvfrom(serSocket, recvData, 10000, 0, (sockaddr *)&remoteAddr, &nAddrLen);  
        if (pktsize > 0){  
            //printf("Addr:%s\r\n",inet_ntoa(remoteAddr.sin_addr));  
            //printf("packet size:%d\r\n",pktsize);  
            //Parse RTP  
            //  
            if(parse_rtp!=0){  
                char payload_str[10]={0};  
                RTP_FIXED_HEADER rtp_header;  
                int rtp_header_size=sizeof(RTP_FIXED_HEADER);  
                //RTP Header  
                memcpy((void *)&rtp_header,recvData,rtp_header_size);  
  
                //RFC3551  
                char payload=rtp_header.payload;  
                switch(payload){  
                case 0:  
                case 1:  
                case 2:  
                case 3:  
                case 4:  
                case 5:  
                case 6:  
                case 7:  
                case 8:  
                case 9:  
                case 10:  
                case 11:  
                case 12:  
                case 13:  
                case 14:  
                case 15:  
                case 16:  
                case 17:  
                case 18: sprintf(payload_str,"Audio");break;  
                case 31: sprintf(payload_str,"H.261");break;  
                case 32: sprintf(payload_str,"MPV");break;  
                case 33: sprintf(payload_str,"MP2T");break;  
                case 34: sprintf(payload_str,"H.263");break;  
                case 96: sprintf(payload_str,"H.264");break;  
                default:sprintf(payload_str,"other");break;  
                }  
  
                unsigned int timestamp=ntohl(rtp_header.timestamp);  
                unsigned int seq_no=ntohs(rtp_header.seq_no);  
  
                fprintf(myout,"[RTP Pkt] %5d| %5s| %10u| %5d| %5d|\n",cnt,payload_str,timestamp,seq_no,pktsize);  
  
                //RTP Data  
                char *rtp_data=recvData+rtp_header_size;  
                int rtp_data_size=pktsize-rtp_header_size;  
                fwrite(rtp_data,rtp_data_size,1,fp1);  
  
                //Parse MPEGTS  
                if(parse_mpegts!=0&&payload==33){  
                    MPEGTS_FIXED_HEADER mpegts_header;  
                    for(int i=0;i<rtp_data_size;i=i+188){  
                        if(rtp_data[i]!=0x47)  
                            break;  
                        //MPEGTS Header  
                        //memcpy((void *)&mpegts_header,rtp_data+i,sizeof(MPEGTS_FIXED_HEADER));  
                        fprintf(myout,"   [MPEGTS Pkt]\n");  
                    }  
                }  
  
            }else{  
                fprintf(myout,"[UDP Pkt] %5d| %5d|\n",cnt,pktsize);  
                fwrite(recvData,pktsize,1,fp1);  
            }  
  
            cnt++;  
        }  
    }  
    closesocket(serSocket);   
    WSACleanup();  
    fclose(fp1);  
  
    return 0;  
}