RTP － 视频流广播

这是用RTP（RFC3350）按RFC2550封装MPEG ES流数据的发送程序。学习RTP的路真的辛苦。在网上收集的有关RTP的程序都是那种只负责RTP数据包发送的库，如jrtplib等，他们的DEMO 程序都只是用来发发字符串，编编聊天程序，无论是国内还是国外，都没有结合真正的应用的DEMO。其实我的目的很简单，就是写发个视频流服务器，不用复杂，只用把基本原理弄懂，因为这样你才能有的放矢。与网上和RTP相关的库没有应用不一让，当你尝试以流媒体服务器、linux来baidu或 google时，你搜出来完非就那么几类：

1.FFSERVER
     FFMPEG2的DEMO,说它有名只是因为这类程序太少了。FFMPEG2是很好用，我现在还在用，但这个DEMO就有很多“炒作”的嫌疑了。好像在做着FFMPEG2库的演示而不是真的视频流服务器。后来想想，这不正是作者想要的吗，但这不是我想要的。编解码部分我会很偏向FFMPEG这个“大杂会 ”，其它部分我会选择其它的“强者”

2.Darwin、Helix
     两个都是非常有名软件，也只能称之为软件了，因为就算Darwin有源码，这种代码规模，也不适合用于嵌入式。说回软件本身，真的很有名。它们都是很真真拿来商业化运行的软件，但我是研发人员，不是视频流服务商，对不起，Apple，对不起，Microsoft。

3.LIVE555
     如果说上面两个和我都相关性为零（当然了，也是困扰了N周以后痛苦得出的结论），那LIVE555真的给了我一条出路，它是一个代码规模非常合适，又非常强大的媒体解决方案（称之为方案是因为它功能非常的丰富）。有长一段时间，我想去弄懂它的源码，不过和网上的很多人一样，最后软下来了，毕竟，去把这么多东西揉在一起，框架会弄得很复杂，因为我们要把这些完全不同的东西不断一层一层的抽像，最后抽像成一样（哲学呀）。它结构复杂是我中断分析它原来的其中一个原因，但不是主要原因。它结构的复杂程度也没胡像很多人网上说的那样严重，如果你是一个C++的热忱爱好者，你反而会迷上这段代码，当然了，对C的爱好都来说，当然是一种折磨了。暂时把我自己归类在C++爱好者范畴吧，呵呵，我很欣赏这段原码。主要原因是我不希望被某一个库绑死。LIVE555是有编解码能力，但我更希望它只做服务器的工作。

     因此，最终后回来的老路上来，没有帮助，就得自己帮自己，从最基础的RFC看起。经过了N天（周）的英文，终于领会了如果在RTP承载MPEG数据包。在这个过程中很得到了一些LIVE555的帮助（通过对Ethereal捕捉的LIVE555数据包进行分析）。先把程序弄上来，原理性的以后有空再写，程序只有一个.cpp文件，在vs.net 2003下编译通过，播放的视频文件在http://www.cnitblog.com/Files/tinnal/ES流解释程序.rar   内，播放的客户端采用VLC，其下载地址为http://www.videolan.org/ 。选择打开网络串流，然后选择“UDP/RTP”端口，输入程序的输出端口1000，然后才运行程序，你将在VLC内看到测试的广播视频，IP不一样的话自己改改就行。其它所谓的原理性的，也就是看RFC 3350、RFC2550以及iso13818－2的一些重点地方。

   // MPEG2RTP.h
#include < stdio.h >
#include < stdlib.h >
#include < conio.h >
#include < string .h >

#include < winsock2.h >
#include < winsock2.h >


// #include "mem.h"

// <! Start code or other signal
#define PACK_STARTCODE                     (unsigned int)0x000001ba
#define SYSTEM_HEADER_STARTCODE     (unsigned int)0x000001bb
#define PICTURE_START_CODE               (unsigned int)0x00000100
#define GROUP_START_CODE            (unsigned int)0x000000B8
#define ISO_11172_ENDCODE             (unsigned int)0x000001b9
#define SEQUENCE_HEADER_CODE        (unsigned int)0x000001b3

#define PACKET_BUFFER_END            (unsigned int)0x00000000


#define MAX_RTP_PKT_LENGTH     1440
#define HEADER_LENGTH        16
#define DEST_IP                "192.168.0.98"
#define DEST_PORT            1000
#define MPA                    14 /*MPEG PAYLOAD TYPE */
#define MPV                    32

typedef struct  
{
    /**/ /* byte 0 */
    unsigned char csrc_len: 4 ;        /**/ /* expect 0 */
    unsigned char extension: 1 ;        /**/ /* expect 1, see RTP_OP below */
    unsigned char padding: 1 ;        /**/ /* expect 0 */
    unsigned char version: 2 ;        /**/ /* expect 2 */
    /**/ /* byte 1 */
    unsigned char payload: 7 ;        /**/ /* RTP_PAYLOAD_RTSP */
    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   {
    // byte 0
    unsigned char TR_high2: 2 ;    /**/ /* Temporal Reference high 2 bits */
    unsigned char T: 1 ;            /**/ /* video specific head extension flag */
    unsigned char MBZ: 5 ;        /**/ /* unused */
    // byte1
    unsigned char TR_low8: 8 ;    /**/ /* Temporal Reference low 8 bits */
    // byte3
    unsigned char P: 3 ;            /**/ /* picture type; 1=I,2=P,3=B,4=D */
    unsigned char E: 1 ;         /**/ /* set if last byte of payload is slice end code */
    unsigned char B: 1 ;            /**/ /* set if start of payload is slice start code */    
    unsigned char S: 1 ;            /**/ /* sequence header present flag */
    unsigned char N: 1 ;            /**/ /* N bit; used in MPEG 2 */
    unsigned char AN: 1 ;        /**/ /* Active N bit */
    // byte4
    unsigned char FFC: 3 ;        /**/ /* forward_f_code */
    unsigned char FFV: 1 ;        /**/ /* full_pel_forward_vector */
    unsigned char BFC: 3 ;        /**/ /* backward_f_code */
    unsigned char FBV: 1 ;        /**/ /* full_pel_backward_vector */
} MPEG_VID_SPECIFIC_HDR; /**/ /* 4 BYTES */


enum reading_status {
    SLICE_AGAIN,
    SLICE_BREAK,
    UNKNOWN,
    SLICE,
    SEQUENCE_HEADER,
    GROUP_START,
    PICTURE
    } ;

void validate_file();
float frame_rate( int buffer_index);
unsigned int read_picture_type( int buffer_index);
unsigned int read_FBV( int buffer_index);
unsigned int read_BFC( int buffer_index);
unsigned int read_FFV( int buffer_index);
unsigned int read_FFC( int buffer_index);
unsigned int extract_temporal_reference( int buffer_index);
unsigned int find_next_start_code(unsigned int   * buffer_index);
void reset_buffer_index( void );
BOOL InitWinsock();

 

//MPEG2RTP.cpp
    // 这个程序主要用于RTP封装MPEG2数据的学习和测试，不作任何其它用途
// 软件在VS.net 2003中编译通过，但在linux下作小量修改也应编译通过。
// 通过VLC测试，VLC能正确接收和解码由本程序发送的TEST.MPV编码流。
//
// 作者：冯富秋 Tinnal
// 邮箱：[email protected]


#include " MPEG2RTP.h "

#pragma   comment(lib, " Ws2_32 " )

unsigned char             buf[MAX_RTP_PKT_LENGTH +   4 ]; // input buffer
enum reading_status        state = SEQUENCE_HEADER;
unsigned int             g_index_in_packet_buffer = HEADER_LENGTH;
static unsigned long     g_time_stamp =   0 ;
static unsigned long     g_time_stamp_current = 0 ;
static   float             g_frame_rate =   0 ;
static unsigned int         g_delay_time =   0 ;
static unsigned int         g_timetramp_increment =   0 ;
FILE    * mpfd;
SOCKET    socket1;
RTP_FIXED_HEADER        * rtp_hdr;
MPEG_VID_SPECIFIC_HDR    * mpeg_hdr;

#if 0
void Send_RTP_Packet(unsigned char   * buf, int bytes)
{

    int i =   0 ;
    int count =   0 ;

    printf( " /nPacket length %d/n " ,bytes);
    printf( " RTP Header: [M]:%s [sequence number]:0x%lx [timestamp]:0x%lx/n " ,
        rtp_hdr -> marker ==   1 ? " TRUE " : " FALSE " ,
        rtp_hdr -> seq_no,
        rtp_hdr -> timestamp);
    printf( " [TR]:%d [AN]:%d [N]:%d [Sequence Header]:%s /
        /n [Begin Slice]: % s [End Slice]: % s /
        /n [Pictute Type]: % d /
        /n [FBV]: % d [BFC]: % d [FFV]: % d [FFC]: % d/n " ,
        (mpeg_hdr -> TR_high2 <<   8   | mpeg_hdr -> TR_low8),
        mpeg_hdr -> AN, mpeg_hdr -> N, mpeg_hdr -> S ==   1 ? " TRUE " : " FALSE " ,
        mpeg_hdr -> B == 1 ? " TRUE " : " FALSE " , mpeg_hdr -> E == 1 ? " TRUE " : " FALSE " ,
        mpeg_hdr -> P,
        mpeg_hdr -> FBV, mpeg_hdr -> BFC, mpeg_hdr -> FFV, mpeg_hdr -> FFC);

    while (bytes -- )
    {
        printf( " %02x " ,buf[count ++ ]);
        if ( ++ i ==   16 )
        {
            i = 0 ;
            printf( " /n " );
        }
    }
    printf( " /n " );

}

#else

Send_RTP_Packet(unsigned char   * buf, int bytes)
{
    return send( socket1,  ( char * ) buf, bytes, 0 );
}

#endif


void main( int argc, char   * argv[])
{
    unsigned int next_start_code;
    unsigned int next_start_code_index;
    unsigned int sent_bytes;
    unsigned short seq_num = 0 ;
    unsigned short stream_num =   10 ;
    struct sockaddr_in server;
    int len = sizeof (server);


#if 0
    mpfd = fopen( " E://tinnal//live555//vc_proj//es//Debug//test.mpv " , " rb " ); 
#else
    if (argc <   2 )
    {
        printf( " /nUSAGE: %s mpegfile/nExiting../n/n " ,argv[ 0 ]);
                exit( 0 );
    }
    mpfd = fopen(argv[ 1 ], " rb " );
#endif



    if (mpfd == NULL )
    {
        printf( " /nERROR: could not open input file %s/n/n " ,argv[ 1 ]);
        exit( 0 );
    }
    rtp_hdr = (RTP_FIXED_HEADER * ) & buf[ 0 ];
    mpeg_hdr = (MPEG_VID_SPECIFIC_HDR * ) & buf[ 12 ];

    memset(( void   * )rtp_hdr, 0 , 12 ); // zero-out the rtp fixed hdr
    memset(( void   * )mpeg_hdr, 0 , 4 ); // zero-out the video specific hdr
    memset(( void   * )buf, 0 ,MAX_RTP_PKT_LENGTH +   4 );

    InitWinsock();

    server.sin_family = AF_INET;
    server.sin_port = htons(DEST_PORT);          // server的监听端口
    server.sin_addr.s_addr = inet_addr(DEST_IP); // server的地址

    socket1 = socket(AF_INET,SOCK_DGRAM, 0 );
    connect(socket1, ( const sockaddr * ) & server, len) ;

    // read the first packet from the mpeg file
    // always read 4 extra bytes in (in case there's a startcode there)
    // but dont send  more than MAX_RTP_PKT_LENGTH in one packet
    fread( & (buf[HEADER_LENGTH]), MAX_RTP_PKT_LENGTH - HEADER_LENGTH + 4 , 1 ,mpfd);

    validate_file();

    do
    {   

        /**/ /* initialization of the two RTP headers */
        rtp_hdr -> seq_no     = htons(seq_num ++ );
        rtp_hdr -> payload     = MPV;
        rtp_hdr -> version     =   2 ;
        rtp_hdr -> marker    =   0 ;
        rtp_hdr -> ssrc        = htonl(stream_num);   

        mpeg_hdr -> S = mpeg_hdr -> E = mpeg_hdr -> B =   0 ;

        do {
            next_start_code = find_next_start_code( & next_start_code_index);


            if ((next_start_code > 0x100 ) && (next_start_code < 0x1b0 ) )
            {
                // <! We reach the first slice start code in current packet buffer.
                // <! Set the B flag of the mpeg special header
                if (state == SEQUENCE_HEADER
                    || state == GROUP_START
                    || state == PICTURE
                    || state == UNKNOWN)
                {
                    state = SLICE;
                    mpeg_hdr -> B =   1 ;
                }
                // <! We reach slice start code in current packet again.
                // <! Set the E flag of the mpeg special header,
                // <! and update the sent_bytes to the last slice data end.
                else   if (state == SLICE || state == SLICE_AGAIN)
                {

                    state = SLICE_AGAIN;
                    sent_bytes = next_start_code_index;
                    mpeg_hdr -> E     =   1 ;
                }
                // <! We reach slice start code(the previous slice end)
                // <! for a broken slice. set the E flag.
                // <! According to RFC2550, we shouldn't put another slice data to this packet,
                // <! instead of sent it out.
                else   if (state == SLICE_BREAK)
                {
                    state = UNKNOWN;
                    sent_bytes = next_start_code_index;
                    mpeg_hdr -> E     =   1 ;
                    goto Sent_Packet;
                }

            }  

            switch (next_start_code)
            {
            case SEQUENCE_HEADER_CODE:
                // <! SEQUENCE_HEADER_CODE after SLICE_START_CODE
                // <! we must sent the packet now, so that, the SEQUENCE_HEADER_CODE
                // <! will appear at the start of the next packet
                if (state == SLICE || state == SLICE_AGAIN)
                {               
                    state = SEQUENCE_HEADER;
                    sent_bytes = next_start_code_index;
                    // <! Accord to RFC 2550,
                    // <! at the end of a frame we should set RTP marker bit to 1.
                    rtp_hdr -> marker =   1 ;
                    goto Sent_Packet;
                }

                state = SEQUENCE_HEADER;
                g_frame_rate = frame_rate(next_start_code_index);
                g_delay_time = (unsigned int )( 1000.0   / g_frame_rate + 0.5 ); // ms
                g_timetramp_increment = (unsigned int )( 90000.0   / g_frame_rate + 0.5 ); // 90K Hz
                mpeg_hdr -> S = 1 ;
                break ;

            case GROUP_START_CODE:
                // <! GROUP_START_CODE after SLICE_START_CODE
                // <! we must sent the packet now, so that, the GROUP_START_CODE
                // <! will appear at the start of the next packet
                if (state == SLICE || state == SLICE_AGAIN)
                {
                    state = GROUP_START;
                    sent_bytes = next_start_code_index;
                    // <! Accord to RFC 2550,
                    // <! at the end of a frame we should set RTP marker bit to 1.
                    rtp_hdr -> marker =   1 ;
                    goto Sent_Packet;
                }    

                state = GROUP_START;

            case PICTURE_START_CODE:
                // <! PICTURE_START_CODE after PICTURE_START_CODE
                // <! we must sent the packet now, so that, the PICTURE_START_CODE
                // <! will appear at the start of the next packet
                if (state == SLICE || state == SLICE_AGAIN)
                {
                    state = PICTURE;
                    sent_bytes = next_start_code_index;
                    // <! Accord to RFC 2550,
                    // <! at the end of a frame we should set RTP marker bit to 1.
                    rtp_hdr -> marker =   1 ;
                    goto Sent_Packet;
                }

                state = PICTURE;

                mpeg_hdr -> TR_high2    = (extract_temporal_reference(next_start_code_index) &   0x300 ) >>   8 ;
                mpeg_hdr -> TR_low8     =   extract_temporal_reference(next_start_code_index) &   0xff ;
                mpeg_hdr -> P             = read_picture_type(next_start_code_index);
                // now read the motion vectors information
                if ( (mpeg_hdr -> P == 2 ) || (mpeg_hdr -> P == 3 ))
                { // if B- or P-type picture, need forward mv
                    mpeg_hdr -> FFV = read_FFV(next_start_code_index);
                    mpeg_hdr -> FFC = read_FFC(next_start_code_index);
                }
                if ( mpeg_hdr -> P == 3 )
                { // if B-type pictue, need backward mv
                    mpeg_hdr -> FBV = read_FBV(next_start_code_index);
                    mpeg_hdr -> BFC = read_BFC(next_start_code_index);
                }

                // <! Time stamp only increate per frame.
                // <! But I or P frame.
                if ( mpeg_hdr -> P ==   1   || mpeg_hdr -> P ==   2 ) {
                    g_time_stamp += g_timetramp_increment;
                    g_time_stamp_current = g_time_stamp;
                } else {
                    g_time_stamp += g_timetramp_increment;
                }
               

                break ;

            case PACKET_BUFFER_END:
                // <! There is one more slice in the packet buffer
                // <! Anyway, we only sent the integrated slice
                if (state == SLICE_AGAIN) {
                    state = UNKNOWN;
                    goto Sent_Packet;
                }

                // <! There is one Slice in the packet buffer.
                // <! But the Slice is to big, so we break the slice.
                if (state == SLICE)
                {
                    state = SLICE_BREAK;
                    sent_bytes = next_start_code_index;
                    goto Sent_Packet;
                }

                // <! There if a broke slice, but in current packet buffer
                // <! we could not find the end of the slice.
                // <! Let it in the broke state.
                if (state == SLICE_BREAK )
                {
                    state = SLICE_BREAK;
                    sent_bytes = next_start_code_index;
                    goto Sent_Packet;
                }

                break ;
            }
        } while (next_start_code != PACKET_BUFFER_END);

Sent_Packet:
        rtp_hdr -> timestamp = htonl(g_time_stamp_current);
        Send_RTP_Packet(buf, sent_bytes);
       
        // copy the tail data to the head of the packet buffer
        memmove( & buf[HEADER_LENGTH], & buf[sent_bytes], MAX_RTP_PKT_LENGTH - sent_bytes);
        // reset the buffer index to zero
        reset_buffer_index();
        // reading data into buffer again
        fread( & (buf[(MAX_RTP_PKT_LENGTH - sent_bytes) + HEADER_LENGTH]), sent_bytes - HEADER_LENGTH , 1 ,mpfd);

        // sleep g_delay_time msec for sending next picture data
        if (rtp_hdr -> marker == 1 ) Sleep( g_delay_time );

    } while ( ! feof(mpfd));
    closesocket(socket1);
    fclose(mpfd);

    printf( " stream end./n " );
}

// ==================================================================

unsigned int find_next_start_code(unsigned int   * next_start_code_index) // NOTE: all start codes ARE byte-aligned
{
    unsigned int byte0 = 0 ,byte1 = 0 ,byte2 = 0 ,byte3 = 0 ,startcode = 0 ;

    // while not startcode and have not exceeded max packet length
    while (g_index_in_packet_buffer < MAX_RTP_PKT_LENGTH)
    {
        if (buf[g_index_in_packet_buffer + 0 ] ==   0
            && buf[g_index_in_packet_buffer + 1 ] ==   0
            && buf[g_index_in_packet_buffer + 2 ] == 1 )
        {
            // printf("FOUND startcode %d/n",indx);
            byte0 = ( int )buf[g_index_in_packet_buffer + 0 ];
            byte1 = ( int )buf[g_index_in_packet_buffer + 1 ];
            byte2 = ( int )buf[g_index_in_packet_buffer + 2 ];
            byte3 = ( int )buf[g_index_in_packet_buffer + 3 ];
            startcode = (byte0 <<   24 ) + (byte1 <<   16 ) + (byte2 <<   8 ) + byte3;
            * next_start_code_index = g_index_in_packet_buffer;
            g_index_in_packet_buffer = g_index_in_packet_buffer + 4 ;
            return (startcode);
        }
        else
            g_index_in_packet_buffer ++ ;
    }

    // <! reach the end of the packet buffer
    if (g_index_in_packet_buffer >= (MAX_RTP_PKT_LENGTH))
    {
        * next_start_code_index = g_index_in_packet_buffer - 1 ;
        g_index_in_packet_buffer = HEADER_LENGTH;
        return PACKET_BUFFER_END;
    }

    printf( " Error reading buffer../n " );
    exit( - 1 );
    return   - 1 ;
}

void reset_buffer_index( void )
{
    g_index_in_packet_buffer = HEADER_LENGTH;
}



// ========================================================
float frame_rate( int buffer_index)
{
    unsigned char frame_rate_code;
    frame_rate_code = (unsigned char )buf[buffer_index + 7 ] &   0xf ;
    switch (frame_rate_code)
    {
    case   0x1 :
        return   23.976 ;
    case   0x2 :
        return   24.0 ;
    case   0x3 :
        return   25.0 ;
    case   0x4 :
        return   29.97 ;
    case   0x5 :
        return   30.0 ;
    case   0x6 :
        return   50.0 ;
    case   0x7 :
        return   59.94 ;
    case   0x8 :
        return   60.0 ;
    default :
        return   0 ;
    }
}
// ========================================================
unsigned int extract_temporal_reference( int buffer_index) // 10 bits
{
    unsigned int low2bits = 0 ,TR = 0 ; // TR = temporal reference;

    TR = (unsigned int ) (buf[buffer_index + 4 ]);
    TR <<=   2 ;
    low2bits = (unsigned int ) (buf[buffer_index + 5 ]);
    TR |= (low2bits >>   6 );
    return (TR);
}

// ========================================================

unsigned int read_picture_type( int buffer_index)
{
    unsigned int pictype = 0 ;

    pictype = (unsigned int ) buf[buffer_index + 5 ];
    pictype = (pictype >>   3 ) & ( 0x7 );
    return (pictype);
}

// =======================================================
unsigned int read_FFV( int buffer_index) // 1 bit
{
    return ( ( int ) ((buf[buffer_index + 7 ] & ( 0x4 )) >>   2 ));
}
// =======================================================
unsigned int read_FFC( int buffer_index) // 3 bits
{
    unsigned int FFC = 0 ,lowbit = 0 ;
    FFC = ( int ) (buf[buffer_index + 7 ] & ( 0x3 ));
    FFC <<=   1 ;
    lowbit = ( int ) ((buf[buffer_index + 8 ]) & ( 0x80 ));
    FFC = FFC | (lowbit >>   7 );
    return (FFC);
}

// =======================================================
unsigned int read_FBV( int buffer_index) // 1 bit
{      
    return ( ( int ) ((buf[buffer_index + 8 ] & ( 0x40 )) >> 6 ) );
}

// =======================================================
unsigned int read_BFC( int buffer_index) // 3 bits
{               
    return ( ( int ) ( (buf[buffer_index + 8 ] & ( 0x38 ) ) >>   3 ) );
}

void validate_file()
{
    /**/ /* to validate the file, ensure the existance of a startcode */
    int j = 0 ,valid = 0 ;

    while ((j ++< MAX_RTP_PKT_LENGTH) && ( ! valid))
    {
        if ( ! (( int )buf[j + 0 ] + ( int )buf[j + 1 ]) && ((( int )buf[j + 2 ]) == 1 ))
            valid = 1 ;
    }
    if ( ! valid)
    {
        printf( " /nERROR: start code not found. /
               /nInput file must be a valid MPEG I file./n " );
        exit( 0 );
    }           
}

BOOL InitWinsock()
{
    int Error;
    WORD VersionRequested;
    WSADATA WsaData;
    VersionRequested = MAKEWORD( 2 , 2 );
    Error = WSAStartup(VersionRequested, & WsaData); // 启动WinSock2
    if (Error != 0 )
    {
        return FALSE;
    }
    else
    {
        if (LOBYTE(WsaData.wVersion) != 2 || HIBYTE(WsaData.wHighVersion) != 2 )
        {
            WSACleanup();
            return FALSE;
        }
       
    }
    return TRUE;
}

 

      完成这个测试程序后，我有了很大的信心，又重复看了RFC3550几编，其实，如果你真看了程序，你发现我只发送了RTP，并没有发送RTCP数据包，因此，我们是不能同步多个RTP流的。我没去编码下去，因为我觉得已经够了。这里强调，没用说的RTP没有了RTCP就不行！接下来的工作，就是把这个程序的下层发包函数去掉，采用RTP库JRTPLIB，我觉得这才应该是JRTPLIB的DEMO！如果有人问，就这样的一个程序就能完成任务了，要 JRTPLIB干嘛，其实，我不写RTCP相关代码的原因为多个：

    1.RTCP里头有很多关于RTCP发送简隔的时间计算，RTP信息的统计，这种操作不是难，而是烦，我不想去写
    2.RTCP和RTP一开始出来的时候并不是因为视频的点播等应用的，而是视频会议。RTCP有管理与会者的层面含义，这一功能在很多场合并不会用到。
    3.我想简单，没有写多个流间的同步，如一个影片的视频和音频流。这些其实是RTCP来完成的。

    我懒得去写，因为这些功作RTP的各个库类都做得很好。我觉得用库的最大优点就在这吧。