不能从RTP数据包中提取RTP有效负载

Question

嗨，我想像udpxy那样将多播视频代理到单播：http://www.udpxy.com，但是在C#中，由于找不到我可以使用的合适的RTP库(它们太复杂了，或者我无法理解如何使用它们)，所以我决定移植一个udpxy使用rtp.c：https://github.com/pcherenkov/udpxy/blob/master/chipmunk/rtp.c。

一切都进行得很顺利(几乎，因为我不想使用指针)，直到我想把RTP_Process翻译成C#

RTP_Process in C

RTP_process( void** pbuf, size_t* len, int verify, FILE* log )
{
    int rtp_padding = -1;
    size_t front_skip = 0, back_skip = 0, pad_len = 0;

    char* buf = NULL;
    size_t pkt_len = 0;

    assert( pbuf && len && log );
    buf = *pbuf;
    pkt_len = *len;

    if( verify && !RTP_verify( buf, pkt_len, log ) )
        return -1;

    if( 0 != RTP_hdrlen( buf, pkt_len, &front_skip, log ) )
        return -1;

    rtp_padding = buf[0] & 0x20;
    if( rtp_padding ) {
        pad_len = buf[ pkt_len - 1 ];
    }

    back_skip += pad_len;

    if( verify && (pkt_len < (front_skip + back_skip)) ) {
        (void) tmfprintf( log, "RTP_process: invalid header "
                "(skip [%lu] exceeds packet length [%lu])\n",
                (u_long)(front_skip + back_skip), (u_long)pkt_len );
        return -1;
    }

    /* adjust buffer/length to skip heading and padding */
    /*
    TRACE( (void)tmfprintf( log, "In: RTP buf=[%p] of [%lu] bytes, "
                "fskip=[%ld], bskip=[%lu]\n",
                (void*)buf, (u_long)pkt_len,
                (u_long)front_skip, (u_long)back_skip ) );
    */

    buf += front_skip;
    pkt_len -= (front_skip + back_skip);

    /*
    TRACE( (void)tmfprintf( log, "Out RTP buf=[%p] of [%lu] bytes\n",
                (void*)buf, (u_long)pkt_len ) );
    */
    *pbuf = buf;
    *len  = pkt_len;

    return 0;
}

RTP_Process in C#

public byte[] RTP_process(int verify)
{
    /* process RTP package to retrieve the payload: set
    * pbuf to the start of the payload area; set len to
    * be equal payload's length
    *
    * @param pbuf      address of pointer to beginning of RTP packet
    * @param len       pointer to RTP packet's length
    * @param verify    verify that it is an RTP packet if != 0
    * @param log       log file
    *
    * @return 0 if there was no error, -1 otherwise;
    *         set pbuf to point to beginning of payload and len
    *         be payload size in bytes
    */
    int rtp_padding = -1;
    int front_skip = 0, back_skip = 0, pad_len = 0;

    int pkt_len = 0;

    //assert(pbuf && len && log);
    //buf = *pbuf;
    pbuf = buf;
    //pkt_len = *len;
    len = pkt_len;

    /*
    if (verify != 1 && RTP_verify() != 1)
        RTPOK = - 1;

    if (0 != RTP_hdrlen(buf, pkt_len, front_skip)) //?????
        RTPOK = - 1;

    */

    rtp_padding = buf[0] & 0x20;
    if (rtp_padding != -1) //???????
    {
        pad_len = buf[pkt_len - 1];
    }

    back_skip += pad_len;

    if ((verify != -1) && (pkt_len < (front_skip + back_skip))) //???????
    {
        Console.WriteLine("RTP_process: invalid header (skip {0} exceeds packet length {1})\n", (long)(front_skip + back_skip), (long)pkt_len);
        RTPOK = - 1;
    }

    /* adjust buffer/length to skip heading and padding */
    /*
    TRACE( (void)tmfprintf( log, "In: RTP buf=[%p] of [%lu] bytes, "
                "fskip=[%ld], bskip=[%lu]\n",
                (void*)buf, (u_long)pkt_len,
                (u_long)front_skip, (u_long)back_skip ) );
    */

    //buf += front_skip;
    //pkt_len -= (front_skip + back_skip);

    /*
    TRACE( (void)tmfprintf( log, "Out RTP buf=[%p] of [%lu] bytes\n",
                (void*)buf, (u_long)pkt_len ) );
    */
    pbuf = buf;
    len = pkt_len;

    RTPOK = 0;
    return pbuf;
}

buf += front_skip;抱怨说，运算符+=不能应用于byte[]和int类型的操作数，那么为什么它在C中的RTP_Process中工作，以及什么是与之对应的C#。

if (rtp_padding != -1) //???????
{
    pad_len = buf[pkt_len - 1]; //There is an exeption trown: System.IndexOutOfRangeException: Index was outside the bounds of the array.

很明显，我解释和翻译了错误的东西，但我只想做的是让MPEG-TS帧离开RTP流，然后将它转发到TCP套接字，这样如果有人能提出更好的方法，我很想听听。

谢谢你的祝福和问候}

Answer 1

首先，我建议仔细阅读RFC-3550，它包含关于RTP -包结构的所有信息(大多数您需要第5节：RTP固定头和扩展)。

1. 然后您必须实现RTP_hdrlen来计算RTP头大小，它必须返回front_skip值作为RTP头大小(包括扩展)。因此，您不必使用buf += front_skip;，RTP有效负载从字节buf[front_skip]开始。
2. 您在这里有错误的数据包长度参数：int pkt_len = 0;，这就是为什么异常在这里抛出的原因，pad_len = buf[pkt_len - 1];。