rfc2435.txt

很多RFC的中文文档

到，接收端才可以正确解码出那个复位间隔中的数据。
一旦解码器接收到一个F和L都为1的包，或者是一连串的包，第一个F为1，最后一
个L为1，它就可以开始解码了。起始MCU在整幅图象中的位置可以通过将复位计数的值与
复位间隔的值相乘来确定。这样的一个包（或一连串包）可以包含任意数量的连续的复位间
隔。
为了兼容生成码流中就包含复位标记但无法按这些复位标记来分片的编码器，将复位计
数域设为0x3FFF并且F和L均为1。这样一种模式意味着解码器必须对整幅图象首先进行重
组，然后才能解码。

5.安全性问题
本文档中所定义的RTP包格式的安全性问题可以遵循[6]和[7]中的建议。这意味着媒体
数据流的安全性是通过加密来实现的。因为对于媒体数据的压缩是端到端的，加密操作可以
在压缩操作之后执行，这样在两种操作之间就不存在任何冲突。
对于解码端计算量不均衡的压缩编码计数，存在一种潜在的拒绝服务的攻击威胁。攻击
者可以在数据流中插入一些恶意的数据包，对于这些包的解码会导致解码器运算量过载。幸
运的是，我们的压缩编码算法并没有明显的计算量不均衡现象。
另一种潜在的拒绝服务威胁存在于我们提出的分片重组机制。接收端应当限制重组数据
的总长，以避免资源耗尽。
对于任何基于IP的协议，接收端在某些情况下可能会因为接收到过多的数据包而发生过
载。网络层的鉴定机制可以将来自不明来源或恶意来源的数据包丢弃，但是这样做所带来的
成本也是相当高的。在组播的环境里，删除某些源的数据包可以通过IGMP[8]的未来版本和
组播路由协议来实现，从而允许用户来选择哪些数据源是允许的，哪些是不允许的。
对于这种荷载格式的安全性考虑并不超出RTP规范中的内容。

原文作者地址
   Lance M. Berc
   Systems Research Center
   Digital Equipment Corporation
   130 Lytton Ave
   Palo Alto CA 94301

   Phone: [phone]
   Email: [e-mail]

   William C. Fenner
   Xerox PARC
   3333 Coyote Hill Road
   Palo Alto, CA 94304

   Phone: [phone]
   Email: [e-mail]

   Ron Frederick
   Xerox PARC
   3333 Coyote Hill Road
   Palo Alto, CA 94304

   Phone: [phone]
   Email: [e-mail]

   Steven McCanne
   University of California at Berkeley
   Electrical Engineering and Computer Science
   633 Soda Hall
   Berkeley, CA 94720

   Phone: [phone]
   Email: [e-mail]

   Paul Stewart
   Xerox PARC
   3333 Coyote Hill Road
   Palo Alto, CA 94304

   Phone: [phone]
   Email: [e-mail]

参考文献

   [1]  ISO DIS 10918-1. Digital Compression and Coding of Continuous-
        tone Still Images (JPEG), CCITT Recommendation T.81.

   [2]  William B. Pennebaker, Joan L. Mitchell, JPEG: Still Image Data
        Compression Standard, Van Nostrand Reinhold, 1993.

   [3]  Gregory K. Wallace, The JPEG Sill Picture Compression Standard,
        Communications of the ACM, April 1991, Vol 34, No. 1, pp. 31-44.

   [4]  The JPEG File Interchange Format.  Maintained by C-Cube
        Microsystems, Inc., and available in
        ftp://ftp.uu.net/graphics/jpeg/jfif.ps.gz.

   [5]  Tom Lane et. Al., The Independent JPEG Group software JPEG
        codec.  Source code available in
        ftp://ftp.uu.net/graphics/jpeg/jpegsrc.v6a.tar.gz.

   [6]  Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
        "RTP: A Transport Protocol for Real-Time Applications", RFC
        1889, January 1996.

   [7]  Schulzrinne, H., "RTP Profile for Audio and Video Conferences
        with Minimal Control", RFC 1890, January 1996.

   [8]  Fenner, W., "Internet Group Management Protocol Version 2", RFC
        2236, November 1997.

   [9]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [10] Kent C., and J. Mogul, "Fragmentation Considered Harmful",
        Proceedings of the ACM SIGCOMM '87 Workshop on Frontiers in
        Computer Communications Technology, August 1987.

附录 A
下面的代码用来通过一个Q因子值生成一个量化表：

/*
?	Table K.1 from JPEG spec.
 */
static const int jpeg_luma_quantizer[64] = {
        16, 11, 10, 16, 24, 40, 51, 61,
        12, 12, 14, 19, 26, 58, 60, 55,
        14, 13, 16, 24, 40, 57, 69, 56,
        14, 17, 22, 29, 51, 87, 80, 62,
        18, 22, 37, 56, 68, 109, 103, 77,
        24, 35, 55, 64, 81, 104, 113, 92,
        49, 64, 78, 87, 103, 121, 120, 101,
        72, 92, 95, 98, 112, 100, 103, 99
};

/*
?	Table K.2 from JPEG spec.
 */
static const int jpeg_chroma_quantizer[64] = {
        17, 18, 24, 47, 99, 99, 99, 99,
        18, 21, 26, 66, 99, 99, 99, 99,
        24, 26, 56, 99, 99, 99, 99, 99,
        47, 66, 99, 99, 99, 99, 99, 99,
        99, 99, 99, 99, 99, 99, 99, 99,
        99, 99, 99, 99, 99, 99, 99, 99,
        99, 99, 99, 99, 99, 99, 99, 99,
        99, 99, 99, 99, 99, 99, 99, 99
};

/*
?	Call MakeTables with the Q factor and two u_char[64] return arrays
 */
void
MakeTables(int q, u_char *lqt, u_char *cqt)
{
  int I;
  int factor = q;

  if (q < 1) factor = 1;
  if (q > 99) factor = 99;
  if (q < 50)
q = 5000 / factor;
  else
q = 200 – factor*2;

  for (I=0; I < 64; I++) {
int lq = (jpeg_luma_quantizer[I] * q + 50) / 100;
int cq = (jpeg_chroma_quantizer[I] * q + 50) / 100;

/* Limit the quantizers to 1 <= q <= 255 */
if (lq < 1) lq = 1;
else if (lq > 255) lq = 255;
lqt[I] = lq;

if (cq < 1) cq = 1;
else if (cq > 255) cq = 255;
cqt[I] = cq;
  }
}

附录 B
下面这段代码用来生成对应于那些RTP/JPEG中不存在的表描述数据的JPEG标记段。

U_char lum_dc_codelens[] = {
        0, 1, 5, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0,
};

u_char lum_dc_symbols[] = {
        0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,
};

u_char lum_ac_codelens[] = {
        0, 2, 1, 3, 3, 2, 4, 3, 5, 5, 4, 4, 0, 0, 1, 0x7d,
};

u_char lum_ac_symbols[] = {
        0x01, 0x02, 0x03, 0x00, 0x04, 0x11, 0x05, 0x12,
        0x21, 0x31, 0x41, 0x06, 0x13, 0x51, 0x61, 0x07,
        0x22, 0x71, 0x14, 0x32, 0x81, 0x91, 0xa1, 0x08,
        0x23, 0x42, 0xb1, 0xc1, 0x15, 0x52, 0xd1, 0xf0,
        0x24, 0x33, 0x62, 0x72, 0x82, 0x09, 0x0a, 0x16,
        0x17, 0x18, 0x19, 0x1a, 0x25, 0x26, 0x27, 0x28,
        0x29, 0x2a, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39,
        0x3a, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48, 0x49,
        0x4a, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58, 0x59,
        0x5a, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68, 0x69,
        0x6a, 0x73, 0x74, 0x75, 0x76, 0x77, 0x78, 0x79,
        0x7a, 0x83, 0x84, 0x85, 0x86, 0x87, 0x88, 0x89,
        0x8a, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97, 0x98,
        0x99, 0x9a, 0xa2, 0xa3, 0xa4, 0xa5, 0xa6, 0xa7,
        0xa8, 0xa9, 0xaa, 0xb2, 0xb3, 0xb4, 0xb5, 0xb6,
        0xb7, 0xb8, 0xb9, 0xba, 0xc2, 0xc3, 0xc4, 0xc5,
        0xc6, 0xc7, 0xc8, 0xc9, 0xca, 0xd2, 0xd3, 0xd4,
        0xd5, 0xd6, 0xd7, 0xd8, 0xd9, 0xda, 0xe1, 0xe2,
        0xe3, 0xe4, 0xe5, 0xe6, 0xe7, 0xe8, 0xe9, 0xea,
        0xf1, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7, 0xf8,
        0xf9, 0xfa,
};

u_char chm_dc_codelens[] = {
        0, 3, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0,
};

u_char chm_dc_symbols[] = {
        0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,

};

u_char chm_ac_codelens[] = {
        0, 2, 1, 2, 4, 4, 3, 4, 7, 5, 4, 4, 0, 1, 2, 0x77,
};

u_char chm_ac_symbols[] = {
        0x00, 0x01, 0x02, 0x03, 0x11, 0x04, 0x05, 0x21,
        0x31, 0x06, 0x12, 0x41, 0x51, 0x07, 0x61, 0x71,
        0x13, 0x22, 0x32, 0x81, 0x08, 0x14, 0x42, 0x91,
        0xa1, 0xb1, 0xc1, 0x09, 0x23, 0x33, 0x52, 0xf0,
        0x15, 0x62, 0x72, 0xd1, 0x0a, 0x16, 0x24, 0x34,
        0xe1, 0x25, 0xf1, 0x17, 0x18, 0x19, 0x1a, 0x26,
        0x27, 0x28, 0x29, 0x2a, 0x35, 0x36, 0x37, 0x38,
        0x39, 0x3a, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48,
        0x49, 0x4a, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58,
        0x59, 0x5a, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68,
        0x69, 0x6a, 0x73, 0x74, 0x75, 0x76, 0x77, 0x78,
        0x79, 0x7a, 0x82, 0x83, 0x84, 0x85, 0x86, 0x87,
        0x88, 0x89, 0x8a, 0x92, 0x93, 0x94, 0x95, 0x96,
        0x97, 0x98, 0x99, 0x9a, 0xa2, 0xa3, 0xa4, 0xa5,
        0xa6, 0xa7, 0xa8, 0xa9, 0xaa, 0xb2, 0xb3, 0xb4,
        0xb5, 0xb6, 0xb7, 0xb8, 0xb9, 0xba, 0xc2, 0xc3,
        0xc4, 0xc5, 0xc6, 0xc7, 0xc8, 0xc9, 0xca, 0xd2,
        0xd3, 0xd4, 0xd5, 0xd6, 0xd7, 0xd8, 0xd9, 0xda,
        0xe2, 0xe3, 0xe4, 0xe5, 0xe6, 0xe7, 0xe8, 0xe9,
        0xea, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7, 0xf8,
        0xf9, 0xfa,
};

u_char *
MakeQuantHeader(u_char *p, u_char *qt, int tableNo)
{
        *p++ = 0xff;
        *p++ = 0xdb;            /* DQT */
        *p++ = 0;               /* length msb */
        *p++ = 67;              /* length lsb */
        *p++ = tableNo;
        memcpy(p, qt, 64);
        return (p + 64);
}

u_char *
MakeHuffmanHeader(u_char *p, u_char *codelens, int ncodes,
                  u_char *symbols, int nsymbols, int tableNo,
                  int tableClass)
{
        *p++ = 0xff;

        *p++ = 0xc4;            /* DHT */
        *p++ = 0;               /* length msb */
        *p++ = 3 + ncodes + nsymbols; /* length lsb */
        *p++ = (tableClass << 4) | tableNo;
        memcpy(p, codelens, ncodes);
        p += ncodes;
        memcpy(p, symbols, nsymbols);
        p += nsymbols;
        return (p);
}

u_char *
MakeDRIHeader(u_char *p, u_short dri) {
        *p++ = 0xff;
        *p++ = 0xdd;            /* DRI */
        *p++ = 0x0;             /* length msb */
        *p++ = 4;               /* length lsb */
        *p++ = dri >> 8;        /* dri msb */
        *p++ = dri & 0xff;      /* dri lsb */
        return (p);
}

/*
?	Arguments:
?	type, width, height: as supplied in RTP/JPEG header
?	lqt, cqt: quantization tables as either derived from
?	the Q field using MakeTables() or as specified
?	in section 4.2.
?	dri: restart interval in MCUs, or 0 if no restarts.
 *
?	p: pointer to return area
 *
?	Return value:
?	The length of the generated headers.
 *
?	Generate a frame and scan headers that can be prepended to the