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📁 通讯类的标准。对要开发SS7的朋友有很大帮助的。（通讯协议）
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<DIV class=im0><IMG height=64 src="002_5.files/right.jpg" width=563 
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<DIV class=im1><IMG height=166 src="002_5.files/002_5_2.jpg" width=384 
border=0></DIV><!-- text starts here --><SPAN class=ps0><NOBR><SPAN 
class=ft0>图<SPAN class=em2>2-2 &nbsp;&nbsp;</SPAN>基本的<SPAN 
class=em2>RTP</SPAN>包报头</SPAN></NOBR></SPAN> <SPAN class=ps1><NOBR>版本（<SPAN 
class=em0>V</SPAN>，<SPAN class=em0>2</SPAN>比特）：这个域标识<SPAN 
class=em0>RTP</SPAN>的版本。<SPAN class=em0>RFC1889</SPAN>定义的版本是<SPAN 
class=em0>2<SPAN class=em1>，</SPAN></SPAN>但是<SPAN 
class=em0>IETF</SPAN>当前工</NOBR></SPAN> <SPAN 
class=ps2><NOBR>作完成后可能会改变。</NOBR></SPAN> <SPAN class=ps3><NOBR>填充（<SPAN 
class=em0>P</SPAN>，<SPAN class=em0>1</SPAN>比特）：如果设置了填充比特位，包的实际数据内容比包本身要小。如果<SPAN 
class=em0>P=1<SPAN class=em1>，</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps4><NOBR>填充位后面的一个字节是应该忽略掉的字节数。某些固定块大小的加密算法可能需要使用填</NOBR></SPAN> <SPAN 
class=ps5><NOBR>充位，一个低层协议数据单元携带几个<SPAN 
class=em0>RTP</SPAN>包也需要使用填充位。</NOBR></SPAN> <SPAN class=ps6><NOBR>扩展（<SPAN 
class=em0>X</SPAN>，<SPAN 
class=em0>1</SPAN>比特）：如果设置了扩展比特位，那么固定报头之后恰好是一个变量长度的报</NOBR></SPAN> <SPAN 
class=ps7><NOBR>头扩展。扩展机制能够应用于传递控制信息，而不需要中间节点的解释。</NOBR></SPAN> <SPAN 
class=ps8><NOBR><SPAN class=ft2>RTP<SPAN 
class=em1>扩展报头是两</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps9><NOBR>个字节，包含一个右对齐的<SPAN class=em0>16</SPAN>比特长度的域，指示扩展的长度。它必须在标准<SPAN 
class=em0>RTP</SPAN>报头（以</NOBR></SPAN> <SPAN class=ps10><NOBR>后我们会看到，要么是<SPAN 
class=em0>SSRC</SPAN>项，要么是<SPAN 
class=em0>CSRC</SPAN>项）的最后一个有效域之后插入。</NOBR></SPAN> <SPAN class=ps11><NOBR><SPAN 
class=ft2>CSRC<SPAN class=em1>数值（<SPAN class=em0>CC</SPAN>，<SPAN 
class=em0>4</SPAN>比特）：<SPAN class=em0>CSRC</SPAN>值是固定报头后</SPAN>CSRC<SPAN 
class=em1>标识符的数目。这个数值用来支</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps12><NOBR>持被称为“混合”的能力。混合器是一个设备，它从多个信源收集多媒体包，把它们合并成</NOBR></SPAN> <SPAN 
class=ps13><NOBR>一个包，然后转发到目的地。但是包还没有被合并的信源需要按顺序明确标识出来，以保证</NOBR></SPAN> <SPAN 
class=ps14><NOBR>得到的包能够被更远的下游使用。第一步是通过在<SPAN 
class=em0>CC</SPAN>域中标识这个包所包含的信源来实现。</NOBR></SPAN> <SPAN 
class=ps15><NOBR>只有信源能够被明确标识出来。可能会存在与这个概念有关的升级问题，但是使用多路复用</NOBR></SPAN> <SPAN 
class=ps16><NOBR>时可能会有其他答案。后面我们将看到，实际的信源是在<SPAN 
class=em0>CSRC</SPAN>列表中标识出来。如果流中只</NOBR></SPAN> <SPAN class=ps17><NOBR>有一个<SPAN 
class=em0>SSRC<SPAN class=em1>，</SPAN>CSRC</SPAN>值被设置为<SPAN 
class=em0>0</SPAN>。</NOBR></SPAN> <SPAN class=ps18><NOBR>标记（<SPAN 
class=em0>M</SPAN>，<SPAN 
class=em0>1</SPAN>比特）：标记的解释是由一个配置来定义的。它可以用来在包流中标记重要</NOBR></SPAN> <SPAN 
class=ps19><NOBR>事件，如帧边界。配置可以定义另外的标记位，或者通过改变有效载荷类型域中的比特数来</NOBR></SPAN> <SPAN 
class=ps20><NOBR>规定不使用标记位。<SPAN class=em0>M</SPAN>比特位意味着谈话的突然开始或者图像帧的结束。在<SPAN 
class=em0>VoIP</SPAN>中，这样就</NOBR></SPAN> <SPAN 
class=ps21><NOBR>很容易知道什么时候谈话突然结束，然后开始发出<SPAN class=em0>CN<SPAN 
class=em1>（</SPAN>Comfort Noise<SPAN 
class=em1>，</SPAN></SPAN>安慰噪音）。</NOBR></SPAN> <SPAN 
class=ps22><NOBR>有效载荷类型（<SPAN class=em0>PT</SPAN>，<SPAN 
class=em0>7</SPAN>比特）：这个域标识了<SPAN 
class=em0>RTP</SPAN>有效载荷类型的格式，并且决定应用对</NOBR></SPAN> <SPAN 
class=ps23><NOBR>它的解释。配置定义了载荷类型编码到载荷格式的静态映射。附加的载荷类型编码可以通过</NOBR></SPAN> <SPAN 
class=ps24><NOBR>非<SPAN 
class=em0>RTP</SPAN>方法动态定义，即使用信令协议。一旦会话建立起来，发送者不能在会话期间使用不</NOBR></SPAN> <SPAN 
class=ps25><NOBR>同的<SPAN 
class=em0>RTP</SPAN>有效载荷类型，所以不可能在同一个包内多路复用不同的介质流。</NOBR></SPAN> <SPAN 
class=ps26><NOBR>顺序编号（<SPAN class=em0>Sequence number</SPAN>，<SPAN 
class=em0>16</SPAN>比特）：每发送一个<SPAN class=em0>RTP</SPAN>数据包，顺序编号就会增加<SPAN 
class=em0>1<SPAN class=em1>，</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps27><NOBR>它可能被接收者用来检测包丢失和恢复包顺序。包重新排序很难实现，而且耗费音频和视频</NOBR></SPAN> <SPAN 
class=ps28><NOBR>质量，这是因为实际上无法预测一个包是真的丢失了还是仅仅迟到而已。当与固定端到端路</NOBR></SPAN> <SPAN 
class=ps29><NOBR>径的<SPAN class=em0>QoS</SPAN>方案结合时，将产生一个虚拟的面向连接的<SPAN 
class=em0>IP</SPAN>会话，也就是前面粗略谈到的矛盾，包</NOBR></SPAN> <SPAN 
class=ps30><NOBR>会按顺序从信源流到目的地。不可预测的顺序编号的简单属性，也就是每个包确定地增加</NOBR></SPAN> <SPAN 
class=ps31><NOBR><SPAN class=ft2>1<SPAN class=em1>，</SPAN></SPAN></NOBR></SPAN> 
<SPAN class=ps32><NOBR>使得在使用报头压缩时不得不排除掉它。</NOBR></SPAN> <SPAN 
class=ps33><NOBR><SPAN class=ft3>第<SPAN 
class=em3>2</SPAN>章&nbsp;包网络的介质传输</SPAN></NOBR></SPAN> <SPAN 
class=ps34><NOBR><SPAN class=ft4>61</SPAN></NOBR></SPAN> <SPAN 
class=ps35><NOBR><SPAN class=ft5>文档</SPAN></NOBR></SPAN> <SPAN 
class=ps36><NOBR><SPAN class=ft6>顺序编号</SPAN></NOBR></SPAN> <SPAN 
class=ps37><NOBR><SPAN class=ft6>时间戳</SPAN></NOBR></SPAN> <SPAN 
class=ps38><NOBR><SPAN class=ft6>同步信源标识符<SPAN 
class=em4>(SSRC)</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps39><NOBR><SPAN 
class=ft6>贡献信源标识符<SPAN class=em4>(CSRC)</SPAN></SPAN></NOBR></SPAN> 
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