002_33.htm
来自「通讯类的标准。对要开发SS7的朋友有很大帮助的。（通讯协议）」· HTM 代码 · 共 254 行
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<DIV class=im0><IMG height=64 src="002_33.files/right.jpg" width=563
border=0></DIV><!-- text starts here --><SPAN class=ps0><NOBR>考虑过。</NOBR></SPAN>
<SPAN class=ps1><NOBR><SPAN class=ft1>2.2 RSVP<SPAN
class=em0>—</SPAN><SPAN class=em0>资源保留协议</SPAN></SPAN></NOBR></SPAN> <SPAN
class=ps2><NOBR><SPAN class=ft2>背景</SPAN></NOBR></SPAN> <SPAN
class=ps3><NOBR>资源保留协议（<SPAN class=em1>ReSource reserVation
Protocol</SPAN>，<SPAN class=em1>RSVP</SPAN>）在<SPAN class=em1>RFC
2205</SPAN>中定义，另外还有几</NOBR></SPAN> <SPAN class=ps4><NOBR>个<SPAN
class=em1>RFC</SPAN>详细说明了它和链路层协议，如<SPAN class=em1>ATM</SPAN>的用法。<SPAN
class=em1>RSVP</SPAN>是为了在两个端点间建立通道的<SPAN class=em1>IP</SPAN></NOBR></SPAN> <SPAN
class=ps5><NOBR>信令，其目标是在介质流上传输高<SPAN class=em1>QoS</SPAN>。<SPAN
class=em1>RSVP</SPAN>并不保证<SPAN class=em1>QoS<SPAN
class=em2>，</SPAN></SPAN>但它尽可能按照分类原则处理</NOBR></SPAN> <SPAN
class=ps6><NOBR>来自特定类别的包，以使端到端传输成为可预测的过程。该协议也适合于包含组播应用，这</NOBR></SPAN> <SPAN
class=ps7><NOBR>样，它必然具有内在的复杂性。它很有希望成为在新的公用网中传输基于<SPAN
class=em1>IP</SPAN>的电话服务的<SPAN class=em1>QoS</SPAN></NOBR></SPAN> <SPAN
class=ps8><NOBR>信令协议。</NOBR></SPAN> <SPAN class=ps9><NOBR><SPAN
class=ft3>RSVP<SPAN class=em2>固定一条路径给两个端点间的包流使用。这时，立刻有一个明显的问题：当</SPAN>IP<SPAN
class=em2>层路</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps10><NOBR>由严格依赖于目标<SPAN
class=em1>IP</SPAN>地址时，如何迫使网络中两端点间路径选择是基于<SPAN
class=em1>QoS</SPAN>的呢？第二个问题</NOBR></SPAN> <SPAN
class=ps11><NOBR>是如何处理下面这种情况：预先确定的通道不能提供要求的</NOBR></SPAN> <SPAN
class=ps12><NOBR><SPAN class=ft3>QoS<SPAN class=em2>，</SPAN><SPAN
class=em2>在呼叫初始化失败之前，</SPAN></SPAN></NOBR></SPAN> <SPAN
class=ps13><NOBR>需要检查其他可选择的路由。</NOBR></SPAN> <SPAN
class=ps14><NOBR>在寻求一个协议以固定无连接服务中两个端点之间的路径时， <SPAN
class=em1>IP</SPAN>界人士的心中有一个宏</NOBR></SPAN> <SPAN
class=ps15><NOBR>伟的目标。这听起来像是一个矛盾，但使得如果在呼叫持续期间内没有特定的传输路径，不</NOBR></SPAN> <SPAN
class=ps16><NOBR>管什么服务，实现任何有意义的<SPAN class=em1>QoS</SPAN>都是没有把握的。为无连接<SPAN
class=em1>IP</SPAN>协议固定一个端到端路径，</NOBR></SPAN> <SPAN
class=ps17><NOBR>这一点说起来容易做起来难。</NOBR></SPAN> <SPAN class=ps18><NOBR><SPAN
class=ft3>RFC<SPAN class=em2>声明中的协议摘要中写到：</SPAN></SPAN></NOBR></SPAN> <SPAN
class=ps19><NOBR>“<SPAN
class=em1>RSVP</SPAN>为组播或单播数据流提供接收者发起的资源保留初始化，它具有良好的升级性和</NOBR></SPAN> <SPAN
class=ps20><NOBR>健壮性。<SPAN class=em1>RSVP</SPAN>协议由主机使用，为特定应用数据流向网络请求特定的<SPAN
class=em1>QoS</SPAN>。<SPAN class=em1>RSVP</SPAN>也可以由</NOBR></SPAN> <SPAN
class=ps21><NOBR>路由器使用，沿着流中的路径将<SPAN
class=em1>QoS</SPAN>请求传送给所有节点，并建立和保持状态以提供给请求</NOBR></SPAN> <SPAN
class=ps22><NOBR>的服务。<SPAN
class=em1>RSVP</SPAN>请求通常会沿着数据路径的每个节点产生资源保留。”</NOBR></SPAN> <SPAN
class=ps23><NOBR><SPAN class=ft3>RSVP<SPAN
class=em2>的一个关键问题是资源保留通常并不能够得到保证。和</SPAN>SS7<SPAN class=em2>信令（将在第</SPAN>3<SPAN
class=em2>章讨论）</SPAN></SPAN></NOBR></SPAN> <SPAN
class=ps24><NOBR>类似，它不能沿路径在交换机之间找到<SPAN
class=em1>TDM</SPAN>网段。但是重要的差别在于它可以很容易发现公</NOBR></SPAN> <SPAN
class=ps25><NOBR>用电话网中是否存在<SPAN
class=em1>TDM</SPAN>干线。干线以提前的方式保留，不需要在相反方向附加字符串。</NOBR></SPAN> <SPAN
class=ps26><NOBR><SPAN class=ft3>PSTN<SPAN class=em2>中的电路保留是一个二元选择。另一方面，<SPAN
class=em1>VoIP</SPAN>电话技术中的资源保留很大程度上依赖</SPAN></SPAN></NOBR></SPAN> <SPAN
class=ps27><NOBR>于运行<SPAN
class=em1>RSVP</SPAN>的路由产品实现的容量算法类型，以及相邻的设备是否能够提前预知<SPAN
class=em1>RSVP</SPAN>请求</NOBR></SPAN> <SPAN
class=ps28><NOBR>能否成功。乍看起来，这会在服务使用频繁期间产生过多的信令，并有很大的故障可能。</NOBR></SPAN> <SPAN
class=ps29><NOBR><SPAN class=ft3>RSVP<SPAN class=em3>并不是路由协议。</SPAN>RSVP<SPAN
class=em3>不是通信量管理。</SPAN>RSVP<SPAN
class=em3>希望和现在的、将来的单</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps30><NOBR><SPAN
class=ft4>播和组播路由协议共同运行。<SPAN class=em4>RSVP</SPAN>是一个传输层控制协议，类似于<SPAN
class=em4>IGMP</SPAN>、<SPAN class=em4>ICMP</SPAN>或</SPAN></NOBR></SPAN> <SPAN
class=ps31><NOBR><SPAN class=ft4>者其他路由协议，但它并不传输数据。</SPAN></NOBR></SPAN> <SPAN
class=ps32><NOBR><SPAN class=ft3>1. <SPAN
class=em3>协议基本原理</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps33><NOBR><SPAN
class=ft3>RSVP<SPAN class=em2>协议由如下高层属性表征：</SPAN></SPAN></NOBR></SPAN> <SPAN
class=ps34><NOBR><SPAN class=ft3>• <SPAN
class=em2>它是面向接收者的，也就是说，发送者提出的实际路径保留是由接收者完成的。主要原</SPAN></SPAN></NOBR></SPAN>
<SPAN class=ps35><NOBR>因在于对组播应用的支持。这是它和<SPAN
class=em1>SS7</SPAN>信令为建立简单电话呼叫而进行电路保留的主要差别。</NOBR></SPAN> <SPAN
class=ps36><NOBR>为支持组播而增加的功能在简单应用中马上就要付出代价。</NOBR></SPAN> <SPAN
class=ps37><NOBR><SPAN class=ft3>• <SPAN
class=em2>它支持为每个组播会话进行异构的保留。它允许多点会话中的接收者请求不同的</SPAN>QoS<SPAN
class=em2>。</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps38><NOBR><SPAN
class=ft4>第<SPAN class=em5>2</SPAN>章 包网络的介质传输</SPAN></NOBR></SPAN> <SPAN
class=ps39><NOBR><SPAN class=ft5>89</SPAN></NOBR></SPAN> <SPAN
class=ps40><NOBR><SPAN class=ft6>文档</SPAN></NOBR></SPAN> </BODY></HTML>
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