L2TP (RFC 2661)

1 简介

PPP [RFC1661] defines an encapsulation mechanism for transporting

 

   multiprotocol packets across layer 2 (L2) point-to-point links.

 

   Typically, a user obtains a L2 connection to a Network Access Server

 

   (NAS) using one of a number of techniques (e.g., dialup POTS, ISDN,

 

   ADSL, etc.)  and then runs PPP over that connection. In such a

 

   configuration, the L2 termination point and PPP session endpoint

 

   reside on the same physical device (i.e., the NAS).

 

 

  L2TP extends the PPP model by allowing the L2 and PPP endpoints to

 

   reside on different devices interconnected by a packet-switched

 

   network.  With L2TP, a user has an L2 connection to an access

 

   concentrator (e.g., modem bank, ADSL DSLAM, etc.), and the

 

   concentrator then tunnels individual PPP frames to the NAS. This

 

   allows the actual processing of PPP packets to be divorced from the

 

   termination of the L2 circuit.

 

L2TP 优点：不需要L2直接连接到远程NAS，只要L2在本地的电路集中器终止，然后在公共设施(如:帧中继或Internet)扩展逻辑上的PPP会话，在用户看来，L2TP和直接通过电路连接在NAS上功能没什么区别；    可以解决multilink hunt-group splitting problem

 

“multilink hunt-group splitting problem”具体是什么问题？

 

LNS 发送Outgoing call 后LAC具体怎么处理？

 

1.2 术语

Attribute Value Pair (AVP)：属性值对。由唯一的属性(Attribute)(表示为整数)和包含该属性标识的实际值的Value组成的变长的组合（The variable length concatenation of a unique Attribute(represented by an integer) and a Value containing the actual value identified by the attribute.）。多个AVP组合就构成了用来建立，维持，终止隧道的控制报文。

 

Control Connection： 控制连接。控制连接通过隧道运行，控制建立，释放和维持会话以及隧道本身。

 

Control Messages ：控制报文。由LAC，LNS 进行交换，通过隧道进行传输，管理该隧道的会话和隧道本身。

 

DSLAM ：不知道什么东东？

 

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avp 什么属性类型，原来都是l2tp 的属性，以及怎么进行属性的设置，属性的长度的计算，