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第二层隧道模块l2tp源码,开发环境为linux

l2tpd version 0.69
==================
Now maintained at:
http://www.l2tpd.org
by:
Jeff McAdams <[e-mail]>

Maintained for a time at:
http://sourceforge.net/projects/l2tpd
by:
Scott Balmos <[e-mail]>
Jeff McAdams <[e-mail]>
David Stipp <[e-mail]>

Based off of l2tpd version 0.60
Copyright (C)1998 Adtran, Inc.
Mark Spencer <[e-mail]>

Introduction:
-------------

l2tpd is an implementation of the layer two tunneling protocol. It works
in userspace completely (although kernel work is planned after the
userspace version is stablized).  l2tpd works by opening a pseudo-tty
for communicating with pppd.  Although l2tpd was written for Linux, the
current version should be highly portable to other UNIX's supported by
pppd.

Legal Information: 
------------------
l2tpd is free software, distributed under the GNU General Public License
(GPL) and you should read the LICENSE File if you are not already
familiar with the GPL before using the product.

Bugs, Patches, and Code Contribution:
-------------------------------------
Please send bug reports and patches to either the l2tp mailng list
([e-mail]).

In order to contribute code, either with patches, or by direct access to
the CVS tree for l2tpd, send e-mail to ([e-mail]).

The word "FIXME" is a place holder for code that needs to be in place,
or checks that need to be done, but haven't been coded yet.  Feel free
to fix the fixme's and submit patches!

Release Notes
-------------
Version 0.69 should be considered ALPHA.  It does NOT completely
implement the l2tp specification (RFC 2661), but its pretty close!

The primary use of this ALPHA level code is to test the ability of l2tpd
to talk with other LAC and LNS implementations.  I hope that everyone
who tests the software will send me results on how it worked or failed
to work for them.  

(theoretically) implemented features
------------------------------------
* Proper payload and control packet handling
* Reliable control packet delivery
* Ability to recover from payload errors
* Ability to handle packets with/without length set
* Ability to handle flow control or no flow control
* Most critical AVP's for normal operation
* Challenge authentication
* Hidden AVP's
* Hello's to detect outages
* Handles sync and async packets
* Can act as LNS
* Can be a source of a virtual LAC call
* Reads configuration file
* Automated LAC dialup via config file, including redial
* Can be configured while running via a file system pipe
* Access Control
* Statistics report when sent a SIGUSR1
* ACCM

Major unimplemented specification features
------------------------------------------
* Rate Adaptive Timeouts
* Out of order packet handling
* Initial/Final LCP states
* Q.931 Result Codes
* Tie Breakers
* Minimum/Maximum BPS
* Call Errors

Important non-specification related features to be added
--------------------------------------------------------
* More configuration options if needed
* Kernel support for *much* improved performance

Usage notes on /var/run/l2tp-control
------------------------------------
There are a few command line options to l2tpd:
    -D  prevents l2tpd from detaching from terminal and running as a
        daemon
    -c <configfile> use alternate config file
    -s <secretfile> use alternate secrets file
    -p <pidfile> use alternate pid file

Upon running l2tpd, a pipe is created in /var/run/l2tp-control.  Simple
commands can then be echoed to this pipe to control l2tp on the fly.
The commands are:  

t <host> - create a tunnel to <host>
c <tid> or <entry> - originate an l2tp call on the tunnel identified
                     locally by <tid>, or dial the entry <entry>
h <cid> - hang up a call with the local identifier <cid>
d <tid> or <entry> - disconnect the tunnel locally identified by <tid>
                     or a lac entry <entry>
s - print out a status summary of currently connected tunnels and
                     sessions

For example, to establish a tunnel to marko.net, one might do:

echo "t marko.net" >/var/run/l2tp-control

l2tpd must be running for this to work.

Various other notes
-------------------
The PPP options used to be hard coded in l2tp.h.  Now they (or at least
some) are configurable - see the example config file and read file.[ch].

Sending a SIGUSR1 to l2tpd will cause it to dump its status.

Mailing List
------------
If you would like to contribute to discussions of the architecture of
l2tpd, file formats, etc, we encourage you to join the l2tpd mailing
list by visiting http://www.l2tpd.org/ and subscribing
to our mailing lists.

Original Project Legal Information: 
-----------------------------------
l2tpd is free software, distributed under the GNU General Public License
(GPL)and you should read the LICENSE File if you are not already
familiar with the GPL before using the product.

If you distribute l2tpd, a modified version, or a derivative product,
you MUST not remove Adtran's name from the product nor modify the terms
of the copyright.