todo

第二层隧道模块l2tp源码,开发环境为linux

Critical:
* Improved performance
* Rate Adaptive Timeouts
* Fork processing code into a kernel module(?)
* Fix protocol correctness issues
	* Are we sending valid and no invalid AVPs in various message types

Other:
* Support tie breakers
* Support proxy authentication
* Support LCP initial/final states
* Maybe do something with private groups
* Tunnel and call lookups should be hashes, not lists
* Meaningful error message about pppd problems...for 
  either not installed, or no kernel support (if possible)

Niceties:
* Command-line tunnel control program, instead of using echo commands

Way-down-the-line:
* GUI configuration