Squid is a fully-featured HTTP/1.0 proxy which is almost (but not
quite - we're getting there!) HTTP/1.1 compliant. Squid offers a rich
access control, authorization and logging environment to develop web
proxy and content serving applications.

Major new features from Squid 2.6:

* Code converted to C++, with significant internal restructuring and rewrites.
* ICAP implementation (RFC 3507 and www.icap-forum.org)
* Edge Side Includes (ESI) implementation (www.esi.org)

This Squid 3.0 is stable release but some functinality lacks from 2.6.

* refresh_stale_hit option. Not yet ported.
* ability to follow X-Forwarded-For. Not yet ported.
* Full caching of Vary/ETag using If-None-Match. Only basic Vary cache
  supported. Not yet ported.
* Mapping of server error messages. Not yet ported.
* http_access2 access directive. Not yet ported.
* Location header rewrites. Not yet ported.
* wais_relay. Feature dropped as it's equivalent to cache_peer +
  cache_peer_access.
* urlgroup. Not yet ported.
* collapsed forwarding. Not yet ported.
* stable Windows support. Irregularly maintained.

And COSS backend dosen't supported now.