Squid is a fully-featured HTTP/1.0 proxy with partial HTTP/1.1 support
The 4 series brings many new features and upgrades to the basic
networking protocols. A short list of the major new features is:

Squid 4 represents a new feature release above 3.5.

The most important of these new features are:

* Configurable helper queue size
* Helper concurrency channels changes
* SSL support removal
* Helper Binary Changes
* Secure ICAP
* Improved SMP support
* Improved process management
* Initial GnuTLS support
* ESI Custom Parser removal