Squid supports the following incoming protocol request types (when the proxy requests are sent in HTTP format)
HyperText Transfer Protocol (HTTP), which is the specification that the WWW is based on.
File Transfer Protocol (FTP)
Wide Area Information (?Server?) (WAIS) (With the appropriate relay server.)
Secure Socket Layer - which is used for secure online transactions.
(? oh bugger ?)
HTTP, which is used for retrieving copies of objects from other caches.
Internet Cache Protocol (ICP). ICP is used to find out if a specific object is in another cache's store.
Cache Digests. This protocol is used to retrieve an index of objects in another cache's store. When a cache receives a request for an object it does not have, it checks this index to determine which cache does have the object.
Simple Network Management Protocol (SNMP). Common SNMP tools can be used to retrieve information about your cache.
Hyper Text Caching Protocol (HTCP). Though HTCP is not widely implemented, Squid is in the process of incorporating the protocol. (?check...1.2.3?)