SOCKS

SOCKS protocol can be used in situations where hosts on one side of the network needs to access hosts on the other side without direct IP reachability.

The main components are SOCKS client and SOCKS Proxy Server.

SOCKS Client works in the layer between Application layer and Transport Layer. SOCKS Proxy Server works in the application layer.

It works in this way:

• SOCKS client makes connection request and sends the authentication methods supported
• SOCKS Proxy Server selects the authentication method
• SOCKS client gets authenticated with server
  
• It sends SOCKS CONNECT request for setting up Proxy circuits
• Relays Application Data

SOCKS protocol is traditionally used for hosts to traverse a firewall from inside to access Servers outside. But it is also used in VPN like scenarious where a client from outside uses SOCKS to communicate to the servers inside the network.

Please go throught this good site which details SOCKS overview, control flow, references - Socks Permeo