u3SOCKS is a generic proxy protocol for ICP/IP-based networking , applications. SOCKS includes two （ ）, the SOCKS server and the SOCKS client. The SOCKS server is implemented at the application layer. The SOCKS client is implemented between applications and the（ ） layer. When an application client needs to connect to an application server, the client connects to a SOCKS proxy server. The proxy server connects to the application server instead of. the client, and （ ）data between the client and the application server. For the application server, the proxy server is the （ ） . SOCKS is also one of the popular （ ）to network firewalls. Because of its simplicity and flexibility, SOCKS has been used as generic application proxy in virtual private network (VPN), and for extranet applications.
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