SSH is a protocol for secure remote login and other secure network services over an insecure network. SSH consists of three major components:

The Transport Layer Protocol [SSH-TRANS] provides server authentication, confidentiality, and integrity. It may optionally also provide compression. The transport layer will typically be run over a TCP/IP connection, but might also be used on top of any other reliable data stream. SSH-Trans provides strong encryption, cryptographic host authentication, and integrity protection. Authentication in this protocol level is host-based; this protocol does not perform user authentication. A higher level protocol for user authentication can be designed on top of this protocol.

The User Authentication Protocol [SSH-USERAUTH] authenticates the client-side user to the server. It runs over the transport layer protocol SSH-TRANS. When SSH-USERAUTH starts, it receives the session identifier from the lower-level protocol (this is the exchange hash H from the first key exchange). The session identifier uniquely identifies this session and is suitable for signing in order to prove ownership of a private key. SSH-USERAUTH also needs to know whether the lower-level protocol provides confidentiality protection.

The Connection Protocol [SSH-CONNECT] multiplexes the encrypted tunnel into several logical channels. It runs over the user authentication protocol. It provides interactive login sessions, remote execution of commands, forwarded TCP/IP connections, and forwarded X11 connections.

The client sends a service request once a secure transport layer connection has been established. A second service request is sent after user authentication is complete. This allows new protocols to be defined and coexist with the protocols listed above. The connection protocol provides channels that can be used for a wide range of purposes. Standard methods are provided for setting up secure interactive shell sessions and for forwarding ("tunneling") arbitrary TCP/IP ports and X11 connections.

Protocol Structure

Secure Shell (SSH) protocols have many messages and each message may have different formats. For details of the message formats, please refer to the Reference documents listed below.

Related protocols: TCP

Sponsor Source: SSH is drafted by IETF (http://www.ietf.org) now

Reference:
http://www.javvin.com/protocol/sshdraft15.pdf: SSH Protocol Architecture
http://www.javvin.com/protocol/sshtransport17.pdf: SSH Transport Layer Protocol
http://www.javvin.com/protocol/sshauth18.pdf: SSH User Authentication Protocol
http://www.javvin.com/protocol/sshconnect18.pdf: SSH Connection Protocol