The Secure Shell (SSH) Connection Protocol
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T. Ylonen, C. Lonvick · 2006-01
Abstract
The Secure Shell (SSH) Connection Protocol provides interactive login sessions, remote execution of commands, forwarded TCP/IP connections, and forwarded X11 connections. All of these channels are multiplexed into a single encrypted tunnel provided by the SSH Transport Layer Protocol (RFC 4253).
Why This RFC Matters
RFC 4254 defines how SSH multiplexes multiple logical channels — interactive shells, exec requests, TCP port forwarding, and X11 forwarding — over a single encrypted transport connection. This design enables use cases far beyond simple login: developers use SSH tunnels to secure database connections, git over SSH uses the exec channel for git-upload-pack, and SFTP (RFC 4253) rides the subsystem channel. The channel flow-control mechanism prevents a busy channel from starving others, making SSH suitable for both interactive and batch workloads simultaneously.