SCTP

SCTP, the Stream Control Transmission Protocol, is a transport-layer protocol in the Internet protocol suite designed to provide reliable, message-oriented data transmission with robust congestion control. It combines features from both TCP and UDP while adding capabilities that address their limitations for certain applications.

Key features include preserved message boundaries, reliable delivery with optional in-order or unordered delivery within individual streams, and multiple simultaneous streams within a single association to reduce head-of-line blocking. SCTP also supports multi-homing, allowing an endpoint to be reachable via multiple IP addresses and providing seamless failover between paths. It employs a four-way handshake to establish associations and uses a cookie mechanism to resist certain kinds of attacks. The protocol includes built-in congestion control, acknowledgments (SACKs), heartbeats, path MTU discovery, and dynamic management of associations and paths. It also provides a Partial Reliability extension to enable time- or event-limited unreliability for specific data.

Protocol operation centers on the SCTP association, a communication structure between two endpoints that can span multiple IP paths. Data are transmitted in chunks, including DATA, SACK, ABORT, SHUTDOWN, COOKIE-ECHO, and COOKIE-ACK. The optional extensions, such as PR-SCTP, allow applications to trade reliability for timeliness in specific scenarios.

Applications and deployment: SCTP is standardized in RFC 4960, with extensions used for specialized needs such as signaling transport in telecom networks (SIGTRAN) and certain data-channel scenarios in WebRTC. It is implemented in major operating systems and networking stacks, including Linux and various BSDs, with supported or available implementations on others.