OSI model, Transport Layer

The protocols of the transport layer provide host-to-host communication services for applications.

It provides services such as connection-oriented data stream support, reliability, flow control, and multiplexing.

The details of implementation and semantics of the Transport Layer of the TCP/IP model (RFC 1122), which is the foundation of the Internet, and the Open Systems Interconnection (OSI) model of general networking, are different.

In the OSI model the transport layer is most often referred to as Layer 4 or L4, while numbered layers are not used in TCP/IP.

The best-known transport protocol of TCP/IP is the Transmission Control Protocol (TCP), and lent its name to the title of the entire suite.

It is used for connection-oriented transmissions, whereas the connectionless User Datagram Protocol (UDP) is used for simpler messaging transmissions.

TCP is the more complex protocol, due to its stateful design incorporating reliable transmission and data stream services.

Other prominent protocols in this group are the Datagram Congestion Control Protocol (DCCP) and the Stream Control Transmission Protocol (SCTP).

Services

• Connection-oriented communication
• Same order delivery
• Reliability
• Flow control
• Congestion avoidance
• Port Multiplexing

Protocols

• ATP AppleTalk Transaction Protocol
• CUDP Cyclic UDP
• DCCP Datagram Congestion Control Protocol
• FCP Datagram Congestion Control Protocol
• IL Fibre Channel Protocol
• MPTCP IL Protocol
• RDP Multipath TCP
• RUDP Reliable User Datagram Protocol
• SCTP Stream Control Transmission Protocol
• SPX Sequenced Packet Exchange
• SST Structured Stream Transport
• TCP Transmission Control Protocol
• UDP User Datagram Protocol
• UDP-Lite User Datagram Protocol
• µTP Micro Transport Protocol