What Are Transport Protocols?

Transport protocols are the set of communication rules that govern end-to-end data delivery between applications running on separate networked hosts. They sit at the fourth layer of the OSI model, above the network layer that routes packets across the Internet, and below the application layer that produces and consumes data. Their responsibilities include multiplexing data streams among concurrent applications (using port numbers), managing reliability, controlling flow to avoid overwhelming a receiver, and responding to congestion signals from the network.

The Internet's transport layer was defined during the 1970s and 1980s alongside the development of the TCP/IP protocol suite. The Internet Engineering Task Force describes transport protocols as foundational technologies that enable applications to send data across the Internet, and its Transport Area working groups continue to standardize both incremental improvements to existing protocols and entirely new designs suited to modern network conditions.

TCP and Reliable Delivery

The Transmission Control Protocol (TCP) is the dominant transport protocol for applications that require ordered, reliable delivery of a byte stream. TCP establishes a connection through a three-way handshake, numbers every byte it sends, acknowledges received data, and retransmits segments that are lost or corrupted. Its congestion control algorithms, including slow start, congestion avoidance, fast retransmit, and fast recovery, regulate the sending rate in response to packet loss or explicit congestion notification (ECN) signals. RFC 9293, published by the IETF in 2022, is the current consolidated specification for TCP, updating the original RFC 793 from 1981. TCP's performance over long-delay or high-bandwidth links is shaped by the bandwidth-delay product: a large product requires a correspondingly large congestion window to keep the pipe full, and window scaling extensions allow windows beyond the original 64 KB limit.

UDP and Low-Latency Delivery

The User Datagram Protocol (UDP) provides a minimal, connectionless transport service: it adds port-number multiplexing and an optional checksum to raw IP packets but offers no reliability, ordering, or congestion control. Applications that can tolerate occasional loss and prioritize low latency choose UDP: real-time voice and video, online games, DNS lookups, and streaming media all use it. UDP also serves as the substrate for protocols that implement their own reliability mechanisms, such as QUIC, which the IETF designed as a UDP-based, stream-multiplexing, encrypted transport protocol with TLS 1.3 integrated by default. QUIC eliminates the TCP handshake round trips that slow down HTTPS connection establishment and is the transport underlying HTTP/3.

Protocol Extensions and Specialized Protocols

Beyond TCP and UDP, the transport layer includes several specialized protocols for particular traffic types. The Stream Control Transmission Protocol (SCTP) supports multi-homing and multi-streaming, making it attractive for telephony signaling over IP. The Datagram Congestion Control Protocol (DCCP) adds congestion control to datagram delivery without requiring reliability, useful for real-time media applications. TCP itself has been extended with mechanisms such as TCP RACK (Recent ACKnowledgment), which improves loss detection using transmission timestamps rather than packet sequence numbers alone, and L4S (Low Latency, Low Loss, Scalable Throughput), which aims to reduce queuing delay across the network. Transport protocols operate over diverse lower-layer carriers including IP networks, radio links, and synchronous optical networks built on SONET or Synchronous Digital Hierarchy (SDH) framing.

Applications

Transport protocols underpin virtually every networked application, with specific protocol choices driven by application requirements:

• Web browsing and file transfer, which use TCP or QUIC for reliable ordered delivery
• Voice over IP and video conferencing, which use UDP or RTP (Real-time Transport Protocol) for low latency
• Domain name resolution and network time synchronization over UDP
• Financial trading systems and database replication over TCP with tuned congestion parameters
• Wireless and radio link networks, where transport protocols must adapt to higher loss rates and variable delay
• SONET/SDH carrier networks, which transport IP packets inside synchronous frames for high-capacity backbone links