Editing Internet protocol suite (section)

==Transport layer==
{{See also|Transport layer}}
The transport layer establishes basic data channels that applications use for task-specific data exchange. The layer establishes host-to-host connectivity in the form of end-to-end message transfer services that are independent of the underlying network and independent of the structure of user data and the logistics of exchanging information. Connectivity at the transport layer can be categorized as either [[connection-oriented]], implemented in TCP, or [[connectionless]], implemented in UDP. The protocols in this layer may provide [[error control]], [[Network segmentation|segmentation]], [[Flow control (data)|flow control]], [[Network congestion|congestion control]], and application addressing ([[port numbers]]).

For the purpose of providing process-specific transmission channels for applications, the layer establishes the concept of the [[network port]]. This is a numbered logical construct allocated specifically for each of the communication channels an application needs. For many types of services, these ''port numbers'' have been standardized so that client computers may address specific services of a server computer without the involvement of [[service discovery]] or [[directory service]]s.

Because IP provides only a [[best-effort delivery]], some transport-layer protocols offer reliability.

TCP is a connection-oriented protocol that addresses numerous reliability issues in providing a [[reliable byte stream]]:
* data arrives in-order
* data has minimal error (i.e., correctness)
* duplicate data is discarded
* lost or discarded packets are resent
* includes traffic congestion control

The newer [[Stream Control Transmission Protocol]] (SCTP) is also a reliable, connection-oriented transport mechanism. It is message-stream-oriented, not byte-stream-oriented like TCP, and provides multiple streams multiplexed over a single connection. It also provides [[multihoming]] support, in which a connection end can be represented by multiple IP addresses (representing multiple physical interfaces), such that if one fails, the connection is not interrupted. It was developed initially for telephony applications (to transport [[Signaling System 7|SS7]] over IP).

Reliability can also be achieved by running IP over a reliable data-link protocol such as the [[High-Level Data Link Control]] (HDLC).

The [[User Datagram Protocol]] (UDP) is a connectionless [[datagram]] protocol. Like IP, it is a best-effort, unreliable protocol. Reliability is addressed through [[error detection]] using a checksum algorithm. UDP is typically used for applications such as streaming media (audio, video, [[Voice over IP]], etc.) where on-time arrival is more important than reliability, or for simple query/response applications like [[DNS]] lookups, where the overhead of setting up a reliable connection is disproportionately large. [[Real-time Transport Protocol]] (RTP) is a datagram protocol that is used over UDP and is designed for real-time data such as [[streaming media]].

The applications at any given network address are distinguished by their TCP or UDP port. By convention, certain [[List of TCP and UDP port numbers|'' well-known ports'']] are associated with specific applications.

The TCP/IP model's transport or host-to-host layer corresponds roughly to the fourth layer in the OSI model, also called the transport layer.

[[QUIC]] is rapidly emerging as an alternative transport protocol. Whilst it is technically carried via UDP packets it seeks to offer enhanced transport connectivity relative to TCP. [[HTTP/3]] works exclusively via QUIC.