Editing Transport layer (section)

==Analysis==
The transport layer is responsible for delivering data to the appropriate application process on the host computers. This involves [[statistical multiplexing]] of data from different application processes, i.e. forming data segments, and adding source and destination port numbers in the header of each transport layer data segment. Together with the source and destination IP address, the port numbers constitute a [[network socket]], i.e. an identification address of the process-to-process communication. In the OSI model, this function is supported by the [[session layer]].

Some transport layer protocols, for example TCP, but not UDP, support [[virtual circuit]]s, i.e. provide [[connection-oriented communication]] over an underlying packet-oriented [[datagram]] network. A byte stream is delivered while hiding the packet mode communication for the application processes. This involves connection establishment, dividing of the data stream into packets called segments, segment numbering and reordering of out-of-order data.

Finally, some transport layer protocols, for example TCP, but not UDP, provide end-to-end reliable communication, i.e. [[error recovery]] by means of [[error detecting code]] and [[automatic repeat request]] (ARQ) protocol. The ARQ protocol also provides [[flow control (data)|flow control]], which may be combined with [[congestion avoidance]].

UDP is a very simple protocol and does not provide virtual circuits, nor reliable communication, delegating these functions to the [[application software|application]] program. UDP packets are called [[datagram]]s, rather than segments.

TCP is used for many protocols, including [[HTTP]] web browsing and email transfer. UDP may be used for [[multicast]]ing and [[Broadcasting (networking)|broadcasting]], since retransmissions are not possible to a large amount of hosts. UDP typically gives higher [[throughput]] and shorter latency and is therefore often used for real-time multimedia communication where packet loss occasionally can be accepted, for example IP-TV and IP-telephony, and for online computer games.

Many non-IP-based networks, such as [[X.25]], [[Frame Relay]] and [[Asynchronous Transfer Mode|ATM]], implement the connection-oriented communication at the network or data link layer rather than the transport layer. In X.25, in telephone network modems and in wireless communication systems, reliable node-to-node communication is implemented at lower protocol layers.

The OSI connection-mode transport layer protocol specification defines five classes of transport protocols: ''TP0'', providing the least error recovery, to ''TP4'', which is designed for less reliable networks.

Due to [[protocol ossification]], TCP and UDP are the only widely used transport protocols on the Internet.{{sfn|Papastergiou|Fairhurst|Ros|Brunstrom|2017|p=620-621}} To avoid [[middlebox]] intolerance, new transport protocols may mimic the [[wire image (networking)|wire image]] of a tolerated protocol, or [[encapsulation (networking)|be encapsulated]] in UDP, accepting some overhead (e.g., due to outer checksums made redundant by inner integrity checks).{{sfn|Papastergiou|Fairhurst|Ros|Brunstrom|2017|p=623-624}} [[QUIC]] takes the latter approach, rebuilding reliable stream transport on top of UDP.{{sfn|Corbet|2018}}