Transport layer

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The transport layer in the Internet protocol stack. Internet Protocol Analysis - Transport Layer.png
The transport layer in the Internet protocol stack.

In computer networking, the transport layer is a conceptual division of methods in the layered architecture of protocols in the network stack in the Internet protocol suite and the OSI model. The protocols of this layer provide end-to-end communication services for applications. [1] :§1.1.3 It provides services such as connection-oriented communication, reliability, flow control, and multiplexing.


The details of implementation and semantics of the transport layer of the Internet protocol suite, [1] which is the foundation of the Internet, and the OSI model of general networking are different. The protocols in use today in this layer for the Internet all originated in the development of TCP/IP. In the OSI model the transport layer is often referred to as Layer 4, or L4, [2] while numbered layers are not used in TCP/IP.

The best-known transport protocol of the Internet protocol suite is the Transmission Control Protocol (TCP). 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. Together, TCP and UDP comprise essentially all traffic on the Internet and are the only protocols implemented in every major operating system. Additional transport layer protocols that have been defined and implemented include the Datagram Congestion Control Protocol (DCCP) and the Stream Control Transmission Protocol (SCTP).


Transport layer services are conveyed to an application via a programming interface to the transport layer protocols. The services may include the following features: [4]


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 circuits, 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, 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 program. UDP packets are called datagrams, rather than segments.

TCP is used for many protocols, including HTTP web browsing and email transfer. UDP may be used for multicasting and 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 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. [6] To avoid middlebox intolerance, new transport protocols may mimic the wire image of a tolerated protocol, or be encapsulated in UDP, accepting some overhead (e.g., due to outer checksums made redundant by inner integrity checks). [7] QUIC takes the latter approach, rebuilding reliable stream transport on top of UDP. [8]


This list shows some protocols that are commonly placed in the transport layers of the Internet protocol suite, the OSI protocol suite, NetWare's IPX/SPX, AppleTalk, and Fibre Channel.

Comparison of Internet transport layer protocols

Feature UDP UDP-Lite TCP Multipath TCP SCTP DCCP RUDP [lower-alpha 1]
Packet header size8 bytes8 bytes20–60 bytes50–90 bytes12 bytes [lower-alpha 2] 12 or 16 bytes14+ bytes
Typical data-packet overhead8 bytes8 bytes20 bytes ?? bytes44–48+ bytes [lower-alpha 3] 12 or 16 bytes14 bytes
Transport-layer packet entityDatagramDatagramSegmentSegmentDatagramDatagramDatagram
Reliable transportNoNoYesYesYesNoYes
Unreliable transportYesYesNoNoYesYesYes
Preserve message boundaryYesYesNoNoYesYesYes
DeliveryUnorderedUnorderedOrderedOrderedOrdered / UnorderedUnorderedUnordered
Data checksum OptionalYesYesYesYesYesOptional
Checksum size16 bits16 bits16 bits16 bits32 bits16 bits16 bits
Partial checksum NoYesNoNoNoYesNo
Path MTU NoNoYesYesYesYes?
Flow control NoNoYesYesYesNoYes
Congestion control NoNoYesYesYesYes?
Explicit Congestion Notification NoNoYesYesYesYes?
Multiple streams NoNoNoNoYesNoNo
Multi-homing NoNoNoYesYesNoNo
Bundling / Nagle NoNoYesYesYesNo?
  1. RUDP is not officially standardized. There have been no standard-related developments since 1999.
  2. Excluding data chunk headers and overhead chunks. Without embedded chunks, an SCTP packet is essentially useless.
  3. Counted as follows: 12 bytes SCTP header + 16 bytes DATA chunk header or 20 bytes I-DATA chunk header + 16+ bytes SACK chunk. Additional non-data chunks (e.g. AUTH) and/or headers for additional data chunks, which might easily increase the overhead with 50 bytes or more, not counted.

Comparison of OSI transport protocols

ISO/IEC 8073/ITU-T Recommendation X.224, "Information Technology - Open Systems Interconnection - Protocol for providing the connection-mode transport service", defines five classes of connection-mode transport protocols designated class 0 (TP0) to class 4 (TP4). Class 0 contains no error recovery and was designed for use on network layers that provide error-free connections. Class 4 is closest to TCP, although TCP contains functions, such as the graceful close, which OSI assigns to the session layer. All OSI connection-mode protocol classes provide expedited data and preservation of record boundaries. Detailed characteristics of the classes are shown in the following table: [10]

Connection-oriented networkYesYesYesYesYes
Connectionless networkNoNoNoNoYes
Concatenation and separationNoYesYesYesYes
Segmentation and reassemblyYesYesYesYesYes
Error recoveryNoYesNoYesYes
Reinitiate connection (if an excessive number of PDUs are unacknowledged)NoYesNoYesNo
Multiplexing and demultiplexing over a single virtual circuit NoNoYesYesYes
Explicit flow controlNoNoYesYesYes
Retransmission on timeoutNoNoNoNoYes
Reliable Transport ServiceNoYesNoYesYes

There is also a connectionless transport protocol, specified by ISO/IEC 8602/ITU-T Recommendation X.234. [11]

Related Research Articles

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The Stream Control Transmission Protocol (SCTP) is a computer networking communications protocol in the transport layer of the Internet protocol suite. Originally intended for Signaling System 7 (SS7) message transport in telecommunication, the protocol provides the message-oriented feature of the User Datagram Protocol (UDP), while ensuring reliable, in-sequence transport of messages with congestion control like the Transmission Control Protocol (TCP). Unlike UDP and TCP, the protocol supports multihoming and redundant paths to increase resilience and reliability.


  1. 1 2 R. Braden, ed. (October 1989). Requirements for Internet Hosts -- Communication Layers. Network Working Group. doi: 10.17487/RFC1122 . STD 3. RFC 1122.Internet Standard 3. Updated by RFC  1349, 4379, 5884, 6093, 6298, 6633, 6864, 8029 and 9293.
  2. "Introducing the Internet Protocol Suite". System Administration Guide, Volume 3.
  3. "X.225 : Information technology – Open Systems Interconnection – Connection-oriented Session protocol: Protocol specification". Archived from the original on February 1, 2021. Retrieved March 10, 2023.
  4. "Transport Layer" (PDF). Galgotias University.
  5. Heena, Khera. "Data Communication and networking" (PDF). Galgotias University. p. 9.
  6. Papastergiou et al. 2017, p. 620-621.
  7. Papastergiou et al. 2017, p. 623-624.
  8. Corbet 2018.
  9. Brian C. Smith, Cyclic-UDP: A Priority-Driven Best-Effort Protocol (PDF), retrieved February 23, 2020
  10. "ITU-T Recommendation X.224 (11/1995) ISO/IEC 8073". Retrieved January 17, 2017.
  11. "ITU-T Recommendation X.234 (07/1994) ISO/IEC 8602". Retrieved January 17, 2017.