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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 the transport layer provide host-to-host communication services for applications.[1] 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 "TCP/IP model,[2] which is the foundation of the "Internet, and the "OSI model of general networking, are easily compared.[3][4][5][6] In both the OSI model and the TCP/IP model the transport layer is referred to as Layer 4.[7]

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 protocols in this group are 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:


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 constitutes 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.


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 transport layer protocols[edit]

Feature "UDP "UDP-Lite "TCP "Multipath TCP "SCTP "DCCP "RUDP[a]
Packet header size 8 bytes 8 bytes 20–60 bytes 50–90 bytes 12 bytes[b] 12 or 16 bytes 14+ bytes
Typical data-packet overhead 8 bytes 8 bytes 20 bytes ?? bytes 44–48+ bytes[c] 12 or 16 bytes 14 bytes
Transport-layer packet entity Datagram Datagram Segment Segment Datagram Datagram Datagram
Connection-oriented No No Yes Yes Yes Yes Yes
Reliable transport No No Yes Yes Yes No Yes
Unreliable transport Yes Yes No No Yes Yes Yes
Preserve message boundary Yes Yes No No Yes Yes Yes
Delivery Unordered Unordered Ordered Ordered Ordered / Unordered Unordered Unordered
Data "checksum Optional Yes Yes Yes Yes Yes Optional
Checksum size 16 bits 16 bits 16 bits 16 bits 32 bits 16 bits 16 bits
Partial "checksum No Yes No No No Yes No
Path "MTU No No Yes Yes Yes Yes ?
"Flow control No No Yes Yes Yes No Yes
"Congestion control No No Yes Yes Yes Yes ?
"Explicit Congestion Notification No No Yes Yes Yes Yes ?
Multiple "streams No No No No Yes No No
"Multi-homing No No No Yes Yes No No
Bundling / "Nagle No No Yes Yes Yes No ?
  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[edit]

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:[8]

Service TP0 TP1 TP2 TP3 TP4
Connection-oriented network Yes Yes Yes Yes Yes
Connectionless network No No No No Yes
Concatenation and separation No Yes Yes Yes Yes
Segmentation and reassembly Yes Yes Yes Yes Yes
Error recovery No Yes No Yes Yes
Reinitiate connection (if an excessive number of "PDUs are unacknowledged) No Yes No Yes No
Multiplexing and demultiplexing over a single "virtual circuit No No Yes Yes Yes
Explicit flow control No No Yes Yes Yes
Retransmission on timeout No No No No Yes
Reliable Transport Service No Yes No Yes Yes

There is also a connectionless transport protocol, specified by ISO/IEC 8602/ITU-T Recommendation X.234.[9]


  1. ^ RFC 1122, §1.1.3.
  2. ^ RFC 1122, Requirements for Internet Hosts – Communication Layers, IETF, R. Braden (Editor), October 1989
  3. ^ Lou Frenzel (2013-10-02). "What's The Difference Between The OSI Seven-Layer Network Model And TCP/IP?". Electronic Design. Retrieved 2017-01-17. 
  4. ^ "Difference between OSI Reference Model and TCP/IP Reference Model | Computer Network Tutorial". Studytonight.com. Retrieved 2017-01-17. 
  5. ^ "Four Layers of TCP/IP model, Comparison and Difference between TCP/IP and OSI models". Omnisecu.com. Retrieved 2017-01-17. 
  6. ^ Tetz, Edward. "Network Basics: TCP/IP and OSI Network Model Comparisons". Dummies.com. Retrieved 2017-01-17. 
  7. ^ "Introducing the Internet Protocol Suite". System Administration Guide, Volume 3. 
  8. ^ "ITU-T Recommendation X.224 (11/1995) ISO/IEC 8073". Itu.int. Retrieved 2017-01-17. 
  9. ^ "ITU-T Recommendation X.234 (07/1994) ISO/IEC 8602". Itu.int. Retrieved 2017-01-17. 
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