In computing and telecommunications, the transport layer is the second highest layer in the four and five layer TCP/IP reference models, where it responds to service requests from the application layer and issues service requests to the Internet layer. It is also the name of layer four of the seven layer OSI model, where it responds to service requests from the session layer and issues service requests to the network layer. The definitions of the transport layer are slightly different in these two models. This article primarily refers to the TCP/IP model. See also the OSI model definition of the transport layer. Memory (Random Access Memory) Look up computing in Wiktionary, the free dictionary. ... Telecommunication involves the transmission of signals over a distance for the purpose of communication. ... The TCP/IP reference model or TCP/IP model, sometimes called the DoD model (DoD, Department of Defense), ARPANET reference model or the Internet reference model, is a layered abstract description for communications and computer network protocol design. ... The application layer is the seventh level of the seven-layer OSI model. ... The network layer is third layer out of seven in OSI model and it is the third layer out of five in TCP/IP model. ... The Open Systems Interconnection Basic Reference Model (OSI Reference Model or OSI Model for short) is a layered, abstract description for communications and computer network protocol design, developed as part of Open Systems Interconnection (OSI) initiative. ... The session layer is level five of the seven level OSI model. ... The network layer is third layer out of seven in OSI model and it is the third layer out of five in TCP/IP model. ... The Open Systems Interconnection Basic Reference Model (OSI Reference Model or OSI Model for short) is a layered, abstract description for communications and computer network protocol design, developed as part of Open Systems Interconnection (OSI) initiative. ...

A transport protocol is a protocol on the transport layer. The two most widely used transport protocols on the Internet are the connection oriented TCP (Transmission Control Protocol), and UDP (User Datagram Protocol). TCP is the more complicated and most common. Other options are the Datagram Congestion Control Protocol (DCCP) and Stream Control Transmission Protocol (SCTP). For other senses of this word, see protocol. ... The Transmission Control Protocol (TCP) is one of the core protocols of the Internet protocol suite, often simply referred to as TCP/IP. Using TCP, applications on networked hosts can create connections to one another, over which they can exchange streams of data using Stream Sockets. ... User Datagram Protocol (UDP) is one of the core protocols of the Internet protocol suite. ... The Datagram Congestion Control Protocol (DCCP) is a message-oriented transport layer protocol that is currently under development in the IETF. Applications that might make use of DCCP include those with timingconstraints on the delivery of data such that reliable in-order delivery, when combined with congestion control, is likely... In the field of computer networking, the IETF Signaling Transport (SIGTRAN) working group defined the Stream Control Transmission Protocol (SCTP) as a transport layer protocol in 2000. ...

The transport layer is typically handled by processes in the host computer operational system, and not by routers and switches. The transport layer usually turns the unreliable and very basic service provided by the Network layer into a more powerful one.

In the TCP/IP model, 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 packets, and adding source and destination port numbers in the header of each transport layer data packet. Together with the source and destination IP address, the port numbers constitutes a network socket, i.e. an identificiation address of the process-to-process communication. In the OSI model, this function is supported by the session layer. Statistical multiplexing is similar to time-division multiplexing (TDM), except that, rather than arbitrarily assigning a time slot to each signal, each signal is assigned a slot according to priority and need. ... An Internet socket (or commonly, a socket or network socket), is a communication end-point unique to a machine communicating on an Internet Protocol-based network, such as the Internet. ... The session layer is level five of the seven level OSI model. ...

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. A virtual circuit (VC) is a communications arrangement in which data from a source user may be passed to a destination user over more than one real communications circuit during a single period of communication, but the switching is hidden from the users. ... A connection-oriented networking protocol is one which identifies traffic flows by some connection identifier rather than by explicitly listing source and destination addresses. ... A packet is the fundamental unit of information carriage in all modern computer networks. ...

Finally, some transport layer protocols, for example TCP but not UDP, provides 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. It has been suggested that this article or section be merged with SEC-DED. (Discuss) In mathematics, computer science, telecommunication, and information theory, error detection and correction has great practical importance in maintaining data (information) integrity across noisy channels and less-than-reliable storage media. ... Automatic Repeat-reQuest (ARQ) is an error control method for data transmission which makes use of acknowledgments and timeouts to achieve reliable data transmission. ... The flow control mechanism is used for controlling the flow of data in a network under well-defined conditions, while congestion control is used for controlling the flow of data when congestion has actually occurred . ... Network congestion avoidance is a process used in computer networks to avoid congestion. ...

UDP is a very simple service, and does not provide virtual circuits, nor reliable communication, leaving these to the application. The UDP packets are called datagrams rather than segments. This article or section does not cite any references or sources. ... A packet is the fundamental unit of information carriage in all modern computer networks. ...

TCP is used for for example 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 therefor often used for realtime multimedia communication where packet loss occasionally can be accepted, for example IP-TV and IP-telephony, and for online computer games. HTTP (for HyperText Transfer Protocol) is the primary method used to convey information on the World Wide Web. ... Multicast is the delivery of information to multiple destinations simultaneously using the most efficient strategy to deliver the messages over each link of the network only once and only create copies when the links to the destinations split. ... Broadcasting is the distribution of audio and/or video signals which transmit programs to an audience. ... In information technology, throughput is the rate at which a computer or network sends or receives data. ...

In many non-IP-based networks, for example X.25, Frame Relay and ATM, the connection oriented communication is implemented at network layer 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. X.25 is an ITU-T standard protocol suite for wide area networks using leased lines, the phone or ISDN system as the networking hardware. ... In the context of computer networking, frame relay (also found written as frame-relay) consists of an efficient data transmission technique used to send digital information quickly and cheaply in a relay of frames to one or many destinations from one or many end-points. ... ATM or atm could refer to: Asynchronous Transfer Mode, a telecommunications protocol Actun Tunichil Muknal, Cave in Belize Adobe Type Manager, typeface management software from Adobe Systems Advanced Traffic Management and Arterial Traffic Management, terms used in the Intelligent Transportation System industry Air Traffic Management (includes air traffic control, air...

In the OSI/X.25 protocol suite, there are five classes of the OSI transport protocol, ranging from class 0 (which is also known as TP0 and provides the least error recovery) to class 4 (which is also known as TP4 and is designed for less reliable networks, similar to the Internet). X.25 is an ITU-T standard protocol suite for wide area networks using the phone or ISDN system as the networking hardware. ... This article or section does not adequately cite its references or sources. ... The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ...

List of transport layer services

There is a long list of services that can be optionally provided by the transport layer. None of them are compulsory, because not all applications want all the services available. Some can be wasted overhead, or even counterproductive in some cases.

Connection-oriented

This is normally easier to deal with than connection-less models, so where the Network layer only provides a connection-less service, often a connection-oriented service is built on top of that in the Transport layer.

Same Order Delivery

The Network layer doesn't generally guarantee that packets of data will arrive in the same order that they were sent, but often this is a desirable feature, so the Transport layer provides it. The simplest way of doing this is to give each packet a number, and allow the receiver to reorder the packets.

Reliable Data

Packets may be lost in routers, switches, bridges and hosts due to network congestion, when the packet queues are filled and the network nodes have to delete packets. Packets may be lost or corrupted in for example Ethernet due to interference and noise, since Ethernet does not retransmit corrupt packets. Packets may be delivered in the wrong order by an underlying network. Some transport layer protocols, for example TCP, can fix this. By means of an error detection code, for example a checksum, the transport protocol may check that the data is not corrupted, and verify that by sending an ACK message to the sender. Automatic repeat request schemes may be used to retransmit lost or corrupted data. By introducing segment numbering in the transport layer packet headers, the packets can be sorted in order. Of course, error free is impossible, but it is possible to substantially reduce the numbers of undetected errors.

Flow Control

The amount of memory on a computer is limited, and without flow control a larger computer might flood a computer with so much information that it can't hold it all before dealing with it. Nowadays, this is not a big issue, as memory is cheap while bandwidth is comparatively expensive, but in earlier times it was more important. Flow control allows the receiver to say "Whoa!" before it is overwhelmed. Sometimes this is already provided by the network, but where it is not, the Transport layer may add it on.

Congestion avoidance

Network congestion occurs when a queue buffer of a network node is full and starts to drop packets. Automatic repeat request may keep the network in a congested state. This situation can be avoided by adding congestion avoidance to the flow control, including slow-start. This keeps the bandwidth consumption at a low level in the beginning of the transmission, or after packet retransmission.

Byte orientation

Rather than dealing with things on a packet-by-packet basis, the Transport layer may add the ability to view communication just as a stream of bytes. This is nicer to deal with than random packet sizes, however, it rarely matches the communication model which will normally be a sequence of messages of user defined sizes.

Ports

(Part of the transport layer in the TCP/IP model, but of the session layer in the OSI model) Ports are essentially ways to address multiple entities in the same location. For example, the first line of a postal address is a kind of port, and distinguishes between different occupants of the same house. Computer applications will each listen for information on their own ports, which is why you can use more than one network-based application at the same time.

A connection-oriented networking protocol is one which identifies traffic flows by some connection identifier rather than by explicitly listing source and destination addresses. ... In computer networking, a reliable protocol is one that ensures data arrival via some internal method, as opposed to an unreliable protocol, which does not guarantee that all the data will arrive intact (or indeed, at all). ... In data networking and queueing theory, network congestion occurs when incremental increases in offered load lead either only to small increases in network throughput, or to an actual reduction in network throughput. ... The Transmission Control Protocol (TCP) is one of the core protocols of the Internet protocol suite, often simply referred to as TCP/IP. Using TCP, applications on networked hosts can create connections to one another, over which they can exchange streams of data using Stream Sockets. ... It has been suggested that this article or section be merged with SEC-DED. (Discuss) In mathematics, computer science, telecommunication, and information theory, error detection and correction has great practical importance in maintaining data (information) integrity across noisy channels and less-than-reliable storage media. ... A checksum is a form of redundancy check, a very simple measure for protecting the integrity of data by detecting errors in data that is sent through space (telecommunications) or time (storage). ... In computing, an ACK (also known as an acknowledgment code) is a signal passed between communicating processes or computers to signify acknowledgement, or receipt of response, as part of a communications protocol. ... Automatic Repeat-reQuest (ARQ) is an error control method for data transmission which makes use of acknowledgments and timeouts to achieve reliable data transmission. ... The flow control mechanism is used for controlling the flow of data in a network under well-defined conditions, while congestion control is used for controlling the flow of data when congestion has actually occurred . ... Network congestion avoidance is a process used in computer networks to avoid congestion. ... In data networking and queueing theory, network congestion occurs when incremental increases in offered load lead either only to small increases in network throughput, or to an actual reduction in network throughput. ... Slow-start is part of the congestion control strategy used by TCP, the data transmission protocol used by many Internet applications, such as HTTP and Secure Shell. ... Byte orientation refers to forms of data processing in which digital data are procesed bytewise. ... It has been suggested that this article or section be merged into Computer port (software). ... The TCP/IP model or Internet reference model, sometimes called the DoD model (DoD, Department of Defense), ARPANET reference model, is a layered abstract description for communications and computer network protocol design. ... The session layer is level five of the seven level OSI model. ...

Transport protocol comparison table

User Datagram Protocol (UDP) is one of the core protocols of the Internet protocol suite. ... The Transmission Control Protocol (TCP) is one of the core protocols of the Internet protocol suite, often simply referred to as TCP/IP. Using TCP, applications on networked hosts can create connections to one another, over which they can exchange streams of data using Stream Sockets. ... The Datagram Congestion Control Protocol (DCCP) is a message-oriented transport layer protocol that is currently under development in the IETF. Applications that might make use of DCCP include those with timingconstraints on the delivery of data such that reliable in-order delivery, when combined with congestion control, is likely... In the field of computer networking, the IETF Signaling Transport (SIGTRAN) working group defined the Stream Control Transmission Protocol (SCTP) as a transport layer protocol in 2000. ... In the TCP and UDP protocols used in computer networking, a port is a special number present in the header of a data packet. ... In computer science and information theory, error correction consists of using methods to detect and/or correct errors in the transmission or storage of data by the use of some amount of redundant data and (in the case of transmission) the selective retransmission of incorrect segments of the data. ... Automatic Repeat-reQuest (ARQ) is an error control method for data transmission which makes use of acknowledgments and timeouts to achieve reliable data transmission. ... A virtual circuit (VC) is a communications arrangement in which data from a source user may be passed to a destination user over more than one real communications circuit during a single period of communication, but the switching is hidden from the users. ... The flow control mechanism is used for controlling the flow of data in a network under well-defined conditions, while congestion control is used for controlling the flow of data when congestion has actually occurred . ... Network congestion avoidance is a process used in computer networks to avoid congestion. ...

Examples