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 network 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.
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).
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 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, 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.
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.
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 realtime multimedia communication where packet loss occasionally can be accepted, for example IP-TV and IP-telephony, and for online computer games.
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.
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).
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.
Transport Layer Services
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 Ethernet due to interference and noise, since Ethernet does not retransmit corrupted 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.
Transport Protocol Comparison
| UDP | TCP | DCCP | SCTP | |
|---|---|---|---|---|
| Packet header size | 8 Bytes | 20 Bytes | Varies | 12 Bytes + Variable Chunk Header |
| Transport layer packet entity | Datagram | Segment | Datagram | Datagram |
| Port numbering | Yes | Yes | Yes | Yes |
| Error detection | Optional | Yes | Yes | Yes |
| Reliability: Error recovery by automatic repeat request (ARQ) | No | Yes | No | Yes |
| Virtual circuits: Sequence numbering and reordering | No | Yes | Yes | Optional |
| Flow control | No | Yes | Yes | Yes |
| Congestion avoidance: Variable congestion window, slow start, time outs | No | Yes | Yes | Yes |
| Multiple streams | No | No | No | Yes |
| ECN support | No | Yes | Yes | Yes |