Transport layer
Encyclopedia
In computer networking, the transport layer or layer 4 provides end-to-end communication services for applications within a layered architecture of network components and protocols. The transport layer provides convenient services such as connection-oriented data stream
support, reliability
, flow control
, and multiplexing
.
Transport layers are contained in both the TCP/IP model (RFC 1122), which is the foundation of the Internet
, and the Open Systems Interconnection
(OSI) model
of general networking. The definitions of the transport layer are slightly different in these two models. This article primarily refers to the TCP/IP model, in which TCP is largely for a convenient application programming interface
to internet hosts, as opposed to the OSI-model definition of the transport layer.
The most well-known transport protocol is the Transmission Control Protocol
(TCP). It lent its name to the title of the entire Internet Protocol Suite
, TCP/IP. 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 prominent protocols in this group are the Datagram Congestion Control Protocol
(DCCP) and the Stream Control Transmission Protocol
(SCTP).
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 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
, 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 datagram
s, 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.
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.
The OSI model defines five classes of transport protocols: TP0, providing the least error recovery, to TP4, which is designed for less reliable networks.
Data stream
In telecommunications and computing, a data stream is a sequence of digitally encoded coherent signals used to transmit or receive information that is in the process of being transmitted....
support, reliability
Reliability (computer networking)
In computer networking, a reliable protocol is one that provides reliability properties with respect to the delivery of data to the intended recipient, as opposed to an unreliable protocol, which does not provide notifications to the sender as to the delivery of transmitted data.A reliable...
, flow control
Flow control
In data communications, flow control is the process of managing the pacing of data transmission between two nodes to prevent a fast sender from outrunning a slow receiver. It provides a mechanism for the receiver to control the transmission speed, so that the receiving node is not overwhelmed with...
, and multiplexing
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
.
Transport layers are contained in both the TCP/IP model (RFC 1122), which is the foundation of the Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
, and the Open Systems Interconnection
Open Systems Interconnection
Open Systems Interconnection is an effort to standardize networking that was started in 1977 by the International Organization for Standardization , along with the ITU-T.-History:...
(OSI) model
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
of general networking. The definitions of the transport layer are slightly different in these two models. This article primarily refers to the TCP/IP model, in which TCP is largely for a convenient application programming interface
Application programming interface
An application programming interface is a source code based specification intended to be used as an interface by software components to communicate with each other...
to internet hosts, as opposed to the OSI-model definition of the transport layer.
The most well-known transport protocol is the Transmission Control Protocol
Transmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
(TCP). It lent its name to the title of the entire Internet Protocol Suite
Internet protocol suite
The Internet protocol suite is the set of communications protocols used for the Internet and other similar networks. It is commonly known as TCP/IP from its most important protocols: Transmission Control Protocol and Internet Protocol , which were the first networking protocols defined in this...
, TCP/IP. It is used for connection-oriented transmissions, whereas the connectionless User Datagram Protocol
User Datagram Protocol
The User Datagram Protocol is one of the core members of the Internet Protocol Suite, the set of network protocols used for the Internet. With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol network without requiring...
(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 prominent protocols in this group are the Datagram Congestion Control Protocol
Datagram Congestion Control Protocol
The Datagram Congestion Control Protocol is a message-oriented Transport Layer protocol. DCCP implements reliable connection setup, teardown, Explicit Congestion Notification , congestion control, and feature negotiation. DCCP was published as RFC 4340, a proposed standard, by the IETF in March,...
(DCCP) and the Stream Control Transmission Protocol
Stream Control Transmission Protocol
In computer networking, the Stream Control Transmission Protocol is a Transport Layer protocol, serving in a similar role to the popular protocols Transmission Control Protocol and User Datagram Protocol...
(SCTP).
Services
There are many services that can be optionally provided by a transport-layer protocol, and different protocols may or may not implement them.- Connection-oriented communication: Interpreting the connection as a data streamData streamIn telecommunications and computing, a data stream is a sequence of digitally encoded coherent signals used to transmit or receive information that is in the process of being transmitted....
can provide many benefits to applications. It is normally easier to deal with than the underlying connection-less models, such as the Transmission Control ProtocolTransmission Control ProtocolThe Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
's underlying Internet ProtocolInternet ProtocolThe Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
model of datagramDatagramA datagram is a basic transfer unit associated with a packet-switched network in which the delivery, arrival time, and order are not guaranteed....
s. - Byte orientation: Rather than processing the messages in the underlying communication system format, it is often easier for an application to process the data streamData streamIn telecommunications and computing, a data stream is a sequence of digitally encoded coherent signals used to transmit or receive information that is in the process of being transmitted....
as a sequence of byteByteThe byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
s. This simplification helps applications work with various underlying message formats. - 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. This is usually done through the use of segment numbering, with the receiver passing them to the application in order. This can cause head-of-line blockingHead-of-line blockingHead-of-line blocking is a performance-limiting phenomenon that occurs in buffered telecommunication network switches.-Description:...
. - ReliabilityReliability (computer networking)In computer networking, a reliable protocol is one that provides reliability properties with respect to the delivery of data to the intended recipient, as opposed to an unreliable protocol, which does not provide notifications to the sender as to the delivery of transmitted data.A reliable...
: Packets may be lost during transport due to network congestionNetwork congestionIn data networking and queueing theory, network congestion occurs when a link or node is carrying so much data that its quality of service deteriorates. Typical effects include queueing delay, packet loss or the blocking of new connections...
and errors. By means of an error detection code, such as a checksumChecksumA checksum or hash sum is a fixed-size datum computed from an arbitrary block of digital data for the purpose of detecting accidental errors that may have been introduced during its transmission or storage. The integrity of the data can be checked at any later time by recomputing the checksum and...
, the transport protocol may check that the data is not corrupted, and verify correct receipt by sending an ACKAcknowledgement (data networks)In data networking, an acknowledgment is a signal passed between communicating processes or computers to signify acknowledgment, or receipt of response, as part of a communications protocol...
or NACKNegative-acknowledge character* In telecommunications, a negative-acknowledge character is a transmission control character sent by a station as a negative response to the station with which the connection has been set up....
message to the sender. Automatic repeat request schemes may be used to retransmit lost or corrupted data. - Flow controlFlow controlIn data communications, flow control is the process of managing the pacing of data transmission between two nodes to prevent a fast sender from outrunning a slow receiver. It provides a mechanism for the receiver to control the transmission speed, so that the receiving node is not overwhelmed with...
: The rate of data transmission between two nodes must sometimes be managed to prevent a fast sender from transmitting more data than can be supported by the receiving data buffer, causing a buffer overrun. This can also be used to improve efficiency by reducing buffer underrunBuffer underrunIn computing, buffer underrun or buffer underflow is a state occurring when a buffer used to communicate between two devices or processes is fed with data at a lower speed than the data is being read from it. This requires the program or device reading from the buffer to pause its processing while...
. - Congestion avoidance: Congestion control can control traffic entry into a telecommunications network, so as to avoid congestive collapse by attempting to avoid oversubscription of any of the processing or linkData linkIn telecommunication a data link is the means of connecting one location to another for the purpose of transmitting and receiving information. It can also refer to a set of electronics assemblies, consisting of a transmitter and a receiver and the interconnecting data telecommunication circuit...
capabilities of the intermediate nodes and networks and taking resource reducing steps, such as reducing the rate of sending packets. For example, automatic repeat requests may keep the network in a congested state; this situation can be avoided by adding congestion avoidance to the flow control, including slow-startSlow-startSlow-start is part of the congestion control strategy used by TCP, the data transmission protocol used by many Internet applications. Slow-start is used in conjunction with other algorithms to avoid sending more data than the network is capable of transmitting, that is, to avoid causing network...
. This keeps the bandwidth consumption at a low level in the beginning of the transmission, or after packet retransmission. - MultiplexingMultiplexingThe multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
: PortsTCP and UDP portIn computer networking, a port is an application-specific or process-specific software construct serving as a communications endpoint in a computer's host operating system. A port is associated with an IP address of the host, as well as the type of protocol used for communication...
can provide multiple endpoints on a single node. For example, the name on a postal address is a kind of multiplexing, and distinguishes between different recipients of the same location. Computer applications will each listen for information on their own ports, which enables the use of more than one network serviceNetwork serviceNetwork services are the foundation of a networked computing environment. Generally network services are installed on one or more servers to provide shared resources to client computers.- Network services in LAN :...
at the same time. It is part of the transport layer in the TCP/IP model, but of the session layerSession layerThe session layer is layer 5 of the seven-layer OSI model of computer networking.The session layer provides the mechanism for opening, closing and managing a session between end-user application processes, i.e., a semi-permanent dialogue. Communication sessions consist of requests and responses...
in the OSI model.
Analysis
The transport layer is responsible for delivering data to the appropriate application process on the host computers. This involves statistical multiplexingStatistical multiplexing
Statistical multiplexing is a type of communication link sharing, very similar to dynamic bandwidth allocation . In statistical multiplexing, a communication channel is divided into an arbitrary number of variable bit-rate digital channels or data streams. The link sharing is adapted to the...
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
Session layer
The session layer is layer 5 of the seven-layer OSI model of computer networking.The session layer provides the mechanism for opening, closing and managing a session between end-user application processes, i.e., a semi-permanent dialogue. Communication sessions consist of requests and responses...
.
Some transport-layer protocols, for example TCP, but not UDP, support virtual circuit
Virtual circuit
In telecommunications and computer networks, a virtual circuit , synonymous with virtual connection and virtual channel, is a connection oriented communication service that is delivered by means of packet mode communication...
s, i.e. provide connection oriented communication over an underlying packet oriented datagram
Datagram
A datagram is a basic transfer unit associated with a packet-switched network in which the delivery, arrival time, and order are not guaranteed....
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
Flow control
In data communications, flow control is the process of managing the pacing of data transmission between two nodes to prevent a fast sender from outrunning a slow receiver. It provides a mechanism for the receiver to control the transmission speed, so that the receiving node is not overwhelmed with...
, 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
Application software
Application software, also known as an application or an "app", is computer software designed to help the user to perform specific tasks. Examples include enterprise software, accounting software, office suites, graphics software and media players. Many application programs deal principally with...
program. UDP packets are called datagram
Datagram
A datagram is a basic transfer unit associated with a packet-switched network in which the delivery, arrival time, and order are not guaranteed....
s, rather than segments.
TCP is used for many protocols, including HTTP web browsing and email transfer. UDP may be used for multicasting and broadcasting
Broadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...
, since retransmissions are not possible to a large amount of hosts. UDP typically gives higher throughput
Throughput
In communication networks, such as Ethernet or packet radio, throughput or network throughput is the average rate of successful message delivery over a communication channel. This data may be delivered over a physical or logical link, or pass through a certain network node...
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.
In many non-IP-based networks, for example X.25
X.25
X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...
, Frame Relay
Frame relay
Frame Relay is a standardized wide area network technology that specifies the physical and logical link layers of digital telecommunications channels using a packet switching methodology...
and ATM
Asynchronous Transfer Mode
Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...
, 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.
The OSI model defines five classes of transport protocols: TP0, providing the least error recovery, to TP4, which is designed for less reliable networks.
Protocols
The exact definition of what qualifies as a transport-layer protocol is not firm. The following is a short list:- ATP, AppleTalk Transaction ProtocolAppleTalkAppleTalk is a proprietary suite of protocols developed by Apple Inc. for networking computers. It was included in the original Macintosh released in 1984, but is now unsupported as of the release of Mac OS X v10.6 in 2009 in favor of TCP/IP networking...
- CUDP, Cyclic UDPCyclic UDPCUDP stands for Cyclic UDP.It is used for streaming media and resides in the Transport layer of the ISO/OSI protocol stack.- External links :*...
- DCCP, Datagram Congestion Control ProtocolDatagram Congestion Control ProtocolThe Datagram Congestion Control Protocol is a message-oriented Transport Layer protocol. DCCP implements reliable connection setup, teardown, Explicit Congestion Notification , congestion control, and feature negotiation. DCCP was published as RFC 4340, a proposed standard, by the IETF in March,...
- FCP, Fiber Channel ProtocolFibre ChannelFibre Channel, or FC, is a gigabit-speed network technology primarily used for storage networking. Fibre Channel is standardized in the T11 Technical Committee of the InterNational Committee for Information Technology Standards , an American National Standards Institute –accredited standards...
- IL, IL ProtocolIL ProtocolThe Internet Link protocol or IL is a connection-based transport layer protocol designed at Bell Labs originally as part of the Plan 9 operating system and is used to carry 9P. It is assigned the Internet Protocol number of 40...
- NBF, NetBIOS Frames protocol
- RDP, Reliable Datagram Protocol
- SCTP, Stream Control Transmission ProtocolStream Control Transmission ProtocolIn computer networking, the Stream Control Transmission Protocol is a Transport Layer protocol, serving in a similar role to the popular protocols Transmission Control Protocol and User Datagram Protocol...
- SPX, Sequenced Packet ExchangeIPX/SPXIPX/SPX stands for Internetwork Packet Exchange/Sequenced Packet Exchange. IPX and SPX are networking protocols used primarily on networks using the Novell NetWare operating systems.-Protocol Layers:...
- SST, Structured Stream TransportStructured Stream TransportStructured Stream Transport is an experimental transport protocol that provides an ordered, reliable byte stream abstraction similar to TCP's, but enhances and optimizes stream management to permit applications to use streams in a much more fine-grained fashion than is feasible with TCP streams....
- TCP, Transmission Control ProtocolTransmission Control ProtocolThe Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
- UDP, User Datagram ProtocolUser Datagram ProtocolThe User Datagram Protocol is one of the core members of the Internet Protocol Suite, the set of network protocols used for the Internet. With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol network without requiring...
- UDP LiteUDP LiteUDP-Lite is a connectionless protocol, very similar to UDP. Unlike UDP, where either all or none of a packet is protected by a checksum, UDP-Lite allows for partial checksums that only cover part of a datagram , and will therefore deliver packets that have been partially corrupted...
- µTP, Micro Transport ProtocolMicro Transport ProtocolMicro Transport Protocol or µTP is an open UDP-based variant of the BitTorrent peer-to-peer file sharing protocol intended to mitigate poor latency and other congestion control issues found in conventional bittorrent over TCP, while providing reliable, ordered delivery.It was devised to...
Comparison of transport-layer protocols
| Feature Name | UDP User Datagram Protocol The User Datagram Protocol is one of the core members of the Internet Protocol Suite, the set of network protocols used for the Internet. With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol network without requiring... |
UDP Lite UDP Lite UDP-Lite is a connectionless protocol, very similar to UDP. Unlike UDP, where either all or none of a packet is protected by a checksum, UDP-Lite allows for partial checksums that only cover part of a datagram , and will therefore deliver packets that have been partially corrupted... |
TCP Transmission Control Protocol The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP... |
SCTP | DCCP | RUDP Reliable User Datagram Protocol In computer networking, the Reliable User Datagram Protocol is a transport layer protocol designed at Bell Labs for the Plan 9 operating system... |
|---|---|---|---|---|---|---|
| Packet header size | 8 Bytes | 8 Bytes | 20-60 Bytes | 12 Bytes | 12 or 16 bytes | |
| Transport-layer packet entity | Datagram | Datagram | Segment | Datagram | Datagram | Datagram |
| Connection oriented | ||||||
| Reliable transport | ||||||
| Unreliable transport | ||||||
| Preserve message boundary | Unsure | |||||
| Ordered delivery | ||||||
| Unordered delivery | ||||||
| Data checksum Checksum A checksum or hash sum is a fixed-size datum computed from an arbitrary block of digital data for the purpose of detecting accidental errors that may have been introduced during its transmission or storage. The integrity of the data can be checked at any later time by recomputing the checksum and... |
Optional | Unsure | ||||
| Checksum size (bits) | 16 | 16 | 16 | 32 | 16 | Unsure |
| Partial checksum Checksum A checksum or hash sum is a fixed-size datum computed from an arbitrary block of digital data for the purpose of detecting accidental errors that may have been introduced during its transmission or storage. The integrity of the data can be checked at any later time by recomputing the checksum and... |
||||||
| Path MTU Maximum transmission unit In computer networking, the maximum transmission unit of a communications protocol of a layer is the size of the largest protocol data unit that the layer can pass onwards. MTU parameters usually appear in association with a communications interface... |
Unsure | |||||
| Flow control Flow control In data communications, flow control is the process of managing the pacing of data transmission between two nodes to prevent a fast sender from outrunning a slow receiver. It provides a mechanism for the receiver to control the transmission speed, so that the receiving node is not overwhelmed with... |
||||||
| Congestion control | Unsure | |||||
| ECN Explicit Congestion Notification Explicit Congestion Notification is an extension to the Internet Protocol and to the Transmission Control Protocol and is defined in RFC 3168 . ECN allows end-to-end notification of network congestion without dropping packets. ECN is an optional feature that is only used when both endpoints... support |
||||||
| Multiple streams | ||||||
| Multi-homing support | ||||||
| Bundling / Nagle Nagle's algorithm Nagle's algorithm, named after John Nagle, is a means of improving the efficiency of TCP/IP networks by reducing the number of packets that need to be sent over the network.... |
Unsure | |||||
| NAT Network address translation In computer networking, network address translation is the process of modifying IP address information in IP packet headers while in transit across a traffic routing device.... friendly |
Comparison of OSI transport protocols
The OSI model 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:| Service | TP0 | TP1 | TP2 | TP3 | TP4 |
|---|---|---|---|---|---|
| Connection oriented network | |||||
| Connectionless network | |||||
| Concatenation and separation | |||||
| Segmentation and reassembly | |||||
| Error Recovery | |||||
| Reinitiate connection (if an excessive number of PDUs Protocol data unit In telecommunications, the term protocol data unit has the following meanings:#Information that is delivered as a unit among peer entities of a network and that may contain control information, address information, or data.... are unacknowledged) |
|||||
| multiplexing and demultiplexing over a single virtual circuit Virtual circuit In telecommunications and computer networks, a virtual circuit , synonymous with virtual connection and virtual channel, is a connection oriented communication service that is delivered by means of packet mode communication... |
|||||
| Explicit flow control | |||||
| Retransmission on timeout | |||||
| Reliable Transport Service |