View source for Transport layer

{{short description|Layer in the OSI and TCP/IP models providing host-to-host communication services for applications}}
{{Technical|date=March 2025}}
{{Use American English|date = March 2019}}
{{Use mdy dates|date = March 2019}}

[[File:Internet Protocol Analysis - Transport Layer.png|thumb|The transport layer in the Internet protocol stack.|alt=Four labeled stacked blocks. The blue block labeled "transport" is the second from the top.]]
{{IPstack}}

In [[computer network]]ing, the '''transport layer''' is a conceptual division of methods in the [[Abstraction layer|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.{{Ref RFC|1122|rsection=1.1.3}} It provides services such as [[connection-oriented communication]], [[reliability (computer networking)|reliability]], [[flow control (data)|flow control]], and [[multiplexing]].

The details of implementation and semantics of the transport layer of the [[Internet protocol suite]],{{Ref RFC|1122}} 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''',<ref>{{cite web|url=https://docs.oracle.com/cd/E19455-01/806-0916/6ja85398m/index.html|work=System Administration Guide, Volume 3|title=Introducing the Internet Protocol Suite}}</ref> 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).

{{OSI model}}

==Services==
Transport layer services are conveyed to an application via a programming interface to the transport layer protocols. The services may include the following features:<ref>{{Cite web |title=Transport Layer |url=http://103.47.12.35/bitstream/handle/1/4881/659.pdf?sequence=1&isAllowed=y |website=Galgotias University }}</ref>
* [[Connection-oriented communication]]:<ref>{{Cite web |last=Heena |first=Khera |title=Data Communication and networking |url=http://103.47.12.35/bitstream/handle/1/4881/659.pdf?sequence=1&isAllowed=y |website=Galgotias University |page=9}}</ref> It is normally easier for an application to interpret a connection as a [[data stream]] rather than having to deal with the underlying connection-less models, such as the [[datagram]] model of the [[User Datagram Protocol]] (UDP) and of the [[Internet Protocol]] (IP).
* 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 blocking]].
* [[Reliability (computer networking)|Reliability]]: Packets may be lost during transport due to [[network congestion]] and errors. By means of an [[error detection code]], such as a [[checksum]], the transport protocol may check that the data is not corrupted, and verify correct receipt by sending an [[acknowledgement (data networks)|ACK]] or [[NACK]] message to the sender. [[Automatic repeat request]] schemes may be used to retransmit lost or corrupted data.
* [[Flow control (data)|Flow control]]: 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 underrun]].
* [[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 [[data link|link]] capabilities of the intermediate nodes and networks and taking resource reducing steps, such as reducing the rate of sending [[Packet (information technology)|packets]]. For example, [[automatic repeat request]]s may keep the network in a congested state; this situation can be avoided by adding congestion avoidance to the flow control, including [[TCP congestion control#Slow start|slow start]]. This keeps the bandwidth consumption at a low level in the beginning of the transmission, or after packet retransmission.
* [[Multiplexing]]: [[TCP and UDP port|Ports]] 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 service]] at the same time. It is part of the transport layer in the [[TCP/IP model]], but of the [[session layer]] 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 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 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 (data)|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 software|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 [[multicast]]ing and [[Broadcasting (networking)|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 [[Asynchronous Transfer Mode|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.{{sfn|Papastergiou|Fairhurst|Ros|Brunstrom|2017|p=620-621}} To avoid [[middlebox]] intolerance, new transport protocols may mimic the [[wire image (networking)|wire image]] of a tolerated protocol, or [[encapsulation (networking)|be encapsulated]] in UDP, accepting some overhead (e.g., due to outer checksums made redundant by inner integrity checks).{{sfn|Papastergiou|Fairhurst|Ros|Brunstrom|2017|p=623-624}} [[QUIC]] takes the latter approach, rebuilding reliable stream transport on top of UDP.{{sfn|Corbet|2018}}

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

{{Div col|colwidth=30em}}
* ATP, [[AppleTalk|AppleTalk Transaction Protocol]]
* CUDP, [[Cyclic UDP]]<ref>{{citation |url=https://www.cs.cornell.edu/zeno/Papers/cyclicudp.pdf |title=Cyclic-UDP: A Priority-Driven Best-Effort Protocol |author=Brian C. Smith |access-date=2020-02-23}}</ref>
* DCCP, [[Datagram Congestion Control Protocol]]
* FCP, [[Fibre Channel Protocol]]
* IL, [[IL Protocol]]
* MPTCP, [[Multipath TCP]]
* NORM, [[NACK-Oriented Reliable Multicast]]
* [[QUIC]]
* RDP, [[Reliable Data Protocol]]
* RUDP, [[Reliable User Datagram Protocol]]
* SCTP, [[Stream Control Transmission Protocol]]
* SPX, [[IPX/SPX|Sequenced Packet Exchange]]
* SST, [[Structured Stream Transport]]
* TCP, [[Transmission Control Protocol]]
* UDP, [[User Datagram Protocol]]
* [[UDP-Lite]]
* μTP, [[Micro Transport Protocol]]
{{div col end}}

=== <span class="anchor" id="COMPARISON1"></span><span class="anchor" id="Comparison of transport layer protocols"></span>Comparison of Internet transport layer protocols ===
{| class="wikitable" style="text-align:center"
|-
! scope="col" | Feature
! scope="col" | [[User Datagram Protocol|UDP]]
! scope="col" | [[UDP-Lite]]
! scope="col" | [[Transmission Control Protocol|TCP]]
! scope="col" | [[Multipath TCP]]
! scope="col" | [[SCTP]]
! scope="col" | [[DCCP]]
! scope="col" | [[RUDP]]{{efn|RUDP is not officially standardized. There have been no standard-related developments since 1999.}}
|-
| style="text-align:left" | Packet header size
| 8 bytes
| 8 bytes
| 20–60 bytes
| 50–90 bytes
| 12 bytes{{efn|Excluding data chunk headers and overhead chunks. Without embedded chunks, an SCTP packet is essentially useless.}}
| 12 or 16 bytes
| 14+ bytes<!--Including 8-byte UDP header-->
|-
| style="text-align:left" | Typical data-packet overhead
| 8 bytes
| 8 bytes
| 20 bytes
| ?? bytes
| 44–48+ bytes{{efn|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.}}
| 12 or 16 bytes
| 14 bytes
|-
| style="text-align:left" | Transport-layer packet entity
| Datagram
| Datagram
| Segment
| Segment
| Datagram
| Datagram
| Datagram
|-
| style="text-align:left" | Connection-oriented
| {{No}}
| {{No}}
| {{Yes}}
| {{Yes}}
| {{Yes}}
| {{Yes}}
| {{Yes}}
|-
| style="text-align:left" | Reliable transport
| {{No}}
| {{No}}
| {{Yes}}
| {{Yes}}
| {{Yes}}
| {{No}}
| {{Yes}}
|-
| style="text-align:left" | Unreliable transport
| {{Yes}}
| {{Yes}}
| {{No}}
| {{No}}
| {{Yes}}
| {{Yes}}
| {{Yes}}
|-
| style="text-align:left" | Preserve message boundary
| {{Yes}}
| {{Yes}}
| {{No}}
| {{No}}
| {{Yes}}
| {{Yes}}
| {{Yes}}
|-
| style="text-align:left" | Delivery
| Unordered
| Unordered
| Ordered
| Ordered
| Ordered / Unordered
| Unordered
| Unordered
|-
| style="text-align:left" | Data [[checksum]]
| {{Optional}}{{efn|Optional in IPv4 but compulsory in IPv6.<ref>{{Cite web|url=https://datatracker.ietf.org/doc/html/rfc8200|title=RFC 8200 |access-date=2025-06-15 |quote=Unlike IPv4, the default behavior when UDP packets are originated by an IPv6 node is that the UDP checksum is not optional.}}</ref>}}
| {{Yes}}
| {{Yes}}
| {{Yes}}
| {{Yes}}
| {{Yes}}
| {{Optional}}
|-
| style="text-align:left" | Checksum size
| 16 bits
| 16 bits
| 16 bits
| 16 bits
| 32 bits
| 16 bits
| 16 bits
|-
| style="text-align:left" | Partial [[checksum]]
| {{No}}
| {{Yes}}
| {{No}}
| {{No}}
| {{No}}
| {{Yes}}
| {{No}}
|-
| style="text-align:left" | Path [[Maximum transmission unit|MTU]]
| {{No}}
| {{No}}
| {{Yes}}
| {{Yes}}
| {{Yes}}
| {{Yes}}
| {{dunno}}
|-
| style="text-align:left" | [[Flow control (data)|Flow control]]
| {{No}}
| {{No}}
| {{Yes}}
| {{Yes}}
| {{Yes}}
| {{No}}
| {{Yes}}
|-
| style="text-align:left" | [[Congestion control]]
| {{No}}
| {{No}}
| {{Yes}}
| {{Yes}}
| {{Yes}}
| {{Yes}}
| {{dunno}}
|-
| style="text-align:left" | [[Explicit Congestion Notification]]
| {{No}}
| {{No}}
| {{Yes}}
| {{Yes}}
| {{Yes}}
| {{Yes}}
| {{dunno}}
|-
| style="text-align:left" | Multiple [[Stream (computing)|streams]]
| {{No}}
| {{No}}
| {{No}}
| {{No}}
| {{Yes}}
| {{No}}
| {{No}}
|-
| style="text-align:left" | [[Multi-homing]]
| {{No}}
| {{No}}
| {{No}}
| {{Yes}}
| {{Yes}}
| {{No}}
| {{No}}
|-
| style="text-align:left" | Bundling / [[Nagle's algorithm|Nagle]]
| {{No}}
| {{No}}
| {{Yes}}
| {{Yes}}
| {{Yes}}
| {{No}}
| {{dunno}}
|}
{{notelist}}

=== {{Anchor|COMPARISON2}}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:<ref>{{cite web
| title = ITU-T Recommendation X.224 (11/1995) ISO/IEC 8073
| url = http://www.itu.int/rec/T-REC-X.224-199511-I/en/
| website = Itu.int
| access-date=2017-01-17
}}</ref>

{| class="wikitable"
|-
! scope="col" | Service
! scope="col" | TP0
! scope="col" | TP1
! scope="col" | TP2
! scope="col" | TP3
! scope="col" | 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 [[Protocol data unit|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.<ref>{{cite web
| title = ITU-T Recommendation X.234 (07/1994) ISO/IEC 8602
| url = http://www.itu.int/rec/T-REC-X.234-199407-I/en/
| website = Itu.int
| access-date=2017-01-17
}}</ref>

==References==
{{Reflist}}

==Bibliography==
* {{ cite web | url = https://lwn.net/Articles/745590/ | title = QUIC as a solution to protocol ossification | date = 29 January 2018 | last = Corbet | first = Jonathan | work = [[LWN.net]] }}
* {{ cite journal | doi = 10.1109/COMST.2016.2626780 | title = De-Ossifying the Internet Transport Layer: A Survey and Future Perspectives | date = 2017 | journal = [[IEEE Communications Surveys & Tutorials]] | last1 = Papastergiou | first1 = Giorgos | last2 = Fairhurst | first2 = Gorry | last3 = Ros | first3 = David | last4 = Brunstrom | first4 = Anna | last5 = Grinnemo | first5 = Karl-Johan | last6 = Hurtig | first6 = Per | last7 = Khademi | first7 = Naeem | last8 = Tüxen | first8 = Michael | last9 = Welzl | first9 = Michael | last10 = Damjanovic | first10 = Dragana | last11 = Mangiante | first11 = Simone | volume = 19 | pages = 619–639 | hdl = 2164/8317 | s2cid = 1846371 | hdl-access = free }}

{{Wikiversity | Transport layer}}

[[Category:OSI model]]