Transport Layer

OSI Model
7	Application Layer
6	Presentation Layer
5	Session Layer
4	Transport Layer
3	Network Layer
2	Data Link Layer LLC sublayer MAC sublayer
1	Physical Layer

In computer networking, the Transport Layer provides end-to-end communication services for applications^[1] 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),^[2] 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).

Internet Protocol Suite
Application Layer
BGP · DHCP · DNS · FTP · HTTP · IMAP · IRC · LDAP · MGCP · NNTP · NTP · POP · RIP · RPC · RTP · SIP · SMTP · SNMP · SSH · Telnet · TLS/SSL · XMPP · (more)
Transport Layer
TCP · UDP · DCCP · SCTP · RSVP · ECN · (more)
Internet Layer
IP (IPv4, IPv6) · ICMP · ICMPv6 · IGMP · IPsec · (more)
Link Layer
ARP/InARP · NDP · OSPF · Tunnels (L2TP) · PPP · Media Access Control (Ethernet, DSL, ISDN, FDDI) · (more)

Contents 1 Overview 2 Services 3 Protocols 4 Comparison of TCP/IP transport protocols 5 Comparison of OSI transport protocols 6 References

Overview

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, 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.

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.

OSI Model
7	Application Layer
6	Presentation Layer
5	Session Layer
4	Transport Layer
3	Network Layer
2	Data Link Layer LLC sublayer MAC sublayer
1	Physical 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 require all available services.

• Connection-oriented: Interpreting the connection as a data stream can provide many benefits. It is normally easier to deal with than the underlying connection-less models, such as the Internet Protocol model of datagrams. In general, 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.
• Reliability: 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 respond 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: Ports provide multiplexing. 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. It is part of the Transport Layer in the TCP/IP model, but of the Session Layer in the OSI model.

Protocols

The exact definition of what qualifies as a transport layer protocol is not firm. The following is a short list:

• ATP, AppleTalk Transaction Protocol
• CUDP, Cyclic UDP
• DCCP, Datagram Congestion Control Protocol
• FCP, Fiber Channel Protocol
• IL, IL Protocol
• NBF, NetBIOS Frames protocol
• SCTP, Stream Control Transmission Protocol
• SPX, Sequenced Packet Exchange
• SST, Structured Stream Transport
• TCP, Transmission Control Protocol
• UDP, User Datagram Protocol
• UDP Lite
• µTP, Micro Transport Protocol

Comparison of TCP/IP transport protocols

Service / Attribute	UDP	TCP	DCCP	SCTP
Packet header size	8 Bytes	20-60 Bytes	12 or 16 bytes	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	No	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
NAT friendly	Yes	Yes	Yes	No

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:^[3]

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

References