Transport layer

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The transport layer in the Internet protocol stack. Internet Protocol Analysis - Transport Layer.png
The transport layer in the Internet protocol stack.

In computer networking, the transport layer is a conceptual division of methods in the 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. [1] :§1.1.3 It provides services such as connection-oriented communication, reliability, flow control, and multiplexing.

Contents

The details of implementation and semantics of the transport layer of the Internet protocol suite, [1] 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, [2] 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).

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: [4]

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 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.

Many non-IP-based networks, such as X.25, Frame Relay and 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. [6] To avoid middlebox intolerance, new transport protocols may mimic the wire image of a tolerated protocol, or be encapsulated in UDP, accepting some overhead (e.g., due to outer checksums made redundant by inner integrity checks). [7] QUIC takes the latter approach, rebuilding reliable stream transport on top of UDP. [8]

Protocols

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

Comparison of Internet transport layer protocols

Feature UDP UDP-Lite TCP Multipath TCP SCTP DCCP RUDP [lower-alpha 1]
Packet header size8 bytes8 bytes20–60 bytes50–90 bytes12 bytes [lower-alpha 2] 12 or 16 bytes14+ bytes
Typical data-packet overhead8 bytes8 bytes20 bytes ?? bytes44–48+ bytes [lower-alpha 3] 12 or 16 bytes14 bytes
Transport-layer packet entityDatagramDatagramSegmentSegmentDatagramDatagramDatagram
Connection-orientedNoNoYesYesYesYesYes
Reliable transportNoNoYesYesYesNoYes
Unreliable transportYesYesNoNoYesYesYes
Preserve message boundaryYesYesNoNoYesYesYes
DeliveryUnorderedUnorderedOrderedOrderedOrdered / UnorderedUnorderedUnordered
Data checksum OptionalYesYesYesYesYesOptional
Checksum size16 bits16 bits16 bits16 bits32 bits16 bits16 bits
Partial checksum NoYesNoNoNoYesNo
Path MTU NoNoYesYesYesYes?
Flow control NoNoYesYesYesNoYes
Congestion control NoNoYesYesYesYes?
Explicit Congestion Notification NoNoYesYesYesYes?
Multiple streams NoNoNoNoYesNoNo
Multi-homing NoNoNoYesYesNoNo
Bundling / Nagle NoNoYesYesYesNo?
  1. RUDP is not officially standardized. There have been no standard-related developments since 1999.
  2. Excluding data chunk headers and overhead chunks. Without embedded chunks, an SCTP packet is essentially useless.
  3. 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.

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: [10]

ServiceTP0TP1TP2TP3TP4
Connection-oriented networkYesYesYesYesYes
Connectionless networkNoNoNoNoYes
Concatenation and separationNoYesYesYesYes
Segmentation and reassemblyYesYesYesYesYes
Error recoveryNoYesNoYesYes
Reinitiate connection (if an excessive number of PDUs are unacknowledged)NoYesNoYesNo
Multiplexing and demultiplexing over a single virtual circuit NoNoYesYesYes
Explicit flow controlNoNoYesYesYes
Retransmission on timeoutNoNoNoNoYes
Reliable Transport ServiceNoYesNoYesYes

There is also a connectionless transport protocol, specified by ISO/IEC 8602/ITU-T Recommendation X.234. [11]

Related Research Articles

The Internet Control Message Protocol (ICMP) is a supporting protocol in the Internet protocol suite. It is used by network devices, including routers, to send error messages and operational information indicating success or failure when communicating with another IP address, for example, an error is indicated when a requested service is not available or that a host or router could not be reached. ICMP differs from transport protocols such as TCP and UDP in that it is not typically used to exchange data between systems, nor is it regularly employed by end-user network applications.

The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet.

The Internet protocol suite, commonly known as TCP/IP, is a framework for organizing the set of communication protocols used in the Internet and similar computer networks according to functional criteria. The foundational protocols in the suite are the Transmission Control Protocol (TCP), the User Datagram Protocol (UDP), and the Internet Protocol (IP). Early versions of this networking model were known as the Department of Defense (DoD) model because the research and development were funded by the United States Department of Defense through DARPA.

<span class="mw-page-title-main">OSI model</span> Model of communication of seven abstraction layers

The Open Systems Interconnection model is a conceptual model from the International Organization for Standardization (ISO) that "provides a common basis for the coordination of standards development for the purpose of systems interconnection." In the OSI reference model, the communications between systems are split into seven different abstraction layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application.

The Real-time Transport Protocol (RTP) is a network protocol for delivering audio and video over IP networks. RTP is used in communication and entertainment systems that involve streaming media, such as telephony, video teleconference applications including WebRTC, television services and web-based push-to-talk features.

The Transmission Control Protocol (TCP) is one of the main protocols of the Internet protocol suite. It originated in the initial network implementation in which it complemented the Internet Protocol (IP). Therefore, the entire suite is commonly referred to as TCP/IP. TCP provides reliable, ordered, and error-checked delivery of a stream of octets (bytes) between applications running on hosts communicating via an IP network. Major internet applications such as the World Wide Web, email, remote administration, and file transfer rely on TCP, which is part of the Transport Layer of the TCP/IP suite. SSL/TLS often runs on top of TCP.

In computer networking, the User Datagram Protocol (UDP) is one of the core communication protocols of the Internet protocol suite used to send messages to other hosts on an Internet Protocol (IP) network. Within an IP network, UDP does not require prior communication to set up communication channels or data paths.

Connectionless communication, often referred to as CL-mode communication, is a data transmission method used in packet switching networks in which each data unit is individually addressed and routed based on information carried in each unit, rather than in the setup information of a prearranged, fixed data channel as in connection-oriented communication.

<span class="mw-page-title-main">Protocol data unit</span> Unit of information transmitted over a computer network

In telecommunications, a protocol data unit (PDU) is a single unit of information transmitted among peer entities of a computer network. It is composed of protocol-specific control information and user data. In the layered architectures of communication protocol stacks, each layer implements protocols tailored to the specific type or mode of data exchange.

A virtual circuit (VC) is a means of transporting data over a data network, based on packet switching and in which a connection is first established across the network between two endpoints. The network, rather than having a fixed data rate reservation per connection as in circuit switching, takes advantage of the statistical multiplexing on its transmission links, an intrinsic feature of packet switching.

In telecommunications and computer networking, a network packet is a formatted unit of data carried by a packet-switched network. A packet consists of control information and user data; the latter is also known as the payload. Control information provides data for delivering the payload. Typically, control information is found in packet headers and trailers.

In computer networking, the Datagram Congestion Control Protocol (DCCP) is a message-oriented transport layer protocol. DCCP implements reliable connection setup, teardown, Explicit Congestion Notification (ECN), congestion control, and feature negotiation. The IETF published DCCP as RFC 4340, a proposed standard, in March 2006. RFC 4336 provides an introduction.

The Open Systems Interconnection protocols are a family of information exchange standards developed jointly by the ISO and the ITU-T. The standardization process began in 1977.

In telecommunications and computer networking, connection-oriented communication is a communication protocol where a communication session or a semi-permanent connection is established before any useful data can be transferred. The established connection ensures that data is delivered in the correct order to the upper communication layer. The alternative is called connectionless communication, such as the datagram mode communication used by Internet Protocol (IP) and User Datagram Protocol, where data may be delivered out of order, since different network packets are routed independently and may be delivered over different paths.

A network socket is a software structure within a network node of a computer network that serves as an endpoint for sending and receiving data across the network. The structure and properties of a socket are defined by an application programming interface (API) for the networking architecture. Sockets are created only during the lifetime of a process of an application running in the node.

Multipurpose Transaction Protocol (MTP) software is a proprietary transport protocol developed and marketed by Data Expedition, Inc. (DEI). DEI claims that MTP offers superior performance and reliability when compared to the Transmission Control Protocol (TCP) transport protocol.

The internet layer is a group of internetworking methods, protocols, and specifications in the Internet protocol suite that are used to transport network packets from the originating host across network boundaries; if necessary, to the destination host specified by an IP address. The internet layer derives its name from its function facilitating internetworking, which is the concept of connecting multiple networks with each other through gateways.

A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any variation of a physical quantity. The protocol defines the rules, syntax, semantics, and synchronization of communication and possible error recovery methods. Protocols may be implemented by hardware, software, or a combination of both.

The Stream Control Transmission Protocol (SCTP) is a computer networking communications protocol in the transport layer of the Internet protocol suite. Originally intended for Signaling System 7 (SS7) message transport in telecommunication, the protocol provides the message-oriented feature of the User Datagram Protocol (UDP), while ensuring reliable, in-sequence transport of messages with congestion control like the Transmission Control Protocol (TCP). Unlike UDP and TCP, the protocol supports multihoming and redundant paths to increase resilience and reliability.

References

  1. 1 2 R. Braden, ed. (October 1989). Requirements for Internet Hosts -- Communication Layers. Network Working Group. doi: 10.17487/RFC1122 . STD 3. RFC 1122.Internet Standard 3. Updated by RFC  1349, 4379, 5884, 6093, 6298, 6633, 6864, 8029 and 9293.
  2. "Introducing the Internet Protocol Suite". System Administration Guide, Volume 3.
  3. "X.225 : Information technology – Open Systems Interconnection – Connection-oriented Session protocol: Protocol specification". Archived from the original on February 1, 2021. Retrieved March 10, 2023.
  4. "Transport Layer" (PDF). Galgotias University.
  5. Heena, Khera. "Data Communication and networking" (PDF). Galgotias University. p. 9.
  6. Papastergiou et al. 2017, p. 620-621.
  7. Papastergiou et al. 2017, p. 623-624.
  8. Corbet 2018.
  9. Brian C. Smith, Cyclic-UDP: A Priority-Driven Best-Effort Protocol (PDF), retrieved February 23, 2020
  10. "ITU-T Recommendation X.224 (11/1995) ISO/IEC 8073". Itu.int. Retrieved January 17, 2017.
  11. "ITU-T Recommendation X.234 (07/1994) ISO/IEC 8602". Itu.int. Retrieved January 17, 2017.

Bibliography