Internet Protocol Analysis - Transport Layer

computer network A computer network is a set of computers sharing resources located on or provided by network nodes. The computers use common communication protocols over digital interconnections to communicate with each other. These interconnections are ...

ing, the transport layer is a conceptual division of methods in the layered architecture of protocols in the network stack in the

Internet protocol suite 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 sui ...

and the OSI model. The protocols of this layer provide end-to-end communication services for applications. It provides services such as

connection-oriented communication Connection-oriented communication is a network communication mode in telecommunications and computer networking, where a communication session or a semi-permanent connection is established before any useful data can be transferred. The establishe ...

reliability Reliability, reliable, or unreliable may refer to: Science, technology, and mathematics Computing * Data reliability (disambiguation), a property of some disk arrays in computer storage * High availability * Reliability (computer networking), a ...

, flow control, and multiplexing. The details of implementation and semantics of the transport layer of the

, which is the foundation of the

Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a '' network of networks'' that consists of private, pub ...

, 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, 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: *

Connection-oriented communication Connection-oriented communication is a network communication mode in telecommunications and computer networking, where a communication session or a semi-permanent connection is established before any useful data can be transferred. The establishe ...

: It is normally easier for an application to interpret a connection as a

data stream In connection-oriented communication, a data stream is the transmission of a sequence of digitally encoded coherent signals to convey information. Typically, the transmitted symbols are grouped into a series of packets. Data streaming has b ...

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 Head-of-line blocking (HOL blocking) in computer networking is a performance-limiting phenomenon that occurs when a line of packets is held up in a queue by a first packet. Examples include input buffered network switches, out-of-order delivery a ...

. *

Reliability Reliability, reliable, or unreliable may refer to: Science, technology, and mathematics Computing * Data reliability (disambiguation), a property of some disk arrays in computer storage * High availability * Reliability (computer networking), a ...

: Packets may be lost during transport due to

network congestion Network congestion in data networking and queueing theory is the reduced quality of service that occurs when a network node or link is carrying more data than it can handle. Typical effects include queueing delay, packet loss or the blocking of ...

and errors. By means of an error detection code, such as a

checksum A checksum is a small-sized block of data derived from another block of digital data for the purpose of detecting errors that may have been introduced during its transmission or storage. By themselves, checksums are often used to verify data ...

, the transport protocol may check that the data is not corrupted, and verify correct receipt by sending an ACK or

NACK Nack is an ''Ortsgemeinde'' – a municipality belonging to a ''Verbandsgemeinde'', a kind of collective municipality – in the Alzey-Worms district in Rhineland-Palatinate, Germany. Geography Location The municipality lies in Rhenish Hesse ...

message to the sender. Automatic repeat request schemes may be used to retransmit lost or corrupted 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 In computer science, a data buffer (or just buffer) is a region of a memory used to temporarily store data while it is being moved from one place to another. Typically, the data is stored in a buffer as it is retrieved from an input device (such a ...

, causing a buffer overrun. This can also be used to improve efficiency by reducing buffer underrun. * Congestion avoidance:

Congestion control Network congestion in data networking and queueing theory is the reduced quality of service that occurs when a network node or link is carrying more data than it can handle. Typical effects include queueing delay, packet loss or the blocking of ...

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 link 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 start. This keeps the bandwidth consumption at a low level in the beginning of the transmission, or after packet retransmission. * Multiplexing:

Ports A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as H ...

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 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 suit ...

, but of the

session layer In the seven-layer OSI model of computer networking, the session layer is layer 5. The session layer provides the mechanism for opening, closing and managing a session between end-user application processes, i.e., a semi-permanent dialogue. ...

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 Statistical multiplexing is a type of communication link sharing, very similar to dynamic bandwidth allocation (DBA). In statistical multiplexing, a communication channel is divided into an arbitrary number of variable bitrate digital channels or ...

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 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 programmin ...

, i.e. an identification address of the process-to-process communication. In the OSI model, this function is supported by the

. Some transport layer protocols, for example TCP, but not UDP, support

virtual circuit A virtual circuit (VC) is a means of transporting data over a data network, based on packet switching and in which a connection is established within the network between two endpoints. The network, rather than having a fixed data rate reservation ...

s, i.e. provide

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 The Hypertext Transfer Protocol (HTTP) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide We ...

web browsing and email transfer. UDP may be used for

multicast In computer networking, multicast is group communication where data transmission is addressed to a group of destination computers simultaneously. Multicast can be one-to-many or many-to-many distribution. Multicast should not be confused with ...

ing and

broadcasting Broadcasting is the distribution of audio or video content to a dispersed audience via any electronic mass communications medium, but typically one using the electromagnetic spectrum ( radio waves), in a one-to-many model. Broadcasting beg ...

, since retransmissions are not possible to a large amount of hosts. UDP typically gives higher

throughput Network throughput (or just throughput, when in context) refers to the rate of message delivery over a communication channel, such as Ethernet or packet radio, in a communication network. The data that these messages contain may be delivered ove ...

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 X.25 is an ITU-T standard protocol suite for packet-switched data communication in wide area networks (WAN). It was originally defined by the International Telegraph and Telephone Consultative Committee (CCITT, now ITU-T) in a series of drafts ...

Frame Relay Frame Relay is a standardized wide area network (WAN) technology that specifies the physical and data link layers of digital telecommunications channels using a packet switching methodology. Originally designed for transport across Integrated Se ...

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.

Protocols

This list shows some protocols that are commonly placed in the transport layers of the

, the

OSI protocol suite The Open Systems Interconnection model (OSI model) is a conceptual model that 'provides a common basis for the coordination of SOstandards development for the purpose of systems interconnection'. In the OSI reference model, the communications ...

, NetWare's

IPX/SPX IPX/SPX stands for Internetwork Packet Exchange/Sequenced Packet Exchange. IPX and SPX are networking protocol, networking protocols used initially on networks using the (since discontinued) Novell NetWare operating systems. They also became wid ...

, AppleTalk, and Fibre Channel. * ATP, AppleTalk Transaction Protocol * CUDP, Cyclic UDP * DCCP, Datagram Congestion Control Protocol * FCP,

Fibre Channel Protocol Fibre Channel Protocol (FCP) is the SCSI interface protocol utilising an underlying Fibre Channel connection. The Fibre Channel standards define a high-speed data transfer mechanism that can be used to connect workstations, mainframes, supercomput ...

* IL, IL Protocol * MPTCP,

Multipath TCP Multipath TCP (MPTCP) is an ongoing effort of the Internet Engineering Task Force's (IETF) Multipath TCP working group, that aims at allowing a Transmission Control Protocol (TCP) connection to use multiple paths to maximize throughput and inc ...

* NORM, NACK-Oriented Reliable Multicast * RDP, Reliable Data Protocol * RUDP, Reliable User Datagram Protocol * SCTP, Stream Control Transmission Protocol * SPX,

Sequenced Packet Exchange Sequenced Packet Exchange (SPX) is a protocol in the IPX/SPX protocol stack that corresponds to a connection-oriented transport layer protocol in the OSI model. Being reliable and connection-oriented, it is analogous to the Transmission Control Pr ...

* SST, Structured Stream Transport * TCP, Transmission Control Protocol * UDP, User Datagram Protocol *

UDP-Lite UDP-Lite (Lightweight User Datagram Protocol) is a connectionless protocol that allows a potentially damaged data payload to be delivered to an application rather than being discarded by the receiving station. This is useful as it allows decisi ...

* µTP, Micro Transport Protocol

Comparison of Internet transport layer protocols

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

References

{{Wikiversity , Transport layer OSI model