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What is a transport layer?

Module by: Brandon Hodgson

What is a transport layer?

The transport layer is layer four of the seven layer OSI model (Wikipedia 2006). It responds to service requests from the session layer and issues service requests to the network layer (Wikipedia 2006).

"The transport layer provides transparent transfer of data between hosts. It is usually responsible for end-to-end error recovery and flow control, and ensuring complete data transfer. In the Internet protocol suite this function is achieved by the connection oriented Transmission Control Protocol (TCP). (The datagram-type transport, User Datagram Protocol (UDP), provides neither error recovery, nor flow control, leaving these to the application.) " (Wikipedia 2006)

The purpose of the Transport layer is to provide transparent transfer of data between end users, thus relieving the upper layers from any concern with providing reliable and cost-effective data transfer (Wikipedia 2006).

The transport layer usually turns the unreliable and very basic service provided by the Network layer into a more powerful one (Wikipedia 2006). There is a long list of services that can be optionally provided at this level (Wikipedia 2006). None of them are compulsory, because not all applications want all the services available (Wikipedia 2006). Some can be wasted overhead, or even counterproductive in some cases (Wikipedia 2006). These services are summarised by (Wikipedia 2006) as:

  • Connection-Oriented. This is normally easier to deal with than connection-less models, so 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.
  • Reliable Data. The underlying network may well be noisy, and the data received may not always be the same as the data sent. The Transport layer can fix this: typically by providing a checksum of the data which detects if there has been a glitch of some kind. Of course, error free is impossible, but it is possible to substantially reduce the numbers of undetected errors. This layer may also retransmit packets which have gone missing en route.
  • 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 say "Whoa!" before it is overwhelmed. Sometimes this is already provided by the network, but where it is not, the Transport layer may add it on.
  • 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.
  • Ports. Ports are essentially ways to address multiple entities in the same location. 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.

Some things, such as connection orientation can be implemented at either Transport or Network layer (Wikipedia 2006). The idea is that the Network layer implements whatever set of options is easiest: for some underlying networks it is easiest to implement connectionless communication, while for others it is easiest to implement connection oriented communication (Wikipedia 2006). The Transport layer uses this simplest set of options to implement whatever combinations of options are actually desired (Wikipedia 2006).

Examples: These examples are extracted from (Wikipedia 2006).

  • AppleTalk Echo Protocol (AEP)
  • AppleTalk Transaction Protocol (ATP)
  • CUDP
  • IL
  • NBP
  • NetBEUI
  • RTMP
  • SPX
  • TCP
  • UDP
  • SCTP
  • RTP

References:

Wikipedia. "Transport layer", Wikimedia Foundation Inc,  http://en.wikipedia.org/wiki/Transport_layer, Last accessed 14 February 2006.

Brandon Hodgson

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