Transport Layer

In computer networking it the transport  layer is where  sessions are  exchanged between  hosts. This layer resides  between  the application layer and  network layer the transport layer is  a central  piece of the layered architecture. It has  the critical  role  of providing  communication services directly  to the application processes running on  different hosts. A transport  layer protocol  provides  logical  communication  between  application  processes running  on different hosts. Figure  illustrates the relationship  of the network transport  and application  layers  and the notation of logical  communication. Transport  layer  protocols  are implemented in the  end systems  but not  in network  routers. On the sending side  the  transport  layer converts the messages receives from  a sending application process into  smaller chunks  and adding a transport layer header to each chunk to create the transport  layer  segment. The  transport  layer then passes the segment to the  network  layer at  sending  end system where the segment is encapsulate d within  a network  layer packet  a datagram  and sent to the destination . it  is important to note that network routes act  only on the network  layer segment encapsulated with the datagram. On  the receiving side  the  network  layer extracts the transport layer segment from the  datagram  and passes the  segment up to the  transport layer the transport layer then processes the  received  segment making available the data in  the segment  making available the data  in the  segment  to the receiving application. 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  is provided with the simplest  way of  doing this is to  give  each packet  a number and allow  the receiver to recorder the packet . the  transport   layer provides some other  services   like byte  orientation flow  control  reliable data and ports.

The  byte  orientation  services provides  rather  than dealing  with things  on a packet by packet  basis. The transport  layer may  add be ability  to view communication just  as a stream of bytes. This is nice  to deal  with than  random  packet  sizes  however it  rarely  matches  the communication  model  which will  normally  be a  sequence of message of use defined sizes.

The  amount  of memory  on a  computer 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 time it was more important.

Reliable  data packets may be lost in routers switches bridges and hosts due to  network congestion when  the packet  queers are filled and the network  nodes  have to  delete  packets. Packets  may be lost of corrupted in Ethernet due to interference and  noise since  Ethernet doesn't  retransmit corrupted packets. Packets may be delivered in the wrong  order by an  underlying network some  transport layer protocol for e, g, TCP can fix all these  problems  by means  of error  detection code  for e, g,  checks acknowledgment message, automatic  repeat request  and segment  numbering of course  error  free is impossible  but it  possible to substantially reduce the numbers  of undeleted  error.

Ports  are essentially  ways to address multiple entities in the same  location. For e ,g,  the first line of a postal address is a kind  of port  and distinguishes between  different  occupants of the same house. computer applications will listen to each  use for  information on their  own ports i ,e one can  use more  than one  network  based applications at the same  time.

Transport  layer  lies just  above the network  layer in  protocol  stack whereas a transport layer protocol provides logical  communication  between  running  on different  hosts  a network  layer  provides  logical  communication  between  hosts.

More than one transport  layer  protocols  may be available to network  applications. For  example  the internet  has two  protocols  transmission  control protocol  and user datagram  protocol  each of there  protocols provides  a different  set of  transport layer  services  to the  invoking application.

The ultimate goal  of the  transport  layer is  provide  efficient  reliable and cost  effective  services  to its  users  normally  processes in the application layer. To  achieve this goal  the transport  layer  make use of the  services  provided  by the  network layer.