What is Transport Layer(Layer 4)?

Ø  Responsible for end-to-end transportation of data between the applications or processes or services.

Ø  The major functions described at transport layer are...

·     Identifying services

·     Multiplexing and De-multiplexing

·     Segmentation

·      Sequencing & Reassembling

·     Error control

·     Flow control

(a)Identification of Services:

Ø  Identification of services is done using port numbers.  

Ø  Port is a logical communication channel.

Ø  Port number is a 16 bit identifier 

Ø  Total number of ports         0 -- 65535

Ø  Reserved ports                      1 -- 1023

Ø  Unreserved ports                  1024 – 65535

Ø  The major protocols which takes care of data transportation at transport layer are...TCP, UDP

TCP	UDP
·     Transmission Control Protocol ·     Connection-oriented ·     Reliable Communication(With ACK’s) ·     Slower data transportation ·     Protocol No. is 6 ·     Ex: HTTP, FTP, SMTP etc	·     User Datagram Protocol ·     Connection less ·     Unreliable Communication(No ACK’s) ·     Faster data transportation ·     Protocol No. is 17 ·     Ex: DNS,DHCP,TFTP etc

(b)Segmentation:

Ø  It is the process of dividing one single message into multiple small pieces known as segments

Ø  Because default MTU value is 1500 bytes for Ethernet , then anything send over 1500 bytes, it will be automatically segmented into small message.

Hello

How

Are

You

                                       1 /4                     2/4                          3 /4                  4/4       

(c)Sequencing:

Ø  Adding the sequence number / label to identify particular segment.

Ø  It defines the order of information. why because there is a possibility that, some of segments can go from one path, other segments might be going from other path ( i.e. sending the segments all possible routes) 

(d)Reassembling:

Ø  once the segments are received on destination, it will rearrange or reassembles the segments in proper order with the help of sequence number

(e)Multiplexing (many to one):

Ø  To combine multiple signals for transmission over a single line or media.

Ø   Transport layer combines the data coming from different applications(processes) and send this data to lower layer and finally at the receiving end data is splitted and forwarded to different applications or processes.

Ø  One user is sending request to Google server, one user is sending request to Yahoo server and one user is sending request to Gmail server then these services / processes/ applications are differentiated by using some frequencies and time i.e. FDM ,TDM etc     

(f)De-multiplexing :(One to many):

Ø  At the receiver site, relationship is one to many.

Ø  The transport layer receives datagrams from network layer.

Ø  After error checking and dropping of header, transport layer delivers each message to appropriate process based on port number 

(g)Flow Control:

Ø  It defines the amount of data a source can send before receiving acknowledgement from destination

Ø  The flow control ensures that sender is not sending the data at the speed that the receiver cannot process

Ø  There is something called windowing flow control where sender is going to send three different segments and receiver will acknowledge.

Ø  There is common Windows size equal to 3.

Ø  if I try to send 5 segments at a time then receiver will tell that I can receive only 3 segments, so next time sender will send 3 segments only          

(h)Error control:

Ø  Error control at transport layer includes mechanisms for detecting corrupted segments or lost segments, out of order segments, duplicate segment.

Ø  Thus it will add reliability.