Connectionless Multiplexing  and De multiplexing

Java program running in a host can create a UDP socket  with the  line

Datagram's socket my socket =  new datagram's socket

When a UDP socket  is created in this  manner the transport  layer  automatically assigns  a port  number to the socket.  In particular  the transport  layer assigns a port  number in the range  1024 to 65535 that  is currently not being used  by any  other UDP  port in the  host.  Alternatively a java  program could create  a socket  with the line.

Datagram socket  my socket =  new  data grams socket 9157

In this  cases the  application assigns a specific port number namely 9157 to the UDP  socket. If  the application  developer writing the code  were implementing the server side  of a well known protocol. Then  the developer would have  to assign the corresponding well  known protocol. Then  the developer  would  have t assign the corresponding  well known  port number typically  the client  side of the application. The transport  layers  automatically  assign  the port  number whereas the server side  of the application assigns a specific  port number

Suppose a process in host a with  USP  port 9157  wants  to send  a chunk of  applications data to  process will UDP  post 64281 in host B. The  transport layer in host  a creates a transport layer segment that  includes the application data the source port number 9157 the destination port  number 64281 and  two other values. The  transport  layer than  passes the resulting segment to the network layer. The  network layer encapsulates the segment in an IP datagram and makes a best effort attempt to deliver the segment to the receiving  host. If  the segment  arrives at the receiving host. B could be running  receiving host examines the destination port number in the segment 64281 and delivers  the segment to its socket identified by port 64281. Note that host B could be  running  multiple processes  each with  it own UDP socket  and associated  port  number. As USP segments  arrive from  the network host B  directs  each segment the  appropriate socket by examining  the segment  destination port number.

It is  important  to note  that UDP  socket is fully  identified by a two  tuple  consisting of a  destination IP address  and destination port number  as a consequence if two USP segments  have different  source  IP addresses  and source port numbers. But  have the same  destination IP address and destination port  number  then the two  segments  will be directed to the  same destination  process via the  same  destination socket.

The  purpose of  source  port number is shown in figure 44. In A to B segment  the source  port number solves as part of a return address when B wants to send a segment back  to A the destination port in  the B to A  segment will take  its vale from  the source  port value  of the A to B segment. In UDP server layer  servers a method to extract the  port number  form the  segment it receives from  the client it the sends a new  segment  to the  client  with  the extracted source  port number  serving as the destination  port number  in this  new  segment.