Length

This is a 16 bit  field  that  defines  the total  length  of the user  datagram, header  plus data. This 16 bit  can define  a total  length  of a 0 to 65, 535 bytes. However the  total  length  needs  to be much  less  because  a UDP is stored in an IP datagram with a total  length  of 65, 535 bytes,

The  length  fields in a UDP user datagram is actually not  necessary. A user datagram  is encapsulated in an IP datagram. There is  a fields  in the IP datagram that defined the total  length. There is another  fields in the IP datagram  that defines  the length  the header. So  if we subtract the value of the second field from  the first  we can find the  length  of a UDP  Dataram that is  encapsulated in IP datagram.

UDP length = IP length - IP header s length

However the designers of the UDP protocols  felt  that it  was more efficient  for the  destination  UDP to calculate the length  of the data  from the  information  provided  in the  UDP user  datagram rather than ask the IP software to supply  this information. We should  remember  that when  the IP  software  delivers the UDP user datagram to the UDP layer. It has  already dropped  the IP header.