Advanced Computer Networks Notes-5

This article contains explanation about address resolution problem,two types of physical address,resolution through direct mapping and dynamic mapping,ARP,ARP refinements.

Mapping Internet Addresses to Physical Addresses (ARP)

The address Resolution Problem

• Supppose there are two sytems A and B in the same physical network.P_A and P_B be their physical address and I_A and I_B be their internet addresses.


• Suppose A has to send data to B and A knows B's internet address I_B only.


• In such case A has to map B's physical address from the internet address I_B.


• Otherwise it may have to send the data through the internet using B's IP address which is time consuming and costly affair as compared to sending data through the physical network using physical address.


• Thus there is a greater need for mapping a system's physical address from its internet address.


• The problem of mapping high-level address to physical address is known as address resolution problem.

Two types of Physical Addresses

There are two basic types of physical addresses, exemplified by the Ethernet,
which has large, fixed physical addresses, and proNET, which has small, easily configured
physical addresses. Address resolution is difficult for Ethernet-like networks, but
easy for networks like proNET.

Resolution through Direct Mapping

• This method is mainly used in proNets.


• In proNets user is allowed to select physical address of the system.


• Typically, one assigns IP addresses with the hostid portion equal to 1, 2, 3, and so on, and then, when installing network interface hardware, selects a physical address that corresponds to the IP address. For example, the system administrator would select physical address 3 for a computer with the IP address 192.5.48.3 because 192.5.48.3 is a class C address with the host portion equal to 3.



• Thus extracting physical address from internet address is easy and it would be equal to the hostid portion of the IP address.


• Thus mapping is easy to maintain and does not require external reference.

Resolution through dynamic binding

• This method is mainly used in ethernet technology


• In ethernet technology since physical addresses are 48 bit long.It can't be encoded with 32 bit IP address.



• A low level protocol ARP(address resolution protocol) is used in ethernet technology for solving address resolution problem


• ARP working:
  
  
  
  • Supppose there are two sytems A and B in the same physical network.P_A and P_B be their physical address and I_A and I_B be their internet addresses.
  
  
  
  • A knows B's internet address I_B only.
  
  
  • So A broadcasts a special packet that asks the host with IP address I_B to respond with its physical address P_B.
  
  
  • All hosts including B receives the request.But only B recognizes its IP address and sends a reply that contains its physical address.
  
  
  
  • When A receives the reply.it uses the physical address to send send the internet packet direct to B.
  
  
  • The Address Resolution Protocol, ARP, allows a host to find the physical address of a target host on the same physical network, given only the target's IP address.
  
  
  
  
  
  
  
  
  
  

Address Resolution Cache and ARP Cache Timeout

• Generally the process of sending and receiveing data continues for a long time between users.


• To avoid sending ARP request again amd again , each hosts maintains a list of mapping between obtained ohysical address and its corresponding IP address.This list is called the ARP cache.


• Suppose a sender is sending data to a host whose physical address is in the cache.After sometimes the receiver fails.Since ethernet is a best delivery system sender doesn't know that the receiver has become inactive and keeps sending the data.


• To avoid this scenario the cache is updated after certain delay by sending ARP message again.This delay is called ARP cache timeout.

ARP Refinements

• While sending ARP request host sends its physical address also with the message,which is saved by the receiver in its cache.


• ARP message is broadcasted to all the stations so that all the stations can update the sender's details in the cache and but only the receiver whose address is specified in the ARP message replies.


• During interface change or reboot the host broadcasts its new physical and IP address to all the stations so that they can update their cache.