Two machines on a physical network can communicate only if they know ech other’s physical network address.Address resolution protocol is the resolution through dynamic binding.For example say Host A want to communicate with host B,then Host A sends ARP request with ipaddress of Host B,to all machines,Inresponse to tat only Host B responds with pair IpAddress of B(32 bit) and Mac address of B(48bit).To reduce communication costs ,computers that use ARP maintain a cache of recently acquired IP to physical address bindings.
Soft state is one common situation in network communication.It happens as say two computers A and B both connected to ethrnet.Assume A has send ARP request and B has replied.Assume after exchange of ARP b crashes
Computer A will not have any information about crash and so it will continue to send packets to B.Thus A has no way of knowing when the information in its ARP cache has become incorrect.
Protocols that implements soft state use timers with the state information being deleted when the timer expires.So whenever address binding information is placed in an ARP cache the protocl requires a timer to set with the typical timeout being 20minutes.when the timer expires the information from arp cache must be removed.
The familiar terms are mentioned below in case of ARP:-
Prxy Arp-Proxy ARP lets router answer ARP requests on one of its networks for a host on another of its networks.this fools the sender of the ARP request into thinking that the router is the destination,when in fact the destination host is “on the other side” of the router.
Gratious Arp-when a host sends an ARP request looking for its own IP address.This is usually done when the interface is configured at bootstrap time.
ARP is normally used on broadcast networks such as ethernet,token ring and FDDI and not on point to point networks.
RARP is Reverse address resolution protocol,it maps a hardware address into an Ip address.It is used when a diskless node is booting.