What is Path MTU and How it is used ?
Q 1) What is Path MTU?
A) Path MTU is the lowest MTU of the Path. From Source to destination, the router whose MTU is lowest is considered as the Path MTU It is applicable only for Routers and not for end PC's
E.g. 1
PC1--Eth0(MTU1500)----------- (MTU 1500 Eth0) Router1--(ETH1 MTU1500)------------------(ETH0 MTU1000)–PC2 ( Eth1 MTU1500),
In above case PC2 will receive the packets up to its MRU value, if mru value is greater then the MTU value it will receive the packet and fragment it and send back according to its own mtu. Generally mtu=mru , depends on vendors.
E.g. 2
PC1—1500---------1500-----Router1---1000------------1000--PC2
a) In this case : normal scenario PC1 sends 1500 packets to PC2, the router second interface fragment the packet send to pc2, pc2 will send back the reply, the path mtu is 1000.
b) in this case if PC1 send 1500 packets with DF bit set, -f option in icmp packet. The router see the other interface mtu which is 1000, as it cannot fragment the packet it will send icmp destination unreachable message with next hop MTU as 1000 to PC1. Now the PC1 knows that the least MTU is 1000 for the same source and destination. It will add one static route with destination 255.255.255.255 at that interface. Now if u ping without DF bit set also, the PC1 will itself fragment the packet with 1000 bytes and send to PC2..
This static route can be seen in PC as route print. If we disable and enable the interface this route gets cleared. Also the path mtu will be one set of source, destination and the TOS value, unique to this path, when the path mtu is discovered.
It is defined in RFC 1191 for IPv4.
http://www.faqs.org/rfcs/rfc1191.html
