Maximum Transfer Unit(MTU):

It is the maximum size of data that can be supported in a network. If the size of packet happens to be more than MTU, data packet is fragmented.

Path MTU detection:

• It’s impossible to know the MTU of each link through which a packet might travel.
• Path MTU detection is a simple process through which a host can detect a path MTU smaller than its interface MTU.
• Two components are key to this process: the Don’t Fragment (DF) bit of the IP header, and a subcode of the ICMP Destination Unreachable message, Fragmentation Needed.

How does it work:

• Host A send an IP packet with FD flag set and size 1500, so once the router R1 receives the packet and sees that the size of packet is more than MTU and DF is 1, it just discards the packet and sends a ICMP error message that Destination Unreachable with type 4 that is (The datagram is too big.Packet fragmentation is required but the DF bit in the IP header is set). This message also contains MTU supported(say: 1400).
• So once the error message is received by the HOST A, it understands that the MTU is lesser than 1500, so it lower the size of packet smaller than 1500. It sends the second packet with DF set to 1 and size 1400.
• So now it passes the router R1 and now the router R2 again sends the same error message and sends its MTU(say:1300).
• The process is repeated until the MTU is small enough to traverse the entire path without fragmentation.

Categories: Networking