This Blog post is explain what is IPv4/IPv6 Fragmentation and Path MTU process.
Why IPv4/IPv6 Fragmentation Require
When Frame/Packet travels in ethernet between wires its size should be according to Ethernet Card which is generaly 1500 bytes. Our TCP/UDP or any Transport layer and Network Layer IP packets are large to fit with Ethernet. so, IP layer fragment IPv4/IPv6 packet to MTU means Ethernet Frame size. For IPv4 there is offset and ID on each packet to identify whole IPv4 packet from fragments. And in IPv6 there is Fragment Header to identify fragments with whole IPv6 Packet
What is Path MTU
To find size of Network Card in network to send packet, Network Stack sends IPv4/IPv6 packets with IPv4 Don’t Fragment bit set or IPv6 Fragment header with More Fragment bit to zero in packet with different size which is trial and error because in network other device network card size is not know. When packet is big and not fit which Ethernet Frame or MTU size (MTU is nothing but Maximum Transfer Unit means max frame size supported by other device) then it returns ICMPv4(IPv4) Error Message (Type 3 & Code 4) saying that Fragmentation require Don’t fragment bit set. And in case of IPv6 which is ICMPv6 error message Packet Too Big (Type 2). This sequence continue until right size of MTU not found. This whole process call Path MTU detection process.
To know More About TCP/IP Stack Refer below Book (Free online reading at http://www.tcpipguide.com/free/index.htm)
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