Here’s a reminder of all the fields and their order, with fragmentation headers highlighted:įragmentation’s operation relies upon three IP header fields (32 bits in total), all of which will have very different values in the fragments compared to the original packet: The processes of fragmentation and reassembly involve a number of IP header fields being set in the fragments. Fragmentation may result in out of order packet delivery and the need for reordering (especially if only some packets are fragmented or if link aggregation or other path splitting technologies are in use).NAT functionality) that rely on inspecting those headers. Only the first fragment contains the high layer headers which can cause issues with firewalls, middle-boxes and routers (i.e.The loss of a single fragment results in all the fragments having to be resent where a reliable transport layer protocol such as TCP is in use (in fact the sender resends one packet and fragmentation occurs once again).IP And higher layer protocols to work across variable and diverse network paths and mediums without the need and overhead of a path discovery protocol (but see the PMTUD section).įragmentation has a number of drawbacks which result in it’s use being avoided where possible, primarily:.

Transport layer protocols to be ignorant of the underlying network architecture, reducing overheads.As I’ve made clear, its the data within it that’s fragmented – the original packet is actually discarded. Typically the language used when discussing fragmentation implies the original packet itself is fragmented. This is called Fragmentation and the data in these fragments is then typically reassembled when they reach their destination. This means it is broken into pieces carried within new packets (fragments) that are equal to or smaller than the lower MTU. In these cases, if the packet size exceeds the lower MTU the data in the packet must be fragmented (if possible). However, packets that are initially transmitted over a network supporting one MTU may need be routed across networks (such as a WAN or VPN tunnel) with a smaller MTU. A typical contemporary office, campus or data centre network provided over Ethernet will have 1500 byte MTUs. This size is determined by the network’s data link and IP Maximum Transmission Units (MTUs) which are usually the same. When a host sends an IP packet onto the network it cannot be larger than the maximum size supported by that local network.

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