Editing IP fragmentation (section)

==IPv4 and IPv6 differences==
[[File:IPv4 Fragmentation Algorithm-en.png|thumb|400px|The fragmentation algorithm in IPv4.]]
[[File:IPv4 Fragmentation example -en.svg|thumb|400px|An example of IPv4 multiple fragmentation. The fragmentation takes place on two levels: in the first one the maximum transmission unit is 4000 bytes, and in the second it is 2500 bytes.]]

Under IPv4, a [[router (computing)|router]] that receives a [[network packet]] larger than the next hop's MTU has two options: drop the packet if the ''Don't Fragment'' (DF) flag bit is set in the packet's header and send an [[Internet Control Message Protocol]] (ICMP) message which indicates the condition ''Fragmentation Needed'' (Type 3, Code 4), or fragment the packet and send it over the link with a smaller MTU. Although originators may produce fragmented packets, IPv6 routers do not have the option to fragment further. Instead, network equipment is required to deliver any IPv6 packets or packet fragments smaller than or equal to 1280 bytes and IPv6 hosts are required to determine the optimal MTU through [[Path MTU Discovery]] before sending packets.

Though the header formats are different for IPv4 and IPv6, analogous fields are used for fragmentation, so the same algorithm can be reused for IPv4 and IPv6 fragmentation and reassembly.

In IPv4, hosts must make a best-effort attempt to reassemble fragmented IP packets with a total reassembled size of up to 576 bytes.  They may also attempt to reassemble fragmented IP packets larger than 576 bytes, but they are also permitted to silently discard such larger packets.  Applications are recommended to refrain from sending packets larger than 576 bytes unless they have prior knowledge that the remote host is capable of accepting or reassembling them.<ref name=RFC791/>{{rp|12}}

In IPv6, hosts must make a best-effort attempt to reassemble fragmented packets with a total reassembled size of up to 1500 bytes, larger than IPv6's minimum MTU of 1280 bytes.<ref name=rfc2460>{{citation |rfc=2460 |title=Internet Protocol, Version 6 (IPv6) Specification |authorlink=Steve Deering |author=S. Deering |author2=R. Hinden |date=December 1998}}</ref>  Fragmented packets with a total reassembled size larger than 1500 bytes may optionally be silently discarded.  Applications relying upon IPv6 fragmentation to overcome a path MTU limitation must explicitly fragment the packet at the point of origin; however, they should not attempt to send fragmented packets with a total size larger than 1500 bytes unless they know in advance that the remote host is capable of reassembly.