July 2002
Beginner
360 pages
11h 19m
English
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Path MTU Discovery
With IPv4, every router can fragment packets, if needed. If a router cannot forward a packet because the MTU of the next link is smaller than the packet it has to send, the router fragments the packet. It cuts it into slices that fit the smaller MTU and sends it out as a set of fragments. The packet is then reassembled at the final destination. Depending on the network design, an IPv4 packet may be fragmented more than once during its travel through the network.
With IPv6, routers do not fragment packets anymore; the sender takes care of it. Path MTU discovery tries to ensure that a packet is sent using the largest possible size that is supported on a certain route. The Path MTU is the smallest link MTU of all links from a source to a destination. The discovery of the Path MTU is described in RFC 1981.
The discovery process works like this. First, a host assumes that the Path MTU is the same as the MTU of the first hop link and it uses that size. If the packet is too big for a certain router along the path to deliver the packet to the next link, the router discards the packet and sends back an ICMPv6 Packet Too Big message. Recall that this message type includes the MTU size of the next hop link. The host now uses this MTU for sending further packets to the same destination. The host will never go below the IPv6 minimum MTU size of 1280 bytes, however. The process of receiving a Packet Too Big message and reducing the size of the packets can happen more than ...
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