5.4.2. A Stateful Solution to MTU Handling
5.4.2. A Stateful Solution to MTU Handling
An ITR stateful solution to handle MTU issues is described as follows and was first introduced in [OPENLISP]:
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The ITR will keep state of the effective MTU for each Locator per Map-Cache entry. The effective MTU is what the core network can deliver along the path between the ITR and ETR.
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When an IPv6-encapsulated packet, or an IPv4-encapsulated packet with the DF bit set to 1, exceeds what the core network can deliver, one of the intermediate routers on the path will send an ICMP Too Big message to the ITR. The ITR will parse the ICMP message to determine which Locator is affected by the effective MTU change and then record the new effective MTU value in the Map-Cache entry.
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When a packet is received by the ITR from a source inside of the site and the size of the packet is greater than the effective MTU stored with the Map-Cache entry associated with the destination EID the packet is for, the ITR will send an ICMP Too Big message back to the source. The packet size advertised by the ITR in the ICMP Too Big message is the effective MTU minus the LISP encapsulation length.
Even though this mechanism is stateful, it has advantages over the stateless IP fragmentation mechanism, by not involving the destination host with reassembly of ITR fragmented packets.