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5.4.1. A Stateless Solution to MTU Handling

5.4.1. A Stateless Solution to MTU Handling​

An ITR stateless solution to handle MTU issues is described as follows:

  1. Define H to be the size, in octets, of the outer header an ITR prepends to a packet. This includes the UDP and LISP header lengths.

  2. Define L to be the size, in octets, of the maximum-sized packet an ITR can send to an ETR without the need for the ITR or any intermediate routers to fragment the packet.

  3. Define an architectural constant S for the maximum size of a packet, in octets, an ITR must receive so the effective MTU can be met. That is, S = L - H.

When an ITR receives a packet from a site-facing interface and adds H octets worth of encapsulation to yield a packet size greater than L octets, it resolves the MTU issue by first splitting the original packet into 2 equal-sized fragments. A LISP header is then prepended to each fragment. The size of the encapsulated fragments is then (S/2 + H), which is less than the ITR's estimate of the path MTU between the ITR and its correspondent ETR.

When an ETR receives encapsulated fragments, it treats them as two individually encapsulated packets. It strips the LISP headers and then forwards each fragment to the destination host of the destination site. The two fragments are reassembled at the

destination host into the single IP datagram that was originated by the source host. Note that reassembly can happen at the ETR if the encapsulated packet was fragmented at or after the ITR.

This behavior is performed by the ITR when the source host originates a packet with the 'DF' field of the IP header set to 0. When the 'DF' field of the IP header is set to 1, or the packet is an IPv6 packet originated by the source host, the ITR will drop the packet when the size is greater than L and send an ICMP Too Big message to the source with a value of S, where S is (L - H).

When the outer-header encapsulation uses an IPv4 header, an implementation SHOULD set the DF bit to 1 so ETR fragment reassembly can be avoided. An implementation MAY set the DF bit in such headers to 0 if it has good reason to believe there are unresolvable path MTU issues between the sending ITR and the receiving ETR.

This specification RECOMMENDS that L be defined as 1500.

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