4. Router Specification

When a router is unable to forward a datagram because it exceeds the MTU of the next-hop network and its Don't Fragment bit is set, the router is required to return an ICMP Destination Unreachable message to the source of the datagram, with the Code indicating "fragmentation needed and DF set". To support the Path MTU Discovery technique specified in this memo, the router MUST include the MTU of that next-hop network in the low-order 16 bits of the ICMP header field that is labelled "unused" in the ICMP specification [7]. The high-order 16 bits remain unused, and MUST be set to zero. Thus, the message has the following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Type = 3    |   Code = 4    |           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           unused = 0          |         Next-Hop MTU          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Internet Header + 64 bits of Original Datagram Data      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The value carried in the Next-Hop MTU field is:

The size in octets of the largest datagram that could be forwarded, along the path of the original datagram, without being fragmented at this router. The size includes the IP header and IP data, and does not include any lower-level headers.

This field will never contain a value less than 68, since every router "must be able to forward a datagram of 68 octets without fragmentation" [8].

4. Router Specification​

4. Router Specification