RFC 1063 (rfc1063) - Page 2 of 11

IP MTU discovery options

RFC 1063                IP MTU Discovery Options               July 1988


   unnecessary packets.  And because network paths often change during
   the lifetime of a TCP connection, probe packets will have to be sent
   on a regular basis to detect any changes in the effective MINMTU.

   Implementors sometimes mistake the TCP MSS option as a mechanism for
   learning the network MINMTU.  In fact, the MSS option is only a
   mechanism for learning about buffering capabilities at the two TCP
   peers.  Separate provisions must be made to learn the IP MINMTU.

   In this memo, we propose two new IP options that, when used in
   conjunction will permit two peers to determine the MINMTU of the
   paths between them.  In this scheme, one option is used to determine
   the lowest MTU in a path; the second option is used to convey this
   MTU back to the sender (possibly in the IP datagram containing the
   transport acknowledgement to the datagram which contained the MTU
   discovery option).

OPTION FORMATS

   Probe MTU Option (Number 11)

      Format

              +--------+--------+--------+--------+
              |00001011|00000100|   2 octet value |
              +--------+--------+--------+--------+

      Definition

      This option always contains the lowest MTU of all the networks
      that have been traversed so far by the datagram.

      A host that sends this option must initialize the value field to
      be the MTU of the directly-connected network.  If the host is
      multi-homed, this should be for the first-hop network.

      Each gateway that receives a datagram containing this option must
      compare the MTU field with the MTUs of the inbound and outbound
      links for the datagram.  If either MTU is lower than the value in
      the MTU field of the option, the option value should be set to the
      lower MTU.  (Note that gateways conforming to RFC-1009 may not
      know either the inbound interface or the outbound interface at the
      time that IP options are processed.  Accordingly, support for this
      option may require major gateway software changes).

      Any host receiving a datagram containing this option should
      confirm that value of the MTU field of the option is less than or
      equal to that of the inbound link, and if necessary, reduce the



Mogul, Kent, Partridge, & McCloghrie