RFC 1063 (rfc1063) - Page 2 of 11
IP MTU discovery options
Alternative Format: Original Text Document
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