RFC 1063 (rfc1063) - Page 1 of 11
IP MTU discovery options
Alternative Format: Original Text Document
Network Working Group J. Mogul
Request For Comments: 1063 C. Kent
DEC
C. Partridge
BBN
K. McCloghrie
TWG
July 1988
IP MTU Discovery Options
STATUS OF THIS MEMO
A pair of IP options that can be used to learn the minimum MTU of a
path through an internet is described, along with its possible uses.
This is a proposal for an Experimental protocol. Distribution of
this memo is unlimited.
INTRODUCTION
Although the Internet Protocol allows gateways to fragment packets
that are too large to forward, fragmentation is not always desirable.
It can lead to poor performance or even total communication failure
in circumstances that are surprisingly common. (For a thorough
discussion of this issue, see [1]).
A datagram will be fragmented if it is larger than the Maximum
Transmission Unit (MTU) of some network along the path it follows.
In order to avoid fragmentation, a host sending an IP datagram must
ensure that the datagram is no larger than the Minimum MTU (MINMTU)
over the entire path.
It has long been recognized that the methods for discovering the
MINMTU of an IP internetwork path are inadequate. The methods
currently available fall into two categories: (1) choosing small MTUs
to avoid fragmentation or (2) using additional probe packets to
discover when fragmentation will occur. Both methods have problems.
Choosing MTUs requires a balance between network utilization (which
requires the use of the largest possible datagram) and fragmentation
avoidance (which in the absence of knowledge about the network path
encourages the use of small, and thus too many, datagrams). Any
choice for the MTU size, without information from the network, is
likely to either fail to properly utilize the network or fail to
avoid fragmentation.
Probe packets have the problem of burdening the network with
Mogul, Kent, Partridge, & McCloghrie