RFC 1063 (rfc1063) - Page 1 of 11

IP MTU discovery options

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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
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