IP MTU discovery options (RFC1063)
Original Publication Date: 1988-Jul-01
Included in the Prior Art Database: 2000-Sep-12
Internet Society Requests For Comment (RFCs)
J.C. Mogul: AUTHOR [+4]
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 ).
Network Working Group J. Mogul
Request For Comments: 1063 C. Kent
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.
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 ).
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
Probe packets have the problem of burdening the network with
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