IP MTU discovery options (RFC1063)
Original Publication Date: 1988-Jul-01
Included in the Prior Art Database: 2019-Feb-15
Internet Society Requests For Comment (RFCs)
J.C. Mogul: AUTHOR [+3]
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.
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.
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 avoid fragmentation.
Probe packets have the problem of burdening the network with
Mogul, Kent, Partridge, & McCloghrie [Page 1]
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).
Probe MTU Option (Number 11)
+--------+--------+--------+--------+ |00001011|00000100| 2 octet value | +--------+--------+--------+--------+
This option always contains the lowest MTU of all...