[e2e] Question on MTU

Matt Mathis mathis at psc.edu
Fri Apr 22 14:21:17 PDT 2005

There is another issue here, which I think is more germane to the original
question.   I quote from -pmtud-method-

   MTU, Maximum Transmission Unit, the size in bytes of the largest IP
      packet, including the IP header and payload, that can be
      transmitted on a link or path.  Note that this could more properly
      be called the IP MTU, to be consistent with how other standards
      organizations use the acronym MTU.

   link MTU, The Maximum Transmission Unit, i.e., maximum IP packet size
      in bytes, that can be conveyed in one piece over a link.  Beware
      that this definition differers from the definition used by other
      standards organizations.

      For IETF documents, link MTU is uniformly defined as the IP MTU
      over the link.  This includes the IP header, but excludes link
      layer headers and other framing which is not part of IP or the IP

      Be aware that other standards organizations generally define link
      MTU to include the link layer headers.

So to make it concrete:  To the IETF, Ethernet has a 1500 Byte MTU, to the
IEEE, it has a 1518 Byte MTU.

This causes endless confusion and errors when people are configuring router
interfaces that have selectable MTUs, and other situations where both
communities might have to share documentation.  I seriously considered trying
to pick a new term to replace "IP MTU", but nothing is as crisp or sufficiently
motivating to re-train everyone who never thinks about layers below IP.

When you read a piece of documentation you can usually tell which MTU the
author meant, however once in a while a new product pops up where the HW
engineer failed to realize that IP MTU is not the total frame size and did it

Matt Mathis      http://www.psc.edu/~mathis
Work:412.268.3319    Home/Cell:412.654.7529
Evil is defined by people who think they know
"The Truth" and use force to apply it to others.

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