[3473] in linux-net channel archive
Re: Speaking of MTU problems
daemon@ATHENA.MIT.EDU (Dennis)
Mon Jun 24 21:54:02 1996
Date: Mon, 24 Jun 1996 17:43:48 -0400
To: inr-linux-net@ms2.inr.ac.ru (really kuznet@ms2.inr.ac.ru)
From: dennis@etinc.com (Dennis)
Cc: linux-net@vger.rutgers.edu
>Dennis (dennis@etinc.COM) wrote:
>
>: Shouldn't the (physical) mtu of a device take into account to header
>: length?
>:.....
>: There are those that will argue that the MTU is the data size, but it is
>: difficult
>: to coordinate with directly connected devices when they have different
>: definitions
>: of the terms. (Ciscos have 2 mtus...phyisical and data). Perhaps its
semantics,
>: but it shouldnt work differently in different scenarios.
>
>RFC 1122
>
> MTU
> The maximum transmission unit, i.e., the size of the
> largest packet that can be transmitted.
> ------
>
>
> The terms frame, packet, datagram, message, and segment are
> illustrated by the following schematic diagrams:
>
> A. Transmission on connected network:
> _______________________________________________
> | LL hdr | IP hdr | (data) |
> |________|________|_____________________________|
>
> <---------- Frame ----------------------------->
> <----------Packet -------------------->
>
>
>
>
>Internet has strict definition of MTU used in all Internet docs,
>if you change it, we will have to translate all RFCs to your language.
The problem is that each layer has its own MTU, so its not clear if the
MTU is for the physical device (which is really what is SHOULD be,
since you can run different protocols with different mtus on the same
device). It seems that a device level mtu should be the physical
capability of the device. The big problem is the question of whether
or not the encapsulation protocol is considered data. Frame relay,
for example has only a 2 byte header, but IETF requires an additional
2. So with a 1500 byte mtu does that mean that only 1498 bytes of IP
data can be sent with IETF encapsulation in force? The encapsulation,
although a header, is really data for the protocol decoder, not the
physical medium.
Dennis
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