Subject: OMNI Interface MTU -> 64000

From: "Templin (US), Fred L" <[email-masked]>
Date: Tue, 9 Nov 2021 19:13:44 +0000

The OMNI interface MTU is currently fixed at 9180. This number was selected to match
IP over ATM, which very closely models the way the OAL operates. However, there is
nothing cast in stone about the magic number 9180, and it is really just an artifact of
investigations from the 1990s.

However, while performance analysis shows an appreciable gain in using 9180 byte packets
(i.e., rather than 1500 or smaller) an even much greater gain is found by pushing all the way
to the largest possible IPv6 packet that can undergo fragmentation. I will not make this
change just yet, but this is a notice to the working group that I intend to increase the
OMNI interface MTU to 64000 in the next draft version.

Fred

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From: "Templin (US), Fred L" <[email-masked]>
Date: Wed, 10 Nov 2021 16:16:47 +0000

Hi, responding to myself this requires a bit of tuning. Since an IP packet admitted into
the OMNI interface will immediately be encapsulated in an IPv6 header plus any per
fragment headers (including Hop-By-Hop and Routing Header ) and with a trailing 2
octet checksum added, the original IP packet length is limited to the largest value that
can be represented in the IPv6 encapsulation header Payload Length field.

The calculation of the largest original IP packet (i.e., the OMNI MTU) is therefore:

   OMNI MTU = (64K-1) - MAX(HBH) - MAX(RH) - 2

Where MAX(HBH) is the maximum-length Hop-By-Hop option permitted and MAX(RH)
is the maximum-length Routing Header permitted. Note that the maximum values must
be consistently included in the calculation even if smaller headers appear in an actual
encapsulation so that the original source receives a consistent representation of the
OMNI MTU.

Based solely on a finger-in-the-wind reckoning, I therefore declare MAX(HBH) as 64
and MAX(RH) as 128. The OMNI MTU is therefore calculated as:

  (64K-1) - 64 - 128 -2 = 65,341

Unless anyone else has a better guesstimate for these MAX values, this is the size that
will appear in the next OMNI draft version.

Comments welcome,

Fred

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> Sent: Tuesday, November 09, 2021 11:14 AM
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> Subject: OMNI Interface MTU -> 64000
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From: "Templin (US), Fred L" <[email-masked]>
Date: Wed, 10 Nov 2021 16:26:25 +0000

Also worth mentioning is that 65,341 is the OMNI interface *maximum* MTU; as with
any interface, administrators may set a smaller MTU but MUST NOT set a larger MTU.

And, it must also be said that MRU for all OMNI interfaces is (64K-1).

Fred

> Sent: Wednesday, November 10, 2021 8:17 AM
> To: [email-masked]
> Subject: RE: OMNI Interface MTU -> 64000
>