Re: [v6ops] draft-ietf-intarea-tunnels concerns

["Templin (US), Fred L" <[e-mail]>]

Eduard, I did a quick pass through the intarea discussions and I see points raised that
have been repeated many times since Y2K and even since much longer before that.
Thankfully, there is now a solution called the “OMNI Adaptation Layer (OAL)”:

https://datatracker.ietf.org/doc/draft-templin-6man-omni/

Some might say that it is the “second-coming of AAL5”, and it is true there are
many similarities. Like IP over ATM, the OMNI interface has an MTU/MRU visible
to the IP layer and set to 9180 bytes since that is the maximum size that can be
well protected by CRC32. Like AAL5, within the OMNI interface the OAL has a
“cell size” that determines the size of each fragment that will be produced by
the adaptation layer below IP.

Unlike AAL5 where the cell size is fixed at 48 octets, however, the OAL sets
both a minimum cell size (termed “minimum Maximum Payload Size (MPS)”)
and a possibly larger per-path MPS discovered using probes if necessary. The
minimum/path MPS is the maximum-sized RFC2473 fragment that the OAL
can sneak through the path and be assured it won’t be dropped (silently or
otherwise) due to a size restriction. Without any probing, the minimum MPS
that can be assumed is 576 minus encapsulation overhead (i.e., the IPv4
minimum EMTU_R). But unlike AAL5, the OAL can always strive to find a
larger per-path MPS.

There is more to be said about the OAL, but I will leave it at that for now.
We can talk about PTB “hard” and “soft” errors later if there is interest.

Fred