Re: Path MTU Hop-by-Hop Option

[Tom Herbert <[e-mail]>]

On Tue, Oct 16, 2018, 9:50 PM Carsten Bormann <[e-mail]> wrote:

> On Oct 17, 2018, at 00:55, Bob Hinden <[e-mail]> wrote:
> >
> > This document specifies a new Hop-by-Hop IPv6 option that is used to
> >   record the minimum Path MTU
>
> A while ago I proposed something similar to collect information about
> adaptation layer fragmentation MTUs (information about which is truly not
> available in any other way):
>
> https://tools.ietf.org/html/draft-bormann-intarea-alfi-04
>
> (I also only looked at the Internet layer here — leaving it as an exercise
> to put in transport/application protocol support.)
>
> After mulling an initial version of the proposal, there was pretty much
> consensus that hop-by-hop options were not really available on the
> Internet.  So I put in an alternative way to collect the information, see
> above if you are interested.  Then at some point SPUD came along and I was
> hoping that would lead to a framework within in which this information
> could be better handled.
>

Carsten,

>
SPUD would put this sort of information in a UDP payload. This would then
have intermediate nodes modifying payloads to record the MTU. That's
fundamentally incorrect since port numbers, which would be used to identify
SPUD, only have meaning at the end points.

If intermediate nodes are to set arbitrary information in packets, then
Hop-by-Hop options are really the only correct and generic way to do that.


> Maybe it’s time to pull the old draft out again.  The hop-by-hop option
> problem of course hasn’t gone away.
>

As the draft states, RFC8200 relaxed the requirements of HBH options so
that intermediate nodes ignore them instead of dropping. That is reasonable
and easy to implement. There are still going to be some drops so
implementations will need to fall back when they don't work for a flow
(same thing would be true SPUD since UDP isn't guaranteed to work across
the whole Internet).

Tom


> Grüße, Carsten
>
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