IPv6 link-local traffic questions

[Toerless Eckert <[e-mail]>]

Sorry for cross posting, but this is both ops and arch questions.

I was trying to find any generic description that/if link-local IPv6
traffic MUST/SHOULD use Hop-count 1, but could not find one in
rfc8200 or rfc4291 and its updates. Instead, text like
MUST set Hop-limit to 1 is repeated in various RFCs using link-local
IPv6 traffic.

Am i overlooking something ? Or is this intentional in the IPv6 architecture ?

If intentional, i guess it means: Any RFC introducing some type of link-local
traffic can specify Hop-limit requirements for its traffic. As in
"our WG likes 42 as the Hop-limit" (silly to me, but should be fine with
IPv6 architecture then, right ?).

In one of my draft document reviews using such link-local IPv6 traffic,
EricV brought up the Q if that traffic shouldn't use RFC5082 instead
of Hop-Count 1 too. But given how we do not seem to have architectural
recommendations for TTL=1 (unless, quite likely i overlook something),
and given how i am not aware of any actual use of GTSM, i am quite
weary to go down that route. Aka: Would like learn of actual uses of
GTSM with IPv6.

There is an additional set of consideration re. link-local traffic
when trying to use it as a HW accelerated tunneling mechanism across links,
a use we both have in ANIMA, and maybe get the same in BIER (probably
other WGs too, not sure).

In implementations, i have seen quite a range of issues with
accelerated forwarding planes across link-local addressed tunnels,
and when using non-link-local addresses (loopback addresses),
the HW plane forwarding might have issues doing HW forwarding, because
usually TTL=1 is only used for control plane traffic. Of course,
this is not a good architectural argument, but if given architectural
leeway to even choose 42, maybe GTSM/255 is easier to be supported
across various HW accelerated forwarding planes than 1.

Opinions and experiences welcome!

Thanks!
    Toerless