Re: [Bier] comments for draft-zhang-bier-bierin6-04.txt

["Nagendra Kumar Nainar (naikumar)" <[e-mail]>]

Hi Toerless,

Thank you for triggering the discussion with 6MAN. I was discussing with the authors offline on the same topic. I think it is better to have a clarity on this topic.

I personally think it is better to use non LL address even when the BFRs are directly connected for behavior consistency and to leverage some of the benefits such as IP-FRR.

Thanks,
Nagendra

From: BIER <[e-mail]> on behalf of Tony Przygienda <[e-mail]>
Date: Thursday, March 12, 2020 at 11:58 AM
To: Toerless Eckert <[e-mail]>
Cc: BIER WG <[e-mail]>
Subject: Re: [Bier] comments for draft-zhang-bier-bierin6-04.txt



>    If the neighbor is directly connected, The destination address in
>    IPv6 header SHOULD be the neighbor's link-local address on this
>    router's outgoing interface, the source destination address SHOULD be
>    this router's link-local address on the outgoing interface, and the
>    IPv6 TTL MUST be set to 1.

I've started a thread on 6man/6ops re. link-local-tunnel traffic as i
have the same also in my ANIMA draft, and in implementations i have
seen quite some issues tunneling traffic HW accelerated across IPv6
link-local addresses and TTL=1.

Would welcome implementers in BIER to chime in if they would see this
as an issue.

If folks feel supporting this HW accelerated might be more complicated
than non-link-local tunnels, then it might be useful to change the
text to also allow/require support for using the BFR address even for
subnet adjacent BFR. In that case one would not use HL=1, and that
too i have seen to be a problem in HW accelerated forwarding plane:
HL/TTL=1 -> MUST punt (software processing).

really? I'm surprised, actually, LL with TTL=1 for link local is an absolutely common case since years for e.g. BFD and other thinks like BGP sessions in many topologies so if a silicon does not support that it's a strange IPv6 silicon. But yes, supporting LL properly in IPv6 is non trivial (spec allows to have e.g. same LL on multiple interfaces in a node). The spec here says SHOULD and not MUST so it's perfectly find using any addresses (one could probably even use BFR prefix but that would lead to ECMP non-deterministic behavior which may or may not be OK depending on deployment).  If that needs to be spelled out (albeit it's redundant) I'm not opposed to that.

Please cc: bier working group on any discussion you are starting with 6man on the side ... Unless I missed my morning mail input ...

--- tony