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

Tony Przygienda <tonysietf@gmail.com> Thu, 12 March 2020 21:27 UTC

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From: Tony Przygienda <tonysietf@gmail.com>
Date: Thu, 12 Mar 2020 14:26:09 -0700
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To: Greg Mirsky <gregimirsky@gmail.com>
Cc: Toerless Eckert <tte@cs.fau.de>, BIER WG <bier@ietf.org>
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Subject: Re: [Bier] comments for draft-zhang-bier-bierin6-04.txt
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I'm all for TTL=1. It's one thing having a BFD frame run away, another a
multicast frame ;-)  But yeah, worth discussion on the list ...

--- tony

On Thu, Mar 12, 2020 at 2:00 PM Greg Mirsky <gregimirsky@gmail.com> wrote:

> Hi Tony,
> I think that the example of LL BFD, i.e. single-hop BFD, is in fact
> opposite, not in support of TTL/HC = 1. RFC 5881, following on the
> extensive discussion of the GTSM in RFC 5082, requires that TTL/HC for the
> single-hop BFD be set to 255 and BFD packet discarded by a receiver if
> TTL/HC != 255. I'll note that we had an extensive discussion with IESG on
> the TTL/HC value in the case of BFD over VXLAN. We've agreed that a VXLAN
> tunnel is as single-hop IP and thus TTL/HC MUST be set to 255. Not sure if
> all this information was helpful. My $.02
>
> Regards,
> Greg
>
> On Thu, Mar 12, 2020 at 8:57 AM Tony Przygienda <tonysietf@gmail.com>
> wrote:
>
>>
>>
>>> >    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
>>
>>
>>
>>
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>