Re: [pim] [MBONED] IGMPv3 backward compatibility issue killing SSM

[Hitoshi Asaeda <[e-mail]>]

Hi Brian,

> On Apr 3, 2024, at 4:09, Brian Haberman <[e-mail]> wrote:
> 
> Hi Hitoshi,
> 
> On 3/20/24 10:01 PM, Hitoshi Asaeda wrote:
>> I thought what we need to clarify in DS IGMPv3/MLDv2 RFCs is that, as Brian mentioned, we can simply add recommended default settings for the compatibility mode variables. That’s all, I thought.
>> But you remind me of the following four situations.
>> 1. querier = SSM capable, forwarder = SSM capable -> no fallback
>> 2. querier = SSM capable, forwarder = non-SSM capable -> fallback
>> 3. querier = non-SSM capable, forwarder = SSM capable -> no fallback (and SSM capable router must become (replace) querier?)
>> 4. querier = non-SSM capable, forwarder = non-SSM capable -> fallback
>> I wonder how above 2 and 3 are clearly considered in some RFCs?
> 
> Which RFCs are you worried about? I think 3376 and 3376bis cover this in section 7.3 with discussion of the compatibility mode and group compatibility mode variables. We would augment that discussion with a recommendation that those compatibility mode variables default to IGMPv3 mode.

I tend to change the discussion point a bit. Sorry if I confused you.

It is often happned for the querier and forwarder to be different routers. My questions are, if querier supports IGMPv3 but forwarder does not support IGMPv3 (=above case-2), what happens? Or, if querier does not support IGMPv3 but forwarder supports IGMPv3 (=above case-3), what happens?
If a querier is not IGMPv3-capable, even if all hosts and forwarders support IGMPv3, SSM services cannot be enabled on that LAN, right?
It is not directly related to the backward compatibility issue but this ambiguity may be related to the querier election mechanism mentioned in section 6.6.2 (Querier Election) of both 3376 and 3376bis.

Nevertheless, I DO NOT think this contradiction between the current simple querier selection mechanism and the appropriate forwarder selection mechanism should be discussed in this document. We can describe how to select an appropriate querier taking into account various points (who should be the forwarder, how to prevent inappropriate routers from becoming queriers, etc.) in a separate document.

> Table 9 in section 7.2.1 already indicates that hosts default to IGMPv3 mode. We need to indicate a similar default in section 7.3.1.

I do agree.
I've not proposed to add new functions for 3376bis and 3810bis at all. For me, both 3376bis and 3810bis are almost ready for publication. IMO, for the backward compatibility issue, what we need to do is to describe the way to configure the backward compatibility, “enable" or “disable".
The remaining point about backward compatibility mode mentioned in these documents is whether "enable" or "disable" should be the default.

Thanks for your work.

Regards,

Hitoshi


> Regards,
> Brian
> 
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