Re: [Mcast-wifi] [pim] Volunteers needed for work on progressing IGMP/MLD on the standards track
Stig Venaas <stig@venaas.com> Fri, 14 December 2018 20:05 UTC
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From: Stig Venaas <stig@venaas.com>
Date: Fri, 14 Dec 2018 12:04:56 -0800
Message-ID: <CAHANBtJq88fr65+vc5pZiqr8JTt6uubyVDobuJniU_Pqf=z03Q@mail.gmail.com>
To: Linus Lüssing <linus.luessing@c0d3.blue>
Cc: pim@ietf.org, mcast-wifi@ietf.org
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Subject: Re: [Mcast-wifi] [pim] Volunteers needed for work on progressing IGMP/MLD on the standards track
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Thanks, this is great input! Hoping to get input like this from more people. Thanks, Stig On Thu, Dec 13, 2018 at 6:32 AM Linus Lüssing <linus.luessing@c0d3.blue> wrote: > > And a five more inconveniences that came to my mind: > > > # IGMP Query 0.0.0.0 source address > > As snooping switches have an interest in learning about multicast > listeneres, they often implement the querier protocol. However, > they might not implement DHCP for instance, leaving them with no > assigned IPv4 address to use for IGMP queries. > > In that case, such snooping switches will usually use the 0.0.0.0 > source address. > > The issue with that however is that according to IGMP they would > always win the querier election as it is the lowest address > possible. > > This is undesirable due to the following two reasons: For one > thing a snooping switch might not always be in a favorable > position topology wise. A real multicast router for instance is > probably in a better spot but would always loose the election. For > another, when multiple such snooping switches use the 0.0.0.0 > address then this creates quite some turbulences: The selected > querier will change frequently. > > If I remember correctly, there was some debate on the Linux bridge > mailinglist in the past regarding how to treat an IGMP query with > a zero-source address. > > For reports, there is a nice section regarding the source address > (RFC3376, section 4.2.13. "IP Source Addresses for Reports"). > However there is no equivilant for that for queries. > > Ideally, 0.0.0.0 would be excluded and ignored from the querier > election mechanism, I think. > > > # MLD Query :: source address > > RFC4541, section 3 ("IPv6 considerations") says: > > "[...] the switch should use the null IP source address (::) when > sending said queries" > > While RFC3810, section 5.1.14 ("Source Addresses for Queries") says: > > "All MLDv2 Queries MUST be sent with a valid IPv6 link-local > source address. If a node (router or host) receives a Query > message with the IPv6 Source Address set to the unspecified address (::), or > any other address that is not a valid IPv6 link-local address, > it MUST silently discard the message and SHOULD log a warning." > > Currently, the Linux bridge for instance adhears to the latter, ignoring > all MLD queries with a :: source address. > > > # Query type for Querier Election > > RFC3376, 6.6.2. Querier Election: > > "When a router receives a query with a lower IP address, it sets the > Other-Querier-Present timer to Other Querier Present Interval and ceases > to send queries on the network if it was the previously elected > querier." > > The "receives a query" should probably be a "receives a general > query". Similarly for MLDv2 (RFC3810, section 7.6.2). > > At least that's what the Linux bridge currently does. It only > processes general queries for the querier election and for > multicast router port additions. > > > # IPv6 link-local multicast to multicast routers > > Currently it seems slightly unclear whether data with an IPv6 > link-local destination needs to be forwarded to multicast routers. > > Ideally, this would not be necessary as link-local multicast by > definition will not be routed. This would potentially save quite > some traffic as in a large broadcast domain there are quite a few > such messages for IPv6 Neighbor Discovery, for instance. > > However, this is probably, unfortunately not do-able right now. > At least when there are IGMPv1/2 / MLDv1 hosts present. Due to > the report suppression link-local multicast will need to be > forwarded to the selected querier / some common host which > received all reports, to avoid snooping switches introducing > multicast packet loss. > > RFC4541 does not seem to make any special recommendations > regarding IPv6 link-local multicast. > > .o(One thought that crossed my mind was, whether it might make sense > to add a flag to Multicast Router Advertisements which if set > explicitly indicates that the according router is not interested > link-local multicast.) > > > # Querier => Multicast Router implication? > > Ideally, RFC4286 ("Multicast Router Discovery") would become the > sole source to detect multicast routers. And RFC4541 would not > (need to) recommend to add queriers to the multicast router list > anymore. Especially as snooping switches often implement IGMP/MLD > querying, too, this is a little inconvenient. > > However, since RFC4286 is neither mandatory nor widely > implemented and the report suppression issue still present this > too is probably, unfortunately more a wish-list item for the > distant future. > > > Regards, Linus
- Re: [Mcast-wifi] [pim] Volunteers needed for work… Linus Lüssing
- Re: [Mcast-wifi] [pim] Volunteers needed for work… Linus Lüssing
- Re: [Mcast-wifi] [pim] Volunteers needed for work… Linus Lüssing
- Re: [Mcast-wifi] [pim] Volunteers needed for work… Stig Venaas