Greg, Thank you very much for the detailed explanation.
As for Router sending MLD query to all-nodes multicast ff02::1, if Explicit Tracing is not turned on, routers usually don't keep all the host IP addresses attached to each port/link, does it? If the ToR switches has MLD proxy, it could suppress reports from some IP hosts. Then, the Router may not receive MLD reports from all hosts attached to one port. Under this scenario, when a host "a" sends Neighbor Solicitation multicast to "b", router may not know which link "b" is attached. As the result router could flood all the ports which has the subnet enabled. The MLD with Explicit Tracking being turned on can make router be refreshed with all hosts in the domain. That can definitely scope the ND multicast. However, MLD with Explicit Tracking could be very resource intensive. "draft-nachum-sarp" gives another approach to scope the ND multicast and alleviates the burden on router when subnets are spread across multiple links/ToRs. Linda > -----Original Message----- > From: Greg Daley [mailto:[email protected]] > Sent: Wednesday, November 21, 2012 6:13 PM > To: Linda Dunbar; Suresh Krishnan > Cc: [email protected]; [email protected] > Subject: RE: IPv6 ND applicability in draft-nachum-sarp-03 > > Hi Linda, > (also noting some overlap with Julien's response) > > The MLDv1 (RFC2710) or v2 (RFC3810) Querier sends out a general query R > times (where R is the reliability factor for the link), which requests > that all devices respond. This goes to all-nodes multicast ff02::1, > and the desitnation of the query doesn't have to match recipients (all > of which listen to that multicast group). > > This scales with responses on order N x R where N is the number of > nodes. > > If a router needs to manage a particular stream, it can send a specific > query to one of the multicast addresses, particularly to verify that > participants. > > The router doesn't actually need to track the solicited nodes multicast > addresses, as these are link-local scoped and not multicast routed off- > link. > > Actually in conformance with RFC 6398/BCP 168 (IP Router Alert > Considerations and Usage)/RFC 6192 (Protecting the Router Control Plane) > it is feasible to construct filters which ignore MLD report messages > which only contain Link-local scoped multicast groups on the router. > With the correct hardware support, this can be done at the line > interface, without CPU processing. The default is to pass all MLD > reports up to the Router though. > > Arguably, the only reason to send MLD Join messages for Solicited > Nodes' multicast addresses _is_ to support MLD snooping switches. > > Please note that MLD snooping is an LRU caching scheme, where the cache > is refreshed for all active users through the background polling of the > MLD querier. The switches do not need to know anything ahead of time. > Devices which do not respond to the query will be dropped from the > cache, and only the number of active hosts will be retained. > > If you changed the IP addresses of every host on the link within a > single query interval, the maximum number of Solicited nodes addresses > cached by an MLD snooping network would be: > > ^A x N x 2 > > Where: > > The original the number of addresses is ^A x N > and ^A is the mean number of addresses per host, and N is the number of > hosts on the link. > > Sincerely > > Greg Daley > > > > > > > > From: [email protected] [mailto:[email protected]] On > Behalf Of Linda Dunbar > > Sent: Thursday, 22 November 2012 2:17 AM > > To: Suresh Krishnan > > Cc: [email protected]; [email protected] > > Subject: Re: [Int-area] IPv6 ND applicability in draft-nachum-sarp-03 > > > > Suresh, > > > > "solicited node multicast address" is formed by taking the low-order > 24 bits of target address (unicast or anycast) and appending those bits > to the prefix > FF02:0:0:0:0:1:FF00::/104 > > > > Routers don't know ahead of time what hosts are present in their > Layer 2 domain. In DC with virtual machines added/deleted consistently, > there could > be millions of possibilities. > > > > What multicast addresses should router send out MLD Query? > > > > What you are proposing is like using MLD snooping to achieve ND > function, which doesn't scale. > > > > Linda > > > -----Original Message----- > > From: Suresh Krishnan [mailto:[email protected]] > > Sent: Tuesday, November 20, 2012 5:58 PM > > To: Linda Dunbar > > Cc: [email protected]; [email protected]; Tal Mizrahi > > Subject: Re: IPv6 ND applicability in draft-nachum-sarp-03 > > > > Hi Linda, > > > > On 11/20/2012 01:05 PM, Linda Dunbar wrote: > > > Suresh, > > > > > > Are you saying that router has to send out MLD Query for all > > potential ND multicast addresses and keep up state for listeners for > > all those multicast addresses? > > > > Nope. Only for those solicited node multicast groups that have at > least > > one host on the subnet join. > > > > > > > > There could be millions of "Solicited-Node multicast addresses". > That > > is a lot of processing on routers. > > > > Not sure that I follow. Do you have millions of hosts on the subnet? > If > > not, I do not see why you have to keep track of "millions of > solicited > > node multicast addresses". > > > > Thanks > > Suresh > > > > _______________________________________________ Int-area mailing list [email protected] https://www.ietf.org/mailman/listinfo/int-area
