Re: I-D Action: draft-ietf-6man-resilient-rs-02.txt

On 1/22/14 12:05 PM, Ralph Droms wrote:
>
> On Jan 22, 2014, at 11:55 AM 1/22/14, Lorenzo Colitti <lorenzo@google.com> wrote:
>
>> [Resending with hopefully correct author mailing list]
>>
>>
>> On Wed, Jan 22, 2014 at 11:46 AM, Lorenzo Colitti <lorenzo@google.com> wrote:
>> On Mon, Oct 21, 2013 at 4:38 PM, <internet-drafts@ietf.org> wrote:
>>     When an interface on a host is initialized, the host transmits Router
>>     Solicitations in order to minimize the amount of time it needs to
>>     wait until the next unsolicited multicast Router Advertisement is
>>     received.  In certain scenarios, these router solicitations
>>     transmitted by the host might be lost.  This document specifies a
>>     mechanism for hosts to cope with the loss of the initial Router
>>     Solicitations.  Furthermore, on some links, unsolicited multicast
>>     Router Advertisements are never sent and the mechanism in this
>>     document is intended to work even in such scenarios.
>>
>> Authors,
>>
>> One comment here.
>>
>> If the intent is that we should be able to run networks that do not send periodic RAs at all, then this document will need to require that hosts send router solicitations when their RAs (or whatever is in the RA contents, including lifetimes of PIOs, RIOs, RDNSS options, etc.) are about to expire - otherwise the hosts will lose connectivity.
>
> I think it's worse than that.
>
> A host on a multicast-capable link on which "unsolicited multicast Router Advertisements are never sent" will have to use the same transmission behavior as a host on a non-multicast-capable link, so that the host can learn of changes in the link prefixes in a timely fashion.
>
> Furthermore, a host on a multicast-capable link won't know if multicast RAs are being sent on the link, so all hosts will have to send periodic RSs.

Even if you jump to a period RS from every host approach, there would 
still be operational questions on what timer to use, and whether there 
is a way for the operator to quickly introduce a new prefix (and retire 
an old prefix).

With multicast RAs the operator can change the list of prefixes (to 
handle some problem) quite quickly. To accomplish the same thing without 
the multicast RA option would imply either
  - frequent periodic RSs from all the hosts, or
  - robustly tracking the set of hosts in the routers so that unicast 
RAs can be used for such changes.

Regards,
    Erik

> - Ralph
>
>
>>
>> This may be difficult to specify correctly. Specifically, it may be hard to distinguish something that is being deprecated by the network (e.g., an RA whose lifetime is counting down because it comes from a PD) from an RA that is about to expire.
>>
>> If the intent is not that we should be able to run networks without periodic RAs, then at least the text in 2b needs to change.
>>
>> Cheers,
>> Lorenzo
>>
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