Subject: [VRRP] And subsecond timers for IPv4

From: Radia Perlman - Boston Center for Networking <[email-masked]>
Date: Fri, 15 Aug 2003 14:52:24 -0400 (EDT)

Doing it for v3 is straightforward, since there's no
compatibility issue, and as Mukesh's note says, the only
issue is what the units should be.

The other issue is whether to make a new version of VRRP for IPv4
that uses subsecond timers.

The issues for that are:

1) what is the encoding of the field, given that it can't be
expanded beyond a byte. I think it was Bill Fenner at the meeting
that said that some spec or another did some exponential type encoding,
and that we might as well copy something someone else has done. If someone
can give a pointer to that, that would be helpful.

2) how to (and whether to) mix VRRP-for-IPv4 routers that only do
units of seconds with ones that do subseconds.

Let's say we want to do this...where the new guys would like to use
subsecond granularity, but will be willing to only be seconds if there's
a VRRPv2 router on the link.

So how do they do this? They could send a new version number (let's say 4),
and it won't confuse the old guy (the spec says it must discard it).
However, if there is an old guy, that means that the new guys have to
speak v2, in units of seconds. Presumably they know an old
guy is there because they see a VRRP message with version=2.
Suppose someone upgrades the last
old guy. How will the new guys know that their neighbors are really new
guys even though those neighbors are emitting v2 messages?

That, by the way, is why I like two fields "version of this message",
and "version I can speak". In IKEv2, FYI, we did it with one additional
bit: a flag that says "I could speak a higher version"

But with the current VRRP encoding, I don't even see any reserved bits
we could use to signal subsecond capability.

Radia

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---

From: danny mitzel <[email-masked]>
Date: Fri, 15 Aug 2003 13:07:12 -0700 (PDT)

--- Radia Perlman - Boston Center for Networking
<[email-masked]> wrote:
> I think it was Bill Fenner at the meeting
> that said that some spec or another did some
> exponential type encoding, and that we might 
> as well copy something someone else has done. 
> If someone can give a pointer to that, that 
> would be helpful.

look at the igmpv3 rfc3376.  membership query
message max response time (secton 4.1.1) or
querier interval (section 4.1.7).  these fields
actually encode both a linear range of values
and an exponential encoding based on a flag bit.

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---

From: "Don Provan" <[email-masked]>
Date: Fri, 15 Aug 2003 14:39:33 -0700

> subsecond granularity, but will be willing to only be seconds
> if there's a VRRPv2 router on the link.

I could see something like this in other protocols, but the fact
that only the master system sends VRRP packets makes it really hard
to negotiate. The only way to hear from the dissenters involves them
sending a packet that takes over the VR MAC address. Add to that
the fact that different systems would be using different intervals
depending on whether they think there are v2 nodes present, and I
don't see much hope.

If we want subsecond timers in IPv4, I think just redefining the
unit for a new version is the most reasonable approach. For
backward compatibility, the spec can suggest or even insist that
implementations support both versions depending on how they're
configured. Once all nodes are upgraded, changing the interval is
simply a matter of reconfiguring all the routers, just like it is
in v2.

-don