Subject: RE: meta thoughts on m/o bits

From: "Soliman, Hesham" <[email-masked]>
Date: Fri, 20 May 2005 14:06:24 -0400

> > This proposal looks better and easy to understand. However,
 > > if we just rely on timeout for concluding the 
 > unavailabiliy of DHCP server,
 > > how does the client re-invoke DHCP if the DHCP server is 
 > available later in 
 > > time? I think we need one bit to inform the clients about the
 > > availabilty of DHCP
 > > services in the network.
 > 
 > I would assume that DHC has mechanisms for discovering when DHC
 > servers come on line. I.e., the client just sits quietly in 
 > background.

=> I'm curious about this, does DHCP broadcast its existence to
clients that never used it when it comes up? Seems a bit strange
to do that. I don't even know how it could speak to a client unsolicited.

Hesham

 > 
 > Sure, an RA bit could also signal the availability of a new server,
 > but I would first like to be convinced that the existing default DHC
 > mechanism isn't good enough for handling this case (better 
 > not to have
 > multiple ways of acheiving the same result).
 > 
 > Thomas
 > 
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From: Syam Madanapalli <[email-masked]>
Date: Fri, 20 May 2005 23:55:38 +0530

> to do that. I don't even know how it could speak to a client unsolicited.

Yes, I do not think there is any message that has currently been defined
in DHCP, that the servers broadcast without client solicitation.

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From: Thomas Narten <[email-masked]>
Date: Fri, 20 May 2005 14:33:00 -0400

> in DHCP, that the servers broadcast without client solicitation.

There are generally two ways to discover the existance of a server:

  - broadcast its existance (e.g., via a DHC broadcast or RA "DHC
    server present" bit)

  - have clients periodically poll the server

Each has different tradeoffs in terms of timeliness of discovering the
server vs. load on the network, etc.

Those assuming a broadcast mechanism is needed may be making the
assumption that if an admin turns on a DHC server, it is a
_requirement_ that all nodes start using DHC effectively
_immediately_.

It is not at all clear to me that this is a requirement.

I.e., it might be just fine to have all clients learn of DHC within
say 1-2 hours (if not longer), in which case background polling is
just fine.

Remember, I don't think we're necessarily talking about the case where
a DHC server goes down. We're presumably talking about the case where
no DHC servers exist (and haven't for a long time), but the admin
decides it's time to enable DHC going forward.

Thomas

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From: Ralph Droms <[email-masked]>
Date: Fri, 20 May 2005 14:45:36 -0400

Comments in line...

>   - broadcast its existance (e.g., via a DHC broadcast or RA "DHC

There is no "DHCP broadcast", and changing the RA bit requires touching
all the relevant routers.

>   - have clients periodically poll the server

RFC 3315 currently specifies that clients poll (one message every two
minutes) forever.

> It is not at all clear to me that this is a requirement.

Which is why a broadcast mechanism in either DHCPv4 or DHCPv6.  The
operating environments for the two protocols are somewhat different; in
general, if there is no DHCP server for IPv4 clients, those clients are
simply unable to use the network, while if there is no DHCP server for
IPv6 clients, those clients may be able to use SLAAC addresses.

Either way, broadcasting the existence of a DHCP server is likely of
value only in a very few, if any, cases.

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From: Ralph Droms <[email-masked]>
Date: Fri, 20 May 2005 14:49:41 -0400

> Which is why a broadcast mechanism in either DHCPv4 or DHCPv6.  The
                                    ^is not defined