Subject: An Extension for Host Monitoring

From: Michal Szymaniak <[email-masked]>
Date: Tue, 29 Jun 2004 18:40:47 +0200 (CEST)

Dear IPv6 contributors,

I was thinking about a simple extension to IPv6 that can potentially
be very useful. It addresses a well-known problem, namely host monitoring
in a wide-area network. I would be grateful, if you expressed your
opinion about this extension.

Consider two hosts, A and B. A relies on B in the sense that A wants to
be sure that B is up and running. Typically, A will probe B periodically
to check whether B is still alive. Once B stops responding to probes,
A will consider B to be dead, which shall trigger an application-specific
reaction.

The problems with this approach are (at least):

* trade-off between the frequency of probing and the time of reaction to
  B's failures: to react faster, A must probe B more often, thus
  injecting more packets into the network.

* lack of response does not necessarily mean that B is dead: other
  reasons include network partition (or even congestion, if probes can be
  lost in transit), or B overload. A is therefore likely to react only
  after B failed to respond several times, which can turn out to be too
  slow, or even unnecessary, if B was actually overloaded/unreachable,
  but still alive.

These problems can be solved by introducing an additional signaling
mechanism. Consider additional host B2, which is frequently communicating
with B and/or can observe the B's behavior, such as the B's router. B2 can
quickly notice that B is dead, for example, because B suddently stopped
injecting any packets to the network, or because B2's own communication
with B has broken.

The actual extension could be as follows. B can ask B2 to notify A once
B2 notices that B is dead. In this way, A can potentially react faster,
and does not have to monitor B continously. It is also possible to use
this extension in combination with low-rate probing, and/or use several
B2 hosts for more reliable monitoring.

This extension is in a way similar to what happens in Mobile IPv6, where
a mobile host has its home agent acting as a mobile node's representative.
In our case, B2 is requested to perform a simple best-effort service: B2
is not required to watch B continously. Instead, it is requested to
notify other hosts once it has noticed the B's death itself. There are
also some security considerations, but they can be discussed later.

Would it be possible to know what you guys think about this idea? Also,
if you think that this message should be sent to some other discussion
list instead, could you please let me know?

Thank you for your time.

Kind regards,
M.
-- 
Michal Szymaniak | [email-masked] | http://www.cs.vu.nl/~michal

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