Re: Pinging routers for network status

[Paul Vixie <vixie () mfnx net>]

sean () donelan com (Sean Donelan) writes:

> Most network providers ping their routers for network status.  Several
> providers even track RTT to detect changes.  But very few customers
> connect to routers.  Comparing the performance you see with HP Openview
> or similar products with the performance customers see remains an
> interesting question.  Sometimes C&W's or AT&T's traffic web site does
> show a problem.  But there are also problems that don't show up in
> intra-network pings.  In particular IGP/BGP routing issues can result
> in severe access network problems, but no problem with the internal
> provider network mesh used for pings.

Similarly, long or deviant ping times against a router can just indicate
that it was processing a routing update when it got your ICMP_ECHO request.
Therefore the RTT isn't nec'ily indicative of link latency.  (SNMP is worse.)

If you want to know how fast your network is, send a normal payload.  Even
source routing will variously have to be "processor switched" or otherwise
escape the "fast path" on many modern routers.  Ping and Traceroute are for
when you already know you're screwed, but you're trying to prove it to other
people or you're trying to draw finer distinctions.  When the network isn't
failing, Ping and Traceroute tell you pretty much nothing.

Of course, using normal payloads to measure network health requires agents
on the far side, and thus, is a more difficult design and deployment problem.
(If you don't think of this as a hard problem, you're not doing it right.)