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Re: T3 Latency

  • From: Charles Scott
  • Date: Sat Feb 17 09:21:09 2001


In response to all the questions below...

  The distance is from Northern Michigan to Chicago, apparently via
Detroit, which is about 500 Miles without knowing which way the fiber
really goes. There are I believe more than one SONET ring involved in the
transport. Also something like 13 cross connects, but no other routers in
that path.
  When asked, the provider suggested that the latency was due to the
router I'm hitting being fairly busy. However, the latency is that same
when I ping it as when I go through it and it's always 20 ms, nothing
less. I'd think that if it's a matter of a low priority response issue,
that the latency would be variable and not be a fixed addition to the path
times going past the router.
  OK, here's the funny thing, I have an account on anohter system here in
town that's also connected via a T3, but is 7 hops to the router that is
my first hop. When I ping the router that's my first hop from that system
I get about 22ms for all 7 hops while I get 20ms for just the first hop.
  I've been looking for any system that would have low latency response on
the other side of that router, but so far nothing on the other side of it
is anything less than 20ms.
  I guess the other question is how much of a marketing liability is this
going to be for my service. We're spending the money on this line to get
us the best connectivity we can from up here. Something tells me that some
dedicated or co-location customer is going to ask me about this latency
issue.

Still wondering...

Chuck Scott





--From Chris

> What distance is it running ?

--From Jeff

You may want to check with the carrier of the circuit and make sure
that it's taking the path you expect. If it's on someting like a SONET
ring, it may be riding a much longer path that you would expect.

--From Brian

keep in mind you are pinging/tracerouteing that is aimed at the router.
ICMP is very low on the routers priority
list. The major providor's aggrigate router is prob pushing a few loaded
links. Better test is ping to a idle host nearby off their router.


--From Jonathan

With certain routers if they have enough stress, the RTT to the router
will be larger then though the router.  This has to do with caching
algorithms the router uses.  Also, some large providers do not have you
connect directly into a rotuer, but into a switch that acts as a MUX, and
then they have an OC48 link up to the router.  If that OC48 has been
oversubscribed you might see latency, though I would hope not that
consistent.

What does your ISP's install engineer say?  For a T3 that is going less
then 100 miles, the latency really should be 10percent of what you are
seeing.









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