We had a similar situation going from Minneapolis to Kansas City via
Chicago. Normal latency from Minneapolis to Chicago via Level3 MPLS
network is about 14msec RTT. When the the circuit from Minneapolis to
Chicago went out for one reason or another, our MPLS link went from
Minneapolis to Tulsa, to Dallas, and then to Chicago.. That added a
little latency in the path from Minneapolis to Chicago.. We didn't need
to exceed the SLA in order to cry foul. They didn't intentionally
create an inefficient path.. The problem was recognized and fixed the
same day.
Latency on an MPLS circuit is the cumulative latency on the Label Switch
Path, and a number of the hops are invisible. The latency per hop is
still the same... you just can't see that your traffic is travelling to
say Denver or Dallas.
Tim Peiffer
Network Support Engineer
Networking and Telecommunications Services
University of Minnesota/NorthernLights GigaPOP
Frank Bulk - iNAME wrote:
Thanks for the added information.
Even if their MPLS path went from the midwest (where I'm located) to San
Francisco and then back to St. Louis (where 12.122.112.22 appears to be), I
don't think that accounts for a 70 msec jump in traffic. And I don't think
they would (intentionally) create such an inefficient MPLS path.
Someone off-list told me they tried to trace to 12.88.71.13, but once they
hit an AT&T router their ICMP traffic appeared to be blocked.
Frank
-----Original Message-----
From: John T. Yocum [mailto:[EMAIL PROTECTED]
Sent: Thursday, June 26, 2008 8:09 PM
To: [EMAIL PROTECTED]
Cc: nanog list
Subject: Re: Possible explanations for a large hop in latency
The explanation I got, was that the latency seen at the first hop was
actually a reply from the last hop in the path across their MPLS
network. Hence, all the following hops had very similar latency.
Personally, I thought it was rather strange for them to do that. And,
I've never seen that occur on any other network.
Perhaps someone from ATT would like to chime in.
--John
Frank Bulk - iNAME wrote:
Did that satisfy you? I guess with MPLS they could tag the traffic and
send
it around the country twice and I wouldn't see it at L3.
Frank
-----Original Message-----
From: John T. Yocum [mailto:[EMAIL PROTECTED]
Sent: Thursday, June 26, 2008 7:04 PM
To: [EMAIL PROTECTED]
Cc: nanog list
Subject: Re: Possible explanations for a large hop in latency
When I asked ATT about the sudden latency jump I see in traceroutes,
they told me it was due to how their MPLS network is setup.
--John
Frank Bulk wrote:
Our upstream provider has a connection to AT&T (12.88.71.13) where I
relatively consistently measure with a RTT of 15 msec, but the next hop
(12.122.112.22) comes in with a RTT of 85 msec. Unless AT&T is sending
that
traffic over a cable modem or to Europe and back, I can't see a reason
why
there is a consistent ~70 msec jump in RTT. Hops farther along the route
are just a few msec more each hop, so it doesn't appear that
12.122.112.22
has some kind of ICMP rate-limiting.
Is this a real performance issue, or is there some logical explanation?
Frank
