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Re: Possible explanations for a large hop in latency

  • From: John T. Yocum
  • Date: Thu Jun 26 21:09:49 2008
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