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