They probably don't propagate TTL w/in their MPLS core. Depending on how they have MPLS implemented, you may only see 2 hops on the network; the ingress and egress routers. If the ingress router was in NYC and the egress in Seattle, you could understandably expect a large jump in RTT. Not an ATT customer but do know other providers run their MPLS core's this way... -Robert On Thu, Jun 26, 2008 at 6:09 PM, John T. Yocum <john@fluidhosting.com> wrote:
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:john@fluidhosting.com] Sent: Thursday, June 26, 2008 7:04 PM To: frnkblk@iname.com 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