Possible explanations for a large hop in latency

John T. Yocum john at fluidhosting.com
Fri Jun 27 01:09:25 UTC 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.


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 at fluidhosting.com] 
> Sent: Thursday, June 26, 2008 7:04 PM
> To: frnkblk at 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 ( where I
>> relatively consistently measure with a RTT of 15 msec, but the next hop
>> ( 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
>> has some kind of ICMP rate-limiting.
>> Is this a real performance issue, or is there some logical explanation?
>> Frank

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