Possible explanations for a large hop in latency

Frank Bulk - iNAME frnkblk at iname.com
Thu Jun 26 20:32:30 CDT 2008


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:john at fluidhosting.com] 
Sent: Thursday, June 26, 2008 8:09 PM
To: frnkblk at iname.com
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: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 (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
>>
>>
>
>






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