Possible explanations for a large hop in latency

Tim Peiffer peiffer at umn.edu
Thu Jun 26 21:03:19 CDT 2008


We had a similar situation going from Minneapolis to Kansas City via 
Chicago. Normal latency from Minneapolis to Chicago via Level3 MPLS 
network is about 14msec RTT.  When the the circuit from Minneapolis to 
Chicago went out for one reason or another, our MPLS link went from 
Minneapolis to Tulsa, to Dallas, and then to Chicago..  That added a 
little latency in the path from Minneapolis to Chicago.. We didn't need 
to exceed the SLA in order to cry foul.  They didn't intentionally 
create an inefficient path.. The problem was recognized and fixed the 
same day.

Latency on an MPLS circuit is the cumulative latency on the Label Switch 
Path, and a number of the hops are invisible.  The latency per hop is 
still the same... you just can't see that your traffic is travelling to 
say Denver or Dallas.

Tim Peiffer
Network Support Engineer
Networking and Telecommunications Services
University of Minnesota/NorthernLights GigaPOP


Frank Bulk - iNAME wrote:
> 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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