Possible explanations for a large hop in latency

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.

--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
>>
>>
> 
> 