Possible explanations for a large hop in latency

Robert Richardson bobrmr at gmail.com
Thu Jun 26 20:19:48 CDT 2008


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 at 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 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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