Possible explanations for a large hop in latency

bensons at riot.queuefull.net bensons at riot.queuefull.net
Fri Jun 27 02:57:53 UTC 2008

Depending on whether TTL is propagated into MPLS, this could be true.

Though it should also be pointed out that ICMP responses aren't exactly a precise scientific tool... The responding router could just be busy, and the response time could be reflective of load more than link latency etc. Similarly, failure to get any response at all from a router isn't necessarily indicative of packet loss...


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-----Original Message-----
From: "Frank Bulk - iNAME" <frnkblk at iname.com>

Date: Thu, 26 Jun 2008 19:54:42 
To:"'John T. Yocum'" <john at fluidhosting.com>
Cc:nanog list <nanog at merit.edu>
Subject: RE: Possible explanations for a large hop in latency

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.


-----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.


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
> 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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