Re: Possible explanations for a large hop in latency

["Robert Richardson" <bobrmr () gmail com>]

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 () 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 () fluidhosting com] Sent: Thursday, June
> > 26, 2008 7:04 PM
> > To: frnkblk () 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