Long hops on international paths

PAUL R BARFORD pb at cs.wisc.edu
Mon Jan 17 23:42:43 UTC 2022

What we're considering specifically are consecutive (layer 3) hops as identified by traceroute.  Thus, TTL is decremented by 1 and no more than 1 (i.e., we have to get full information (not *****) from consecutive hops to consider the link).  I have asked my colleague to put together a set of examples.  We assume that there are multiple layer 1 and 2 links, and possibly layer 3 hops masked from traceroute by MPLS.  But what we're seeing in terms of hops exposed by traceroute make it look like a single (TTL decremented by 1) hop.

I'll post the examples when I get them.

From: morrowc.lists at gmail.com <morrowc.lists at gmail.com>
Sent: Monday, January 17, 2022 5:13 PM
To: PAUL R BARFORD <pb at cs.wisc.edu>
Cc: Pengxiong Zhu <pzhu011 at ucr.edu>; nanog at nanog.org <nanog at nanog.org>
Subject: Re: Long hops on international paths

On Mon, Jan 17, 2022 at 5:31 PM PAUL R BARFORD <pb at cs.wisc.edu<mailto:pb at cs.wisc.edu>> wrote:
Dear Pengxiong,

Thanks for your questions:

  1.  We are using CAIDA’s Internet Topology Data Kit (ITDK) that uses the MIDAR alias resolution method to infer IP addresses assigned to the same router.
  2.  We understand the concerns about IP geolocation.  Interfaces of the router in question are assigned similar domain names e.g., “chi-b2-link.ip.twelve99.net<http://chi-b2-link.ip.twelve99.net>” ( We also used CAIDA’s ITDK, which provides geolocation information, and indicates that this router is located in Chicago.  We cross-reference with Maxmind where possible.  In this particular case, there is the telltale in the use of "chi" in the domain name.

I think nick's point about ttl expiry and missing some context on topology still stands.
I'd be that the paths between 2 continents do not actually land in chicago... that you're seeing (or not seeing) missing hops between the coast(s) and chicago inside 1299's network in the US.


Hope that helps.

Regards, PB
From: Pengxiong Zhu <pzhu011 at ucr.edu<mailto:pzhu011 at ucr.edu>>
Sent: Monday, January 17, 2022 3:23 PM
To: PAUL R BARFORD <pb at cs.wisc.edu<mailto:pb at cs.wisc.edu>>
Cc: nanog at nanog.org<mailto:nanog at nanog.org> <nanog at nanog.org<mailto:nanog at nanog.org>>
Subject: Re: Long hops on international paths

Hi Paul,

Just curious. How do you determine they are the same routers? Is it based on IP address or MAC addresses? Or using CAIDA’s router alias database?

Also how do you draw the conclusion that the AS1299 router is indeed in Chicago? IP-geolocation based on rDNS is not always accurate though.


On Mon, Jan 17, 2022 at 10:03 AM PAUL R BARFORD <pb at cs.wisc.edu<mailto:pb at cs.wisc.edu>> wrote:

I am a researcher at the University of Wisconsin.  My colleagues at Northwestern University and I are studying international Internet connectivity and would appreciate your perspective on a recent finding.

We're using traceroute data from CAIDA's Ark project for our work.  We've observed that many international links (i.e., a single hop on an end-to-end path that connects two countries where end points on the hop are identified via rDNS) tend to originate/terminate at the same routers.  Said another way, we are observing a relatively small set of routers in different countries tend to have a majority of the international connections - this is especially the case for hops that terminate in the US.  For example, there is a router operated by Telia (AS1299) in Chicago that has a high concentration of such links.  We were a bit surprised by this finding since even though it makes sense that the set of providers is relatively small (i.e., those that offer global connectivity), we assumed that the set of routers that used for international connectivity within any one country would tend to be more widely distributed (at least with respect to how they appear in traceroute data - MPLS notwithstanding).

We're interested in whether or not this is indeed standard practice and if so, the cost/benefit for configuring international connectivity in this way?

Any thoughts or insights you might have would be greatly appreciated - off-list responses are welcome.

Thank you.

Regards, PB

Paul Barford
University of Wisconsin - Madison


Pengxiong Zhu
Department of Computer Science and Engineering
University of California, Riverside
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