Seeking Comcast Contact: need to troubleshoot packet loss and/or asymmetric routing issue between Comcast & Onvoy
Craig D. Rice
cdr at stolaf.edu
Thu Aug 2 13:30:13 UTC 2007
For four months dozens of our users who are Comcast subscribers have had
difficulty reaching St. Olaf College's and Carleton College's network services.
We have worked through everything we can think of with our Onvoy (regional
ISP) network engineers. We have isolated the problem a couple of Comcast's
IP subnets, but need a contact within Comcast to further troubleshoot.
The behavior in a nutshell:
User A on Comcast Subnet B browses to www.stolaf.edu (http or https, other
web sites on-site and @carleton.edu behave the same). Our access_log shows
an initial "GET /" of our homepage, then very slow (if any) subsequent
requests (for our stylesheet or homepage images). Ping's look fine;
traceroute's look as reasonable. Telnet's to port 80 and other services do
seem to respond, albeit very slowly.
User A has the same problem with access @carleton.edu but can access
everything else (including other Onvoy customers) without any trouble
If User A then removes his Linksys router and connects his computer directly
to the cable modem, he acquires an IP address in Comcast Subnet C. Then,
everything works fine, including access to www.stolaf.edu and
www.carleton.edu. He puts the Linksys router back in (which still has the IP
address in Comcast Subnet B), and the problem returns.
The problem IP subnets are completely consistent.
Known WORKING IP Subnets: 184.108.40.206, 24.x
Known NON-WORKING IP Subnets: 71.x, 73.x
We have already attempted the usual troubleshooting and have eliminated user
problems, computer problems, server problems, cable modem problems, and
Linksys router problems. Traceroutes have been somewhat inconclusive since
Onvoy blocks ICMP within its network.
So, why just St. Olaf and Carleton services? We are on a shared physical
link from Onvoy, though on different VLANs. Onvoy has verified everything
they can (routing, packet loss, etc.) between them and us, and I'm not sure
what additional questions I can ask of them to test. Suggestions?
Maybe Comcast has a broken transparent proxy on part(s) of their network?
But they have told us they have nothing like this anywhere on their network.
Maybe there is some asymmetric routing somewhere, though all the
investigation there has come up empty.
A third possibility is some kind of packet loss, but there is little if any
evidence of that.
So, we are really at a loss and seek any suggestions you all might have. And
a contact in Comcast network engineering would be especially useful to
continue our troubleshooting.
Craig D. Rice Associate Director of Information Systems
cdr at stolaf.edu Information and Instructional Technologies
+1 507 786-3631 St. Olaf College
+1 507 786-3096 FAX 1510 St. Olaf Avenue
http://www.stolaf.edu/people/cdr Northfield, MN 55057-1097 USA
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