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 
whatsoever.

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: 75.72.0.0, 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.

With thanks,
Craig
-- 
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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