data link Story: Step 2: Use traceroute to test the routing

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Step 2: Use traceroute to test the routing

Now that you know you can reach a target remote system from your workstation, learning about the path packets will follow to get to their destination is the next step. The traceroute utility reveals the route your packets take from one machine to another.

Before you use the traceroute utility, make sure your institution is currently connected to either vBNS or Abilene. Both vBNS and Abilene list their connected institutions online. You can take advantage of the vBNS and Abilene backbone services only if your traffic is routed through those backbones.

The example below uses traceroute to check the route from NCSA's modi4 to SDSC's golden. (The output was formatted to improve readability.)

modi4 58% /usr/etc/traceroute golden.sdsc.edu
traceroute to golden.sdsc.edu (132.249.40.55), 30 hops max, 40 byte packets
1 lucy.ncsa.uiuc.edu (141.142.7.228 ) 2 ms 1 ms 1 ms
2 charlie-atm1-0-8.ncsa.uiuc.edu (141.142.11.134 ) 2 ms 1 ms 1 ms
3 cs-atm0-0-12.ncsa.vbns.net (141.142.11.1 ) 2 ms 4 ms 2 ms
4 cs-atm0-0-5.sdsc.vbns.net (204.147.129.69 ) 56 ms 56 ms 56 ms
5 medusa.sdsc.edu (198.17.46.10 ) 57 ms 57 ms 58 ms
6 tigerfish.sdsc.edu (132.249.30.11 ) 58 ms 56 ms 56 ms
7 golden.sdsc.edu (132.249.40.55 ) 59 ms 60 ms 61 ms

The traceroute utility sends packets with an increasing TTL (time-to-live) field at the IP header. It sets TTL to 0 and gets a time-exceeded message back from the first router on the path. Then it sets TTL to 1 and gets another message from the second router, and so on. Three packets are usually send in a row and three replies are sent back. The traceroute output shows the name of the router followed by three RTTs of the packets. If a message is lost, the router won’t reply, or if the reply is lost, traceroute prints out an asterisk (*) indicating an error.

Look carefully at the name of the routers in the traceroute output above. If your packet is traversing the vBNS, you will see some routers in the middle of the output with the vbns.net domain name (see lines #3 and #4 above). If you don't see a vbns.net domain name, it is a strong indication that your vBNS routing is not in place. Check with the network operations staff at your local institution or contact the NLANR engineering service team for help.

If your site is connected to Abilene, use traceroute to check if your Abilene routing is in place. Some routers should show the abilene.ucaid.edu domain name. The example below shows a traceroute from modi4 at NCSA to the web server at the University of Kansas. The university is connected to Abilene, but not vBNS. The packet issued from NCSA goes through NCSA's vBNS backbone and then continues to the Abilene backbone (see lines #5 and #6 below) on its way to Kansas.

modi4 62% /usr/etc/traceroute www.ku.edu
traceroute to raven.cc.ukans.edu (129.237.33.3), 30 hops max, 40 byte packets
1 lucy.ncsa.uiuc.edu (141.142.7.228 ) 2 ms 2 ms 1 ms
2 charlie-atm1-0-8.ncsa.uiuc.edu (141.142.11.134 ) 1 ms 1 ms 9 ms
3 cs-atm0-0-12.ncsa.vbns.net (141.142.11.1 ) 3 ms 2 ms 1 ms
4 cs-atm0-0-6.dng.vbns.net (204.147.129.246) 5 ms 5 ms 5 ms
5 ipls-aads-nap.abilene.ucaid.edu (198.32.130.83 ) 9 ms 9 ms 10 ms
6 kscy-ipls.abilene.ucaid.edu (198.32.8.5 ) 19 ms 20 ms 18 ms
7 ks-2-p00.r.greatplains.net (164.113.248.130) 19 ms 19 ms 24 ms
8 ks-2-ku.r.greatplains.net (164.113.240.205) 27 ms 21 ms 22 ms
9 raven.cc.ukans.edu (129.237.33.3 ) 21 ms 20 ms 21 ms

A healthy routing is usually symmetric: traffic from system1 to system2 and from system2 to system1 traverse the same router set in reverse order. If you run traceroute in both directions and get a different routing, you will likely have asymmetric TCP performance as well. If you record asymmetric routing, pass the data along to your local network operations staff or to ncne@ncne.nlanr.net.