Distributed Systems Department Collaboration Technologies Group

Mtrace information

Mtrace

• This tool will allow you to trace a route from a receiver to a source working backwards using a particular multicast address. If either the receiver or the source is not participating in a multicast on the specified address then mtrace may not work.
• If any of the lines in your mtrace output say "default" then it generally means that the router had no multicast route for that source and so sent you down its unicast default route which is probably not where you wanted to go.
• Vic and vat can automatically launch the mtrace utility. To have mtrace determine the path to or from a particular site in the multicast session simply hold down on the left mouse button on the site and choose "mtrace to" or "mtrace from". If you have not set mtrace to suid root then this will not work unless you are root.
• If mtrace succeeds then you have a working MBone path between you and the site.
• To determine why you are experiencing high loss rates or complete loss of a particular source you generally need to look at the statistics (run mtrace twice in a row to improve the accuracy of the statistics). The interesting statistic is usually the Lost/Sent column. If a particular site has lost a large number of the packets then that router or the next router downstream is likely the problem. If there is a question mark in the Lost field then you might be experiencing a problem with that router or the one downstream from it. This sometimes is caused by an incorrect pruning entry in one of the routers' caches. In the current version of mrouted flushing the cache will not get rid of the incorrect entry, the mrouted or router must be restarted.
• If you need to regularly diagnose the path between a particular pair of sites connected via the MBone, one strategy is to run mtrace when the route is working and store the results. When things go wrong try running mtrace again and if that doesn't work, then start running mrinfo on the routers along the old path and see if there is an obvious problem.
• If you are not running vic or vat on both the site that you are mtracing from and the site that you want to mtrace to then you will not have enough traffic flowing along the route to diagnose where packets are being lost. You can still use mtrace if the site you are trying the mtrace to is advertising a multicast session. To do this mtrace should be invoked with the sdr multicast address which is sap.mcast.net and use the argument -S 300 to tell it to gather statistics over a 300 second interval or use -l to tell it to use a continuous interval. Basically, you can't figure out where the packets are being dropped if none where sent by the remote machine during the statistics interval.

---

Introduction to the MBone Page

---

Please also visit the

Lawrence Berkeley National Laboratory Home Page.

Distributed Systems Department Home Page.

Distributed Collaboratoratories Home Page.

---

This document was last updated on July 31, 2001, and is located at http://www-itg.lbl.gov/OldMisc/mbone/mtrace.tips.html.

To report Web page problems, e-mail webmaster-george@george.lbl.gov.

Support Credits identify the funding sources and the organizational context of the work described in this document.

Privacy and site security notice to Users .

Deb Agarwal (DAAgarwal@lbl.gov) is responsible for this WWW document.