Overview
To allow Access Grid sessions to accommodate large-scale meetings across tens of sites while minimizing the impact on network resources, audio and video feeds from a venue are transmitted over the Internet using IP multicast. Unlike with unicast sessions, which are the standard method for data transfer across the Internet, multicast problems are extremely difficult to debug from end stations. Instead, diagnosing multicast failures usually require information stored only within the underlying network infrastructure.
A "looking glass" is a web interface which allows access to information about a particular network and is generally used as a view into a certain point within the network. Abilene, the production network of the Internet2 consortium, has a looking glass system which allows queries to most of their routers via http://ratt.uits.iu.edu/routerproxy/abilene
 | For the debugging techniques described in this document, one should avoid using the router "IPLS-v6 [Indianapolis, Indiana]" as this device is dedicated to IP version 6 information. Any of the other routers, especially those ending with "ng", can be used. |
For inter-site multicast debugging, there are three different protocols which are important: mbgp, mtrace, and msdp.
mBGP (multiprotocol border gateway protocol) - mBGP is a protocol used to advertise networks which are multicast enabled.
MSDP (multicast source discovery protocol) - MSDP is a protocol used to advertise that a particular host has traffic to send to anyone who belongs to a particular session or multicast group.
Mtrace (multicast traceroute) - Mtrace is a protocol used to determine if a particular router can send multicast traffic to a particular host and along which path this traffic will take.
Typically, multicast issues between institutions result in either a failure to receive any audio and video sessions or prevent other sites from seeing your streams. The MTRACE command is useful for diagnosing why an institution cannot receive traffic, because it shows the path that multicast data takes and where this traffic might be blocked. If others are having problems receiving an institution's sessions, then MSDP should be checked first to ensure that the sources are being properly advertised. In either case, one should check mBGP to ensure that the networks are properly flagged as multicast enabled.