“Multicast: Beacons & Bridges”
Technical Director - Cambridge eScience Centre
Access Grid on the Desktop - Tuesday 15th June 2004
Multicast: an overview
Multicast is a one-to-many distribution model for network traffic,
especially used for video conferencing and other forms of data
Machines express an interest in receiving multicast traffic by joining
a “multicast group”, e.g. 126.96.36.199. Multicast packets are sent
to this group, represented by a special sort of IP address in the range
188.8.131.52-184.108.40.206. (There is also an associated port number.)
How do I know if I have working multicast?
• Ask your local network admin
• Try the ‘mtrace’ tool - a multicast version of ‘traceroute’
• Install the NLANR multicast beacon client...
The NLANR beacon
• Application developed by The National Laboratory for Applied
Network Research (NLANR), USA
• v0.8 written in Java - OS/platform independent
(latest v0.9 in Perl. We will only consider v0.8 here.)
• client-server architecture
- clients exchange multicast packets (with time stamps) and
calculate % packet loss, delay, etc.
- server collects these parameters from the clients and presents
them to the user via a web interface.
• Open source (BSD-type license) allows modification, unlimited
distribution provided you acknowledge NLANR.
of multicast traffic
Installing the beacon client
• Prerequisites: a working Java run-time environment,
and an accurate system clock! (use NTP/Win XP time sync.)
• Download the source (v0.8 available from NIEeS)
• Install in a new directory
mkdir beacon; unzip ukbeacon-v0.8.zip
• Edit the configuration file to point to a beacon server and
a multicast group + port number
(e.g. ulcc.beacon.ja.net/19997, 220.127.116.11/55524)
• Start it up
Interpreting the beacon display
• At its most basic:
green=good, yellow=warning, red=trouble,
grey/white=no information available (usually indicates no
working multicast, also bad!)
• The beacon webpage usually indicates how much % packet
loss/time delay etc. corresponds to which colour
• The columns indicate statistics for multicast senders,
the rows for receivers.
Note that multicast traffic is not necessarily symmetric.
What to do when things look bad...
• Talk to your (friendly) local network support staff
• Contact the Access Grid support centre:
Useful information they’ll likely need: IP address/hostnames,
address for the multicast group you’re attempting to join.
• Your local e-Science Centre may also be able to help
Luckily for the multicast-deprived, it is possible to “bridge”
to a multicast capable network. Usually considered a temporary
solution until multicast can be enabled, or as a workaround during
a multicast outage.
Runs on a multicast capable machine. Clients connect to the bridge
and it forwards all multicast traffic to/from a specific multicast group
on behalf of the client.
Developed by Mike Daw (Manchester) and Stephen Booth (EPCC)
Available for download at
(also covers the multi-session bridge for ANL virtual venues.)
1. Local network does not support multicast at all
May require the co-operation of a friendly remote site with
multicast to run the bridge for you.
2. AG node behind a firewall that will not forward multicast
(but the local network beyond the firewall does), therefore
run a bridge yourself on a machine just outside the firewall.
• Choose a machine to run the bridge software. (Linux)
N.B. this should not be running other AG software
• Download the source from
• Uncompress in a new directory
tar -zxvf QuickBridge.tar.gz
• Compile the C source code
cc -o QuickBridge QuickBridge.c
Starting the server on a multicast-capable machine:
Multicast groups and port numbers are defined for each AG
virtual venue in the vv.dat file.
e.g. the entry for the University of Cambridge virtual venue reads:
Cam a 18.104.22.168 59756 50304 University of Cambridge
Cam v 22.214.171.124 59754 50306 University of Cambridge
./QuickBridge.ksh -s -r Cam
This will output the unicast port numbers for audio and video.
The remote user connecting to the bridge will need these port
numbers and the hostname of the machine running the bridge.
Connecting to a bridge from an Access Grid session:
In command line mode:
cd C:\ag\agapps\bin (or similar)
audio: rat <QuickBridge hostname>/<port>
video: vrm <QuickBridge hostname>/<port>