“Multicast: Beacons & Bridges”

Mark Hayes 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 distribution.

Machines express an interest in receiving multicast traffic by joininga “multicast group”, e.g. 224.2.177.155. Multicast packets are sentto this group, represented by a special sort of IP address in the range224.0.0.0-239.255.255.255. (There is also an associated port number.)

http://www.tldp.org/HOWTO/Multicast-HOWTO.html

How do I know if I have working multicast?

• Ask your local network admin

• Try the ‘mtrace’ tool - a multicast version of ‘traceroute’ftp://limestone.uoregon.edu/pub/multicast/kddlabs-mirror/mtrace/

• Install the NLANR multicast beacon client...

The NLANR beacon

• Application developed by The National Laboratory for AppliedNetwork 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.

http://dast.nlanr.net/Projects/Beacon/

Beacon client

Beacon clientBeacon client

Multicast traffic

Multicast trafficMulticast traffic

Beacon server

Traffic statistics(reports)

Traffic statistics(reports)

Traffic statistics(reports)

WWW presentationof multicast traffic

statistics

Beacon architecture

Screenshothttp://ulcc.beacon.ja.net/

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, 233.3.18.1/55524)• Start it up start_beacon.bat

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: http://www.agsc.ja.net/ support@agsc.ja.netUseful 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

Questions?

Multicast-Unicast bridge

Luckily for the multicast-deprived, it is possible to “bridge”to a multicast capable network. Usually considered a temporarysolution until multicast can be enabled, or as a workaround duringa multicast outage.

QuickBridge

Runs on a multicast capable machine. Clients connect to the bridgeand it forwards all multicast traffic to/from a specific multicast groupon behalf of the client.

Developed by Mike Daw (Manchester) and Stephen Booth (EPCC)

Available for download at http://people.man.ac.uk/~zzcgumd/software/QuickBridge.tar.gz

Documentation at http://www.accessgrid.org/agdp/guide/network-bridging.html

(also covers the multi-session bridge for ANL virtual venues.)

Usage scenarios

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.

Installing QuickBridge

• Choose a machine to run the bridge software. (Linux) N.B. this should not be running other AG software

• Download the source from http://people.man.ac.uk/~zzcgumd/software/QuickBridge.tar.gz

• Uncompress in a new directory tar -zxvf QuickBridge.tar.gz

• Compile the C source code cc -o QuickBridge QuickBridge.c

Using QuickBridge

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 233.2.171.193 59756 50304 University of CambridgeCam v 233.2.171.193 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.

Using QuickBridge

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>

Questions?