Support for IP Multicast on the UC Berkeley Campus Network

IP Multicast is widely-deployed on the Berkeley campus network as a "best effort" feature of the network. It is not a fully supported network service. Campus users are encouraged to use IP multicast applications where feasible, but are cautioned against relying exclusively on IP multicast applications for any critical or very important academic or business activities. For example, IP multicast should not be used to broadcast classroom sessions unless there is a non-multicast mechanism available as a backup, or it is acceptable for multicast to not work correctly at times.

"Best effort" support in this context means:

• IP multicast is available on most, but not all, campus subnets. IP multicast does not work on AirBears wireless networks. There are specific subnets on wired networks where multicast routing has been disabled because of network performance degradation or other operational problems. The small number of wired networks on which IP multicast routing is currently disabled are in all cases either shared 10 Mb/s Ethernets operating on ancient network equipment or remote networks connected to campus by relatively low bandwidth (1.5 Mb/s) T-1 circuits.

• IST will accept trouble reports and will attempt to resolve IP Multicast routing problems, including problems with off-campus connectivity. Users are cautioned however that troubleshooting and fixing IP multicast problems can be very time-consuming, can take significantly long periods of time, and can require significant interaction between users and IST technical staff including scheduled testing and reporting. There may be instances in which IST is unable to satisfactorily resolve the problem.

• There is little or no support from IST for installation, configuration, and operation of IP multicast application software on users' computers.

Suggestions for a Pleasant IP Multicast Experience

In spite of the limited support for IP multicast at UCB, IST believes that IP multicast generally "just works" on the campus network without any special effort. However, there will be times when it will not work; here are a few suggestions for improving the probability that your experience will be a pleasant one:

Test! And then, test again.
If an IP multicast application will be used for an important activity, be sure to test in advance. Be prepared for it to not work correctly the first time, and allow time for testing and troubleshooting. IST suggests initial testing occur at least two weeks in advance of the first instance of the real activity. Once it is working, test again at decreasing intervals prior to the event. Finally, test shortly (one hour) before the scheduled activity. Don't assume that because it worked a week before, it will work again when needed.

This recommendation for testing applies to regularly scheduled activities as well as one-time activities: examples of successful use of IP multicast for broadcasting class lectures and weekly or monthly seminars typically include setup and testing for one hour before every lecture or seminar. This allows some time for problem resolution, should something not work.

Firewalls and IP Multicast
IP multicast employs several protocols that are often disabled by firewalls, either in their "out of the box" default configurations or by firewall administrators who fear protocols that they do not adequately understand.

• IGMP (Internet Group Management Protocol) is used on the local subnet to signal hosts' desire to join or leave multicast groups to the router. Any firewall on the subnet, including host firewall software, must permit IGMP in order for multicast to work.

• Most IP multicast content is carried in UDP (User Datagram Protocol) with high port numbers. Firewalls must permit UDP packets with high port numbers in order for multicast to work. On many firewalls, it is possible to do this specifically for IP multicast packets, which can be identified by their destination IP addresses which are in the 224.0.0.0/4 address block (224.0.0.1 through 239.255.255.255). The source IP address in an IP multicast packet is a standard unicast IP address and may fall anywhere in the usual IP address range, i.e. 1.0.0.1 through 223.255.255.255.

If a user behind a firewall is having trouble with IP multicast, please find a way to test with the firewall temporarily disabled before contacting IST to report a problem.

Please send questions or comments about using IP multicast at UCB to noc@berkeley.edu.