Knowledgebase
Insight Multicast Usage
Posted by on 04 April 2011 02:54 PM

Overview

While Insight defaults to IP-Broadcast (255.255.255.255) when sending data from a “Teacher” machine to multiple “Student” machines, it can easily be reconfigured to use IP-Multicast. The primary benefit of IP-Multicast is twofold: the data packets can then traverse to different network segments and the network infrastructure can isolate Insight network traffic to just the areas which contain Insight Student machines. For simplicity, this paper will use the term “Teacher” and “Student” as data source and sink respectively.

Teacher Channel

Insight assigns a unique “Teacher Channel” number to every Teacher machine. This is a number between 1 and 16000 and is usually assigned at install time. The Teacher Channel number is used to group Students to a particular Teacher. For example, if a particular Teacher were assigned Channel 212, then Students in that same classroom should also be assigned to Channel 212. (While schools are free to use whatever numbering scheme for Teacher Channel assignment, most schools will use the teacher’s room number or a combination of building and room number.) The Teacher Channel number is included in the beginning of each Insight data packet so that receiving Students will be able to discard packets not intended for their current Teacher Channel number. If Insight is using IP-Broadcast, then all Insight UDP traffic will go to all Students and each Student will either process or discard the packet depending on his currently assigned Teacher Channel and the Teacher Channel in the beginning of the data packet.

Multicast Addresses

Every Insight Student machine will join two Multicast Groups. The first is the “global” Insight group: 239.0.208.0. A Teacher will send packets to this group when it is intended for ALL Students, regardless of Teacher Channel. The second is a “Channel Specific” group. It is determined by the currently assigned Teacher Channel. The low-order byte of Teacher Channel is used as the low-order byte of the IP-Multicast group: 239.0.208.xxx. For example, if the Teacher were assigned Teacher Channel 212, then she would send packets to 239.0.208.212. Since this addressing scheme has only 256 unique addresses, it is quite possible that multiple Teacher channels will map to the same “Channel Specific” group. (For example, Channel’s 212, 468, and 724 will all map to group 239.0.208.212.) Since the actual Teacher Channel is included at the beginning of each data packet, student can easily discard packets which are not destined for their current Teacher Channel. The design goal is to spread out the Insight traffic to several Multicast groups and hopefully reduce the overall reach of Insight traffic.

Limiting Insight Traffic

By default, the TTL (Time To Live) for all Multicast packets is set to 6. This is designed to get the traffic through enough routers to be live school-wide without going “world-wide”. This can be altered by adding to the registry the REG_DWORD value ”TimeToLive” on the Teacher machine at HKEY_LOCAL_MACHINE\SOFTWARE\Insight. By adjusting this value, Insight traffic can be adjusted to stay within a subnet, a school, a campus, etc… Another way to limit Insight to a specific area is by limiting Multicast traffic on port 796. That is the UDP port used by Insight. Most routers can be configured to not route (or block) traffic destined to a specific port.
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