Appendix E. Glossary - Black Box ACR1000A-CTL-24 Manual

iPATH Agility Controller

Appendix E. Glossary

E.1 Internet Group Management Protocol
Where a Agility transmitter is required to stream video to two or more receivers, multicasting is the method used.
Multicasting involves the delivery of identical data to multiple receivers simultaneously without the need to maintain individual
links. When multicast data packets enter a subnet, the natural reaction of the switches that bind all the hosts together within
the subnet, is to spread the multicast data to all of their ports. This is referred to as Multicast flooding and means that the hosts
(or at least their network interfaces) are required to process plenty of data that they didn't request. IGMP offers a partial
solution.
The Internet Group Management Protocol (IGMP) is designed to prevent multicast flooding by allowing Layer 3 switches to check
whether host computers within their care are interested in receiving particular multicast transmissions. They can then direct
multicast data only to those points that require it and can shut off a multicast stream if the subnet has no recipients.
There are currently three IGMP versions: 1, 2 and 3, with each version building upon the capabilities of the previous one:
• IGMPv1 allows host computers to opt into a multicast transmission using a Join Group message, it is then incumbent on the
router to discover when they no longer wish to receive; this is achieved by polling them (see IGMP Querier below) until they no
longer respond.
• IGMPv2 includes the means for hosts to opt out as well as in, using a Leave Group message.
• IGMPv3 encompasses the abilities of versions 1 and 2 but also adds the ability for hosts to specify particular sources of
multicast data.
Agility units make use of IGMPv2 when performing multicasts to ensure that no unnecessary congestion is caused.
E.1.2 IGMP Snooping
The IGMP messages are effective but only operate at layer 2 - intended for routers to determine whether multicast data should
enter a subnet. A relatively recent development has taken place within the switches that glue together all of the hosts within each
subnet: IGMP Snooping. IGMP snooping means these layer 2 devices now have the ability to take a peek at the IGMP messages.
As a result, the switches can then determine exactly which of their own hosts have requested to receive a multicast – and only
pass on multicast data to those hosts.
E.1.3 IGMP Querier
When IGMP is used, each subnet requires one Layer 3 switch to act as a Querier. In this lead role, the switch periodically sends
out IGMP Query messages and in response all hosts report which multicast streams they wish to receive. The Querier device and
all snooping Layer 2 switches, then update their lists accordingly (the lists are also updated when Join Group and Leave Group
(IGMPv2) messages are received).
E.1.4 IGMP Fast-Leave (aka Immediate Leave)
When a device/host no longer wishes to receive a multicast transmission, it can issue an IGMP Leave Group message as
mentioned above. This causes the switch to issue an IGMP Group-Specific Query message on the port (that the Leave Group was
received on) to check no other receivers exist on that connection that wish to remain a part of the multicast. This process has a
cost in terms of switch processor activity and time.
Where Agility units are connected directly to the switch (with no other devices on the same port) then enabling IGMP Fast-Leave
mode means that switches can immediately remove receivers without going through a full checking procedure. Where multiple
units are regularly joining and leaving multicasts, this can speed up performance considerably.
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