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administratively scoped addresses – 22.214.171.124 through 126.96.36.199; used for private network multicasting
CGMP – Cisco group management protocol. Used by a router to inform a switch of multicast MACs it should add or remove from its CAM table. The switch then knows which ports to forward multicast traffic to, rather than flooding to all ports.
GLOP addressing – 188.8.131.52 through 184.108.40.206. Used by ASN owners to get a block of 256 multicast addresses as allocated by the IANA.
IGMP – internet group management protocol. Hosts and routers use IGMP to communicate about the desire to join or leave a multicast application.
IGMP snooping – a switch inspects IGMP messages at L3 to determine what switch ports should participate in multicast group traffic
IGMPv1 host membership query – sent by the v1 router every 60 seconds by default; asks whether anyone on the segment wants to receive multicast traffic for any group
IGMPv1 host membership report – a host sends to all hosts that he wants to receive multicast traffic for a specific group
IGMPv2 group-specific query – the router sends this after receiving a v2 leave message; asks if there’s anyone left who wants to receive traffic for the multicast group
IGMPv2 host membership query – sent by the v2 router asking if anyone on the segment wants to receive multicast traffic for any group, every 125 seconds by default
IGMPv2 host membership report – sent by the host to all on the segment announcing the group it wants to receive multicast traffic for
IGMPv2 leave – sent by a v2 host when it no longer wishes to receive traffic for a specific multicast group.
IGMPv3 host membership query – sent by the v3 router asking if anyone on the segment wants to receive multicast traffic for any group, every 125 seconds by default
IGMPv3 host membership report – sent by v3 hosts to 220.127.116.11 on the segment announcing the group it wants to receive multicast traffic for. In v3, the source IP can also be specified.
joining a group – firing up a multicast application on a host.
MLD – multicast listener discovery protocol; used in IPv6 to determine what hosts are listening on the link-local for multicast traffic
MRT – maximum response time – a host must respond to an IGMP query with an IGMP report in this amount of time, 10 seconds by default.
multicast address range – 18.104.22.168 through 22.214.171.124
multicast address structure – 4 bits of 1110 + 28 bits of whatever. There is no concept of a subnet mask.
multicast MAC address – 01.00.5E (24 bits) + 0 (1 bit) + last 23 bits of L3 multicast address (23 bits)
multicasting – relaying a single stream of data to multiple recipients without having multiple unicasts, one for each receiver.
permanent multicast group – addresses assigned by the IANA
querier election – in IGMPv2 and v3, this is the process of determining which multicast router will send IGMP queries onto the segment. IGMPv1 relies on multicast routing protocols to determine this.
report suppression mechanism – this prevents all hosts from responding to IGMP queries, since 1 host is sufficient to keep the multicast data flowing into the segment. Each host selects a random time in which they’ll respond, up to the MRT. Everyone sees the responses, so hosts know not to send a report if someone else already has.
RGMP – Router-Port group management protocol – allows a router to inform a switch of what multicast traffic he wishes to see.
source-specific addresses – 126.96.36.199 through 188.8.131.52 allocated by IANA for use with source-specific multicast destinations.
SSM – source specific multicast – an IGMPv3 feature that allows hosts to specify which source addresses they are willing to receive a multicast from.
transient multicast group – multicast addresses that are not predefined and managed by the IANA.
Ethan Banks writes & podcasts about IT, new media, and personal tech.
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