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606 | Multicast Listener Discovery
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• Maximum Response Delay—the maximum amount of time that the Querier waits to receive a
response to a General or Multicast-Address-Specific Query. The value is zero in reports and Done
messages.
• Multicast Address — set to zero in General Queries, and set to the relevant multicast address in
multicast-address-specific queries and done messages.
Figure 27-1. MLD version 1 Packet Structure
MLD Querier Router
MLD routers periodically ask connected hosts in which, if any, multicasts groups they are interested. For
any subnet, only on router solicit hosts for this information; this router is called the Querier, and all the
other routers on the subnet are non-queriers. Initially, each router assumes that it is the Querier, and
transmits queries. If a router receives a query with a source IP address lower than its own, it stops
transmitting queries, and so the router with the lowest IP address is ultimately elected the Querier for the
subnet.
Joining a Multicast Group
The Querier periodically sends a General Query to the all-nodes multicast address FF02::1. A host that
wants to join a multicast group responds to the general query with a report that contains (in the MLD
Multicast Address field, Figure 27-1) the group address; the report is also addressed to the group (in the
IPv6 Destination Address field). To avoid duplicate reporting, any host that hears a report from another
host for the same group in which it itself is interested cancels its report for that group.
A host does not have to wait for a General Query to join a group. If a host wants to become a member of a
group for which the router is not currently forwarding traffic, it should send an unsolicited report.
When a router receives a report for a group, it either creates a new entry in the group membership table, or
it updates an existing entry by adding the interface on which the report arrived to the outgoing interface list
for the group.
Version
(6)
Traffic Class
Flow Label
Payload Length
Next Header
(0)
Hop Limit
(1)
ICMPv6 Packet
Preamble
Start Frame
Delimiter
Destination MAC
Source MAC
Ethernet Type
IPv6 Packet
Padding
MLD 0
01
Type
Code
(0)
Multicast Address
Checksum
Code: 130: Multicast Listener Query
131: Multicast Listener Report
132: Multicast Listener Done
FCS
Next Header
(58)
Header Extension
Length
(0)
Options
(Router Alert)
Max. Response
Delay
Reserved