Transcript Group Management

Group Management
Introduction
 Internet Group Management Protocol (IGMP)
 Multicast Listener Discovery (MLD) protocol
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The Need for Multicast Group
Management
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By definition a multicast message is sent from a source to a group of
destination hosts
In IP multicasting, multicast groups have an ID called multicast group ID
If a process in a host wants to receive a multicast message sent to a
particular group, it needs to somehow listens to all messages sent to that
particular group
If the source and destinations of a multicast packet share a common bus
(i.e. Ethernet bus) , each host only needs to know what groups have
members among the processes of that host
If the source and destinations are not on the same LAN, forwarding the
multicast messages to the destinations become more complicated
Hosts need to join a group by informing the multicast router on their
subnetwork
Basic Host Model
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Strive to make the host model simple
–When sourcing data, just send the data
»Map network layer address to link layer address
»Routers will figure out where receivers are and
are not
–When receiving data, need to perform two actions
»Tell routers what group you’re interested in (via IGMP)
»Tell your LAN controller to receive for link-layer mapped address
Transmission and Delivery of Multicast Datagram
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Local subnetwork transmission
 The source station simply addresses the IP packet to the multicast group, the
network interface card maps the Class D address to the corresponding
Ethernet multicast address, and the frame is sent
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 Receivers that wish to capture the frame notify their IP layer
that they want to receive datagrams addressed to the group
Transmission throughout the Internet
 Routers are required to implement a multicast routing protocol that permits
the construction of multicast delivery trees and supports multicast data
packet forwarding
 Each router needs to implement a group membership protocol (IGMP) that
allows it to learn about the existence of group members on its directly
attached subnetwork
Introduction to Group Management
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Group Management:
– Router accept host as members of groups or
delete them from a certain group, and
maintenance of the router’s information about
the group membership
Introduction to IGMP
•Internet Group Management Protocol (IGMP) is used by
hosts and routers that support multicasting
•It lets all the systems on a physical network know which
hosts currently belong to which multicast groups
•This information helps multicast routers know which
multicast datagrams to forward onto which interfaces
•IGMP is considered to be part of IP layer
•IGMP messages are transmitted in IP datagrams
Versions of IGMP
There are 3 versions of IGMP.
• Versions 1 and 2 can only communicate group information
to a multicast router.
• Version 3 adds source specific messages so that sources of
a multicast group can be selectively received
•The motivation behind this additional functionality is a
reduction in network traffic
Internet Group Management Protocol
(IGMP)
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Signaling protocol
IGMPv1
– joining message
– host membership query (not to a certain group address, but to the
address 224.0.0.1, using flooding)
IGMPv1 Package Format
0
3 4
4-bit
IGMP
4-bit
IGMP
version (1)
type (1-2)
7 8
15 16
(unused)
31
16-bit checksum
32-bit group address (class D IP address)
8 bytes
An IGMP package consists of 8 bytes. The IGMP version is 1.
An IGMP type of 1 is a query sent by a multicast router, and 2 is
a response sent by a host
The group address is a class D IP address. In a query the group
address is set to 0, and in a report it contains the group address
being reported
IGMPv2
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Extensions to IGMPv1:
– If there are more routers, then it chooses one to be responsible
for querying the subnetwork
– The routers can query the hosts according to a certain group
with a group specific query
– Explicit group leave message
» The router after receiving the group leave message — by using
the group specific query — does not have to wait for timing out to
check if there is member of the group or not in the subnetwork.
» This decreases the latency time of the protocol
Phases of IGMP
IGMP has 2 phases:
1) When a host joins a new multicast group, it sends an
IGMP message to the “all hosts” multicast address
declaring its membership
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Local multicast gateways receive the message and
establish necessary routing by propagating the group
membership information to other multicast gateways
throughout the Internet
Phases of IGMP
2) Because membership is dynamic, local multicast gateways
periodically poll hosts on the local network to determine
which hosts remain members of which groups
•If no host reports membership in a group after a poll, the
multicast gateway assumes that no host on the network
remains in that group, and stops advertising group
membership to other multicast routers
IGMP Reports and Queries
1) A host sends an IGMP report when a process joins a group
2) A host does not send a report when a process leaves a group
3) A multicast router sends an IGMP query at regular intervals
to see if any hosts still have processes belonging to any
groups
4) A host responds to an IGMP query by sending one IGMP
report for each group that still contains at least one process
IGMPv3
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New property: source-filtering
– The capability of the system to accept the packages addressed to
a certain multicast address from a given source address, only
– however, in a certain configuration accepting the packages from
any source addresses except some source addresses
– with this the source specific joining and leaving are introduced,
in such a way the hosts can (un)subscribe to/from certain
sources in a group independently
– By using the source filtering the receiver can subscribe to a
Source Specific Multicast (SSM) channel
Implementation of IGMPv3
•To implement source specific IGMP messages, two new
types of reports were created:
•an inclusion and
•an exclusion group-source report
•These reports allow a host to communicate what sources in
a group it wants to receive; a host utilizes an inclusion report
to specify (by IP-address) what sources it wishes to receive,
or an exclusion report specifying what sources it does not
wish to receive
Multicast Listener Discovery (MLD)
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Group management in IPv6
Listeners = that nodes, which want to receive the multicast
messages
With the MLD it can be determined that whitch listener is
interested in which multicast group
Two versions: MLDv1 and MLDv2
The MLDv2, similarly to IGMPv3 gives the source filtering,
Determining the required source in case of each group
The MLD enable the IPv6 routers to perceive the presence of
the listeners