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Networking
Multicasting
© Janice Regan, 2006-2013
© Janice Regan, 2006-2013
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Types of Multicasting (1)
Multicasting is useful in several contexts (RFC 3569)
SSM, source specific multicast: Information is sent to all members of the multicast group from a single source , members do not communicate directly with each other.
Multimedia “broadcast” to many destinations (hosts and networks)
Multicast to hosts on a local network for resource discovery
© Janice Regan, 2006-2013
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Types of Multicasting (2)
Multicasting is useful in several contexts (RFC 3569)
ASM, any source multicast: many sources and many destinations, no limit on source or number of hosts
Supports applications such asTeleconferencing, gaming, and distributed computing
An IP datagram is transmitted to a “host group“
A “host group” is a set of >=0 end-hosts (or routers) identified by a single IP destination address (224.0.0.0 – 239.255.255.255 ).
End-hosts may join and leave the group any time,
Any host may send to a group
Only members of the group may receive from that group
protocols include ICMPv2 with DVRMP (or PIM or MOSPF)
© Janice Regan, 2006-2013
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Types of Multicasting (3)
Multicasting is useful in several contexts (RFC 3569)
SFM, source filtered multicast can specify one type of filtering either
INCLUDE: Specify the group and the members of the group from which incoming data will be accepted,
EXCLUDE: Specify hosts from which multicast data will not be accepted
A variant of ASM (using IGMPv3 for IPv4 and MLDv2 for IPv6) with source filtering added
© Janice Regan, 2006-2013
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ASM Multicasting Overview
Transmit to a group of hosts on several networks.
This group of hosts includes a subset of hosts on each of the several networks. (subset may be entire network)
Membership in the group is dynamic. Hosts may join or leave groups as they wish.
A multicast router (mrouted, pimd) used to route between networks with members in a multicast group
It is the responsibility of the multicast router to monitor and update changes in membership of groups, both within and outside the attached networks
A single multicast address in the range (224.0.0.0 – 229.255.255.255) refers to the group of hosts
© Janice Regan, 2006-2013
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Multicasting Requirements(1)
Addressing Scheme: locally autonomous assignment of addresses with global applicability
IPv4 – Class D – start 1110
Address range 224.0.0.0 – 239.255.255.255
IPv6 – 8 bit prefix, all 1, 4 bit flags field (permanently assigned or dynamic using IGMP), 4 bit scope field (single network to global), 112 bit group identifier
A multicast address can only be a destination address, not a source address.
A multicast datagram cannot generate an ICMPv4 message
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Special multicast addresses
Just as some unicast IP addresses are reserved for special purposed, some multicast addresses are used for special multicast groups
A list of examples are given in your text (figure 16.2).
Addresses 224.0.0.0 through 224.0.0.255 are reserved for special groups and 239.252.0.0 through 239.255.255.255 are restricted to use on a single site
You will see and use examples of some special multicast addresses in your laboratory experiments
© Janice Regan, 2006-2013
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Special multicast addresses
224.0.0.0 Reserved network base address
224.0.0.1 All systems on this subnet (all hosts)
224.0.0.2 All routers on this subnet (all routers)
224.0.0.4 All DVMRP Routers
224.0.0.5 All OSPF Routers
224.0.0.8 All OSPF Designated Routers
224.0.0.13 All PIMv2 Routers
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Multicasting In a LAN (1)
Many LANs provide hardware support for broadcast or multicast
To send the Protocol stack on the host must map the IP multicast address onto the appropriate (MAC) multicast or broadcast addresses.
To receive the protocol’s network layer must keep track of processes belonging to each multicast group (allowing processes to join or leave the group)
© Janice Regan, 2006-2013
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Multicasting In a LAN (2)
If the LAN does not provide hardware support, (LAN is a non broadcast multi access or NBMA network)
map the multicast addresses to local link layer unicast addresses at the multicast router.
The multicast router can then send the appropriate unicast messages to group members
© Janice Regan, 2006-2013
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Address Translation (1)
Within an Ethernet the interface to each host is assigned an Ethernet address (when manufactured for IPv4)
The Ethernet address belongs to the interface, not the host, if the interface is replaced the Ethernet address will change
48 bit Ethernet addresses can indicate one interface, a network broadcast address or a multicast address.
© Janice Regan, 2006-2013
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Address Translation (2)
Within an Ethernet each interface to a host is assigned an ethernet address
The interface to a host will extract from the network packets with
The Ethernet address of the interface itself
The Ethernet network broadcast address
The Ethernet local broadcast address
The Ethernet all hosts multicast group and other special groups
Any Ethernet multicast addresses of groups the host is a member of
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Address Translation (3)
A multicast router attached to the ethernet will translate IP addresses to the ethernet multicast or broadcast addresses, recognizable by the interfaces
To translate: the lowest 23 bits of the IP multicast address is placed in the special ethernet address 01.00.5e.00.00.00 (Not a unique mapping as IP has 28 significant bits)
The non unique mapping means that a multicast host must check the IP address on a received multicast packet to assure it is actually a recipient of the packet (not one of the other 31 of 32 hosts with the same ethernet multicast address.)
Address Translation (4)
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The first four bits of the IP address are fixed (they indicate a multicast address)
The next 5 bits (dark blue) are not used in the Ethernet address
Therefore there are 25 = 32 IP addresses with the same Ethernet broadcast address
© Janice Regan, 2006-2013
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Multicasting Requirements (2)
Effective notification and delivery
Hosts need a mechanism to notify routers about which groups are of interests to them (IGMP), that is a mechanism to join and leave multicast groups
Routers need a mechanism to deliver the multicast packets to the hosts attached to them
This mechanism is determined by the type of network
Broadcast network
Non broadcast multiple access network (NBMA)
© Janice Regan, 2006-2013
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Multicasting Requirements (3)
Effective notification and delivery
Routers need a mechanism to deliver the multicast packets to the hosts attached to them
In a broadcast network the router must translate between IP multicast address and network multicast address and multicast packet to group members
In an NBMA network the router must deliver translate between the multicast address and a list of unicast IP address in the group then unicast the packet to all hosts on that list
© Janice Regan, 2006-2013
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Multicasting Requirements(4)
An Efficient, Dynamic Forwarding Facility
Router must translate between IP multicast addresses and list of networks containing group members
Routers must exchange information
Which networks include members of given group at a given time
Sufficient path cost information to work out shortest path to each network
Routers must determine ‘shortest’ routing paths based on source and destination addresses
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Constraints for efficient multicast routing:
Every member host of a multicast group should receive one copy of the packet
No host not a member of the multicast group should receive the packet
Each router should receive the packet only once
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Delivery to multicast group members
Multiple Unicast
Send a copy of the packet to each member of the multicast group
Not efficient, creates a larger load than necessary
Is used only by multicast routers to send information to members of a multicast group on an NBMA network
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Multicast
Determine least cost path to each network that has host in group subject to these constraints
Gives spanning tree configuration containing networks with group members
Transmit one packet along the resulting spanning tree
Routers replicate packets at branch points of the spanning tree
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Multicast Example
Stallings 2003:
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Spanning Tree
from Router C to Multicast Group
Must determine multicast routing paths on the basis of both source and destination addresses
Stallings 2003:
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Multicast vs Multiple Unicast
Stallings 2003:
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