CMPT 471 Networking II

1

CMPT 471Networking II

Multicasting

© Janice Regan, 2006-2013

2© Janice Regan, 2006-2013

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


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)


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


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


Multicasting In a LAN (1) Many LANs provide hardware support for

broadcast or multicast To send 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)


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


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.


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 Any Ethernet multicast addresses of groups the

host is a member of


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.)

11

Address Translation (4)

© Janice Regan, 2006-2013

1 1 1 0 0 0 0 1 0 0 0 0 0 0 1 0 10 0 0 1 0 1 10 1 1 1 1 1 0 1

0 1 0 1 1 1 1 0 0 0 0 0 0 0 1 0 10 0 0 1 0 1 10 1 1 1 1 1 0 10 0 0 0 0 0 1 0 0 0 0 0 0 0 00

225 125 22 5

01 00 5E

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 addressTherefore there are 25 = 32 IP addresses with the same Ethernet broadcast address


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


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


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


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)


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


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


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


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


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


Multicast Example

Stallings 2003:


Spanning Tree from Router C to Multicast Group Must determine multicast routing paths on the

basis of both source and destination addresses

Stallings 2003:


Multicast vs Multiple Unicast

Stallings 2003:

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