S7C9 - Multicasts

Characteristics

Management

Configuration

Traffic Types

• Unicast– Single copy to every client unicast address

• Concern is number of user connections & replication

• Eat up bandwidth– Includes replication at router and across links

– Suitable for small numbers of destinations

• NOTE: IP TV is a streaming video server capable of both unicast and multicast

Broadcast

• One copy of each packet to broadcast address– Multimedia broadcasts and be as high as 7

Mbps or more of data– Rarely implemented with multimedia

transmissions

Multicast

• Packet sent to special multicast address– Single data stream to multiple clients– Saves bandwidth and controls network traffic– Reduces network and host processing

Multicast Traffic Characteristics

• Facilitates transmission of an IP datagram to a multicast group identified by single address

• Delivers multicast datagram to all members of the multicast group with best effort delivery

• Supports dynamic membership of a multicast group• Supports all multicast groups regardless of location or number

of members• Supports membership of single host in one or more groups• Upholds multiple data streams at application level for single

group address• Supports single group address for multiple applications on host

Multicast Address StructureDestination address only

• 224.0.0.0 to 239.255.255.255– 224.0.0.1 all hosts group– 224.0.0.2 all routers– 224.0.0.4 all distance vector multicast

routing protocol– 224.0.0.5 OSPF routers– 224.0.0.6 OSPF designated routers– 224.0.0.9 RIP2 routers– 224.0.0.13 PIM (Protocol Independent Multicast)

routers

Multimedia Traffic Issues

• Coordinating multicast operations of different devices in the network

• Establishing a path between source and destination devices forwarding multicast traffic through the network– Traffic transmitted via a distribution tree

• Tree connects all hosts in group

• Different protocols use different techniques to construct trees

• Routers need to know which hosts belong to group

Subscribing and MaintainingGroups

• Issues in facilitating multimedia traffic– Coordinating multicast operations of different devices– Establishing a path between source and destination

devices– Forwarding multicast traffic through the network

• IGMP provides a means to control and limit the flow of multicast traffic through network– Query messages discover which network devices are

members of multicast group– Report messages respond to query messages

IGMP v1

• Basic protocol designed to facilitate a device when joining a multicast group– Responsible for communications between host and

router• Determines which hosts can join• Decides when host no longer needs to be part of group

– Datagram has version number, type, unused, checksum, and group address fields

» Addressed to 224.0.0.1 with TTL=1» Type identifies query or report status

• Membership report can be sent without receipt of query• Queries can be sent in rapid succession on startup

IGMP v2

• Router can transmit to selected group• Four types of messages (not 2)

– Membership query– Version 2 membership report– Leave report– Version 1 membership report

• Frame format is type, response time, checksum, and group address

• Router builds table detailed interfaces with one or more hosts in a group

IGMP v2 Querier Election

• Elects multicast querier for each network segment– Multicast router with lowest IP address on LAN

segment is elected– All routers initially act as querier; when they

receive a query message with a lower number, they stop sending query messages

Maintaining a Group

• Queries go to 224.0.0.1 group address– Only one member responds; others suppress

– General query sets delay times – random times

– Responding host sets delay timer – random values

• Leaving a group– Host transmits a leave message 224.0.0.2

• Pruning takes place when there is no response to group-specific query

Multicast Traffic in the Switch

• Multicast traffic delivered to all ports of a layer 2 switch– Switches must be capable of forwarding to a large

number of members without overloading the switch fabric

– Switches need some degree of multicast awareness• VLANs can be defined to correspond to multicast group

boundaries

• Layer 2 switches can snoop IGMP queries and reports to learn port mappings of multicast group members

CGMP

• Cisco developed protocol– Enables switch to learn about existence of

multicast clients from Cisco routers and layer 3 switches

– Based on client/server model; router is server and switch is client

• Router creates a CGMP packet; sent to address to which all switches listen; switch creates proper entry in switching table

Multicast Routing

• Routers interact with each other to exchange information about neighboring routers– Designated router constructs a tree to connect all

members of IP multicast group• Specifies a unique forwarding path between source’s subnet

and each subnet containing multicast group members

• Only one loop-free path between pairs of routers– Must be dynamically updated

– Two types of trees- - source specific and shared distribution

Source Specific Tree

• Requires finding shortest path from sends to each receiver– Builds a spanning tree for each potential source of

subnetwork

– Use a technique called Reverse Path Forwarding (RPF)

– Multicast packet interface is called parent link

• RPF algorithm reduces unnecessary packet duplication

Shared Distribution Tree

• Use distribution centers and construct single multicast tree– Low-overhead and higher end-to-end delay

– Single delivery tree shared by all group members

– Devices wanting to receive traffic must explicitly join the shared delivery tree

– Multicast traffic sent over same tree regardless of source

– Can involve single router or group of routers

Treshhold

• TTL field controls packet live time

• TTL in multicasting uses threshhold concept– Each interface is assigned threshold value– Packets with greater TTL than threshhold are

forwarded– Router compares TTL and decrements by 1

before sending out interface

Threshhold Scopes

• Default value is 255 – different routing protocols look for different TTL

• 0 restricted to same host• 1 restricted to same subnet• 15 restricted to same site• 63 restricted to same region• 127 worldwide• 191 worldwide; limited bandwidth• 255 unrestricted in scope; global

Multicast Routing Protocols

• Responsible for constructing multicast delivery trees and forwarding packets– Dense Mode Routing Protocols

• Assume almost all routers need to distribute – DVMRP– MOSPF (RFC 1584) – single routing domain

» Not supported by Cisco routers– PIM DM

» Floods and then prunes

– Distance Vector Multicast Routing Protocol• DVMRP (RFC 1075) used on Internet Backbone (Mbone) –

uses reverse flooding

PIM Dense Mode

• Useful when– Senders and receivers are in close proximity– Few senders and many receivers– Volume of multicast traffic is high– Stream of multicast traffic is constant

Sparse Mode Routing Protocols

• Multicast members are sparsely distributed– CBT (Core-based trees) – RFC 2201

• Single tree shared by all members of group

• Core router constructs tree

– PIM SM ( Protocol-independent Multicast Sparse Mode)

• Used when few receivers in a group

• Used when traffic is intermittent

IP Multicasting Requirements

• IP protocol stack that supports multicasting (RFC 1112(

• Servers and clients need applications• NICS on receiving hosts must be

configured to monitor multicast packets• High performance backbone with layer 2

and 3 switching• Switches that can handle multicasting

Cisco Software Supports

• PIM

• IGMP

• CGMP

• DVMRP

Basic Configuration Tasks

• Enable IP multicast routing

• Enable PIM on interfaces

• Configure rendezvous port

• Configure TTL threshhold

• Join a multicast group

• Change IGMP version

• Enable CGMP

Configuration Commands

• Ip multicast-routing• Ip pim [dense-mode|sparse-mode|dense-sparse-

mode]• Show ip pim interface s0

– Shows next hop IP address, int type, PIM mode, PIM neighbor count, query frequency, IP address of designated router

• Show IP pim neighbor e2– Shows neighbor address, neighbor int, uptime, expire

time, mode, and DR

Outgoing List Member Requirements

• PIM neighbor was heard on interface

• Host serviced by interface has joined a group

• Interface manually configured to join a group

Command List

Show cgmp Displays cgmp settings

Cgmp hold-time 5 Global configuration

Set cgmp enable Enables cgmp

Show cgmp statistics 10 Displays for each vlan

Show config Displays configuration

IP cgmp Router will support cgmp

IP multicast-routing Enables

IP pim rp-address Specifies rp address for PIM