Transcript S7C9 - Multicasts

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 Structure
Destination 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 Maintaining
Groups
• 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)
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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
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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
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PIM
IGMP
CGMP
DVMRP
Basic Configuration Tasks
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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-sparsemode]
• 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
Cgmp hold-time 5
Set cgmp enable
Show cgmp statistics 10
Show config
IP cgmp
IP multicast-routing
IP pim rp-address
Displays cgmp settings
Global configuration
Enables cgmp
Displays for each vlan
Displays configuration
Router will support cgmp
Enables
Specifies rp address for
PIM