Network Training - VITEC

8/2/2019 Network Training - VITEC

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Networking FundamentalsKobi Levy, March 2012


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Agenda

Why Multicast?

Multicast VS Unicast

Multicast forwarding

IGMPv1 vs IGMPv2

IGMP Snooping


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Why Multicast?

Used when sending same data to multiple receivers

Better bandwidth utilization

Less host/router processing

Used when receivers addresses unknown

Used when simultaneous delivery for a group of receivers is requir(simulcast)

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Multicast Vs Unicast

Pure Unicast: Send the same copy of data multiple times

Web Technologies: Webcastingpush the same data to multipl

Traditionally, receivers subscribed to data and pulled itperiodically

The underlying transport for those Technologies has bee

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Host

Router

Unicast

Host

Router


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The sender (source) sends one copy of a single packet addressedreceivers - multicast group

Multicast routers replicate and forward the packet to all the branchreceivers (may) exist

Receivers express their interest in multicast traffic by sending contto routers

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Multicast advantages

Enhanced Efficiency: Controls network traffic and reduces server a

Optimized Performance: Eliminates traffic redundancy

Distributed Applications: Makes multipoint applications possible

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Example: Audio StreamingAll clients listening to the same 8-Kbps audio

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Traffic

Mbps

1 20 40 60 80 100

Multicast

Unicast


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Multicast advantages

Fewer resources required bandwidth and host processing power

Almost simultaneous delivery is assured (one packet is simultaneoforwarded across the networks)

Foundation for a whole range of new applications not possible in th

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Multicast disadvantages

Multicast is UDP based.

Best Effort Delivery:Drops are to be expected. Multicast applications must nodelivery of data and should be designed accordingly. Reliable multicast will address

No Congestion Avoidance:Lack of TCP windowing and slow-start mechanetwork congestion. If possible, multicast applications should attempt to detect and conditions.

Duplicates:Some multicast protocol mechanisms result in the occasional generatpackets. Multicast applications should be designed to expect occasional duplicate pa

Out of sequence delivery:Network topology changes affect the order of delivapplication must properly address the issue.

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Multicast applications

One-to-many: A single host sending to two or more (n) receive

Many-to-many: Any number of hosts sending to the same mugroup hosts are also members of the group (sender = receiver)

Other: e.g. many-to-one: Any number of receivers sending data source (via unicast or multicast)

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Typical Multicast Network

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Source Source segment

First-hop router

Last-hop router

Multicastdistributiontree

Receiversegment

Multicast data flow


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Multicast Forwarding


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Multicast routing works the opposite way of unicast routing

Unicast routing is concerned with where the packet is going

Multicast routing is concerned with where the packet comes from

Multicast routing uses Reverse Path Forwarding to prevent forwar

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Dense Mode

Uses the push model

Traffic Flooded throughout network

Pruned back where it is unwanted

Flood and prune behavior (typically every three minutes)

Sparse Mode Uses the pull model

Traffic sent only to where it is requested

Explicit join behavior

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Dense Mode

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Initial Flooding

Source

Receiver

Multicast Packets

(S, G) state created ineveryrouter in the network!


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Dense Mode

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Pruning Unwanted Traffic

Source

Receiver

Multicast Packets

Prune Messages


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Dense Mode

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Source

Results after Pruning

Receiver

Multicast Packets

Flood and Prune processrepeats every three minutes!!!

(S, G) state still exists ineveryrouter in the network!


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Dense Mode

Uses the push model

Traffic Flooded throughout network

Pruned back where it is unwanted

Flood and prune behavior (typically every three minutes)

Sparse Mode Uses the pull model

Traffic sent only to where it is requested

Explicit join behavior

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Sparse Mode

Protocol Independent works with any of the underlying unicast roprotocols

Supports both source and shared trees

Based on an explicit pull model

Uses a rendezvous point (RP)

Senders and receivers meet each other

Senders are registered with RP by their first-hop router

Receivers are joined to the shared tree (rooted at the RP) by their local de(DR)

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PIM-SM Shared Tree Join

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Receiver

RP

(*, G) Join

Shared Tree

(*, G) State created onlyalong the Shared Tree.


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PIM-SM Sender Registration

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Receiver

RP

(S, G) Join

Source

Shared Tree

(S, G) Register (unicast)

Source Tree

(S, G) State created onlyalong the Source Tree.Traffic Flow


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PIM-SM Sender Registration

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Receiver

RPSource

Shared Tree

Source TreeRP sends a Register-Stopback to the first-hop routerto stop the Registerprocess.(S, G) Register-Stop (unicast)

Traffic Flow

(S, G) Register (unicast)

(S, G) traffic begins arrivingat the RP via the Sourcetree.


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IGMP


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IGMP

Internet Group management protocol (IGMP),

a multicasting protocol in the internet protocols family,

is used by IP hosts to report their host group memberships to any

immediately neighboring multicast routers. IGMP messages are

encapsulated in IP datagrams, with an IP protocol number of 2.

IGMP has versions IGMP v1, v2 and v3.

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IGMP v1

IGMPv1: Hosts can join multicast groups.

There were no leave messages. Routers were using a time-out

Based mechanism to discover the groups that are of no interest

To the members.

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IGMP v2

IGMPv2: Leave messages were added to the protocol.

Allow group membership termination to be quickly reported to

The routing protocol, which is important for high-bandwidth

Multicast groups and/or subnets with highly volatile group membersh

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IGMP v3

IGMPv3: Major revision of the protocol.

It allows hosts to specify the list of hosts from which they want to

Receive traffic from. Traffic from other hosts is blocked inside the

Network. It also allows hosts to block inside the network packets

That come from sources that sent unwanted traffic.

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IGMP Snooping

IGMP snooping is designed to prevent hosts on a local network from traffic for a multicast group they have not explicitly joined.

It provides switches with a mechanism to prune multicast traffic from not contain a multicast listener (an IGMP client).

A switch will, by default, flood multicast traffic to all the ports in a broaMulticast can cause unnecessary load on host devices by requiring thprocess packets they have not solicited.

When purposefully exploited this is known as one variation of a deniaattack.

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IGMP Snooping

IGMP snooping allows a switch to only forward multicast traffic to thehave solicited them.

Essentially, IGMP snooping is a layer 2 optimization for the layer 3 IG

IGMP snooping takes place internally on switches and is not a protoc

Snooping is therefore especially useful for bandwidth-intensive IP muapplications such as IPTV.

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