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IP Multicast – Single Source & Multiple Source

Brian J.S. CheeAssociate Director: Advanced Network Computing Laboratory

University of Hawaii

Karl AuerbachAdvanced Internet Architectures, Office of the CSO

Cisco Systems

The iLabs Multicast Team:Jim Martin, Nortel Networks

Helen GaryBrian Chee, Univ. of Hawaii ICS Dept.

Karl Auerbach, Cisco Systems

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Multicast-Overview

What is Multicast anyway? What are its two main flavors:

• Single Source Multicast (SSM)

• Any Source Multicast (ASM)

Why the difference? What are the ramifications? Multicast Routing Protocols

• Intra-Domain

• Inter-Domain

Conclusions

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What is IP Multicast?

IP Multicast exists at Layer 3• Don’t confuse it with Layer 2 multicast

One IP packet, many receivers The set of receivers is expressed as an IP address

• 224/8 (I.e. 224.x.x.x through 239.255.255.255)

• There addresses are very different from normal IP addresses.– These addresses represent a dynamic “group” of receivers– Receivers can come and go from the group as they please.

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SSM and ASM (Classic IP Multicast)

Both send to a group SSM has a single sender

• One to many.

• Uses addresses 232/8– 232.0.0.0 … 232.255.255.255

ASM (Classic IP Multicast) allows anyone and everyone to be a sender• Many to many

• Uses addresses 224/8 through 239/8 (with a hole for SSM at 232/8)

– 224.0.0.0 … 231.255.255.255– 233.0.0.0 … 239.255.255.255

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Many Receivers? How Many?

The number of receivers can range from zero to infinity• The number of receivers can change as receivers join and leave

the group at any time.

• The sender is not aware of the number of-, or identity of-, the receivers.

Packets from the source(s) are delivered to all destinations that have expressed an interest.• IGMP is used to express that interest.

• In ASM, a receiver needs to merely indicate the group address

• In SSM a receiver needs to indicate both the source and the group address.

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What Kind of Data Delivery Service?

IP Multicast is a datagram service.• One can layer on streaming services.

Today most IP Multicast applications use UDP encapsulation• UDP provides data checksums

– IP itself checks only the header, not the data

• UDP provides further demultiplexing via “ports”.

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IP Multicast and Reliability

IP Multicast is an “unreliable” datagram service• Don’t read the word “unreliable” too strongly.

– It merely means that there are no guarantees made

• IP multicast carries datagrams– That means that it doesn’t have the notion of a connection.

There are several protocols that build reliable data distribution services on top of IP multicast.

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Why SSM and ASM?

Any Source Multicast (ASM) is a well established technology• Standard IP multicast is a many-to-many medium

• It’s been running for more than a decade

• However it often requires babysitting.

Single Source Multicast (SSM) is a new approach• SSM is a one-to-many medium.

• It may prove to require much less supporting mechanism than many-to-many multicast.

Both forms can coexist• Indeed SSM uses many of the infrastructure elements of

standard IP multicast.

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Multicast Basics

R

RR

R*

sourcereceiver

R

receiver

Multicast Tree links

receiver

• Data flow is controlled by routers

• Data only flows to those that request the data through IGMP “Joins” (ASM) or “Subscribes” (SSM)

• Data is addressed to a Class “D” IP addresses

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Internet Group Management Protocol (IGMP)

Not a Routing Protocol Common to ALL IP multicast systems Receivers join a multicast group by sending a IGMP REPORT

message.• Routers send IGMP QUERY messages to establish

continued receiver interest. One receiver responds with an IGMP REPORT for each group in use.

• (added to V2) Receivers send IGMP LEAVE messages when they are no longer interested in a group.

• (added to V3) Receiver specifies source(s) of interest.

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IGMP (continued)

Version History• Version 1 · Basic REPORT and QUERY mechanism• Version 2 · Adds LEAVE messages and variable response

timers• Version 3 · Adds ability to do source specific joins and

leaves (This is NOT widely available yet, new spec)

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Multicast routing issues - DVMRP

Distance Vector Multicast Routing Protocol• Uses “flood & prune” to control who receives the stream

– Floods data from new sources across the entire network. – Routers without receivers then prune back any sources that

are not of interest, upstream routers do the same, until only links that have downstream users have the data

– Periodically, prune state is removed and the process begins again.

• Takes time to for receiver joins/leaves to propagate.

• Can result in traffic floods when things go wrong.

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Flood and Prune…

receiver

R2

G1

Multicast data flow

R1R3

Sender, S1

Data

Control

PruneS1,G1

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PIM:Protocol Independent Multicast

PIM-SM (sparse mode) & PIM-DM (dense mode) are VERY different protocols….!!!• Dense Mode is more like DVMRP

– it uses flood and prune

• Sparse Mode uses Rendezvous Points to control flows– Qualified routers advertise willingness to be an RP, and the

actual RP is elected from those advertising– Senders send traffic to the RP for distribution

– If warranted, a sender also distributes the multicast traffic directly.

– Rendezvous Point’s are configured as part of PIM-Sparse mode

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PIM-Sparse Mode

How PIM-SM works• Advertises RP-to-multicast-group mapping

• If a host joins a group, the serving router joins the shared tree rooted at the Rendezvous Point (RP)

• Sender’s router forwards group data to the RP

• RP forwards data on shared tree

• Once data is received, an endpoint router can switch to an efficient Shortest Path Tree (SPT)

– Router sends join towards source

• Shared tree must be used to support Shortest Path Tree (SPT)

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PIM-Sparse Mode (cont.)

The shared tree gives a common place to setup the flow of traffic from sender to receivers

The Rendezvous Point provides a common control point to switch over from a less efficient “shared tree” to a more efficient “shortest path tree”.

Alternate Rendezvous Points provide for less chance of a single point of failure.

Transitions from RP to source base tree (and vice-versa) can be an operational nuisance.

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receiver

RP

G1

Multicast data flow

RR

sender

Join: G1 Shared tree

Data

Control

Join MulticastGroup G1

Shared Tree to Shortest Path Tree (SPT)

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receiver

RP

Multicast data flow

RR

sender

Join: SPT

Data

Control

Shared Tree to Shortest Path Tree (SPT)

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receiver

RP

Multicast data flow

RR

sender

PruneShared Tree

Data

Control

Shared Tree to Shortest Path Tree (SPT)

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receiver

RP

Multicast data flow

RR

sender

Data

Control

Shared Tree to Shortest Path Tree (SPT)

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Single Source Multicast (SSM)

Needs IGMP v3 that has ability to include sender information for the “subscribe”s and “unsubscribe”s

No longer needs rendezvous points• Everything is now shortest path trees since you already know

who the sender is

Allows reuse of multicast address space Removes the need for VERY complex router code

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Multicast Basics - Source Specific Multicast

R

RR

R*

sourcereceiver

R

receiver

Multicast Tree links

receiver

Subscribe channel “n” at IP Addr “x”

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References - ASM

Deploying Ip Multicast in the Enterprise by Thomas A. Maufer

Deering, S., "Host Extensions for IP Multicasting," RFC 1112, August 1989.

Fenner, W., "Internet Group Management Protocol, Version 2," RFC 2236, November 1997.

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References – ASM (continued)

Fenner, W., "Internet Group Management Protocol, Version 2," RFC 2236, November

1997.

Deering, S., Estrin, D., Farinacci, D., Jacobson, V., Helmy, A., Meyer, D., and L. Wei,

"Protocol Independent Multicast Version 2 Dense Mode Specification," Work in

Progress.

Estrin, D., Farinacci, D., Helmy, A., Thaler, D., Deering, S., Handley, M., Jacobson,

V., Liu, C., Sharma, P. and L. Wei, "Protocol Independent Multicast-Sparse Mode

(PIM-SM): Protocol Specification," RFC 2362, June 1998.

Waitzman, D., Partridge, C., and S. Deering., "Distance Vector Multicast Routing

Protocol," RFC 1075, Nov 1988.

Meyer, D., "Administratively Scoped IP Multicast", BCP 23, RFC 2365, July, 1998.

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References - SSM

http://search.ietf.org/internet-drafts/draft-holbrook-ssm-00.txt

http://search.ietf.org/internet-drafts/draft-holbrook-ssm-arch-00.txt

http://search.ietf.org/internet-drafts/draft-holbrook-ssm-arch-00.txt

http://search.ietf.org/internet-drafts/draft-holbrook-idmr-igmpv3-ssm-00.txt

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The players….

Jim Martin - Nortel Innovations Lab (Lead)

Helen Garey - Independent Consultant

Brian Chee - University of Hawaii ICS Dept.

Karl Auerbach - Cisco Systems

Jeff Danley - Berkeley

Daniel Bui - Ixia

Angus Robertson - Adtech

The I-labs is designed to highlight emerging technologies…not

everything is fully baked yet...