multi cast routing

8/20/2019 Multi Cast Routing

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© J. Liebeherr, All rights reserved1

Multicast Routing


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Multicasting

• Multicast communications refers to one-to-many or many-to-many communications.

IP Multicasting refers to the implementation of multicast

communication in the Internet

Multicast is driven b receivers! "eceivers indicate interest in

receiving data

Unicast Broadcast Multicast


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#

Multicast Groups

• The set of receivers for a multicast transmission is called amulticast group

– A multicast group is identified by a multicast address

– A user that wants to receive multicast transmissions joins

the corresponding multicast group, and becomes amember of that group

• After a user joins, the network builds the necessary routing

paths so that the user receives the data sent to the multicastgroup


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$

Multicasting over a Packet Network

• Without support for multicast at the network layer

Multiple copies

of the same

message istransmitted on

the same link


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%

Multicasting over a Packet Network

• With support for multicast at the network layer

• Requires a set of mechanisms !"# $acket forwarding can send multiple

copies of same packet

!%# Multicast routing algorithm which builds

a spanning tree !dynamically#


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Multicast !ddressing in the "nternet

• All &lass ' addresses are multicast addresses

• Multicast addresses are dynamically assigned.

• An ($ datagram sent to a multicast address is forwarded to everyone

who has joined the multicast group

• (f an application is terminated, the multicast address is !implicitly#

released.

&lass ' 1 multicast group id %) bits

01 1

&lass *rom To

# %%+.,.,., %.%//.%//.%//


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"GMP

• The "nternet Group Management Protocol $"GMP% is asimple protocol for the support of ($ multicast.

• (0M$ is defined in 1*& """%.

• (0M$ operates on a physical network !e.g., single 2thernet

3egment.• (0M$ is used by multicast routers to keep track of

membership in a multicast group.

• 3upport for –

4oining a multicast group – 5uery membership

– 3end membership reports


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"GMP Protocol


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• A host sends an (0M$ report when it joins a multicast group!6ote multiple processes on a host can join. A report is sent

only for the first process#.

• 6o report is sent when a process leaves a group

–&hanged in version %

• A multicast router regularly multicasts an (0M$ 7uery to all

hosts !group address is set to 8ero#.

• A host responds to an (0M$ 7uery with an (0M$ report.

• Multicast router keeps a table on the multicast groups that have joined

hosts. The router only forwards a packet, if there is a host still joined.

• 6ote 1outer does not keep track which host is joined.

"GMP Protocol


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1*

"GMP Protocol


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Multicast Routing Protocols

• Goal 9uild a spanning tree between all members of amulticast group


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Multicast routing as a graph problem

• Problem 2mbed a tree such that allmulticast group members are

connected by the tree


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1#


•Problem 2mbed a tree such that allmulticast group members are


•&olution ' &hortest Path (ree orsource)based tree

9uild a tree that minimi8es the path

cost from the source to each receiver – Good tree if there is a single sender

– If there are multiple senders, need one

tree per sender

– Easy to compute


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1$


•Problem 2mbed a tree such that allmulticast group members are


•&olution * Minimum)+ost (ree 9uild a tree that minimi8es the total

cost of the edges – Good solution if there are multiple

senders

– Very expensive to compute (not practical

for more than 30 nodes)


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1%

Multicast routing in practice

•

1outing $rotocols implement one of two approaches

', &ource Based (ree – 2ssentially implements 3olution ". – 9uilds one shortest path tree for each sender – Tree is built from receiver to the sender reverse shortest path :

reverse path forwarding

*, &hared (ree – 9uild a single distribution tree that is shared by all senders – 'oes not use 3olution % !because it is too e;pensive# – 3elects one router as a


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1&

Multicast Routing table

•

1outing table entries for source-based trees and for core-basedtrees are different

– &ource)based tree !3ource, 0roup# or !3, 0# entry.

– &hared tree !>, 0# entry.

+ource IPaddress

Multicastgroup

Incoming interface"P- interface

/utgoinginterface list

+1 01 I1 I2, I#

02 I2 I1, I#


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Reverse Path -orwarding $RP-%

•

1$* builds a shortest path tree in a distributed fashion by taking advantage of theunicast routing tables.

• Main concept 0iven the address of the root of the tree !e.g., the sending host#, a

router selects as its upstream neighbor in the tree the router which is the ne;t-hop

neighbor for forwarding unicast packets to the root.

• This concept leads to a reverse shortest

path from any router to the sending host.

The union of reverse shortest paths builds

a reverse shortest path tree.

RP- -orwarding

*orward a packet

only if it is receives

from an 1$* neighbor

?"

3ource

1"

R.

1+

1%

1/

R!

interface

for "#

R! neigh$or of R3

for "#

nicast routing table of router "#!

Destination Next Hop

31

4

"2


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Multicast routing in practice

•1outing algorithms in practice implement one of twoapproaches

', &ource Based (ree (ree –

2stablish a reverse path to the source*, &hared (ree

– 2stablish a reverse path to the core


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1)

Building a source)based tree

•3et routing tablesaccording to 1$*

forwarding

•

-lood)and)Prune ?%

?

?/?+

?"3ource

1"

1%

1@

1+

1/

1

1)1A

joined

joined


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2*


•3et routing tablesaccording to 1$*

forwarding

•

-lood)and)Prune

*loodB

*orward packets that

arrive on 1$* interface

on all non-1$*interfaces


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•3et routing tablesaccording to 1$*forwarding

• -lood)and)Prune

$runeB3end a prune messagewhen a packet isreceived on a non-1$*

interface or when thereare no receiversdownstream$rune message disablesrouting table entry


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2#

Pruning

•$rune message temporarily disables a routing table entry

• /ffect 1emoves a link from the multicast tree

• 6o multicast messages are sent on a pruned link

• $rune message is sent in response to a multicast packet• %uestion& 'hy is routing ta$le only temporarily disa$led

• Who sends prune messagesC• A router with no group members in its local network and no

connection to other routers !sent on 1$* interface#

• A router with no group members in its local network which hasreceived a prune message on all non-1$* interfaces !sent on 1$*interface#

• A router with group members which has received a packet from anon-1$* neighbor !to non-1$* neighbor#


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2$


•When a receiver

joins, one needs tore-activate a prunedrouting table entry

• Grafting3ending a 0raftmessage disablesprune, and re-activatesrouting table entry.


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2%

!lternative method for building a source)based tree

•This only worksif the receiverknows thesource

• /0plicit)1oin – 1eceiver sends

a 4oin messageto 1$* neighbor

– 4oin message

creates !3,0#routing tableentry

– 4oin message ispassed on

?%

?-

?/?+

?"

3ource

1"

1%

1@

1/

1-

1)1A

joined

joined

1+


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2&

Building a shared tree

•Dne router is thecore

• 1eceiver sends a 4oin

message to 1$*

neighbor with respect tocore

• 4oin message creates

!>, 0# routing table

entry

?%

?-

?/?+

?"

3ource

1"

1%

1@

1+

1/

1-

1)1A

joined

joined

4 oi n

joined

+ore


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2'

Building a shared tree

•3ource sends datato the core

• &ore forwards data

according to

routing table entry ?%

?

?/?+

?"3ource

1"

1%

1@

1+

1/

1

1)1

joined

joined joined

+ore


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2(

Multicast routing protocols in the "nternet

•

#istance 2ector Multicast Routing Protocol $#2MRP% – *irst multicast routing protocol

– Assumes an


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2)

MB3N/ ) 3riginal Multicast #eplo4ment

•

M9one !Multicast 9ackbone# started multicast deployment in "%• M9one consists of multicast routers that e;change ($ multicast

datagrams over a unicast ($ network

• 'FM1$ is the routing protocol for the M9one

Multicast

router

Multicastrouter

unicast network

Multicast

router

($-in-($ tunnel


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#*

(unneling

•

M9one routers connect via ($ tunnels• With tunneling, ($ packets are encapsulated by another ($

header !"P)in)"P encapsulation#


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#1

P"M Messages $P"M version *%

PIM56M messages 7pe PIM56M PIM5+M

3ello *

"egister 1

"egister5+top 2

Join8Prune #

9ootstrap $

Assert %

0raft &

0raft5Ac: '

• 2ncapsulated in ($

datagrams with protocol

number ".

• $(M messages can be

sent as unicast ormulticast packet

• %%+..." is reserved as

the **+I+Routers

group

Fersion

!B %# Type 1eserved &hecksum

% bit

Message type specific part


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P"M)#M P"M #ense Mode

•

$(M-'M implementsflood-and-prune

• Drange packet

Multicast packet !B'ata#• 9lue packet

$(M message


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##

P"M)&M P"M &parse Mode

•

&ore is calledrende8vous-point $RP%

• 1eceivers know 1$

!statically configured ordynamically elected#

• When receiver joins, a

4oin message is sent to1$ on 1$*.


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P"M)&M P"M &parse Mode

•

?ost ? joins4oin message is only

forwarded until the first

router that is part of the

shared tree.


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#%

P"M)&M #ata transmission

•

3ource sends multicastpacket to 1$

• $acket is attached to an

1$ 1egister message

• When packet reaches

1$, it is forwarded in the

tree

• Also 1$ sends a 4oin

message on reverse

path to 3"


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•

When 4oin messagesreaches 1", it sends a

native multicast packet

to the 1$ !in addition to

the packet attached to

the register message#


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•

When 1$ receivesnative multicast packet it

sends a register stop

message to 1". This

message stops the

transmission of register

messages from 1".


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•

1esulting, one copy ofdata flows

– *rom 3" to 1$

– *rom 1$ to 1


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#)

P"M)&M &witching to source)based tree

•

When data to receiverse;ceeds a threshold,

routers switch to a

source-based tree

• This is done by sending

an e;plicit join message

to the source

• There may be duplicate

packets being sent for

some time


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P"M)&M &witching to source)based tree

•

When data arrives fromsource !as opposed to

1$#, a $rune message

is sent to the 1$T

• 6ow data is forwarded

only along the shortest-

path tree

multi cast routing

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