IP multicastAdvisor: Prof. Wanjiun Liao

Instructor: De-Nian Yang

e-mail: tony@kiki.ee.ntu.edu.tw

Multicast

• Why multicast?

• MBONE– Internet’s Multicast Backbone

– More than 2800 subnets in over 25 country by 1996 March

– A semi-permanent IP multicast testbed

– Use IP tunneling to send data– Multicast routing protocol: DVMRP, MOSPF, PIM

– How to join MBONE?

Related protocols• Group management protocol:

– IGMP(Internet Group Management Protocol)

• Multicast routing protocol:– Source-based tree

• Multicast tree per (S,G)

• DVMRP, MOSPF, PIM-DM

– Shared tree• Multicast tree per (*,G)

• CBT, PIM-SM

Ethernet

Host A Host B

RouterQuery Query

IGMP• The multicast router use IGMP to obtain the

group-membership information of the LAN.

Report Report

Periodically

Source-based tree

Source B

A

． Source A and B use the same multicast address to send data.． Intermediate routers use (S,G) to forward data.． Intermediate routers create forward entry while receiving data.

DVMRP

• Distance vector multicast routing protocol• Rely on distance vector based unicast routing prot

ocol(RIP)• For each (S,G), grow a shortest-path tree rooted on

the source• Use distance vector routing algorithm to support r

everse path multicasting(RPM)

DVMRP(cont’d)Receiver

Sender

SPT branch

data

DVMRP(cont’d)Receiver

Sender

SPT branch

Prune message

data

DVMRP(cont’d)Receiver

Sender

SPT branch

Prune message

data

． Data and Prune messages will be sent periodically.

DVMRP(cont’d)Receiver

Sender

SPT branch

Graft message

． Receiver sends a graft message toward the to join the group quickly.

MOSPF

• Multicast open shortest path first routing protocol• Rely on link state based unicast routing protocol

(OSPF)• For each (S,G), grow a shortest-path tree rooted on

the sources• Use Dijkstra’s algorithm and group-membership L

SAs to compute shortest path tree

MOSPF(con’t)Group-membershipLSA

Receiver

MOSPF(con’t)

．MOSPF creates tree without prune messages

Shared tree

Source B

A

Sender

Receiver

Core

． Intermediate routers use (*,G) to forward data.． Receivers join the multicast tree before receiving data.

CBT

• Core based trees multicast routing protocol• For each group, grow a shortest-path tree rooted o

n the core.• To join a group, the DR sends a Join_Request tow

ard the core of the group.• Join_Ack is returned by an on-tree router or the co

re.

CBT(cont’d)

• To leave a group, the DR sends a Quit_Notification to its parent router.

• An on-tree router sends Echo_Requests and Echo_Replys to maintain the multicast tree.

• If the sender doesn’t want to join the group, it encapsulates data packets and unicasts them to the core.

Comparison of source-based tree and shared tree

• Source-based tree mechanisms scales worse than shared tree counterparts– Table size:

• Source-based tree: group * source

• Shared tree: group

– Large overhead• DVMRP: periodically sending prune message if group density

is low

• MOSPF: periodically flooding link states

Comparison of source-based tree and shared tree

• Shared tree mechanisms also have drawbacks– Core might be point of failure

– Larger delay than source-based tree counterparts

– Traffic concentration

• PIM uses hybrid architecture to gain advantages of source-based tree and shared tree.

PIM-SM

Data from S2

Data from S1

While the data rates of S1 and S2 areboth low, data travel through RP to the receiver.

PIM-SM

Data from S2

Data from S1

While the data rate of S2 is above somethreshold, DR of the receiver decides to switch the traffic from shared tree to source-based tree.

PIM-SM

Data from S2

Data from S1

DR of the receiver sends Join_Requesttoward S2.

Join_Request

PIM-SM

Data from S2

Data from S1

While receiving data from S2 directly, theintermediate router sends a prune message to RP.

Prune message

More about multicast• Why shortest path tree? Is it reasonable?• Multicast with QoS

– QoS routing

– Multi-path reservation

– IP multicast in MPLS and differentiated service network

– QoS extension to legacy multicast routing protocols

• Multicast with mobility• Reliable multicast• Secure Internet multicast

Reference

• MBONE– http://www.cs.columbia.edu/~hgs/internet/mbone-faq.h

tml

– http://www.savetz.com/mbone/

• Related protocols can be found on http://www.ietf.org