CS640Introduction to Computer Networks

Multicast

Nov 29th, 1999Prof. Lawrence H. Landweber

Prof. Jun Murai

One to many communication

• Application level one to many communication• multiple unicasts

• IP multicast

S S

R

R

R

R

R

R

Why Multicast

• When sending same data to multiple receivers– better bandwidth utilization

– less host/router processing

– quicker participation

• Application– Video/Audio broadcast (TV, radio type)

– Video conferencing

– Real time news distribution

IP multicast service model

• RFC1112 : Host Extensions for IP Multicasting

• Transmission of an IP datagram to a "host group", • host group identified by a class D IP address

• Members of the group could be present anywhere in the Internet

• Members join and leave the group and indicate this to the routers

• Senders and receivers are distinct: i.e., a sender need not be a member

• Routers listen to all multicast addresses and use multicast routing protocols to manage groups (IGMP, Internet Group Management Protocol, RFC2236)

IP multicast group address

• Class D address space– high-order three 3bits are set– 224.0.0.0 ~ 239.255.255.255

• Well-known address designated by IANA– RFC1700– 224.0.0.0 ~ 224.0.0.25– 224.0.0.1 (ALL-SYSTEMS.MCAST.NET)

• all multicast hosts on the subnet

– 224.0.0.2 (ALL-ROUTERS.MCAST.NET)• all multicast routers on the subnet

IGMP – Joining a group

R

Example : R joins to Group 224.2.0.1

• R sends IGMP Membership-Reportto 224.2.0.1

• DR receives it. DR will start forwarding packets for 224.2.0.1 to Network A

• DR periodically sends IGMP Membership-Query to 224.0.0.1 (ALL-SYSTEMS.MCAST.NET)

• R answers IGMP Membership-Report to 224.2.0.1

R: ReceiverDR: Designated Router

Data to 224.2.0.1

IGMP Membership-Report

Network A

Network B

DR

IGMP – Leaving a group

R

DR

Example : R leaves from a Group 224.2.0.1

• R sends IGMP Leave-Group to 224.0.0.2 (ALL-ROUTERS.MCAST.NET)

• DR receives it.

• DR stops forwarding packets for 224.2.0.1 to Network A if no more 224.2.0.1 group members on Network A.

R: ReceiverDR: Designated Router

Data to 224.2.0.1

IGMP Leave-Group

Network A

Network B

Challenges in this model

• How to select receivers by a sender ?– need authentication, authorization– encryption of data– key distribution– still under research phase

IP multicast routing

• Purpose: share a Group information among routers, to implement a better routing for data distribution

• Distribution tree structure– Source tree vs shared tree

• Data distribution policy– ACK type vs NACK type

Source distribution tree

Receiver 1

E

BA D F

SourceNotation: (S, G) S = Source G = Group

C

Receiver 2

S

R R

Shared distribution tree

Receiver 1

E

BA D F

Source

Notation: (*, G) * = all sources G = Group

C

Receiver 2

S1

R R

Shared Root

S2

Source tree characteristics

• Source tree– More memory O (G x S ) in routers – optimal path from source to receiver, minimizes

delay

• good for– small number of senders, many receivers such

as Radio broadcasting application

Shared tree characteristics

• Shared tree– less memory O (G) in routers– Sub-optimal path from source to receiver, may introduce

extra delay (source to root)– May have duplicate data transfer (possible duplication of

a path from source to root and a path from root to receivers)

• good for– Many senders with low bandwidth– environment such as most part of the shared tree is identi

cal to the source tree

Data distribution policy

• NACK type– Start with “broadcasting” then prune brunches with no

receivers, to create a distribution tree– a lot of un-used traffic when small receivers in wide

area

• ACK type– forward only to the hosts which explicitly joined to the

group– latency by join propagation and high costs when many

receivers

Protocol types

• Dense mode protocols– assumes dense group membership– Source distribution tree and NACK type– DVMRP (Distance Vector Multicast Routing Protocol)– PIM-DM (Protocol Independent Multicast, Dense Mode)

• Sparse mode protocol– assumes sparse group membership– Shared distribution tree and ACK type– PIM-SM (Protocol Independent Multicast, Sparse Mode)– CBT

DVMRPexchange distance vector

• Maintain ‘multicast routing table’ by exchanging distance vector information among routers – consistent view of a tree among all routers– convenient to have separate paths for unicast versus multicast data

grams;( ie tunnelings)• ‘Dependent routers’ information

– information about responsible downstream routers for a source– A downstream router send “poison Reverse” to a selected upstrea

m router to indicate the dependency.– Poison Reverse : metric + 32 (inf)– entry will be removed from the list by prune message

• Designated Forwarder– multiple routers on the same LAN– lower metric or lower IP address becomes the designated forwarde

r (discover when exchanging metric info.)

DVMRPbroadcast & prune

• Forward multicast packets based on RPF (Reverse path forwarding) rule

• leaf routers check and sends prune message to upstream router when no group member on the network

• upstream router prune the interface with no dependent downstream router.

• Graft message to create a new branch for late participants

• Restart forwarding after prune lifetime (standard : 720 minutes)

• draft-ietf-idmr-dvmrp-v3-09.txt (September 1999)

RPF(reverse path forwarding)

• RPF check– A packet received through interface I, from S (source) to G (multicas

t group) – packet (S,G)

– A router looks into the routing table to find an interface used to send packet to S, I(parent).

– If I != I (parent), I is a wrong interface to receive (S,G).

– if I = I(parent), I is a correct interface to receive (S, G).

• If the RPF check succeeds, the datagram is forwarded to all interfaces except I.

• If the RPF check fails, the datagram is typically silently discarded

• Packet is nevernever forwarded back out the RPF interface!

DVMRP (1) form a source tree by exchanging metric

Source

Receiver 1

S

R1

DF

source tree

DVMRP (2) broadcast

Source

Receiver 1

S

R1

DF

source treedatagram

DVMRP (3) prune

Source

Receiver 1

S

R1

DF

source treedatagramIGMP DVMRP-Prune

DVMRP (4) X and Y pruned

Source

Receiver 1

S

R1

DF X

Y

source treedatagram

DVMRP (4) New member

Source

Receiver 1

S

R1

DF X

Y

source treedatagram

R2

Receiver 2

IGMP DVMRP-Graft

DVMRP (4) New branch

Source

Receiver 1

S

R1

DF X

Y

source treedatagram

R2

Receiver 2

IGMP DVMRP-Graft

PIM• PIM : Protocol Independent Multicast

– independent of particular unicast routing protocol

– Pros: simple, less overhead

– Cons: may cause more broadcast-and-prunes

• All routers in a domain support PIM

• No multicast tunnel

• PIM v2 is assigned new protocol 103

PIM DM overview(1)

• Broadcast and prune ideal for dense group

• Source tree created on demand based on RPF rule

• If the source goes inactive, the tree is torn down

• Easy plug-and-play

• Branches that don’t want data are pruned

PIM DM overview(2)

• Grafts used to join existing source tree

• Uses Asserts to determine the forwarder for multi-access LAN

• Prunes on non-RPF P2P links

• Rate-limited prunes on RPF P2P links

PIM DM Forwarding

• PIM DM interfaces are placed on the “oilist” for a multicast group if;– PIM neighbor heard on interface

– Host on this interface has just joined the group

– Interface has been manually configured to join group

• Packets are “flooded” out all interfaces in “oilist”– If a PIM neighbor is present, DM assumes EVERYON

E wants to receive the group so it gets flooded to that link by definition

PIM Assert Mechanism

• Routers receive packet on an interface in their “oilist”– Only one router should continue sending to avid duplic

ate packets.

• Routers sends “PIM assert” messages– Compare distance and metric values

– Router with best route to source wins

– If metric & distance equal, highest IP addr wins

– Losing router stops sending (prunes interface)

PIM DM State Maintenace

• State is maintained by the “flood and prune” behavior of Dense mode.– Received Multicast packets reset(S,G) entry “ex

piration” timers.– When (S,G) entry “expiration” timers count do

wn to zero, the entry is deleted.

• Interface prune state timers out every 210 seconds causing periodic reflooding and pruning

PIM-DM(1)Initial flood of data

Source

Receiver 2

Receiver 1

S

R1

A

R2

B

C D F

G

H

IE

PIM-DM(2)prune non-RPF p2p link

Source

Receiver 2

Receiver 1

S

R1

A

R2

B

C D F

G

H

IE

IGMP PIM-Prune

PIM-DM(3) C and D Assert to Determine

Forwarder for the LAN, C Wins

Source

Receiver 2

Receiver 1

S

R1

A

R2

B

C D F

G

H

IE

IGMP PIM-Assertwith its own IP address

PIM-DM(4)I, E, G send Prune

H send Join to override G’s Prune

Source

Receiver 2

Receiver 1

S

R1

A

R2

B

C D F

G

H

IE

IGMP PIM-PruneIGMP PIM-Join

PIM-DM(5)I Gets Pruned

E’s Prune is IgnoredG’s Prune is Overridden

Source

Receiver 2

Receiver 1

S

R1

A

R2

B

C D F

G

H

IE

PIM-DM(6)New Receiver, I send Graft

Source

Receiver 2

Receiver 1

S

R1

A

R2

B

C D F

G

H

IE

IGMP PIM-Graft

Receiver 3

R3

PIM-DM(6)new branch

Source

Receiver 2

Receiver 1

S

R1

A

R2

B

C D F

G

H

IE

IGMP PIM-Graft

Receiver 3

R3

PIM-SM overview (1)

• Shared Tree for a group with RP (Rendez-Vous Point) as a root

• Explicit Join Model– Receivers send Join towards the RP

– Sender Register with RP

– Last hop routers can join source tree if the data rate warrants by sending joins to the source

• RPF check depends on tree type– For shared trees, uses RP address

– For source tress, uses Source address

PIM-SM overview(2)

• Only one RP is chosen for a particular group• RP statically configured or dynamically learned

(Auto-RP, PIM v2 candidate RP advertisements)• Data forwarded based on the source state (S, G)

if it exists, otherwise use the shared state (*, G)• RFC2362 – “PIM Sparse Mode Protocol Spec”

(experimental)• Internet Draft: draft-ietf-pim-v2-sm-00.txt

(October 1999)

PIM-SM Basics

• PIM Neighbor Discovery• PIM SM Forwarding• PIM SM Joining• PIM SM REgistering• PIM SM SPT-Swichover• PIM SM Pruning• PIM SM Bootstrap• PIM SM State Maintenance

PIM Neighbor Discovery

• PIM Hellos are multicast to the “All-PIM-Routers” (224.0.0.13, ff02::d) multicast group address periodically

• Highest IP address elected as “DR” (Designated Router)

• If “DR” times-out, a new “DR” is elected

PIM SM Bootstrap

• A set of routers are configured as candidate Bootstrap Router(BSR)s – single BSR is selectd for the domain

• Candidate RPs periodically unicast Candidate-RP-Advertisement messages (C-RP-Advs) to BSR

• The BSR periodically sends Bootstrap messages containing the set of Candidate-RPs – Bootstrap message are flooded in the domain

• Routers receive and store Bootstrap messages originated by the BSR.

PIM SM State Maintenance

• Periodic Join/Prunes are sent to all PIM neighbors

• Periodic Joins refresh interfaces in a PIM neighbor’s oilists

• Periodic Prunes refresh prune state in a PIM neighbor

• Received Multicast packets reset(S,G) entry expiration timers.

PIM-SM(1)

Receiver 1

Source

Receiver 2

S

R1

A B RP D

C E

R2

PIM-SM(2)

Receiver 1

Source

Receiver 2

S

R1

A B RP D

C E

R2

Receiver 1 Joins Group GC Creates (*, G) State, Sends(*, G) Join to the RP

Join

PIM-SM(3)

Receiver 1

Source

Receiver 2

S

R1

A B RP D

C E

R2

RP Creates (*, G) State

PIM-SM(4)

Receiver 1

Source

Receiver 2

S

R1

A B RP D

C E

R2

Source Sends DataA Sends Registers to the RP

Register

Data

PIM-SM(5)

Receiver 1

Source

Receiver 2

S

R1

A B RP D

C E

R2

RP de-encapsulates RegistersForwards Data Down the Shared TreeSends Joins Towards the Source

joinjoin

PIM-SM(6)

Receiver 1

Source

Receiver 2

S

R1

A B RP D

C E

R2

RP Sends Register-Stop OnceData Arrives Natively

Register-Stop

PIM-SM(7)SPT Switchover

Receiver 1

Source

Receiver 2

S

R1

A B RP D

C E

R2

C Sends (S, G) Joins to Join theShortest Path (SPT) Tree

join

PIM-SM(8)

Receiver 1

Source

Receiver 2

S

R1

A B RP D

C E

R2

C starts receiving Data natively

PIM-SM(9)

Receiver 1

Source

Receiver 2

S

R1

A B RP D

C E

R2

C Sends Prunes Up the RP tree forthe Source. RP Deletes (S, G) OIF andSends Prune Towards the Source

Prune

PrunePrune

PIM-SM(10)

Receiver 1

Source

Receiver 2

S

R1

A B RP D

C E

R2

B, RP pruned

PIM-SM(11)

Receiver 1

Source

Receiver 2

S

R1

A B RP D

C E

R2

join

New receiver2 joinsE Creates State and Sends (*, G) Join

PIM-SM(12)

Receiver 1

Source

Receiver 2

S

R1

A B RP D

C E

R2

C Adds Link Towards E to the OIFList of Both (*, G) and (S, G)Data from Source Arrives at E

PIM-SM(13)

Receiver 1

Source

Receiver 2

S

R1

A B RP D

C E

R2

C Adds Link Towards E to the OIFList of Both (*, G) and (S, G)Data from Source Arrives at E

Inter-Domain Multicast Routing

• BGP4+ (Multicast BGP) for short-term solution

• MASC : Multicast Address Set and Claim

• BGMP : Border Gateway Multicast Protocol

MASC

• Hierarchical multicast address allocation

• dynamic allocation (lease) by “set and claim with collision”

domain A

domain B domain Cdomain E

domain D

MASC• Assume Addr(A) is allocated to domain A, A notify to B C and E.• B selects Addr(B) , subset of Addr(A) and send Claim (addr(B)) m

essage to A and C• A forward it to all children except B.• If A’s children is already using Addr(B), report collision to A.• A notify the collision to B to select other address space.• B wait for a certain period before using it.• Address space information is used to create distribution tree using

BGMP.• Stored in M-RIB (Multicast Routing Information Base)

BGMP• Use a PIM-like protocol between domains (“BGP for multicas

t”)• BGMP builds shared tree of domains for a group

– So we can use a rendezvous mechanism at the domain level– Shared tree is bidirectional– Root of shared tree of domains is at root domain

• Runs in routers that border a multicast routing domain• Runs over TCP like BGP• Joins and prunes travel across domains• Can build unidirectional source trees• M-IGP(multicast Intra-Gateway Protocol) tells the borders about group

membership

BGMPA host in C joins to Group G

DomainA

DomainE

DomainC

DomainD

DomainB

DomainF

Root domain

C1

A2

E1

A1A4

A3

D1

B1B2

F1F2

M-IGPjoin

join

M-IGPjoin

BGMPTree constructed, data goes to C

DomainA

DomainE

DomainC

DomainD

DomainB

DomainF

Root domain

C1

A2

E1

A1A4

A3

D1

B1B2

F1F2

BGMPDomain E joins to G

DomainA

DomainE

DomainC

DomainD

DomainB

DomainF

join

C1

A2

E1

A1A4

A3

D1

B1B2

F1F2

join

BGMPtree constructed. Data goes to E

DomainA

DomainE

DomainC

DomainD

DomainB

DomainF

C1

A2

E1

A1A4

A3

D1

B1B2

F1F2

Multicast Routers

• mrouted (Xerox PARC) : DVMRP

• GateD (Merit) : DVMRP, PIM-DM, PIM-SM

• Cisco IOS : DVMRP, PIM-DM, PIM-SM

M-Bone

• Wide area IP multicast test bed using IP-in-IP tunneling technology

• Routing protocol– DVMRP is used– Transition to PIM (DM, SM) is ongoing

• Started in March 1992 for audio broadcasting of IETF meeting (San Diego)

• Latest tolopology– ftp://ftp.parcftp.xerox.com/pub/net-research/mbone/maps/mbone-map-big.ps

– About 6000 (S,G) entries

• Discussion in by mbone@isi.edu

M-BONE in 1994

M-BONE in 1996

M-BONE in 1998

Future Mulicast Service

• Current multicast service - latency and packet drop

• Research for “Reliable multicast” is actively going on for;– large scale interactive game on the Internet– Distributed database– large scale news distribution etc.

Reliable multicast technology

• SRM ( scalable Reliable Multicast)– multicast with re-transmit (with random back-off)– All nodes can re-transmit datagram (Multicast/Unica

st)

• MTP (Multicast Transport Protocol: RFC1301)

• FEC (Forward Error Correction)– error packet recovery by redundant packets

IPv6 Multicast Test10sites in Japan

PIM-SM over IPv6 (KAME)Digital Video streaming

Nov 27th, 1999