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Multicast

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Title: Multicast

1
Multicast

Outline
Multicast revisited
Protocol Independent Multicast - SM
Future Directions

2
Multicast Revisited

Motivation multiple hosts wish to receive the
same data from one or more senders
Multicast routing defines extensions to IP
routers to support broadcasting data in IP
networks
Until now IP has only facilitated a point to
point routing
Multicast data is sent and received at a
multicast address which defines a group
Simple notion of a group is a TV channel
Data is sent and received in multicast groups via
routing trees from sender(s) to receivers.
Protocols are principally concerned with setting
up and maintaining trees
Note All multicast messaging is sent via unicast

3
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)
Example Company-wide announcement
Sparse mode protocol
assumes sparse group membership
Shared distribution tree and ACK type
PIM-SM (Protocol Independent Multicast, Sparse
Mode)
Examples a Shuttle Launch

4
PIM-SM overview (1)

Developed due to scaling issues
Flooding is generally a real bad idea
Based on creating routing tree for a group with
Rendezvous Point (RP) as a root for the tree
RP is a focus for both senders and receivers
Explicit join model
Receivers send Join towards the RP
Sender send Register towards the RP
Supports both shared trees (default) and source
trees
RPF check depends on tree type
For shared tree (between RP and receivers), uses
RP address
For source tree (between RP and source), uses
Source address

5
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)
(,G) means all senders
RFC2362 PIM Sparse Mode Protocol Spec
(experimental)
Internet Draft draft-ietf-pim-v2-sm-00.txt
(October 1999)

6
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

7
PIM SM Tree Maintenance

Periodic Join/Prunes are sent to all PIM
neighbors
Periodic Joins refresh interfaces in a PIM
neighbors downstream list
Periodic Prunes refresh pruned state of a PIM
neighbor
There is a designated router (DR) for each local
network and all other routers get pruned
Received multicast packets reset (S,G) entry
expiration timers.
(S,G) entries are deleted if timers expire

8
PIM-SM(1)
S
Source
A
B
RP
D
C
E
R2
R1
Receiver 1
Receiver 2
9
PIM-SM(2)
S
Receiver 1 Joins Group GC Creates (, G) State,
Sends(, G) Join to the RP
Source
A
B
RP
D
Join
C
E
R2
R1
Receiver 1
Receiver 2
10
PIM-SM(3)
S
RP Creates (, G) State
Source
A
B
RP
D
C
E
R2
R1
Receiver 1
Receiver 2
11
PIM-SM(4)
S
Source Sends DataA Sends Registration to the RP
Source
IP tunnel between A and RP since multicast tree
is not established
Register
Data
A
B
RP
D
C
E
R2
R1
Receiver 1
Receiver 2
12
PIM-SM(5)
S
RP decapsulates RegistrationForwards Data Down
the Shared TreeSends Joins Towards the Source
Source
join
join
A
B
RP
D
C
E
R2
R1
Receiver 1
Receiver 2
13
PIM-SM(6)
S
RP Sends Register-Stop OnceData Arrives Natively
Source
Register-Stop
A
B
RP
D
C
E
R2
R1
Receiver 1
Receiver 2
14
PIM-SM(7)SPT Switchover
S
C Sends (S, G) Joins to Join theShortest Path
Tree (SPT)
Source
A
B
RP
D
join
C
E
R2
R1
Receiver 1
Receiver 2
15
PIM-SM(8)
S
C starts receiving Data natively
Source
A
B
RP
D
C
E
R2
R1
Receiver 1
Receiver 2
16
PIM-SM(9)
S
C Sends Prunes Up the RP tree forthe Source. RP
Deletes (S, G) OIF andSends Prune Towards the
Source
Source
Prune
Prune
A
B
RP
D
Prune
C
E
R2
R1
Receiver 1
Receiver 2
17
PIM-SM(10)
S
B, RP pruned
Source
A
B
RP
D
C
E
R2
R1
Receiver 1
Receiver 2
18
PIM-SM(11)
S
Source
New receiver2 joinsE Creates State and Sends (,
G) Join
A
B
RP
D
C
E
join
R2
R1
Receiver 1
Receiver 2
19
PIM-SM(12)
S
Source
C Adds Link Towards E to the OIFList of Both (,
G) and (S, G)Data from Source Arrives at E
A
B
RP
D
C
E
R2
R1
Receiver 1
Receiver 2
20
Inter-Domain Multicast Routing

BGP4 (Multicast BGP) for short-term solution
Tweeks to BGP4 to support multicast
Multicast Address Set and Claim (MASC)
Hierarchical multicast address allocation at
domain level
Dynamic allocation (not permanent) of addresses
by set and claim with collision
Border Gateway Multicast Protocol (BGMP)
Use a PIM-like protocol between domains (BGP for
multicast)

21
MASC

Assume Addr(A) is allocated to domain A and
domains B and C sit below A in a domain
hierarchy
B selects Addr(B) which is subset of Addr(A) and
send claim (addr(B)) message to A and C
A forwards claim to all children except B.
If any of As children is already using Addr(B)
they will report a collision to A.
A will notify B of the collision and B will
select other address space.
Address space information is used to create
distribution tree using BGMP.
Stored in M-RIB (Multicast Routing Information
Base)

22
BGMP

BGMP builds shared tree of domains for a group
Uses 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
Joins and prunes travel across domains
Can build unidirectional source trees
M-IGP (multicast Intra-Gateway Protocol) tells
the borders about group membership

23
Multicast Routers