Multicast

1
Multicast Multimedia

• CS731
• Wei Tsang Ooi

2
Overview

• What is Multicast ?
• Group Management (IGMP)
• Routing Protocols (DVMRP, MOPSF, CBT, PIM-DM,
  PIM-SM)
• Adaptivity (DSG, RLM, ThinStreams, SCUBA, MEGA)

3
What is Multicast ?
4
What is Multicast ?

• Sending a packet to multiple destination using a
  single address

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Unicast
S
A
A
B
Router
C
B
Router
Router
Router
C
6
Multicast
S
A
G
Router
B
Router
Router
Router
C
7
Multicast
S
A
Router
G
B
Router
Router
Router
G
C
8
Multicast
S
G
A
Router
B
Router
Router
G
Router
C
9
Questions

• How does a router know if some host in its subnet
  is member of some group ?
• How does a router know where to forward a packet
  ?
• What if a packet is lost ?
• What if the members have different network
  capacity ?

10
Questions

• How does a router know if some host in its subnet
  is member of some group ?
• How does a router know where to forward a packet
  ?
• What if a packet is lost ?
• What if the members have different network
  capacity ?

11
Group Management
12
Group Management

• Routers maintain local host group membership
  table
• which group has a member in my subnet ?

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IGMP v1.0

• JOIN messageA I want to join group G.
• QUERY messageR Which group have you joined ?

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IGMP v1.0

• MEMBERSHIP ReportA I am a member of group
  GA I am a member of group HB I am a
  member of group G

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Avoiding Implosion

• Select random delay t
• After time t, if nobody belongs to the same
  group, send membership report.
• Resend the report after some delay just to be
  safe.

16
Leave

• If nobody tell the router that they belongs to
  some group G after a few query messages, G will
  be removed from local host membership table.
• Long leave latency (minutes)

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IGMP v2.0

• LEAVE messageI want to leave group G
• Group-Specific QueryAnybody else belongs to
  group G ?

18
IGMP v3.0

• Group-Source InclusionI want to listen to S
  from group G
• Group-Source ExclusionI do not want to listen
  to S from group G

19
Questions

• How does a router know if some host in its subnet
  is member of some group ?
• How does a router know where to forward a packet
  ?
• What if a packet is lost ?
• What if the members have different network
  capacity ?

20
Routing Protocols
21
Routing Protocols

• Generic Methods
• Form a tree to all routers with members
• Deliver the packets along the tree

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Rounting Protocols

• DVMRP
• MOSPF
• CBT
• PIM

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Rounting Protocols

• DVMRP
• MOSPF
• CBT
• PIM

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From S to G
Q
R
P
T
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Is R on the shortest path to S ?
Q
R
P
T
26
If no.. ignore the packet
Q
R
P
T
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If yes.. continue
Where should I forward it to ?
Q
R
P
T
28

Q
R
P
T
29

Q
R
P
T
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Truncation

• Router checks local host membership table.
• Duplicate the packets only if there is a member
  in the subnet.

31

Q
?
R
P
T
?
32

U
S
Q
?
R
P
T
?
33
Exchanging Routing Table

• Routers periodically sends routing tables to
  their neighbours
• If neighbour is going to ignore my packets, dont
  need to send the packets to it.

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Pruning

• Default Always send to neighbouring routers,
  unless told otherwise.
• Routers who received a useless packet send a
  prune message back.Dont send me packet
  addressed to G anymore !

35

Q
R
P
T
36

Q
R
P
T
PRUNE
37

Q
R
P
T
38

Q
R
P
T
GRAFT
39

Q
R
P
T
40

Q
PRUNE
R
P
T
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Problems of DVMRP

• One tree for each pair (source, group)
• Rebuilt tree periodically
• So need to exchange routing tables periodically
• First packet may visits more routers than it
  needs to

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Rounting Protocols

• DVMRP
• MOSPF
• CBT
• PIM

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MOSPF

• Based on OSPF
• For intra-gateway routing
• Routers flood membership infomation to all other
  routers

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MOPSF

• All routers must have the same topological/group
  members information.
• Each of them compute a shortest path tree.

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Problems of MOSPF

• Different minimal tree leads to wastage.

P
P
R
Q
Q
R
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Solution

• Some convention to make sure all tree computed
  are the same

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Problems of MOSPF

• One tree for each (source, group) pair
• Computational intensive

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Solution

• Compute the tree when received the first packet.

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Example of MOSPF
P
Q
R
T
U
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P computes the tree
P
Q
R
T
U
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Q computes the tree
P
Q
R
T
U
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R U compute the tree
P
Q
R
T
U
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T computes the tree
P
Q
R
T
U
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MOSPF vs DVMRP

• MOSPF only forward packets down the path that
  leads to members.

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Problems with MOSPF/DVMRP

• Not scalable O(SG)
• Does not work well over sparsely distributed
  group
• Also known as dense-mode routing protocols

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Rounting Protocols

• DVMRP
• MOSPF
• CBT
• PIM

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Core Based Tree

• Designed for
• sparse-mode
• better scalability
• A router is desinated as a core (how?)

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Join
R
U
join
core
P
Q
V
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Join
R
U
ack
core
P
Q
V
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Intercept Join
R
U
(G, v, core)
join
core
P
Q
V
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Intercept Join
R
U
ack
core
P
Q
V
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Send (on the tree)
R
U
core
P
Q
V
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Send (not on the tree)
R
U
core
P
Q
V
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Send (not on the tree)
R
U
core
P
Q
V
65
Core Router Discoveries

• Position of core affect performance.
• No perfect solutions in choosing core.

66
Core Discovery - Bootstrap

• A router is elected as Bootstrap Router
• Other routers send core candidate message to
  the bootstrap router
• Bootstrap router send a vector of candidate core
  routers to all routers
• Routers hash the group address, index into the
  vector to find the core router.

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Core Discovery - Manual

• Configure each routers with (core, group) table.

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CBT Summaries

• One shared tree per group
• Scalablity O(G)
• No need to broadcast routing tables or flood link
  states
• Worst case delay is twice of the shortest path
  tree (average 1.4 times)
• Single point of failure

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Rounting Protocols

• DVMRP
• MOSPF
• CBT
• PIM

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Protocol Independent Multicast

• Get the best of both world
• dense mode shortest path
• sparse mode shared tree
• Independent of unicast routing protocols.

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Join
R
join
P
core
Q
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Send (SM)
S
R
P
core
Q
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Switch
S
R
switch
P
core
Q
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Switch
S
R
P
core
Q
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Send (DM)
S
R
P
core
Q
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Questions

• How does a router know if some host in its subnet
  is member of some group ?
• How does a router know where to forward a packet
  ?
• What if a packet is lost ?
• What if the members have different network
  capacity ?

77
Questions

• How does a router know if some host in its subnet
  is member of some group ?
• How does a router know where to forward a packet
  ?
• What if a packet is lost ?
• What if the members have different network
  capacity ?

78
Multicasting over Heterogenous Network
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Problems

• Different receivers reside on different networks
  with different capacities, what should the source
  transmit ?

ISDN
S
T3
28.8K Modem
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Solutions

• Source decides what to send based on feedback
  from receivers
• Receivers decides what to receive from the source
• Network decides what the receivers will received

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Source Driven

• Source decides what to send based on feedback
  from receivers
• Receivers decides what to receive from the source
• Network decides what the receivers will received

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Jean Bolot et. al. (1994)

• Networks condition is categorized as
• Unloaded
• Loaded
• Congested
• Objectives Stays in loaded region

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Naive Approach

• If sender received one complain about congestions
  in the network, sender reduce the sending rate.
• Works in unicast case but not in the case of
  multicase.

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A Better Solution

• If at least x of the receivers are congested,
  reduce sending rate.
• If at least y of the receivers are loaded, do
  nothing.
• Else increase sending rate

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How to Get Feedback ?

• Sender cannot ask all receivers at once and
  receivers cannot all answer at once.

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Bolots Idea

• Senders and receivers generate random 16-bit
  key
• If a the first k bit of receivers key match the
  first k bit of the senders, the receivers
  responds.

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Bolots Idea

• First iteration, senders use the receiver
  responds to estimate the group size.
• Subsequently, receivers only responds if the
  network condition is worst than the sender
  thought.

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Receiver Driven

• Source decides what to send based on feedback
  from receivers
• Receivers decides what to receive from the source
• Network decides what the receivers will received

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Destination Set GroupingS.Y. Cheung et. al. 1995

• Source transmit same data in different streams
  with different quality
• Receivers can feedback to the source, to adjust
  the rate of a stream (intra-stream), or they can
  move to different streams (inter-stream)

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Simulcast Streams
stream 1
2
3
high
low
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Problems

• Receivers might decide to join higher quality
  streams, but later find out that they are not
  able to handle it.
• Side Effects quality of streams might be
  lowered.

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Solutions

• inter-stream switch is by invitation of source
  only
• punishment cannot switch again within a time
  inverval

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RLMMcCanne et. al. 1996

• Source transmits data in different layers.
• One layer per group
• Need better quality, add a layer
• Congestion, drop a layer

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Join-Experiment

• Receivers find out if they can join the next
  higher layer by experiments.
• Join the layer, if congested, drop it.

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Join Experiment
4
3
tjoin
2
1
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Problems

• If two or more receivers perform experiments at
  once, they interfere with each other.

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Solutions

• Before performing an experiment, a receiver
  announce its intention to others.
• Others will refrain from performing the
  experiment.

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More Problems

• New receiver needs to quickly subscribe to
  layers.

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Solution

• join-experiments for lower layer is allowed to
  overlap with higher layer experiments.
• The receivers needs to compensate for the
  overlapping when analyzing the result of the
  experiment.

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Problems

• Buffer space needed by router is
  layerBandwidthtotalExperimentTime
• Packets lost if buffer space is not large enough
• Congestions is detected after it occurs.

101
ThinStreams Linda, Brian Rosen 1997

• Each layer is thin has a fixed thickness
• Use throughput as a measurement instead of packet
  losses
• Use clock signal to sync join-experiment
• Enforce link-sharing by making it harder to join
  higher group.

102
SCUBA Elan Amir et. al 1997

• Reflect receivers interest in adapting bandwidth
• Receiver sends interest report to source.
• Source adjust bandwidth according to its weight

103
SCUBA Example
R1
S1
R2
S2
R3
104
Interest Report
S1s weight ( 0.8 0.8 0.5 )/ 3 0.7 S2s
weight ( 0.2 0.2 0.5 )/ 3 0.3
105
Result

• S1 will transmit at 70 total bandwidth
• S2 will transmit at 30 total bandwidth
• Others will share the rest of 5

106
Problems

• Receiver Heterogeneity

107
Layers
S0
S1
108
Network Driven

• Source decides what to send based on feedback
  from receivers
• Receivers decide what to receive from the source
• Network decides what the receivers will received

109
MeGaElan Amir et. al. 1995

• Position a application level gateway at strategic
  point in the network.

S
64Kbps
100Mbps
MeGa
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The End