Multicast & Multimedia
description
Transcript of Multicast & Multimedia
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Multicast & Multimedia
CS731Wei Tsang Ooi
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Overview
• What is Multicast ?• Group Management (IGMP)• Routing Protocols (DVMRP, MOPSF,
CBT, PIM-DM, PIM-SM)• Adaptivity (DSG, RLM,
ThinStreams, SCUBA, MEGA)
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What is Multicast ?
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What is Multicast ?
• Sending a packet to multiple destination using a single address
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Unicast
RouterRouter
Router
Router
A
B
C
SABC
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Multicast
RouterRouter
Router
Router
A
B
C
SG
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Multicast
RouterRouter
Router
Router
A
B
C
S
G
G
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Multicast
RouterRouter
Router
Router
A
B
C
S G
G
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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 ?
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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 ?
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Group Management
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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 Report
A : “I am a member of group G”A : “I am a member of group H”B : “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.
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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 message“I want to leave group G”
• Group-Specific Query“Anybody else belongs to group G ?”
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IGMP v3.0
• Group-Source Inclusion“I want to listen to S from group G”
• Group-Source Exclusion“I do not want to listen to S from group G”
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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 ?
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Routing Protocols
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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
R P
Q
T
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R P
Q
T
Is R on the shortest path to S ?
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R P
Q
T
If no.. ignore the packet
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R P
Q
T
If yes.. continue
Where should I
forward it to ?
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R P
Q
T
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R P
Q
T
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Truncation
• Router checks local host membership table.
• Duplicate the packets only if there is a member in the subnet.
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R P
Q
T
?
?
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R P
Q
T
US
?
?
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Exchanging Routing Table
• Routers periodically sends routing tables to their neighbours
• If neighbour is going to ignore my packets, don’t 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.
“Don’t send me packet addressed to G anymore !”
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R P
Q
T
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R P
Q
TPRUNE
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R P
Q
T
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R P
Q
TGRAFT
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R P
Q
T
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R P
Q
T
PRUNE
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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
RQ
P
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
TU
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P computes the tree
P
Q R
TU
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Q computes the tree
P
Q R
TU
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R & U compute the tree
P
Q R
TU
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T computes the tree
P
Q R
TU
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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
core PQ
RU
V
join
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Join
core PQ
RU
V
ack
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Intercept Join
core PQ
RU
V
join
(G, v, core)
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Intercept Join
core PQ
RU
V
ack
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Send (on the tree)
core PQ
RU
V
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Send (not on the tree)
core PQ
RU
V
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Send (not on the tree)
core PQ
RU
V
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Core Router Discoveries
• Position of core affect performance.
• No perfect solutions in choosing core.
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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
P coreQ
R
join
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Send (SM)
P coreQ
R
S
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Switch
P coreQ
R
S
switch
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Switch
P coreQ
R
S
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Send (DM)
P coreQ
R
S
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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 ?
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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 ?
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Multicasting over
Heterogenous
Network
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Problems
• Different receivers reside on different networks with different capacities, what should the source transmit ?
ISDN
28.8KModem
T3S
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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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Bolot’s Idea
• Senders and receivers generate random 16-bit “key”
• If a the first k bit of receiver’s key match the first k bit of the sender’s, the receivers responds.
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Bolot’s 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 Grouping
S.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
1
2
3
4
tjoi
n
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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 : layerBandwidth*totalExperimentTime
• Packets lost if buffer space is not large enough
• Congestions is detected after it occurs.
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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.
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SCUBA Elan Amir et. al 1997
• Reflect receiver’s interest in adapting bandwidth
• Receiver sends interest report to source.
• Source adjust bandwidth according to its weight
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SCUBA Example
S1
S2R3
R2
R1
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Interest Report
S1 S2R1 0.8 0.2
R2 0.8 0.2
R3 0.5 0.5
S1’s weight =( 0.8 + 0.8 + 0.5 )/ 3 = 0.7S2’s weight =( 0.2 + 0.2 + 0.5 )/ 3 = 0.3
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Result
• S1 will transmit at 70% total bandwidth
• S2 will transmit at 30% total bandwidth
Others will share the rest of 5%
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Problems
• Receiver Heterogeneity
Solution
• Layered Multicast
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1
2
3
4
1
2
3
4
S0 S1
Layers
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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
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MeGaElan Amir et. al. 1995
• Position a application level gateway at strategic point in the network.
MeGa
S
64Kbps100Mbps
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The End