IP Multicast

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IP Multicast

• CS5248

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You are Here
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Communication Models
receiver
sender
Traditional Applications One-to-One
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Communication Models
sender
receivers
Media Applications One-to-Many
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Communication Models
Media Applications Many-to-Many
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Naïve Solution

• Create M unicast connection for M clients

NOT Scalable!
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Famous Example

• Victorias Secret Spring Fashion Show
• Live Broadcast in 1999
• Video bit-rate 28 Kbps
• Number of viewers 1.5 Millions

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IP Multicast
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New Model IP Multicast
sender
receivers
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Group and Members
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Sending to a Group
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Joining and Leaving
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Anyone can Send
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Multicast Address

• Group ID or Multicast address
• 224.0.0.0 239.255.255.255

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Unicast
S
A
A
B
Router
C
B
Router
Router
Router
C
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Multicast
S
A
G
Router
B
Router
Router
Router
C
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Multicast
S
A
Router
G
B
Router
Router
Router
G
C
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Multicast
S
G
A
Router
B
Router
Router
G
Router
C
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Question 1
should I forward this packet to my subnet?
Router
?
G
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Question 2
which neighbors should I forward this packet?
Router
Router
Router
Router
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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 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.

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Leave

• If no one in group G after a few query messages,
  remove G from local host membership table.
• Problem 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 ?

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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

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Question 2
which neighbors should I forward this packet?
Router
Router
Router
Router
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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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Shortest Path Tree

• One tree for each source for each group

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Shared Tree

• One tree for each group

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Routing Protocols

• DVMRP
• CBT
• PIM

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DVMRP

• Distance Vector Multicast Routing Protocol

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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
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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
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Forward to subnet?
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.

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Truncation
Q
R
P
T
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Forward to Neighbors?
Q
?
R
P
T
?
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Observation

• If neighbor is going to ignore my packets, dont
  need to send the packets to it.

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Exchanging Routing Tables
Dest Next Hop Cost
S P 4
A T 3
B P 2
Q
R
P
T
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Flooding

• Default Always send to neighboring routers,
  unless told otherwise.

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Pruning

• Routers who received a useless packet send a
  prune message back.Dont send me packets
  addressed to G anymore !

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Q
R
P
T
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Q
R
P
T
PRUNE
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Q
R
P
T
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Q
R
P
T
GRAFT
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Q
R
P
T
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Q
PRUNE
R
P
T
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A router needs to remember..

• If it has any member for group G in its subnet
• Where to forward packets from source S to group G
• Which neighbors will not throw my packets away
• Which sub-trees are pruned

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Periodically Rebuild Tree

• DVMRP periodically forget about everything it
  learned
• Why?
• To clean-up left-over states for groups that
  are no longer active.

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Problems of DVMRP

• Not Scalable
• O(SG)
• Not Efficient
• Flooding initially
• Periodically exchange routing tables

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CBT

• Core-based Tree

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Core Based Tree

• One tree per group
• Pick a router as core
• Identity of core can be pre-configured or
  computing using some multicast address to core
  address mapping.

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A Shared Tree
W
R
U
core
P
Q
V
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V Joins G
W
R
U
JOIN
core
P
Q
V
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V Joins G
W
R
U
JOIN
core
P
Q
V
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V Joins G
W
R
U
ACK
core
P
Q
V
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V Joins G
W
R
U
ACK
core
P
Q
V
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V Joins G
W
R
U
core
P
Q
V
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U Joins G
W
R
U
core
P
Q
V
JOIN
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U Joins G
W
R
U
core
P
Q
V
ACK
intercept!
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Echo
W
R
U
core
P
Q
V
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Flush!
W
R
U
core
P
Q
V
X
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Send (on Tree)
W
R
U
core
P
Q
V
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Send (Not on Tree)
W
R
U
core
P
Q
V
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CBT Strengths

• Scalable
• O(G) states
• No flooding
• No exchange of states

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CBT Weaknesses

• Core placement matters
• Single point of failure
• Core can become bottleneck

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SPT vs. ST
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Experiment 1

• Shared tree has worst delay than shortest path
  tree. How much worst?
• Generate 500 random networks, 50 nodes each,
  10-member groups

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Experimental Results
Delay Ratio (CBT/SPT)
1.4
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Node Degree
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Experiment 2

• CBT causes traffic to concentrate on a set of
  links. How bad is this concentration?
• Generate 500 random networks, 50 nodes, 300
  groups, 40 members, 32 senders

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Experimental Results
Max Number of Flows/Link
9K
CBT
SPT
6K
Node Degree
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PIM

• Protocol Independent Multicast

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PIM

• Get the best of both world
• dense mode shortest path
• sparse mode shared tree

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PIM-DM

• Similar to DVMRP, except
• send to neighbors even if they will ignore my
  packets

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PIM-SM

• Similar to CBT except
• links are uni-directional

W
R
U
core
P
Q
V
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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 (Join S)
S
R
switch
P
core
Q
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Switch
S
R
switch
P
core
Q
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Prune
S
R
P
core
Q
prune
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Summary

• What is IP Multicast?
• How to route packets
• IGMP
• DVMRP/CBT/PIM

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So, why cant we multicast?

• Who assign multicast address?
• Who pay for multicast traffic?
• How to inter-operate between protocols?
• How can we prevent DoS?

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System Design Considerations

• or What I learned from Multicast Routing
  Protocols

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Practical Issues

• Money matters
• Security matters

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Scalability

• Minimize states
• Avoid implosion
• random timer
• Avoid hot spot

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Robustness

• Periodically forget (soft-states)
• Echo messages (alive messages)

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Minimize Interface /Coupling

• do not assume unicast routing protocol

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Middle Ground

• If you have two schemes with different pros/cons,
  think
• middle ground

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Possible Survey and Project Topics
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Application-Level Multicast
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Single Source Multicast

• Most media applications do not require
  many-to-many communication.
• Single Source Multicast protocols have been
  proposed

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Transport Layer
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TCP vs UDP

• TCP offers
• packet ordering
• reliability
• flow control
• UDP
• just send!

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TCP vs UDP

• Continuous media uses UDP
• Retransmission may not be useful
• Flow control makes throughput unpredictable
• Multicast TCP has problems

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UDP Header

• struct UDPHeader
• short src_port
• short dst_port
• short length
• short checksum

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Is UDP Enough?

• Who sent this packet?
• How do I interpret this packet?
• When was this packet generated?
• Which packets come first?

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RTP

• Real-Time Transport Protocol
• (not really a transport protocol)
• Provides
• source identification
• sequence number
• time stamping
• payload identification