nus.soc.cs5248 ooi wei tsang ip multicast cs5248
TRANSCRIPT
NUS.SOC.CS5248 Ooi Wei Tsang
IP Multicast
CS5248
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You are Here
Network
Encoder
Sender
Middlebox
Receiver
Decoder
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Communication Models
Traditional Applications:One-to-One
receiversender
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Communication Models
Media Applications:One-to-Many
sender
receivers
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Communication Models
Media Applications:Many-to-Many
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Naïve SolutionCreate M unicast connection for M
clients
NOT Scalable!
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Famous Example
Victoria’s 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 AddressGroup ID or “Multicast address”
224.0.0.0 – 239.255.255.255
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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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Question 1
Router
G?
should I forward this packet to my
subnet?
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Question 2
RouterRouter
Router
Router
which neighbors should I
forward this packet?
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Group Management
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Group ManagementRouters 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 tAfter 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 v2.0LEAVE message
“I want to leave group G”
Group-Specific Query“Anybody else belongs to group G ?”
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IGMP v3.0Group-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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Question 2
RouterRouter
Router
Router
which neighbors should I
forward this packet?
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Routing Protocols
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Routing ProtocolsGeneric Methods :
Form a tree to all routers with members
Deliver the packets along the tree
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Shortest Path TreeOne tree for each source for each
group
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Shared TreeOne tree for each group
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Routing ProtocolsDVMRP CBTPIM
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DVMRP
Distance Vector Multicast Routing Protocol
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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.. continueWhere
should I forward it
to ?
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R P
Q
T
Forward to subnet?
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TruncationRouter 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
Truncation
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R P
Q
T
?
?
Forward to Neighbors?
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Observation If neighbor is going to ignore my
packets, don’t need to send the packets to it.
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R P
Q
T
Exchanging Routing Tables
Dest Next Hop Cost
S P 4
A T 3
B P 2
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FloodingDefault : Always send to
neighboring routers, unless told otherwise.
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PruningRouters who received a “useless”
packet send a prune message back.
“Don’t send me packets 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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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 TreeDVMRP 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 DVMRPNot Scalable
O(|S||G|)
Not Efficient Flooding initially Periodically exchange routing
tables
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CBT
Core-based Tree
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Core Based TreeOne 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
core PQ
RU
V
W
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V Joins G
core PQ
RU
V
JOIN
W
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V Joins G
core PQ
RU
V
JOIN
W
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V Joins G
core PQ
RU
V
ACK
W
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V Joins G
core PQ
RU
V
ACK
W
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V Joins G
core PQ
RU
V
W
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U Joins G
core PQ
RU
VJOIN
W
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U Joins G
core PQ
RU
VACK
W
intercept!
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Echo
core PQ
RU
V
W
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Flush!
core PQ
RU
VX
W
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Send (on Tree)
core PQ
RU
V
W
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Send (Not on Tree)
core PQ
RU
V
W
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CBT StrengthsScalable
O(|G|) states
No floodingNo exchange of states
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CBT WeaknessesCore placement mattersSingle point of failureCore 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
Node Degree
Delay Ratio (CBT/SPT)
1
~1.4
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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
Node Degree
Max Number of Flows/Link
6K
9K
CBT
SPT
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PIM
Protocol Independent Multicast
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PIMGet the best of both world :
dense mode : shortest path sparse mode : shared tree
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PIM-DMSimilar to DVMRP, except
send to neighbors even if they will ignore my packets
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PIM-SMSimilar to CBT except
links are uni-directional
core PQ
RU
V
W
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Join
P coreQ
R
join
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Send (SM)
P coreQ
R
S
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Switch (Join S)
P coreQ
R
S
switch
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Switch
P coreQ
R
S
switch
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Prune
P coreQ
R
S
prune
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SummaryWhat is IP Multicast?How to route packets
IGMP DVMRP/CBT/PIM
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So, why can’t 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 IssuesMoney mattersSecurity matters
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ScalabilityMinimize states
Avoid implosion random timer
Avoid hot spot
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RobustnessPeriodically 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 MulticastMost 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 UDPTCP offers
packet ordering reliability flow control
UDP just send!
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TCP vs UDPContinuous 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