Transcript Dynamic Routing Algorism: Link State

Dynamic Routing Algorism:
Link State
2000. 10.23
Bang, Sun lee
Database Research Lab.
Dept. of Computer Science
Distance Vector Routing’s Problems
• Queue length as the delay metrics, not considered
link bandwidth
• Long convergence time
• Distance vector routing was replaced in 1979 by a
link state routing
• Link State Algorism have Network topology
information
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Link State Algorism’s Basic Operation
• Discover its neighbors and learn their network
address
– Sending HELLO packet
• Measure the delay or cost to each of its neighbors
– Sending ECHO packet
• Construct a packet telling all it has just learned
• Send this packet to all other routers
• Compute the shortest path to every other router
– Dijkstra’s algorism
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Learning about the Neighbors
• Sending a special HELLO packet
• The router on the other end send back a reply
telling who it is
• Name must be globally unique
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Measuring Line Cost
• Reasonable estimate, of the delay to each of
neighbors
– Sending a special ECHO packet
Measuring the round-trip time and dividing it by two
– To factor the load in
• timer must start when the ECHO packet is queued
– To ignore the load
• timer should be start when the ECHO packet reached
the front of the queue
West
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East
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Building Link State Packets
• Once information is collected,next
Step is for each router to build
a packet containing all the data.
A
• When to build packets?
1.periodically
2.specific events
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<subnet>
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Seq.
Sequence number
Age
age
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List of neighbors
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Sender’s identity
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Seq.
Seq.
Seq.
Seq.
Seq.
Seq.
Age
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<link state packet>
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Distributing the Link State Packets
• Basic distribution algorithm
– Flooding to distribute the LSP
– Seq. # is incremented for each new packet sent
– Checked against the list of packets already seen
• New : packet is forwarded on all lines
• Duplicate : packet is discarded
• Greater seq. number is newer
– Problems
• If seq. # wrap around, confusion will reign
– Solution. 32-bit seq. #
• If router crashes,lose track of seq. #
• If seq.# is corrupted and 65540(1-bit error) is received instead of 4,
packet 5 is rejected
– Solution
• Include the age of packet
– Ageing
• Puts time out value in the header
• Decrement it once per second
• If Age hits zero then router is discarded
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Distributing the Link State Packets
• Refinement algorism
– Holding area
– To guard against errors on the router-router lines,
LSP are acknowledged
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<subnet>
Source
Send flags ACK flags
Seq.
Age
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0 1 1 1 0 0
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1 1 0 0 0 1
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0 1 0 1 0 1
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1 0 1 0 1 0
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1 0 0 0 1 1
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Computing the New Routes
• Once a router has a full set of link state packets,
the entire subnet graph can be constructed
• Dijkstra’s shortest path algorithm
1 Initialization:
2 N = {A}
3 for all nodes v
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if v adjacent to A
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then D(v) = c(A,v)
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else D(v) = infinity
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8 Loop
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find w not in N such that D(w) is a minimum
10 add w to N
11 update D(v) for all v adjacent to w and not in N:
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D(v) = min( D(v), D(w) + c(w,v) )
13 /* new cost to v is either old cost to v or known
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shortest path cost to w plus cost from w to v */
15 until all nodes in N
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Computing the New Routes
• For n routers and each with k neighbors
we need memory proportional to kn
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step
N
D(B),p(B)
D(C),p(C)
D(D),p(D)
D(E),p(E)
D(F),p(F)
0
A
1,A
6,A
9,A
INFINITY
INFINITY
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6,A
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9,C
INFINITY
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ABCD
9,C
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ABCDE
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Computing the New Routes
B(1)
D(9)
C(6)
A(0)
A(0)
A(0)
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Computing the New Routes
A(0)
B(1)
B(1)
B(1)
C(6)
D(8)
C(6)
C(6)
E(9)
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Used in actual Network
• OSPF(Open Shortest Path First) protocol
– Increasingly being used in the Internet
• IS-IS(Intermediate system-Intermediate system)
– Numerous Internet backbones(include the old NSFNET
backbone)
– Digital cellular systems such as CDPD
– Novell NetWare uses a minor variant of IS-IS for routing
IPX packets
• Almost same between IS-IS and OSPF
– important difference
• IS-IS : it is easy and natural to simultaneously
carry information about multiple network layer
protocol
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OSPF protocol
database1
database2
• Link-State routing Protocol ‘s feature
– All routers has same routing database
– Each database of router implement topology of router
domain
– Runs the same shortest path algorism, this algorism
modify Routing table
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