Routing Protocols

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Routing Protocols
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Routing

• Forwarding versus Routing
• forwarding to select an output port based on
  destination address and routing table
• routing process by which routing table is built
• Network as a Graph

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Routing

• Problem Find the lowest cost path between any
  two nodes
• Factors
• Static topology, SOL delays
• Dynamic load, congestion
• Scalable

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Distance Vector (RIP)

• Each node maintains a set of triples
• (Destination, Cost, NextHop)
• Each node sends updates to (and receives updates
  from) its directly connected neighbors
• periodically (on the order of several seconds)
• whenever its table changes (called triggered
  update)
• Used by Routing Information Protocol (RIP)

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Routing

• Each update is a list of pairs
• (Destination, Cost)
• Update local table if receive a better route
• smaller cost
• came from next-hop
• Refresh existing routes delete if they time out

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Example
Routing table at node B
Destination A C D E F G
Cost 1 1 inf inf inf inf
Next Hop A C
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Example

• Routing table at node B

B
C
A
D
E
F
G
Destination A C D E F G
Cost 1 1 2 2 2 3
Next Hop A C C A A A
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Routing Loops

• Example 1
• F detects that link to G has failed
• F sets distance to G to infinity and sends update
  to A
• A sets distance to G to infinity since it uses F
  to reach G
• A receives periodic update from C with 2-hop path
  to G
• A sets distance to G to 3 and sends update to F
• F decides it can reach G in 4 hops via A

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Example 2

• Link from A to E fails
• A advertises distance of infinity to E
• B and C advertise a distance of 2 to E
• B and C both set the distance to E to infinity
• B decides it can reach E in 3 hops through C
  advertises this to A
• A decides it can reach E in 4 hops through B
  advertises this to C
• C decides that it can reach E in 5 hops......

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Heuristics to break routing loops

• set infinity to 16
• split horizon
• Dont send routes learned from particular
  neighbor to that neighbor (B doesnt send route
  E,2,A to A)
• split horizon with poison reverse
• Include negative information (B sends route E,?
  to A)
• Only work with routing loop with simple loop,
  more complex techniques later

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Example 2 with poison

• Link from A to E fails
• A advertises distance of infinity to E
• B and C advertise a distance of 2 to E to each
  other
• B decides it can reach E in 3 hops through C
  advertises this to A, sends ? to C,
• since C was using B to get to E, C advertises a
  distance of ? to E
• Since B was using C to get to E, B advertises a
  distance of ? to E

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RIP Network
C advertises to router A and D, cost 0 to 2,3,
cost 1 to 5,6,1 and cost 2 to 4
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Summary Distance Vector

• Send entire routing table (Large amount of data
  (O(N)) )
• Send it only to directly connected neighbors (to
  a few endpoints (O(1))
• Routers do not have global information about
  connections in the network

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Link State (OSPF)

• Strategy Send to all nodes (not just neighbors)
  information about directly connected links (not
  entire routing table).
• Link State Packet (LSP)
• id of the node that created the LSP
• cost of link to each directly connected neighbor
• sequence number (SEQNO)
• time-to-live (TTL) for this packet
• Example Open Shortest Path First Protocol (OSPF)

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Reliable Flooding

• store most recent LSP from each node
• forward LSP to all nodes but one that sent it
• generate new LSP periodically increment SEQNO
• start SEQNO at 0 when reboot
• decrement TTL of each stored LSP discard when
  TTL0

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Reliable Flooding
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What do you do with LSPs

• Want to calculate shortest path from D to every
  other node
• Naïve approach explores every possibility
• O(NN)
• NP Complete multicriteria

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Route Calculation (Dijkstras) O(N2)
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Edsger Wybe Dijkstra 1930-2002

• Dijkstra is renowned for the insight that
  mathematical logic is and must be the basis for
  sensible computer program construction and for
  his contributions to mathematical methodology.
• thirteen hundred written works
• leader in the abolition of the GOTO statement
  from programming
• "The question of whether computers can think is
  like the question of whether submarines can swim

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Route Calculation (in practice)

• Forward search algorithm
• Each switch maintains two lists
• Tentative and Confirmed
• Each list contains a set of triples
• (Destination, Cost, NextHop)

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(destination, cost, nexthop)
Step 1. 2. 3. 4.
Confirmed (D,0,-) (D,0,-) (D,0,-) (C,2,C) (D,0
,-) (C,2,C)
Tentative (B,11,B) (C,2,C) (B,11,B) (B,5,C) (
A,12,C)
Step 5. 6. 7.
Confirmed (D,0,-) (C,2,C) (B,5,C) (D,0,-) (C,2,C)
(B,5,C) (D,0,-) (C,2,C) (B,5,C) (A,10,C)
Tentative (A,12,C) (A,10,C)
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Link State vs distance-vector

• Distance vector (original ARPANET)
• Node talks to direct neighbors
• Sends everything it has learned
• Link State (Later ARPANET)
• Node talks to everyone
• Only sends what it knows for sure