CSCI 5273 Computer Networks Stevens, Chapter 9 Routing

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CSCI 5273Computer NetworksStevens, Chapter
9Routing

• Dirk GrunwaldAssoc. ProfessorDept. of Computer
  ScienceUniversity of Colorado, Boulder

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Outline

• Routing domains levels
• RIP routing
• Distance Vector Routing
• Examples of RIP information management
• OSPF routing
• Link state routing

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Gateway Hierarchy
InternetCore
AutonomousSystem(AS)
AutonomousSystem(AS)
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Two levels of Routing Protocols
RoutingDomain
RoutingDomain
IGP
IGP
EGP
RoutingDomain
EGP
EGP
Intra-domainrouting protocol
Exteriorrouting protocol
IGP
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Routing Protocols

• Intra-domain Gateway Protocols
• RIP
• RIP V2
• OSPF - open shortest path first
• IS-IS (similar to OSPF)
• Exterior Gateway Protocols
• EGP
• BGP

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RIP

• Distance vector routing algorithm based on hops
  that communicates between routers using UDP
• On initailization, router determines all
  available interfaces and sends a REQUEST packet
  out each interface. Special request for send
  everything
• On receipt of request,
• Either return everything
• Or, for each requested destination, return
  distance 1
• On response
• Update routing tables

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RIP V1 Protocol
Command
Version
MBZ
Address Family
MBZ
32-bit IP address
MBZ
MBZ
Metric (value of 1..16)
Up to 24 more routes in same format...
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Metrics
N2 is 1 hop
N1
R1
Route to N3via R2 withhop count of 2
N3 is 1 hop
N2
N1 is 1 hop
R2
N3
N2 is 1 hop
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Problems

• Hop count limited to 15
• Can only be used within an AS where maxiumum
  network diameter of 15
• Its based on HOPS, not e.g., latency or
  bandwidth
• No notion of subnet addressing in RIP V1

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RIP V2 Protocol
Command
Version
Routing domain
Address Family
Route tag
32-bit IP address
32-bit subnet mask
32-bit next-hop IP address
Metric (value of 1..16)
Up to 24 more routes in same format...
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RIP V2

• Routing domain is an identifier of the routing
  daemon
• Process ID in UNIX
• So you can run multiple instances of RIP
• Route tag carries an autonomous system number for
  EGP and BGP
• Next op address is where packets corresponding to
  that (sub)network should be sent. A value of zero
  means send to the system sending RIP info.
• Simple authentication scheme with clear-text
  password

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Distance Vector Routing

• Also called Bellman-Ford or Ford-Fulkerson
  algorithms
• Used by RIP
• Each router is responsible for keeping track and
  informing its neighbors of its distance to each
  destination
• The router computes its distance to a destination
  based on its neighbors distance to the
  destination
• Router must know its own ID and the cost of its
  links to each neighbor

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Distance Vector Routing For Address D
17
2
Link number
1
2
R
35
3
5
4
41
5
Link cost
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Distance Vector Routing For Address D
97
81
17
2
Cost from neighbor to destination D
1
2
R
35
62
3
5
4
41
5
29
118
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Distance Vector Routing For Address D
97
81
99
17
2
Cost for Rto get to Dvia this link
98
1
2
R
35
62
3
5
97
4
41
5
70
123
29
118
Minimumcost route
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Distance Vector Routing For Address D
70
70
17
2
Cost fromR to D
1
2
R
35
70
3
5
4
41
5
70
70
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Problems With Distance Vector

• Slow convergence to the lowest cost route
• Slow recovery time
• Slow recovery leads to routing problems during
  recovery
• Router loops
• Count to infinity

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Routing Loops
A
A
C
C
B
B
D
D
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Count To Infinity (worse case loop)
A
A
A
A
B
B
B
B
C
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OSPF - Open Shortest Path First

• OSPF uses IP directly (I.e., like ICMP)
• Routes calculated based on TOS
• Each interface is assigned a dimensionless cost,
  for each TOS
• If several equal-cost routes are available,
  traffic is load-balanced
• Subnets are associated with each advertised route
• Supports authentication
• Uses multicast to distribute information

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Link State Routing

• Used by OSPF and IS-IS
• Construct a Link State Packet that lists
  neighbors and costs to get to those neighbours
• Use Dijkstras algorithm to compute global routes
  as a tree from the current router

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Dijkstras Algorithm

• Two sets of paths PATH and TENT(ative)
• (destination, path cost, forwarding direction)
• (1) Place (self, 0,0) in path
• (2) For any (D,C,F) placed in PATH, examine Ds
  LSP. For each Neighbour of D listed in Ds LSP
  with a link cost to N of c, see if (N, c, ) is
  already in TENT or PATH. If not listed, or if c
  is less than existing cost, add (N, Cc,) to
  TENT
• (3) Terminate if TENT is empty, otherwise find
  (D,C,F) with minimum C and add that to PATH. Goto
  (2)

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Example of Dijkstras Algorithm
C(0)
F(2)
B(2)
G(5)
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Example of Dijkstras Algorithm
C(0)
Place F in path
F(2)
B(2)
G(5)
E(6)
G(3)
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Example of Dijkstras Algorithm
C(0)
F(2)
B(2)
G(5)
E(6)
Better pathto G exists
G(3)
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Example of Dijkstras Algorithm
Add B to Path
C(0)
F(2)
B(2)
A(8)
E(6)
E(3)
G(3)
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Example of Dijkstras Algorithm
C(0)
F(2)
B(2)
A(8)
E(6)
E(3)
G(3)
Better pathto E exists
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Example of Dijkstras Algorithm
C(0)
Add E to path
F(2)
B(2)
A(8)
E(3)
G(3)
D(5)
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Original Distribution of LSPs

• Neighbor-to-neighbor protocol for LSP
  distribution
• Eash LSP consists of
• Identity of router generating the LSP
• A sequence number
• The time left until the LSP should be discarded
• A list of neighbors, and the cost of links to
  each
• LSP is broadcast to all neighbors
• LSPs have a 64 second lifetime - each router
  must resend each minute
• Wrap-around comparison on sequence numbers used

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Problem in 1991

• A router announced LSPs with random TTLs
• a lt b lt c lta
• Each router that processed one of these LSPs
  over-wrote the one in memory and generated more
  copies of the bad LSP, since it thought that this
  LSP was newer needed to be propagated to
  neighbors
• The TTL field never changed because of the rapid
  rate of updates

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Revised method using 32-bit sequences

• LSPs compared only by their sequence numbers -
  if you get LSP with sequence S T, and S lt T,
  then T is more recent (no matter what TTL says)
• LSPs are purged when old, based on TTL. Every
  router decrements TTL
• When TTL reaches 0, router refloods the LSP.
  Given (sequence, TTL) (S,x) (T,y) then S lt T if
  x 0, but not otherwise
• LSPs generated much less frequently (1hr)
• Router starts with the lowest sequence number. If
  the network retains old LSP, it will be flooded
  back to source, when then knows current
  sequence number to use

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BGP

• Replacement for EGP
• AS classifications
• Stub - only has a single connection to another AS
• Multihomed - multiple connections, but doesnt
  carry transit traffic
• Transit - has connections to more than one other
  AS and is designed to carry both local and
  transit traffic
• BGP supports Policy Based Routing
• Did you pay for this?

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BGP

• Uses TCP
• Distance vector protocol, but BGP enumerates the
  route to each destination (using a sequence of AS
  numbers)
• Each AS is identified by a 16-bit number

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CIDR - Classless inter-domain routing

• Based on hierarchy of address prefixes