CS 352 Internet Technology Routing

1
CS 352Internet TechnologyRouting

• Richard Martin
• Dept. of Computer Science
• Rutgers University

2
Forwarding vs Routing

• Routing is the decision which path to take
• E.g. From Rutgers to Manhattan
• Route 1 Hoes ln, Centennial Ave, 287, GSP, 78,
  Route 1/9, Holland tunnel
• Route 2 Rt. 18, Turnpike, Lincoln tunnel
• Factors Length, cost, congestion
• Forwarding is the lookup once the decision has
  been made
• Youre at the intersection of Rt. 18 the
  Turnpike. Which ramp to take?
• These functions are typically decoupled in real
  systems

3
Routing Algorithms

• A routing algorithm decides which output link an
  incoming packet should be transmitted on
• A routing table contains the mappings from the
  networks and host addresses to output ports on
  the router
• The routing algorithm builds this table
• Typically a more advanced data structure is used,
  but conceptually its a table

4
Forwarding

• A forwarding algorithm performs the lookup of
  address to output port
• In connection-oriented service, the forwarding
  algorithm is performed by switches and only
  during connection setup
• In connectionless service, the forwarding
  algorithm is performed by routers every time a
  packet arrives

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Example Routing Table

• remus/ug/u1/rmartinnetstat -rn
• Routing Table
• Destination Gateway Flags
  Ref Use Interface
• -------------------- -------------------- -----
  ----- ------ --------
• 128.6.13.0 128.6.13.3 U
  3 4259 hme0
• 224.0.0.0 128.6.13.3 U
  3 0 hme0
• default 128.6.13.21 UG
  02184931
• 127.0.0.1 127.0.0.1 UH
  0 346400 lo0

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Shortest Path Routing

• For a pair of communicating hosts, there is a
  shortest path between them
• Shortness may be defined by
• Number of router/switch hops
• Geographic distance
• Link delay

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Shortest Path
What is the shortest path between A and F?
2
B
3
4
D
F
1
A
2
2
1
C
E
3
Edge Weight (Distance)
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Shortest Path Intuition

• To find the shortest path from X to Y
• Start with node X
• Select the next shortest path from X to a node we
  have not yet found a shortest path to
• Eventually well select node Y
• We must keep selecting shortest paths from other
  nodes because some path to Y through a node Z
  might be shorter

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Computing the Shortest Path

• Dijkstras Shortest Path Algorithm
• Step 1 Draw nodes as circles. Fill in a circle
  to mark it as a permanent node.
• Step 2 Set the current node equal to the source
  node
• Step 3 For the current node
• Mark the cumulative distance from the current
  node to each non-permanent adjacent node. Also
  mark the name of the current node. Erase this
  marking if the adjacent node already has a
  shorter cumulative distance marked
• Mark the non-permanent node with the shortest
  listed cumulative distance as permanent and set
  the current node equal to it. Repeat step 3
  until all nodes are marked permanent.

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Dijskstras Shortest Path AlgorithmExample
7
B
C
2
3
2
3
2
A
E
F
D
1
2
2
6
4
G
H
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Dijskstras Shortest Path AlgorithmExample
7
B
C
2
3
2
3
2
A
E
F
D
1
2
2
6
4
G
H
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Dijskstras Shortest Path AlgorithmExample
7
B
C
2
3
2
3
2
A
E
F
D
1
2
2
6
4
G
H
Selected edge
Candidate edge
Not visited
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Dijskstras Shortest Path AlgorithmExample
7
B
C
2
3
2
3
2
A
E
F
D
1
2
2
6
4
G
H
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Example
Confirmed Edge List
Candidate Edge List
Step
(neighbor,cost,nexthop)
(A,0,-)
1
(A,2,B)
(A,0,-)
Add candidates
2
(A,6,G)
(A,0,-)
Select Edge
(G,6,G)
3
(A,2,B)
(A,0,-)
(G,6,G)
Add candidates
(A,2,B)
(B,9,C)
4
(B,4,E)
(A,0,-)
(G,6,G)
Select Edge
5
(A,2,B)
(B,9,C)
(B,4,E)
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Shortest Path Routing (contd)

• Non adaptive to congestion, if
• geographical distances are used as edge weights
• maximum link throughputs are used as edge weights
• Number of hops are used as edge weights

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Problems with Centralized Approaches

• Reliability
• If the control center goes down, no more routing
• Scalability
• The control center must handle a great deal of
  routing information, especially for larger
  networks
• How many routers does a given system support?
  10s, 100s?, 1000s, 1000000s?
• Decentralized control
• Why should one organization depend on another for
  internal routing?

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Distributed Routing Algorithms

• Hot Potato
• Exchange based
• Each router periodically exchanges routing
  information (e.g., estimated time delay, queue
  length, etc.) with its neighbors
• Examples
• Distance Vector Routing
• Link State Routing

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Hot Potato Routing

• When a packet arrives, the router tries to get
  rid of it as fast as it can by putting it on the
  output line that has the shortest queue
• Hot potato does not care where the output line
  leads
• Not useable for large networks
• Can work in small networks

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Distance Vectors

• Each router maintains lists of best-known
  distances to all other known routers. These
  lists are called vectors.
• Each router is assumed to know the exact distance
  (in delay, hop count, etc.) to other routers
  directly connected to it.
• Periodically, vectors are exchanged between
  adjacent routers, and each router updates its
  vectors.

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Distance Vectors (contd)
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Convergence Speed

• Supposed all routers know the current state of
  the network
• Next, a change in link-status happens
• How fast the routers learn new state of the
  network is the convergence speed

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Problem Count-to-Infinity

• With distance vector routing, links coming on
  line converge quickly but link failures converge
  slowly
• This problem for DV routing algorithms is call
  count-to-infinity

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Count-to-Infinity
A
B
C
D
E
1
2
3
4
Initially
3
2
3
4
After 1 exchange
3
4
3
4
After 2 exchanges
5
4
5
4
After 3 exchanges
5
6
5
6
After 4 exchanges
7
6
7
6
After 5 exchanges
etc to infinity
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Split Horizon

• Insight Its not useful to claim reachablity for
  a destination to the neighbor from which the
  route was learned
• Dont report routes back to node from which the
  route was learned
• E.g. If I hear from X has the shortest route to
  Y, dont report to X I have a route to Y

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Poison Reverse

• Not reporting a route means having to wait for a
  timeout
• Report split-horizon routes as infinity to
  break loops on the first routing exchange.

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Split Horizon with Poisoned Reverse
A
B
C
D
E
inf.
2
3
4
B learns A is dead
C reports to B that As metric is inf.
inf.
inf.
2
3
4
inf.
inf.
3
4
After 1 exchange
inf.
inf.
inf.
4
After 2 exchanges
inf.
inf.
inf.
inf.
After 3 exchanges
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Split Horizon Failure
B
A
If D goes down, A and B will still count to
infinity. Split-Horizon infinity messages are
sent from A-C and B-C, not AB
C
D
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Link State Routing

• Each router measures the distance (in delay, hop
  count, etc.) between itself and its adjacent
  routers
• The router builds a packet containing all these
  distances. The packet also contains a sequence
  number and an age field.
• Each router distributes these packets using
  flooding
• Each router builds map of the entire network,
  uses a shortest-path algorithm

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Link State Routing (contd)

• To control flooding, the sequence numbers are
  used by routers to discard flood packets they
  have already seen from a given router
• The age field in the packet is an expiration
  date. It specifies how long the information in
  the packet is good for.
• Once a router receives all the link state packets
  from the network, it can reconstruct the complete
  topology and compute a shortest path between
  itself and any other node using Dijsktras
  algorithm.

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Hierarchical Routing

• All routing algorithms have difficulties as the
  network becomes large
• For large networks, the routing tables grow very
  quickly, and so does the number of flood packets
• How can this be reduced? Hierarchical routing

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Hierarchical Routing (contd)

• Segment the network into regions
• Called autonomous systems, (AS) typically a
  single organizational entity
• Routers in a single AS know all the details about
  other routers in that region, but none of the
  details about routers in other regions
• Analogy Telephone area codes

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Hierarchical Routing (contd)
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RIP

• Route Information Protocol
• One of the routing algorithms used by the
  Internet
• Based on distance vector routing
• Did not scale well
• suffered the count-to-infinity problem
• weak security
• RIP is phased out for large backbones
• but still widely used for smaller networks

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OSPF

• Open Shortest Path First
• Routing algorithm now used in the Internet
• OSPF uses the Link State Routing algorithm with
  modifications to support
• Multiple distance metrics (geographical distance,
  delay, throughput)
• Support for real-time traffic
• Hierarchical routing
• Security

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OSPF (contd)

• OSPF divides the network into several
  hierarchies
• Autonomous Systems (ASs)
• groups of subnets
• Areas
• Groups of routers within an AS
• Backbone Areas
• Groups of routers that connect other areas
  together

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OSPF Model (contd)
Autonomous System
Backbone Area
Area
Area
Autonomous System
Area
Backbone Area
Area
Area
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OSPF (contd)

• Routers are distinguished by the functions they
  perform
• Internal routers
• Only route packets within one area
• Area border routers
• Connect to areas together
• Backbone routers
• Reside only in the backbone area
• AS boundary routers
• Routers that connect to a router outside the AS

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OSPF Modified Link State Routing

• Recall
• In link state routing, routers flood their
  routing information to all other routers in the
  network
• In OSPF, routers only send their information to
  adjacent routers, not to all routers.
• Adjacent does NOT mean nearest-neighbor in OSPF
• One router in each area is marked as the
  designated router
• Designated routers are considered adjacent to all
  other routers in the area
• OSPF combines link state routing with centralized
  adaptive routing

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OSPF Adjacency
Designated Router
To backbone area
F
B
D
A
Area
E
C
Example C is adjacent to B but not to A or E B
is adjacent to all routers in the area