Virtual Private Networks and Distance Vector Routing

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• Virtual Private Networks and Distance Vector
  Routing

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Virtual Private Networks
Reality Two private corporate networks. Traffic
from one network is invisible to the other
network (ideal privacy). Leased lines, however,
may be too expensive to keep.
Virtual Two corporate networks sharing some of
the same physical links. Want to give the
functionality and the privacy of private networks
for each corporation.
Question What are the challenges to making this
happen?
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Implementing a VPN
Routing table
Encapsulate traffic from R1 to R2 inside IP
packets addressed to R2. Together with
encryption, this tunneling of packets is an
effective way to implement a VPN.
Question Can you think of other uses for
tunneling?
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Routing vs. Forwarding

• Definition forwarding consists of taking a
  packet, identifying its destination, looking up
  in a table the interface that leads to the
  destination, and sending it off in that
  direction.
• Definition routing is the process by which
  forwarding tables are built.

Routing table
Forwading table
vs.
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Network as a Graph
Routing is a problem of graph theory. You are
given a set of nodes V and a set of edges E such
that e (vi,vj) in E for vi and vj in V. These
two sets define a graph G(V,E). You are also
given a weight function w(.), which maps each e
in E to some real valued weight.
The problem is to find the lowest-cost path
between any two nodes, where the cost of a path
equals the sum of the weights of all edges in the
path. This can be re-stated as the all-pairs
shortest path problem.
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CSCI 311 FlashbackSingle-Source Shortest Paths

• In a single-source shortest paths problem we are
  given a weighted, directed graph G(V,E), with a
  weight function on E that maps edges to real
  valued weights. The weight of a path is defined
  as the sum of its constituent edges

A shortest path from vertex u to vertex v is
defined as any path p with weight
We will consider this problem only for
non-negative weights.
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Dijkstras Algorithm
S 1 for i 2 to n do Di C1i for
i 1 to n 1 do choose a vertex w in V-S
such that Dw is a minimum add w to
S for each vertex v in V-S do Dv
min (Dv, Dw Cwv)
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Distance Vector Routing
Global view
As view vector
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Distance Vector Routing
Every node starts by building its own local view
of what nodes are 1 hop away. Next, every node
sends its vector to its directly connected
neighbors.
F tells A that it can reach G at cost 1. A knows
it can reach F at cost 1, so it updated its own
vector to indicate that it can reach G at cost 2.
If A were to discover another route to G at a
cost higher than 2, it would ignore it and leave
its vector as it is. After a few iterations of
these exchanges, the routing table converges to a
consistent state.
Question How would this method deal with link
failures?
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Distance Vector Routing
Periodic updates Every t seconds, send your
local info to your neighbors. This allows other
nodes to know that you are running.
Triggered updates Every time you learn new
information from a neighbor that leads you to
update your local vector, you send the recomputed
vector to all your neighbors.
Question How can you detect that a node has
failed?
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Final Routing Table
Global view
As view vector
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The Count-to-infinity Problem
Link (A,E) goes down. A periodic update kicks in
and A advertises that its distance to E is
infinity. At about the same time, B tells A it
can reach E in 2 hops. A took the advice and
updates its table to reach E in 3 hops through B.
In next update, B will update its costs to E in 4
hops through A
The routing table doesnt converge or stabilize.
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Solutions for 2-node loops

• Count-to-infinity Cap infinity at some maximum
  number of hops that allow a packet to go all the
  way across the network.
• Reduce time to convergence Split-horizon when
  a node sends a routing update to its neighbors,
  it does not send the routes it learned from a
  neighbor back to that neighbor.
• Reduce time to convergence Split-horizon with
  poison reverse communicate back to the sending
  neighbor but poison the route with negative
  information (infinity) so that it doesnt end up
  used as intermediate node in a route.

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Routing Information Protocol (RIP)
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RIP

• Routing within an autonomous system
• Hop count metric
• Unreliable transport
• Broadcast or multicast delivery
• Support for default route propagation
• Distance vector algorithm
• Passive version for hosts