ITI 510

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ITI 510 Computer NetworksMeeting 3

• Rutgers University Internet Institute
• Instructor Chris Uriarte

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Meeting 3 - Agenda

• Dynamic Routing Protocols
• Transport Layer Protocols
• UDP
• TCP

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Dynamic Routing Protocols

• Recall, the routing table on a host or router
  maintains the next hop information for
  destination hosts and networks.
• In simple networks, there is often only one path
  that packets can take one default route.
• As networks become larger and more complex, a
  packet may have multiple paths to one
  destination. Dynamic Routing Protocols help keep
  routing tables up-to-date with accurate routing
  information.

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Dynamic Routing Protocols If Routers Could Talk

• Dynamic Routing Protocols are languages that
  routers use to share route information with each
  other.

Network A
Hey everyone, I can reach network B!
Network B
Hey everyone, I can reach network A!
Router B
Router A
Router C
Hey everyone, I can reach network C!
Network C
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Dynamic vs. Static Routing

• Sometimes, network topologies dont change very
  often and interconnected routers dont need to
  share any route information with each other. In
  this case, we use static routing, where route
  information is manually entered into a router and
  requires a manual change when the network
  topology changes.
• In other cases, a network topology may change
  very often, or there can be multiple paths to a
  particular destination. In cases like this,
  Dynamic Routing Protocols may be used.

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Simple Example of Dynamic Routing Protocols (If
Routers Could Talk II)

• Routers exchange information with each other

• I am directly attached to Network B.
• I Can reach network A in 1 hop.
• I can reach network C in 1 hop.

• I am directly attached to Network A.
• I Can reach network C in 1 hop.
• I can reach network B in 1 hop.

Network A
Network B
Router B
Router A

• I am directly attached to Network C.
• I Can reach network A in 1 hop.
• I can reach network B in 1 hop.

Router C
Network C
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A Change In Network Topology

• A change in network topology will cause routers
  to change the routes they advertise.

• I am directly attached to Network B.
• I Can reach network A in 1 hop.
• I can reach network C in 1 hops.

• I am directly attached to Network A.
• I Can reach network C in 1 hop.
• I can reach network B in 1 hop.

Network A
Network B
Router B
Router A
Link between AB goes down.
X

• I am directly attached to Network C.
• I Can reach network A in 1 hop.
• I can reach network B in 2 hops.

Router C
Network C
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Classes of Routing Protocols Measuring Routing
Metrics

• How can we decide if one router is better than
  another? We can use different types of metrics
  (distance measurements) to compare two routes.

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Distance Vector Protocols

• The most simple type of dynamic routing protocol.
• Routers keep simple routing metrics for each
  link. Metrics for each link may include
• The number of hops to a particular destination
• The bandwidth, delay, reliability or cost-of-use
  for a link.
• Distance Vector algorithms base routing decisions
  on metric values alone.

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Example Distance Vector Protocol

• For this example, lets base routing decisions
  solely on the least amount of hops needed to
  reach a destination.

Net E
E
Net B
B
Net A
A
D
C
Net C
In the above example, packets from Network A
destined for Network E will chose the path A-B-E
since that represents the least amount of hops -
Even though packets have an alternate path to
Network E (A-C-D-E)
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Example 2 Distance Vector Protocols

• In this example, lets assign a weight to each
  link the greater the weight, the faster the
  link. Routing decisions will be made based on
  the quickest path.

Net E
E
Net B
10
B
Net A
3
15
A
D
2
5
C
Net C
Here, Packets from Net A-gtE will choose the path
A-C-D-E because the path is faster.
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RIP Routing Information Protocol

• RIP is a simple distance vector protocol where
  each link is assigned a value of 1. The total
  metric of the path is the sum of all the hop
  costs.

Net E
E
Net B
B
Net A
A
D
C
Net C
Source Destination Cost Next Hop Net A Net
B 1 Rtr. B Net A Net E 2 Rtr. B Net A Net
C 1 Rtr. C
RIP Logic Table for Router A
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Propagation of RIP Routes

• The sharing of route information between routers
  is known as route propagation.
• RIP packets are broadcast on the network by
  RIP-enabled routers. RIP routers listen for
  routes advertised by other RIP routers.
• RIP routers will ONLY advertise network routes to
  neighbor routers - routers they are directly
  attached to.
• When a peer router received a routing metric from
  another router, the route information is added to
  the routing table UNLESS the recipient router
  already has a BETTER route.
• Refer to Feit. Pg. 225 for a good description of
  the RIP propegation process.

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RIP Routes, cont.

• If a link is down between 2 routers, a router
  will set the metric for any relitive destinations
  to 16, which means I cant get there. When
  neighbors receive the advertised router of 16,
  they will attempt to recalculate routing paths.
• RIP is a very simple routing protocol that is
  useful in small networks.

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RIP Version 1

• RIP packets are sent to neighbor rourters every
  30 seconds.
• If a neighbor router advertises a route of 16,
  other routers will begin to calculate new paths
  to the relevant destination networks.
• If a router does not receive RIP updates from its
  neighbor in 3 minutes, the router will assume the
  neighbor is down and will mark all routes as
  unreachable (metric 16). The router will
  then search for another route for up to 2 minutes
  before totally deleting the entry from its
  routing tables.

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Disadvantages of RIP

• The maximum metric for any path is 15 since 16
  means I cant get there. This is not effective
  for big networks.
• When there is a change of topology, RIP is often
  very slow to re-establish optimal routes this
  is known as slow convergence.
• RIP does not have the ability to split traffic
  across multiple links.