Routing Information Protocol (RIP)

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Routing Information Protocol (RIP)

• NETE0514
• Presented by
• Dr.Apichan Kanjanavapastit

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Intra-and Interdomain Routing

• An internet is divided into autonomous systems.
  An autonomous system (AS) is a group of networks
  and routers under the authority of a single
  administration
• Routing inside an autonomous system is referred
  to as intradomain routing
• Routing between autonomous system is referred to
  as interdomain routing

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Popular Routing Protocols

• In this chapter, we discuss two intradomain
  routing protocols distance vector and link state
• For the interdomain routing protocol, we
  introduce path vector
• Routing Information Protocol (RIP) is the
  implementation of the distance vector protocol
• Open Shortest Path First (OSPF) is the
  implementation of the link state protocol
• Border Gateway Protocol (BGP) is the
  implementation of the path vector protocol

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Distance Vector Routing

• In distance vector routing, the least cost route
  between any two nodes is the route with minimum
  distance
• In this protocol each node maintains a vector
  (table) of minimum distances to every node
• The table at each node also guides the packets to
  the desired node by showing the next stop in the
  route (next-hop routing)

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Initialization

• At the beginning, each node can know only the
  distance between itself and its immediate
  neighbors, those directly connected to it

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Sharing

• The whole idea of distance vector routing is the
  sharing of information between neighbors
• Although node A does not know about node E, node
  C does. So if node C shares its routing table
  with A, node A can also know how to reach node E

Note
In distance vector routing, each node shares its
routing table with its immediate neighbors
periodically and when there is a change.
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Updating

• When a node receives a 2-column table from a
  neighbor, it needs to update its routing table.
  Updating takes 3 steps
• The receiving node needs to add the cost between
  itself and the sending node to each value in the
  second column
• The receiving node needs to add the name of the
  sending node to each row as the 3rd column if the
  receiving node uses information from any row. The
  sending node is the next node in the route

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Updating (cont.)

• The receiving node needs to compare each row of
  its old table with the corresponding row of the
  modified version of the received table.
• If the next-node entry is different, the
  receiving node chooses the row with the smaller
  cost. If there is a tie, the old one is kept
• If the next-node entry is the same, the receiving
  node chooses the new row. For example, if node C
  has previously advertised a route to node X with
  distance 3. Suppose that now there is no path
  between C and X node C now advertises this route
  with a distance of infinity. Node A must not
  ignore this value even though its old entry is
  smaller.

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Updating (cont.)
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When to Share

• The table is sent both periodically and when
  there is a change in the table
• Periodic Update. A node sends its routing table,
  normally every 30 seconds, in a periodic update.
  The period depends on the protocol that is using
  distance vector routing
• Triggered Update. A node sends its 2-column
  routing table to its neighbors any time there is
  a change in its routing table. This is called a
  triggered update. The change can result from the
  following.
• A node receive a table from a neighbor resulting
  in changes in its own table after updating
• A node detects some failure in the neighboring
  links which results in a distance change to
  infinity

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Initial routing tables in a small autonomous
system
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Final routing tables for the previous figure
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Two-Node Loop Instability

• A problem with distance vector routing is
  instability, which means that a network using
  this protocol can become unstable

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Solutions to two-node instability

• Defining Infinity. Most implementation of the
  distance vector protocol define the distance
  between each node to be 1 and define 16 as
  infinity. Therefore, the distance vector cannot
  be used in large systems
• Split Horizon. In this strategy, instead of
  flooding the table through each interface, each
  node sends only part of its table through each
  interface. If node B thinks that the optimum
  route to reach X is via A, it does not need to
  advertise this piece of information to A.

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Solutions to two-node instability (cont.)
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Solutions to two-node instability (cont.)

• Split Horizon and Poison Reverse. This strategy
  is a combination between split horizon and poison
  reverse where node B can still advertise the
  value of X, but if the source of information is
  A, it can replace the distance with infinity as a
  warning Dont use this value what I know about
  this route comes from you.

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Solutions to two-node instability (cont.)
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Three-Node Instability
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Routing Information Protocol (RIP)

• The Routing Information Protocol (RIP) is an
  intradomain routing protocol used inside an
  autonomous system
• It is a very simple protocol based on distance
  vector routing
• RIP implements distance vector directly with some
  considerations
• In an autonomous, we are dealing with routers and
  networks (links). The routers have routing
  tables, networks dont
• The destination in a routing table is a network,
  which means the first column defines a network
  address

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Routing Information Protocol (RIP) (cont.)

1. The metric used by RIP is the number of links
   that have to be used to reach the destination
   which is called hop count
2. Infinity is defined as 16
3. The next node column defines the address of the
   router to which the packet is to be sent to reach
   its destination

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RIP Message Format
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RIP Message Format (cont.)

• Command. This 8-bit field specifies the type of
  message request (1) or response (2)
• Version. This 8-bit defines the version. In the
  textbook, we use version 1
• Family. This 16-bit field defines the family of
  the protocol used. For TCP/IP the value is 2.
• Network address. RIP has allocated 14 bytes for
  this field to be applicable to any protocol.
• Distance. This 32-bit field defines the hop count
  from the advertising router to the destination
  network

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Request and Response

• Request. A request message is sent by a router
  that has just come up or by a router that has
  some time-out entries. A request can ask about
  specific entries or all entries
• Response. A solicited response is sent only in
  answer to a request. It contains information
  about the destination specified in the
  corresponding request. An unsolicited response is
  sent periodically, every 30 s or when there is a
  change in the routing table

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Request Messages
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Example 1
What is the periodic response sent by router R1?
Assume R1 knows about the whole autonomous system.
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Solution
R1 can advertise three networks 144.2.7.0,
144.2.9.0, and 144.2.12.0. The periodic response
(update packet) is shown below.
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Example 2
Figure 14.11 shows the update message sent from
router R1 to router R2 in Figure 14.8. The
message is sent out of interface 130.10.0.2.
See Next Slide
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The message is prepared with the combination of
split horizon and poison reverse strategy in
mind. Router R1 has obtained information about
networks 195.2.4.0, 195.2.5.0, and 195.2.6.0 from
router R2. When R1 sends an update message to R2,
it replaces the actual value of the hop counts
for these three networks with 16 (infinity) to
prevent any confusion for R2. The figure also
shows the table extracted from the message.
Router R2 uses the source address of the IP
datagram carrying the RIP message from R1
(130.10.02) as the next hop address.
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Timer in RIP

• RIP uses 3 timers. The periodic timer controls
  the sending of messages, the expiration timer
  governs the validity of a route, and the garbage
  collection timer advertises the failure of a route

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Periodic Timer

• The periodic timer controls the advertising of
  regular update messages
• The working model uses a random number between 25
  and 30 s
• This is to prevent any possible synchronization
  and therefore overload on an internet if routers
  update simultaneously

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Expiration Timer

• The expiration timer governs the validity of a
  route
• When a router receives update information for a
  route, the expiration timer is set to 180 s for
  that particular route
• Every time a new update for the route is
  received, the timer is reset
• If the timer is expired, the hop count of the
  route is set to 16, which means the destination
  is unreachable

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Garbage Collection Timer

• When the information about a route becomes
  invalid, the router does not immediately purge
  that route from its table
• Instead, it continues to advertise the route with
  a metric value of 16
• At the same time, the garbage collection timer is
  set to 120 s for that route
• When the count reaches zero, the route is purged
  from the table

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Example 3
A routing table has 20 entries. It does not
receive information about five routes for 200 s.
How many timers are running at this time?
SolutionThe 21 timers are listed below
Periodic timer 1 Expiration timer 20 - 5
15 Garbage collection timer 5
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RIP Version 2

• RIP version 2 was designed to overcome some of
  the shortcomings of version 1
• The designers of version 2 have not augmented the
  length of the message for each entry
• They have only replaced those fields in version 1
  that were filled with 0s for the TCP/IP protocol
  with some new fields

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Message Format
16
8
24
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AS NUMBER
VERSION (2)
COMMAND (1-5)
AUTHENTICATION TYPE
FFFF
AUTHENTICATION HEADER
MUST BE ZERO
FAMILY OF NET 1
ADDRESS OF NET 1
MASK
NEXT HOP
DISTANCE TO NET 1

• Route tag. This field carries information such as
  the autonomous system number. It can be used to
  enable RIP to receive information from an
  interdomain routing protocol
• Subnet mask. This is a 4-byte field that carries
  the subnet mask. This means that RIP2 supports
  classless addressing and CIDR
• Next-hop address. If the sending router want to
  specify another router IP address to be the next
  hop router.