IEG3090 Tutorial 2 IntraDomain Routing

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IEG3090 Tutorial 2 Intra-Domain Routing

• Fong Chi Hang, Bosco

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Outline

• Routing
• Routing Protocol 1 Routing Information Protocol
  (RIP)
• Routing Protocol 2 Open Shortest Path First
  (OSPF)

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Goal of routing

• You may be able to send the datagram directly to
  the destination,
• However, the interesting case is when the
  destination is not directly reachable.
• In this case, the host or gateway attempts to
  send the datagram to a gateway that is nearer the
  destination.
• The goal of a routing protocol is very simple It
  is to supply the information that is needed to do
  routing.

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Routing Information Protocol (RIP)

• RIP is a Distance Vector protocol.
• Why we still use it
• RIP is one of the easiest to configure and least
  resource-demanding of all the routing protocols.
• It was included in the popular Berkeley Standard
  Distribution (BSD) of UNIX starting in 1982. In
  fact, support for RIP has been built into
  operating systems for as long as TCP/IP itself
  has existed

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Fundamentals
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Fundamentals
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Comments

• RIP is designed so that a routing entry is only
  replaced if information is received about a
  shorter route.
• Naturally, this same propagation scheme will
  occur for all the other networks as well.
• This propagation of network routing information
  occurs on a regular basis.
• Regular period is normally 30 seconds.

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Time out Timer

• A special Timeout timer is started whenever a
  route entry is installed in the routing table.
• Whenever the router receives another RIP Response
  with information about that route, the route is
  considered refreshed and its Timeout timer is
  reset.
• The default time-out value for this timer is 120
  seconds.

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Garbage collection Timer

• When a route is marked for deletion, a
  Garbage-Collection timer is also started.
• When this Timer timeout, the marked invalid route
  entry will be deleted,
• The default time-out value for this timer is 120
  seconds.

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RIP Messages

• Communication between RIP software elements is
  done with RIP messages.
• These messages are sent using the UDP, with
  reserved UDP port 520 for RIP-1 and RIP-2.
• even though RIP is considered part of layer 3
  like other routing protocols, it behaves more
  like an application in terms of how it sends
  messages.

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Message Types

• RIP Request
• A message sent by a router to another router
  asking it to send back all or part of its routing
  table.
• RIP Response
• A message sent by a router containing all or part
  of its routing table.
• Note that despite the name, this message is not
  sent just in response to an RIP Request message.
  So it's not really a good name oh well.

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Message format
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Count-To-Infinity Problem

• At first everything works fine.
• Later Link (A, B) is broken. Router B observed
  it, but in his routing table he sees, that router
  C has a route to A with 2 hops.

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Count-To-Infinity Problem (cont.)

• The problem is, router B doesn't know that C
  routes to A through itself.
• So B increases his cost to A to 3 later C
  increases his cost to A to 4.
• Now lets count to infinity

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Limitations of RIP

• slow convergence (Triggered Updates)
• routing loops
• counting to infinity (Split Horizon)
• small infinity
• Metric
• Hop Count as a distance metric
• Lack of support for dynamic (real-time) metrics

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Triggered Updates

• Whenever a router changes the metric for a route
  it is required to (almost) immediately send out
  an RIP Response to tell its immediate neighbor
  routers about the change.
• Why almost
• waits a random amount of time, from 1 to 5
  seconds
• Reduce the load on network

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Split Horizon

• Something is non-sense
• a simple solution
• have Router C not mention the route to Router A
  in any RIP Response messages it sends to Router
  B.
• New rule
• when a router sends out an RIP Response on any of
  the networks to which it is connected, it omits
  any route information that was originally learned
  from that network.
• Split Horizon
• the router effectively splits its view of the
  network, sending different information on certain
  interfaces than on others.

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Split Horizon With Poisoned Reverse

• Enhancement
• Previous omitting routes learned from a
  particular interface when sending RIP Response
  messages on that interface
• Now include those routes in the message but set
  their metric to RIP infinity---16.
• Poisoned reverse
• Poisoning the routes that we want to make sure
  routers on that interface don't use.
• Router B will see Router C advertise a route to
  Router A but with a cost of 16,
• Provide more insurance

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Split Horizon With Poisoned Reverse
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Split Horizon Limitation

• Note, however, that split horizon may not always
  solve the counting to infinity problem,
  especially in the case where multiple routers are
  connected indirectly.
• Classic example
• three routers configured in a triangle. See the
  following.

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Count-To-Infinity Problem (cont.)

• Simple example to show split horizon cannot help
  much

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Hold Down

• A timer is started when they first receive
  information about a network that is unreachable.
• Until the timer expires, the router will discard
  any subsequent route messages that indicate the
  route is in fact reachable.
• A typical hold-down time-out value is 180
  seconds.
• It provides a period of time for out-of-date
  information to be flushed from the system

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Summary

• Split horizon and split horizon with poisoned
  reverse prevent having a router send invalid
  route information back to the router from which
  it originally learned the route.
• Triggered updates reduce the slow convergence
  problem by causing immediate propagation of
  changed route information.
• Finally, hold-down may be used to provide
  robustness when information about a failed route
  is received.

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Open Shortest Path First (OSPF)

• Why we use it
• RIP has a long history and work well in a small
  group of routers.
• However, poorly-suited to larger AS or those with
  specific performance issues.
• OSPF allows routes to be selected dynamically
  based on the current state of the network, not
  just a static picture of how routers are
  connected.
• Also note that it is a complicated protocol,
  which means it is often not used unless it is
  really needed.

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Fundamentals

• Link state database
• Each router in an autonomous system maintains a
  copy of this database, which contains information
  in the form of a directed graph that describes
  the current state of the autonomous system.
• Each link to a network or another router is
  represented by an entry in the database, and each
  has an associated cost (or metric).

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Link-State Advertisement (LSA)

• In flat topology
• A router's LSDB represents the topology of the
  entire AS, including links between routers that
  may be rather distant from it.
• the routers communicate as peers using link-state
  advertisements (LSAs).
• The routers that connect the AS to other AS are
  often called boundary routers. (OSPF/BGP)

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Hierarchical Topology

• The autonomous system is divided into constructs
  called areas, each of which contains a number of
  contiguous routers and networks.
• These areas are numbered, and managed
  independently by the routers within them.
• The areas are interconnected so that routing
  information can be shared between areas, across
  the entire AS.

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Router roles

• There are three different labels applied to
  routers in this configuration
• Internal Routers These are routers that are only
  connected to other routers or networks within a
  single area.
• Area Border Routers These are routers that
  connect to routers or networks in more than one
  area.
• Backbone Routers These are routers that are part
  of the OSPF backbone.

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Hierarchical Topology
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Route determination

• The key data structure link-state database
  (LSDB).
• The LSDB contains a representation of the
  topology of either the entire AS (in basic
  topology) or a single area (in hierarchical
  topology).
• Route determination
• Taking the information in the LSDB and
  transforming the graph into a shortest path first
  tree or SPF tree.

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SPF Tree
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Operation and message

• Unlike RIP, OSPF does not send its information
  using the User Datagram Protocol (UDP).
• Instead, OSPF forms IP datagrams directly,
  packaging them using protocol number 89 for the
  IP Protocol field.
• OSPF defines five different message types, for
  various types of communication

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Message types

• Hello
• allow a router to discover other adjacent routers
  on its local links and networks.
• Database Description
• contain descriptions of the topology of the AS or
  area.

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Message types

• Link State Request
• These messages are used by one router to request
  updated information about a portion of the LSDB
  from another router.
• Link State Update
• These messages contain updated information about
  the state of certain links on the LSDB. They are
  sent in response to a Link State Request message,
  and also broadcast or multicast by routers on a
  regular basis.
• Link State Acknowledgment
• These messages provide reliability to the
  link-state exchange process, by explicitly
  acknowledging receipt of a Link State Update
  message.

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Summary

• Hello messages are used to establish contact
  between routers, and Database Description
  messages to initialize a routers link-state
  database.
• Routine LSDB updates are sent using Link State
  Update messages, which are acknowledged using
  Link State Acknowledgments.
• A device may also request a specific update using
  a Link State Request.
• All of these are highly simplified descriptions

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• Thank you!

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Message Format