Internet Protocol: Routing IP Datagrams

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Internet ProtocolRouting IP Datagrams

• D. E. Comer, Internetworking with TCP/IP
  Principles, Protocols and Architectures, Ch. 8,
  Prentice Hall, 2000

presented by Roozbeh Farahbodrfarahbo_at_sfu.ca
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Routing

• Routing The process of choosing a path over
  which to send packets.
• Router A computer in general making this
  choice.
• Routing occurs at several levels
• From node to node in a simple LAN
• From LAN to LAN in a WAN

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Internet, Router, Host

• Internet is composed of multiple physical
  networks interconnected by computers called
  routers.
• Routers have direct connections to two or more
  networks.
• A Host usually connects directly to one physical
  network.

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Direct / Indirect Delivery

• Routing can be divided in to two forms
• Direct Delivery
• When two machines are both attached to the same
  underlying physical transmission system (i.e. a
  single Ethernet)
• Indirect Delivery
• When two machines are not directly attached to
  the same network and packets must go through at
  least one router for delivery.

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Direct Delivery

• Delivery from A to C
• A encapsulates the datagram in a physical frame
• Maps the destination IP address to a physical
  address (MAC address)
• Uses the network hardware to deliver it
• How does A know whether C is in the same network?

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Network Prefix

• IP addresses are divided into a Network Prefix
  and a Host Suffix
• By checking the network prefix of the destination
  IP address, sender will know if it is directly
  connected to the destination machine or not.

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Indirect Delivery

• B wants to deliver a datagram to D
• B checks the network prefix and realizes that D
  is outside of L1.
• In an internet, every host can reach a router
  directly.
• B sends the packet to R1 directly and lets R1
  handle the delivery.

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Table-Driven Routing

• How does B decide to send the datagram to R1 and
  not to R2?
• How does R1 know where to send the datagram?
• The usual IP routing algorithm employs an
  Internet Routing Table or IP Routing Table.
• Both hosts and routers have IP routing tables.
• IP routing tables, based on the destination
  address, tell the router where to send a datagram.

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Information Hiding

• Do we need to keep the list of all possible
  destination addresses?
• Taking the advantage of Network Prefix
• A routing table keeps a set of pairs (Network,
  Path)

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Next-Hop

• Do we need to keep the whole path to a
  destination address?
• Every router only needs to know what is the next
  router in the path.
• This next router is called the next hop.

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Next-Hop Routing

• Each router in a routing table can be reached via
  a direct connection.

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Default Routes

• Another technique used to hide information
• If the destination network was not in the routing
  table, use the default route
• Example
• For hosts like H that attach to a single network,
  only one row in the routing table required

Routing Table for host H
To Network Route To
40.0.0.0 Direct Delivery
Default Routing S
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The IP Routing

• RouteDatagram(Datagram, RoutingTable)
• Extract destination IP address in D
• Extract the network prefix in N
• if N matches any directly connected network
• deliver datagram directly to destination D over
  that network
• else if the table contains a host-specific route
  for D
• send datagram to the next-hop specified in the
  table
• else if the table contains a route for network N
• send datagram to the next-hop specified in the
  table
• else if the table contains a default route
• send datagram to the default router specified
• else
• declare a routing error!

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Routing with IP address

• IP routing does not alter the original datagram
  except for
• Decrementing the Time-To-Live
• Re-computing the checksum
• When IP executes the routing, it selects the
  next-hop IP address and forwards the datagram to
  that using the network interface layer.
• The network layer then binds the IP address to a
  physical addressand sends the datagram to its
  destination in form of frames.

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Why IP Address?

• Converting IP addresses every time routing
  occurs? Inefficient!
• Why not using physical addresses in routing
  tables?
• Routing table provides a clean interface between
  IP software that routes and high-level software
  that manipulates routes.
• The whole point of IP is to hide the details of
  the underlying network.

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Incoming Datagrams

• When a router receives a datagram
• If the destination IP is the routers IP (for
  each of its network connections), it passes the
  datagram to higher levels.
• Otherwise, it routes the datagram.
• Hosts are forbidden from forwarding datagrams
  that are accidentaly routed to them.
• Reasons
• Something has gone wrong!
• It will cause unnecessary network traffic
• Simple errors can cause chaos.
• Routers report errors, while hosts not!

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Summary

• IP uses routing information to route datagrams.
• Direct delivery is considered as the final step
  in routing.
• The result of routing is the IP address of the
  next hop.
• Physical address and physical frame vs. IP
  address and IP datagram
• IP routing algorithm is table-driven and in most
  cases based on the network addresses.
• Using a default route keeps the routing tables
  small.

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Presented forEngineering Communication Systems a
course byDr. Uwe Glaesser School of Computing
ScienceSimon Fraser University October 2002