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Internet Protocol: Routing IP Datagrams D. E. Comer, “Internetworking with TCP/IP: Principles, Protocols and Architectures” , Ch. 8, Prentice Hall, 2000 presented by Roozbeh Farahbod [email protected] Routing Routing : The process of choosing a path over which to send packets.

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Internet protocol routing ip datagrams l.jpg

Internet Protocol:Routing IP Datagrams

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

presented by Roozbeh [email protected]


Routing l.jpg
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 R1directly 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


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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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Internet Layer

Datagram + The next-hop IP address

Network Layer

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 router’s 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


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