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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 Routing Routing : The process of choosing a path over which to send packets.

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internet protocol routing ip datagrams

Internet Protocol:Routing IP Datagrams

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

presented by Roozbeh

  • 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
internet router host
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.
direct indirect delivery
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.
direct delivery
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?
network prefix
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.
indirect delivery
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.
table driven routing
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.
information hiding
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)
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.
next hop routing
Next-Hop Routing
  • Each router in a routing table can be reached via a direct connection.
default routes
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

the ip routing
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!
routing with ip address

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.
why ip address
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.
incoming datagrams
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!
  • 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.

Presented forEngineering Communication Systems

a course byDr. Uwe Glaesser

School of Computing ScienceSimon Fraser University

October 2002