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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]
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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
Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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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.
Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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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.
Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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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?
Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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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.
Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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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.
Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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Table-Driven Routing


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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.
Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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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)
Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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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.
Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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Next-Hop Routing

Each router in a routing table can be reached via
a direct connection.
Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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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
Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
S
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The IP Routing
RouteDatagram(Datagram, RoutingTable)


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Extract destination IP address in D
Extract the network prefix in N
if N matches any directly connected 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 default route


send datagram to the next-hop specified in the table
else if the table contains a route for network N


deliver datagram directly to destination D over that network
send datagram to the default router specified
else

declare a routing error!
Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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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 address
and sends the datagram to its
destination in form of frames.
Internet Layer
Datagram
+
The next-hop IP address
Network Layer
Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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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.
Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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Incoming Datagrams

When a router receives a datagram:


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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!
Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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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.
Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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Presented for
Engineering Communication Systems
a course by
Dr. Uwe Glaesser
School of Computing Science
Simon Fraser University
October 2002
Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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