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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 [email protected]
2Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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
3Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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.
4Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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.
5Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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?
6Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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.
7Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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.
8Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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.
9Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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)
10Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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.
11Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
Next-Hop Routing
Each router in a routing table can be reached via a direct connection.
12Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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
To Network Route To
40.0.0.0 Direct Delivery
Default Routing S
Routing Table for host H
13Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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!
14Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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.
Internet Layer
Network Layer
Datagram + The next-hop IP address
15Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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.
16Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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!
17Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
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.
18Routing IP Datagrams – by Roozbeh Farahbod, [email protected]
Presented for
Engineering Communication Systemsa course by
Dr. Uwe Glaesser
School of Computing ScienceSimon Fraser University
October 2002
