1. IP: The Internet Protocol

2. Computer Networks Applications
LAN---Bus Topology
Each computer
is connected to a cable.
Coordinates with others to send a message
BUS
Spring 2002
Computer Networks Applications

3. Issues in Sharing Communication Media
Generally, most networks types involve sharing:
Bus, ring in LANs
High speed backbone in WANs
Granting one party exclusive access may block all others for intolerable periods of time;
Ex: to send a file of 5megabytes over a network that may transfer 56,000 bits/s will require 12 minutes.
Spring 2002
Computer Networks Applications

4. Computer Networks Applications
Solution:
Divide a message in small blocks, called packets;
Computers take turns in sending packets;
insures fair, prompt access to the shared resources;
Ex. a packet of 8000 bits is transmitted in s over a network that transfers 56,000 bits/s.
easier to detect and recover from errors
Spring 2002
Computer Networks Applications

5. Computer Networks Applications
Packets Transmission
All computers connected to a network have a unique number, called address;
Each type of network uses its own format for addresses: from few digits to as much as 16 digits;
At the beginning of a packet, there is a header which contains the addresses of the sender and the destination;
The hardware (NIC card) looks at every packet; if its for the local machines it copies it; A C c c D D D B
Spring 2002
Computer Networks Applications

6. Packets Transmission (cont.)
Packets are not all the same size
any size can be sent up to a maximum
Ex: a keystroke can be sent as one individual packet.
Sender and receiver have to agree on how to specify the beginning and ending of each frame;
Ex: have dedicated characters for beginning and end;
Spring 2002
Computer Networks Applications

7. Packets Transmission (cont.)
Packet transmission is very fast:
Typical LAN transfer ~ large 1000 packets/s
Packet switching systems adapt automatically as computers start/end sending data fair access to shared resources
Most networks, including Internet are a packet switching system
Spring 2002
Computer Networks Applications

8. Internet: a Collection of Disparate Networks
Different goals:
Speed, cost, distance;
Different standards for:
Expected carrier;
Coding bits;
Detecting and recovering from errors;
Protocols for transmitting messages: bus, token ring,…
Packets sizes, and encoding for the start/end of packets,…
Types of computer addresses
Spring 2002
Computer Networks Applications

9. Computer Networks Applications
Solution: Routers
High speed
connection
Router
Routers: computers design to interconnect different networks
Spring 2002
Computer Networks Applications

10. Solution: Internet Protocol (IP)
IP hides the details of physical networks
IP specifies:
Packet format;
How routers should forward packets
Define address format
Every computer connected to the Internet must run IP software
Spring 2002
Computer Networks Applications

11. The illusion of a single network
The internet concept:
the illusion of a single network that IP provides;
(b) the underlying
physical structure.
Spring 2002
Computer Networks Applications

12. Computer Networks Applications
Datagrams
A packet following IP specification is called a datagram;
The header of a datagram contains the addresses of the sender and the destination;
But, each network type:
defines its own packet format;
Accepts and delivers only packets that adhere to its own format.
How can IP datagrams be sent across networks that do not recognize IP format?
Spring 2002
Computer Networks Applications

13. Computer Networks Applications
Datagrams (cont.)
A datagram travels across a given network inside that network packet;
When the packet arrives at the next router, the router opens the packet and extracts the datagram;
If the datagram has to be sent to another network, the router:
Creates a new network packet;
Encloses the datagram inside that packet;
Sends it to the next router along the path;
If the datagram arrived at the destination: the receiver processes it.
Spring 2002
Computer Networks Applications

14. Datagram transmission
Spring 2002
Computer Networks Applications

15. Computer Networks Applications
Internet Addressing
To make datagram routing and delivery possible, each computer is assigned a unique address, called Internet address or IP address;
Each address is a 32-bit binary number;
To make routing efficient, each address is divided into two parts: a prefix and a suffix;
Prefix: identifies the physical network to which the computer is attached;
Suffix: identifies each computer attached to that network.
Spring 2002
Computer Networks Applications

16. Internet Addressing (cont.)
To insure uniqueness:
two networks cannot be assigned the same address
network numbers are distributed by a centralized authority, called Internet Assigned Number Authority
two computers on the same network cannot be assigned the same number
suffixes can be assigned locally without global coordination
Spring 2002
Computer Networks Applications

17. Dotted decimal notation
Convenient way to express IP addresses
Each 8-bit section represented as a decimal number;
Uses periods to separate the sections;
Spring 2002
Computer Networks Applications

18. Computer Networks Applications
Forwarding a datagram
Each router along the path, uses the destination address to determine the next hop to which it has to be sent.
Each IP router keeps relevant information into a routing table;
Each entry specifies a destination and the next hop used to reach it;
Each destination is a network (an internet contains over 1000 times more hosts than networks)
Spring 2002
Computer Networks Applications

19. Routing table---an example
An internet formed by 4 networks and 3 routers;
The conceptual routing table of router 2
Spring 2002
Computer Networks Applications

20. Computer Networks Applications
Address resolution
IP addresses are abstractions provided by software---differ usually from hardware addresses;
Mapping between a hardware address and an IP address is called address resolution;
Is used by routers/computers when need to send a packet on the same physical network.
Address resolution techniques:
Table lookup: mappings are stored in memory, which the software searches
Message exchange: a computer sends a message that requests an address binding, and another computer sends a reply that contains the requested info.
Spring 2002
Computer Networks Applications

21. Address resolution with table lookup
Spring 2002
Computer Networks Applications

22. An example trip through an internetY
Spring 2002
Computer Networks Applications