IP: The Internet Protocol

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

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

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 0.143 s over a network that transfers 56,000 bits/s. easier to detect and recover from errors Spring 2002 Computer Networks Applications

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

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

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

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

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

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

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

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

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

Datagram transmission Spring 2002 Computer Networks Applications

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

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

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

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

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

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

Address resolution with table lookup Spring 2002 Computer Networks Applications

An example trip through an internet Y Spring 2002 Computer Networks Applications