Part 3: Internetworking Internet architecture, addressing, encapsulation, reliable transport and the TCP/IP protocol suite

Internetworking: concepts, architecture and protocols Motivation, architecture, routers, TCP/IP protocols, internet reference model

Motivation A large organisation will use several networking technologies Inter-organisational communication is significant Universal service - any two computers should be able to communicate However, different network technologies cannot just be wired together

Internetworking Interconnect heterogeneous networks and provide universal service – Hardware: routers connect different networks – Internet protocols: provide universal service by creating a single virtual network

Internet architecture Although a single router can connect many networks, most organisations use multiple routers

Virtual network The illusion that there is a single universal network

Internetworking protocols The TCP/IP Internet Protocols – begun in the 1970s – The Internet has emerged into the public domain in the 1990s – Controlled by the Internet Engineering Task Force (IETF)

Internet Reference Model

Host computers TCP/IP used the term host computer to refer to any system that connects to an Internet and that runs applications Both hosts and routers use TCP/IP protocol software

IP: Internet protocol addresses Uniform addressing, the IP address hierarchy, address classes, dotted decimal notation, special addresses, routers and addresses, address resolution

Uniform addressing Internet protocols deal in packets and provide uniform addressing The addressing scheme is defined in software and is used transparently by applications Internet addressing is specified in the IP protocol Each host is assigned a unique 32 bit address

The IP address hierarchy Each 32 bit address is divided into two parts – prefix: physical network to which the host is attached - the network number – suffix: a host attached to a given physical network Prefixes are coordinated globally and suffixes locally

Classes of IP address Size of prefix and suffix determines maximum number of networks and maximum number of hosts per network IP defines different classes of address with different sized prefixes and suffixes The first four bits of the address specify its class

The five classes of IP address

Dotted decimal notation Makes it easier to for humans to use addresses (names are also possible)

Classes and dotted decimal

Division of the address space Public Internet network numbers are assigned by Internet Service providers (ISPs) and these are coordinated by the Internet Assigned Number Authority

An addressing example

Classless Addressing The Internet is running out of addresses Allow division between prefix and suffix to appear at an arbitrary boundary Consider network with only 9 hosts – Only need four bits for host suffix – Class C (smallest) address uses 8 bits for host suffix – Can subdivide a class C address into 16 addresses with a 28 bit prefix and 4 bit suffix Extend dotted decimal notation – /28, /28, …, /28

Special IP addresses

Routers and IP addressing Routers are assigned two or more IP addresses So are multi-homed computers

Binding protocol addresses An Internet packet passes through a series of routers – each hop takes it over a particular network, either to a specific computer on that network or to the next router – in either case, the sending router has to map between the protocol (IP) address and a hardware address – this is called address resolution

Address resolution techniques Table lookup Closed-form computation Message exchange – send message to specific server computers – broadcast message, only the required computer responds

Pros and cons of techniques

Address resolution protocol TCP/IP defines the Address Resolution Protocol (ARP) which defines the format of resolution requests and responses This technique is usually combined with local caching of hardware addresses

Dynamic Host Configuration Protocol (DHCP) Special DHCP server that assigns IP addresses to hosts Newly booted machine broadcasts a DHCP discover packet DHCP server sends back an IP address – Permanent IP addresses Manually assigned by administrator – Automatic IP address from a pool of addresses to be allocated on demand Leased for a finite period of time

DHCP Operation DHCP server does not need to be on the same network as the host

Summary Uniform addressing Address classes Dotted decimal notation Classless addressing Special IP addresses Address resolution (ARP) and ssignment (DHCP)