Internetworking An internetwork is typically comprised of many

Internetworking • An internetwork is typically comprised of many physical networks over which data travels • There are many different types of physical networks: – Ethernet – FDDI – ATM • Goals of internetworking: – Make all the diverse network technologies function as a coordinated whole – Hide the details of the underlying network hardware – Provide universal communication services

Review: Ethernet • A popular packet-switched LAN technology invented at Xerox PARC in the 1970’s

Properties of an Ethernet • 10 – 100 Mbps • Broadcast bus • Best-effort delivery • Distributed access control (CSMA/CD)

Distributed Access Control • Carrier Sense Multiple Access (CSMA) • If two transceivers both sense that the network is idle and begin transmission simultaneously a collision occurs • Collision Detection (CD) • Recovery

Ethernet Addressing • Each machine connected to the network has a unique 48 -bit number (it’s address) • To send a packet a computer puts the destination address in the destination field • That address can be: – A unicast address – A broadcast address – A multicast address

Ethernet Frame Format • Variable length (no shorter than 64 octets and no larger than 1518 octets) • Fields: – – – Preamble Destination address Source address Frame type Data Cyclic Redundancy Check (CRC)

An Ethernet Frame

Extending an Ethernet • Repeaters - hardware devices used to relay electrical signals from one cable to another • Advantages • Disadvantages

Extending an Ethernet (cont) • Bridges - receives valid frames and retransmits it on another cable • Advantages • Disadvantages

Access Control Alternatives • Ethernet - distributed • Advantages • Disadvantages • Can you think of other alternatives?

Fiber Distributed Data Interconnect (FDDI) • Popular LAN technology • Higher bandwidth (100 Mbps) than Ethernet • Uses optical fiber • Implements a token ring

FDDI (cont)

FDDI’s Self-Healing Capability • Two rings • Counter rotating • Uses the backup ring to bypass a failed computer or link

FDDI Self-Healing (cont)

Asynchronous Transfer Mode (ATM) • A high-speed connection-oriented network technology • Operates from 100 Mbps to >1 Gbps • Expensive

Recall: How Internetworking Works

Goals of Internetworking • Make all the diverse network technologies function as a coordinated whole • Hide the details of the underlying network hardware • Provide universal communication services

Application-Level Interconnection • One application-level program per machine cooperating/interoperating with other programs • Example? • Advantages • Disadvantages

Network-Level Interconnection • Have a set of communication conventions • The O. S. for each computer provides communication services for all application programs • Example? • Advantages • Disadvantages

Internetworking • Takes the network-level interconnection approach • Functions as a virtual packet-switched network • Attempts to build a unified, cooperative interconnection of networks that supports a universal communication service

Properties of the Internet • Hides underlying internet architecture from the user • Does not mandate a network interconnection topology • Establishes network independence in the user interface

Network Interconnection • Internet router – A small computer that physically attaches to two networks – Passes packets from one network to the other – As the Internet’s topology becomes more complex, routers need to know about that topology – More on routers later…

Routing Decisions • Can be complex • Want to keep routing hardware simple and cheap • So routers only use the destination network (not the destination machine) to make routing decisions • Routing table is proportional to the # of networks in the internet (not the # of hosts)

All Networks Are Equal • Any communication system capable of transferring packets counts as a single network independent of: – its delay – throughput characteristics – packet size – scale

Big Advantages of Internetworking • Users perceive a single, virtual network through which all machines can be reached • Users do not need to know the underlying details of the network • Application-level programs do not need to know the underlying details of the network

Big Responsibilities of Internetworking • Cooperation – Communication conventions – Each participating network agrees to handle traffic to/from other hosts in return for their handling of its traffic

Unanswered Questions • What is the addressing scheme in an internet? • What do packets look like? • How is routing done? • What happens when routers/hosts malfunction? • What happens when routers/hosts get overloaded?