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 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?