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?