Internetworking

Internetworking
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Internetworking

Terminology - General

Internet:
- A collection of interconnected communications networks
Intranet:
- An internet used by a single organization
Subnetwork:
- A constituent network (in an internet)
End System (ES):
- A device on an internet that supports end-user applications/services
Intermediate System (IS):
- A device that connects two (or more) networks

Terminology - Hardware

Bridge:
- An IS that connects two subnetworks that use the same protocols (and, hence does not need to modify PDUs but only needs to resolve addresses)
- (Typically operates at the data link layer of OSI, unlike a hub that operates at the physical level)
Router:
- An IS that connects two (usually dissimilar) networks
- (Typically operates at the network layer of OSI)

What's Required for Internetworking?

Encapsulation

Defined:
- The addition of control information to data to create a protocol data unit (PDU)
Examples of Control Information:
- Address
- Error Detection/Correction Information
- Protocol headers/trailers

Addressing

An Observation:
- To transmit data between devices across networks we must be able to identify the devices
Issues that Arise:
- Scope
- Multiplexing
- Mode

Addressing (cont.) - Scope

Some Observations:
- Different layers of the architecture may use addresses
- Addresses at different layers may conflict
Types:
- Global - an address that is unique and applicable in any system
- Local - enables an individual network to deliver data (e.g., MAC addresses on IEEE802/Ethernet; host addresses on ATM)

Addressing (cont.) - Multiplexing

The Issue:
- One might want to communicate with a particular application on a particular device
A Common Solution:
- Ports (which can be thought of as sending a letter to a particular person at an address)

Addressing (cont.) - Modes

The Issue:
- An address can refer to a single device/port or multiple devices/ports
Modes:
- Unicast
- Multicast
- Broadcast

Routing

The Issue:
- There might be multiple paths that can be used to transmit information form a sender to a receiver
An Observation:
- The issues that arise here are similar to those that arise in all data communications that involve "complicated" networks

Fragmentation/Segmentation and Reassembly

Defined:
- Dividing data into smaller units and then re-assembling
Rationale:
- Lower layers often place limits on the size of data
- Improved error control (e.g., fewer errors, smaller re-transmissions)
- More equitable/fair access (e.g., reduces hogging)

Ordered Delivery

The Issue:
- Since PDUs might take different paths, they might not arrive in the same order in which they were transmitted
- This can be a particular problem in connection-oriented communications
An Obvious Approach:
- Use a sequence number

Flow Control

The Issue:
- Since different networks have different performance characteristics, it may be necessary to limit data rates
Common Approaches:
- Stop and wait (i.e., each PDU must be acknowledged before the next is transmitted)
- Sliding window (i.e., multiple PDUs can be sent without an acknowledgment)

Error Control

The Issue:
- Since different networks have different error properties it may be necessary to add error control to internetwork communications
An Observation:
- The issues that arise here are similar to those that arise in all data communications (e.g., detection, correction, re-transmission)