Networking Discussion Board
sb230529
CH10.pptxChapter 10 Connecting Computers
Chapter 10 Overview
Digital networking and reliable transmission
Building and operating a small-scale LAN
Reliability and error detection
Network protocols and the protocol stack
Network applications and resource sharing
Potential Networking Risks
Physical theft of network hardware
Subversion of network switches or wiring
Disclosure – sniffing of sensitive messages
Forgery – constructing a bogus message
Masquerade – lying about the message source
Denial of service – make the net unusable
An Example: Sharing a Printer
The Old Masonic building tenants share a LAN with a printer
Requirements
All tenants shall contribute for utility bills and supplies
All tenants shall have use of the shared network and printer
Access shall only be granted to tenants
Potential Controls
Physical – isolate the net from attackers
Mechanical – lock the building doors
Logical – restrict network traffic to the suite
Functional – provide no risky actions on the network
Procedural – only suitemates have keys to the suite
Cryptographic – use crypto to protect the net
Deferred until Chapter 14
Physical Network Security
Host and Network Integrity
Network worms
Like the Morris worm, only up-to-date
Use the latest Internet vulnerabilities
The botnet risk
A botnet is a network of subverted computers
Malware embeds special software
Allows a botnet herder to operate the subverted computers (“bots”) remotely
The bot software is hard to detect
Transmitting Information
Message switching – send whole messages
Sending and receiving are independent
Recipient gets all of message or nothing
Message size limits and longer delays
Circuit switching – connect two speakers
Send and receive one or many messages
Sender and recipient must both be available
Packet switching – send message in pieces
More efficient, but requires complex endpoints
Circuit Switching
Packet Switching
Packets Are Like Postcards
Easily carry short messages
Longer messages broken into segments
Recipient must reassemble the messages
Each message has a header that contains
Destination address
Damage detection, like a checksum
Data to carry
Sender's address: Optional and unreliable
Transmitting Packets Reliably
Packet Duplication
Reliability and Efficiency
ACK protocol may produce duplicate packets
This is the price of reliability
“NAK” protocols can't work, since the recipient doesn't always know when data is expected
Approximate packet efficiency:
Size of the data field in bits
Total packet size in bits
Ethernet – A Modern LAN
Defined by IEEE 802 LAN standards
Wired, optical, wireless
Traditionally a “bus” connection between hosts
A party line – everyone sees all traffic
Ethernet wiring
Original coaxial
10baseT – twisted pair wires, 10Mb/sec
100baseT – twisted pair “Cat 5,” 100Mb/sec
1000baseT – Cat 5e or 6, 1000Mb/sec
Packet (“Frame”) Format
Packet Contents
Source and destination “MAC” addresses
Two-part address, unique across all networks
3 bytes indicate the interface manufacturer
3 bytes indicate specific interface
All hosts hear all packets
Hosts discard all packets not for them
Packet type – indicates the type of message it carries
CRC – an error detection code
Finding a MAC Address
Keyboard commands
Windows MSDOS: ipconfig
Unix/Linix/Mac OS X: ifconfig
Graphical interface
Windows: Via Network and Sharing Center
“Details” under “Network Status”
Mac OS: Via Network System Preferences
“Ethernet tab” under the “Advanced” button
Building a LAN
Most use twisted pair wire: Cat 5, 5+, or 6
Connections are from hosts to network “nodes” that pass the traffic between hosts
A node “downlink” connects to a host
A node “uplink” connects to other parts of the network
Two types of nodes in typical small LANs
Hubs: 10- or 100baseT; broadcast to all hosts
Switches: All speeds; connects hosts pairwise
Ethernet and Collisions
If two hosts transmit at once, neither message goes through – they “collide”
Wired Ethernet uses “CSMA/CD”
CSMA – carrier sense multiple access
Each host listens (senses) whether other hosts are transmitting. If so, then the host waits before it transmits its own message
All hosts do this to take turns
CD – collision detection
Hosts listen for garbling (collision)
Wireless Collision Handling
Network Protocol Software
We organize network software in layers, similar to I/O and file system software
The layers form the protocol stack
Lower parts (network device drivers) may be in kernel mode
Higher parts are privileged OS programs
Highest parts are user application programs
Protocol Stack
A Simple, LAN-Oriented Stack
Top layer: Application layer
An API is provided between the application programs and network software built into the OS
First layer below the API: Transport layer
Links processes between hosts
Next layer: Link layer
Builds packets to travel between hosts
Bottom layer: Device drivers/physical layer
Actually transfers data to physical devices
Layering Example: Ordering Pizza
Packet Formats and Protocol Stacks
Layers in a Simple LAN
Network Applications: Client/Server
Resource Sharing
Sharing Files on an LAN
Delegating Access Rights
