Transcript Computer Networking From LANs to WANs: Hardware, Software

Computer Networking From LANs
to WANs: Hardware, Software, and
Security
Chapter 7
Low-Level Protocols
Objectives
• Describe the format of a serial data transmission
• List the differences between SLIP and PPP
• Explain the operation of the Logical Link Control
sublayer
• Discuss the role of NetBEUI, NetBIOS, and
NetBIOS over TCP/IP in a Windows network
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Serial Data Communication
• Communication over single communication line
– One bit at a time (least significant bit first)
• 11-bit transmission waveform standard
– Used for transmitting 7-bit ASCII characters
– First bit: start bit (always low)
• Identifies beginning of new transmission
– Next seven bits: ASCII code
– Last data bit: parity bit
– Last two transmission frame bits: stop bits (always
high)
• May be one, one and one-half, two stop bits
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Serial Data Communication (cont’d.)
• Parity bits
– Determine error in the received character
– Odd parity
• Occurs if odd number of 1s
– Even parity
• Occurs if even number of 1s
• Transmission error:
– Received parity not in line with expected parity
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Serial Data Communication (cont’d.)
• Example: ASCII character lower case I
– ASCII code is 69 hexadecimal (1101001 binary)
Figure 7-1 11-bit transmission code
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Serial Data Communication (cont’d.)
• Each waveform bit takes same amount of time
– Related to baud rate of the serial transmission
– Baud rate
• Generally: number of bits per second in a transmission
• Actually: number of transitions per second
• Advantages
– Simple connection requirements
• Single transmit wire, single receive wire (plus ground)
• Asynchronous communication not possible
– Start bit used to synchronize transmitter and receiver
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Serial Data Communication (cont’d.)
• UARTs (universal asynchronous receiver
transmitter)
– Digital devices handling serial data transmission
– Parallel input data converted to serial output data
– Serial input data converted into parallel output data
Figure 7-2 The UART
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SLIP
• First protocol to transmit TCP/IP over dial-up lines
• Provides basic method to encapsulate TCP/IP data
• Not provided
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Error detection
Internet Protocol (IP) address assignments
Link testing
Synchronous communication
Transmission protocols other than TCP/IP
• Compressed SLIP (CSLIP) available
• Replaced Point-to-Point Protocol (PPP)
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PPP
• Provides serial connection encapsulation
– TCP/IP, IPX, NetBEUI, AppleTalk, other protocols
• PPP frame provides:
– Error detection
• Checksum value tests data validity
– Ability to assign IP addresses
• As needed during session duration
– Link testing
• Mechanism to periodically test PPP link operation
status
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PPP (cont’d.)
• PPP frame provides: (cont’d.)
– Synchronous, asynchronous communication modes
– Security (user name, password authentication)
• Password Authentication Protocol (PAP)
• Challenge Handshake Authentication Protocol (CHAP)
– Compression
• Operates on layers 1 (Physical) and 2 (Data-Link)
Figure 7-3 PPP frame format
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PPP (cont’d.)
Table 7-1 Comparing SLIP and PPP
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PPPoE
• Allows multiple LAN users to share a single Internet
connection
– Encapsulates PPP within an Ethernet frame
• User establishes PPPoE session through the single
ISP Internet connection
– Each PPPoE user session can be monitored (billing)
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PPPoE (cont’d.)
• Code field: PPPoE frame type (discovery, session)
• Session ID, Ethernet frame’s source and destination
MAC addresses
– Uniquely identify PPPoE session
• Length field: payload size or user data, sent
Figure 7-4 PPPoE frame format
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Logical Link Control
• Data-Link layer contains two parts
Figure 7-5 Data-Link layer details
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Logical Link Control (cont’d.)
• Three types of service:
– Type 1: Connectionless communication (unreliable)
– Type 2: Connection-oriented communication (reliable)
– Type 3: Acknowledged connectionless
communication
• LLC protocol:
– Based on HDLC (High-Level Data-Link Control)
• Provides mechanism for sending commands and
responses over a communication link
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Logical Link Control (cont’d.)
• DSAP and SSAP fields
– Indicate how payload interpreted
• SNAP (subnetwork access protocol) payload use
– When high-level protocol handled by LLC sublayer
Figure 7-6 Logical Link Control PDU
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Logical Link Control (cont’d.)
• MAC sublayer
– Interfaces LLC sublayer with different network
technologies
• LLC sublayer does not know details
– Allows the same LLC sublayer to work with different
network technologies
– Additional responsibilities
• Controlling access to network media
• Recovering from errors
• Addressing (working with MAC addresses)
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NetBIOS
• Provides all functionality to share resources
between networked computers
• Utilizes three types of services:
– Name: Finding and naming machines
– Session: Connection-oriented reliable transfer of
messages
– Datagram: Connectionless non-reliable datagram
transfer
• NetBIOS message main component
– SMB (server message block)
• Provide all functionality possible under NetBIOS
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NetBIOS (cont’d.)
Table 7-2 Sample NetBIOS commands
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NetBIOS (cont’d.)
• Machine names
– NetBIOS names that identify each machine
• Up to 15 characters (letters, numbers, limited set of
symbols)
Figure 7-8 Invalid NetBIOS computer
name error message
Figure 7-7 Windows 9x Network Identification display
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NetBEUI
• Transport mechanism to deliver NetBIOS messages
over a LAN
• Does not conform to OSI model
– Uses Transport, Network, LLC part of Data-Link
• Not a routable protocol
– Uses MAC addresses to specify source, destination
computers
• Only used on small networks
• Windows Internet Name Service (WINS)
– Maps NetBIOS names to IP addresses
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NetBEUI (cont’d.)
• NetBIOS over TCP/IP
– NetBIOS messages transported using TCP
– Used to share resources over a WAN
• View Network properties window
• NETSTAT displays active NetBIOS ports
– Some may cause computer vulnerability
Table 7-3 NetBIOS over TCP/IP port assignments
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Figure 7-9 Active network connections on a Windows system
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Figure 7-10(a) Network Properties showing
protocol bindings on a Windows 9x computer
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NetBEUI (cont’d.)
• Communication channel
– Formed between protocol and adapter
• When protocol bound to network adapter
• Protocol cannot use an adapter until it is bound to it
• Multiple protocols
– May be bound to single adapter or multiple adapters
• NetBEUI originally used by IBM LAN Manager
network
– Now adapted by Microsoft for use in Windows
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Figure 7-10(b) Windows 9x NetBEUI Properties window
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Figure 7-10(c) Windows 9x NetBEUI Properties window
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Figure 7-11 NetBEUI Properties windows
on Windows 2003 server
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Troubleshooting Techniques
• Protocol information sources
– http://www.protocols.com
– http://www.whatis.com
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Summary
• Low-level protocols
– Establish serial communication
– Exchange data over different hardware technologies
– Provide peer-to-peer communication
• Serial data communication
– One bit at a time over a single communication line
– Parity bits help determine errors in the received
waveform
• Many protocols available
– SLIP, PPP, PPPoE, NetBIOS, NetBEUI
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