CSCI 45508556 Computer Networks

1
CSCI 4550/8556Computer Networks

• Comer, Chapter 9
• Hardware Addressing and Frame Type Identification

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Introduction

• The previous chapter on LAN technology described
  techniques for providing connectivity between
  computers.
• We need to devise techniques for delivering
  messages through the LAN medium to a single,
  specific destination computer.
• The sending computer uses a hardware address to
  identify the intended destination of a frame.
• The sending computer also identifies the type of
  data carried in the frame.

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Specifying a Destination

• Data sent across a shared network reaches all
  attached stations this is true for all LAN
  topologies.
• The interface hardware detects delivery of the
  frame and extracts the frame from the medium.
• But most applications want data to be delivered
  to one specific application on another computer,
  not to all computers.

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Hardware Addressing

• Most network technologies have a hardware
  addressing scheme that identifies stations on the
  network.
• Each station is assigned a numeric hardware
  address or physical address .
• The sender includes the hardware address of the
  destination in each transmitted frame.
• With most LAN technologies, the sender also
  includes its own hardware address in each
  transmitted frame.

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LAN Hardware and Packet Filtering
Note that the network interface is placed
between the processor/memory and the LAN.
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LAN Hardware and Packet Filtering

• The LAN interface handles all details of frame
  transmission and reception
• Adds hardware addresses, error detection codes,
  etc. to outgoing frames
• May use DMA to copy frame data directly from main
  memory
• Obeys access rules (e.g., CSMA/CD) when
  transmitting
• Checks error detection codes on incoming frames
• Checks destination address on incoming frames
• If the destination address on an incoming frame
  matches the local station's address, a copy of
  the frame is passed to the attached computer.
• Frames not addressed to the local computer are
  ignored and don't affect the local computer in
  any way.

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Hardware Address Properties

• Each hardware address is numeric, not symbolic.
• The size (in bytes) is selected for the
  particular network technology.
• Common lengths are between one and six bytes.

8
Assigning Hardware Addresses

• Hardware addresses must be unique on a LAN
• How can those address be assigned and who is
  responsible for uniqueness?
• Static the hardware manufacturer assigns a
  unique permanent address to each interface it
  manufactures.
• Dynamic the end user sets the hardware address,
  either by using switches or jumpers on the
  interface, or through software. System
  administrators must coordinate this activity to
  avoid conflicts.
• Automatic the interface automatically assigns
  itself a hardware address each time it is powered
  up. The scheme must be reliable to avoid
  conflicts.

9
Broadcasting

• Some applications need to broadcast messages to
  all stations on the LAN
• Shared communication channels can make
  broadcasting efficient the message is delivered
  to all network interfaces on the LAN.
• Special broadcast addresses are used to identify
  broadcast messages, which are captured by all
  stations.

10
Identifying Packet Contents

• The destination must get some clue about how to
  interpret frame data.
• This can be done using an
• Explicit frame type - an identifying value
  included with the frame describing the type of
  included data or
• Implicit frame type - the receiver must infer
  the frame type from the frame data.

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Headers and Frame Formats

• LAN technology standards define the frame format
  for each technology.
• All contemporary standards use the following
  general format

• The frame header has addresses and other
  identifying information.
• The information is typically in fields with fixed
  sizes and locations.
• The data area may vary in size.

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Example Frame Format (Ethernet)

• Preamble for receiver synchronization
• Destination addr. identifies the intended
  receiver
• Source addr. identifies the frames sender
• Frame type identifies type of data carried in
  the frame
• Data the frames payload
• CRC 32-bit cyclical redundancy code

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Ethernet Fields

• Preamble and CRC often not shown
• Destination address of all 1s is the broadcast
  address
• Special values are reserved for frame type field

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Frames Without Type Fields

• Some LAN technologies do not include a type
  field.
• In these cases, the sender and receiver can agree
  on interpretation
• Agree on a single data format and use only that
  format.
• Limits LAN to one type of data
• All computers on LAN must use one format
• Agree to encode the data format in the first few
  bytes of the data field.

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Encoding the Frame Type

• To ensure interoperability, the format of the
  encoding area must be universally agreed upon.
• The format is typically set by standards body.
• Illustration of using data area to encode data
  type

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IEEE 802.2 LLC

• IEEE 802.2 standard includes Logical Link Control
  (LLC) SubNetwork Attachment Point (SNAP) header
• SNAP/LLC format widely used e.g., by Ethernet
• LLC portion indicates SNAP field to follow
• OUI (Organizationally Unique Identifier)
  identifies Ethernet specification organization
• TYPE field interpreted as in Ethernet (in this
  case, IP)
• SNAP/LLC format widely used e.g., by Ethernet

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Unknown Types

• For either encoding format, some computers may
  not be prepared to accept frames of some types.
• Protocol type not installed
• Newly defined type
• In this case, the receiving computer examines the
  the type field and discards any frames with an
  unknown type.

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Network Analyzers

• A network analyzer or network monitor or network
  snifferis used to examine the performance of or
  debug a network.
• It can report statistics such as capacity
  utilization, distribution of frame size,
  collision rate or token circulation time.
• It can record and display specific frames, to
  understand and debug packet transmissions and
  exchanges.

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Operation of a Network Analyzer

• The basic network analyzer is just a computer
  with a network interface that receives all
  frames.
• This is sometimes called the promiscuous mode of
  operation of the interface.
• Many desktop computers have an interface that can
  be configured for promiscuous mode.
• Combined with appropriate software, the computer
  can examine any frame on a LAN.
• Thus communication across a LAN is not guaranteed
  to be private!
• The computer receives and displays or tabulates
  (but does not respond to) frames on the LAN.

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Filtering Incoming Frames

• An analyzer can be configured to filter and
  process frames
• Count frames of a specific type or size
• Display only frames from or to specific
  computers or
• In general, be configured to match the value of
  any field and capture only those frames meeting
  the filter specification.
• An analyzer can display real-time performance by
  computing running totals over specific time
  periods.

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Summary

• LAN technologies use hardware addresses to
  identify the source and destination for frames
  sent across the shared communication channel.
• Each LAN technology defines its own hardware
  format.
• Addresses may be statically assigned,
  configurable or automatically assigned
• Each station must have a unique address on the
  LAN segment.
• Frames include a header with fields for
  destination, source and other information such as
  frame type.
• The frame type defines how to interpret frame
  data.
• A network analyzer can receive all frames and
  display statistics or aid in debugging problems.