Hardware Addressing and Frame Type Identification

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Hardware Addressing and Frame Type Identification
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Addressing

• Most LAN technologies have addressing scheme
• Each station has unique physical address, often
  called medium access controller (MAC) address
• Each frame in LAN uses physical address to send
  frames

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Addressing Continued

• Every station receives every frame but only pays
  attention to those addressed to it
• Each frame includes both destination and source
  physical addresses
• Including source hastens reply
• Each frame has a fixed header that is broken into
  fields

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LAN Hardware

• The hardware in a computer that talks to LAN
  completely separate from CPU - NIC
• This LAN interface HW has to own processor and
  can do work independently of the CPU
• LAN interface hardware checks frame size, CRC,
  etc.
• Discards bad frames
• CPU knows nothing of all this
• Efficient use of hardware

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LAN Hardware Continued

• LAN interface uses physical addressing to ignore
  frames not intended for this computer
• If physical address matches, frame contents given
  to CPU

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Physical Addresses(Dont confuse with IP
Addresses)

• Type and Formats
• Static set by manufacturer and never changed
  (must be globally unique! fit anywhere)
• Configurable customer may set via switches or
  EPROM. Typically configured once stay fixed
  across reboots
• Dynamic address given during each boot
• Can be random, clock based
• Check to see if others using

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Physical Addresses Continued

• Physical addresses must be unique on single LAN
• Not necessarily globally unique

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Broadcasting

• E.g. computer wants to know all printers on LAN
• Medium shared so broadcasting easy
• Need mechanism for all station to pay attention
  to frame
• Have special physical address that everyone
  listens to

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Broadcasting Continued

• Create broadcast address
• LAN interface card also forwards these to CPU

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Multicasting

• Broadcasting can be inefficient if only small
  group needs message
• Broadcasts involve every CPU
• Multicasting like broadcasting in that only one
  frame sent on shared medium
• Interface does not forward to CPU by default
• Interface must be programmed to do this

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Multicasting Continued

• Reserves some addresses for multicast
  transmissions
• Application notifies interface card it wants to
  receive multicasts
• Subscribes to multicast group
• Summary
• NIC can be set up to recognize its own address,
  all broadcast addresses, and programmed
  multicast addresses.
• NIC passes frames with these into CPU.

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Frame Content Identification Frame Type

• Explicit have content description in frame
  frame type in header
• Implicit sender and receiver must agree
  (coordinate) before frames transferred frame
  type embedded in payload somewhere

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Ethernet Frame Format

• 64-bit preamble
• Used to synchronize sender and receiver
• Destination and source addresses
• 48 bit physical addresses are used
• 16-bit frame type
• Data
• CRC

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Ethernet Frame with type field in header

• See page 130 for possible values of frame type
  field for self-identifying frames

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Standard

• Specifies all aspects of frame
• E.g. all 1s in destination is broadcast
• All others starting with 1 are multicast

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Frame Type Not in Header

• Use data field at beginning of payload
• Use 8 octet IEEE LLC/SNAP layout for type
  identification
• Octets LLC (3 octets) followed by SNAP (5)
• LLC logical link control - AA AA 03
• SNAP sub-network attachment point
• Organizationally unique ID (3 octets - 00 00 00
  for all Ethernet) specify standards
  organization
• Two octet type field specific frame type within
  organizations standards

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Network Analyzers

• Used to debug networks
• Interface cards receive all packets
• Can forward all to CPU instead of discarding
• Called promiscuous mode
• No special card or computer required can just
  be NIC and laptop with the right software