Ethernet Encapsulation

1
Ethernet Encapsulation

• RFC 894

2
Ethernet

• The term Ethernet generally refers to a standard
  published in 1982 by Digital Equipment Corp.,
  Intel Corp., and Xerox Corp.
• It is the predominant form of Local Area Networks
  technology used with TCP/IP today.
• It uses an access method called CSMA/CD, which
  stands for Carrier Sense, Multiple Access with
  Collision Detection.
• It operates at 10 Mbits /sec and uses 48-bit
  addresses

3
Ethernet Encapsulation
4
Ethernet Encapsulation

• 48-bit (6bytes) source and destination addresses.
  These are what we call hardware addresses.
• The Ethernet type field identifies the type of
  data that follows.
• The data field is the actual payload and must be
  at least 46 bytes
• If needed (frames with less than 46 bytes of
  payload), Pad bytes are inserted to assure that
  the frame is long enough.
• The CRC field is a cyclic Redundancy check (a
  checksum) that detects errors in the frame. (This
  is also called FCS or Frame Check Sequence)

5
Ethernet and MTU

• As you can see there is a limit on the size of an
  Ethernet frame.
• This limits the number of bytes of data to 1500
  bytes.
• This characteristics of the data link layer is
  called MTU, or Maximum Transmission Unite

6
Typical MTUs
7
Ethernet and MTU

• When two hosts on the same network are
  communicating with each other, it is the MTU of
  the network that is important.
• But, when two hosts are communicating across
  multiple networks, each link can have a different
  MTU. The important numbers are the MTUs of the
  two networks to which the two hosts connect, but
  rather the smallest MTU of any data link that
  packets traverse between the two hosts.

8
Ethereal Exercise

• Do the following using Ethereal
• Capture packets
• Filter IP packets. What is the value of the
  Ethernet type field and what does it mean?
• Clear the previous filter
• Filter ARP packets. What is the value of the
  Ethernet type field and what does it mean?

9
Next Week

• Make sure that you have completed Ch. 2 Quiz
  online.
• Make sure that you have read Ch. 3 Network
  Protocols
• Binary and Hexadecimal Exercises
• Address Resolution Protocol Lab
• Remember to buy the cable installation kit.
  (will not be used next week, but must have it
  before we get to chapter 4. All cable labs will
  be moved to chapter 4)
• ? Have a good Week