Ethernet Encapsulation

RFC 894

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

Ethernet Encapsulation

Destination Address

6 Bytes

Source Address

6 Bytes

Type

2

Data

46-1500 Bytes

CRC

4 Bytes

Type

0800

2B

IP Datagram

46-1500 Bytes

Type 0806

2B

ARP Request/Reply

28 Bytes

PAD

18 Bytes

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)

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

Typical MTUs

Network MTU (bytes)

16 Mbits /sec token ring (IBM)

17914

FDDI 4352

Ethernet 1500

Frame Relay 1500

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.

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?

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