Transcript Ethernet Encapsulation

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
Source
Address
Type
Data
CRC
6 Bytes
6 Bytes
2
46-1500 Bytes
4 Bytes
Type
0800
IP Datagram
2B
Type
0806
2B
46-1500 Bytes
ARP Request/Reply
PAD
28 Bytes
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