Learning by Doing: CISCO Certified Network ... - SCN Research

The Start of Frame Delimiter (SOF) further helps to set up the transmission and
reception of the information and synchronization. This is only a 2-bit portion with just
two one’s. No matter how many zeros and one’s come before the SOF the NIC does
nothing until it gets to the one-one (SOF). This information is stripped by the NIC and
the NIC can “do its work” on the rest of the packet. (In hex: 3 In binary: 11) You will
not see this with a protocol sniffer because it is stripped and dumped.
Used in de-encapsulation:
The Destination Address (DA) is the physical address (MAC) of the networking device
the information is going to be sent to. This is 48 bits in hexadecimal. This will be the
first “bits” of information you will see with a protocol inspector.
The Source Address (SA) is the physical address (MAC) of the networking device
sending the information. This is 48 bits in hexadecimal.
The Type indicates what types of request will follow. This will be given in hexadecimal.
This field is usually 2 bytes. A 0800 in the type field indicates an IP datagram will
follow. A 0806 in the type field indicates an ARP request will follow. A 0835 in the
type field indicates a RARP request will follow. Current type codes can be found at
http://www.iana.org/numbers.html#
The Data is what it sounds like…it’s the “meat” of the information transmitted. For
“generic” Ethernet this can be as small as 46 bytes and up to 1500 bytes. The first part of
the data field contains the IP header information. See the discussion below on the
composition of the data field for both types of Ethernet packets.
The Frame Check Sequence (FCS) is the CRC information for error control. This is 4
bytes in hexadecimal. There are many different error control calculations. (Is it a
coincidence there are many flavors of Jell-O too?) I described one in an earlier lab using
unique prime numbers. Another FCS calculation is called “AUTODIN II.” It is
calculated using this formula:
(X 32 + X 26 + X 23 + X 22 + X 16 + X 12 + X 11 + X 10 + X 8 + X 7 + X 5 + X 4 + X 2 + X 1 +1)
802.2/802.3 Ethernet (RFC 1042)
802.3 MAC Information 802.2 Info
Preamble SOF DA SA Length LLC SNAP Data FCS
Figure 2—Ethernet SNAP packet structure.
The “Standard for the Transmission of IP Datagrams Over IEEE 802 Networks” was
written by Postel and Reynolds in 1988 (ftp://ftp.isi.edu/in-notes/rfc1042.txt ). This is
more commonly used today.
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