Справочник Пользователя для IBM CTI 2572

CTI 2572 Technical Overview Manual

14

The Ethernet Version 2 standard defines the packet as follows:

6 bytes

6 bytes

2 bytes

Variable

4 bytes

The type field is used to define the upper level protocol being used (e.g. TCP/IP, DEC LAT,
etc.).

The IEEE 802.3 specification substitutes a length field for the type field and has the following
format:

6 bytes

6 bytes

2 bytes

Variable

4 bytes

Even though the Ethernet II and 802.3 packet formats are different, there are generally accepted
conventions for placing information in the Type/Length field which allows communications
software to differentiate between the two.  As a result, both packet types can co-exist on the same
network.  However, a node which sends and receives only Ethernet II packets cannot
communicate with a node which sends and receives only 802.3 packets.

Since IEEE 802.3 does not contain any information which identifies the upper level protocol
being used, it is usually combined with the 802.2 Logical Link Control (LLC) specification.  The
LLC header does provide information about the upper layer protocol and is placed in the first few
bytes of the 802.3 data area. 
However, the 802.2 specification
allows only one byte for the upper
level protocol identifier (called a
Service Address Point).  Thus, a
special version of 802.2 LLC
called SNAP (Sub Network
Access Protocol) was created to
contain the 2 byte TYPE field
used in the Ethernet II packet.

Destination

SAP

170 = SNAP

Source

SAP

170 = SNAP

LLC

Control
03 = UI

Organization

Code

= 000

EtherType

0800 = IP

1 Byte

1 Byte

1 Byte

3 Bytes

2 Bytes

Destination

Address

Source

Address

Data

Length

Data

CRC

6 Bytes

6 Bytes

2 Bytes

Variable (up to 1500 Bytes)

4 Bytes

IEEE 802.2

LLC Header

Data