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IEEE 802.2 LLC

The IEEE 802.2 standard is identical for all network topologies. It provides a general interface between the different protocols (IPX, TCP/IP, etc.) and the different network types (Ethernet, Token Ring, etc.). The positioning within the OSI Reference model:

2Data LinkLLC or LLC + SNAP

The LLC is responsible for the logical link functions of one or more logical links, hence the name Logical Link Control. The command packets generated by the LLC are called PDUs (Protocol Data Units).

The LLC provides the following services to the Network Layer (layer 3):

What the LLC expects from the MAC sub-layer:

Frame format
The LLC format looks like this:

DSAP addressSSAP addressControl fieldInformation field
8 bits8 bits8 or 16 bitsN * 8 bits
Destination Service Access Point
Source Service Access Point
Integer equal to or greater than 0

The DSAP field has 8 bits which contain the following information:


For I/G = 0 the address is individual and for I/G=1 it's a group address.

The SSAP field has 8 bits which contain the following information:


For C/R = 0 it indicates a command and for C/R=1 it's a response.

Notes on IEEE 802.2
The most common SAP values:

IEEE 802.2: SNAP

SNAP is the layer between the Network layer and the MAC layer. SNAP stands for SubNetwork Attachment Point and is part of the LLC 802.2 standard.

The main usage for SNAP/LLC is in Apple Macintosh networks using the AppleTalk protocol.

Frame format
The LLC indicates if there is a SNAP header. The entire header looks like this:

 3 octet OUI2 octet TYPE-field

The AAs are the DSAP and SSAP fields from the LLC and the 03 is the control field.

The OUI (Organizational Unique Identifier) identifies the standards organization and the TYPE field defines the protocol type of the information. In this case IP.

For AppleTalk phase II the packet would look like this:

AA AA 03 00 00 00 08 9B
 3 octet OUI2 octet TYPE-field