IEEE 802.2. LLC, Logical Link Control

Type:

Data link protocol.

This Data Link Layer is divided into two sublayers:

Logical Link Control (LLC). This sublayer is responsible for the data transmission between computers or devices on a network.
Media Access Control (MAC). On a network, the network interface card (NIC) has an unique hardware address which identifies a computer or device. The physical address is utilized for the MAC sublayer addressing.

The function of the Logical Link Control (LLC) is to manage and ensure the integrity of data transmissions. The LLC provides Data Link Layer links to services for the Network Layer protocols. This is accomplished by the LLC Service Access Points (SAPs) for the services residing on network computers. Also, there is a LLC Control field for delivery requests or services.

The Logical Link Control (LLC) has several service types:

Service type 1, is a connectionless service with no establishment of a connection, and an unacknowledged delivery.
Service type 2, is a connection logical service with an acknowledgement of delivery.
Service type 3, is a connectionless service with an acknowledgement of delivery.

Service classes furthermore support sundry permutations of these LLC service types:

Class I supports only service type 1.
Class II supports both service type 1 and type 2.
Class III support both service type 1 and type 3.
Class IV support all three service types.

The SubNetwork Access Protocol (SNAP) is an augmentation of the IEEE 802.2 LLC header. SNAP provides a method by which to utilize non-IEEE protocols on IEEE 802 networks.

RFC 1042:

IEEE 802 networks may be used as IP networks of any class (A, B, or C). These systems use two Link Service Access Point (LSAP) fields of the LLC header in much the same way the ARPANET uses the "link" field. Further, there is an extension of the LLC header called the Sub-Network Access Protocol (SNAP).

IP datagrams are sent on IEEE 802 networks encapsulated within the 802.2 LLC and SNAP data link layers, and the 802.3, 802.4, or 802.5 physical networks layers. The SNAP is used with an Organization Code indicating that the following 16 bits specify the EtherType code.

Normally, all communication is performed using 802.2 type 1 communication. Consenting systems on the same IEEE 802 network may use 802.2 type 2 communication after verifying that it is supported by both nodes. This is accomplished using the 802.2 XID mechanism.

However, type 1 communication is the recommended method at this time and must be supported by all implementations. The rest of this specification assumes the use of type 1 communication. The IEEE 802 networks may have 16-bit or 48-bit physical addresses. This specification allows the use of either size of address within a given IEEE 802 network.

RFC 1042, page 3:

The mapping of 32-bit Internet addresses to 16-bit or 48-bit IEEE 802 addresses must be done via the dynamic discovery procedure of the Address Resolution Protocol (ARP). Internet addresses are assigned arbitrarily on Internet networks. Each host's implementation must know its own Internet address and respond to Address Resolution requests appropriately. It must also use ARP to translate Internet addresses to IEEE 802 addresses when needed.

MAC Header

IEEE 802.2 LLC

Data :::

CRC

IEEE 802.2 LLC packet format:

00	01	02	03	04	05	06	07
DSAP
SSAP
Control :::

DSAP, Destination Service Access Point. 8 bits.
The LLC service access point address for the destination Service Access Point (SAP).

SSAP, Source Service Access Point. 8 bits.
The LLC service access point address for the source Service Access Point (SAP).

Control. 8 or 16 bits.
This field length depends upon whether the service has been requested or supplied.

Data.
This field contains the encapsulated protocol.

CRC, Cycle Redundancy Check. 4 bytes.
Error checking.

Packet Header:

MAC Header

IEEE 802.2 LLC and SNAP

Data :::

CRC

IEEE_SNAP Frame format.

Size (Bytes)	Value	Description	IEEE Types
1	AA	DSAP, Destination Service Access Point.	802.2 LLC
1	AA	SSAP, Source Service Access Point.	802.2 LLC
1 or 2	03	Control.	802.2 LLC
3		Organization Code.	802.2 SNAP
2		EtherType.	802.2 SNAP

IEEE SNAP. Organization Code and EtherType fields follow the LLC fields.

Organization Code. 3 bytes.
For the organization who assigned the EtherType field.

EtherType. 2 bytes.
For the non-IEEE protocol.

Data. Variable length.
This field contains the encapsulated protocol.

CRC, Cycle Redundancy Check. 4 bytes.
Error checking.

Glossary:

Ethernet.
A set of network protocols which serves the physical and data link layers. Ethernet utilizes a linear bus or star topology. The Ethernet served as the basis for the IEEE 802.3 standard.

SNAP.
(RFC 1042, page 2) IP datagrams are sent on IEEE 802 networks encapsulated within the 802.2 LLC and SNAP data link layers, and the 802.3, 802.4, or 802.5 physical networks layers. The SNAP is used with an Organization Code indicating that the following 16 bits specify the EtherType code.

(RFC 1010, page 13) To implement SNAP, the DSAP and SSAP are set to hex AA and the control field is set to hex 03. Five bytes are allocated to differentiate the vendor (Organization) and the media type (EtherType).

RFCs:

[RFC 1042] A Standard for the Transmission of IP Datagrams over IEEE 802 Networks.

Obsoletes: RFC 948.

Publications:

Naugle, Matthew. Network Protocol Handbook. McGraw-Hill, Inc., 1994.

Networking Technologies, Novell Education, Novell Inc.

Stevens, W. Richard. TCP/IP Illustrated - The Protocols, Volume 1. Addison-Wesley Publishing Company, 1994.

Obsolete RFCs:

[RFC 948] TWO METHODS FOR THE TRANSMISSION OF IP DATAGRAMS OVER IEEE 802.3 NETWORKS.

Obsoleted by: RFC 1042.