IEEE 802.2 LLC Protocol

The logical Link Control (LLC) provides a common interface between different protocols (IPX, TCP/IP, etc.) and different network types (Ethernet, Token Ring network, etc.). The LLC provides a way for the upper layer to handle any type of MAC, for example, Ethernet IEEE 802.3 CSMA/CD or Token ring IEEE 802.5 token delivery (token passing). The LLC is developed on the basis of high-level Data link control (Hdlc:high-level data-link control) and uses a subset of the HDLC specification. The LLC defines three types of data communication operations: Type 1: no connection mode. 　　This method of sending information is usually not guaranteed to be received. Type 2: Connection oriented. This approach provides four services: connection establishment, acknowledgment and acknowledgment response, error recovery (via request for re-sending of received erroneous data), and sliding window (factor: 128). 　　The data transfer rate can be improved by changing the sliding window. 　　Type 3: No connection acknowledgment response service. The LLC no connection service of type 1 specifies a static frame format and supports running network protocols. The Transport Layer network protocol is typically used in service Type 1 mode. The LLC for type 2 is a connection-oriented service that supports reliable data transfer and is used in LAN environments where the network layer and transport layer protocols are not required to be invoked.
Frame Format for LLC:
SSAP,DSAP is the address of the source service access point, where the DSAP format (i/g| d| d| d| d| d| d| D) i/g=0 indicates that the address is invalid, =1 indicates the group address; SSAP (c/r| s| s| s| s| s| s| S) c/r=0 indicates a command, =1 indicates a response. Some of the most commonly used SAP are:-04-IBM Sna-06-ip-80-3com-aa-snap-bc-banyan-e0-novell-f4-lan Manager Fe-clns
Subnet Access Protocol (SNAP) is part of the logical link control standard used to encapsulate IP packets, Address Resolution Protocol (ARP) requests and replies on an IEEE 802 network. The SNAP protocol header is located behind the LLC protocol header and contains the organization code that represents the next 16 bits of the Etheric class (EtherType) code. All communication processes are typically implemented with 802.2 type 1.
In the case of a snap header, the frame header format (16bit after the organizational Unique identifier represents the protocol type of the information body):
The common ether type has eth_p_ip =0X0800,ETH_P_ARP=0806,ETH_P_IPV6=0X08DD

Reference 1.  Http://ckp.made-it.com/ieee8022.html 2. Http://blog.chinaunix.net/uid-23241716-id-134091.html