Hello Smitesh,
LLC is used only for node-to-node flow and error control in ealry days
I believe this is not entirely correct. Logical Link Control as you describe it was indeed used predominantly in HDLC-based networks where it covered sequencing, acknowledgements and other stuff that dealt with reliable communication over a Layer2 link. However, it is not limited to that.
Each Ethernet 802.2 LLC-formatted frame contains at least these three additional fields: Destination Service Access Point (DSAP), Source Service Access Point (SSAP), Control. The SSAP and DSAP identify the service originating the frame and service for which the payload of this frame is intended. In different words, these fields allow for multiplexing of different services (and hence protocols) over the same Ethernet link. This is something you need to do all the time, as we are using Ethernet in a multiprotocol environment nowadays. For example, STP, RSTP, MSTP, LACP, LLDP, GARP/MRP, GVRP/MVRP - all these protocols use the 802.2 LLC format and out of all functions provided by the LLC, they use a small subset - multiplexing and demultiplexing various types of services over the same Ethernet link, thanks to identifying the particular service in the DSAP/SSAP field. The DSAP/SSAP here serve a very similar function to the EtherType field in the Ethernet_II frame format. We could even say that in the Ethernet_II frame format, the EtherType provides a function falling into the LLC's domain of responsibility.
In addition, the LLC is the basic way how to send SNAP-encapsulated frames, whereby the use of the SNAP encapsulation is identified by the SNAP service access point value 0xAA in both DSAP and SSAP fields. SNAP is an application of LLC and is used extensively - think of proprietary L2 protocols like CDP/VTP/DTP/UDLD/PVST+/RPVST+/PAgP, or SNAP-encapsulated packets in Frame Relay/ATM/DSL bridging.
So the bottom line is - LLC is very much used, even though not in full extent of its capabilities.
Stefi, to your questions:
- Mapping of IPX to a physical network card driver.
No, this would not be an LLC function. However, if an LLC-encapsulated IPX packet arrived to your NIC, the values of DSAP and SSAP would help the NIC driver know that this packet should be handed over to the IPX driver.
- Mapping of a physical network card to a network card driver.
This is not an LLC function, either - LLC is about handling the communication that already occurs over the Ethernet. There is no such communication here - it is the driver's internal procedure how to find and "bind" to a particular physical card in a system.
- Mapping of a physical network card driver to IP.
This is similar to the question about IPX. LLC does not map a physical network card driver to IP - simply because such mapping has no meaning. LLC, as should be enough for CCNA level, helps identify the contents of the frame so they can be processed by an appropriate service. In this case, LLC would help identify that the frame carries an IP packet, so the NIC driver can hand off the frame payload to the IP driver.
- Mapping of IP to more than one physical network card driver.
Again, this is not a function of LLC. This is something done internally in the operating system without a need for a protocol.
Best regards,
Peter
