| Chapter 3 introduced two reference models where the lower layers up to the network layer were structured in a different way. In the Internet reference model (TCP/IP model) there is only the data-link layer with the network adapter, and no other instance underneath the Internet protocol (network layer). In the ISO/OSI basic reference model, there are two different layers (physical layer and data-link layer), where the data-link layer is expanded by the media-access layer (Layer 2a) when using local area networks. This book deals mainly with the protocols of the Internet world, and one assumes that the Internet reference model would best describe the structure of the Linux network architecture. Interestingly, the classification of the ISO/OSI reference model matches the structure of communication systems in local area networks much better. When taking a closer look at the IEEE 802 standards for local area networks, which are actually always used in the Internet, and their implementation in the Linux kernel, we can clearly recognize the structuring of the ISO/OSI model. For this reason, the following discussion assumes a structuring as shown in Figure 6-1: The OSI layers 1 (physical layer) and 2a (media-access control layer ?MAC) are implemented in network adapters. The logical-link control (LLC) layer is implemented in the operating system kernel; network adapters are connected to the operating system kernel by the network devices described in Chapter 5.
Figure 6-1. Standardization of layers 1 and 2 in IEEE 802 and their implementation in the Linux network architecture. 
6.1.1 IEEE Standard for Local Area Networks (LANs) With its IEEE 802.x standards, the IEEE (Institute of Electrical and Electronics Engineers) found a very extensive proliferation for local area networks (LANs). The best known LAN technologies are 802.3 (CSMA/CD), 802.5 (Token Ring), and 802.11 (wireless LANs). Figure 6-1 gives a rough overview of the 802.x standards and classifies them within the ISO/OSI layer model. As mentioned above, the data-link layer is divided into a logical-link control (LLC) and a media-access control (MAC) layer for networks with jointly used media. The LLC layer hides all media-specific differences and should provide a uniform interface for protocols to the higher layers; the MAC layer reflects the differences between different transmission technologies. To hide the characteristics of the underlying transmission technology, the LLC layer should offer three services, regardless of this technology: Unreliable datagram service (LLC type 1): This very simple service offers no flow control or error control, so it doesn't even guarantee that data is transmitted. The removal of errors is left to the protocols of the higher layers. Connection-oriented service (LLC type 2): This service establishes a logical connection between the sender and the receiver, and it supports flow control and error control. Reliable datagram service (LLC type 3): This service combines LLC types 1 and 2 it is connectionless, but it supports both flow control and error control.
The very simple service (LLC type 1) is used mainly in local area networks, probably for its simplicity. No connection has to be established, and the higher-layer protocols offer an integrated error-handling feature (e.g., TCP in the transport layer). The protocol header of the LLC type-1 protocol consists of three fields: DSAP and SSAP specify the service access points in the sender and receiver. It is unclear why a protocol identification is stated for both the sender and the receiver, especially because no example is known where the two values would be different. Both fields have a width of only eight bits, so very few protocols can be defined. For this reason, the SNAP extension described below was defined. The Control field always takes the value 0x03 for LLC type 1. This corresponds to an Unnumbered Information Frame in the HDLC protocol, on which the LLC protocols are based.
For these reasons, the LLC layer and the relevant protocol-control information (LLC packet header) can no longer be recognized in some variants of local network protocols, because they were integrated into the packet headers of MAC PDUs. The best-known example is probably 802.3, which has the protocol control information (PCI) of the LLC layer in its protocol field. One major drawback of this integrated solution is that many organizations and companies try to integrate their proprietary standards into a MAC PDU. To ensure that duplicate assignments of some identifications by different organizations are prevented, the IEEE invented a packet format for the LLC PDU, which allows each organization to define its own packet types. This packet format is called the SNAP extension of the LLC protocol. In the SNAP extension, the SSAP and DSAP fields take the constant 0xAA, indicating that they expand the LLC packet header by five bytes (SNAP extension). These five bytes can be used to identify a large number of new protocols. In addition, the extension field is divided into a part for the assigning organization and another part for the actual protocol identification, to prevent conflicts in the assigning of protocol identifiers. |
