Subnetwork Access Protocol
The Subnetwork Access Protocol (SNAP) is a mechanism for multiplexing, on networks using IEEE 802.2 LLC, more protocols than can be distinguished by the eight-bit 802.2 Service Access Point (SAP) fields. SNAP supports identifying protocols by EtherType field values; it also supports vendor-private protocol identifier spaces. It is used with IEEE 802.3, IEEE 802.4, IEEE 802.5, IEEE 802.11 and other IEEE 802 physical network layers, as well as with non-IEEE 802 physical network layers such as FDDI that use 802.2 LLC.
Background[edit]
The OSI model uses a Service Access Point (SAP) to define the communication between layers (like Network, Transport, Session, and the other layers of the seven-layer model), that is to identify which protocol should process an incoming message. Within a given layer, programs can exchange data by a mutually agreed-upon protocol mechanism. A pair of programs that do not support a common protocol cannot communicate with each other. Thus for multiple protocols to coexist within a layer, it is necessary to determine which protocol is invoked to process a service data unit delivered by the lower layer.
The most common reference to SAP, including a Source Service Access Point (SSAP) and a Destination Service Access Point (DSAP) refers to the boundary between the Data Link Layer and the Network Layer. It is common to think of SAP only in terms of its use at Layer 2, specifically in its Logical Link Control (LLC) sub-layer as defined in the IEEE 802.2 standards. Link Service Access Point (LSAP) includes both Destination Service Access Point (DSAP) and Source Service Access Point (SSAP). It enables a MAC station to communicate with upper layers via different protocols.
Standard Network layer protocols have been assigned reserved LLC addresses, as recorded in ISO/IEC TR 11802-1. One half of the LLC address space is reserved for such assignment. Other protocols are accommodated in two ways. One way is by local assignment of LSAPs, for which the other half of the LLC address space is available.
The second way is to use a particular reserved LLC address value that has been assigned for use in conjunction with the Sub-network Access Protocol (SNAP) is called the SNAP address. The SNAP address identifies, at each MAC SAP, a single LSAP. Thus, each protocol using SNAP must employ a protocol identifier. Thus, the Subnetwork Access Protocol (SNAP) is a mechanism for multiplexing, on networks using IEEE 802.2 LLC, more protocols than can be distinguished by the 8-bit 802.2 Service Access Point (SAP) fields. SNAP supports identifying protocols by Ethernet type field values; it also supports vendor-private protocol identifier spaces. It is used with IEEE 802.3, IEEE 802.4, IEEE 802.5, IEEE 802.11 and other IEEE 802 physical network layers, as well as with non-IEEE 802 physical network layers such as FDDI that use 802.2 LLC.