On Ethernet, the 8 octets occupied by the LLC and SNAP headers reduce the size of the available payload for protocols such as the Internet Protocol to 1492 bytes, compared to the use of the Ethernet II framing therefore, for protocols that have EtherType values, packets are usually transmitted with Ethernet II headers rather than with LLC and SNAP headers. SNAP is usually used with Unnumbered Information 802.2 protocol data units (PDUs), with a control field value of 3, and the LSAP values are usually hexadecimal AA, so the 802.2 LLC header for a SNAP packet is usually AA AA 03 however, SNAP can be used with other PDU types as well. If the OUI is an OUI for a particular organization, the protocol ID is a value assigned by that organization to the protocol running on top of SNAP.
If the OUI is zero, the protocol ID is the registered EtherType value for the protocol running on top of SNAP. The SNAP header consists of a 3-octet IEEE organizationally unique identifier (OUI) followed by a 2-octet protocol ID. The 5-octet SNAP header follows the 802.2 LLC header if the destination SAP (DSAP) and the source SAP (SSAP) contain hexadecimal values of AA or AB: The SNAP is an extension of the 802.2 LLC specified in the IEEE 802 Overview and Architecture document. 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. SNAP supports identifying protocols by Ethernet type field values it also supports vendor-private protocol identifier spaces. 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. Thus, each protocol using SNAP must employ a protocol identifier.
ISNAP NETWORK MAC
The SNAP address identifies, at each MAC SAP, a single LSAP. 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. One way is by local assignment of LSAPs, for which the other half of the LLC address space is available. Other protocols are accommodated in two ways. One half of the LLC address space is reserved for such assignment. Standard Network layer protocols have been assigned reserved LLC addresses, as recorded in ISO/IEC TR 11802-1. It enables a MAC station to communicate with upper layers via different protocols.
Link Service Access Point (LSAP) includes both Destination Service Access Point (DSAP) and Source Service Access Point (SSAP). 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. 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. 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. A pair of programs that do not support a common protocol cannot communicate with each other.
Within a given layer, programs can exchange data by a mutually agreed-upon protocol mechanism. 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-Layered Model), that is to identify which protocol should process an incoming message.
ISNAP NETWORK DRIVER
The SNAP and LSAP fields are added to the packets at the transmitting node in order to allow the receiving node to pass each received frame to an appropriate device driver which understands given protocol. SNAP supports identifying protocols by EtherType field values it also supports vendor-private protocol identifier spaces. 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. JSTOR ( January 2018) ( Learn how and when to remove this template message).Unsourced material may be challenged and removed.įind sources: "Subnetwork Access Protocol" – news Please help improve this article by adding citations to reliable sources. This article needs additional citations for verification.
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