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 layer
, as well as with non-IEEE 802 physical network layers such as FDDI
that use 802.2 LLC.
SNAP is included in an extension of the 802.2 LLC header; it is specified in the document. A packet with an LLC header with a destination SAP of hexadecimal AA or AB and a source SAP of hexadecimal AA or AB is a SNAP packet. The SNAP header follows the 802.2 header; it has a 5-octet protocol identification field, consisting of a 3-octet IEEE Organizationally Unique Identifier
(OUI) followed by a 2-octet protocol ID. If the OUI is hexadecimal 000000, the protocol ID is the Ethernet type (EtherType
) field value for the protocol running on top of SNAP; 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.
SNAP is usually used with Unnumbered Information 802.2 protocol data units
(PDUs), with a control field value of hexadecimal 03, 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... Read More