Subnetwork Access Protocol: Difference between revisions

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{{refimprove|date=January 2018}}

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 8eight-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 layer|physical network layer]]s, as well as with non-IEEE 802 physical network layers such as [[FDDI]] that use 802.2 LLC.

The SNAP and [[IEEE 802.2#LSAP values|LSAP]] fields are [[Encapsulation (networking)|added]] to the [[Network packet|packets]] at the transmitting node in order to allow the receiving node to [[demultiplex|pass]] each received [[Frame (networking)|frame]] to an appropriate [[device driver]] which understands the given [[Communications protocol|protocol]].

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==Use==

The SNAP is an extension of the 802.2 LLC specified in the IEEE 802 Overview and Architecture document.<ref>{{citation |url=http://standards.ieee.org/about/get/802/802.html |archive-url=https://web.archive.org/web/20101129155708/http://standards.ieee.org/about/get/802/802.html |url-status=dead |archive-date=November 29, 2010 |title=IEEE 802 Overview and Architecture |publisher=[[IEEE]] |accessdate=2014-08-02}}</ref> 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:

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One might ask, "why is a separate sub-network header necessary?". The answer is that it was to augment a decision made during the layout of the LLC header. At the time that the LLC header was being designed, it was thought that a single octet (256 possible values) in the header would be enough to specify all the protocol values that vendors would want to register. As the values began to be reserved, it was discovered that the LLC header would soon run out of open values. The hexadecimal AA and AB values were reserved, and an additional header—the SNAP header—was developed; it can support all EtherType values and multiple spaces of private protocol values.

As per [[IETF]] RFC 1042, IP datagrams and ARP datagrams are transmitted over IEEE 802 networks using LLC and SNAP headers,{{Ref RFC|1042}} except on Ethernet/IEEE 802.3, where they are transmitted with Ethernet II headers, as per.{{Ref RFC |894.}}

==References==