Oct 27, 2004

Ethernet_II, 802.3, 802.2 vs. SNAP

There are four types of ethernet frame type:

Ether_II: has a field named "protocal", which specifys the upper layer(network layer)protocal(e.g. IP, IPX, etc.)

802.3: without a "protocal" field but has a "length"

802.2: Since the 802.3 Ethernet frame cannot by itself identify the upper-layer (Network)protocol, it obviously needs some help. The IEEE defined the 802.2LLC specifications to provide this function and more. an 802.2 frame is an 802.3 frame with the LLC information in the data field of the header. 802.2 frame is identified by "Dest. SAP" and "Source SAP" fields, these two fields specifys the upper layer protocal

SNAP: The SNAP frame has its own protocol field to identify the upper-layer protocol.This is really a way to allow an Ethernet_II Ether-Type field to be usedin an 802.3 frame. SNAP frame is identified by its "Dest. SAP" and "Source SAP" fields, always have the value of "0xAA" - SNAP, the upper layer protocal has already been specified in "protocal" field

In short words:
originally we have Ether_II or 802.3(created by Novell, with no Type field, then accepted by IEEE), then IEEE introduced 802.2 and SNAP to modify 802.3 so 802.3 can identify upper level protocol. when we say 802.2 frame, we mean a 802.3 frame with 802.2 header; when we say SNAP frame, we mean on top of 802.3, we have SNAP header. In the meantime, SNAP is a
special 802.2 frame with certain values on SSAP, DSAP, and Command fields.