Dave_Katz@UM.CC.UMICH.EDU (02/18/90)
For starters, you should decouple 802.2 from 802.3, they are quite separate. In fact, you won't see much 802.3 framing on LANs, except for OSI packets. SNAP framing for IP, in particular, is virtually never done on 802.3; rather, ethernet framing is used instead. 802.2/SNAP is the only way to go on token ring, token bus, and FDDI, however. > 1. Is the ethertype same as the ethertype in ethernet packet header? Yes. This provides a way of representing Ethernet protocols on 802 media, since Ethernet identifies the protocol above within the MAC frame, whereas 802 effectively does not. > 2. Will dsap and ssap ever be different i.e. What is the justification for > having two numbers? In the case of the SNAP SAP, SSAP will typically equal DSAP. There are some exception cases; for instance, an 802.2 XID command can be sent to the broadcast SAP, and each active SAP in the station would need to respond (so in this case, in the request, SSAP=x, DSAP=BCAST, and in the response, SSAP=SNAP, DSAP=x). 802.2 itself does not constrain the use of SAPs, except to identify special cases (null, broadcast, and group). See IEEE 802.2 for gory details. > 3. In practice which protocols use SNAP. I think IP uses SNAP, do > other protocols such as DECNET, Apple Talk, Novell etc. which > have ethertype assigned to them always use SNAP or do they have > newer dsap and ssap numbers assigned to them? Right now, IP and ARP are the biggest users of SNAP (though on other 802 media, not 802.3). It appears likely that "translucent" bridges that connect ethernets together over other 802 media (in particular, FDDI, which is "802-like") will enapsulate ethernet frames in SNAP headers to get them across the foreign media, so any ethernet protocol could conceivably be carried this way. SSAP/DSAP address space is scarce (126 values, or thereabouts), so they are not given away lightly. All OSI protocols use a single SAP value, and are demuxed at the network layer instead. SAP values will tend to be assigned to families of protocols, rather than specific ones. > 4. Which protocols do not use SNAP? What are the current assigned > values of dsap and ssap? OSI protocols do not use SNAP. IEEE keeps the registry of SAP values. I recall that x'FE' means "OSI network layer protocol". There are probably others, but not too many. > 4. When is protid not zero? This field is really the Organizationally Unique Indentifier, or OUI. This allows proprietary protocols to be demuxed. Organizations can presumably stick their 23-bit IEEE-assigned org code in this field and do whatever they want with the rest. 0-0-0 is a special value assigned for ethernet encapsulation.