neeraj@matrix.UUCP (neeraj sangal) (02/13/90)
I understand Ethernet packets and now I am trying to understand IEEE802 packets. (Please bear with me if these are naive questions). According to what I understand by looking at the limited documentation I have, the 802.3+802.2 packet looks as follows: destination 6 bytes source 6 bytes length 2 bytes (must be less than 0x0600) dsap 1 byte ssap 1 byte control 1 byte If dsap = ssap = 170 then the following fields (SNAP header) are also included: protid 3 bytes (always 0) ethertype 2 bytes Questions: 1. Is the ethertype same as the ethertype in ethernet packet header? 2. Will dsap and ssap ever be different i.e. What is the justification for having two numbers? 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? 4. Which protocols do not use SNAP? What are the current assigned values of dsap and ssap? 4. When is protid not zero? Any answers or pointers to where answers could be found will be greatly appreciated. Please email directly to me. Thanks. Neeraj Sangal Matrix Computer Systems, Inc. 7 1/2 Harris Rd, Nashua, NH 03062 uunet!matrix!neeraj (603) 888-7790
morgan@jessica.Stanford.EDU (RL "Bob" Morgan) (02/20/90)
> 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? AppleTalk Phase 2 uses 802.2/SNAP encapsulation on 802.3 (Ethernet) and 802.5 (Token Ring). AppleTalk Phase 1 uses Ethernet encapsulation on Ethernets. - RL "Bob" Morgan Networking Systems Stanford