art@ACC.ARPA (06/25/86)
> "I assume this applies to the protocol field as well?" > > The IEEE doesn't think anybody needs a protocol field. They use that > word for the packet length. Thus I doubt very much if they are > interested in assigning values. I don't know for sure though. > > ... > > OK, my ignorance is showing. Given that what used to be the protocol > field is now the length, how does one determine what layer (protocol) > the packet is really for? I assume there is still a 16 bit field > SOMEPLACE. Who assigns those numbers? IEEE 802 divides link level services into two sublayers, Media Access Control (MAC) and Logical Link Control (LLC). Different MAC layers are defined for various LAN types, 802.3 for CSMA/CD (ethernet), 802.4 Token Bus (used by MAP), and 802.5 Token Ring (the IBM ring). All MAC layers share a common LLC (802.2) which is supposed to provide a link level service which is independent of LAN type. The addressing of the next higher level user (protocol) has been moved into the LLC protocol header. The 802.3 MAC level header is nearly identical to the old ethernet header with the exception of the protocol field becoming a length field. LLC identifies its users through Link Service Access Points (LSAPs in ISOease). LSAPs are 8 bit quantities with two of those bits reserved for indicating single/multi recipient addressing and local/global SAP administration. This leaves room for 64 standard protocol identifiers. IEEE has assigned values for ISO IP and DARPA IP, but none for ARP. This has generated a lot of activity from the folks trying to convert TCP/IP/Ethernet systems to 802.3. There is a proposal to use a reserved LSAP value to identify an expanded LLC header with more addressing space. <Art@ACC.ARPA> ------