alex@sapphire.idbsu.edu (Alex Feldman) (01/31/91)
This isn't about tcp/ip, but someone in this group must know this... The specifications of an ethernet frame in 8802/3 standard (5th printing, May 1988) and the one in Comer's book (Internetworking with TCP/IP, 2nd ed (1991)) differ. My question is, why the difference, and what happens to the type field in the 8802/3 version? Comer lists all these types for different kinds of potential data... do those go away? How are types distinguished? -- --alex alex@opal.idbsu.edu Boise State University doesn't have any opinions. Therefore, these are not the opinions of Boise State University.
lstowell@pyrnova.pyramid.com (Lon Stowell) (01/31/91)
In article <1991Jan30.213139.4067@sapphire.idbsu.edu> alex@sapphire.idbsu.edu (Alex Feldman) writes: >This isn't about tcp/ip, but someone in this group must know this... > >The specifications of an ethernet frame in 8802/3 standard (5th >printing, May 1988) and the one in Comer's book (Internetworking >with TCP/IP, 2nd ed (1991)) differ. My question is, why the difference, >and what happens to the type field in the 8802/3 version? Comer >lists all these types for different kinds of potential data... >do those go away? How are types distinguished? Not sure if you mean the diff between Ethernet and 802.3x frame formats or not. Ethernet has the Type code right after addressing, 802.3 has the "LLC Length" field in the same position. LLC Length is the length of the LLC data within the 802.3 frame. IF, and it is a big if, the protocol stack conforms to 88022 LLC, the type code is inside the LLC data and actually serves as a source and destination "address" for data contained within the Frame (which have their own physical level addresses) These LLC addresses are known as SAP (service access points). The format of the LLC data within the frame is: DSAP SSAP C-Field LLC layer I-Field Addx Addx > It is not required that the DSAP and SSAP be the same value, sor example in SNA, it is permissible for the SNA SAP '04' to address LLC commands to the Null SAP '00' in order to query the LLC of the destination for routing and/or status information. Typically user data flows over like SAP's, but this is not a requirement.....the SAP is just the protocol handler within the device to which the LLC layer will hand the incoming data. It is up to the upper layer to handle the data appropriately.
postel@VENERA.ISI.EDU (02/01/91)
Lon Stowell: See also RFC 1042 and RFC 894. --jon.