Date: Sun, 28 Mar 1999 11:53:44 +0200 From: Gerald Heinig <heinig@hdz-ima.rwth-aachen.de> To: justin@apple.com Cc: net@FreeBSD.ORG Subject: Re: OSI layering Query.. Please Help ME Message-ID: <36FDFC28.A4A971E9@hdz-ima.rwth-aachen.de> References: <199903280454.UAA00698@walker3.apple.com>
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"Justin C. Walker" wrote: > find detailed discussions of the model, and its applicabiliity to IP, > in many places. See, e.g., Stallings, "Handbook of Computer > Communications Standards, V. 1, The Open Systems Interconnect model > ..." (MacMillan/Stallings); Stevens' "TCP/IP Illustrated, V1" > (Addison Wesley), discusses layering as it applies specifically to > TCP and IP (i.e., up through what OSI calls the transport layer). > > > And what exactly are PDU? > Protocol Data Units - this is OSI-speak for "packets", > although it allows you to talk about packets at the various protocol > layers (e.g., an IP packet as distinct from a TCP packet or an > ethernet packet). > > > What exactly is SAP?? > Service Access Point - it's a protocol addressing term. > E.g., in IP terms, an SAP at the network layer would be a port (and > is known as an NSAP). You'd need to delve more deeply into the OSI > model to appreciate the subtlety of the concept :-}. Hmmm. Yet another interpretation :-(. Did you get this out of the books you mentioned above? The books I took my definitions out of are "Data communications, computer networks and OSI" by Fred Halsall and "Internetworking with TCP/IP volume 1" by Douglas Comer. The Halsall book is very abstract and hardly mentions TCP/IP, but the Comer book does. It says, for example: "Level 3: A reference to transport level communication derived from the ISO 7-layer reference model. For TCP/IP internets, level 3 refers to IP and the IP datagram format. Thus, a level 3 address is an IP address." Since level 3 is defined as the network layer, the point at which you access this layer is the logical network interface and you use a network service access point address to do this. Or so the logic goes. The Halsall book is very hazy on this and seems to contradict itself at several points. Worse, nobody seems to be able to agree on this... :-( Gerald -- "Would you like to buy an encyclopaedia to help your child get to college?" "He doesn't need it. He takes the bus!" To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message
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