nigel@modcomp.UUCP (Nigel Gamble) (07/30/88)
There seems to be some confusion as to the position of the TLI (Transport Layer Interface) in a typical comms protocol stack. The TLI was defined by AT&T for UNIX System V because the OSI standards only specify the data that must flow between protocol layers in a very general way. Note that the DATA itself is precisely defined, but not the details of how it is passed from one layer to another. OSI does not define exactly what the interface looks like in terms of a particular set of library routines in a particular programming language on a particular operating system; it is up to each implementor to define these details for themselves. However, it is obviously better if a standard for these details exists for a particular OS, so that layers implemented by different people can be easily slotted together. Hence AT&T defined the TLI for UNIX System V, which provides the 'glue' between the Session layer and the Transport layer. Note that the TLI is only the first to be defined of SEVEN possible xLI's. This was probably because it is strategically the most important, in terms of the number of existing (non OSI) protocols which can, nevertheless, be implemented so as to be accessible via the TLI (such as TCP/IP), or to run over the TLI (such as uucp, RFS, FTP etc.). This has the obvious advantage that existing applications do not have to be changed at all to be able to run over both TCP/IP and the newer OSI Transport protocols. Perhaps the following diagram showing the standard ISO/OSI (International Standards Organization/ Open Systems Interconnection) 7 layer reference model may help to clarify the issue. _________________ | |\ | Application | \ |_______________| \ ( Ideally, these should be standard | | \ ( OSI protocol layers, but anything | Presentation | >-- ( that is able to use the Transport |_______________| / ( Layer Interface (such as uucp in | | / ( SVR3) can replace them | Session | / |_______________|/______ ( TLI provides a standard INTERFACE | |\ ( to the Transport layer | Transport | \ |_______________| \ | | \ ( Ideally, these should be standard | Network | \ ( OSI protocol layers, but anything |_______________| \__( that provides approximately the same | | / ( functionality (such as TCP/IP) can | Link | / ( replace them |_______________| / | | / | Physical | / |_______________|/ You can see that, whereas PROTOCOL layers are represented by BOXES in the diagram, the TLI is represented by a horizontal LINE between two boxes. If (or when?) each of the other horizontal lines is defined in terms of a standard set of interface routines in the UNIX environment, then, as well as a TLI, we will also have an ALI, PLI, SLI, NLI, LLI and PhLI. -- Nigel Gamble "Everything should be made as simple as possible, MODCOMP/AEG but not simpler." Albert Einstein. uunet!modcomp!nigel