howard@COS.COM (Howard C. Berkowitz) (06/16/87)
In article <920@bloom-beacon.MIT.EDU>, martillo@athena.mit.edu (Yakim Martillo) writes: > Personally, I have always found this confusing because I have > read the ISO documents and have never seen anything which implies > there must be only one protocol at every layer. Further, I thought > swap in and swap out of protocol layers should be possible. It is emphatically true that there need not be one protocol per layer; examples are given below. The ISO OSI Reference Model is a context for defining specific layer services and protocols; Implementors' Agreements and Functional Profiles details the options to be used if interoperability is the goal. The Corporation for Open Systems (COS) currently has a number "protocol stacks" defined for future conformance certification; these stacks have a fair variation of lower-layer protocols( i.e., various LAN and WAN technologies) supporting upper-layer messaging (MHS/X.400) and file transfer, access, and management (FTAM). More stacks are being defined. > > TCP and IP are tailored to each other but I have seen nothing > which implies that TCP could not run over ISO IP or which implies > TP4 could not run over IP. > > In fact there might be reason to tailor a transport protocol for > certain classes of protocols. To some extent, this is being done now. ISO defines five classes of the transport protocol, Class 0 having minimum functionality and Class 4 maximum (essentially that of TCP). COS uses Class 4 for most applications, but allows Class 0 for certain modes of operation over X.25 networks, which provide lower-layer error recovery not offered by LANs. > > In any case I don't quite understand why IP and ISO IP can't be > coresident in the network. This should be possible. Note, however, that their functionality is quite similar, and DoD has stated a policy of migration to ISO. >The communications subnet protocol will > have some way of indentifying the level 3 protocol to the host level 3 > layer software (like the type field in ethernet 2.0 or like ssap/dsap > tuples in IEEE 802.2) > > IEEE 802.2 and ISO level 2 puzzle me as well. These are protocols for > the communications subnet and I don't quite understand why IEEE and > ISO are trying to define communications subnet protocols for all time. Realize first that ISO has split Layer 2 into two sublayers: the Logical Link Control (LLC) sublayer, where 802.2 lives, and the Media Access Control (MAC) Layer, which defines the mechanism used to control access to the shared medium, as opposed to the medium-dependent Layer 1 mechanisms used to transmit over it. 802.2 is medium independent, and basically provides framing and limited error detection. It is practically necessary, in an implementation context, to shield upper layers from some of the details of interrupt handling, buffer management, etc. An 802.2 interface can be convenient at the communications chip level, to give a relatively constant interface to upper layers using quite different lower layers. IEEE and ISO definitely are not trying to define communications subnet protocols for all applications. Again, this comment seems to represent the common misconception that the OSI community prescribes one protocol per layer; it simply does not. At the MAC sublayer, there are three especially common protocols using different media access control approaches: 802.3 (using Carrier Sense Multiple Access/Collision Detection), 802.4 (using Token Passing Bus), and 802.5 (using Token Passing Ring). 802.3, essentially equivalent to Phase 2 ETHERNET, is a robust technology, rather easy to implement. It does not offer deterministic delay, which is needed for process control applications [note: there is much theological debate about whether it is; I leave that to the theologians.]. 802.4 is the preferred LAN protocol of the Manufacturing Automation Protocol (MAP) user group. 802.5 offers high reliability and compatibility with certain non-OSI protocols as seen in SNA architecture. MAC and LLC protocols can mix and match on top of specific media. For example, 802.3 protocols have well-defined physical subtypes appropriate for different environments, identified as 802.3 XYZ, where X is the data rate in megabits per second, Y is the medium, and Z is the maximum physical length in hundreds of meters: 10Base5 is thickwire "ETHERNET" cable -- 10 MBPS for 500 M 1Base5 is "STARLAN" type twisted pair 10Broad36 is a broadband implementation... Similar variants exist for 802.4 and 802.5. Other LAN protocols are emerging for specific communities of interest, such as FDDI very-high-speed optical fiber with kilometer range. The key point here is not that there is one protocol, but a whole family of protocols with standardized interfaces between layers. Internetworking is a Layer 3 problem, not Layer 2, so, for example, the goal is to have a common Layer 3 for a variety of quite necessary LANs. ---------------------------------- SEMI-DISCLAIMER I am a conformance testing system developer for the Corporation for Open Systems, and primary instructor for the COS seminar. Information in this posting are accurate to the best of my knowledge, as a worker in the field. Nevertheless, statements here are not necessarily the final word of the Corporation for Open Systems, ISO, ANSI, IEEE, CCITT, the Internal Revenue Service, etc. > Shouldn't communications subnet protocols be medium-dependent? MAC and PHY (Layer 1) protocols are, MAC somewhat so and PHY definitely.