Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools

Problem, Formatting or Query -  Send Feedback

Was this page helpful?-10+1



   < INC-PROJECT, MAP-CRITIQUE.NLS.10, >, 12-Aug-83 11:46 AMW ;;;;
   RFC 874                                            September 1982
                          A CRITIQUE OF X.25
                            M.A. PADLIPSKY
                         THE MITRE CORPORATION
                        Bedford, Massachusetts
        The widely touted network interface protocol, "X.25", and
   its attendant conceptual framework, the International Standards
   Organization's Reference Model for Open System Interconnection
   (ISORM), are analyzed and found wanting.  The paper is a
   companion piece to M82-48, and M82-51.
                          A CRITIQUE OF X.25
                            M. A. Padlipsky
        According to some sources, the International Standards
   Organization's (ISO) "Open System Interconnection" (OSI) effort
   has adopted the International Consultative Committee on Telephony
   and Telegraphy (CCITT) developed X.25 protocol(s) as its Levels
   1-3. ("Loose constructionists" of the ISORM would hold that X.25
   is a mechanization of L1-L3 rather than the mechanization, and at
   least one British source holds that "we in the U.K. don't believe
   that ISO have adopted X.25.")  In the U.S. Government arena,
   where the author spends much of his time, the Government
   Accounting Office (GAO) has suggested that the Department of
   Defense (DoD) ought to consider adopting "X.25 networks,"
   apparently in preference to networks based on protocols developed
   by the DoD-sponsored intercomputer networking research community.
   That intercomputer networking research community in turn has,
   with a few recent exceptions, adhered to its commitment to the
   Oral Tradition and not taken up the cudgels against X.25 in the
   open literature, even though X.25 is an object of considerable
   scorn in personal communications.
        Although the DoD Protocol Standards Technical Panel has
   begun to evolve a "Reference Model" different from ISO's for
   reasons which will be touched on below, there seems to be a need
   to address the deficiencies of X.25 on their own demerits as soon
   as possible. Without pretending to completeness*, this paper will
   attempt to do just that.
        The overall intent is to deal with X.25 in the abstract;
   because of who pays the bills, though, a necessary preliminary is
   to at least sketch the broad reasons why the DoD in particular
   should not
   *  Various versions of X.25 and ISO documentation were employed;
      one incompleteness of note, however, is that no attempt has
      been made to do proper bibliographic citation.  Another
      incompleteness lies in the area of "tutoriality"; that is,
      appropriate prior knowledge is assumed on the part of the
      reader.  (The author apologizes for the omissions but hasn't
      the time or the energy to be overly scholarly.  Reference [3]
      might be of use to the reader who feels slighted.)
   RFC 874                                            September 1982
   employ intercomputer networks which base their protocol suites on
   the ISO Reference Model (ISORM) with X.25 as Levels 1-3.  (Note
   that this is a different formulation from "use communications
   subnetworks which present an X.25 interface.")  Very briefly, the
   DoD has concerns with "survivability," reliability, security,
   investment in prior art (i.e., its research community has a
   working protocol suite in place on many different operating
   systems), procurability (i.e., ISORM-related protocol suites do
   not as yet fully exist even on paper and the international
   standardization process is acknowledged even by its advocates to
   require several years to arrive at full suite specification, much
   less offer available interoperable implementation), and
   interoperability with a much wider range of systems than are ever
   likely to receive vendor-supplied implementations of ISORM
   protocol suites.  Regardless of which particular concerns are
   considered to dominate, the DoD cannot be expected to await
   events in the ISO arena.  (Particularly striking is the fact that
   DoD representatives are not even permitted under current doctrine
   to present their specific concerns in the area of security in the
   sort of unclassified environment the ISO arena constitutes.)
        Some zealous ISORM advocates have suggested that the DoD
   research community suffers from a "Not Invented Here" syndrome
   with respect to ISORM-related protocols, though, so even if the
   various reasons just cited were to prevail, there would still be
   an open issue at some level.  At least one or two zealous members
   of the research community have asserted that the problem is not
   Not Invented Here, but Not Invented Right, so an assessment of
   the apparent keystone of the ISORM suite, X.25, from the
   perspective of whether it's "good art" ought to be appropriate.
   That's what we're up to here.
   RFC 874                                            September 1982
   Problems With the Conceptual Model*
        There is confusion even amongst its advocates as to the real
   conceptual model of X.25-based ISO networking.  Some draw their
   Reference  Model as two "highrises," others draw "parking
   garages" beside each highrise.  That is, some draw the seven
   ISORM layers in large rectangles (representing Hosts) next to one
   another, showing each layer in communication with its "peer" in
   the other Host/Open System; this implies an "end-to-end" view of
   X.25.  Others draw smaller rectangles between the larger ones,
   with Levels 1-3 having peer relationships from the Host-OS ("Data
   Terminal Equipment") to the Comm Subnet Node ("Data Circuit
   Terminating Equipment"); this implies a "link-by-link" view of
   X.25.  This ambiguity does not engender confidence in the
   architects, but perhaps the real problem is with the spectators.
   Yet it is indisputable that when internetting with X.75, the
   model becomes "hop-by-hop" (and it is likely it's meant to be
   link-by-link even on a single comm subnet).
        A major problem with such a model is that the designers have
   chosen to construe it as requiring them to break the "virtual
   circuit" it is supposed to be supporting whenever there is
   difficulty with any one of the links.  Thus, if internetting, and
   on some interpretations even on one's proximate net, rerouting of
   messages will not occur when needed, and all the upper levels of
   protocols will have to expend space-time resources on
   reconstituting their own connections with their counterparts.
   (Note that the success of the reconstitution under DCE failure
   appears to assume a certain flexibility in routing which is not
   guaranteed by the Model.)  This can scarcely be deemed sound
   design practice for an intercomputer networking environment,
   although many have conjectured that it probably makes sense to
   *  Note that we are assuming an ISO-oriented model rather than a
      CCITT-oriented one (X.25/X.28/X.29) because the latter appears
      to offer only "remote access" functionality whereas the sort
      of intercomputer networking we are interested in is concerned
      with the full "resource-sharing" functionality the former is
      striving for.  This might be somewhat unfair to X.25, in that
      it is taking the protocol(s) somewhat out of context; however,
      it is what ISO has done before us, and if what we're really
      accomplishing is a demonstration that ISO has erred in so
      doing, so be it.  As a matter of fact, it can also be argued
      that X.25 is itself somewhat unfair--to its users, who are
      expecting real networking and getting only communication; cf.
      Padlipsky, M. A., "The Elements of Networking Style", M81-41,
      The MITRE Corporation, October 1981, for more on the extremely
      important topic of resource sharing vs. remote access.
   RFC 874                                            September 1982
        Indeed, it appears the virtual circuit metaphor is in some
   sense being taken almost literally (with the emphasis on the
   "circuit" aspect), in that what should be an environment that
   confers the benefits of packet-switching is, at the X.25 level,
   reduced to one with the limitations of circuit-switching.  On the
   other hand, the metaphor is not being taken literally enough in
   some other sense (with the emphasis on the "virtual" aspect), for
   many construe it to imply that the logical connection it
   represents is "only as strong as a wire."  Whether the whole
   problem stems from the desire to "save bits" by not making
   addresses explicitly available on a per-transmission basis is
   conjectural, but if such be the case it is also unfortunate.
        (As an aside, it should be noted that there is some evidence
   that bit saving reaches fetish--if not pathological--proportions
   in X.25:  For instance, there does not even appear to be a Packet
   Type field in data packets; rather--as best we can determine--for
   data packets the low order bit of the "P(R)" field, which
   overlaps/stands in the place of the Packet Type is always 0,
   whereas in "real" Packet Type fields it's always 1.  [That may,
   by the way, not even be the way they do it--it's hard to tell ...
   or care.])
        There is also confusion even amongst its advocates as to
   what implications, if any, the protocol(s) has (have) for comm
   subnet node to comm subnet node (CSN) processing.  Those who draw
   just two highrises seem to be implying that from their
   perspective the CSN (or "DCE") is invisible.  This might make a
   certain amount of sense if they did not assert that each floor of
   a highrise has a "peer-relationship" with the corresponding floor
   of the other highrise--for to do so implies excessively long
   wires, well beyond the state of the wire-drawing art, when one
   notices that the first floor is the physical level.  (It also
   appears to disallow the existence of concatenated comm subnets
   into an internet, or "catenet," unless the CSN's are all
   identically constituted.  And those who hold that the ISORM
   dictates single protocols at each level will have a hard time
   making an HDLC interface into a Packet Radio Net, in all
        Those who, on the other hand, "draw parking garages," seem
   to be dictating that the internal structure of the CSN also
   adhere to X.25 link and physical protocols.  This implies that
   Packet Radio or satellite CSNs, for example, cannot "be X.25."
   Now that might be heartening news to the designers of such comm
   subnets, but it presumably wasn't intended by those who claim
   universality for X.25--or even for the ISO Reference Model.
   RFC 874                                            September 1982
        Even granting that ambiguities in the conceptual model do
   not constitute prima facie grounds for rejecting the protocol(s),
   it is important to note that they almost assuredly will lead to
   vendor implementations based on differing interpretations that
   will not interoperate properly. And the unambiguous position that
   virtual circuits are broken whenever X.25 says so constitutes a
   flaw at least as grave as any of the ambiguities.
        Another, in our view extremely severe, shortcoming of the
   X.25 conceptual model is that it fails to address how programs
   that interpret its protocol(s) are to be integrated into their
   containing operating systems.  (This goes beyond the shortcoming
   of the X.25 specifications in this area, for even the advocates
   of the ISORM--who, by hypothesis at least, have adopted X.25 for
   their Levels 1-3--are reticent on the topic in their literature.)
   Yet, if higher level protocols are to be based on X.25, there
   must be commonality of integration of X.25 modules with operating
   systems at least in certain aspects.  The most important example
   that comes to mind is the necessity for "out-of-band signals" to
   take place.  Yet if there is no awareness of that sort of use
   reflected in the X.25 protocol's specification, implementers need
   not insert X.25 modules into their operating systems in such a
   fashion as to let the higher level protocols function properly
   when/if an X.25 Interrupt packet arrives.
        Yet much of the problem with the conceptual model might turn
   out to stem from our own misunderstandings, or the
   misunderstandings of others.  After all, it's not easy to infer a
   philosophy from a specification.  (Nor, when it comes to
   recognizing data packets, is it easy even to infer the
   specification--but it might well say something somewhere on that
   particular point which we simply overlooked in our desire to get
   the spec back on the shelf rapidly.) What other aspects of X.25
   appear to be "bad art"?
   "Personality Problems"
        When viewed from a functionality perspective, X.25 appears
   to be rather schizophrenic, in the sense that sometimes it
   presents a deceptively end-to-end "personality" (indeed, there
   are many who think it is usable as an integral Host-Host, or
   Transport, and network interface protocol, despite the fact that
   its specification itself--at least in the CCITT "Fascicle"
   version--points out several functional omissions where a
   higher-level protocol is expected--and we have even spoken to one
   or two people who say they actually do -- use it as an end-to-end
   protocol, regardless); sometimes it presents a comm subnet
   network interface personality (which all would agree it must);
   and sometimes (according to some observers) it presents a
   RFC 874                                            September 1982
   "Host-Front End Protocol" personality.  Not to push the "bad art"
   methaphor too hard, but this sort of violation of "the Unities"
   is, if demonstrable, grounds for censure not only to literary
   critics but also to those who believe in Layering.  Let's look at
   the evidence for the split-personality claim:
        X.25 is not (and should not be) an "end-to-end" protocol in
   the sense of a Transport or Host-to-Host protocol.  Yet it has
   several end-to-end features.  These add to the space-time expense
   of implementation (i.e., consume "core" and CPU cycles) and
   reflect badly on the skill of its designers if one believes in
   the design principles of Layering and Least Mechanism.  (Examples
   of end-to-end mechanisms are cited below, as mechanisms
   superfluous to the network interface role.)  The absence of a
   datagram mode which is both required and "proper" (e.g., not Flow
   Controlled, not Delivery Confirmed, not Non-delivery mechanized)
   may also be taken as evidence that the end-to-end view is very
   strong in X.25.  That is, in ISO Reference Model (ISORM) terms,
   even though X.25 "is" L1-3, it has delusions of L4-ness; in
   ARPANET Reference Model (ARM) terms, even though X.25 could "be"
   L I, it has delusions of L II-ness.*
        X.25 is at least meant to specify an interface between a
   Host (or "DTE") and a comm subnet processor (or "DCE"),
   regardless of the ambiguity of the conceptual model about whether
   it constrains the CSNP "on the network side."  (Aside:  that
   ambiguity probably reflects even more badly on certain X.25
   advocates than it does on the designers, for there is a strong
   sense in which "of course it can't" is the only appropriate
   answer to the question of whether it is meant to constrain
   generic CSN processors (CSNP's) in the general case.  Note,
   though, that it might well be meant to constrain specific DCE's;
   that is, it started life as a protocol for PTT's--or Postal,
   Telephone, and Telegraph monopolies--and they are presumably
   entitled to constrain themselves all they want.)  Yet the
   end-to-end features alluded to above are redundant to the
   interfacing role, and, as noted, extraneous features have
   space-time consequences. There are also several features which,
   though not end-to-end, seem superfluous to a "tight" interface
   protocol.  Further, the reluctance of the designers to
   incorporate a proper "datagram" capability in the protocol (what
   they've got doesn't seem to be
   *  For more on the ARM, see Padlipsky, M. A., "A Perspective on
      the ARPANET Reference Model", M82-47, The MITRE Corporation,
      September 1982; also available in Proc. INFOCOM '83.  (Some
      light may also be cast by the paper on the earlier-mentioned
      topic of Who Invented What.)
   RFC 874                                            September 1982
   usable as a "pure"--i.e., uncontrolled at L3 but usable without
   superfluous overheard by L4--datagram, but instead entails
   delivery confirmation traffic like it or not; note that "seem" is
   used advisedly:  as usual, it's not easy to interpret the
   Fascicle) suggests at least that they were confused about what
   higher-level protocols need from interfaces to CSNP's, and at
   worst that there is some merit to the suggestion that, to
   paraphrase Louis Pouzin, "the PTT's are just trying to drum up
   more business for themselves by forcing you to take more service
   than you need."
        Examples of mechanisms superfluous to the interface role:
         1.  The presence of a DTE-DTE Flow Control mechanism.
         2.  The presence of an "interrupt procedure" involving the
             remote DTE.
         3.  The presence of "Call user data" as an end-to-end item
             (i.e., as "more" than IP's Protocol field).
         4.  The "D bit" (unless construed strictly as a "RFNM" from
             the remote DCE).
         5.  The "Q bit" (which we find nearly incomprehensible, but
             which is stated to have meaning of some sort to
             X.29--i.e., to at least violate Layering by having a
             higher-level protocol depend on a lower level
             machanism--and hence can't be strictly a network
             interface mechanism).
   RFC 874                                            September 1982
        The final "personality problem" of X.25 is that some of its
   advocates claim it can and should be used as if it were a
   Host-Front End protocol.*  Yet if such use were intended, surely
   its designers would have offered a means of differentiating
   between control information destined for the outboard
   implementation of the relevant protocols and data to be
   transmitted through X.25, but there is no evidence of such
   mechanisms in the protocol.  "Borrowing" a Packet Type id for
   H-FP would be risky, as the spec is subject to arbitrary
   alteration.  Using some fictitious DTE address to indicate the
   proximate DCE is also risky, for the same reason.  Further, using
   "Call user data" to "talk to" the counterpart H-FP module allows
   only 15 octets (plus, presumably, the 6 spare bits in the 16th
   octet) for the conversation, whereas various TCP and IP options
   might require many more octets than that.  Granted that with
   sufficient ingenuity--or even by the simple expedient of
   conveying the entire H-FP as data (i.e., using X.25 only to get
   channels to demultiplex on, and DTE-DCE flow control, with the
   "DCE" actually being an Outboard Processing Environment that gets
   its commands in the data fields of X.25 data packets)--X.25 might
   be used to "get at" outboard protocol interpreters, but its
   failure to address the issue explicitly again reflects badly on
   its designers' grasp of intercomputer networking issues.
   (Another possibility is that the whole H-FP notion stems from the
   use of X.25 as a Host-Host
   *  That is, as a distributed processing mechanism which allows
      Host operating systems to be relieved of the burden of
      interpreting higher level protocols "inboard" of themselves by
      virtue of allowing Host processes to manipulate "outboard"
      interpreters of the protocols on their behalf.  Note that the
      outboarding may be to a separate Front-End processor or to the
      CSNP itself.  (The latter is likely to be found in
      microprocessor-based LAN "BIU's.")  Note also that when
      dealing with "process-level" protocols (ARM L III;
      approximately ISORM L5-7), only part of the functionality is
      outboarded (e.g., there must be some Host-resident code to
      interface with the native File System for a File Transfer
      Protocol) and even when outboarding Host-Host protocols (ARM L
      II; approximately ISORM L4 plus some of 5) the association of
      logical connections (or "sockets") with processes must be
      performed inboard--which is why, by the way, it's annoying to
      find ISO L5 below ISO L6: because, that is, you'd like to
      outboard "Presentation" functionality but its protocol expects
      to interact with the "Session" protocol, the functionality of
      which can't be outboarded.  (Although this approach, not the
      proper context for a full treatment of the H-FP approach, it
      is also of interest that the approach can effectively insulate
      the Host from changes in the protocol suite, which can be a
      major advantage in some environments.)
   RFC 874                                            September 1982
   protocol so that some might think of it in its Host aspect as
   "simply" a way of getting at the H-HP.  This interpretation does
   give rise to the interesting observation that DCE's seem to need
   a protocol as strong as TCP amongst themselves, but doesn't
   strike the author as particularly convincing evidence for viewing
   X.25 as anything like a proper H-FP--if for no other reason than
   that a central premise of Outboard Processing is that the
   Host-side H-FP module must be compact relative to an inboard
   generic Network Control Program.)
        X.25, then, is rather schizophrenic:  It exceeds its brief
   as an  interface protocol by pretending to be end-to-end
   (Host-Host) in some respects; it is by no means a full end-to-end
   protocol (its spec very properly insists on that point on several
   occasions); it's at once too full and too shallow to be a good
   interface; and it's poorly structured to be treated as if it were
   "just" an H-FP.  (Some would phrase the foregoing as "It's
   extremely ill layered"; we wouldn't argue.)
   A Note on "Gateways"*
        Although it was at least implied in the discussion of
   conceptual model problems, one aspect of X.25/X.75 internetting
   is sufficiently significant to deserve a section of its own:  Not
   only does the link-by-link approach taken by CCITT make it
   unlikely that alternate routing can take place, but it is also
   the case that ARPANET Internet Protocol (IP) based internetting
   not only permits alternate routing but also could alt-route over
   an "X.25 Subnet."  That is, in IP's conceptual model, Gateways
   attach to two or more comm subnets "as if they (the Gateways)
   were Hosts."  This means that they interpret the appropriate
   Host-comm subnet processor protocol of whatever comm subnets
   they're attached to, giving as the "proximate net address" of a
   given transmission either the ultimate (internet addressed)
   destination or the address of another Gateway "in the right
   direction."  And an implementation of IP can certainly employ an
   implementation of ("DTE") X.25 to get a proximate net, so ... at
   least "in an emergency" X.25 interface presenting Public Data
   Networks can indeed carry IP traffic.  (Note also that only the
   proximate net's header has to be readable by the nodal processor
   of/on the proximate net, so if some appropriate steps were taken
   to render the data portion of such transmissions unintelligible
   to the nodal processors, so much the better.)
   *  This section was added to address the ill-founded concerns of
      several ISORMites that "TCP/IP won't let you use Public Data
      Nets in emergencies."
   RFC 874                                            September 1982
        (Further evidence that X.75 internetting is undesirable is
   found in the fact that the U.S. National Bureau of Standards has,
   despite its nominal adoption of the ISORM, inserted IP at
   approximately L3.5 in its version of the Reference Model.)
   The Off-Blue Blanket
        Although touched on earlier, and not treatable at much
   length in the present context, the topic of security deserves
   separate mention.  We are familiar with one reference in the open
   literature [1] which appears to make a rather striking point
   about the utility of X.25 in a secure network.   Dr. Kent's point
   that the very field sizes of X.25 are not acceptable from the
   point of view of encryption devices would, if correct (and we are
   neither competent to assess that, nor in a position to even if we
   were), almost disqualify X.25 a priori for use in many arenas.
   Clearly, uncertified "DCE's" cannot be permitted to read
   classified (or even "private") data and so must be "encrypted
   around," after all.
        It would probably be the case, if we understand Dr. Kent's
   point, that X.25 could be changed appropriately--if its
   specifiers were willing to go along.  But this is only one
   problem out of a potentially large number of problems, and,
   returning briefly to our concern with the interplay of X.25 and
   the DoD, those persons in the DoD who know best what the problems
   are and/or could be are debarred from discussing them with the
   specifiers of X.25.  Perhaps a sufficiently zealous ISORM
   advocate would be willing to suggest that Professor Kuo's
   publisher be subsidized to come out with a new edition whenever a
   problem arises so that if Dr. Kent happens to spot it advantage
   can continue to be taken of his ability to write for the open
   literature--but we certainly hope and trust that no ISORMite
   would be so tone-deaf as to fail to recognize the facetiousness
   of that suggestion.
        In short, it appears to be difficult to dispute the
   assertion that whatever sort of security blanket X.25 could
   represent would at best be an off shade of blue.
   Space-Time Considerations
        Another topic touched on earlier which deserves separate
   mention, if only to collect the scattered data in a single
   section, is that of what have been called space-time
   considerations.  That is, we are concerned about how well X.25 in
   particular and the ISORM-derived protocols in general will
   implement, both in terms of size of protocol interpreters (PI's)
   and in terms of execution and delay times.
   RFC 874                                            September 1982
        On the space heading, certainly the fact that X.25 offers
   more functionality in its end-to-end guise than is required to
   fulfill its network interface role suggests that X.25 PI's will
   be bigger than they need be.  As an aside--but a striking one--it
   should be noted that X.25's end-to-end functions are at variance
   with the ISORM itself, for the "peer entity" of a DTE X.25 entity
   must surely be the local DCE X.25.  Perhaps a later version of
   the ISORM will introduce the polypeer and give rise to a whole
   new round of Layering-Theologic controversy.*  Speaking of the
   ISORM itself, those who hold that each layer must be traversed on
   each transmission are implicitly requiring that space (and time)
   be expended in the Session and Presentation Levels even for
   applications that have no need of their services.  The Well-Known
   Socket concept of the ARM's primary Host-Host protocol, the
   Transmission Control Protocol (TCP), lets Session functionality
   be avoided for many applications, on the other hand--unless ISORM
   L5 is to usurp the Host's user identification/authentication role
   at some point.  (Yes, we've heard the rumors that "null layers"
   might be introduced into the ISORM; no, we don't want to get into
   the theology of that either.)
        On the time heading, X.25's virtual circuit view can be
   debilitating--or even crippling--to applications such as
   Packetized Speech where prompt delivery is preferred over ordered
   or even reliable delivery.  (Some hold that the X.25 datagram
   option will remedy that; others hold that it's not "really
   datagrams"; we note the concern, agree with the others, and pass
   on.)  Speaking of reliable delivery, as noted earlier some
   observers hold that in order to present an acceptable virtual
   circuit X.25 must have a protocol as strong as TCP "beneath"
   itself; again, we're in sympathy with them.  Shifting focus again
   to the ISORM itself, it must be noted that the principle that
   "N-entities" must communicate with one another even in the same
   Host via "N-1 entities" even in the same Host is an over-zealous
   application of the Principle of Layering that must consume more
   time in the interpreting of the N-1 protocol than would a direct
   interface between N-level PI's or such process-level protocols as
   FTP and Telnet, as is done in the ARPANET-derived model.
        Other space-time deficiencies could be adduced, but perhaps
   a shortcut will suffice.  There is a Law of Programming
   (attributed to Sutherland) to the effect that "Programs are like
   waffles: you should always throw the first one out."  Its
   relevance should become
   *  And perhaps we now know why some just draw the highrises.
   RFC 874                                            September 1982
   clear when it is realized that (with the possible exception of
   X.25) ISORM PI's are in general either first implementations or
   not even implemented yet (thus, the batter, as it were, is still
   being mixed).  Contrast this with the iterations the
   ARPANET-derived PI's--and, for that matter, protocols--have gone
   through over the years and the grounds for our concern over
   X.25/ISORM space-time inefficiency become clear irrespective of
   corroborative detail. Factor in the consideration that space-time
   efficiency may be viewed as contrary to the corporate interests
   of the progenitors of X.25 ("the PTT's") and at least the current
   favorite for ISORM Level 4 (ECMA--the European Computer
   Manufacturers' Association), and it should become clear why we
   insist that space-time considerations be given separate mention
   even though touched upon elsewhere.*
   Getting Physical
        Still another area of concern over X.25 is that it dictates
   only one means of attaching a "DTE" to a "DCE."  That is, earlier
   references to "the X.25 protocol(s)" were not typographical
   errors. Most of the time, "X.25" refers to ISORM Level 3;
   actually, though, the term subsumes L2 and L1 as well.  Indeed,
   the lowest levels constitute particular bit serial interfaces.
   This is all very well for interfacing to "Public Data Nets"
   (again, it must be recalled that X.25's roots are in CCITT), but
   is scarcely appropriate to environments where the communications
   subnetwork may consist of geosynchronous communications satellite
   channels, "Packet Radios," or whatever.  Indeed, even for
   conventional Local Area Networks it is often the case that a
   Direct Memory Access arrangement is desired so as to avoid
   bottlenecking--but DMA isn't HDLC, and the "vendor supported X.25
   interface" so prized by some won't be DMA either, one imagines.
   (Speaking of LAN's, at least the evolving standard in that
   arena--"IEEE 802"--apparently will offer multiple physical
   interfaces depending on comm subnet style [although there is some
   disagreement on this point amongst readers of their draft specs];
   we understand, however, that their Level 2 shares X.25's end-end
   aspirations--and we haven't checked up on DMA capability.)  X.25,
   then, imposes constraints upon its users with regard to interface
   technology that are inappropriate.
   *  The broad issue of design team composition is amplified in
      Padlipsky, M. A., "The Illusion of Vendor Support", M82-49,
      The MITRE Corporation, September 1982.
   RFC 874                                            September 1982
   Other Observers' Concerns
        This paper owes much to conversations with a number of
   people, although the interpretations of their concerns are the
   author's responsibility.  Mention should be made, however, of a
   few recent documents in the area:  The Defense Communications
   Agency (DCA Code J110) has sent a coordinated DoD position [2] to
   NBS holding that X.25 cannot be the DoD's sole network interface
   standard; Dr. Vinton Cerf of the ARPA Information Processing
   Technology Office made a contribution to the former which
   contains a particularly lucid exposition of the desirability of
   proper "datagram" capability in DoD comm subnets [3]; Mr. Ray
   McFarland of the DoD Computer Security Evaluation Center has also
   explored the limitations of X.25 [4]. Whether because these
   authors are inherently more tactful than the present author, or
   whether their positions are more constraining, or even whether
   they have been more insulated from and hence less provoked by
   uninformed ISORMite zealots, none has seen fit to address the
   "quality" of X.25.  That this paper chooses to do so may be
   attributed to any one of a number of reasons, but the author
   believes the key reason is contained in the following:
        X.25 is not a good thing.
   [1] Kent, S. T., "Security in Computer Networks," in Kuo, F.,
       Ed., Protocols and Techniques for Data Communications
       Networks, Prentice-Hall, 1981, pp. 369-432.
   [2] Letter to NBS from P. S. Selvaggi, Chief, Interoperability
       and Standards Office, 7 April 1982.
   [3] Cerf, V. G., "Draft DoD Position Regarding X.25" in undated
       letter to P. S. Selvaggi.
   [4] Personal communications.
/data/webs/external/dokuwiki/data/pages/rfc/rfc874.txt · Last modified: 1992/09/23 20:20 (external edit)