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     RFC 874                                            September 1982
                                                                M82-50







                            A CRITIQUE OF X.25





















                              M.A. PADLIPSKY
                           THE MITRE CORPORATION
                          Bedford, Massachusetts





                                 ABSTRACT




          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.











































                                     i




                            A CRITIQUE OF X.25

                              M. A. Padlipsky




     Introduction

          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.)





                                     1
     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.
























                                     2
     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
     telephonists.

     ________________
     *  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.





                                     3
     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
     probability.)

          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.








                                     4
     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






                                     5
     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.)






                                     6
     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).

























                                     7
     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.)




                                     8
     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."







                                     9
     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.






                                    10
     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.








                                    11
     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.










                                    12
     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:

     Conclusion

          X.25 is not a good thing.

     References

     [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.
















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