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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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