Gateways, architectures, and heffalumps

     RFC 875                                            September 1982
                                                                M82-51







                  Gateways, Architectures, and Heffalumps





















                              M.A. PADLIPSKY
                           THE MITRE CORPORATION
                          Bedford, Massachusetts

                                 ABSTRACT




          The growth of autonomous intercomputer networks has led to a
     desire on the part of their respective proprietors to "gateway"
     from one to the other.  Unfortunately, however, the implications
     and shortcomings of gateways which must translate or map between
     differing protocol suites are not widely understood.  Some
     protocol sets have such severe functionality mismatches that
     proper T/MG's cannot be generated for them; all attempts to mesh
     heterogeneous suites are subject to numerous problems, including
     the introduction of "singularity points" on logical connections
     which would otherwise be able to enjoy the advantages of
     communications subnetwork alternate routing, loss of
     functionality, difficulty of Flow Control resolution, higher cost
     than non-translating/mapping Gateways, and the necessity of
     re-creating T/MG's when a given suite changes.  The preferability
     of a protocol-compatible internet is also touched upon, as is the
     psychology of those soi-disant architects who posit T/MG's.
































                                     i

                  Gateways, Architectures, and Heffalumps

                              M. A. Padlipsky




          In our collective zeal to remain (or become) abreast of the
     State of the Art, we sometimes fall into one or the other (or
     both) of a couple of pitfalls.  Only one of these pitfalls is
     particularly well-known:  "Buzzwords" -- and even here merely
     knowing the name doesn't necessarily effect a spontaneous
     solution.  The other deserves more attention:  inadequate
     familiarity with The Relevant Literature.

          The key is the notion of what's really relevant.  Often,
     it's the Oral Tradition that matters; published papers, in their
     attempts to seem scholarly, offer the wrong levels of abstraction
     or, because of the backgrounds of their authors, are so
     ill-written as to fail to communicate well.  Sometimes, however,
     that which is truly relevant turns out to be unfindable by a
     conventional literature searcher because it isn't "in" the field
     of search.

          I wandered into an instructive case in point recently, when
     it took me over an hour to convince a neophyte to the mysteries
     of intercomputer networking (who is quite highly regarded in at
     least one other area of computer science, and is by no means a
     dummy) that a particular Local Area Network architecture proposal
     which casually appealed to the notion of "gatewaying" to three or
     four other networks it didn't have protocols in common with was a
     Very Bad Thing.  "Gateways" is, of course, another one of those
     bloody buzzwords, and in some contexts it might have been enough
     just to so label it.  But this was a conversation with a bright
     professional who'd recently been reading up on networks and who
     wanted really to understand what was so terrible.

          So I started by appealing to the Oral Tradition, pointing
     out that in the ARPA internetworking research community (from
     which we probably got the term "Gateway" in the first place --
     and from which we certainly get the proof of concept for
     internets) it had been explicitly decided that it would be too
     hard to deal with connecting autonomous networks whose protocol
     sets differed "above" the level of
     Host-to-Communications-Subnetwork-Processor protocol.  That is,
     the kind of Gateway we know how to build -- and, indeed, anything
     one might call a Gateway -- attaches to two (or more) comm
     subnets as if it were a Host on each, by appropriately
     interpreting their respective H-CSNP protocols and doing the
     right things in hardware (see Figure 1), but for ARPA Internet
     Gateways each net attached to is assumed to have the same
     Host-Host Protocol (TCP/IP, in fact
     or, anyway, IP and either TCP or some other common-to-both-nets
     protocol above it), and the same process level protocols (e.g.,
     Telnet, FTP, or whatever).  The reason for this assuming of
     protocol set homogeneity is that they "knew" the alternative was
     undesirable, because it would involve the translation or mapping
     between different protocol sets in the Gateways and such T/MG's
     were obviously to be avoided.

          Well, that didn't do the trick.  "Why is a T/MG a Bad
     Thing?" he wanted to know.  "Because of the possibility of
     irreconcilable mismatches in functionality."  "For instance?"
     "Addressing is the most commonly cited."  "Addressing?"

          Assuming the reader is as bored as I am with the dialogue
     bit, I'll try to step through some specifics of the sorts of
     incompatibility one can find between protocol sets in a less
     theatric manner.  Note that the premise of it all is that we
     don't want to change either pre-existing protocol set.  Let's
     assume for convenience that we are trying to attach just two nets
     together with a T/MG, and further assume that one of the nets
     uses the original ARPANET "NCP" -- which consists, strictly
     speaking, of the unnamed original ARPANET Host-Host Protocol and
     the unfortunately named "1822", or ARPANET Host-IMP Protocol --
     and the other uses TCP/IP.

          Host addressing is the most significant problem.  NCP-using
     hosts have "one-dimensional" addresses.  That is, there's a field
     in the Host-IMP "leader" where the Host number goes.  When you've
     assigned all the available values in that field, your net is full
     until and unless you go back and change all the IMP's and NCP's
     to deal with a bigger field.  Using IP, on the other hand,
     addresses of Hosts are "two-dimensional".  That is, there's an IP
     header field in which to designate the foreign network and
     another field in which to designate the foreign Host.  (The
     foregoing is a deliberate oversimplification, by the way.)  So if
     you wanted a Host on an NCP-based net to communicate with a Host
     on another, TCP-based net you'd have a terrible time of it if you
     also didn't want to go mucking around inside of all the different
     NCP implementations, because you don't have a way of expressing
     the foreign address within your current complement of addressing
     mechanisms.

          There are various tricks available, of course.  You could
     find enough spare bits in the Host-IMP leader or Host-Host header
     perhaps, and put the needed internet address there.  Or you could
     change the Initial Connection Protocol, or even make the internet
     address be the first thing transmitted as "data" by the User side
     of each process-level protocol.  The common failing of all such
     ploys is that you're changing the pre-existing protocols, though,
     and if
     that sort of thing were viewed with equanimity by system
     proprietors you might as well go the whole hog and change over to
     the new protocol set across the board.  Granted, that's a big
     jump; but it must be realized that this is just the first of
     several problems.

          (It is the case that you could get around the addressing
     problem by having the T/MG become more nearly a real Host and
     terminate the NCP-based side in an application program which
     would "ask" the user what foreign Host he wants to talk to on the
     TCP-based side -- at least for Telnet connections.  When there's
     no user around, though, as would be the case in most file
     transfers, you lose again, unless you fiddle your FTP.  In
     general, this sort of "Janus Host" -- after the Roman deity with
     two faces, who was according to some sources the god of gateways
     (!) -- confers extremely limited functionality anyway; but in
     some practical cases it can be better than trying for full
     functionality and coming up empty.)

          Then there's the question of what to do about RFNM's.  That
     is, NCP's follow the discipline of waiting until the foreign IMP
     indicates a Ready for Next Message state exists before sending
     more data on a given logical connection, but if you're talking to
     a T/MG, its IMP is the one you'll get the RFNM from (the real
     foreign Host might not even be attached to an IMP).  Now, I've
     actually seen a proposal that suggested solving this problem by
     altering the T/MG's IMP to withhold RFNM's, but that doesn't make
     me think it's a viable solution.  At the very least, the T/MG is
     going to have to go in for buffering in a big way (see Figure 2).
     In a possible worst case, the foreign net might not even let you
     know your last transmission got through without changing its
     protocols.

          Going beyond the NCP-TCP example, a generic topic fraught
     with the peril of functionality mismatch is that of the
     Out-of-Band Signal.  (There are some who claim it's also an
     NCP-TCP problem.) The point is that although "any good Host-Host
     protocol" should have some means of communicating aside from
     normal messages "on" logical connections, the mechanizations and
     indeed the semantics of such Out-of-Band Signals often differ.
     The fear is that the differences may lead to  incompatibilities.
     For example, in NCP the OOBS is an Interrupt command "on" the
     control link, whereas in TCP it's an Urgent bit in the header of
     a message "on" the socket.  If you want Urgent to be usable in
     order to have a "virtual quit button", the semantics of the
     protocol must make it very clear that Urgent is not merely the
     sort of thing the NBS/ECMA Host-Host protocol calls "Expedited
     Data".  If, that is, the intent of the mechanism is to cause the
     associated process/job/task to take special action rather than
     merely the associated protocol interpreter (which need not be
     part of the process), you'd better say so -- and none of the
     ISO-derived protocols I've seen yet does so.  And there's not
     much a T/MG  can do if it gets an NCP Interrupt on a control
     link, notices a Telnet Interrupt Process control code on the
     associated socket, and doesn't have anything other than
     Expediting Data to do with it on its other side.  (Expedited
     Data, it may be noted, bears a striking resemblance to taking an
     SST across the Atlantic, only to find no one on duty in the
     Customs shed -- and the door locked from the other side.)

          Functionality mismatch is not, of course, limited to
     Host-Host protocols.  Indeed, the following interesting situation
     was observed at University College London:  In their "Terminal
     Gateway", which translates/maps ARPANET Telnet and "Triple X"
     (CCITT X.25, X.28, X.29), they were able to get data across, as
     might be expected, but only one option (echoing), which is rather
     worse than might be expected.  (And the UCL people are quite
     competent, so the problem almost certainly doesn't have to do
     with inadequate ingenuity.)

          It could be argued that the real problem with Expedite Data
     and Triple X is that some protocol sets are a lot worse than
     others.  I wouldn't dispute that.  But it's still the case, to
     re-use a Great Network One-liner, that:

                   sometimes, when you try to turn an apple into an
                   orange, you get back a lemon.

          Nor is the likelihood of encountering irresolvable
     functionality  mismatches the only technical shortcoming of
     Translating/Mapping Gateways.  A somewhat subtle but rather
     fascinating point arises if we ask what happens when traffic is
     heavy enough to warrant more than one T/MG between a given pair
     of protocol-incompatible nets (or even if we'd like to add some
     reliability, regardless of traffic).  What happens, if we think
     about it a little, is a big problem.  Suppose you actually could
     figure out a way to translate/map between two given sets of
     protocols.  That would mean that for each logical connection you
     had open, you'd have a wealth of state information about it for
     each net you were gatewaying.  But "you" now stand revealed as a
     single T/MG -- and your clone next door doesn't have that state
     information, so any logical connection that started its life with
     you has to spend its life with you, in a state of perpetual
     monogamy, as it were.  Naturally, this epoxied pair-bonding could
     perhaps be dealt with by still another new protocol between
     T/MG's, but it's abundantly clear that there will be no easy
     analogue to no-fault divorce.  That is, to put it less
     metophorically, it becomes at best extremely complex to do
     translating/mapping at more
     than one T/MG for the same logical connection.  As with the
     broader issue of reconciling given protocol sets at all, doing so
     at multiple loci of control may or may not turn out to be
     feasible in practice and certainly will be a delicate and complex
     design task.

          One more NCP/TCP problem:  When sending mail on an NCP-based
     net, the mail (actually, File Transfer) protocol currently only
     uses the addressee's name, because the Host was determined by the
     Host-Host Protocol.  If you're trying to get mail from an
     NCP-based net to a TCP-based net, though, you're back in the Host
     addressing bind already discussed.  If you don't want to change
     NCP (which, after all, is being phased out), you have to do
     something at the process level.  You can, but the "Simple Mail
     Transfer Protocol" to do it takes 62 pages to specify in ARPANET
     Request for Comments 788.

          If things get that complicated when going from NCP to TCP,
     where there's a close evolutionary link between the Host-Host
     protocols, and the process-level protocols are nominally the
     same, what happens when you want to go from DECNET, or from SNA,
     or from the as-yet incomplete NBS or ISO protocol sets?  There
     may or may not turn out to be any aspects that no amount of
     ingenuity can reconcile, but it's abundantly clear that
     Translating/Mapping Gateways are going to have to be far more
     powerful systems than IP Gateways (which are what you use if both
     nets use the same protocol sets above the Host to Comm Subnet
     Processor protocol).  And you're going to need a different T/MG
     for each pair of protocol sets.  And you may have to tinker with
     CSNP internals....  An analogy to the kids' game of Telephone (or
     Gossip) comes to mind:  How much do you lose each time you
     whisper to your neighbor who in turn whispers to the next
     neighbor?  What, for that matter, if we transplant the game to
     the United Nations and have the whisperers be translators who
     have speakers of different languages on each side?

          Other problem areas could be adduced.  For example, it's
     clear that interpreting two protocol sets rather than one would
     take more time, even if it could be done.  Also, it should be
     noted that the RFNM's Problem generalizes into a concern over
     resolving Flow Control mismatches for any pair of protocol sets,
     and could lead to the necessity of having more memory for buffers
     on the T/MG than on any given Host even for those cases where
     it's doable in principle. But only one other problem area seems
     particularly major, and that is the old Moving Target bugaboo:
     For when any protocol changes, so must all the T/MG's involving
     it, and as there have already been three versions of SNA,
     presumably a like number of versions of DECNET, and as there are
     at least two additional levels which ISO should be acknowledging
     the existence of, the fear of having to re-do T/MG's should serve
     as a considerable deterrent to doing them
     in the first place.  (This apparent contravention of the
     Padlipsky's Law to the effect that Implemented Protocols Have
     Barely Finite Inertia Of Rest is explained by a brand-new
     Padlipsky's Law:  To The Technologically Naive, Change Equals
     Progress; To Vendors, Change Equals Profit.)

          At any rate, it's just not clear that a given Translating/
     Mapping Gateway can even be built; you have to look very closely
     at the protocol sets in question to determine even that.  It's
     abundantly clear that if a given one can be built it won't be
     easy to do (see Figure 3).  Yet "system architect" after "system
     architect", apparently in good faith, toss such things into their
     block diagrams.  Assuming that the architectural issue isn't
     resolved by a fondness for the Gothic in preference to the more
     modern view that form should follow function, let's pause briefly
     to visualize an immense, turreted, crenellated, gargoyled  ...
     microprocessor, and return to the question of why this sort of
     thing happens.

          It's clear that buzzwording is a factor.  After all, "system
     architects" in our context are usually employees of contractors
     and their real role in life is not to build more stately mansions
     but to get contracts, so it's not surprising to find appeal to
     the sort of salesmanship that relies more heavily on fast patter
     than precision. Another good analogy: I once went to one of the
     big chain electronics stores in response to an ad for a cassette
     recorder that "ran on batteries or house current" for $18, only
     to find that they wanted an additional $9 for the (outboard) AC
     adaptor.  Given the complexities of T/MG's, however, in our case
     it's more like an $18 recorder and a $36 adaptor.

          But is buzzwording all there is?  Clearly not, for as
     mentioned earlier there's also ignorance of the Oral Tradition in
     play. Whether the ignorance is willful or not is probably better
     left unexamined, but if we're willing to entertain the notion
     that it's not all a bait-and-switch job akin to the
     separately-priced AC adaptor, we see that those who casually
     propose T/MG's haven't done enough homework as to the real state
     of the art.

          What ever became of that early reference to The Relevant
     Literature, though?  Surely you didn't think I'd never ask.  The
     answers are both implied in the assertion that:

                          Gateways are Heffalumps

     as you'll plainly see once you've been reminded of what
     Heffalumps are.  Dipping into The Relevant Literature, then,
     let's reproduce the opening of the Heffalumps story:

                  One day, when Christopher Robin and Winnie-the-Pooh
             and Piglet were all talking together, Christopher Robin
             finished the mouthful he was eating and said carelessly:
             "I saw a Heffalump today, Piglet."
                  "What was it doing?"  asked Piglet.
                  "Just lumping along," said Christopher Robin.
             "I don't think it saw me."
                  "I saw one once," said Piglet. "At least, I think
             I did," he said.  "Only perhaps it wasn't."
                  "So did I," said Pooh, wondering what a Heffalump
             was like.
                  "You don't often see them," said Christopher Robin
             carelessly.
                  "Not now," said Piglet.
                  "Not at this time of year," said Pooh.
                  Then they all talked about something else, until it
             was time for Pooh and Piglet to go home together.

          (To satisfy the lazy reader -- who'd actually be better off
     searching for it in both -- it's from Winnie-the Pooh, not The House  at
     Pooh Corner.)

          Pooh, in case you still don't recall, decides to make a Heffalump
     Trap.  (Piglet is sorry he didn't think of it first.)  He baits it with
     a jar of honey, after making sure that it really was honey all the way
     to the bottom, naturally.  In the middle of the night, he goes to the
     Trap to get what's left of the honey and gets his head stuck in the jar.
     Along comes Piglet, who sees this strange creature with a jar-like head
     making frightful noises, and, having known no more than Pooh what
     Heffalumps really were, assumes that a Heffalump has indeed been Trapped
     and is duly terrified.

          It would probably be too moralistic to wonder how much Christopher
     Robin actually knew about Heffalumps in the first place. The
     "Decorator", based on the picture on page 60 of my edition, clearly
     thinks C.R. thought they were elephants, but I still wonder. At best,
     though, he knew no more about them than the contractor did about
     Gateways in the proposal that started this whole tirade off.

          NOTE:  FIGURE 1.  Defining Characteristic of All Flavors of
     Gateways, FIGURE 2.  Gateway and Translating/Mapping Gateway,
     Approximately to Scale, and FIGURE 3.  Respective Internals Schematics,
     may be obtained by writing to:  Mike Padlipsky, MITRE Corporation, P.O.
     Box 208, Bedford, Massachusetts, 01730, or sending computer mail to
     Padlipsky@ISIA.