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INFORMATIONAL
RFC 872                                            September 1982                                                                M82-48                                                                  TCP-ON-A-LAN                                                                                                                                       M.A. PADLIPSKY                           THE MITRE CORPORATION                          Bedford, Massachusetts

                            Abstract                              The sometimes-held position that the DoD Standard     Transmission Control Protocol (TCP) and Internet Protocol (IP)     are inappropriate for use "on" a Local Area Network (LAN) is     shown to be fallacious.  The paper is a companion piece to     M82-47, M82-49, M82-50, and M82-51.                                                                                                                                                                                                                                                  i

                         "TCP-ON-A-LAN"                                   M. A. Padlipsky          Thesis               It is the thesis of this paper that fearing "TCP-on-a-LAN"     is a Woozle which needs slaying.  To slay the "TCP-on-a-LAN"     Woozle, we need to know three things:  What's a Woozle?  What's a     LAN?  What's a TCP?          Woozles               The first is rather straightforward [1]:                    One fine winter's day when Piglet was brushing away the          snow in front of his house, he happened to look up, and          there was Winnie-the-Pooh.  Pooh was walking round and round          in a circle, thinking of something else, and when Piglet          called to him, he just went on walking.               "Hallo!" said Piglet, "what are you doing?"               "Hunting," said Pooh.               "Hunting what?"               "Tracking something," said Winnie-the-Pooh very          mysteriously.               "Tracking what?" said Piglet, coming closer.               "That's just what I ask myself.  I ask myself, What?"               "What do you think you'll answer?"               "I shall have to wait until I catch up with it," said          Winnie-the-Pooh.  "Now look there."  He pointed to the          ground in front of him.  "What do you see there?               "Tracks," said Piglet, "Paw-marks."  he gave a little          squeak of excitement.  "Oh, Pooh!  Do you think it's a--a--a          Woozle?"               Well, they convince each other that it is a Woozle, keep     "tracking," convince each other that it's a herd of Hostile     Animals, and get duly terrified before Christopher Robin comes     along and points out that they were following their own tracks     all the long.               In other words, it is our contention that expressed fears     about the consequences of using a particular protocol named "TCP"     in a particular environment called a Local Area Net stem from     misunderstandings of the protocol and the environment, not from     the technical facts of the situation.                                                                   1

RFC 872                                            September 1982               LAN's               The second thing we need to know is somewhat less     straightforward:  A LAN is, properly speaking [2], a     communications mechanism (or subnetwork) employing a transmission     technology suitable for relatively short distances (typically a     few kilometers) at relatively high bit-per-second rates     (typically greater than a few hundred kilobits per second) with     relatively low error rates, which exists primarily to enable     suitably attached computer systems (or "Hosts") to exchange bits,     and secondarily, though not necessarily, to allow terminals of     the teletypewriter and CRT classes to exchange bits with Hosts.     The Hosts are, at least in principle, heterogeneous; that is,     they are not merely multiple instances of the same operating     system.  The Hosts are assumed to communicate by means of layered     protocols in order to achieve what the ARPANET tradition calls     "resource sharing" and what the newer ISO tradition calls "Open     System Interconnection."  Addressing typically can be either     Host-Host (point-to-point) or "broadcast." (In some environments,     e.g., Ethernet, interesting advantage can be taken of broadcast     addressing; in other environments, e.g., LAN's which are     constituents of ARPA- or ISO-style "internets", broadcast     addressing is deemed too expensive to implement throughout the     internet as a whole and so may be ignored in the constituent LAN     even if available as part of the Host-LAN interface.)               Note that no assumptions are made about the particular     transmission medium or the particular topology in play.  LAN     media can be twisted-pair wires, CATV or other coaxial-type     cables, optical fibers, or whatever.  However, if the medium is a     processor-to-processor bus it is likely that the system in     question is going to turn out to "be" a moderately closely     coupled distributed processor or a somewhat loosely coupled     multiprocessor rather than a LAN, because the processors are     unlikely to be using either ARPANET or ISO-style layered     protocols.  (They'll usually -- either be homogeneous processors     interpreting only the protocol necessary to use the transmission     medium, or heterogeneous with one emulating the expectations of     the other.)  Systems like "PDSC" or "NMIC" (the evolutionarily     related, bus-oriented, multiple PDP-11 systems in use at the     Pacific Data Services Center and the National Military     Intelligence Center, respectively), then, aren't LANs.               LAN topologies can be either "bus," "ring," or "star".  That     is, a digital PBX can be a LAN, in the sense of furnishing a     transmission medium/communications subnetwork for Hosts to do     resource sharing/Open System Interconnection over, though it     might not present attractive speed or failure mode properties.     (It might, though.)  Topologically, it would probably be a     neutron star.                                                    2

RFC 872                                            September 1982                    For our purposes, the significant properties of a LAN are     the high bit transmission capacity and the good error properties.     Intuitively, a medium with these properties in some sense     "shouldn't require a heavy-duty protocol designed for long-haul     nets," according to some.  (We will not address the issue of     "wasted bandwidth" due to header sizes. [2], pp. 1509f, provides     ample refutation of that traditional communications notion.)     However, it must be borne in mind that for our purposes the     assumption of resource-sharing/OSI type protocols between/among     the attached Hosts is also extremely significant.  That is, if     all you're doing is letting some terminals access some different     Hosts, but the Hosts don't really have any intercomputer     networking protocols between them, what you have should be viewed     as a Localized Communications Network (LCN), not a LAN in the     sense we're talking about here.          TCP               The third thing we have to know can be either     straightforward or subtle, depending largely on how aware we are     of the context estabished by ARPANET-style prococols:  For the     visual-minded, Figure 1 and Figure 2 might be all that need be     "said."  Their moral is meant to be that in ARPANET-style     layering, layers aren't monoliths.  For those who need more     explanation, here goes:  TCP [3] (we'll take IP later) is a     Host-Host protocol (roughly equivalent to the functionality     implied by some of ISO Level 5 and all of ISO Level 4).  Its most     significant property is that it presents reliable logical     connections to protocols above itself.  (This point will be     returned to subsequently.)  Its next most significant property is     that it is designed to operate in a "catenet" (also known as the,     or an, "internet"); that is, its addressing discipline is such     that Hosts attached to communications subnets other than the one     a given Host is attached to (the "proximate net") can be     communicated with as well as Hosts on the proximate net.  Other     significant properties are those common to the breed:  Host-Host     protocols (and Transport protocols) "all" offer mechanisms for     flow Control, Out-of-Band Signals, Logical Connection management,     and the like.               Because TCP has a catenet-oriented addressing mechanism     (that is, it expresses foreign Host addresses as the     "two-dimensional" entity Foreign Net/Foreign Host because it     cannot assume that the Foreign Host is attached to the proximate     net), to be a full Host-Host protocol it needs an adjunct to deal     with the proximate net.  This adjunct, the Internet Protocol (IP)     was designed as a separate protocol from TCP, however, in order     to allow it to play the same role it plays for TCP for other     Host-Host protocols too.                                                         3

RFC 872                                            September 1982                    In order to "deal with the proximate net", IP possess the     following significant properties:  An IP implementation maps from     a virtualization (or common intermediate representation) of     generic proximate net qualities (such as precedence, grade of     service, security labeling) to the closest equivalent on the     proximate net. It determines whether the "Internet Address" of a     given transmission is on the proximate net or not; if so, it     sends it; if not, it sends it to a "Gateway" (where another IP     module resides).  That is, IP handles internet routing, whereas     TCP (or some other Host-Host  protocol) handles only internet     addressing.  Because some proximate nets will accept smaller     transmissions ("packets") than others, IP, qua protocol, also has     a discipline for allowing packets to be fragmented while in the     catenet and reassembled at their destination.  Finally (for our     purposes), IP offers a mechanism to allow the particular protocol     it was called by (for a given packet) to be identified so that     the receiver can demultiplex transmissions based on IP-level     information only. (This is in accordance with the Principle of     Layering:  you don't want to have to look at the data IP is     conveying to find out what to do with it.)               Now that all seems rather complex, even though it omits a     number of mechanisms.  (For a more complete discussion, see     Reference [4].)  But it should be just about enough to slay the     Woozle, especially if just one more protocol's most significant     property can be snuck in.  An underpublicized member of the     ARPANET suite of protocols is called UDP--the "User Datagram     Protocol."  UDP is designed for speed rather than accuracy.  That     is, it's not "reliable."  All there is to UDP, basically, is a     mechanism to allow a given packet to be associated with a given     logical connection. Not a TCP logical connection, mind you, but a     UDP logical connection.  So if all you want is the ability to     demultiplex data streams from your Host-Host protocol, you use     UDP, not TCP.  ("You" is usually supposed to be a Packetized     Speech protocol, but doesn't have to be.)  (And we'll worry about     Flow Control some other time.)          TCP-on-a-LAN               So whether you're a Host proximate to a LAN or not, and even     whether your TCP/IP is "inboard" or "outboard" of you, if you're     talking to a Host somewhere out there on the catenet, you use IP;     and if you're exercising some process-level/applications protocol     (roughly equivalent to some of some versions of ISO L5 and all of     L6 and L7) that expects TCP/IP as its Host-Host protocol (because     it "wants" reliable, flow controlled, ordered delivery [whoops,     forgot that "ordered" property earlier--but it doesn't matter all     that much for present purposes] over logical connections which     allow it to be                                                         4

RFC 872                                            September 1982               addressed via a Well-Known Socket), you use TCP "above" IP     regardless of whether the other Host is on your proximate net or     not.  But if your application doesn't require the properties of     TCP (say for Packetized Speech), don't use it--regardless of     where or what you are.  And if you want to make the decision     about whether you're talking to a proximate Host explicitly and     not even go through IP, you can even arrange to do that (though     it might make for messy implementation under some circumstances).     That is, if you want to take advantage of the properties of your     LAN "in the raw" and have or don't need appropriate applications     protocols, the Reference Model to which TCP/IP were designed     won't stop you.  See Figure 2 if you're visual.  A word of     caution, though:  those applications probably will need protocols     of some sort--and they'll probably need some sort of Host-Host     protocol under them, so unless you relish maintaining "parallel"     suites of protocols....  that is, you really would be better off     with TCP most of the time locally anyway, because you've got to     have it to talk to the catenet and it's a nuisance to have     "something else" to talk over the LAN--when, of course, what     you're talking requires a Host-Host protocol.               We'll touch on "performance" issues in a bit more detail     later. At this level, though, one point really does need to be     made:  On the "reliability" front, many (including the author) at     first blush take the TCP checksum to be "overkill" for use on a     LAN, which does, after all, typically present extremely good     error properties. Interestingly enough, however, metering of TCP     implementations on several Host types in the research community     shows that the processing time expended on the TCP checksum is     only around 12% of the per-transmission processing time anyway.     So, again, it's not clear that it's worthwhile to bother with an     alternate Host-Host protocol for local use (if, that is, you need     the rest of the properties of TCP other than "reliability"--and,     of course, always assuming you've got a LAN, not an LCN, as     distinguished earlier.)               Take that, Woozle!          Other Significant Properties               Oh, by the way, one or two other properties of TCP/IP really     do bear mention:               1.   Protocol interpreters for TCP/IP exist for a dozen or               two different operating systems.               2.   TCP/IP work, and have been working (though in less               refined versions) for several years.                                                              5

RFC 872                                            September 1982                    3.   IP levies no constraints on the interface protocol               presented by the proximate net (though some protocols               at that level are more wasteful than others).               4.   IP levies no constraints on its users; in particular,               any proximate net that offers alternate routing can be               taken advantage of (unlike X.25, which appears to               preclude alternate routing).               5.   IP-bearing Gateways both exist and present and exploit               properties 3 and 4.               6.   TCP/IP are Department of Defense Standards.               7.   Process (or application) protocols compatible with               TCP/IP for Virtual Terminal and File Transfer               (including "electronic mail") exist and have been               implemented on numerous operating systems.               8.   "Vendor-style" specifications of TCP/IP are being               prepared under the aegis of the DoD Protocol Standards               Technical Panel, for those who find the               research-community-provided specs not to their liking.               9.   The research community has recently reported speeds in               excess of 300 kb/s on an 800 kb/s subnet, 1.2 Mb/s on a               3 Mb/s subnet, and 9.2 kbs on a 9.6 kb/s phone               line--all using TCP.  (We don't know of any numbers for               alternative protocol suites, but it's unlikely they'd               be appreciably better if they confer like               functionality--and they may well be worse if they               represent implementations which haven't been around               enough to have been iterated a time or three.)               With the partial exception of property 8, no other     resource-sharing protocol suite can make those claims.               Note particularly well that none of the above should be     construed as eliminating the need for extremely careful     measurement of TCP/IP performance in/on a LAN.  (You do, after     all, want to know their limitations, to guide you in when to     bother ringing in "local" alternatives--but be very careful:  1.     they're hard to measure commensurately with alternative     protocols; and 2.  most conventional Hosts can't take [or give]     as many bits per second as you might imagine.)  It merely     dramatically refocuses the motivation for doing such measurement.     (And levies a constraint or two on how you outboard, if you're     outboarding.)                                                              6

RFC 872                                            September 1982               Other Contextual Data               Our case could really rest here, but some amplification of     the aside above about Host capacities is warranted, if only to     suggest that some quantification is available to supplement the a     priori argument:  Consider the previously mentioned PDSC.  Its     local terminals operate in a screen-at-a-time mode, each     screen-load comprising some 16 kb.  How many screens can one of     its Hosts handle in a given second?  Well, we're told that each     disk fetch requires 17 ms average latency, and each context     switch costs around 2 ms, so allowing 1 ms for transmission of     the data from the disk and to the "net" (it makes the arithmetic     easy), that would add up to 20 ms "processing" time per screen,     even if no processing were done to the disk image.  Thus, even if     the Host were doing nothing else, and  even if the native disk     I/O software were optimized to do 16 kb reads, it could only     present 50 screens to its communications mechanism     (processor-processor bus) per second.  That's 800 kb/s. And     that's well within the range of TCP-achievable rates (cf.  Other     Significant Property 9).  So in a realistic sample environment,     it would certainly seem that typical Hosts can't necessarily     present so many bits as to overtax the protocols anyway.  (The     analysis of how many bits typical Hosts can accept is more     difficult because it depends more heavily on system internals.     However, the point is nearly moot in that even in the intuitively     unlikely event that receiving were appreciably faster in     principle [unlikely because of typical operating system     constraints on address space sizes, the need to do input to a     single address space, and the need to share buffers in the     address space among several processes], you can't accept more     than you can be given.)          Conclusion               The sometimes-expressed fear that using TCP on a local net     is a bad idea is unfounded.          References          [1]  Milne, A. A., "Winnie-the-Pooh", various publishers.          [2]  The LAN description is based on Clark, D. D.  et al., "An          Introduction to Local Area Networks,"  IEEE Proc., V. 66, N.          11, November 1978, pp. 1497-1517, several year's worth of          conversations with Dr. Clark, and the author's observations          of both the open literature and the Oral Tradition (which          were sufficiently well-thought of to have prompted The MITRE          Corporation/NBS/NSA Local Nets "Brain Picking Panel" to have                                                              7

RFC 872                                            September 1982                    solicited his testimony during the year he was in FACC's          employ.*)          [3]  The TCP/IP descriptions are based on Postel, J. B.,          "Internet Protocol Specification," and "Transmission Control          Specification" in DARPA Internet Program Protocol          Specifications, USC Information Sciences Institute,          September, 1981, and on more than 10 years' worth of          conversations with Dr. Postel, Dr. Clark (now the DARPA          "Internet Architect") and Dr. Vinton G. Cerf (co-originator          of TCP), and on numerous discussions with several other          members of the TCP/IP design team, on having edited the          referenced documents for the PSTP, and, for that matter, on          having been one of the developers of the ARPANET "Reference          Model."          [4]  Padlipsky, M. A., "A Perspective on the ARPANET Reference          Model", M82-47, The MITRE Corporation, September 1982; also          available in Proc. INFOCOM '83.          ________________     *  In all honesty, as far as I know I started the rumor that TCP        might be overkill for a LAN at that meeting.  At the next TCP        design meeting, however, they separated IP out from TCP, and        everything's been alright for about three years now--except        for getting the rumor killed.  (I'd worry about Woozles        turning into roosting chickens if it weren't for the facts        that:  1.  People tend to ignore their local guru; 2.  I was        trying to encourage the IP separation; and 3.  All I ever        wanted was some empirical data.)          NOTE:  FIGURE 1. ARM in the Abstract, and FIGURE 2.  ARMS,        Somewhat Particularized, may be obtained by writing to:  Mike        Padlipsky, MITRE Corporation, P.O. Box 208, Bedford,        Massachusetts, 01730, or sending computer mail to        Padlipsky@USC-ISIA.                                                                                                                          8

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