RFC 872 September 1982
M82-48
TCP-ON-A-LAN
M.A. PADLIPSKY
THE MITRE CORPORATION
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.
"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.
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.
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.
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
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.
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.)
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
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.
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