In other words, the the main goal was to enable a useful interconne...
These are the protocols defined by the International Organization f...
The Internet ended up connecting a number of already existing netwo...
It’s important to underline geo-political landscape of the decades ...
This paragraph provides a great description of the fundamental arch...
The top three goals on this list ended up having the most significa...
At the Future Internet Design project (2008), David Clark, the auth...
X.25 is a protocol suite for packet switched networks. It is one of...
The easiest way to support a wide variety of networks is by making ...
Security and trust are clearly not one of the big goals, but it’s i...
Am I correct to assume that this is the “grave misunderstanding” me...
Early TCP implementations faced a number of issues such has poor re...
ACM SIGCOMM -1- Computer Communication Review
The Design Philosophy of the DARPA Internet Protocols
David D. Clark
*
Massachusetts Institute of Technology
Laboratory f or Computer Science
Cambridge, MA. 02139
(Originally published in Pr oc. SIGCO M M ‘88, Com puter Comm unicat ion Review Vol. 18, No. 4,
August 1988, pp. 106–114)
*
Abstract
The Internet protocol suite, TCP/IP, was first proposed
fifteen years ago. It was developed by the Defense
Advanced Research Projects Agency (DARPA), and
has been used widely in military and commercial
systems. While there have been papers and
specifications that describe how the protocols work, it is
sometimes difficult to deduce from these why the
protocol is as it is. For example, the Internet protocol is
based on a connectionless or datagram mode of service.
The motivation for this has been greatly misunderstood.
This paper attempts to capture some of the early
reasoning which shaped the Internet protocols.
1. Introduction
For the last 15 years
1
, the Advanced Research Proj ects
Agency of the U.S. Department of Defense has been
developing a suite of protocols for packet switched
networking. These protocols, which include the Internet
Protocol (IP), and the Transmission Control Protocol
(TCP), are now U.S. Department of Defense standards
for internetworking, and are in wide use in the
commercial networking environment. The ideas
developed in this effort have also influenced other
protocol suites, most importantly the connectionless
configuration of the ISO protocols
2
,
3
,
4
.
While specific information on the DOD protocols is
fairly generally available
5
,
6
,
7
, it is sometimes difficult to
determine the motivation and reasoning which led to the
design.
In fact, the design philosophy has evolved considerably
from the first proposal to the current standards. For
example, the idea of the datagram, or connectionless
service, does not receive particular emphasis in the first
paper, but has come to be the defining characteristic of
the protocol. Another example is the layering of the
architecture into the IP and TCP layers. This seems
basic to the design, but was also not a part of the
original proposal. These changes in the Internet design
arose through the repeated pattern of implementation
and testing that occurred before the standards were set.
The Internet architecture is still evolving. Sometimes a
new extension challenges one of the design principles,
but in any case an understanding of the history of the
design provides a necessary context for current design
extensions. The connectionless configuration of ISO
protocols has also been colored by the history of the
Internet suite, so an understanding of the Internet design
philosophy may be helpful to those working with ISO.
This paper catalogs one view of the original objectives
of the Internet architecture, and discusses the relation
between these goals and the important features of the
protocols.
2. Fundamental Goal
The top level goal for the DARPA Internet Architecture
was to develop an effective technique for multiplexed
utilization of existing interconnected networks. Some
elaboration is appropriate to make clear the meaning of
that goal.
The components of the Internet were networks, which
were to be interconnected to provide some larger
service. The original goal was to connect together the
original ARPANET
8
with the ARPA packet radio
network
9
,
10
, in order to give users on the packet radio
network access to the large service machines on the
ARPANET. At the time it was assumed that there would
be other sorts of networks to interconnect, although the
local area network had not yet emerged.
An alternative to interconnecting existing networks
would have been to design a unified system which
incorporated a variety of different transmission media, a
ACM SIGCOMM -2- Computer Communication Review
multi-media networ k. While this might have permitted a
higher degree of integration, and thus better
performance, it was felt that it was necessary to
incorporate the then existing network architectures if
Internet was to be useful in a practical sense. Further,
networks represent administrative boundaries of
control, and it was an ambition of this project to come
to grips with the problem of integrating a number of
separately administrated entities into a common utility.
The technique selected for multiplexing was packet
switching. An alternative such as circuit switching could
have been considered, but the applications being
supported, such as remote login, were naturally served
by the packet switching paradigm, and the networks
which were to be integrated together in this proj ect were
packet switching networks. So packet switching was
accepted as a fundamental component of the Internet
architecture.
The final aspect of this fundamental goal was the
assumption of the particular technique for inter-
connecting these networks. Since the technique of store
and forward packet switching, as demonstrated in the
previous DARPA project, the ARPANET, was well
understood, the top level assumption was that networks
would be interconnected by a layer of Internet packet
switches, which were called gateways.
From these assumptions comes the fundamental
structure of the Internet: a packet switched communica-
tions facility in which a number of distinguishable
networks are connected together using packet communi-
cations processors called gateways which implement a
store and forward packet forwarding algorithm.
3. Second Level Goals
The top level goal stated in the previous section
contains the word "effective," without offering any
definition of what an effective interconnection must
achieve. The following list summarizes a more detailed
set of goals which were established for the Internet
architecture.
1. Internet communication must continue despite loss
of networks or gateways.
2. The Internet must support multiple types of
communications service.
3. The Internet architecture must accommodate a
variet y of networks.
4. The Internet architecture must permit distributed
management of its resources.
5. The Internet architecture must be cost effective.
6. The Internet architecture must permit host
attachment with a low level of effort.
7. The resources used in the internet architecture must
be accountable.
This set of goals might seem to be nothing more than a
checklist of all the desirable network features. It is
important to understand that these goals are in order of
importance, and an entirely different network
architecture would result if the order were changed. For
example, since this network was designed to operate in
a military context, which implied the possibility of a
hostile environment, survivability was put as a first
goal, and accountability as a last goal. During wartime,
one is less concerned with detailed accounting of
resources used than with mustering whatever resources
are available and rapidly deploying them in an
operational manner. While the architects of the Internet
were mindful of accountability, the problem received
very little attention during the early stages of the design,
and is only now being considered. An architecture
primarily for commercial deployment would clearly
place these goals at the opposite end of the list.
Similarly, the goal that the architecture be cost effective
is clearly on the list, but below certain other goals, such
as distributed management, or support of a wide variety
of networks. Other protocol suites, including some of
the more popular commercial architectures, have been
optimized to a particular kind of network, for example a
long haul store and forward network built of medium
speed telephone lines, and deliver a very cost effective
solution in this context, in exchange for dealing
somewhat poorly with other kinds of nets, such as local
area nets.
The reader should consider carefully the above list of
goals, and recognize that this is not a " motherhood" list,
but a set of priorities which strongly colored the design
decisions within the Internet architecture. T he following
sections discuss the relationship between this list and
the features of the Internet.
4. Survivability in the Face of Failure
The most important goal on the list is that the Internet
should continue to supply communications service, even
though networks and gateways are failing. In p articular,
this goal was interpreted to mean that if two entities are
communicating over the Internet, and some failure
causes the Internet to be temporarily disrupted and
reconfigured to reconstitute the service, then the entities
communicating should be able to continue without
having to reestablish or reset the high level state o f their
conversation. More concretely, at the service interface
of the transport layer, this architecture provides no
ACM SIGCOMM -3- Computer Communication Review
facility to communicate to the client of the transport
service that the synchronization between the sender and
the receiver may have been lost. It was an assumption in
this architecture that synchronization would never be
lost unless there was no physical path over which any
sort of communication could be achieved. In other
words, at the top of transport, there is only one failure,
and it is total partition. The architecture was to mask
completely any transient failure.
To achieve this goal, the state information which
describes the on-going conversation must be protected.
Specific examples of state information would be the
number of packets transmitted, the number of packets
acknowledged, or the number of outstanding flow
control permissions. If the lower layers of the archi-
tecture lose this information, they will not be able to tell
if data has been lost, and the app lication layer will have
to cope with the loss of synchrony. This architecture
insisted that this disruptio n not occur, which meant that
the state information must be protected from loss.
In some network architectures, this state is stored in the
intermediate packet switching nodes of the network. In
this case, to protect the information from loss, it must
replicated. Because of the distributed nature of the
replication, algorithms to ensure robust replication are
themselves difficult to build, and few networks with
distributed state information provide any sort of
protection against failure. The alternative, which this
architecture chose, is to take this information and gather
it at the endpoint of the net, at the entity which is
utilizing the service of the network. I call this approach
to reliability "fate-sharing." The fate-sharing model
suggests that it is acceptable to lose the state
information associated with an entity if, at the same
time, the entity itself is lost. Specifically, information
about transport level synchronization is stored in the
host which is attached to the net and using its
communication service.
There are two important advantages to fate-sharing over
replication. First, fate-sharing protects against any
number of intermediate failures, whereas replication can
only protect against a certain number (less than the
number of replicated copies). Second, fate-sharing is
much easier to engineer than replication.
There are two consequences to the fate-sharing
approach to survivability. First, the intermediate packet
switching nodes, or gateways, must not have any
essential state information about on-going connections.
Instead, they are stateless packet switches, a class of
network design sometimes called a " d atagr am" network.
Secondly, rather more trust is placed in the host
machine than in an architecture where the network
ensures the reliable delivery of data. If the host resident
algorithms that ensure the sequencing and
acknowledgment of data fail, applications on that
machine are prevented from operation.
Despite the fact that survivability is the first goal in the
list, it is still second to the top level goal of
interconnection of existing networks. A more survivable
technology might have resulted from a single multi-
media network design. For example, the Internet makes
very weak assumptions about the ability of a network to
report that it has failed. Internet is thus forced to detect
network failures using Internet level mechanisms, with
the potential for a slower and less specific error
detection.
5. Types of Service
The second goal of the Internet architecture is that it
should support, at the transport service level, a variety
of types of service. Different types of service are
distinguished by differing requirements for such things
as speed, latency and reliability. The traditional type of
service is the bi-directional reliable delivery of data.
This service, which is sometimes called a "virtual
circuit" service, is appropriate for such applications as
remote login or file transfer. It was the first service
provided in the Internet architecture, using the
Transmission Control Protocol (TCP)
11
. It was early
recognized that even this service had multiple variants,
because remote login required a service with low delay
in delivery, but low requirements for bandwidth, while
file transfer was less concerned with delay, but very
concerned with high throughput. TCP attempted to
provide both these types of service.
The initial concept of TCP was that it could be general
enough to support any needed type of service. However,
as the full range of needed services became clear, it
seemed too difficult to build support for all of them into
one protocol.
The first example of a service outside the range of TCP
was support for XNET
12
, the cross-Internet debugger.
TCP did not seem a suitable transport for XNET for
several reasons. First, a debugger protocol should not
be reliable. This conclusion may seem odd, but under
conditions of stress or failure (which may be exactly
when a debugger is needed) asking for reliable
communications may prevent any communications at
all. It is much better to build a service which can deal
with whatever gets through, rather than insisting that
every byte sent be delivered in order. Second, if TCP is
general enough to deal with a broad range of clients, it
is presumably somewhat complex. Again, it seemed
wrong to expect support for this complexity in a
debugging environment, which may lack even basic
ACM SIGCOMM -4- Computer Communication Review
services expected in an operating system (e.g. support
for timers.) So XNET was designed to run directly on
top of the datagram service provided by Internet.
Another service which did not fit TCP was real time
delivery of digitized speech, which was needed to
support the teleconferencing aspect of command and
control applications. In real time digital speech, the
primary requirement is not a reliable service, but a
service which minimizes and smoothes the delay in the
delivery of packets. The application layer is digitizing
the analog speech, packetizing the resulting bits, and
sending them out ac ross the network on a regular b asis.
They must arrive at the receiver at a regular basis in
order to be converted back to the analog signal. If
packets do not arrive when expected, it is impossible to
reassemble the signal in real time. A surprising
observation about the control of variation in delay is
that the most serious source of delay in networks is the
mechanism to provide reliable delivery. A typical
reliable transport protocol responds to a missing packet
by requesting a retransmission and delaying the delivery
of any subseq uent packets until the lost packet has been
retransmitted. It then delivers that packet and all
remaining ones in sequence. The delay while this occurs
can be many times the round trip delivery time of the
net, and may completely disrupt the speech reassembly
algorithm. In contrast, it is very easy to cope with an
occasional missing packet. The missing speech can
simply be replaced by a short period of silence, which
in most cases does not impair the intelligibility of the
speech to the listening human. If it does, high level error
correction can occur, and the listener can ask the
speaker to repeat the damaged phrase.
It was thus decided, fairly early in the development of
the Internet architecture, that more than one transport
service would be required, and the architecture must be
prepared to tolerate simultaneously transports which
wish to constrain reliability, delay, or bandwidth, at a
minimum.
This goal caused TCP and IP, which originally had been
a single protocol in the architecture, to be separated into
two layers. TCP provided one particular type of service,
the reliable sequenced data stream, while IP attempted
to provide a basic building block out of which a variety
of types of service could be built. This building block
was the datagram, which had also been adopted to
support survivability. Since the reliability associated
with the delivery of a datagram was not guaranteed, but
"best effort," it was possible to build out of the
datagram a service that was reliable (by acknowledging
and retransmitting at a higher level), or a service which
traded reliability for the primitive delay characteristics
of the underlying network substrate. The User Datagram
Protocol (UDP)
13
was created to provide a application-
level interface to the basic datagram service of Internet.
The architecture did not wish to assume that the
underlying networks themselves support multiple types
of services, because this would violate the goal of using
existing networks. Instead, the hope was that multiple
types of service could be constructed out of the basic
datagram building block using algorithms within the
host and the gateway. For example , (altho ugh this is no t
done in most current implementations) it is possible to
take datagrams which are associated with a controlled
delay but unreliable service and place them at the head
of the transmission queues unless their lifetime has
expired, in which case they would be discarded; while
packets associated with reliable streams would be
placed at the back of the queues, but never discarded,
no matter how long they had been in the net.
It proved more difficult than first hoped to provide
multiple types of service without explicit support from
the underlying networks. The most seri ous prob lem was
that networks designed with one particular type of
service in mind were not flexible enough to support
other services. Most commonly, a network will have
been designed under the assumption that it should
deliver reliable service, and will inject delays as a part
of producing reliable service, whether or not this
reliability is desired. The interface behavior defined by
X.25, for example, implies reliable delivery, and there
is no way to turn this feature off. Therefore, although
Internet operates successfully over X.25 networks it
cannot deliver the desired variability of type service in
that context. Other networks which have an intrinsic
datagram service are much more flexible in the type of
service they will permit, but these networks are much
less common, especially in the long-haul context.
6. Varieties of Networks
It was very important for the success of the Internet
architecture that it be able to incorporate and utilize a
wide variety of network technolo gies, includ ing military
and commercial facilities. The Internet architecture has
been very successful in meeting this goal; it is o perated
over a wide variety of networks, including long haul
nets (the ARPANET itself and various X.25 networks),
local area nets (Ethernet, ringnet, etc.), broadcast
satellite nets (the DARPA Atlantic Satellite
Network
14
,
15
operating at 64 kilobits per second and the
DARPA Experimental Wideband Satellite Net,
16
operating within the United States at 3 megabits per
second), packet radio networks (the DARPA packet
radio network, as well as an experimental Br itish packet
radio net and a network developed by amateur radio
operators), a variety of serial links, ranging from 1200
ACM SIGCOMM -5- Computer Communication Review
bit per second asynchronous connections to T1 links,
and a variety of other ad hoc facilities, including
intercomputer busses and the transport service provided
by the higher layers of other network suites, such as
IBM’s HASP.
The Internet architecture achieves this flexibility by
making a minimum set of assumptions about the
function which the net will provide. The basic
assumption is that network can transport a packet or
datagram. The packet must be of reasonable size,
perhaps 100 bytes minimum, and should be delivered
with reasonable but not perfect reliability. The network
must have some suitable form of addressing if it is more
than a poi nt to point link.
There are a number of services which are explicitly not
assumed from the network. These include reliable or
sequenced delivery, network level broadcast or
multicast, priority ranking of transmitted packet,
support for multiple types of service, and internal
knowledge of failures, speeds, or delays. If these
services had been required, then in order to
accommodate a network within the Internet, it would be
necessary either that the network support these services
directly, or that the network interface software provide
enhancements to simulate these services at the endpoint
of the network. It was felt that this was an undesirable
approach, because these services would have to be re-
engineered and reimplemented for every single network
and every single host interface to every network. By
engineering these services at the transport, for example
reliable delivery via TCP, the engineering must be done
only once, and the implementation must be done only
once for each host. After that, the implementation of
interface software for a new network is usually very
simple.
7. Other Goals
The three goals discussed so far were those which had
the most profound impact on the design on the
architecture. The remaining goals, because they were
lower in importance, were perhaps less effectively met,
or not so completely engineered. The goal of permitting
distributed management of the Internet has certainly
been met in certain respects. For example, not all of the
gateways in the Internet are implemented and managed
by the same agency. There are several different
management centers within the deployed Internet, each
operating a subset of the gateways, and there is a two-
tiered routing algorithm which permits gateways from
different administrations to exchange routing tables,
even though they do not completely trust each other,
and a variety of private routing algorithms used among
the gateways in a single administration. Similarly, the
various organizations which manage the gateways are
not necessarily the same organizations that manage the
networks to which the gateways are attached.
On the other hand, some of the most significant
problems with the Internet today relate to lack of
sufficient tools for distributed management, especially
in the area of routing. In the large internet being
currently operated, routing decisions need to be
constrained by policies for resource usage. Today this
can be done only in a very limited way, which requires
manual setting of tables. This is error-prone and at the
same time not sufficiently powerful. T he most important
change in the Internet architecture over the next few
years will probably be the development of a new
generation of tools for management of resources in the
context of multiple administrations.
It is clear that in certain circumstances, the Internet
architecture does not produce as cost effective a
utilization of expensive communication resources as a
more tailored architecture would. The headers of
Internet packets are fairly long (a typical header is 40
bytes), and if short packets are sent, this overhead is
apparent. The worse case, of course, is the single
character remote login packets, which carry 40 bytes of
header and one byte of data. Actually, it is very difficult
for any protocol suite to claim that these sorts of
interchanges are carried out with reasonable efficiency.
At the other extreme, large packets for file transfer, with
perhaps 1,000 bytes of data, have an overhead for the
header of only four percent.
Another possible source of inefficiency is
retransmission of lost packets. Since Internet does not
insist that lost packets be recovered at the network
level, it may be necessary to retransmit a lost packet
from one end of the Internet to the other. This means
that the retransmitted packet may cross several
intervening nets a second time, whereas recovery at the
network level would not generate this repeat traffic.
This is an example of the tradeoff resulting from the
decision, discussed above, of providing services from
the end-points. The network interface code is much
simpler, but the overall efficiency is potentially less.
However, if the retransmission rate is low enough (for
example, 1%) then the incremental cost is tolerable. As
a rough rule of thumb for networks incorporated into
the architecture, a loss of one packet in a hundred is
quite rea sonable, but a loss of one packet in ten suggests
that reliability enhancements be added to the network if
that type of service is required.
The cost of attaching a host to the Internet is perhaps
somewhat higher than in other architectures, because all
of the mechanisms to provide the desired types of
service, such as acknowledgments and retransmission
ACM SIGCOMM -6- Computer Communication Review
strategies, must be implemented in the host rather than
in the network. Initially, to programmers who were not
familiar with protocol implementation, the effort of
doing this seemed somewhat daunting. Implementors
tried such things as moving the transport protocols to a
front end processor, with the idea that the protocols
would be implemented only once, rather than again for
every type of host. However, this required the invention
of a host to front end protocol which some thought
almost as complicated to implement as the original
transport protocol. As experience with protocols
increases, the anxieties associated with implementing a
protocol suite within the host seem to be decreasing,
and implementations are now available for a wide
variety of machines, including personal computers and
other machines with very limited computing resources.
A related problem arising from the use of host-resident
mechanisms is that poor implementation of the
mechanism may hurt the network as well as the host.
This problem was tolerated, because the initial
experiments involved a limited number of host
implementations which could be controlled. However,
as the use of Internet has grown, this problem has
occasionally surfaced in a serious way. In this respect,
the goal of robustness, which led to the method of fate-
sharing, which led to host-resident algorithms, contri-
butes to a loss o f robustness if the host mis-behaves.
The last goal was accountability. In fact, accounting
was discussed in the first paper by Cerf and Kahn as an
important function of the protocols and gateways.
However, at the present time, the Internet architecture
contains few tools for accounting for packet flows. This
problem is only now being studied, as the scope of the
architecture is being expanded to include non-military
consumers who are seriously concerned with under-
standing and monitoring the usage of the resources
within the internet.
8. Architecture and Implementation
The p revious discussio n clearly suggests that one o f the
goals of the Internet architecture was to provide wide
flexibility in the service offered. Different transport
protocols could be used to provide different types of
service, and different networks could be incorporated.
Put another way, the architecture tried very hard not to
constrain the range of service which the Internet could
be engineered to provide. This, in turn, means that to
understand the service which can be offered by a
particular implementation of an Internet, one must look
not to the architecture, but to the actual engineering of
the software within the particular hosts and gateways,
and to the particular networks which have been
incorporated. I will use the term "realization" to
describe a particular set of networks, gateways and
hosts which have been connected together in the context
of the Internet architecture. Realizations can differ by
orders of magnitude in the service which they offer.
Realizations have been built out of 1200 bit per second
phone lines, and out of networks only with speeds
greater than 1 megabit per second. Clearly, the
throughput expectations which one can have of these
realizations differ by orders of magnitude. Similarly,
some Internet realizations have delays measured in tens
of milliseconds, where others have delays measured in
seconds. Certain applications such as real time speech
work fundamentally differently across these two
realizations. Some Internets have been engineered so
that there is great redundancy in the gateways and paths.
These Internets are survivable, because resources exist
which can be reconfigured after failure. Other Internet
realizations, to reduce cost, have single points of
connectivity through the realization, so that a failure
may partitio n the Internet into two halves.
The Internet architecture tolerates this variety of
realization by design. However, it leaves the designer of
a particular realization with a great deal of engineering
to do. One of the major struggles of this architectural
develo p ment was to understa nd ho w to give guid anc e to
the designer of a realization, guidance which would
relate the engineering of the realization to the types of
service which would result. For example, the designer
must answer the following sort of question. What sort of
bandwidths must be in the underlying networks, if the
overall service is to deliver a throughput of a certain
rate? Given a certain model of possible failures within
this realization, what sorts of redundancy ought to be
engineered into the realization?
Most of the known network design aids did not seem
helpful in answering these sorts of questions. Protocol
verifiers, for example, assist in confirming that
protocols meet specifications. However, these tools
almost never deal with performance issues, which are
essential to the idea of the type of service. Instead, they
deal with the much more restricted idea of logical
correctness of the protocol with respect to specification.
While tools to verify logical correctness are useful, both
at the specification and implementation stage, they do
not help with the severe problems that often arise
related to performance. A typical implementation
experience is that even after logical correctness has
been demonstrated, design faults are discovered that
may cause a performance degradation of an order of
magnitude. Exploration of this problem has led to the
conclusion that the difficulty usually arises, not in the
protocol itself, but in the operating system on which the
protocol runs. This being the case, it is difficult to
address the problem within the context of the
ACM SIGCOMM -7- Computer Communication Review
architectural specification. However, we still strongly
feel the need to give the implementor guidance. We
continue to struggle with this proble m today.
The other class of design aid is the simulator, which
takes a particular realization and explores the service
which it can deliver under a variety of loadings. No one
has yet attempted to construct a simulator which take
into account the wide variability of the gateway
implementation, the host implementation, and the
network performance which one sees within possible
Internet realizations. It is thus the case that the analysis
of most Internet realizations is done on the back of an
envelope. It is a comment on the goal structure of the
Internet architecture that a back of the envelope
analysis, if done by a sufficiently knowledgeable
person, is usually sufficient. The designer of a particular
Internet realization is usually less concerned with
obtaining the last five percent po ssible in line utilizatio n
than knowing whether the desired type of service can be
achieved at all given the resources at hand at the
moment.
The relationship between architecture and performance
is an extremely challenging one. The designers of the
Internet architecture felt very strongly that it was a
serious mistake to attend only to logical correctness and
ignore the issue of performance. However, they
experienced great difficulty in formalizing any aspect of
performance constraint within the architecture. These
difficulties arose both because the goal of the
architecture was not to constrain performance, but to
permit variability, and secondly (and perhaps more
fundamentally), because there seemed to be no useful
formal tools for describing performance.
This problem was particularly aggravating because the
goal of the Internet project was to produce specification
documents which were to become military standards. It
is a well known problem with government contracting
that one cannot expect a contractor to meet any criteria
which is not a part of the procurement standard. If the
Internet is concerned about performance, therefore, it
was mandatory that performance requirements be put
into the procurement specification. It was trivial to
invent specifications which constrained the perform-
ance, for example to specify that the implementation
must be capable of passing 1,000 packets a second.
However, this sor t of constrai nt could not b e part o f the
architecture, and it was therefore up to the individual
performing the procurement to recognize that these
performance constraints must be added to the specifica-
tion, and to specify them properly to achieve a
realization which provides the required types of service.
We do not have a good idea how to offer guidance in
the architecture for the person performing this task.
9. Datagrams
The fundamental architectural feature of the Internet is
the use of datagrams as the entity which is transported
across the underlying networks. As this paper has
suggested, there are several reasons why datagrams are
important within the architecture. First, they eliminate
the need for connection state within the intermediate
switching nodes, which means that the Internet can be
reconstituted after a failure without concern about state.
Secondly, the datagram provides a basic building block
out of which a variety of types of service can be
implemented. In contrast to the virtual circuit, which
usually implies a fixed type of service, the datagram
provides a more elemental service which the endpoints
can combine as appropriate to build the type of service
needed. Third, the datagram represents the minimum
network service assumption, which has permitted a wide
variety of networks to be incorporated into various
Internet realizations. The decision to use the datagram
was an extremely successful one, which allowed the
Internet to meet its most important goals very
successfully.
There is a mistaken assumption often associated with
datagrams, which is that the motivation for datagrams is
the support of a higher level service which is essentially
equivalent to the datagram. In other words, it has
sometimes been suggested tha t the d atagr am is pr o vided
because the transport service which the application
requires is a datagram service. In fact, this is seldom the
case. While some applications in the Internet, such as
simple queries of date servers or name servers, use an
access method based on an unreliable datagram, most
services within the Internet would like a more
sophisticated transport model than simple datagram.
Some services would like the reliability enhanced, some
would like the delay smoothed and buffered, but almost
all have some expectation more complex than a
datagram. It is important to understand that the role of
the datagram in this respect is as a building block, and
not as a service in itself.
10. TCP
There were several interesting and controversial design
decisions in the development of TCP, and TCP itself
went through several major versions before it became a
reasonably stable standard. Some of these design
decisions, such as window management and the nature
of the port address structure, are discussed in a series of
implementation notes published as part of the TCP
protocol handbook.
17
,
18
But again the motivation for the
decision is sometimes lacking. In this section, I attempt
to capture some of the early reasoning that went into
parts of TCP. This section is of necessity incomplete; a
ACM SIGCOMM -8- Computer Communication Review
complete review of the history of TCP itself would
require another paper of this length.
The original ARPANET host-to host protocol provided
flow control based on both bytes and packets. This
seemed overly complex, and the designers of TCP felt
that only one form of regulation would be sufficient.
The choice was to regulate the delivery of bytes, rather
than packets. Flow control and acknowledgment in TCP
is thus based on byte number rather than packet number.
Indeed, in TCP there is no significance to the
packetization of the data.
This decision was motivated by several considerations,
some of which became irrelevant and others of which
were more important that anticipated. One reason to
acknowledge bytes was to permit the insertion of
control information into the sequence space of the
bytes, so that control as well as data could be
acknowledged. That use of the sequence space was
dropped, in favor of ad hoc techniques for dealing with
each control message. While the original idea has
appealing generality, it caused complexity in practice.
A second reason for the byte stream was to permit the
TCP packet to be broken up into smaller packets if
necessary in order to fit through a net with a small
packet size. But this function was moved to the IP layer
when IP was split from TCP, and IP was forced to
invent a different method of fra gmentation.
A third reason for acknowledging bytes rather than
packets was to permit a number of small packets to be
gathered together into one larger packet in the sending
host if retransmission of the data was necessary. It was
not clear if this advantage would be impo rtant; it turned
out to be critical. Systems such as UNIX which have a
internal communication model based on single character
interactions often send many packets with one byte of
data in them. (One might argue from a network
perspective that this behavior is silly, but it was a
reality, and a necessity for interactive remote login.) It
was often observed that such a host could produce a
flood of packets with one byte of data, which would
arrive much faster than a slow host could process them.
The result is lost packets and retransmission.
If the retransmission was of the original packets, the
same problem would repeat on every retransmission,
with a performance impact so intolerable as to prevent
operation. But since the bytes were gathered into one
packet for retransmission, the retransmission occurred
in a much more effective way which permitted practical
operation.
On the other hand, the acknowledgment of bytes could
be seen as creating this problem in the first place. If the
basis of flow control had been packets rather than bytes,
then this flood might never have occurred. Control at
the packet level has the effect, however, of providing a
severe limit on the thro ughput if small packets are sent.
If the receiving host specifies a number of packets to
receive, without any knowledge of the number of bytes
in each, the actual amount of data received could vary
by a factor of 1000, depending on whether the sending
host puts one or one thousand bytes in each packet.
In retrospect, the correct design decision may have been
that if TCP is to provide effective support of a variety
of services, both packets and bytes must be regulated, as
was done in the original ARPANET protocols.
Another design decision related to the byte stream was
the End-Of-Letter flag, or EOL. This has now vanished
from the protocol, replaced by the Push flag, or PSH.
The original idea of EOL was to break the byte stream
into records. It was implemented by putting data from
separate records into separate packets, which was not
compatible with the idea of combining packets on
retransmission. So the semantics of EOL was changed
to a weaker form, meaning only that the data up to this
point in the stream was one or more complete
application-level elements, which should occasion a
flush of any internal buffering in TCP or the network.
By saying "one or more" rather than "exactly one", it
became possible to combine several together and
preserve the goal of compacting data in reassembly. But
the weaker semantics meant that various applications
had to invent an ad hoc mechanism for delimiting
records on top of the data stream.
In this evolution of EOL semantics, there was a little
known intermediate form, which generated great deba te.
Depending on the buffering strategy of the host, the
byte stream model of TCP can cause great problems in
one improbable case. Consider a host in which the
incoming data is put in a sequence of fixed size buffers.
A buffer is returned to the user either when it is full, or
an EOL is received. Now consider the case of the
arrival of an out-of-order packet which is so far out of
order to lie beyond the current buffer. Now further
consider that after receiving this out-of-order packet, a
packet with an EOL causes the current buffer to be
returned to the user only partially full. This particular
sequence of actions has the effect of causing the out of
order data in the next buffer to be in the wrong place,
because of the empty bytes in the buffer returned to the
user. Coping with this generated book-keeping
problems in the host which seemed unnecessary.
To cope with this it was proposed that the EOL should
"use up" all the sequence space up to the next value
which was zero mod the buffer size. In other words, it
was proposed that EOL should be a tool for mapping
the byte stream to the buffer management of the host.
ACM SIGCOMM -9- Computer Communication Review
This idea was not well received at the time, as it seemed
much too ad hoc, and only one host seemed to have this
problem.
*
In retrospect, it may have been the correct
idea to incorp orate into T CP some means of relating the
sequence space and the buffer management algorithm of
the host. At the time, the designers simply lacked the
insight to see how that might be done in a sufficiently
general manner.
11. Conclusion
In the context of its priorities, the Internet architecture
has been very successful. The protocols are widely used
in the commercial and military environment, and have
spawned a number of similar architectures. At the same
time, its success has made clear that in certain
situations, the priorities of the designers do not match
the needs of the actual users. More attention to such
things as accounting, resource management and
operation of regions with separate administrations are
needed.
While the datagram has served very well in solving the
most impo rtant goals of the Internet, it has no t served so
well when we attempt to address some of the goals
which were further down the priority list. For example,
the goals of resource management and accountability
have proved difficult to achieve in the context of
datagrams. As the previous section discussed, most
datagrams are a part of some sequence of packets from
source to destination, rather than isolated units at the
application level. Ho wever, the gateway cannot directly
see the existence of this sequence, because it is forced
to deal with each packet in isolation. Therefore,
resource management decisions or accounting must be
done on each packet separately. Imposing the datagram
model on the internet layer has deprived that layer of an
important source of information which it could use in
achieving these goals.
This suggests that there may be a better building block
than the datagram for the next generation of
architecture. The general characteristic of this building
block is that it would identify a sequence of packets
traveling from the source to the destination, without
assuming any particular type of service with that
service. I have used the word "flow" to characterize this
building block. It would be necessary for the gateways
to have flow state in order to remember the nature of the
flows which are passing through them, but the state
*
This use of EOL was properly called "Rubber EOL" but its
detractors quickly called it "rubber baby buffer bumpers" in an
attempt to ridicule the idea. Credit must go to the creator of the
idea, Bill Plummer, for sticking to his guns in the face of detractors
saying the above to him ten times fast.
information would not be critical in maintaining the
desired type of service associated with the flow. Instead,
that type of service would be enforced by the end
points, which would periodically send messages to
ensure that the proper type of service was being
associated with the flow. In this way, the state
information associated with the flow could be lost in a
crash without permanent disruption of the service
features being used. I call this concept "soft state," and
it may very well permit us to achieve our primary goals
of survivability and flexibility, while at the same time
doing a better job of dealing with the issue of resource
management and accountability. Exploration of alterna-
tive building blocks constitute one of the current
directions for research within the DARPA Internet
Program.
12. Acknowledgments — A Historical
Perspective
It would be impossible to acknowledge all the
contributors to the Internet project; there have literally
been hundreds over the 15 years of development:
designers, implementors, writers and critics. Indeed, an
important topic, which probably deserves a paper in
itself, is the process by which this pr oject was managed.
The participants came from universities, research
laboratories and corporations, and they united (to some
extent) to achieve this common goal.
The original vision for TCP came from Robert Kahn
and Vinton Cerf, who saw very clearly, back in 1973,
how a protocol with suitable features might be the glue
that would pull together the various emerging network
technologies. From their position at DARPA, they
guided the project in its early days to the point where
TCP and IP became standards for the DoD.
The author of this paper joined the project in the mid-
70s, and took over architectural responsibility for
TCP/IP in 1981. He would like to thank all those who
have worked with him, and particularly those who took
the time to reconstruct some of the lost history in this
paper.
References
1. V. Cerf, and R. Kahn, "A Protocol for Packet
Network Intercommunication" , IEEE Transactions
on Communications, Vol. 22, No. 5, May 1974, pp.
637-648.
2. ISO, "Transport Protocol Specification", Tech.
report IS-8073, International Organization for
Standardization, September 1984.
ACM SIGCOMM -10- Computer Communication Review
3. ISO, " Protocol for Providing the Connection-
lessMode Network Service’, Tech. report DIS8473,
International Organization for Standardization,
1986
4. R. Callon, "Internetwork Protocol’, Proceedings of
the IEEE, Vol. 71, No. 12, December 1983, pp.
1388-1392.
5. Jonathan B. Postel "Internetwork Protocol
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6. Jonatha n B. P ostel, Carl A. Sunshine, Danny
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Discussion

What I struggle to get out of this is when is IP used vs TCP? If most users have moved to IP based systems and only select industries use X.25, where does TCP slot into the picture? As mentioned earlier in the paper, originally they planned for having IP and TCP effectively be one protocol. Eventually they realized is that it made sense to have the basic building block be the datagram (in other words, sending a snippet of data without strong delievery guarantees). This effectively is the IP layer. TCP is built on top and gives you a two-way communication channel with delivery guarantees, congestion control, etc. As the authors explore, one of the reasons for this is that there are some applications with specific requirements that are in tension with the tradeoffs involved in TCP. For instance, for a video chat application maybe you would rather have lower latency at the risk of loosing a few packages rather than having perfect deliverability and higher latency. This paragraph provides a great description of the fundamental architecture of the internet that remains accurate to this day. These are the protocols defined by the International Organization for Standardization (ISO). As an example, here is the 1984 [ISO Transport Protocol Specification](https://tools.ietf.org/html/rfc905) The top three goals on this list ended up having the most significant impact on the design of the network. In other words, the the main goal was to enable a useful interconnection among heterogeneous networks that were controlled by different administrative entities (universities, military, research institutions, etc). At the Future Internet Design project (2008), David Clark, the author of this paper, proposed a list of requirements that a new architecture might take into account: 1. Security 2. Availability and resilience 3. Economic viability 4. Better management 5. Meet society’s needs 6. Longevity 7. Support for tomorrow’s computing 8. Exploit tomorrow’s networking 9. Support tomorrow’s applications 10. Fit for purpose (it works…) It’s important to underline geo-political landscape of the decades when a lot of this early work was done. During the Cold War, defense analysts saw robust communications networks as a necessity in any kind of nuclear confrontation. An effective response strategy required political leaders to continue communicating during an escalating nuclear exchange. Therefore the need for “survivable communications” - essential preserving central command and control - achieved the highest priority. Security and trust are clearly not one of the big goals, but it’s interesting to see the author bring up the issue of trust between administrators as a significant topic. X.25 is a protocol suite for packet switched networks. It is one of the oldest packet switched protocols available. Once popular, it is now used in a number of select industries (e.g. aviation) as most users have moved to IP based systems. The easiest way to support a wide variety of networks is by making the requirements to join the network as simple and easy to implement as possible. Early TCP implementations faced a number of issues such has poor retransmission behavior. One of the consequences of this was unnecessary network congestion. This was first identified as a possible problem in 1984. It was first observed on the Internet in October of 1986, when the NSFnet backbone actually saw its capacity be decreased ~1000 fold from 32kbit/s to 40bit/s due to congestion collapse. The Internet ended up connecting a number of already existing networks **ARPANET** - one of the earliest packet-switching networks which connected together a number of universities, military sites and research laboratories via leased land lines. **ARPA Packet Radio Network** - network built using packet-switched, store-and-forward radio communications in order to provide computer networking in a mobile environment. This project was initiated in 1973, and later became known as DARPA PRNET. **SATNET** - also known as the Atlantic Packet Satellite Network, was an early satellite network that provided connectivity between the US and Europe Am I correct to assume that this is the “grave misunderstanding” mentioned in the abstract?