TheDesignPhilosophyoftheDARPAInternetProtocols

Design

Philosophy

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ACM SIGCOMM -1- Computer Communication Review

The Design Philosophy of the DARPA Internet Protocols

David D. Clark

Massachusetts Institute of Technology

Laboratory for Computer Science

Cambridge, MA. 02139

(Originally published in Proc. SIGCOMM ‘88, Computer Communication Review Vol. 18, No. 4,

August 1988, pp. 106–114)

This work was supported in part by the Defense Advanced Research Projects Agency (DARPA) under Contract No. N00014-83-K-0125

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

, the Advanced Research Projects

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

While specific information on the DOD protocols is

fairly generally available

, 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

with the ARPA packet radio

network

, 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 network. 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 project 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

variety 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. The 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 particular,

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 of their

conversation. More concretely, at the service interface

of the transport layer, this architecture provides no

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xuews

2014-04-16

经典的论文，很好！
w526482656

2014-10-08

别处找不到原文这个很好啦
xy-liu11

2013-10-26

这是pdf英文版，比百度百科上的详细多了。赞一个！
lsq10

2013-03-05

好论文，同时有高亮
keepClimbing

2014-05-13

找了很久了，百度上还要这么多积分，这里太好了

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