IAB B. Carpenter
Internet Draft (editor)
January 1996
Architectural Principles of the Internet
Abstract
draft-iab-principles-01.txt
The Internet and its architecture have grown in evolutionary fashion from
modest beginnings, rather than from a Grand Plan. While this process of
evolution is one of the main reasons for the technology's success, it
nevertheless seems useful to record a snapshot of the current
principles of the Internet architecture. This is intended for general
guidance and general interest, and is in no way intended to be a
formal or invariant reference model.
Discussion of this draft should take place on the ietf@ietf.org list
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
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Table of Contents:
Status of this Memo.............................................1
1. Constant Change..............................................2
2. Is there an Internet Architecture?...........................2
3. General Design Issues........................................5
4. Name and address issues......................................6
5. External Issues..............................................6
6. Related to Confidentiality and Authentication................6
7. Security considerations......................................7
Acknowledgements................................................7
References......................................................8
Editor's Address................................................8
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1. Constant Change
In searching for Internet architectural principles, we must remember
that technical change is continuous in the information technology
industry. The Internet reflects this. Over the 25 years since the
ARPANET started, various measures of the size of the Internet have
increased by factors between 1000 (backbone speed) and 1000000
(number of hosts). In this environment, some architectural principles
inevitably change. Principles that seemed inviolable a few years ago
are deprecated today. Principles that seem sacred today will be
deprecated tomorrow. The notion of a fixed Reference Model into which
all designs must fit is anathema to the Internet. The principle of
constant change is perhaps the only principle of the Internet that
should survive indefinitely.
A good analogy for the development of the Internet is that of
constantly renewing the individual streets and buildings of a city,
rather than razing the city and rebuilding it. The architectural
principles therefore aim to provide a framework for creating
cooperation and standards, as a small "spanning set" of rules that
generates a large, varied and evolving space of technology.
Some current technical triggers for change include the limits to the
scaling of IPv4, the fact that gigabit/second networks and multimedia
present fundamentally new challenges, and the need for quality of
service and security guarantees in the commercial Internet.
As Lord Kelvin stated in 1895, "Heavier-than-air flying machines are
impossible." We would be foolish to imagine that the principles
listed below are more than a snapshot of our current understanding.
2. Is there an Internet Architecture?
2.1 Many members of the Internet community would argue that there is
no architecture, but only a tradition, which was not written down for
the first 25 years (or at least not by the IAB). However, in very
general terms, the community believes that the goal is connectivity,
the tool is the Internet Protocol, and the intelligence is end to end
rather than hidden in the network.
The current exponential growth of the network seems to show that
connectivity is its own reward, and is more valuable than any
individual application such as mail or the World-Wide Web. This
connectivity requires technical cooperation between service
providers, and flourishes in the increasingly liberal and competitive
commercial telecommunications environment.
The key to global connectivity is a single protocol at the inter-
networking layer. The key to exploiting this single protocol over
diverse hardware providing global connectivity is the "end to end
argument".
2.2 It is generally felt that there should be one, and only one,
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protocol at the Internet level (but perhaps two versions for an
extended period during transition). There can be many protocols at
other levels.
This single protocol must be independent of the hardware medium and
hardware addressing. This approach allows the Internet to exploit
any new digital transmission technology of any kind, and to decouple
its addressing mechanisms from the hardware. It allows the Internet
to be the easy way to interconect fundamentally different
transmission media, and to offer a single platform for a wide variety
of Information Infrastructure applications and services. There is a
good exposition of this model, and other important fundemental
issues, in [Clark].
2.3 It is also generally felt that end-to-end functions can best be
realised by end-to-end protocols.
The end-to-end argument is discussed in depth in [Saltzer]. The
basic argument is that, as a first principle, certain required end-
to-end functions can only be performed correctly by the end-systems
themselves. A specific case is that any network, however carefully
designed, will be subject to failures of transmission at some
statistically determined rate. The best way to cope with this is to
accept it, and give responsibility for the integrity of communication
to the end systems. Another specific case is end-to-end security.
To quote from [Saltzer], "The function in question can completely and
correctly be implemented only with the knowledge and help of the
application standing at the endpoints of the communication system.
Therefore, providing that questioned function as a feature of the
communication system itself is not possible. (Sometimes an incomplete
version of the function provided by the communication system may be
useful as a performance enhancement.)"
This principle has important consequences if we require applications
to survive partial network failures. An end-to-end protocol design
should not rely on the maintenance of state (i.e. information about
the state of the end-to-end communication) inside the network. Such
state should be maintained only in the endpoints, in such a way that
the state can only be destroyed when the endpoint itself breaks
(known as fate-sharing). An immediate consequence of this is that
datagrams are better than classical virtual circuits. The network's
job is to route datagrams as efficiently and flexibly as possible.
Everything else should be done at the fringes.
To perform its services, the network maintains some state
information: routes, QoS guarantees that it makes, session
information where that is used in header compression, compression
histories for data compression, and the like. This state must be
self-healing; adaptive procedures or protocols must exist to derive
and maintain that state, and change it when the topology or activity
of the network changes. The volume of this state must be minimized,
and the loss of the state must not result in more than a temporary
denial of service given that connectivity exists.
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2.4 Fortunately, nobody owns the Internet, there is no centralized
control, and nobody can turn it off. Its evolution depends on rough
consensus about technical proposals, and on running code.
Engineering feed-back from real implementations is more important
than any architectural principles
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3. General Design Issues
3.1 Heterogeneity is inevitable and must be supported by design.
Multiple types of hardware must be allowed for, e.g. transmission
speeds differing by at least 7 orders of magnitude, various computer
word lengths, and hosts ranging from memory-starved microprocessors
up to massively parallel supercomputers. Multiple types of
application protocol must be allowed for, ranging from the simplest
such as remote login up to the most complex such as distributed
databases.
3.2 If there are several ways of doing the same thing, choose one.
If a previous Internet design has successfully solved the same
problem, choose the same solution unless there is a good technical
reason not to. Duplication of the same protocol functionality should
be avoided as far as possible, without of course using this argument
to reject improvements.
3.3 All designs must scale readily to very many nodes per site and to
many millions of sites.
3.4 Performance and cost must be considered as well as functionality.
3.5 Keep it simple. When in doubt during design, choose the simplest
solution. If you can keep things separate, do so.
3.6 In many cases it is better to adopt an almost complete solution
now, rather than to wait until a perfect solution can be found.
3.7 Avoid options and parameters whenever possible. Any options and
parameters should be configured or negotiated dynamically rather than
manually.
3.8 Be strict when sending and tolerant when receiving.
Implementations must follow specifications precisely when sending to
the network, and tolerate faulty input from the network. When in
doubt, discard faulty input silently, without returning an error
message unless this is required by the specification.
3.9 Be parsimonious with unsolicited packets, especially multicasts
and broadcasts.
3.10 Circular dependencies must be avoided.
For example, routing must not depend on look-ups in the
Domain Name System (DNS), since the updating of DNS servers
depends on successful routing.
3.11 Objects should be self decribing (include type and size), within
reasonable limits. Only type codes and other magic numbers assigned
by the Internet Assigned Numbers Authority (IANA) may be used.
3.12 All specifications should use the same terminology and notation,
and the same bit- and byte-order convention.
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3.13 And perhaps most important: Nothing gets standardised until
there are multiple instances of running code.
4. Name and address issues
4.1 Avoid any design that requires addresses to be hard coded or
stored on non-volatile storage (except of course where this is an
essential requirement as in a name server or configuration server).
In general, user applications should use names rather than addresses.
4.2 A single naming structure should be used.
4.3 Public (i.e. widely visible) names should be in case-independent
ASCII.
4.4 Addresses must be unambiguous (unique within any scope where they
may appear).
4.5 Upper layer protocols must be able to identify end-points
unambiguously. In practice today, this means that addresses must be
the same at start and finish of transmission.
5. External Issues
5.1 Prefer unpatented technology, but if the best technology is
patented and is available to all at reasonable terms, then
incorporation of patented technology is acceptable.
5.2 The existence of export controls on some aspects of Internet
technology is only of secondary importance in choosing which
technology to adopt into the standards. All of the technology
required to implement Internet standards can be fabricated in each
country, so world wide deployment of Internet technology does not
depend on its exportability from any particular country or countries.
5.3 Any implementation which does not include all of the required
components cannot claim conformance with the standard.
5.4 Designs should be "internationally user-friendly". In particular,
there should be a uniform approach to character set support for
information content.
6. Related to Confidentiality and Authentication
6.1 All designs must fit into the IP security architecture.
6.2 Confidentiality and authentication are the responsibility of end
users and must be implemented in the protocols used by the end users.
Endpoints should not depend on the confidentiality or integrity of
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the carriers, and in particular, it is not the responsibility of the
carriers to ensure confidentiality. Carriers may choose to provide
some level of protection, but this is secondary to the primary
responsibility of the end users to protect themselves.
6.3 Wherever a cryptographic algorithm is called for in a protocol,
the protocol should be designed to permit alternative algorithms to
be used and the specific algorithm employed in a particular
implementation should be explicitly labeled. Official labels for
algorithms are to be recorded by the IANA.
(It can be argued that this principle could be generalised beyond the
security area.)
6.4 In choosing algorithms, the algorithm should be one which is
widely regarded as strong enough to serve the purpose. Among
alternatives all of which are strong enough, preference should be
given to algorithms which have stood the test of time and which are
not unnecessarily inefficient.
7. Security considerations
We just did that.
Acknowledgements
This document is a collective work of the Internet community,
published by the Internet Architecture Board. Special thanks to Fred
Baker, Noel Chiappa, Donald Eastlake, Frank Kastenholz, and Neal
McBurnett.
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References
Note that the references have been deliberately limited to two
fundamental papers on the Internet architecture.
[Clark] The Design Philosophy of the DARPA Internet Protocols,
D.D.Clark, Proc SIGCOMM 88, ACM CCR Vol 18, Number 4, August 1988,
pages 106-114 (reprinted in ACM CCR Vol 25, 1995, pages 102-111).
[Saltzer] End-To-End Arguments in System Design, J.H. Saltzer,
D.P.Reed, D.D.Clark, ACM TOCS, Vol 2, Number 4, November 1984, pp
277-288.
Editor's Address
Brian E. Carpenter
Group Leader, Communications Systems Phone: +41 22 767-4967
Computing and Networks Division Fax: +41 22 767-7155
CERN
European Laboratory for Particle Physics Email: brian@dxcoms.cern.ch
1211 Geneva 23, Switzerland
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