Architectural Principles of the Internet

Network Working Group                               B. Carpenter, Editor
Request for Comments: 1958                                           IAB
Category: Informational                                        June 1996


                Architectural Principles of the Internet

Status of This Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Abstract

   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.

Table of Contents

      1. Constant Change..............................................1
      2. Is there an Internet Architecture?...........................2
      3. General Design Issues........................................4
      4. Name and address issues......................................5
      5. External Issues..............................................6
      6. Related to Confidentiality and Authentication................6
      Acknowledgements................................................7
      References......................................................7
      Security Considerations.........................................8
      Editor's Address................................................8

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 principle of constant change is perhaps the
   only principle of the Internet that should survive indefinitely.

   The purpose of this document is not, therefore, to lay down dogma
   about how Internet protocols should be designed, or even about how
   they should fit together. Rather, it is to convey various guidelines
   that have been found useful in the past, and that may be useful to
   those designing new protocols or evaluating such designs.

   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 the inter-networking layer.  The
   key to exploiting this layer over diverse hardware providing global
   connectivity is the "end to end argument".

   2.2 It is generally felt that in an ideal situation there should be
   one, and only one, protocol at the Internet level.  This allows for
   uniform and relatively seamless operations in a competitive, multi-
   vendor, multi-provider public network.  There can of course be
   multiple protocols to satisfy different requirements at other levels,
   and there are many successful examples of large private networks with
   multiple network layer protocols in use.

   In practice, there are at least two reasons why more than one network
   layer protocol might be in use on the public Internet. Firstly, there
   can be a need for gradual transition from one version of IP to
   another.  Secondly, fundamentally new requirements might lead to a
   fundamentally new protocol.

   The Internet level 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 transmit 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.  Manually
   configured state must be kept to an absolute minimum.

   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.

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 design, in the Internet context or elsewhere, 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.

   3.6 Modularity is good. If you can keep things separate, do so.

   3.7 In many cases it is better to adopt an almost complete solution
   now, rather than to wait until a perfect solution can be found.

   3.8 Avoid options and parameters whenever possible.  Any options and
   parameters should be configured or negotiated dynamically rather than
   manually.

   3.9 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.

   3.10 Be parsimonious with unsolicited packets, especially multicasts
   and broadcasts.

   3.11 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.

   3.12 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.

   3.13 All specifications should use the same terminology and notation,
   and the same bit- and byte-order convention.

   3.14 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.  Specifically, this refers to DNS names, and to protocol
   elements that are transmitted in text format.

   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 fully international, with support for
   localisation (adaptation to local character sets). In particular,
   there should be a uniform approach to character set tagging for
   information content.

6. Related to Confidentiality and Authentication

   6.1 All designs must fit into the IP security architecture.

   6.2 It is highly desirable that Internet carriers protect the privacy
   and authenticity of all traffic, but this is not a requirement of the
   architecture.  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 the carriers. 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.

   6.5 To ensure interoperation between endpoints making use of security
   services, one algorithm (or suite of algorithms) should be mandated
   to ensure the ability to negotiate a secure context between
   implementations. Without this, implementations might otherwise not
   have an algorithm in common and not be able to communicate securely.

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, Neal
   McBurnett, Masataka Ohta, Jeff Schiller and Lansing Sloan.

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, Number 1, January 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.

Security Considerations

   Security issues are discussed throughout this memo.

Editor's Address

   Brian E. Carpenter
   Group Leader, Communications Systems
   Computing and Networks Division
   CERN
   European Laboratory for Particle Physics
   1211 Geneva 23, Switzerland

   Phone:  +41 22 767-4967
   Fax:    +41 22 767-7155
   EMail: brian@dxcoms.cern.ch