Network Working Group|
Request for Comments: 1958
B. Carpenter, Editor|
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.
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.
1. Constant Change
2. Is there an Internet Architecture?
3. General Design Issues
4. Name and address issues
5. External Issues
6. Related to Confidentiality and Authentication
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.
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".
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].
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.
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.
(It can be argued that this principle could be generalised beyond the security area.)
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.
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 issues are discussed throughout this memo.
Brian E. Carpenter
Group Leader, Communications Systems
Computing and Networks Division
European Laboratory for Particle Physics
1211 Geneva 23, Switzerland
Phone: +41 22 767-4967
Fax: +41 22 767-7155 EMail: email@example.com