Internet DRAFT - draft-iab-e2e-futures
draft-iab-e2e-futures
James Kempf
Internet Draft Rob Austein
Document: draft-iab-e2e-futures-05.txt IAB
Expires: August 2004 Feburary 2004
The Rise of the Middle and the Future of End to End:
Reflections on the Evolution of the Internet Architecture
Status of this Memo
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Abstract
The end to end principle is the core architectural guideline of the Internet.
In this document, we briefly examine the development of the end to end
principle as it has been applied to the Internet architecture over the years.
We discuss current trends in the evolution of the Internet architecture in
relation to the end to end principle, and try to draw some conclusion about the
evolution of the end to end principle, and thus for the Internet architecture
which it supports, in light of these current trends.
Table of Contents
1.0 Introduction..........................................................2
2.0 A Brief History of the End to End Principle...........................2
3.0 Trends Opposed to the End to End Principle............................4
4.0 Whither the End to End Principle?.....................................7
5.0 Internet Standards as an Arena for Conflict...........................8
6.0 Conclusions...........................................................9
7.0 Acknowledgements......................................................9
8.0 References............................................................9
9.0 Security Considerations..............................................10
10.0 IANA Considerations................................................10
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11.0 Author Information.................................................10
12.0 Full Copyright Statement...........................................11
1.0 Introduction
One of the key architectural guidelines of the Internet is the end to end
principle in the papers by Saltzer, Reed, and Clark [1][2]. The end to end
principle was originally articulated as a question of where best not to put
functions in a communication system. Yet, in the ensuing years, it has evolved
to address concerns of maintaining openness, increasing reliability and
robustness, and preserving the properties of user choice and ease of new
service development as discussed by Blumenthal and Clark in [3]; concerns that
were not part of the original articulation of the end to end principle.
In this document, we examine how the interpretation of the end to end principle
has evolved over the years, and where it stands currently. We examine trends in
the development of the Internet that have led to pressure to define services in
the network, a topic that has already received some amount of attention from
the IAB in RFC 3238 [5]. We describe some considerations about how the end to
end principle might evolve in light of these trends.
2.0 A Brief History of the End to End Principle
2.1 In the Beginning...
The end to end principle was originally articulated as a question of where best
to put functions in a communication system:
The function in question can completely and correctly be implemented only
with the knowledge and help of the application standing at the end points 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.) [1].
A specific example of such a function is delivery guarantees [1]. The original
ARPANET returned a message "Request for Next Message" whenever it delivered a
packet. Although this message was found to be useful within the network as a
form of congestion control, since the ARPANET refused to accept another message
to the same destination until the previous acknowledgment was returned, it was
never particularly useful as an indication of guaranteed delivery. The problem
was that the host stack on the sending host typically doesn't want to know just
that the network delivered a packet, but rather the stack layer on the sending
host wants to know that the stack layer on the receiving host properly
processed the packet. In terms of modern IP stack structure, a reliable
transport layer requires an indication that transport processing has
successfully completed, such as given by TCP's ACK message [4], and not simply
an indication from the IP layer that packet arrived. Similarly, an application
layer protocol may require an application-specific acknowledgement that
contains, among other things, a status code indicating the disposition of the
request.
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The specific examples given in [1] and other references at the time [2]
primarily involve transmission of data packets: data integrity, delivery
guarantees, duplicate message suppression, per packet encryption, and
transaction management. From the viewpoint of today's Internet architecture, we
would view most of these as transport layer functions (data integrity, delivery
guarantees, duplicate message suppression, and perhaps transaction management),
others as network layer functions with support at other layers where necessary
(for example, packet encryption), and not application layer functions.
2.2 ...In the Middle...
As the Internet developed, the end to end principle gradually widened to
concerns about where best to put the state associated with applications in the
Internet: in the network or at end nodes. The best example is the description
in RFC 1958 [6]:
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.
The original articulation of the end to end principle - that knowledge and
assistance of the end point is essential and that omitting such knowledge and
implementing a function in the network without such knowledge and assistance is
not possible - took a while to percolate through the engineering community, and
had evolved by this point to a broad architectural statement about what belongs
in the network and what doesn't. RFC 1958 uses the term "application" to mean
the entire network stack on the end node, including network, transport, and
application layers, in contrast to the earlier articulation of the end to end
principle as being about the communication system itself. "Fate-sharing"
describes this quite clearly: the fate of a conversation between two
applications is only shared between the two applications; the fate does not
depend on anything in the network, except for the network's ability to get
packets from one application to the other.
The end to end principle in this formulation is specifically about what kind of
state is maintained where:
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.[6]
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In this formulation of the end to end principle, state involved in getting
packets from one end of the network to the other is maintained in the network.
The state is "soft state," in the sense that it can be quickly dropped and
reconstructed (or even required to be periodically renewed) as the network
topology changes due to routers and switches going on and off line. "Hard
state", state upon which the proper functioning of the application depends, is
only maintained in the end nodes. This formulation of the principle is a
definite change from the original formulation of the principle, about end node
participation being required for proper implementation of most functions.
In summary, the general awareness both of the principle itself and of its
implications for how unavoidable state should be handled grew over time to
become a (if not the) foundation principle of the Internet architecture.
2.3 ...And Now.
An interesting example of how the end to end principle continues to influence
the technical debate in the Internet community is IP mobility. The existing
Internet routing architecture severely constrains how closely IP mobility can
match the end to end principle without making fundamental changes. Mobile IPv6,
described in the Mobile IPv6 specification by Johnson, Perkins, and Arkko [7],
requires a routing proxy in the mobile node's home network (the Home Agent) for
maintaining the mapping between the mobile node's routing locator, the care of
address, and the mobile node's node identifier, the home address. But the local
subnet routing proxy (the Foreign Agent), which was a feature of the older
Mobile IPv4 design that compromised end to end routing, has been eliminated.
The end node now handles its own care of address. In addition, Mobile IPv6
includes secure mechanisms for optimizing routing to allow end to end routing
between the mobile end node and the correspondent node, removing the need to
route through the global routing proxy at the home agent. These features are
all based on end to end considerations. However, the need for the global
routing proxy in the Home Agent in Mobile IPv6 is determined by the aliasing of
the global node identifier with the routing identifier in the Internet routing
architecture, a topic that was discussed in an IAB workshop and reported in RFC
2956 [9], and that hasn't changed in IPv6.
Despite this constraint, the vision emerging out of the IETF working groups
developing standards for mobile networking is of a largely autonomous mobile
node with multiple wireless link options, among which the mobile node picks and
chooses. The end node is therefore responsible for maintaining the integrity of
the communication, as the end to end principle implies. This kind of innovative
application of the end to end principle derives from the same basic
considerations of reliability and robustness (wireless link integrity, changes
in connectivity and service availability with movement, etc.) that motivated
the original development of the end to end principle. While the basic
reliability of wired links and routing and switching equipment has improved
considerably since the end to end principle was formalized 15 years ago, the
reliability or unreliability of wireless links is governed more strongly by the
basic physics of the medium and the instantaneous radio propagation conditions.
3.0 Trends Opposed to the End to End Principle
While the end to end principle continues to provide a solid foundation for much
IETF design work, the specific application of the end to end principle
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described in RFC 1958 has increasingly come into question from various
directions. The IAB has been concerned about trends opposing the end to end
principle for some time, for example RFC 2956 [9] and RFC 2775 [12]. The
primary focus of concern in these documents is the reduction in transparency
due to the introduction of NATs and other address translation mechanisms in the
Internet, and the consequences to the end to end principle of various scenarios
involving full, partial, or no deployment of IPv6. More recently, the topic of
concern has shifted to the consequences of service deployment in the network.
The IAB opinion on Open Pluggable Edge Services (OPES) in RFC 3238 [5] is
intended to assess the architectural desirability of defining services in the
network and to raise questions about how such services might result in
compromises of privacy, security, and end-to-end data integrity. Clark, et al.
in [10] and Carpenter in RFC 3234 [11] also take up the topic of service
definition in the network.
Perhaps the best review of the forces militating against the end to end
principle is by Blumenthal and Clark in [3]. The authors make the point that
the Internet originally developed among a community of like-minded technical
professionals who trusted each other, and was administered by academic and
government institutions who enforced a policy of no commercial use. The major
stakeholders in the Internet are quite different today. As a consequence, new
requirements have evolved over the last decade. Examples of these requirements
are discussed in the following subsections. Other discussions about pressures
on the end to end principle in today's Internet can be found in the discussion
by Reed [13] and Moors' paper in the 2002 IEEE International Communications
Conference [14].
3.1 Need for Authentication
Perhaps the single most important change from the Internet of 15 years ago is
the lack of trust between users. Because the end users in the Internet of 15
years ago were few, and were largely dedicated to using the Internet as a tool
for academic research and communicating research results (explicit commercial
use of the Internet was forbidden when it was run by the US government), trust
between end users (and thus authentication of end nodes that they use) and
between network operators and their users was simply not an issue in general.
Today, the motivations of some individuals using the Internet are not always
entirely ethical, and, even if they are, the assumption that end nodes will
always co-operate to achieve some mutually beneficial action, as implied by the
end to end principle, is not always accurate. In addition, the growth in users
who are either not technologically sophisticated enough or simply uninterested
in maintaining their own security has required network operators to become more
proactive in deploying measures to prevent naive or uninterested users from
inadvertently or intentionally generating security problems.
While the end to end principle does not require that users implicitly trust
each other, the lack of trust in the Internet today requires that application
and system designers make a choice about how to handle authentication, whereas
that choice was rarely apparent 15 years ago. One of the most common examples
of network elements interposing between end hosts are those dedicated to
security: firewalls, VPN tunnel endpoints, certificate servers, etc. These
intermediaries are designed to protect the network from unimpeded attack or to
allow two end nodes whose users may have no inherent reason to trust each other
to achieve some level of authentication. At the same time, these measures act
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as impediments for end to end communications. Third party trust intermediaries
are not a requirement for security, as end to end security mechanisms, such as
S/MIME [15], can be used instead, and where third party measures such as PKI
infrastructure or keys in DNS are utilized to exchange keying material, they
don't necessarily impinge on end to end traffic after authentication has been
achieved. Even if third parties are involved, ultimately it is up to the
endpoints and their users in particular, to determine which third parties they
trust.
3.2 New Service Models
New service models inspired by new applications require achieving the proper
performance level as a fundamental part of the delivered service. These service
models are a significant change from the original best effort service model.
Email, file transfer, and even Web access aren't perceived as failing if
performance degrades, though the user may become frustrated at the time
required to complete the transaction. However, for streaming audio and video,
to say nothing of real time bidirectional voice and video, achieving the proper
performance level, whatever that might mean for an acceptable user experience
of the service, is part of delivering the service, and a customer contracting
for the service has a right to expect the level of performance for which they
have contracted. For example, content distributors sometimes release content
via content distribution servers that are spread around the Internet at various
locations to avoid delays in delivery if the server is topologically far away
from the client. Retail broadband and multimedia services are a new service
model for many service providers.
3.3 Rise of the Third Party
Academic and government institutions ran the Internet of 15 years ago. These
institutions did not expect to make a profit from their investment in
networking technology. In contrast, the network operator with which most
Internet users deal today is the commercial ISP. Commercial ISPs run their
networks as a business, and their investors rightly expect the business to turn
a profit. This change in business model has led to a certain amount of pressure
on ISPs to increase business prospects by deploying new services.
In particular, the standard retail dialup bit pipe account with email and shell
access has become a commodity service, resulting in low profit margins. While
many ISPs are happy with this business model and are able to survive on it,
others would like to deploy different service models that have a higher profit
potential and provide the customer with more or different services. An example
is retail broadband bit pipe access via cable or DSL coupled with streaming
multimedia. Some ISPs that offer broadband access also deploy content
distribution networks to increase the performance of streaming media. These
services are typically deployed so that they are only accessible within the
ISP's network, and as a result, they do not contribute to open, end to end
service. From an ISP's standpoint, however, offering such service is an
incentive for customers to buy the ISP's service.
ISPs are not the only third party intermediary that has appeared within the
last 10 years. Unlike the previous involvement of corporations and governments
in running the Internet, corporate network administrators, and governmental
officials have become increasingly demanding of opportunities to interpose
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between two parties in an end to end conversation. A benign motivation for this
involvement is to mitigate the lack of trust, so the third party acts as a
trust anchor or enforcer of good behavior between the two ends. A less benign
motivation is for the third parties to insert policy for their own reasons,
perhaps taxation or even censorship. The requirements of third parties often
have little or nothing to do with technical concerns, but rather derive from
particular social and legal considerations.
4.0 Whither the End to End Principle?
Given the pressures on the end to end principle discussed in the previous
section, a question arises about the future of the end to end principle. Does
the end to end principle have a future in the Internet architecture or not? If
it does have a future, how should it be applied? Clearly, an unproductive
approach to answering this question is to insist upon the end to end principle
as a fundamentalist principle that allows no compromise. The pressures
described above are real and powerful, and if the current Internet technical
community chooses to ignore these pressures, the likely result is that a market
opportunity will be created for a new technical community that does not ignore
these pressures but which may not understand the implications of their design
choices. A more productive approach is to return to first principles and re-
examine what the end to end principle is trying to accomplish, and then update
our definition and exposition of the end to end principle given the
complexities of the Internet today.
4.1 Consequences of the End to End Principle
In this section, we consider the two primary desirable consequences of the end
to end principle: protection of innovation and provision of reliability and
robustness.
4.1.1 Protection of Innovation
One desirable consequence of the end to end principle is protection of
innovation. Requiring modification in the network in order to deploy new
services is still typically more difficult than modifying end nodes. The
counterargument - that many end nodes are now essentially closed boxes which
are not updatable and that most users don't want to update them anyway - does
not apply to all nodes and all users. Many end nodes are still user
configurable and a sizable percentage of users are "early adopters," who are
willing to put up with a certain amount of technological grief in order to try
out a new idea. And, even for the closed boxes and uninvolved users,
downloadable code that abides by the end to end principle can provide fast
service innovation. Requiring someone with a new idea for a service to convince
a bunch of ISPs or corporate network administrators to modify their networks is
much more difficult than simply putting up a Web page with some downloadable
software implementing the service.
4.1.2 Reliability and Trust
Of increasing concern today, however, is the decrease in reliability and
robustness that results from deliberate, active attacks on the network
infrastructure and end nodes. While the original developers of the Internet
were concerned by large-scale system failures, attacks of the subtlety and
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variety that the Internet experiences today were not a problem during the
original development of the Internet. By and large, the end to end principle
was not addressed to the decrease in reliability resulting from attacks
deliberately engineered to take advantage of subtle flaws in software. These
attacks are part of the larger issue of the trust breakdown discussed in
Section 3.1. Thus, the issue of the trust breakdown can be considered another
forcing function on the Internet architecture.
The immediate reaction to this trust breakdown has been to try to back fit
security into existing protocols. While this effort is necessary, it is not
sufficient. The issue of trust must become as firm an architectural principle
in protocol design for the future as the end to end principle is today. Trust
isn't simply a matter of adding some cryptographic protection to a protocol
after it is designed. Rather, prior to designing the protocol, the trust
relationships between the network elements involved in the protocol must be
defined, and boundaries must be drawn between those network elements that share
a trust relationship. The trust boundaries should be used to determine what
type of communication occurs between the network elements involved in the
protocol and which network elements signal each other. When communication
occurs across a trust boundary, cryptographic or other security protection of
some sort may be necessary. Additional measures may be necessary to secure the
protocol when communicating network elements do not share a trust relationship.
For example, a protocol might need to minimize state in the recipient prior to
establishing the validity of the credentials from the sender in order to avoid
a memory depletion DoS attack.
4.2 The End to End Principle in Applications Design
The concern expressed by the end to end principle is applicable to applications
design too. Two key points in designing application protocols are to ensure
they don't have any dependencies that would break the end to end principle and
to ensure that they can identify end points in a consistent fashion. An example
of the former is layer violations - creating dependencies that would make it
impossible for the transport layer, for example, to do its work appropriately.
Another issue is the desire to insert more applications infrastructure into the
network. Architectural considerations around this issue are discussed in RFC
3238 [5]. This desire need not result in a violation the end to end principle
if the partitioning of functioning is done so that services provided in the
network operate with the explicit knowledge and involvement of endpoints, when
such knowledge and involvement is necessary for the proper functioning of the
service. The result becomes a distributed application, in which the end to end
principle applies to each connection involved in implementing the application.
5.0 Internet Standards as an Arena for Conflict
Internet standards have increasingly become an arena for conflict [10]. ISPs
have certain concerns, businesses and government have others, and vendors of
networking hardware and software still others. Often, these concerns conflict,
and sometimes they conflict with the concerns of the end users. For example,
ISPs are reluctant to deploy interdomain QoS services because, among other
reasons, every known instance creates a significant and easily exploited
DoS/DDoS vulnerability. However, some end users would like to have end-to-end,
Diffserv or Intserv-style QoS available to improve support for voice and video
multimedia applications between end nodes in different domains, as discussed by
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Huston in RFC 2990 [16]. In this case, the security, robustness and reliability
concerns of the ISP conflict with the desire of users for a different type of
service.
These conflicts will inevitably be reflected in the Internet architecture going
forward. Some of these conflicts are impossible to resolve on a technical
level, nor would it even be desirable, because they involve social and legal
choices that the IETF is not empowered to make (for a counter argument in the
area of privacy, see Goldberg, et al. [17]). But for those conflicts that do
involve technical choices, the important properties of user choice and
empowerment, reliability and integrity of end to end service, supporting trust
and "good network citizen behavior," and fostering innovation in services
should be the basis upon which resolution is made. The conflict will then play
out on the field of the resulting architecture.
6.0 Conclusions
The end to end principle continues to guide technical development of Internet
standards, and remains as important today for the Internet architecture as in
the past. In many cases, unbundling of the end to end principle into its
consequences leads to a distributed approach in which the end to end principle
applies to interactions between the individual pieces of the application, while
the unbundled consequences, protection of innovation and reliability and
robustness, apply to the entire application. While the end to end principle
originated as a focused argument about the need for the knowledge and
assistance of end nodes to properly implement functions in a communication
system, particular second order properties developed by the Internet as a
result of the end to end principle have come to be recognized as being as
important, if not more so, than the principle itself. End user choice and
empowerment, integrity of service, support for trust, and "good network citizen
behavior" are all properties that have developed as a consequence of the end to
end principle. Recognizing these properties in a particular proposal for
modifications to the Internet has become more important than before as the
pressures to incorporate services into the network have increased. Any proposal
to incorporate services in the network should be weighed against these
properties before proceeding.
7.0 Acknowledgements
Many of the ideas presented here originally appeared in the works of Dave
Clark, John Wroclawski, Bob Braden, Karen Sollins, Marjory Blumenthal, and Dave
Reed on forces currently influencing the evolution of the Internet. The authors
would particularly like to single out the work of Dave Clark, who was the
original articulator of the end to end principle and who continues to inspire
and guide the evolution of the Internet architecture, and John Wroclawski, with
whom conversations during the development of this paper helped to clarify
issues involving tussle and the Internet.
8.0 References
[1] Saltzer, J.H., Reed, D.P., and Clark, D.D., "End to End Arguments in
System Design," ACM TOCS, Vol 2, Number 4, November 1984, pp 277-288.
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[2] Clark, D., "The Design Philosophy of the DARPA Internet Protocols,"
Proc SIGCOMM 88, ACM CCR Vol 18, Number 4, August 1988, pp. 106-114.
[3] Blumenthal, M., Clark, D.D., "Rethinking the design of the Internet:
The end to end arguments vs. the brave new world", ACM Transactions on
Internet Technology, Vol. 1, No. 1, August 2001, pp 70-109.
[4] Postel, J., "Transmission Control Protocol," RFC 793, September, 1981.
[5] Floyd, S., and Daigle, L., "IAB Architectural and Policy Considerations
for Open Pluggable Edge Services", RFC 3238, January 2002.
[6] Carpenter, B. (editor), "Architectural Principles of the Internet," RFC
1958, June, 1996.
[7] Johnson, D., Perkins, C., and Arkko, J., "Mobility Support in IPv6,"
draft-ietf-mobileip-ipv6-20.txt, a work in progress.
[8] Perkins, C., editor, "Mobility Support in IPv4", RFC 3220, August,
2002.
[9] Kaat, M., "Overview of 1999 IAB Network Layer Workshop," RFC 2956,
October, 2000.
[10] Clark, D.D., Wroclawski, J., Sollins, K., and Braden, B., "Tussle in
Cyberspace: Defining Tommorow's Internet", Proceedings of Sigcomm 2002.
[11] Carpenter, B., and Brim, S., "Middleboxes: Taxonomy and Issues," RFC
3234, February, 2002.
[12] Carpenter, B., "Internet Transparency," RFC 2775, February, 2000.
[13] Reed, D., "The End of the End-to-End Argument?",
http://www.reed.com/dprframeweb/dprframe.asp?section=paper&fn=endofendt
oend.html, April, 2000.
[14] Moors, T., "A Critical Review of End-to-end Arguments in System
Design," Proc. 2000 IEEE International Conference on Communications,
pp. 1214-1219, April, 2002.
[15] Ramsdell, B. (editor), "S/MIME Version 3 Message Specification", RFC
2633, June, 1999.
[16] Huston, G., "Next Steps for the IP QoS Architecture", RFC 2990,
November, 2000.
[17] Goldberg, I., Wagner, D., and Brewer, E., "Privacy-enhancing
technologies for the Internet," Proceedings of IEEE COMPCON 97, pp.
103-109, 1997.
9.0 Security Considerations
This document does not propose any new protocols, and therefore does not
involve any security considerations in that sense. However, throughout this
document there are discussions of the privacy and integrity issues and the
architectural requirements created by those issues.
10.0 IANA Considerations
There are no IANA considerations regarding this document.
11.0 Author Information
Internet Architecture Board
EMail: iab@iab.org
IAB Membership at time this document was completed:
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Bernard Aboba
Harald Alvestrand
Rob Austein
Leslie Daigle
Patrik F„ltstr÷m
Sally Floyd
Jun-ichiro Itojun Hagino
Mark Handley
Geoff Huston
Charlie Kaufman
James Kempf
Eric Rescorla
Mike St. Johns
This draft was created in Feburary 2004.
12.0 Full Copyright Statement
Copyright (C) The Internet Society (2004). All Rights Reserved. This
document and translations of it may be copied and furnished to others, and
derivative works that comment on or otherwise explain it or assist in its
implementation may be prepared, copied, published and distributed, in whole
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derivative works. However, this document itself may not be modified in any
way, such as by removing the copyright notice or references to the Internet
Society or other Internet organizations, except as needed for the purpose of
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