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INFORMATIONAL

Network Working Group                                   S. Floyd, Editor
Request for Comments: 3426                   Internet Architecture Board
Category: Informational                                    November 2002


            General Architectural and Policy Considerations

Status of this Memo

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

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   This document suggests general architectural and policy questions
   that the IETF community has to address when working on new standards
   and protocols.  We note that this document contains questions to be
   addressed, as opposed to guidelines or architectural principles to be
   followed.

1.  Introduction

   This document suggests general architectural and policy questions to
   be addressed in our work in the IETF.  This document contains
   questions to be addressed, as opposed to guidelines or architectural
   principles to be followed.  These questions are somewhat similar to
   the "Security Considerations" currently required in IETF documents
   [RFC2316].

   This document is motivated in part by concerns about a growing lack
   of coherence in the overall Internet architecture.  We have moved
   from a world of a single, coherent architecture designed by a small
   group of people, to a world of complex, intricate architecture to
   address a wide-spread and heterogeneous environment.  Because
   individual pieces of the architecture are often designed by
   sub-communities, with each sub-community having its own set of
   interests, it is necessary to pay increasing attention to how each
   piece fits into the larger picture, and to consider how each piece is
   chosen.  For example, it is unavoidable that each of us is inclined
   to solve a problem at the layer of the protocol stack and using the
   tools that we understand the best;  that does not necessarily mean
   that this is the most appropriate layer or set of tools for solving
   this problem in the long-term.



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   Our assumption is that this document will be used as suggestions (but
   not a checklist!) of issues to be addressed by IETF members in
   chartering new working groups, in working in working groups, and in
   evaluating the output from other working groups.  This document is
   not a primer on how to design protocols and architectures, and does
   not provide answers to anything.

2.  Relationship to "Architectural Principles of the Internet"

   RFC 1958 [RFC1958] outlines some architectural principles of the
   Internet, as "guidelines that have been found useful in the past, and
   that may be useful to those designing new protocols or evaluating
   such designs." An example guideline is that "it is also generally
   felt that end-to-end functions can best be realized by end-to-end
   protocols." Similarly, an example design issue from [RFC1958] is that
   "heterogeneity is inevitable and must be supported by design."

   In contrast, this document serves a slightly different purpose, by
   suggesting additional architectural questions to be addressed.  Thus,
   one question suggested in this document is the following: "Is this
   proposal the best long-term solution to the problem?  If not, what
   are the long-term costs of this solution, in terms of restrictions on
   future development, if any?" This question could be translated to a
   roughly equivalent architectural guideline, as follows: "Identify
   whether the proposed protocol is a long-term solution or a short-term
   solution, and identify the long-term costs and the exit strategy for
   any short-term solutions."

   In contrast, other questions are more open-ended, such as the
   question about robustness: "How robust is the protocol, not just to
   the failure of nodes, but also to compromised or malfunctioning
   components, imperfect or defective implementations, etc?" As a
   community, we are still learning about the degree of robustness that
   we are able to build into our protocols, as well as the tools that
   are available to ensure this robustness.  Thus, there are not yet
   clear architectural guidelines along the lines of "Ensure that your
   protocol is robust against X, Y, and Z."

3.  Questions

   In this section we list some questions to ask in designing protocols.
   Each question is discussed more depth in the rest of this paper.  We
   aren't suggesting that all protocol design efforts should be required
   to explicitly answer all of these questions; some questions will be
   more relevant to one document than to another.  We also aren't
   suggesting that this is a complete list of architectural concerns.





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   DESIGN QUESTIONS:

   Justifying the Solution:

   * Why are you proposing this solution, instead of proposing something
   else, or instead of using existing protocols and procedures?

   Interactions between Layers:

   * Why are you proposing a solution at this layer of the protocol
   stack, rather than at another layer?  Are there solutions at other
   layers of the protocol stack as well?

   * Is this an appropriate layer in terms of correctness of function,
   data integrity, performance, ease of deployment, the diagnosability
   of failures, and other concerns?

   * What are the interactions between layers, if any?

   Long-term vs. Short-term Solutions:

   * Is this proposal the best long-term solution to the problem?

   * If not, what are the long-term costs of this solution, in terms of
   restrictions on future development, if any?  What are the
   requirements for the development of longer-term solutions?

   The Whole Picture vs. Building Blocks:

   * Have you considered the larger context, while appropriately
   restricting your own design efforts to one part of the whole?

   * Are there parts of the overall solution that will have to be
   provided by other IETF Working Groups or by other standards bodies?

   EVALUATION QUESTIONS:

   Weighing Benefits against Costs:

   * How do the architectural benefits of a proposed new protocol
   compare against the architectural costs, if any?  Have the
   architectural costs been carefully considered?

   Robustness:

   * How robust is the protocol, not just to the failure of nodes, but
   also to compromised or malfunctioning components, imperfect or
   defective implementations, etc?



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   * Does the protocol take into account the realistic conditions of the
   current or future Internet (e.g., packet drops and packet corruption;
   packet reordering; asymmetric routing; etc.)?

   Tragedy of the Commons:

   * Is performance still robust if everyone is using this protocol?
   Are there other potential impacts of widespread deployment that need
   to be considered?

   Protecting Competing Interests:

   * Does the protocol protect the interests of competing parties (e.g.,
   not only end-users, but also ISPs, router vendors, software vendors,
   or other parties)?

   Designing for Choice vs. Avoiding Unnecessary Complexity:

   * Is the protocol designed for choice, to allow different players to
   express their preferences where appropriate?  At the other extreme,
   does the protocol provide so many choices that it threatens
   interoperability or introduces other significant problems?

   Preserving Evolvability?

   * Does the protocol protect the interests of the future, by
   preserving the evolvability of the Internet?  Does the protocol
   enable future developments?

   * If an old protocol is overloaded with new functionality, or reused
   for new purposes, have the possible complexities introduced been
   taken carefully into account?

   * For a protocol that introduces new complexity to the Internet
   architecture, how does the protocol add robustness and preserve
   evolvability, and how does it also introduce new fragilities to the
   system?

   Internationalization:

   * Where protocols require elements in text format, have the possibly
   conflicting requirements of global comprehensibility and the ability
   to represent local text content been properly weighed against each
   other?







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   DEPLOYMENT QUESTIONS:

   * Is the protocol deployable?

   Each of these questions is discussed in more depth in the rest of
   this paper.

4.  Justifying the Solution

   Question: Why are you proposing this solution, instead of proposing
   something else, or instead of using existing protocols and
   procedures?

4.1.  Case study: Integrated and Differentiated Services

   A good part of the work of developing integrated and differentiated
   services has been to understand the problem to be solved, and to come
   to agreement on the architectural framework of the solution, and on
   the nature of specific services.  Thus, when integrated services were
   being developed, the specification of the Controlled Load [RFC2211]
   and Guaranteed [RFC2212] services each required justification of the
   need for that particular service, of low loss and bounded delay
   respectively.

   Later, when RFC 2475 on "An Architecture for Differentiated Services"
   proposed a scalable, service differentiation architecture that
   differs from the previously-defined architecture for integrated
   services, the document also had to clearly justify the requirements
   for this new architecture, and compare the proposed architecture to
   other possible approaches [RFC2475].  Similarly, when the Assured
   Forwarding [RFC2597] and Expedited Forwarding [RFC3246] Per-Hop
   Behaviors of differentiated services were proposed, each service
   required a justification of the need for that service in the
   Internet.

5. Interactions between Layers

   Questions: Why are you proposing a solution at this layer of the
   protocol stack, rather than at another layer?  Are there solutions at
   other layers of the protocol stack as well?

   Is this an appropriate layer in terms of correctness of function,
   data integrity, performance, ease of deployment, the diagnosability
   of failures, and other concerns?

   What are the interactions between layers, if any?





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5.1.  Discussion: The End-to-End Argument

   The classic 1984 paper on "End-To-End Arguments In System Design"
   [SRC84] begins a discussion of where to place functions among modules
   by suggesting that "functions placed at low levels of a system may be
   redundant or of little value when compared with the cost of providing
   them at that low level.  Examples discussed in the paper include bit
   error recovery, security using encryption, duplicate message
   suppression, recovery from system crashes, and delivery
   acknowledgement.  Low level mechanisms to support these functions are
   justified only as performance enhancements."  The end-to-end
   principle is one of the key architectural guidelines to consider in
   choosing the appropriate layer for a function.

5.2.  Case study: Endpoint Congestion Management

   The goal of the Congestion Manager in Endpoint Congestion Management
   is to allow multiple concurrent flows with the same source and
   destination address to share congestion control state [RFC3124].
   There has been a history of proposals for multiplexing flows at
   different levels of the protocol stack; proposals have included
   adding multiplexing at the HTTP (WebMux) and TCP (TCP Control Blocks)
   layers, as well as below TCP (the Congestion Manager) [Multiplexing].

   However, the 1989 article on "Layered Multiplexing Considered
   Harmful" suggests that "the extensive duplication of multiplexing
   functionality across the middle and upper layers is harmful and
   should be avoided" [T89].  Thus, one of the key issues in providing
   mechanisms for multiplexing flows is to determine which layer or
   layers of the protocol stack are most appropriate for providing this
   functionality.  The natural tendency of each researcher is generally
   to add functionality at the layer that they know the best; this does
   not necessarily result in the most appropriate overall architecture.

5.3.  Case study: Layering Applications on Top of HTTP

   There has been considerable interest in layering applications on top
   of HTTP [RFC3205].  Reasons cited include compatibility with widely-
   deployed browsers, the ability to reuse client and server libraries,
   the ability to use existing security mechanisms, and the ability to
   traverse firewalls.  As RFC 3205 discusses, "the recent interest in
   layering new protocols over HTTP has raised a number of questions
   when such use is appropriate, and the proper way to use HTTP in
   contexts where it is appropriate." Thus, RFC 3205 addresses not only
   the benefits of layering applications on top of HTTP, but also
   evaluates the additional complexity and overhead of layering an
   application on top of HTTP, compared to the costs of introducing a
   special-purpose protocol.



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   The web page on "References on Layering and the Internet
   Architecture" has pointers to additional papers discussing general
   layering issues in the Internet architecture [Layering].

6.  Long-term vs. Short-term Solutions

   Questions: Is this proposal the best long-term solution to the
   problem?

   If not, what are the long-term costs of this solution, in terms of
   restrictions on future development, if any?  What are the
   requirements for the development of longer-term solutions?

6.1.  Case study: Traversing NATs

   In order to address problems with NAT middleboxes altering the
   external address of endpoints, various proposals have been made for
   mechanisms where an originating process attempts to determine the
   address (and port) by which it is known on the other side of a NAT.
   This would allow an originating process to be able to use address
   data in the protocol exchange, or to advertise an external address
   from which it will receive connections.

   The IAB in [RFC3424] has outlined reasons why these proposals can be
   considered, at best, short-term fixes to specific problems, and the
   specific issues to be carefully evaluated before standardizing such
   proposals.  These issues include the identification of the
   limited-scope problem to be fixed, the description of an exit
   strategy for the short-term solution, and the description of the
   longer-term problems left unsolved by the short-term solution.

7.  Looking at the whole picture vs. making a building block

   For a complex protocol which interacts with protocols from other
   standards bodies as well as from other IETF working groups, it can be
   necessary to keep in mind the overall picture while, at the same
   time, breaking out specific parts of the problem to be standardized
   in particular working groups.

   Question: Have you considered the larger context, while restricting
   your own design efforts to one part of the whole?

   Question: Are there parts of the overall solution that will have to
   be provided by other IETF Working Groups or by other standards
   bodies?






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7.1.  Case Study: The Session Initiation Protocol (SIP)

   The Session Initiation Protocol (SIP) [RFC2543], for managing
   connected, multimedia sessions,  is an example of a complex protocol
   that has been broken into pieces for standardization in other working
   groups.  SIP has also involved interaction with other standardization
   bodies.

   The basic SIP framework is being standardized by the SIP working
   group.  This working group has focused on the core functional
   features of setting up, managing, and tearing down multimedia
   sessions.  Extensions are considered if they relate to these core
   features.

   The task of setting up a multimedia session also requires a
   description of the desired multimedia session.  This is provided by
   the Session Description Protocol (SDP).  SDP is a building block that
   is supplied by the Multiparty Multimedia Session Control (MMUSIC)
   working group.  It is not standardized within the SIP working group.

   Other working groups are involved in standardizing extensions to SIP
   that fall outside of core functional features or applications.  The
   SIPPING working group is analyzing the requirements for SIP applied
   to different tasks, and the SIMPLE working group is examining the
   application of SIP to instant messaging and presence.  The IPTEL
   working group is defining a call processing language (CPL) that
   interacts with SIP in various ways.  These working groups
   occasionally feed requirements back into the main SIP working group.

   Finally, outside standardization groups have been very active in
   providing the SIP working group with requirements.  The Distributed
   Call Signaling (DCS) group from the PacketCable Consortium, 3GPP, and
   3GPP2 are all using SIP for various telephony-related applications,
   and members of these groups have been involved in drafting
   requirements for SIP.  In addition, there are extensions of SIP which
   are under consideration in these standardization bodies.  Procedures
   are under development in the IETF to address proposed extensions to
   SIP, including a category of preliminary, private, or proprietary
   extensions, and to provide for the safe management of the SIP
   namespace [MBMWOR02].

8.  Weighing architectural benefits against architectural costs

   Questions: How do the architectural benefits of a proposed new
   protocol compare against the architectural costs, if any?  Have the
   architectural costs been carefully considered?





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8.1.  Case Study: Performance-enhancing proxies (PEPs)

   RFC 3135 [RFC3135] considers the relative costs and benefits of
   placing performance-enhancing proxies (PEPs) in the middle of a
   network to address link-related degradations.  In the case of PEPs,
   the potential costs include disabling the end-to-end use of IP layer
   security mechanisms; introducing a new possible point of failure that
   is not under the control of the end systems; adding increased
   difficulty in diagnosing and dealing with failures; and introducing
   possible complications with asymmetric routing or mobile hosts.  RFC
   3135 carefully considers these possible costs, the mitigations that
   can be introduced, and the cases when the benefits of
   performance-enhancing proxies to the user are likely to outweigh the
   costs.

8.2.  Case Study: Open Pluggable Edge Services (OPES)

   One of the issues raised by middleboxes in the Internet involves the
   end-to-end integrity of data.  This is illustrated in the recent
   question of chartering the Open Pluggable Edge Services (OPES)
   Working Group.  Open Pluggable Edge Services are services that would
   be deployed as application-level intermediaries in the network, for
   example, at a web proxy cache between the origin server and the
   client.  These intermediaries would transform or filter content, with
   the explicit consent of either the content provider or the end user.

   One of the architectural issues that arose in the process of
   chartering the OPES Working Group concerned the end-to-end integrity
   of data.  As an example, it was suggested that "OPES would reduce
   both the integrity, and the perception of integrity, of
   communications over the Internet, and would significantly increase
   uncertainly about what might have been done to content as it moved
   through the network", and that therefore the risks of OPES outweighed
   the benefits [CDT01].

   As one consequence of this debate, the IAB wrote a document on "IAB
   Architectural and Policy Considerations for OPES", considering both
   the potential architectural benefits and costs of OPES [RFC3238].
   This document did not recommend specific solutions or mandate
   specific functional requirements, but instead included
   recommendations of issues such as concerns about data integrity that
   OPES solutions standardized in the IETF should be required to
   address.








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9.  General Robustness Questions

   Questions: How robust is the protocol, not just to the failure of
   nodes, but also to compromised or malfunctioning components,
   imperfect or defective implementations, etc?

   Does the protocol take into account the realistic conditions of the
   current or future Internet (e.g., packet drops and packet corruption,
   packet reordering, asymmetric routing, etc.)?

9.1.  Discussion: Designing for Robustness

   Robustness has long been cited as one of the overriding goals of the
   Internet architecture [Clark88].  The robustness issues discussed in
   [Clark88] largely referred to the robustness of packet delivery even
   in the presence of failed routers;  today robustness concerns have
   widened to include a goal of robust performance in the presence of a
   wide range of failures, buggy code, and malicious actions.

   As [ASSW02] argues, protocols need to be designed somewhat
   defensively, to maximize robustness against inconsistencies and
   errors.  [ASSW02] discusses several approaches for increasing
   robustness in protocols, such as verifying information whenever
   possible; designing interfaces that are conceptually simple and
   therefore less conducive to error; protecting resources against
   attack or overuse; and identifying and exposing errors so that they
   can be repaired.

   Techniques for verifying information range from verifying that
   acknowledgements in TCP acknowledge data that was actually sent, to
   providing mechanisms for routers to verify information in routing
   messages.

   Techniques for protecting resources against attack range from
   preventing "SYN flood" attacks by designing protocols that don't
   allocate resources for a single SYN packet, to partitioning resources
   (e.g., preventing one flow or aggregate from using all of the link
   bandwidth).

9.2.  Case Study: Explicit Congestion Notification (ECN)

   The Internet is based on end-to-end congestion control, and
   historically the Internet has used packet drops as the only method
   for routers to indicate congestion to the end nodes.  ECN [RFC3168]
   is a recent addition to the IP architecture to allow routers to set a
   bit in the IP packet header to inform end-nodes of congestion,
   instead of dropping the packet.




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   The first, Experimental specification of ECN [RFC3168] contained an
   extensive discussion of the dangers of a rogue or broken router
   "erasing" information from the ECN field in the IP header, thus
   preventing indications of congestion from reaching the end-nodes.  To
   add robustness, the standards-track specification [RFC3168] specified
   an additional codepoint in the IP header's ECN field, to use for an
   ECN "nonce".  The development of the ECN nonce was motivated by
   earlier research on specific robustness issues in TCP [SCWA99].  RFC
   3168 explains that the addition of the codepoint "is motivated
   primarily by the desire to allow mechanisms for the data sender to
   verify that network elements are not erasing the CE codepoint, and
   that data receivers are properly reporting to the sender the receipt
   of packets with the CE codepoint set, as required by the transport
   protocol." Supporting mechanisms for the ECN nonce are needed in the
   transport protocol to ensure robustness of delivery of the ECN-based
   congestion indication.

   In contrast, a more difficult and less clear-cut robustness issue for
   ECN concerns the differential treatment of packets in the network by
   middleboxes, based on the TCP header's ECN flags in a TCP SYN packet
   [RFC3360].  The issue concerns "ECN-setup" SYN packets, that is, SYN
   packets with ECN flags set in the TCP header to negotiate the use of
   ECN between the two TCP end-hosts.  There exist firewalls in the
   network that drop "ECN-setup" SYN packets, others that send TCP Reset
   messages, and yet others that zero ECN flags in TCP headers.  None of
   this was anticipated by the designers of ECN, and RFC 3168 added
   optional mechanisms to permit the robust operation of TCP in the
   presence of firewalls that drop "ECN-setup" SYN packets.  However,
   ECN is still not robust to all possible scenarios of middleboxes
   zeroing ECN flags in the TCP header.  Up until now, transport
   protocols have been standardized independently from the mechanisms
   used by middleboxes to control the use of these protocols, and it is
   still not clear what degree of robustness is required from transport
   protocols in the presence of the unauthorized modification of
   transport headers in the network.  These and similar issues are
   discussed in more detail in [RFC3360].

10.  Avoiding Tragedy of the Commons

   Question: Is performance still robust if everyone is using the new
   protocol?  Are there other potential impacts of widespread deployment
   that need to be considered?

10.1.  Case Study: End-to-end Congestion Control

   [RFC2914] discusses the potential for congestion collapse if flows
   are not using end-to-end congestion control in a time of high
   congestion.  For example, if a new transport protocol was proposed



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   that did not use end-to-end congestion control, it might be easy to
   show that an individual flow using the new transport protocol would
   perform quite well as long as all of the competing flows in the
   network were using end-to-end congestion control.  To fully evaluate
   the new transport protocol, it is necessary to look at performance
   when many flows are competing, all using the new transport protocol.
   If all of the competing flows were using the more aggressive
   transport protocol in a time of high congestion, the result could be
   high steady-state packet drop rates and reduced overall throughput,
   with busy links carrying packets that will only be dropped
   downstream.  This can be viewed as a form of tragedy of the commons,
   when a practice that is productive if done by only one person (e.g.,
   adding a few more sheep to the common grazing pasture) is instead
   counter-productive when done by everyone [H68].

11.  Balancing Competing Interests

   Question: Does the protocol protect the interests of competing
   parties (e.g., not only end-users, but also ISPs, router vendors,
   software vendors, or other parties)?

11.1.  Discussion: balancing competing interests

   [CWSB02] discusses the role that competition between competing
   interests plays in the evolution of the Internet, and takes the
   position that the role of Internet protocols is to design the playing
   field in this competition, rather than to pick the outcome.  The IETF
   has long taken the position that it can only design the protocols,
   and that often two competing approaches will be developed, with the
   marketplace left to decide between them [A02].  (It has also been
   suggested that "the marketplace" left entirely to itself does not
   always make the best decisions, and that working to identify and
   adopt the technically best solution is sometimes helpful.  Thus,
   while the role of the marketplace should not be ignored, the
   decisions of the marketplace should also not be held as sacred or
   infallible.)

   An example cited in [CWSB02] of modularization in support of
   competing interests is the decision to use codepoints in the IP
   header to select QoS, rather than binding QoS to other properties
   such as port numbers.  This separates the structural and economic
   issues related to QoS from technical issues of protocols and port
   numbers, and allows space for a wide range of structural and pricing
   solutions to emerge.

   There have been perceived problems over the years of individuals
   adding unnecessary complexity to IETF protocols, increasing the
   barrier to other implementations of those protocols.  Clearly such



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   action would not be protecting the interests of open competition.
   Underspecified protocols can also serve as an unnecessary barrier to
   open competition.

12.  Designing for Choice vs. Avoiding Unnecessary Complexity:

   Is the protocol designed for choice, to allow different players to
   express their preferences where appropriate?  At the other extreme,
   does the protocol provide so many choices that it threatens
   interoperability or introduces other significant problems?

12.1.  Discussion: the importance of choice

   [CWSB02] suggests that "the fundamental design goal of the Internet
   is to hook computers together, and since computers are used for
   unpredictable and evolving purposes, making sure that the users are
   not constrained in what they can do is doing nothing more than
   preserving the core design tenet of the Internet.  In this context,
   user empowerment is a basic building block, and should be embedded
   into all mechanism whenever possible."

   As an example of choice, "the design of the mail system allows the
   user to select his SMTP server and his POP server" [CWSB02].  More
   open-ended questions about choice concern the design of mechanisms
   that would enable the user to choose the path at the level of
   providers, or to allow users to choose third-party intermediaries
   such as web caches, or providers for Open Pluggable Edge Services
   (OPES).  [CWSB02] also notes that the issue of choice itself reflects
   competing interests.  For example, ISPs would generally like to lock
   in customers, while customers would like to preserve their ability to
   change among providers.

   At the same time, we note that excessive choice can lead to "kitchen
   sink" protocols that are inefficient and hard to understand, have too
   much negotiation, or have unanticipated interactions between options.
   For example, [P99] notes that excessive choice can lead to difficulty
   in ensuring interoperability between two independent, conformant
   implementations of the protocol.

   The dangers of excessive options are also discussed in [MBMWOR02],
   which gives guidelines for responding to the "continuous flood" of
   suggestions for modifications and extensions to SIP (Session
   Initiation Protocol).  In particular, the SIP Working Group is
   concerned that proposed extensions have general use, and do not
   provide efficiency at the expense of simplicity or robustness.
   [MBMWOR02] suggests that other highly extensible protocols developed
   in the IETF might also benefit from more coordination of extensions.




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13.  Preserving evolvability?

   Does the protocol protect the interests of the future, by preserving
   the evolvability of the Internet?  Does the protocol enable future
   developments?

   If an old protocol is overloaded with new functionality, or reused
   for new purposes, have the possible complexities introduced been
   taken into account?

   For a protocol that introduces new complexity to the Internet
   architecture, does the protocol add robustness and preserve
   evolvability?  Does it also introduce unwanted new fragilities to the
   system?

13.1.  Discussion: evolvability

   There is an extensive literature and an ongoing discussion about the
   evolvability, or lack of evolvability, of the Internet
   infrastructure; the web page on "Papers on the Evolvability of the
   Internet Infrastructure" has pointers to some of this literature
   [Evolvability].  Issues range from the evolvability and overloading
   of the DNS; the difficulties of the Internet in evolving to
   incorporate multicast, QoS, or IPv6; the difficulties of routing in
   meeting the demands of a changing and expanding Internet; and the
   role of firewalls and other middleboxes in limiting evolvability.

   [CWSB02] suggests that among all of the issues of evolvability,
   "keeping the net open and transparent for new applications is the
   most important goal."  In the beginning, the relative transparency of
   the infrastructure was sufficient to ensure evolvability, where a
   "transparent" network simply routes packets from one end-node to
   another.  However, this transparency has become more murky over time,
   as cataloged in [RFC3234], which discusses the ways that middleboxes
   interact with existing protocols and increase the difficulties in
   diagnosing failures.  [CWSB02] realistically suggests the following
   guideline: "Failures of transparency will occur - design what happens
   then," where examples of failures of transparency include firewalls,
   application filtering, connection redirection, caches, kludges to
   DNS, and the like.  Thus, maintaining evolvability also requires
   mechanisms for allowing evolution in the face of a lack of
   transparency of the infrastructure itself.

   One of the ways for maintaining evolvability is for designers of new
   mechanisms and protocols to be as clear as possible about the
   assumptions that are being made about the rest of the network.  New





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   mechanisms that make unwarranted assumptions about the network can
   end up placing unreasonable constraints on the future evolution of
   the network.

13.2.  Discussion: overloading

   There has been a strong tendency in the last few years to overload
   some designs with new functionality, with resulting operational
   complexities.  Extensible protocols could be seen as one of the tools
   for providing evolvability.  However, if protocols and systems are
   stretched beyond their reasonable design parameters, then scaling,
   reliability, or security issues could be introduced.  Examples of
   protocols that have been seen as either productively extended, or as
   dangerously overloaded, or both, include DNS [K02,RFC3403], MPLS
   [A02a], and BGP [B02].  In some cases, overloading or extending a
   protocol may reduce total complexity and deployment costs by avoiding
   the creation of a new protocol; in other cases a new protocol might
   be the simpler solution.

   We have a number of reusable technologies, including component
   technologies specifically designed for re-use.  Examples include
   SASL, BEEP and APEX.  TCP and UDP can also be viewed as reusable
   transport protocols, used by a range of applications.  On the other
   hand, re-use should not go so far as to turn a protocol into a Trojan
   Horse, as has happened with HTTP [RFC3205].

13.3.  Discussion: complexity, robustness, and fragility

   [WD02] gives a historical account of the evolution of the Internet as
   a complex system, with particular attention to the tradeoffs between
   complexity, robustness, and fragility.  [WD02] describes the
   robustness that follows from the simplicity of a connectionless,
   layered, datagram infrastructure and a universal logical addressing
   scheme, and, as case studies, describes the increasing complexity of
   TCP and of BGP.  The paper describes a complexity/robustness spiral
   of an initially robust design and the appearance of fragilities,
   followed by modifications for more robustness that themselves
   introduce new fragilities.  [WD02] conjectures that "the Internet is
   only now beginning to experience an acceleration of this
   complexity/robustness spiral and, if left unattended, can be fully
   expected to experience arcane, irreconcilable, and far-reaching
   robustness problems in the not-too-distant future."  Citing [WD02],
   [BFM02] focuses on the ways that complexity increases capital and
   operational expenditures in carrier IP network, and views complexity
   as the primary mechanism that impedes efficient scaling.






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14.  Internationalization

   Where protocols require elements in text format, have the possibly
   conflicting requirements of global comprehensibility and the ability
   to represent local text content been properly weighed against each
   other?

14.1.  Discussion: internationalization

   RFC 1958 [RFC1958] included a simple statement of the need for a
   common language:

   "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."

   The IETF has studied character set issues in general [RFC 2130] and
   made specific recommendations for the use of a standardized approach
   [RFC 2277].  The situation is complicated by the fact that some uses
   of text are hidden entirely in protocol elements and need only be
   read by machines, while other uses are intended entirely for human
   consumption.  Many uses lie between these two extremes, which leads
   to conflicting implementation requirements.

   For the specific case of DNS, the Internationalized Domain Name
   working group is considering these issues.  As stated in the charter
   of that working group, "A fundamental requirement in this work is to
   not disturb the current use and operation of the domain name system,
   and for the DNS to continue to allow any system anywhere to resolve
   any domain name."  This leads to some very strong requirements for
   backwards compatibility with the existing ASCII-only DNS.  Yet since
   the DNS has come to be used as if it was a directory service, domain
   names are also expected to be presented to users in local character
   sets.

   This document does not attempt to resolve these complex and difficult
   issues, but simply states this as an issue to be addressed in our
   work.  The requirement that names encoded in a text format within
   protocol elements be universally decodable (i.e. encoded in a
   globally standard format with no intrinsic ambiguity) does not seem
   likely to change.  However, at some point, it is possible that this
   format will no longer be case-independent ASCII.









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15.  Deployability

   Question: Is the protocol deployable?

15.1.  Discussion: deployability

   It has been suggested that the failure to understand deployability
   considerations in the current environment is one of the major
   weakness of the IETF.  As examples of deployment difficulties, RFC
   2990 [RFC2990] discusses deployment difficulties with Quality of
   Service (QoS) architectures, various documents of the MBONE
   Deployment Working Group address deployment problems with IP
   Multicast, and the IPv6 Working Group discusses a wealth of issues
   related to the deployment of IPv6.  [CN02] discusses how the
   deployment of Integrated Services has been limited by factors such as
   the failure to take into account administrative boundaries.
   Additional papers on difficulties in the evolution of the Internet
   architecture are available from [Evolvability].

   Issues that can complicate deployment include whether the protocol is
   compatible with pre-existing standards, and whether the protocol is
   compatible with the installed base.  For example, a transport
   protocol is more likely to be deployable if it performs correctly and
   reasonably robustly in the presence of dropped, reordered,
   duplicated, delayed, and rerouted packets, as all of this can occur
   in the current Internet.

16.  Conclusions

   This document suggests general architectural and policy questions to
   be addressed when working on new protocols and standards in the IETF.

   The case studies in this document have generally been short
   illustrations of how the questions proposed in the document have been
   addressed in working groups in the past.  However, we have generally
   steered away from case studies of more controversial issues, where
   there is not yet a consensus in the IETF community.  Thus, we
   side-stepped suggestions for adding a case study for IKE (Internet
   Key Exchange) as an possible example of a protocol with too much
   negotiation, or of adding a case study of network management
   protocols as illustrating the possible costs of leaving things to the
   marketplace to decide.  We would encourage others to contribute case
   studies of these or any other issues that may shed light on some of
   the questions in this document;  any such case studies could include
   a careful presentation of the architectural reasoning on both sides.






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   we would conjecture that there is a lot to be learned, in terms of
   the range of answers to the questions posed in this document, by
   studying the successes, failures, and other struggles of the past.

   We would welcome feedback on this document for future revisions.
   Feedback could be send to the editor, Sally Floyd, at floyd@icir.org.

17.  Acknowledgements

   This document has borrowed text freely from other IETF RFCs, and has
   drawn on ideas from [ASSW02], [CWSB02], [M01] and elsewhere.  This
   document has developed from discussions in the IAB, and has drawn
   from suggestions made at IAB Plenary sessions and on the ietf general
   discussion mailing list.  The case study on SIP was contributed by
   James Kempf, an early case study on Stresses on DNS was contributed
   by Karen Sollins, and Keith Moore contributed suggestions that were
   incorporated in a number of places in the document.  The discussions
   on Internationalization and on Overloading were based on an earlier
   document by Brian Carpenter and Rob Austein.  We have also benefited
   from discussions with Noel Chiappa, Karen Sollins, John Wroclawski,
   and others, and from helpful feedback from members of the IESG.  We
   specifically thank Senthilkumar Ayyasamy, John Loughney, Keith Moore,
   Eric Rosen, and Dean Willis and others for feedback on various stages
   of this document.

18.  Normative References

19.  Informative References

   [A02]          Harald Alvestrand, "Re: How many standards or
                  protocols...", email to the ietf discussion mailing
                  list, Message-id:  <598204031.1018942481@localhost>,
                  April 16, 2002.

   [A02a]         Loa Andersson, "The Role of MPLS in Current IP
                  Network", MPLS World News, September 16, 2002.  URL
                  "http://www.mplsworld.com/archi_drafts/focus/analy-
                  andersson.htm".

   [ASSW02]       T. Anderson, S. Shenker, I. Stoica, and D. Wetherall,
                  "Design Guidelines for Robust Internet Protocols",
                  HotNets-I, October 2002.

   [BFM02]        Randy Bush, Tim Griffin, and David Meyer, "Some
                  Internet Architectural Guidelines and Philosophy",
                  Work in Progress.





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   [B02]          Hamid Ould-Brahim, Bryan Gleeson, Peter Ashwood-Smith,
                  Eric C. Rosen, Yakov Rekhter, Luyuan Fang, Jeremy De
                  Clercq, Riad Hartani, and Tissa Senevirathne, "Using
                  BGP as an Auto-Discovery Mechanism for Network-based
                  VPNs", Work in Progress.

   [CDT01]        Policy Concerns Raised by Proposed OPES Working Group
                  Efforts, email to the IESG, from the Center for
                  Democracy & Technology, August 3, 2001.  URL
                  "http://www.imc.org/ietf-openproxy/mail-
                  archive/msg00828.html".

   [Clark88]      David D. Clark, The Design Philosophy of the DARPA
                  Internet Protocols, SIGCOMM 1988.

   [CN02]         Brian Carpenter, Kathleen Nichols, "Differentiated
                  Services in the Internet", Technical Report, February
                  2002, URL "http://www.research.ibm.com/resources/
                  paper_search.shtml".

   [CWSB02]       Clark, D., Wroslawski, J., Sollins, K., and Braden,
                  R., "Tussle in Cyberspace: Defining Tomorrow's
                  Internet", SIGCOMM 2002.  URL
                  "http://www.acm.org/sigcomm/sigcomm2002/papers/
                  tussle.html".

   [Evolvability] Floyd, S., "Papers on the Evolvability of the Internet
                  Infrastructure".  Web Page, URL
                  "http://www.icir.org/floyd/evolution.html".

   [H68]          Garrett Hardin, "The Tragedy of the Commons", Science,
                  V. 162, 1968, pp. 1243-1248.  URL
                  "http://dieoff.org/page95.htm".

   [K02]          John C. Klensin, "Role of the Domain Name System",
                  Work in Progress.

   [Layering]     Floyd, S., "References on Layering and the Internet
                  Architecture", Web Page, URL
                  "http://www.icir.org/floyd/layers.html".

   [Multiplexing] S. Floyd, "Multiplexing, TCP, and UDP: Pointers to the
                  Discussion", Web Page, URL
                  "http://www.icir.org/floyd/tcp_mux.html".







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RFC 3426        Architectural and Policy Considerations    November 2002


   [MBMWOR02]     Mankin, A., Bradner, S., Mahy, R., Willis, D., Ott, J.
                  and B. Rosen, "Change Process for the Session
                  Initiation Protocol (SIP)", BCP 67, RFC 3427, November
                  2002.

   [M01]          Tim Moors, A Critical Review of End-to-end Arguments
                  in System Design, 2001.  URL
                  "http://uluru.poly.edu/~tmoors/".

   [P99]          Radia Perlman, "Protocol Design Folklore", in
                  Interconnections Second Edition: Bridges, Routers,
                  Switches, and Internetworking Protocols, Addison-
                  Wesley, 1999.

   [RFC1958]      Carpenter, B.,  "Architectural Principles of the
                  Internet", RFC 1958, June 1996.

   [RFC2211]      Wroclawski, J., "Specification of the Controlled Load
                  Quality of Service", RFC 2211, September 1997.

   [RFC2212]      Shenker, S., Partridge, C., and R. Guerin,
                  "Specification of Guaranteed Quality of Service", RFC
                  2212, September 1997.

   [RFC2316]      Bellovin, S., "Report of the IAB Security Architecture
                  Workshop", RFC 2316, April 1998.

   [RFC2475]      Blake, S., Black, D., Carlson, M., Davies, E., Wang,
                  Z.  and W. Weiss, "An Architecture for Differentiated
                  Services", RFC 2475, December 1998.

   [RFC2543]      Handley, M., Schulzrinne, H., Schooler, B. and J.
                  Rosenberg, "SIP: Session Initiation Protocol", RFC
                  25434, March 1999.

   [RFC2597]      Heinanen, J., Baker, F., Weiss, W. and J. Wroclawski,
                  "Assured Forwarding PHB Group", RFC 2597, June 1999.

   [RFC2990]      Huston, G., "Next Steps for the IP QoS Architecture",
                  RFC 2990, November 2000.

   [RFC3124]      Balakrishnan, H. and S. Seshan, "The Congestion
                  Manager", RFC 3124, June 2001.

   [RFC3135]      Border, J., Kojo, M., Griner, J., Montenegro, G. and
                  Z.  Shelby, "Performance Enhancing Proxies Intended to
                  Mitigate Link-Related Degradations", RFC 3135, June
                  2001.



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   [RFC3168]      Ramakrishnan, K. K., Floyd, S. and D. Black, "The
                  Addition of Explicit Congestion Notification (ECN) to
                  IP", RFC 3168, September 2001.

   [RFC3205]      Moore, K., "On the use of HTTP as a Substrate", BCP
                  56, RFC 3205, February 2002.

   [RFC3221]      Huston, G., "Commentary on Inter-Domain Routing in the
                  Internet", RFC 3221, December 2001.

   [RFC3234]      Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
                  Issues", RFC 3234, February 2002.

   [RFC3238]      Floyd, S. and L. Daigle, "IAB Architectural and Policy
                  Considerations for Open Pluggable Edge Services", RFC
                  3238, January 2002.

   [RFC3246]      Davie, B., Charny, A., Bennet, J. C. R., Benson, K.,
                  Le Boudec, J. Y., Courtney, W., Davari, S., Firoiu, V.
                  and D. Stiliadis, "An Expedited Forwarding PHB (Per-
                  Hop Behavior)", RFC 3246, March 2002.

   [RFC3360]      Floyd, S., "Inappropriate TCP Resets Considered
                  Harmful", BCP 60, RFC 3360, August 2002.

   [RFC3403]      Mealling, M., "Dynamic Delegation Discovery System
                  (DDDS) Part Three: The Domain Name System (DNS)
                  Database", RFC 3403, October 2002.

   [RFC3424]      Daigle, L., "IAB Considerations for UNilateral Self-
                  Address Fixing (UNSAF)", RFC 3424, November 2002.

   [SCWA99]       Stefan Savage, Neal Cardwell, David Wetherall, Tom
                  Anderson, "TCP Congestion Control with a Misbehaving
                  Receiver", ACM Computer Communications Review, October
                  1999.

   [SRC84]        J. Saltzer, D. Reed, and D. D. Clark, "End-To-End
                  Arguments In System Design", ACM Transactions on
                  Computer Systems, V.2, N.4, p.  277-88. 1984.

   [T89]          D. Tennenhouse, "Layered Multiplexing Considered
                  Harmful", Protocols for High-Speed Networks, 1989.

   [WD02]         Walter Willinger and John Doyle, "Robustness and the
                  Internet: Design and Evolution", draft, March 2002,
                  URL "http://netlab.caltech.edu/internet/".




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20.  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.

21.  IANA Considerations

   There are no IANA considerations regarding this document.

Authors' Addresses

   Internet Architecture Board
   EMail:  iab@iab.org

   IAB Membership at time this document was completed:

   Harald Alvestrand
   Ran Atkinson
   Rob Austein
   Fred Baker
   Leslie Daigle
   Steve Deering
   Sally Floyd
   Ted Hardie
   Geoff Huston
   Charlie Kaufman
   James Kempf
   Eric Rescorla
   Mike St. Johns



















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Full Copyright Statement

   Copyright (C) The Internet Society (2002).  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 or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and 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
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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