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Network Working Group                                   S. Floyd, EditorRequest for Comments: 3426                   Internet Architecture BoardCategory: Informational                                    November 2002General Architectural and Policy ConsiderationsStatus 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.Floyd                        Informational                      [Page 1]

RFC 3426        Architectural and Policy Considerations    November 2002   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.Floyd                        Informational                      [Page 2]

RFC 3426        Architectural and Policy Considerations    November 2002   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?Floyd                        Informational                      [Page 3]

RFC 3426        Architectural and Policy Considerations    November 2002   * 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?Floyd                        Informational                      [Page 4]

RFC 3426        Architectural and Policy Considerations    November 2002   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, whenRFC 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?Floyd                        Informational                      [Page 5]

RFC 3426        Architectural and Policy Considerations    November 20025.1.  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.  AsRFC 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.Floyd                        Informational                      [Page 6]

RFC 3426        Architectural and Policy Considerations    November 2002   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?Floyd                        Informational                      [Page 7]

RFC 3426        Architectural and Policy Considerations    November 20027.1.  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?Floyd                        Informational                      [Page 8]

RFC 3426        Architectural and Policy Considerations    November 20028.1.  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.RFC3135 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.Floyd                        Informational                      [Page 9]

RFC 3426        Architectural and Policy Considerations    November 20029.  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.Floyd                        Informational                     [Page 10]

RFC 3426        Architectural and Policy Considerations    November 2002   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].RFC3168 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, andRFC 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 proposedFloyd                        Informational                     [Page 11]

RFC 3426        Architectural and Policy Considerations    November 2002   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 suchFloyd                        Informational                     [Page 12]

RFC 3426        Architectural and Policy Considerations    November 2002   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.Floyd                        Informational                     [Page 13]

RFC 3426        Architectural and Policy Considerations    November 200213.  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.  NewFloyd                        Informational                     [Page 14]

RFC 3426        Architectural and Policy Considerations    November 2002   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.Floyd                        Informational                     [Page 15]

RFC 3426        Architectural and Policy Considerations    November 200214.  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: internationalizationRFC 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.Floyd                        Informational                     [Page 16]

RFC 3426        Architectural and Policy Considerations    November 200215.  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,RFC2990 [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.Floyd                        Informational                     [Page 17]

RFC 3426        Architectural and Policy Considerations    November 2002   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 References19.  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.Floyd                        Informational                     [Page 18]

RFC 3426        Architectural and Policy Considerations    November 2002   [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".Floyd                        Informational                     [Page 19]

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",RFC2212, 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",RFC25434, 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.Floyd                        Informational                     [Page 20]

RFC 3426        Architectural and Policy Considerations    November 2002   [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",BCP56,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",RFC3238, 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/".Floyd                        Informational                     [Page 21]

RFC 3426        Architectural and Policy Considerations    November 200220.  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. JohnsFloyd                        Informational                     [Page 22]

RFC 3426        Architectural and Policy Considerations    November 2002Full 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.Floyd                        Informational                     [Page 23]