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Network Working Group R. Bush Request for Comments: 3439 D. Meyer Updates: 1958 December 2002 Category: Informational

       Some Internet Architectural Guidelines and Philosophy

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.


 This document extends RFC 1958 by outlining some of the philosophical
 guidelines to which architects and designers of Internet backbone
 networks should adhere.  We describe the Simplicity Principle, which
 states that complexity is the primary mechanism that impedes
 efficient scaling, and discuss its implications on the architecture,
 design and engineering issues found in large scale Internet

Table of Contents

 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . .  2
 2. Large Systems and The Simplicity Principle . . . . . . . . .  3
 2.1. The End-to-End Argument and Simplicity   . . . . . . . . .  3
 2.2. Non-linearity and Network Complexity   . . . . . . . . . .  3
 2.2.1. The Amplification Principle. . . . . . . . . . . . . . .  4
 2.2.2. The Coupling Principle . . . . . . . . . . . . . . . . .  5
 2.3. Complexity lesson from voice. . . . .  . . . . . . . . . .  6
 2.4. Upgrade cost of complexity. . . . . .  . . . . . . . . . .  7
 3. Layering Considered Harmful. . . . . . . . . . . . . . . . .  7
 3.1. Optimization Considered Harmful . . .  . . . . . . . . . .  8
 3.2. Feature Richness Considered Harmful .  . . . . . . . . . .  9
 3.3. Evolution of Transport Efficiency for IP.  . . . . . . . .  9
 3.4. Convergence Layering. . . . . . . . . . .  . . . . . . . .  9
 3.4.1. Note on Transport Protocol Layering. . . . . . . . . . . 11
 3.5. Second Order Effects   . . . . . . . . . . . . . . . . . . 11
 3.6. Instantiating the EOSL Model with IP   . . . . . . . . . . 12
 4. Avoid the Universal Interworking Function. . . . . . . . . . 12
 4.1. Avoid Control Plane Interworking . . . . . . . . . . . . . 13

Bush, et. al. Informational [Page 1] RFC 3439 Internet Architectural Guidelines December 2002

 5. Packet versus Circuit Switching: Fundamental Differences . . 13
 5.1. Is PS is inherently more efficient than CS?  . . . . . . . 13
 5.2. Is PS simpler than CS? . . . . . . . . . . . . . . . . . . 14
 5.2.1. Software/Firmware Complexity . . . . . . . . . . . . . . 15
 5.2.2. Macro Operation Complexity . . . . . . . . . . . . . . . 15
 5.2.3. Hardware Complexity. . . . . . . . . . . . . . . . . . . 15
 5.2.4. Power. . . . . . . . . . . . . . . . . . . . . . . . . . 16
 5.2.5. Density. . . . . . . . . . . . . . . . . . . . . . . . . 16
 5.2.6. Fixed versus variable costs. . . . . . . . . . . . . . . 16
 5.2.7. QoS. . . . . . . . . . . . . . . . . . . . . . . . . . . 17
 5.2.8. Flexibility. . . . . . . . . . . . . . . . . . . . . . . 17
 5.3. Relative Complexity  . . . . . . . . . . . . . . . . . . . 17
 5.3.1. HBHI and the OPEX Challenge. . . . . . . . . . . . . . . 18
 6. The Myth of Over-Provisioning. . . . . . . . . . . . . . . . 18
 7. The Myth of Five Nines . . . . . . . . . . . . . . . . . . . 19
 8. Architectural Component Proportionality Law. . . . . . . . . 20
 8.1. Service Delivery Paths . . . . . . . . . . . . . . . . . . 21
 9. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . 21
 10. Security Considerations . . . . . . . . . . . . . . . . . . 22
 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 23
 12. References. . . . . . . . . . . . . . . . . . . . . . . . . 23
 13. Authors' Addresses. . . . . . . . . . . . . . . . . . . . . 27
 14. Full Copyright Statement. . . . . . . . . . . . . . . . . . 28

1. Introduction

 RFC 1958 [RFC1958] describes the underlying principles of the
 Internet architecture.  This note extends that work by outlining some
 of the philosophical guidelines to which architects and designers of
 Internet backbone networks should adhere.  While many of the areas
 outlined in this document may be controversial, the unifying
 principle described here, controlling complexity as a mechanism to
 control costs and reliability, should not be.  Complexity in carrier
 networks can derive from many sources.  However, as stated in
 [DOYLE2002], "Complexity in most systems is driven by the need for
 robustness to uncertainty in their environments and component parts
 far more than by basic functionality".  The major thrust of this
 document, then, is to raise awareness about the complexity of some of
 our current architectures, and to examine the effect such complexity
 will almost certainly have on the IP carrier industry's ability to
 The rest of this document is organized as follows: The first section
 describes the Simplicity Principle and its implications for the
 design of very large systems.  The remainder of the document outlines
 the high-level consequences of the Simplicity Principle and how it
 should guide large scale network architecture and design approaches.

Bush, et. al. Informational [Page 2] RFC 3439 Internet Architectural Guidelines December 2002

2. Large Systems and The Simplicity Principle

 The Simplicity Principle, which was perhaps first articulated by Mike
 O'Dell, former Chief Architect at UUNET, states that complexity is
 the primary mechanism which impedes efficient scaling, and as a
 result is the primary driver of increases in both capital
 expenditures (CAPEX) and operational expenditures (OPEX).  The
 implication for carrier IP networks then, is that to be successful we
 must drive our architectures and designs toward the simplest possible

2.1. The End-to-End Argument and Simplicity

 The end-to-end argument, which is described in [SALTZER] (as well as
 in RFC 1958 [RFC1958]), contends that "end-to-end protocol design
 should not rely on the maintenance of state (i.e., information about
 the state of the end-to-end communication) inside the network.  Such
 state should be maintained only in the end points, in such a way that
 the state can only be destroyed when the end point itself breaks."
 This property has also been related to Clark's "fate-sharing" concept
 [CLARK].  We can see that the end-to-end principle leads directly to
 the Simplicity Principle by examining the so-called "hourglass"
 formulation of the Internet architecture [WILLINGER2002].  In this
 model, the thin waist of the hourglass is envisioned as the
 (minimalist) IP layer, and any additional complexity is added above
 the IP layer.  In short, the complexity of the Internet belongs at
 the edges, and the IP layer of the Internet should remain as simple
 as possible.
 Finally, note that the End-to-End Argument does not imply that the
 core of the Internet will not contain and maintain state.  In fact, a
 huge amount coarse grained state is maintained in the Internet's core
 (e.g., routing state).  However, the important point here is that
 this (coarse grained) state is almost orthogonal to the state
 maintained by the end-points (e.g., hosts).  It is this minimization
 of interaction that contributes to simplicity.  As a result,
 consideration of "core vs. end-point" state interaction is crucial
 when analyzing protocols such as Network Address Translation (NAT),
 which reduce the transparency between network and hosts.

2.2. Non-linearity and Network Complexity

 Complex architectures and designs have been (and continue to be)
 among the most significant and challenging barriers to building cost-
 effective large scale IP networks.  Consider, for example, the task
 of building a large scale packet network.  Industry experience has
 shown that building such a network is a different activity (and hence
 requires a different skill set) than building a small to medium scale

Bush, et. al. Informational [Page 3] RFC 3439 Internet Architectural Guidelines December 2002

 network, and as such doesn't have the same properties.  In
 particular, the largest networks exhibit, both in theory and in
 practice, architecture, design, and engineering non-linearities which
 are not exhibited at smaller scale.  We call this Architecture,
 Design, and Engineering (ADE) non-linearity.  That is, systems such
 as the Internet could be described as highly self-dissimilar, with
 extremely different scales and levels of abstraction [CARLSON].  The
 ADE non-linearity property is based upon two well-known principles
 from non-linear systems theory [THOMPSON]:

2.2.1. The Amplification Principle

 The Amplification Principle states that there are non-linearities
 which occur at large scale which do not occur at small to medium
 COROLLARY: In many large networks, even small things can and do cause
 huge events.  In system-theoretic terms, in large systems such as
 these, even small perturbations on the input to a process can
 destabilize the system's output.
 An important example of the Amplification Principle is non-linear
 resonant amplification, which is a powerful process that can
 transform dynamic systems, such as large networks, in surprising ways
 with seemingly small fluctuations.  These small fluctuations may
 slowly accumulate, and if they are synchronized with other cycles,
 may produce major changes.  Resonant phenomena are examples of non-
 linear behavior where small fluctuations may be amplified and have
 influences far exceeding their initial sizes.  The natural world is
 filled with examples of resonant behavior that can produce system-
 wide changes, such as the destruction of the Tacoma Narrows bridge
 (due to the resonant amplification of small gusts of wind).  Other
 examples include the gaps in the asteroid belts and rings of Saturn
 which are created by non-linear resonant amplification.  Some
 features of human behavior and most pilgrimage systems are influenced
 by resonant phenomena involving the dynamics of the solar system,
 such as solar days, the 27.3 day (sidereal) and 29.5 day (synodic)
 cycles of the moon or the 365.25 day cycle of the sun.
 In the Internet domain, it has been shown that increased inter-
 connectivity results in more complex and often slower BGP routing
 convergence [AHUJA].  A related result is that a small amount of
 inter-connectivity causes the output of a routing mesh to be
 significantly more complex than its input [GRIFFIN].  An important
 method for reducing amplification is ensure that local changes have
 only local effect (this is as opposed to systems in which local
 changes have global effect).  Finally, ATM provides an excellent
 example of an amplification effect: if you lose one cell, you destroy

Bush, et. al. Informational [Page 4] RFC 3439 Internet Architectural Guidelines December 2002

 the entire packet (and it gets worse, as in the absence of mechanisms
 such as Early Packet Discard [ROMANOV], you will continue to carry
 the already damaged packet).
 Another interesting example of amplification comes from the
 engineering domain, and is described in [CARLSON].  They consider the
 Boeing 777, which is a "fly-by-wire" aircraft, containing as many as
 150,000 subsystems and approximately 1000 CPUs.  What they observe is
 that while the 777 is robust to large-scale atmospheric disturbances,
 turbulence boundaries, and variations in cargo loads (to name a few),
 it could be catastrophically disabled my microscopic alterations in a
 very few large CPUs (as the point out, fortunately this is a very
 rare occurrence).  This example illustrates the issue "that
 complexity can amplify small perturbations, and the design engineer
 must ensure such perturbations are extremely rare." [CARLSON]

2.2.2. The Coupling Principle

 The Coupling Principle states that as things get larger, they often
 exhibit increased interdependence between components.
 COROLLARY: The more events that simultaneously occur, the larger the
 likelihood that two or more will interact.  This phenomenon has also
 been termed "unforeseen feature interaction" [WILLINGER2002].
 Much of the non-linearity observed large systems is largely due to
 coupling.  This coupling has both  horizontal and vertical
 components.  In the context of networking, horizontal coupling is
 exhibited between the same protocol layer, while vertical coupling
 occurs between layers.
 Coupling is exhibited by a wide variety of natural systems, including
 plasma macro-instabilities (hydro-magnetic, e.g., kink, fire-hose,
 mirror, ballooning, tearing, trapped-particle effects) [NAVE], as
 well as various kinds of electrochemical systems (consider the custom
 fluorescent nucleotide synthesis/nucleic acid labeling problem
 [WARD]).  Coupling of clock physical periodicity has also been
 observed [JACOBSON], as well as coupling of various types of
 biological cycles.
 Several canonical examples also exist in well known network systems.
 Examples include the synchronization of various control loops, such
 as routing update synchronization and TCP Slow Start synchronization
 [FLOYD,JACOBSON].  An important result of these observations is that
 coupling is intimately related to synchronization.  Injecting
 randomness into these systems is one way to reduce coupling.

Bush, et. al. Informational [Page 5] RFC 3439 Internet Architectural Guidelines December 2002

 Interestingly, in analyzing risk factors for the Public Switched
 Telephone Network (PSTN), Charles Perrow decomposes the complexity
 problem along two related axes, which he terms "interactions" and
 "coupling" [PERROW].  Perrow cites interactions and coupling as
 significant factors in determining the reliability of a complex
 system (and in particular, the PSTN).  In this model, interactions
 refer to the dependencies between components (linear or non-linear),
 while coupling refers to the flexibility in a system.  Systems with
 simple, linear interactions have components  that affect only other
 components that are functionally downstream.  Complex system
 components interact with many other components in different and
 possibly distant parts of the system.  Loosely coupled systems are
 said to have more flexibility in time constraints, sequencing, and
 environmental assumptions than do tightly coupled systems.  In
 addition, systems with complex interactions and tight coupling are
 likely to have unforeseen failure states (of course, complex
 interactions permit more complications to develop and make the system
 hard to understand and predict); this behavior is also described in
 [WILLINGER2002].  Tight coupling also means that the system has less
 flexibility in recovering from failure states.
 The PSTN's SS7 control network provides an interesting example of
 what can go wrong with a tightly coupled complex system.  Outages
 such as the well publicized 1991 outage of AT&T's SS7 demonstrates
 the phenomenon: the outage was caused by software bugs in the
 switches' crash recovery code.  In this case, one switch crashed due
 to a hardware glitch.  When this switch came back up, it (plus a
 reasonably probable timing event) caused its neighbors to crash When
 the neighboring switches came back up, they caused their neighbors to
 crash, and so on [NEUMANN] (the root cause turned out to be a
 misplaced 'break' statement; this is an excellent example of cross-
 layer coupling).  This phenomenon is similar to the phase-locking of
 weakly coupled oscillators, in which random variations in sequence
 times plays an important role in system stability [THOMPSON].

2.3. Complexity lesson from voice

 In the 1970s and 1980s, the voice carriers competed by adding
 features which drove substantial increases in the complexity of the
 PSTN, especially in the Class 5 switching infrastructure.  This
 complexity was typically software-based, not hardware driven, and
 therefore had cost curves worse than Moore's Law.  In summary, poor
 margins on voice products today are due to OPEX and CAPEX costs not
 dropping as we might expect from simple hardware-bound

Bush, et. al. Informational [Page 6] RFC 3439 Internet Architectural Guidelines December 2002

2.4. Upgrade cost of complexity

 Consider the cost of providing new features in a complex network.
 The traditional voice network has little intelligence in its edge
 devices (phone instruments), and a very smart core.  The Internet has
 smart edges, computers with operating systems, applications, etc.,
 and a simple core, which consists of a control plane and packet
 forwarding engines.  Adding an new Internet service is just a matter
 of distributing an application to the a few consenting desktops who
 wish to use it.  Compare this to adding a service to voice, where one
 has to upgrade the entire core.

3. Layering Considered Harmful

 There are several generic properties of layering, or vertical
 integration as applied to networking.  In general, a layer as defined
 in our context implements one or more of
  Error Control:     The layer makes the "channel" more reliable
                     (e.g., reliable transport layer)
  Flow Control:      The layer avoids flooding slower peer (e.g.,
                     ATM flow control)
  Fragmentation:     Dividing large data chunks into smaller
                     pieces, and subsequent reassembly (e.g., TCP
                     MSS fragmentation/reassembly)
  Multiplexing:      Allow several higher level sessions share
                     single lower level "connection" (e.g., ATM PVC)
  Connection Setup:  Handshaking with peer (e.g., TCP three-way
                     handshake, ATM ILMI)
  Addressing/Naming: Locating, managing identifiers associated
                     with entities (e.g., GOSSIP 2 NSAP Structure
 Layering of this type does have various conceptual and structuring
 advantages.  However, in the data networking context structured
 layering implies that the functions of each layer are carried out
 completely before the protocol data unit is passed to the next layer.
 This means that the optimization of each layer has to be done
 separately.  Such ordering constraints are in conflict with efficient
 implementation of data manipulation functions.  One could accuse the
 layered model (e.g., TCP/IP and ISO OSI) of causing this conflict.
 In fact, the operations of multiplexing and segmentation both hide
 vital information that lower layers may need to optimize their

Bush, et. al. Informational [Page 7] RFC 3439 Internet Architectural Guidelines December 2002

 performance.  For example, layer N may duplicate lower level
 functionality, e.g., error recovery hop-hop versus end-to-end error
 recovery.  In addition, different layers may need the same
 information (e.g., time stamp): layer N may need layer N-2
 information (e.g., lower layer packet sizes), and the like [WAKEMAN].
 A related and even more ironic statement comes from Tennenhouse's
 classic paper, "Layered Multiplexing Considered Harmful"
 [TENNENHOUSE]: "The ATM approach to broadband networking is presently
 being pursued within the CCITT (and elsewhere) as the unifying
 mechanism for the support of service integration, rate adaptation,
 and jitter control within the lower layers of the network
 architecture.  This position paper is specifically concerned with the
 jitter arising from the design of the "middle" and "upper" layers
 that operate within the end systems and relays of multi-service
 networks (MSNs)."
 As a result of inter-layer dependencies, increased layering can
 quickly lead to violation of the Simplicity Principle.  Industry
 experience has taught us that increased layering frequently increases
 complexity and hence leads to increases in OPEX, as is predicted by
 the Simplicity Principle.  A corollary is stated in RFC 1925
 [RFC1925], section 2(5):
    "It is always possible to agglutinate multiple separate problems
    into a single complex interdependent solution.  In most cases
    this is a bad idea."
 The first order conclusion then, is that horizontal (as opposed to
 vertical) separation may be more cost-effective and reliable in the
 long term.

3.1. Optimization Considered Harmful

 A corollary of the layering arguments above is that optimization can
 also be considered harmful.  In particular, optimization introduces
 complexity, and as well as introducing tighter coupling between
 components and layers.
 An important and related effect of optimization is described by the
 Law of Diminishing Returns, which states that if one factor of
 production is increased while the others remain constant, the overall
 returns will relatively decrease after a certain point [SPILLMAN].
 The implication here is that trying to squeeze out efficiency past
 that point only adds complexity, and hence leads to less reliable

Bush, et. al. Informational [Page 8] RFC 3439 Internet Architectural Guidelines December 2002

3.2. Feature Richness Considered Harmful

 While adding any new feature may be considered a gain (and in fact
 frequently differentiates vendors of various types of equipment), but
 there is a danger.  The danger is in increased system complexity.

3.3. Evolution of Transport Efficiency for IP

 The evolution of transport infrastructures for IP offers a good
 example of how decreasing vertical integration has lead to various
 efficiencies.  In particular,
  | IP over ATM over SONET  -->
  | IP over SONET over WDM  -->
  | IP over WDM
 Decreasing complexity, CAPEX, OPEX
 The key point here is that layers are removed resulting in CAPEX and
 OPEX efficiencies.

3.4. Convergence Layering

 Convergence is related to the layering concepts described above in
 that convergence is achieved via a "convergence layer".  The end
 state of the convergence argument is the concept of Everything Over
 Some Layer (EOSL).  Conduit, DWDM, fiber, ATM, MPLS, and even IP have
 all been proposed as convergence layers.  It is important to note
 that since layering typically drives OPEX up, we expect convergence
 will as well.  This observation is again consistent with industry
 There are many notable examples of convergence layer failure.
 Perhaps the most germane example is IP over ATM.  The immediate and
 most obvious consequence of ATM layering is the so-called cell tax:
 First, note that the complete answer on ATM efficiency is that it
 depends upon packet size distributions.  Let's assume that typical
 Internet type traffic patterns, which tend to have high percentages
 of packets at 40, 44, and 552 bytes.  Recent data [CAIDA] shows that
 about 95% of WAN bytes and 85% of packets are TCP.  Much of this
 traffic is composed of 40/44 byte packets.
 Now, consider the case of a a DS3 backbone with PLCP turned on.  Then
 the maximum cell rate is 96,000 cells/sec.  If you multiply this
 value by the number of bits in the payload, you get: 96000 cells/sec
 * 48 bytes/cell * 8 = 36.864 Mbps.  This, however, is unrealistic
 since it

Bush, et. al. Informational [Page 9] RFC 3439 Internet Architectural Guidelines December 2002

 assumes perfect payload packing.  There are two other things that
 contribute to the ATM overhead (cell tax): The wasted padding and the
 8 byte SNAP header.
 It is the SNAP header which causes most of the problems (and you
 can't do anything about this), forcing most small packets to consume
 two cells, with the second cell to be mostly empty padding (this
 interacts really poorly with the data quoted above, e.g., that most
 packets are 40-44 byte TCP Ack packets).  This causes a loss of about
 another 16% from the 36.8 Mbps ideal throughput.
 So the total throughput ends up being (for a DS3):
           DS3 Line Rate:              44.736
           PLCP Overhead              - 4.032
           Per Cell Header:           - 3.840
           SNAP Header & Padding:     - 5.900
                                       30.960 Mbps
 Result: With a DS3 line rate of 44.736 Mbps, the total overhead is
 about 31%.
 Another way to look at this is that since a large fraction of WAN
 traffic is comprised of TCP ACKs, one can make a different but
 related calculation.  IP over ATM requires:
           IP data (40 bytes in this case)
           8 bytes SNAP
           8 bytes AAL5 stuff
           5 bytes for each cell
           + as much more as it takes to fill out the last cell
 On ATM, this becomes two cells - 106 bytes to convey 40 bytes of
 information.  The next most common size seems to be one of several
 sizes in the 504-556 byte range - 636 bytes to carry IP, TCP, and a
 512 byte TCP payload - with messages larger than 1000 bytes running
 One would imagine that 87% payload (556 byte message size) is better
 than 37% payload (TCP Ack size), but it's not the 95-98% that
 customers are used to, and the predominance of TCP Acks skews the

Bush, et. al. Informational [Page 10] RFC 3439 Internet Architectural Guidelines December 2002

3.4.1. Note on Transport Protocol Layering

 Protocol layering models are frequently cast as "X over Y" models.
 In these cases, protocol Y carries protocol X's protocol data units
 (and possibly control data) over Y's data plane, i.e., Y is a
 "convergence layer".  Examples include Frame Relay over ATM, IP over
 ATM, and IP over MPLS.  While X over Y layering has met with only
 marginal success [TENNENHOUSE,WAKEMAN], there have been a few notable
 instances where efficiency can be and is gained.  In particular, "X
 over Y efficiencies" can be realized when there is a kind of
 "isomorphism" between the X and Y (i.e., there is a small convergence
 layer).  In these cases X's data, and possibly control traffic, are
 "encapsulated" and transported over Y.  Examples include Frame Relay
 over ATM, and Frame Relay, AAL5 ATM and Ethernet over L2TPv3
 [L2TPV3]; the simplifying factors here are that there is no
 requirement that a shared clock be recovered by the communicating end
 points, and that control-plane interworking is minimized.  An
 alternative is to interwork the X and Y's control and data planes;
 control-plane interworking is discussed below.

3.5. Second Order Effects

 IP over ATM provides an excellent example of unanticipated second
 order effects.  In particular, Romanov and Floyd's classic study on
 TCP good-put [ROMANOV] on ATM showed that large UBR buffers (larger
 than one TCP window size) are required to achieve reasonable
 performance, that packet discard mechanisms (such as Early Packet
 Discard, or EPD) improve the effective usage of the bandwidth and
 that more elaborate service and drop strategies than FIFO+EPD, such
 as per VC queuing and accounting, might be required at the bottleneck
 to ensure both high efficiency and fairness.  Though all studies
 clearly indicate that a buffer size not less than one TCP window size
 is required, the amount of extra buffer required naturally depends on
 the packet discard mechanism used and is still an open issue.
 Examples of this kind of problem with layering abound in practical
 networking.  Consider, for example, the effect of IP transport's
 implicit assumptions of lower layers.  In particular:
  o Packet loss: TCP assumes that packet losses are indications of
    congestion, but sometimes losses are from corruption on a wireless
    link [RFC3115].
  o Reordered packets: TCP assumes that significantly reordered
    packets are indications of congestion.  This is not always the
    case [FLOYD2001].

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  o Round-trip times: TCP measures round-trip times, and assumes that
    the lack of an acknowledgment within a period of time based on the
    measured round-trip time is a packet loss, and therefore an
    indication of congestion [KARN].
  o Congestion control: TCP congestion control implicitly assumes that
    all the packets in a flow are treated the same by the network, but
    this is not always the case [HANDLEY].

3.6. Instantiating the EOSL Model with IP

 While IP is being proposed as a transport for almost everything, the
 base assumption, that Everything over IP (EOIP) will result in OPEX
 and CAPEX efficiencies, requires critical examination.  In
 particular, while it is the case that many protocols can be
 efficiently transported over an IP network (specifically, those
 protocols that do not need to recover synchronization between the
 communication end points, such as Frame Relay, Ethernet, and AAL5
 ATM), the Simplicity and Layering Principles suggest that EOIP may
 not represent the most efficient convergence strategy for arbitrary
 services.  Rather, a more CAPEX and OPEX efficient convergence layer
 might be much lower (again, this behavior is predicted by the
 Simplicity Principle).
 An example of where EOIP would not be the most OPEX and CAPEX
 efficient transport would be in those cases where a service or
 protocol needed SONET-like restoration times (e.g., 50ms).  It is not
 hard to imagine that it would cost more to build and operate an IP
 network with this kind of restoration and convergence property (if
 that were even possible) than it would to build the SONET network in
 the first place.

4. Avoid the Universal Interworking Function

 While there have been many implementations of Universal Interworking
 unction (UIWF), IWF approaches have been problematic at large scale.
 his concern is codified in the Principle of Minimum Intervention
 "To minimise the scope of information, and to improve the efficiency
 of data flow through the Encapsulation Layer, the payload should,
 where possible, be transported as received without modification."

Bush, et. al. Informational [Page 12] RFC 3439 Internet Architectural Guidelines December 2002

4.1. Avoid Control Plane Interworking

 This corollary is best understood in the context of the integrated
 solutions space.  In this case, the architecture and design
 frequently achieves the worst of all possible worlds.  This is due to
 the fact that such integrated solutions perform poorly at both ends
 of the performance/CAPEX/OPEX spectrum: the protocols with the least
 switching demand may have to bear the cost of the most expensive,
 while the protocols with the most stringent requirements often must
 make concessions to those with different requirements.  Add to this
 the various control plane interworking issues and you have a large
 opportunity for failure.  In summary, interworking functions should
 be restricted to data plane interworking and encapsulations, and
 these functions should be carried out at the edge of the network.
 As described above, interworking models have been successful in those
 cases where there is a kind of "isomorphism" between the layers being
 interworked.  The trade-off here, frequently described as the
 "Integrated vs.  Ships In the Night trade-off" has been examined at
 various times and  at various protocol layers.  In general, there are
 few cases in which such integrated solutions have proven efficient.
 Multi-protocol BGP [RFC2283] is a subtly different but notable
 exception.  In this case, the control plane is  independent of the
 format of the control data.  That is, no control plane data
 conversion is required, in contrast with control plane interworking
 models such as the ATM/IP interworking envisioned by some soft-switch
 manufacturers, and the so-called "PNNI-MPLS SIN" interworking

5. Packet versus Circuit Switching: Fundamental Differences

 Conventional wisdom holds that packet switching (PS) is inherently
 more efficient than circuit switching (CS), primarily because of the
 efficiencies that can be gained by statistical multiplexing and the
 fact that routing and forwarding decisions are made independently in
 a hop-by-hop fashion [[MOLINERO2002].  Further, it is widely assumed
 that IP is simpler that circuit switching, and hence should be more
 economical to deploy and manage [MCK2002].  However, if one examines
 these and related assumptions, a different picture emerges (see for
 example [ODLYZKO98]).  The following sections discuss these

5.1. Is PS is inherently more efficient than CS?

 It is well known that packet switches make efficient use of scarce
 bandwidth [BARAN].  This efficiency is based on the statistical
 multiplexing inherent in packet switching.  However, we continue to
 be puzzled by what is generally believed to be the low utilization of

Bush, et. al. Informational [Page 13] RFC 3439 Internet Architectural Guidelines December 2002

 Internet backbones.  The first question we might ask is what is the
 current average utilization of Internet backbones, and how does that
 relate to the utilization of long distance voice networks?  Odlyzko
 and Coffman [ODLYZKO,COFFMAN] report that the average utilization of
 links in the IP networks was in the range between 3% and 20%
 (corporate intranets run in the 3% range, while commercial Internet
 backbones run in the 15-20% range).  On the other hand, the average
 utilization of long haul voice lines is about 33%.  In addition, for
 2002, the average utilization of optical networks (all services)
 appears to be hovering at about 11%, while the historical average is
 approximately 15% [ML2002].  The question then becomes why we see
 such utilization levels, especially in light of the assumption that
 PS is inherently more efficient than CS.  The reasons cited by
 Odlyzko and Coffman include:
    (i).   Internet traffic is extremely asymmetric and bursty, but
           links are symmetric and of fixed capacity (i.e., don't know
           the traffic matrix, or required link capacities);
    (ii).  It is difficult to predict traffic growth on a link, so
           operators tend to add bandwidth aggressively;
    (iii).  Falling prices for coarser bandwidth granularity make it
           appear more economical to add capacity in large increments.
 Other static factors include protocol overhead, other kinds of
 equipment granularity, restoration capacity, and provisioning lag
 time all contribute to the need to "over-provision" [MC2001].

5.2. Is PS simpler than CS?

 The end-to-end principle can be interpreted as stating that the
 complexity of the Internet belongs at the edges.  However, today's
 Internet backbone routers are extremely complex.  Further, this
 complexity scales with line rate.  Since the relative complexity of
 circuit and packet switching seems to have resisted direct analysis,
 we instead examine several artifacts of packet and circuit switching
 as complexity metrics.  Among the metrics we might look at are
 software complexity, macro operation complexity, hardware complexity,
 power consumption, and density.  Each of these metrics is considered

Bush, et. al. Informational [Page 14] RFC 3439 Internet Architectural Guidelines December 2002

5.2.1. Software/Firmware Complexity

 One measure of software/firmware complexity is the number of
 instructions required to program the device.  The typical software
 image for an Internet router requires between eight and ten million
 instructions (including firmware), whereas a typical transport switch
 requires on average about three million instructions [MCK2002].
 This difference in software complexity has tended to make Internet
 routers unreliable, and has notable other second order effects (e.g.,
 it may take a long time to reboot such a router).  As another point
 of comparison, consider that the AT&T (Lucent) 5ESS class 5 switch,
 which has a huge number of calling features, requires only about
 twice the number of lines of code as an Internet core router [EICK].
 Finally, since routers are as much or more software than hardware
 devices, another result of the code complexity is that the cost of
 routers benefits less from Moore's Law than less software-intensive
 devices.  This causes a bandwidth/device trade-off that favors
 bandwidth more than less software-intensive devices.

5.2.2. Macro Operation Complexity

 An Internet router's line card must perform many complex operations,
 including processing the packet header, longest prefix match,
 generating ICMP error messages, processing IP header options, and
 buffering the packet so that TCP congestion control will be effective
 (this typically requires a buffer of size proportional to the line
 rate times the RTT, so a buffer will hold around 250 ms of packet
 data).  This doesn't include route and packet filtering, or any QoS
 or VPN filtering.
 On the other hand, a transport switch need only to map ingress time-
 slots to egress time-slots and interfaces, and therefore can be
 considerably less complex.

5.2.3. Hardware Complexity

 One measure of hardware complexity is the number of logic gates on a
 line card [MOLINERO2002].  Consider the case of a high-speed Internet
 router line card: An OC192 POS router line card contains at least 30
 million gates in ASICs, at least one CPU, 300 Mbytes of packet
 buffers, 2 Mbytes of forwarding table, and 10 Mbytes of other

Bush, et. al. Informational [Page 15] RFC 3439 Internet Architectural Guidelines December 2002

 state memory.  On the other hand, a comparable transport switch line
 card has 7.5 million logic gates, no CPU, no packet buffer, no
 forwarding table, and an on-chip state memory.  Rather, the line-card
 of an electronic transport switch typically contains a SONET framer,
 a chip to map ingress time-slots to egress time-slots, and an
 interface to the switch fabric.

5.2.4. Power

 Since transport switches have traditionally been built from simpler
 hardware components, they also consume less power [PMC].

5.2.5. Density

 The highest capacity transport switches have about four times the
 capacity of an IP router [CISCO,CIENA], and sell for about one-third
 as much per Gigabit/sec.  Optical (OOO) technology pushes this
 complexity difference further (e.g., tunable lasers, MEMs switches.
 e.g., [CALIENT]), and DWDM multiplexers provide technology to build
 extremely high capacity, low power transport switches.
 A related metric is physical footprint.  In general, by virtue of
 their higher density, transport switches have a smaller "per-gigabit"
 physical footprint.

5.2.6. Fixed versus variable costs

 Packet switching would seem to have high variable cost, meaning that
 it costs more to send the n-th piece of information using packet
 switching than it might in a circuit switched network.  Much of this
 advantage is due to the relatively static nature of circuit
 switching, e.g., circuit switching can take advantage of of pre-
 scheduled arrival of information to eliminate operations to be
 performed on incoming information.  For example, in the circuit
 switched case, there is no need to buffer incoming information,
 perform loop detection, resolve next hops, modify fields in the
 packet header, and the like.  Finally, many circuit switched networks
 combine relatively static configuration with out-of-band control
 planes (e.g., SS7), which greatly simplifies data-plane switching.
 The bottom line is that as data rates get large, it becomes more and
 more complex to switch packets, while circuit switching scales more
 or less linearly.

Bush, et. al. Informational [Page 16] RFC 3439 Internet Architectural Guidelines December 2002

5.2.7. QoS

 While the components of a complete solution for Internet QoS,
 including call admission control, efficient packet classification,
 and scheduling algorithms, have been the subject of extensive
 research and standardization for more than 10 years, end-to-end
 signaled QoS for the Internet has not become a reality.
 Alternatively, QoS has been part of the circuit switched
 infrastructure almost from its inception.  On the other hand, QoS is
 usually deployed to determine queuing disciplines to be used when
 there is insufficient bandwidth to support traffic.  But unlike voice
 traffic, packet drop or severe delay may have a much more serious
 effect on TCP traffic due to its congestion-aware feedback loop (in
 particular, TCP backoff/slow start).

5.2.8. Flexibility

 A somewhat harder to quantify metric is the inherent flexibility of
 the Internet.  While the Internet's flexibility has led to its rapid
 growth, this flexibility comes with a relatively high cost at the
 edge: the need for highly trained support personnel.  A standard rule
 of thumb is that in an enterprise setting, a single support person
 suffices to provide telephone service for a group, while you need ten
 computer networking experts to serve the networking requirements of
 the same group [ODLYZKO98A].  This phenomenon is also described in

5.3. Relative Complexity

 The relative computational complexity of circuit switching as
 compared to packet switching has been difficult to describe in formal
 terms [PARK].  As such, the sections above seek to describe the
 complexity in terms of observable artifacts.  With this in mind, it
 is clear that the fundamental driver producing the increased
 complexities outlined above is the hop-by-hop independence (HBHI)
 inherent in the IP architecture.  This is in contrast to the end to
 end architectures such as ATM or Frame Relay.
 [WILLINGER2002] describes this phenomenon in terms of the robustness
 requirement of the original Internet design, and how this requirement
 has the driven complexity of the network.  In particular, they
 describe a "complexity/robustness" spiral, in which increases in
 complexity create further and more serious sensitivities, which then
 requires additional robustness (hence the spiral).

Bush, et. al. Informational [Page 17] RFC 3439 Internet Architectural Guidelines December 2002

 The important lesson of this section is that the Simplicity
 Principle, while applicable to circuit switching as well as packet
 switching, is crucial in controlling the complexity (and hence OPEX
 and CAPEX properties) of packet networks.  This idea is reinforced by
 the observation that while packet switching is a younger, less mature
 discipline than circuit switching, the trend in packet switches is
 toward more complex line cards, while the complexity of circuit
 switches appears to be scaling linearly with line rates and aggregate

5.3.1. HBHI and the OPEX Challenge

 As a result of HBHI, we need to approach IP networks in a
 fundamentally different way than we do circuit based networks.  In
 particular, the major OPEX challenge faced by the IP network is that
 debugging of a large-scale IP network still requires a large degree
 of expertise and understanding, again due to the hop-by-hop
 independence inherent in a packet architecture (again, note that this
 hop-by-hop independence is not present in virtual circuit networks
 such as ATM or Frame Relay).  For example, you may have to visit a
 large set of your routers only to discover that the problem is
 external to your own network.  Further, the debugging tools used to
 diagnose problems are also complex and somewhat primitive.  Finally,
 IP has to deal with people having problems with their DNS or their
 mail or news or some new application, whereas this is usually not the
 case for TDM/ATM/etc.  In the case of IP, this can be eased by
 improving automation (note that much of what we mention is customer
 facing).  In general, there are many variables external to the
 network that effect OPEX.
 Finally, it is important to note that the quantitative relationship
 between CAPEX, OPEX, and a network's inherent complexity is not well
 understood.  In fact, there are no agreed upon and quantitative
 metrics for describing a network's complexity, so a precise
 relationship between CAPEX, OPEX, and complexity remains elusive.

6. The Myth of Over-Provisioning

 As noted in [MC2001] and elsewhere, much of the complexity we observe
 in today's Internet is directed at increasing bandwidth utilization.
 As a result, the desire of network engineers to keep network
 utilization below 50% has been termed "over-provisioning".  However,
 this use of the term over-provisioning is a misnomer.  Rather, in
 modern Internet backbones the unused capacity is actually protection
 capacity.  In particular, one might view this as "1:1 protection at
 the IP layer".  Viewed in this way, we see that an IP network
 provisioned to run at 50% utilization is no more over-provisioned
 than the typical SONET network.  However, the important advantages

Bush, et. al. Informational [Page 18] RFC 3439 Internet Architectural Guidelines December 2002

 that accrue to an IP network provisioned in this way include close to
 speed of light delay and close to zero packet loss [FRALEIGH].  These
 benefits can been seen as a "side-effect" of 1:1 protection
 There are also other, system-theoretic reasons for providing 1:1-like
 protection provisioning.  Most notable among these reasons is that
 packet-switched networks with in-band control loops can become
 unstable and can experience oscillations and synchronization when
 congested.  Complex and non-linear dynamic interaction of traffic
 means that congestion in one part of the network will spread to other
 parts of the network.  When routing protocol packets are lost due to
 congestion or route-processor overload, it causes inconsistent
 routing state, and this may result in traffic loops, black holes, and
 lost connectivity.  Thus, while statistical multiplexing can in
 theory yield higher network utilization, in practice, to maintain
 consistent performance and a reasonably stable network, the dynamics
 of the Internet backbones favor 1:1 provisioning and its side effects
 to keep the network stable and delay low.

7. The Myth of Five Nines

 Paul Baran, in his classic paper, "SOME PERSPECTIVES ON NETWORKS--
 PAST, PRESENT AND FUTURE", stated that "The tradeoff curves between
 cost and system reliability suggest that the most reliable systems
 might be built of relatively unreliable and hence low cost elements,
 if it is system reliability at the lowest overall system cost that is
 at issue" [BARAN77].
 Today we refer to this phenomenon as "the myth of five nines".
 Specifically, so-called five nines reliability in packet network
 elements is consider a myth for the following reasons: First, since
 80% of unscheduled outages are caused by people or process errors
 [SCOTT], there is only a 20% window in which to optimize.  Thus, in
 order to increase component reliability, we add complexity
 (optimization frequently leads to complexity), which is the root
 cause of 80% of the unplanned outages.  This effectively narrows the
 20% window (i.e., you increase the likelihood of people and process
 failure).  This phenomenon is also characterized as a
 "complexity/robustness" spiral [WILLINGER2002], in which increases in
 complexity create further and more serious sensitivities, which then
 requires additional robustness, and so on (hence the spiral).
 The conclusion, then is that while a system like the Internet can
 reach five-nines-like reliability, it is undesirable (and likely
 impossible) to try to make any individual component, especially the
 most complex ones, reach that reliability standard.

Bush, et. al. Informational [Page 19] RFC 3439 Internet Architectural Guidelines December 2002

8. Architectural Component Proportionality Law

 As noted in the previous section, the computational complexity of
 packet switched networks such as the Internet has proven difficult to
 describe in formal terms.  However, an intuitive, high level
 definition of architectural complexity might be that the complexity
 of an architecture is proportional to its number of components, and
 that the probability of achieving a stable implementation of an
 architecture is inversely proportional to its number of components.
 As described above, components include discrete elements such as
 hardware elements, space and power requirements, as well as software,
 firmware, and the protocols they implement.
 Stated more abstractly:
       A   be a representation of architecture A,
       |A| be number of distinct components in the service
           delivery path of architecture A,
       w   be a monotonically increasing function,
       P   be the probability of a stable implementation of an
           architecture, and let
       Complexity(A) = O(w(|A|))
       P(A)          = O(1/w(|A|))
     O(f) = {g:N->R | there exists c > 0 and n such that g(n)
     < c*f(n)}
     [That is, O(f) comprises the set of functions g for which
     there exists a constant c and a number n, such that g(n) is
     smaller or equal to c*f(n) for all n. That is, O(f) is the
     set of all functions that do not grow faster than f,
     disregarding constant factors]
 Interestingly, the Highly Optimized Tolerance (HOT) model [HOT]
 attempts to characterize complexity in general terms (HOT is one
 recent attempt to develop a general framework for the study of
 complexity, and is a member of a family of abstractions generally
 termed "the new science of complexity" or "complex adaptive

Bush, et. al. Informational [Page 20] RFC 3439 Internet Architectural Guidelines December 2002

 systems").  Tolerance, in HOT semantics, means that "robustness in
 complex systems is a constrained and limited quantity that must be
 carefully managed and protected." One focus of the HOT model is to
 characterize heavy-tailed distributions such as Complexity(A) in the
 above example (other examples include forest fires, power outages,
 and Internet traffic distributions).  In particular, Complexity(A)
 attempts to map the extreme heterogeneity of the parts of the system
 (Internet), and the effect of their organization into highly
 structured networks, with hierarchies and multiple scales.

8.1. Service Delivery Paths

 The Architectural Component Proportionality Law (ACPL) states that
 the complexity of an architecture is proportional to its number of
 COROLLARY: Minimize the number of components in a service delivery
 path, where the service delivery path can be a protocol path, a
 software path, or a physical path.
 This corollary is an important consequence of the ACPL, as the path
 between a customer and the desired service is particularly sensitive
 to the number and complexity of elements in the path.  This is due to
 the fact that the complexity "smoothing" that we find at high levels
 of aggregation [ZHANG] is missing as you move closer to the edge, as
 well as having complex interactions with backoffice and CRM systems.
 Examples of architectures that haven't found a market due to this
 effect include TINA-based CRM systems, CORBA/TINA based service
 architectures.  The basic lesson here was that the only possibilities
 for deploying these systems were "Limited scale deployments (such) as
 in Starvision can avoid coping with major unproven scalability
 issues", or "Otherwise need massive investments (like the carrier-
 grade ORB built almost from scratch)" [TINA].  In other words, these
 systems had complex service delivery paths, and were too complex to
 be feasibly deployed.

9. Conclusions

 This document attempts to codify long-understood Internet
 architectural principles.  In particular, the unifying principle
 described here is best expressed by the Simplicity Principle, which
 states complexity must be controlled if one hopes to efficiently
 scale a complex object.  The idea that simplicity itself can lead to
 some form of optimality has been a common theme throughout history,
 and has been stated in many other ways and along many dimensions.
 For example, consider the maxim known as Occam's Razor, which was
 formulated by the medieval English philosopher and Franciscan monk
 William of Ockham (ca. 1285-1349), and states "Pluralitas non est

Bush, et. al. Informational [Page 21] RFC 3439 Internet Architectural Guidelines December 2002

 ponenda sine neccesitate" or "plurality should not be posited without
 necessity." (hence Occam's Razor is sometimes called "the principle
 of unnecessary plurality" and " the principle of simplicity").  A
 perhaps more contemporary formulation of Occam's Razor states that
 the simplest explanation for a phenomenon is the one preferred by
 nature.  Other formulations of the same  idea can be found in the
 KISS (Keep It Simple Stupid) principle and the Principle of Least
 Astonishment (the assertion that the most usable system is the one
 that least often leaves users astonished).  [WILLINGER2002] provides
 a more theoretical discussion of "robustness through simplicity", and
 in discussing the PSTN, [KUHN87] states that in most systems, "a
 trade-off can be made between simplicity of interactions and
 looseness of coupling".
 When applied to packet switched network architectures, the Simplicity
 Principle has implications that some may consider heresy, e.g., that
 highly converged approaches are likely to be less efficient than
 "less converged" solutions.  Otherwise stated, the "optimal"
 convergence layer may be much lower in the protocol stack that is
 conventionally believed.  In addition, the analysis above leads to
 several conclusions that are contrary to the conventional wisdom
 surrounding  packet networking.  Perhaps most significant is the
 belief that packet switching is simpler than circuit switching.  This
 belief has lead to conclusions such as "since packet is simpler than
 circuit, it must cost less to operate".  This study finds to the
 contrary.  In particular, by examining the metrics described above,
 we find that packet switching is more complex than circuit switching.
 Interestingly, this conclusion is borne out by the fact that
 normalized OPEX for data networks is typically significantly greater
 than for voice networks [ML2002].
 Finally, the important conclusion of this work is that for packet
 networks that are of the scale of today's Internet or larger, we must
 strive for the simplest possible solutions if we hope to build cost
 effective infrastructures.  This idea is eloquently stated in
 [DOYLE2002]: "The evolution of protocols can lead to a
 robustness/complexity/fragility spiral where complexity added for
 robustness also adds new fragilities, which in turn leads to new and
 thus spiraling complexities".  This is exactly the phenomenon that
 the Simplicity Principle is designed to avoid.

10. Security Considerations

 This document does not directly effect the security of any existing
 Internet protocol.  However, adherence to the Simplicity Principle
 does have a direct affect on our ability to implement secure systems.
 In particular, a system's complexity grows, it becomes  more
 difficult to model and analyze, and hence it becomes more difficult

Bush, et. al. Informational [Page 22] RFC 3439 Internet Architectural Guidelines December 2002

 to find and understand the security implications inherent in its
 architecture, design, and implementation.

11. Acknowledgments

 Many of the ideas for comparing the complexity of circuit switched
 and packet switched networks were inspired by conversations with Nick
 McKeown.  Scott Bradner, David Banister, Steve Bellovin, Steward
 Bryant, Christophe Diot, Susan Harris, Ananth Nagarajan, Andrew
 Odlyzko, Pete and Natalie Whiting, and Lixia Zhang made many helpful
 comments on early drafts of this document.

12. References

 [AHUJA]         "The Impact of Internet Policy and Topology on
                 Delayed Routing Convergence", Labovitz, et. al.
                 Infocom, 2001.
 [ATMMPLS]       "ATM-MPLS Interworking Migration Complexities Issues
                 and Preliminary Assessment", School of
                 Interdisciplinary Computing and Engineering,
                 University of Missouri-Kansas City, April 2002
 [BARAN]         "On Distributed Communications", Paul Baran, Rand
                 Corporation Memorandum RM-3420-PR,
       ", August,
                 FUTURE", Paul Baran,  Information Processing 77,
                 North-Holland Publishing Company, 1977,
 [BRYANT]        "Protocol Layering in PWE3", Bryant et al, Work in
 [CARLSON]       "Complexity and Robustness", J.M. Carlson and John
                 Doyle, Proc. Natl. Acad. Sci. USA, Vol. 99, Suppl. 1,
                 2538-2545, February 19, 2002.
 [CIENA]         "CIENA Multiwave CoreDiretor",

Bush, et. al. Informational [Page 23] RFC 3439 Internet Architectural Guidelines December 2002

 [CLARK]         "The Design Philosophy of the DARPA Internet
                 Protocols", D. Clark, Proc. of the ACM SIGCOMM, 1988.
 [COFFMAN]       "Internet Growth: Is there a 'Moores Law' for Data
                 Traffic", K.G. Coffman and A.M. Odlyzko, pp. 47-93,
                 Handbook of Massive Data Stes, J. Elli, P. M.
                 Pardalos, and M. G. C. Resende, Editors. Kluwer,
 [DOYLE2002]     "Robustness and the Internet: Theoretical
                 Foundations", John C. Doyle, et. al. Work in
 [EICK]          "Visualizing Software Changes", S.G. Eick, et al,
                 National Institute of Statistical Sciences, Technical
                 Report 113, December 2000.
 [MOLINERO2002]  "TCP Switching: Exposing Circuits to IP", Pablo
                 Molinero-Fernandez and Nick McKeown, IEEE January,
 [FLOYD]         "The Synchronization of Periodic Routing Messages",
                 Sally Floyd and Van Jacobson, IEEE ACM Transactions
                 on Networking, 1994.
 [FLOYD2001]     "A Report on Some Recent Developments in TCP
                 Congestion Control, IEEE Communications Magazine, S.
                 Floyd, April 2001.
 [FRALEIGH]      "Provisioning IP Backbone Networks to Support Delay-
                 Based Service Level Agreements", Chuck Fraleigh,
                 Fouad Tobagi, and Christophe Diot, 2002.
 [GRIFFIN]       "What is the Sound of One Route Flapping", Timothy G.
                 Griffin,  IPAM Workshop on Large-Scale Communication
                 Networks: Topology, Routing, Traffic, and Control,
                 March, 2002.
 [HANDLEY]       "On Inter-layer Assumptions (A view from the
                 Transport Area), slides from a presentation at the
                 IAB workshop on Wireless Internetworking", M.
                 Handley,  March 2000.
 [HOT]           J.M. Carlson and John Doyle, Phys. Rev. E 60, 1412-
                 1427, 1999.

Bush, et. al. Informational [Page 24] RFC 3439 Internet Architectural Guidelines December 2002

 [ISO10589]      "Intermediate System to Intermediate System
                 Intradomain Routing Exchange Protocol (IS-IS)".
 [JACOBSON]      "Congestion Avoidance and Control", Van Jacobson,
                 Proceedings of ACM Sigcomm 1988, pp. 273-288.
 [KARN]          "TCP vs Link Layer Retransmission" in P. Karn et al.,
                 Advice for Internet Subnetwork Designers, Work in
 [KUHN87]        "Sources of Failure in the Public Switched Telephone
                 Network", D. Richard Kuhn, EEE Computer, Vol. 30, No.
                 4, April, 1997.
 [L2TPV3]        Lan, J., et. al., "Layer Two Tunneling Protocol
                 (Version 3) -- L2TPv3", Work in Progress.
 [MC2001]        "U.S Communications Infrastructure at A Crossroads:
                 Opportunities Amid the Gloom", McKinsey&Company for
                 Goldman-Sachs, August 2001.
 [MCK2002]       Nick McKeown, personal communication, April, 2002.
 [ML2002]        "Optical Systems", Merril Lynch Technical Report,
                 April, 2002.
 [NAVE]          "The influence of mode coupling on the non-linear
                 evolution of tearing modes", M.F.F. Nave, et al, Eur.
                 Phys. J. D 8, 287-297.
 [NEUMANN]       "Cause of AT&T network failure", Peter G. Neumann,
 [ODLYZKO]       "Data networks are mostly empty for good reason",
                 A.M. Odlyzko, IT Professional 1 (no. 2), pp. 67-69,
                 Mar/Apr 1999.
 [ODLYZKO98A]    "Smart and stupid networks: Why the Internet is like
                 Microsoft".  A. M. Odlyzko, ACM Networker, 2(5),
                 December, 1998.
 [ODLYZKO98]     "The economics of the Internet: Utility, utilization,
                 pricing, and Quality of Service", A.M. Odlyzko, July,

Bush, et. al. Informational [Page 25] RFC 3439 Internet Architectural Guidelines December 2002

 [PARK]          "The Internet as a Complex System: Scaling,
                 Complexity and Control", Kihong Park and Walter
                 Willinger, AT&T Research, 2002.
 [PERROW]        "Normal Accidents: Living with High Risk
                 Technologies", Basic Books, C. Perrow, New York,
 [PMC]           "The Design of a 10 Gigabit Core Router
                 Architecture", PMC-Sierra, http://www.pmc-
 [RFC1629]       Colella, R., Callon, R., Gardner, E. and Y. Rekhter,
                 "Guidelines for OSI NSAP Allocation in the Internet",
                 RFC 1629, May 1994.
 [RFC1925]       Callon, R., "The Twelve Networking Truths", RFC 1925,
                 1 April 1996.
 [RFC1958]       Carpenter, B., Ed., "Architectural principles of the
                 Internet", RFC 1958, June 1996.
 [RFC2283]       Bates, T., Chandra, R., Katz, D. and Y. Rekhter,
                 "Multiprotocol Extensions for BGP4", RFC 2283,
                 February 1998.
 [RFC3155]       Dawkins, S., Montenegro, G., Kojo, M. and N. Vaidya,
                 "End-to-end Performance Implications of Links with
                 Errors", BCP 50, RFC 3155, May 2001.
 [ROMANOV]       "Dynamics of TCP over ATM Networks", A. Romanov, S.
                 Floyd, IEEE JSAC, vol. 13, No 4, pp.633-641, May
 [SALTZER]       "End-To-End Arguments in System Design", J.H.
                 Saltzer, D.P. Reed, and D.D. Clark, ACM TOCS, Vol 2,
                 Number 4, November 1984, pp 277-288.
 [SCOTT]         "Making Smart Investments to Reduce Unplanned
                 Downtime", D. Scott, Tactical Guidelines, TG-07-4033,
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 [SPILLMAN]      "The Law of Diminishing Returns:, W. J. Spillman and
                 E. Lang, 1924.
 [STALLINGS]     "Data and Computer Communications (2nd Ed)", William
                 Stallings, Maxwell Macmillan, 1989.

Bush, et. al. Informational [Page 26] RFC 3439 Internet Architectural Guidelines December 2002

 [TENNENHOUSE]   "Layered multiplexing considered harmful", D.
                 Tennenhouse, Proceedings of the IFIP Workshop on
                 Protocols for High-Speed Networks, Rudin ed., North
                 Holland Publishers, May 1989.
 [THOMPSON]      "Nonlinear Dynamics and Chaos". J.M.T. Thompson and
                 H.B. Stewart, John Wiley and Sons, 1994, ISBN
 [TINA]          "What is TINA and is it useful for the TelCos?",
                 Paolo Coppo, Carlo A. Licciardi, CSELT, EURESCOM
                 Participants in P847 (FT, IT, NT, TI)
 [WAKEMAN]       "Layering considered harmful", Ian Wakeman, Jon
                 Crowcroft, Zheng Wang, and Dejan Sirovica, IEEE
                 Network, January 1992, p. 7-16.
 [WARD]          "Custom fluorescent-nucleotide synthesis as an
                 alternative method for nucleic acid labeling",
                 Octavian Henegariu*, Patricia Bray-Ward and David C.
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 [WILLINGER2002] "Robustness and the Internet: Design and evolution",
                 Walter Willinger and John Doyle, 2002.
 [ZHANG]         "Impact of Aggregation on Scaling Behavior of
                 Internet Backbone Traffic", Sprint ATL Technical
                 Report TR02-ATL-020157 Zhi-Li Zhang, Vinay Ribeiroj,
                 Sue Moon, Christophe Diot, February, 2002.

13. Authors' Addresses

 Randy Bush
 David Meyer

Bush, et. al. Informational [Page 27] RFC 3439 Internet Architectural Guidelines December 2002

14. Full Copyright Statement

 Copyright (C) The Internet Society (2002).  All Rights Reserved.
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 The limited permissions granted above are perpetual and will not be
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Bush, et. al. Informational [Page 28]

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