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RFC 5290 - Comments on the Usefulness of Simple Best-Effort Traf


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Network Working Group                                           S. Floyd
Request for Comments: 5290                                     M. Allman
Category:  Informational                                            ICSI
                                                               July 2008

        Comments on the Usefulness of Simple Best-Effort Traffic

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.

IESG Note

   The content of this RFC was at one time considered by the IETF, and
   therefore it may resemble a current IETF work in progress or a
   published IETF work.

   This RFC is not a candidate for any level of Internet Standard.  The
   IETF disclaims any knowledge of the fitness of this RFC for any
   purpose and notes that the decision to publish is not based on IETF
   review apart from IESG review for conflict with IETF work.  The RFC
   Editor has chosen to publish this document at its discretion.  See
   RFC 3932 for more information.

Abstract

   This document presents some observations on "simple best-effort
   traffic", defined loosely for the purposes of this document as
   Internet traffic that is not covered by Quality of Service (QOS)
   mechanisms, congestion-based pricing, cost-based fairness, admissions
   control, or the like.  One observation is that simple best-effort
   traffic serves a useful role in the Internet, and is worth keeping.
   While differential treatment of traffic can clearly be useful, we
   believe such mechanisms are useful as *adjuncts* to simple best-
   effort traffic, not as *replacements* of simple best-effort traffic.
   A second observation is that for simple best-effort traffic, some
   form of rough flow-rate fairness is a useful goal for resource
   allocation, where "flow-rate fairness" is defined by the goal of
   equal flow rates for different flows over the same path.

Table of Contents

   1. Introduction ....................................................2
   2. On Simple Best-Effort Traffic ...................................3
      2.1. The Usefulness of Simple Best-Effort Traffic ...............4
      2.2. The Limitations of Simple Best-Effort Traffic ..............4
           2.2.1. Quality of Service (QoS) ............................4
           2.2.2. The Avoidance of Congestion Collapse and the
                  Enforcement of Fairness..............................6
           2.2.3. Control of Traffic Surges ...........................6
   3. On Flow-Rate Fairness for Simple Best-Effort Traffic ............6
      3.1. The Usefulness of Flow-Rate Fairness .......................7
      3.2. The Limitations of Flow-Rate Fairness ......................8
           3.2.1. The Enforcement of Flow-Rate Fairness ...............8
           3.2.2. The Precise Definition of Flow-Based Fairness .......9
   4. On the Difficulties of Incremental Deployment ..................11
   5. Related Work ...................................................12
      5.1. From the IETF .............................................12
      5.2. From Elsewhere ............................................13
   6. Security Considerations ........................................14
   7. Conclusions ....................................................14
   8. Acknowledgements ...............................................14
   9. Informative References .........................................14

1.  Introduction

   This document gives some observations on the role of simple best-
   effort traffic in the Internet.  For the purposes of this document,
   we define "simple best-effort traffic" as traffic that does not
   *rely* on the *differential treatment* of flows either in routers or
   in policers, enforcers, or other middleboxes along the path and that
   does not use admissions control.  We define the term "simple best-
   effort traffic" to avoid unproductive semantic discussions about what
   the phrase "best-effort traffic" does or does not include.  We note
   that our definition of "simple best-effort traffic" includes traffic
   that is not necessarily "simple", including mechanisms common in the
   current Internet such as pairwise agreements between ISPs, volume-
   based pricing, firewalls, and a wide range of mechanisms in
   middleboxes.

   "Simple best-effort traffic" in the current Internet uses end-to-end
   transport protocols (e.g., TCP, UDP, or others), with minimal
   requirements of the network in terms of resource allocation.
   However, other implementations of simple best-effort service would be
   possible, including those that would rely on Fair Queueing or some
   other form of per-flow scheduling in congested routers.  Our
   intention is to define "simple best-effort traffic" to include the
   dominant traffic class in the current Internet.

   In contrast to "simple best-effort traffic", intserv- or diffserv-
   enabled traffic relies on differential scheduling mechanisms at
   congested routers, with packets from different intserv or diffserv
   classes receiving different treatment.  Similarly, in contrast to
   "simple best-effort traffic", cost-based fairness [B07] would most
   likely require the deployment of traffic marking (e.g., Explicit
   Congestion Notification (ECN)) at congested routers, along with
   policing mechanisms near the two ends of the connection providing
   differential treatment for packets in different flows or in different
   traffic classes.  Intserv/diffserv, cost-based fairness, and
   congestion-based pricing could also require more complex pairwise
   economic relationships among Internet Service Providers (ISPs), and
   between end-users and ISPs.

   This document suggests that it is important to retain the class of
   "simple best-effort traffic" (though hopefully augmented by a wider
   deployment of other classes of service).  Further, this document
   suggests that some form of rough flow-rate fairness is an appropriate
   goal for simple best-effort traffic.  We do not argue in this
   document that flow-rate fairness is the *only possible* or *only
   desirable* resource allocation goal for simple best-effort traffic.
   We maintain, however, that it is an appropriate resource allocation
   goal for simple best-effort traffic in the current Internet, evolving
   from the Internet's past of end-point congestion control.

   This document was motivated by [B07], a paper titled "Flow Rate
   Fairness:  Dismantling a Religion" that asserts in the abstract that
   "comparing flow rates should never again be used for claims of
   fairness in production networks."  This document does not attempt to
   be a rebuttal to [B07], or to answer any or all of the issues raised
   in [B07], or to give the "intellectual heritage" for flow-based
   fairness in philosophy or social science, or to commit the authors of
   this document to an extended dialogue with the author of [B07].  This
   document is simply a separate viewpoint on some related topics.

2.  On Simple Best-Effort Traffic

   This section makes some observations on the usefulness and
   limitations of the class of simple best-effort traffic, in comparison
   with traffic receiving differential treatment.

2.1.  The Usefulness of Simple Best-Effort Traffic

   We now list some useful aspects of simple best-effort traffic.

   Minimal technical demands on the network infrastructure:

      Simple best-effort traffic, as implemented in the current
      Internet, makes minimal technical demands on the infrastructure.
      There are no technical requirements for scheduling, queue
      management, or enforcement mechanisms in routers.

   Minimal demands in terms of economic infrastructure:

      Simple best-effort traffic makes minimal demands in terms of
      economic infrastructure, relying on fairly simple pair-wise
      economic relationships among ISPs, and between a user and its
      immediate ISP.  In contrast, Section 4 discusses some of the
      difficulties in the incremental deployment of infrastructure for
      additional classes of service.

   Usefulness in the real world:

      Simple best-effort traffic has been shown to work in the Internet
      for the past 20 years, however imperfectly.  Simple best-effort
      traffic has supported everything from simple file and e-mail
      transfer and web traffic to video and audio streaming and voice
      communications.

      As discussed below, simple best-effort traffic is not optimal.
      However, experience in the Internet has shown that there has been
      significant value in the mechanism of simple best-effort traffic,
      generally allowing all users to get a portion of the resources
      while still preventing congestion collapse.

2.2.  The Limitations of Simple Best-Effort Traffic

   We now discuss some limitations of simple best-effort traffic.

2.2.1.  Quality of Service (QoS)

   Some users would be happy to pay for more bandwidth, less delay, less
   jitter, or fewer packet drops.  It is desirable to accommodate such
   goals within the Internet architecture while preserving a sufficient
   amount of bandwidth for simple best-effort traffic.

   One of the obvious dangers of simple differential traffic treatment
   implementations that do not take steps to protect simple best-effort
   traffic would be that the users with more money *could* starve users

   with less money in times of congestion.  There seems to be fairly
   widespread agreement that this would not be a desirable goal.  As a
   sample of the range of positions, the Internet Society's Internet
   2020 Initiative, titled "The Internet is (still) for Everyone",
   states that "we remain committed to the openness that ensures equal
   access and full participation for every user" [Internet2020].

   The wide-ranging discussion of "network neutrality" in the United
   States includes advocates of several positions, including that of
   "absolute non-discrimination" (with no QoS considerations), "limited
   discrimination without QoS tiering" (no fees charged for higher-
   quality service), and "limited discrimination and tiering" (including
   higher fees allowed for QoS) [NetNeutral].  The proponents of
   "network neutrality" are opposed to charging based on content (e.g.,
   based on applications or the content provider).

   As the "network neutrality" discussion makes clear, there are many
   voices in the discussion that would disagree with a resource
   allocation goal of maximizing the combined aggregate utility
   (advocated in [B07a]), particularly where a user's utility is
   measured by the user's willingness to pay.  "You get what you pay
   for" ([B07], page 5) does not appear to be the consensus goal for
   resource allocation in the community or in the commercial or
   political realms of the Internet.  However, there is a reasonable
   agreement that higher-priced services, as an adjunct to simple best-
   effort traffic, can play an important role in helping to finance the
   Internet infrastructure.

   Briscoe argues for cost-fairness [B07], so that senders are made
   accountable for the congestion they cause.  There are, of course,
   differences of opinion about how well cost-based fairness could be
   enforced, and how well it fits the commercial reality of the
   Internet, with [B07] presenting an optimistic view.  Another point of
   view, e.g., from an earlier paper by Roberts titled "Internet
   Traffic, QoS, and Pricing", is that "many proposed schemes are overly
   concerned with congestion control to the detriment of the primary
   pricing function of return on investment" [R04].

   With *only* simple best-effort traffic, there would be fundamental
   limitations to the performance that real-time applications could
   deliver to users.  In addition to the obvious needs for high
   bandwidth, low delay or jitter, or low packet drop rates, some
   applications would like a fast start-up, or to be able to resume
   their old high sending rate after a relatively long idle period, or
   to be able to rely on a call-setup procedure so that the application
   is not even started if network resources are not sufficient.  There
   are severe limitations to how effectively these requirements can be
   accommodated by simple best-effort service in a congested

   environment.  Of course, Quality of Service architectures for the
   Internet have their own limitations and difficulties, as discussed in
   [RFC2990] and elsewhere.  We are not going to discuss these
   difficulties further here.

2.2.2.  The Avoidance of Congestion Collapse and the Enforcement of
        Fairness

   As discussed in Section 3.2 below, there are well-known problems with
   the enforcement of fairness and the avoidance of congestion collapse
   [RFC2914] with simple best-effort traffic.  In the current Internet,
   end-to-end congestion control is relied upon to deal with these
   concerns; this use of end-to-end congestion control essentially
   requires cooperation from end-hosts.

2.2.3.  Control of Traffic Surges

   Simple best-effort traffic can suffer from sudden aggregate
   congestion from traffic surges (e.g., Distributed Denial of Service
   (DDoS) attacks, flash crowds), resulting in degraded performance for
   all simple best-effort traffic sharing the path.  A wide range of
   approaches for detecting and responding to sudden aggregate
   congestion in the network has been proposed and used, including deep
   packet inspection and rate-limiting traffic aggregates.  There are
   many open questions about both the goals and mechanisms of dealing
   with aggregates within simple best-effort traffic on congested links.

3.  On Flow-Rate Fairness for Simple Best-Effort Traffic

   This section argues that rough flow-rate fairness is an acceptable
   goal for simple best-effort traffic.  We do not, however, claim that
   flow-rate fairness is necessarily an *optimal* fairness goal or
   resource allocation mechanism for simple best-effort traffic.  Simple
   best-effort traffic and flow-rate fairness are in general not about
   optimality, but instead are about a low-overhead service (best-effort
   traffic) along with a rough, simple fairness model (flow-rate
   fairness).

   Within simple best-effort traffic, it would be possible to have
   explicit fairness mechanisms that are implemented by the end-hosts in
   the network (as in proportional fairness or TCP fairness), explicit
   fairness mechanisms enforced by the routers (as in max-min fairness
   with Fair Queueing), or a traffic class with no explicit fairness
   mechanisms at all (as in the Internet before TCP congestion control).

   This document does *not* address the issues about the implementation
   of flow-rate fairness.  In the current Internet, rough flow-rate
   fairness is achieved by the fact that *most* of the traffic in the

   Internet uses TCP, and *most* of the TCP connections in fact use
   conformant TCP congestion control [MAF05].  However, rough flow-rate
   fairness could also be achieved by the use of per-flow scheduling at
   congested routers [DKS89] [LLSZ96], by related router mechanisms
   [SSZ03], or by congestion-controlled transport protocols other than
   TCP.  This document does not address the pros and cons of TCP-
   friendly congestion control, equation-based congestion control
   [FHPW00], or any of the myriad of other issues concerning mechanisms
   for approximating flow-rate fairness.  Le Boudec's tutorial on rate
   adaption, congestion control, and fairness gives an introduction to
   some of these issues [B00].

3.1.  The Usefulness of Flow-Rate Fairness

   We note that the limitations of flow-rate fairness are many, with a
   long history in the literature.  We discuss these limitations in the
   next section.  While the benefits of simple best-effort traffic and
   rough flow-rate fairness are rarely discussed, this does *not* mean
   that benefits do not exist.  In this section, we discuss the benefits
   of flow-rate fairness.  We note that many of the useful aspects of
   simple best-effort traffic discussed above also qualify as useful
   aspects of rough flow-rate fairness.  For simple best-effort traffic
   with rough flow-rate fairness, the quote from Winston Churchill about
   democracy comes to mind: "Democracy is the worst form of government
   except all those other forms that have been tried from time to time"
   [C47].

   Minimal technical demands on the network infrastructure:

      First, the rough flow-rate fairness for best-effort traffic
      provided by TCP or other transport protocols makes minimal
      technical demands on the infrastructure, as TCP's congestion
      control algorithms are wholly implemented in the end-hosts.
      However, mechanisms for *enforcement* of the flow-rate fairness
      *would* require some support from the infrastructure.

   Minimal demands in terms of economic infrastructure:

      A system based on rough flow-rate fairness for simple best-effort
      traffic makes minimal demands in terms of economic relationships
      among ISPs or between users and ISPs.  In contrast, Section 4
      discusses some of the difficulties in the incremental deployment
      of infrastructure for cost-based fairness or other fairness
      mechanisms.

   Usefulness in the real world:

      The current system -- based on rough flow-rate fairness and simple
      best-effort traffic -- has shown its usefulness in the real world.

   Getting a share of the available bandwidth:

      A system based on rough flow-rate fairness and simple best-effort
      traffic gives all users a reasonable chance of getting a share of
      the available bandwidth.  This seems to be a quality that is much
      appreciated by today's Internet users (as discussed above).

3.2.  The Limitations of Flow-Rate Fairness

   This section discusses some of the limitations of flow-rate fairness
   for simple best-effort traffic.

3.2.1.  The Enforcement of Flow-Rate Fairness

   One of the limitations of rough flow-rate fairness is the difficulty
   of enforcement.  One possibility for implementing flow-rate fairness
   would be an infrastructure designed from the start with a requirement
   for ubiquitous per-flow scheduling in routers.  However, when
   starting with an infrastructure such as the current Internet with
   best-effort traffic largely served by First-In First-Out (FIFO)
   scheduling in routers and a design preference for intelligence at the
   ends, enforcement of flow-rate fairness is difficult at best.
   Further, a transition to an infrastructure that provides actual
   flow-rate fairness for best-effort traffic enforced in routers would
   be difficult.

   A second possibility, which is largely how the current Internet is
   operated, would be simple best-effort traffic where most of the
   connections, packets, and bytes belong to connections using similar
   congestion-control mechanisms (in this case, those of TCP congestion
   control), with few if any enforcement mechanisms.  Of course, when
   this happens, the result is a rough approximation of flow-rate
   fairness, with no guarantees that the simple best-effort traffic will
   continue to be dominated by connections using similar congestion-
   control mechanisms or that users or applications cannot game the
   system for their benefit.  That is our current state of affairs.  The
   good news is that the current Internet continues to successfully
   carry traffic for many users.  In particular, we are not aware of
   reports of frequent congestion collapse, or of the Internet being
   dominated by severe congestion or intolerable unfairness.

   A third possibility would be simple best-effort traffic with flow-
   rate fairness provided by the congestion control mechanisms in the
   transport protocols, with some level of enforcement, either in
   congested routers, in middleboxes, or by other mechanisms [MBFIPS01]
   [MF01] [SSZ03].  There seems to us to be considerable promise that
   incentives among the various players (ISPs, vendors, customers,
   standards bodies, political entities, etc.) will align somewhat, and
   that further progress will be made on the deployment of various
   enforcement mechanisms for flow-rate fairness for simple best-effort
   traffic.  Of course, this is not likely to turn in to a fully
   reliable and ubiquitous enforcement of flow-rate fairness, or of any
   related fairness goals, for simple best-effort traffic, so this is
   not likely to be satisfactory to purists in this area.  However, it
   may be enough to continue to encourage most systems to use standard
   congestion control.

3.2.2.  The Precise Definition of Flow-Based Fairness

   A second limitation of flow-based fairness is that there is seemingly
   no consensus within the research, standards, or technical communities
   about the precise form of flow-based fairness that should be desired
   for simple best-effort traffic.  This area is very much still in
   flux, as applications, transport protocols, and the Internet
   infrastructure evolve.

   Some of the areas where there is a range of opinions about the
   desired goals for rough flow-based fairness for simple best-effort
   traffic include the following:

   *  Granularity: What is the appropriate fairness granularity?  That
      is, for flow-based fairness, what is the definition of a 'flow'?
      (This question has been explicitly posed in [RFC2309], [RFC2914],
      and many other places.)  Should fairness be assessed on a per-
      connection basis?  Should fairness take into account multiple
      connections between a pair of end-hosts (e.g., as suggested by
      [RFC3124])?  If congestion control applies to each individual
      connection, what controls (if any) should constrain the number of
      connections opened between a pair of end-hosts?  As an example,
      RFC 2616 specifies that with HTTP 1.1, a single-user client SHOULD
      NOT maintain more than two persistent connections with any server
      or proxy [RFC2616] (Section 8.1.4).  For peer-to-peer traffic,
      different operating systems have different limitations on the
      maximum number of peer-to-peer connections; Windows XP Pro has a
      limit of ten simultaneous peer-to-peer connections, Windows XP
      Home (for the client) has a limit of five, and an OS X client has
      a limit of ten [P2P].

   *  RTT fairness: What is the desired relationship between flow
      bandwidth and round-trip times, for simple best-effort traffic?
      As shown in Section 3.3 of [FJ92], it would be straightforward to
      modify TCP's congestion control algorithms so that flows with
      similar packet drop rates but different round-trip times would
      receive roughly the same throughput.  This question is further
      studied in [HSMK98].  It remains an open question what would be
      the desired relationship between throughput and round-trip times
      for simple best-effort traffic, particularly for applications or
      transport protocols using some form of feedback-based congestion
      control.

   *  Multiple congested routers: What is the desired relationship
      between flow bandwidth and the number of congested routers along
      the path, for simple best-effort traffic?  It is well established
      that for TCP traffic in particular, flows that traverse multiple
      congested routers receive a higher packet drop rate, and therefore
      lower throughput, than flows with the same round-trip time that
      traverse only one congested router [F91].  There is also a long-
      standing debate between max-min fairness [HG86] and proportional
      fairness [KMT98], and no consensus within the research community
      on the desired fairness goals in this area.

   *  Bursty vs. smooth traffic: What is the desired relationship
      between flow bandwidth and the burstiness in the sending rate of
      the flow?  Is it a goal for a bursty flow to receive the same
      average or maximum bandwidth as a flow with a smooth sending rate?
      How does the goal depend on the time scale of the burstiness of
      the flow [K96]?  For instance, a flow that is bursty on time
      scales of less than a round-trip time has different dynamics than
      a flow that is bursty on a time scale of seconds or minutes.

   *  Packets or bytes: Should the rough fairness goals be in terms of
      packets per second or bytes per second [RFC3714]?  And if the
      fairness goals are in terms of bytes per second, does this include
      the bandwidth used by packet headers (e.g., TCP and IP headers)?

   *  Different transport protocols: Should the transport protocol used
      (e.g., UDP, TCP, SCTP, DCCP) or the application affect the rough
      fairness goals for simple best-effort traffic?

   *  Unicast vs. multicast: What should the fairness goals be between
      unicast and multicast traffic [FD04] [ZOX05]?

   *  Precision of fairness:  How precise should the fairness goals be?
      Is the precision that is possible from per-flow scheduling the
      right benchmark?  Or, is a better touchstone the rough fairness
      over multiple round-trip times achieved by TCP flows over FIFO

      scheduling?  Or, is a goal of even more rough fairness of an order
      of magnitude or more between flows using different transport
      protocols right?

      There is a range of literature for each of these topics, and we
      have not attempted to cite it all above.  Rough flow-based
      fairness for simple best-effort traffic could evolve with a range
      of possibilities for fairness in terms of round-trip times, the
      number of congested routers, packet size, or the number of
      receivers per flow.  (Further discussion can be found in
      [RFC5166].)

   Fairness over time:

      One issue raised in [B07] concerns how fairness should be
      integrated over time.  For example, for simple best-effort
      traffic, should long flows receive less bandwidth in bits per
      second than short flows?  For cost-based fairness or for QoS-based
      traffic, it seems perfectly viable for there to be some scenarios
      where the cost is a function of flow or session lifetime.  It also
      seems viable for there to be some scenarios where the cost of
      QoS-enabled traffic is independent of flow or session lifetime
      (e.g., for a private Intranet that is measured only by the
      bandwidth of the access link, but where any traffic sent on that
      Intranet is guaranteed to receive a certain QoS).

      However, for simple best-effort traffic, the current form of rough
      fairness seems acceptable, with fairness that is independent of
      session length.  That is, in the current Internet, a user who
      opens a single TCP connection for ten hours *might* receive the
      same average throughput in bits per second, during that TCP
      connection, as a user who opens a single TCP connection for ten
      minutes and then goes off-line.  Similarly, a user who is online
      for ten hours each day *might* receive the same throughput in bits
      per second, and pay roughly the same cost, as a user who is online
      for ten minutes each day.  That seems acceptable to us.  Other
      pricing mechanisms between users and ISPs seem acceptable also.
      The current Internet includes a wide range of pricing mechanisms
      between users and ISPs for best-effort traffic.

4.  On the Difficulties of Incremental Deployment

   One of the advantages of simple best-effort service is that it is
   currently operational in the Internet, along with the rough flow-rate
   fairness that results from the dominance of TCP's congestion control.

   While additional classes of service would clearly be of use in the
   Internet, the deployment difficulties of such mechanisms have been
   non-trivial [B03].  The problems of deploying interlocking changes to
   the infrastructure do not necessarily have an easy fix as they stem
   in part from the underlying architecture of the Internet.  As
   explained in RFC 1958 titled "Architectural Principles of the
   Internet":  "Fortunately, nobody owns the Internet, there is no
   centralized control, and nobody can turn it off" [RFC1958].  Some of
   the difficulties of making changes in the Internet infrastructure,
   including the difficulties imposed by the political and economic
   context, have been discussed elsewhere (e.g., [CMB07]).  The
   difficulty of making changes to the Internet infrastructure is in
   contrast to the comparative ease in making changes in Internet
   applications.

   The difficulties of deployment for end-to-end intserv or diffserv
   mechanisms are well-known, having in part to do with the difficulties
   of deploying the required economic infrastructure [B03].  It seems
   likely that cost-based schemes based on re-ECN could also have a
   difficult deployment path, involving the deployment of ECN-marking at
   routers, policers at both ends of a connection, and a change in
   pairwise economic relationships to include a congestion metric [B07].
   Some infrastructure deployment problems are sufficiently difficult
   that they have their own working groups in the IETF [MBONED].

5.  Related Work

5.1.  From the IETF

   This section discusses IETF documents relating to simple best-effort
   service and flow-rate fairness.

   RFC 896 on congestion control: Nagle's RFC 896 titled "Congestion
   Control in IP/TCP", from 1984, raises the issue of congestion
   collapse, and says that "improved handling of congestion is now
   mandatory" [RFC896].  RFC 896 was written in the context of a heavily
   loaded network, the only private TCP/IP long-haul network in
   existence at the time (that of Ford Motor Company, in 1984).  In
   addition to introducing the Nagle algorithm for minimizing the
   transmission of small packets in TCP, RFC 896 considers the
   effectiveness of ICMP Source Quench for congestion control, and
   comments that future gateways should be capable of defending
   themselves against obnoxious or malicious hosts.  However, RFC 896
   does not raise the question of fairness between competing users or
   flows.

   RFC 2309 on unresponsive flows: RFC 2309, an Informational document
   from the End-to-End Research Group titled "Recommendations on Queue
   Management and Congestion Avoidance in the Internet" from 2000,
   contains the following recommendation: "It is urgent to begin or
   continue research, engineering, and measurement efforts contributing
   to the design of mechanisms to deal with flows that are unresponsive
   to congestion notification or are responsive but more aggressive than
   TCP" [RFC2309].

   RFC 2616 on opening multiple connections: RFC 2616, the standards-
   track document for HTTP/1.1, specifies that "clients that use
   persistent connections SHOULD limit the number of simultaneous
   connections that they maintain to a given server" (Section 8.1.4 of
   [RFC2616]).

   RFC 2914 on congestion control principles: RFC 2914, a Best Current
   Practice document, from 2000 titled "Congestion Control Principles",
   discusses the issues of preventing congestion collapse, maintaining
   some form of fairness for best-effort traffic, and optimizing a
   flow's performance in terms of throughput, delay, and loss for the
   flow in question.  In the discussion of fairness, RFC 2914 outlines
   policy issues concerning the appropriate granularity of a "flow", and
   acknowledges that end nodes can easily open multiple concurrent flows
   to the same destination.  RFC 2914 also discusses open issues
   concerning fairness between reliable unicast, unreliable unicast,
   reliable multicast, and unreliable multicast transport protocols.

   RFC 3714 on the amorphous problem of fairness: Section 3.3 of RFC
   3714, an Informational document from the IAB (Internet Architecture
   Board) discussing congestion control for best-effort voice traffic,
   has a discussion of "the amorphous problem of fairness", discussing
   complicating issues of packet sizes, round-trip times, application-
   level functionality, and the like [RFC3714].

   RFCs on QoS: There is a long history in the IETF of the development
   of QoS mechanisms for integrated and differentiated services
   [RFC2212, RFC2475].  These include lower effort per-domain behaviors
   that could be used to protect best-effort traffic from lower-priority
   traffic [RFC3662].

5.2.  From Elsewhere

   This section briefly mentions some of the many papers in the
   literature on best-effort traffic or on fairness for competing flows
   or users.  [B07] also has a section on some of the literature
   regarding fairness in the Internet.

   Fairness with AIMD: Fairness with AIMD (Additive Increase
   Multiplicative Decrease) congestion control was studied by Chiu and
   Jain in 1987, where fairness is maximized when each user or flow gets
   equal allocations of the bottleneck bandwidth [CJ89].  Van Jacobson's
   1988 paper titled "Congestion Avoidance and Control" defined TCP's
   AIMD-based congestion control mechanisms [J88].

   Fair Queueing: The 1989 paper on Fair Queueing by Demers et al.
   promoted Fair Queueing scheduling at routers as providing fair
   allocation of bandwidth, lower delay for low-bandwidth traffic, and
   protection from ill-behaved sources [DKS89].

   Congestion-based pricing: One of the early papers on congestion-based
   pricing in networks is the 1993 paper titled "Pricing the Internet"
   by MacKie-Mason and Varian [MV93].  This paper proposed a "Smart
   Market" to price congestion in real time, with a per-packet charge
   reflecting marginal congestion costs.  Frank Kelly's web page at
   [Proportional] has citations to papers on proportional fairness,
   including [K97] titled "Charging and Rate Control for Elastic
   Traffic".

   Other papers on pricing in computer networks include [SCEH96], which
   is in part a critique of some of the pricing proposals in the
   literature at the time.  [SCEH96] argues that usage charges must
   remain at significant levels even if congestion is extremely low.

6.  Security Considerations

   This document does not propose any new mechanisms for the Internet,
   and so does not require any security considerations.

7.  Conclusions

   This document represents the views of the two authors on the role of
   simple best-effort traffic in the Internet.

8.  Acknowledgements

   We thank Ran Atkinson, Roland Bless, Bob Briscoe, Mitchell Erblich,
   Ted Faber, Frank Kelly, Tim Shephard, and members of the Transport
   Area Working Group for feedback on this document.

9.  Informative References

   [B00]     J.-Y. Le Boudec, Rate adaptation, Congestion Control and
             Fairness: A Tutorial, 2000.  URL
             "http://citeseer.ist.psu.edu/boudec00rate.html" or
             "http://ica1www.epfl.ch/PS_files/LEB3132.pdf".

   [B03]     G. Bell, Failure to Thrive: QoS and the Culture of
             Operational Networking, Proceedings of the ACM SIGCOMM
             Workshop on Revisiting IP QoS: What Have We Learned, Why Do
             We Care?, pp. 115-120, 2003, URL
             "http://doi.acm.org/10.1145/944592.944595".

   [B07]     B. Briscoe, Flow Rate Fairness: Dismantling a Religion, ACM
             SIGCOMM Computer Communication Review, V.37 N.2, April
             2007.

   [B07a]    B. Briscoe, "Flow Rate Fairness: Dismantling a Religion",
             Work in Progress, July 2007.

   [CJ89]    Chiu, D.-M., and Jain, R., Analysis of the Increase and
             Decrease Algorithms for Congestion Avoidance in Computer
             Networks, Computer Networks and ISDN Systems, V. 17, pp.
             1-14, 1989.  [The DEC Technical Report DEC-TR-509 was in
             1987.]

   [CMB07]   kc claffy, Sascha D. Meinrath, and Scott O. Bradner, The
             (un)Economic Internet?, IEEE Internet Computing, vol. 11,
             no. 3, pp. 53--58, May 2007.  URL
             "http://www.caida.org/publications/papers/2007/ieeecon/".

   [C47]     Churchill, W., speech, House of Commons, November 11, 1947.
             URL
             "http://www.askoxford.com/quotations/827?view=uk".

   [DKS89]   A. Demers, S. Keshav, and S. Shenker, Analysis and
             Simulation of a Fair Queueing Algorithm, SIGCOMM, 1989.

   [F91]     Floyd, S., Connections with Multiple Congested Gateways in
             Packet-Switched Networks Part 1: One-way Traffic, Computer
             Communication Review, Vol.21, No.5, October 1991.

   [FD04]    F. Filali and W. Dabbous, Fair Bandwidth Sharing between
             Unicast and Multicast Flows in Best-Effort Networks,
             Computer Communications, V.27 N.4, pp. 330-344, March 2004.

   [FHPW00]  Floyd, S., Handley, M., Padhye, J., and Widmer, J,
             Equation-Based Congestion Control for Unicast Applications,
             SIGCOMM, August 2000.

   [FJ92]    On Traffic Phase Effects in Packet-Switched Gateways,
             Floyd, S. and Jacobson, V., Internetworking: Research and
             Experience, V.3 N.3, September 1992.

   [HG86]    E. Hahne and R. Gallager, Round Robin Scheduling for Fair
             Flow Control in Data Communications Networks, IEEE
             International Conference on Communications, June 1986.

   [HSMK98]  Henderson, T.R., E. Sahouria, S. McCanne, and R.H.  Katz,
             On Improving the Fairness of TCP Congestion Avoidance,
             Globecom, November 1998.

   [Internet2020]
             Internet Society, An Internet 2020 Initiative: The Internet
             is (still) for Everyone, 2007.  URL "http://
             www.isoc.org/orgs/ac/cms/uploads/docs/2020_vision.pdf".

   [J88]     V. Jacobson, Congestion Avoidance and Control, SIGCOMM '88,
             August 1988.

   [K96]     F. Kelly, Charging and Accounting for Bursty Connections,
             In L. W. McKnight and J. P. Bailey, editors, Internet
             Economics. MIT Press, 1997.

   [K97]     F. Kelly, Charging and Rate Control for Elastic Traffic,
             European Transactions on Telecommunications, 8:33--37,
             1997.

   [KMT98]   F. Kelly, A. Maulloo and D. Tan, Rate Control in
             Communication Networks: Shadow Prices, Proportional
             Fairness and Stability.  Journal of the Operational
             Research Society 49, pp.  237-252, 1998.  URL
             "http://citeseer.ist.psu.edu/kelly98rate.html".

   [LLSZ96]  C. Lefelhocz, B. Lyles, S. Shenker, and L. Zhang,
             Congestion Control for Best-effort Service: Why We Need a
             New Paradigm, IEEE Network, vol. 10, pp. 10-19, Jan. 1996.

   [MAF05]   A. Medina, M. Allman, and S. Floyd, Measuring the Evolution
             of Transport Protocols in the Internet, Computer
             Communications Review, April 2005.

   [MBFIPS01]
             R. Manajan, S. Bellovin, S. Floyd, J. Ioannidis, V.
             Paxson, and S. Shenker, Controlling High Bandwidth
             Aggregates in the Network, Computer Communications Review,
             V.32 N.3, July 2002.

   [MBONED]  MBONE Deployment Working Group, URL
             "http://www.ietf.org/html.charters/mboned-charter.html".

   [MF01]    Mahajan, R., and Floyd, S., Controlling High-Bandwidth
             Flows at the Congested Router, ICNP 2001, November 2001.

   [MV93]    J. K. MacKie-Mason and H. Varian, Pricing the Internet, in
             the conference on Public Access to the Internet, JFK School
             of Government, May 1993.

   [NetNeutral]
             Network Neutrality, Wikipedia.  URL
             "http://en.wikipedia.org/wiki/Net_neutrality".

   [P2P]     "Maximum Number of Peer-to-Peer Connections", MAC OS X
             Hints web site, February 2007, URL
             "http://forums.macosxhints.com/showthread.php?t=67237".

   [Proportional]
             Kelly, F., papers on Proportional Fairness.  URL
             "http://www.statslab.cam.ac.uk/~frank/pf/".

   [R04]     J. Roberts, Internet Traffic, QoS, and Pricing, Proceedings
             of the IEEE, V.92 N.9, September 2004.

   [RFC896]  Nagle, J., "Congestion control in IP/TCP internetworks",
             RFC 896, January 1984.

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

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

   [RFC2309] Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
             S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
             Partridge, C., Peterson, L., Ramakrishnan, K., Shenker, S.,
             Wroclawski, J., and L. Zhang, "Recommendations on Queue
             Management and Congestion Avoidance in the Internet", RFC
             2309, April 1998.

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

   [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter,
             L., Leach, P., and T. Berners-Lee, "Hypertext Transfer
             Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC2914] Floyd, S., "Congestion Control Principles", BCP 41, RFC
             2914, September 2000.

   [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.

   [RFC3662] Bless, R., K. Nichols, and K. Wehrle, "A Lower Effort Per-
             Domain Behavior (PDB) for Differentiated Services", RFC
             3662, December 2003.

   [RFC3714] Floyd, S., Ed., and J. Kempf, Ed., "IAB Concerns Regarding
             Congestion Control for Voice Traffic in the Internet", RFC
             3714, March 2004.

   [RFC5166] Floyd, S., Ed., "Metrics for the Evaluation of Congestion
             Control Mechanisms", RFC 5166, March 2008.

   [SCEH96]  Shenker, D. D. Clark, D. Estrin, and S. Herzog, Pricing in
             Computer Networks: Reshaping the Research Agenda, ACM
             Computer Communication Review, vol. 26, April 1996.

   [SSZ03]   I. Stoica, S. Shenker, and H. Zhang, Core-Stateless Fair
             Queueing: a Scalable Architecture to Approximate Fair
             Bandwidth Allocations in High-speed Networks, IEEE/ACM
             Transactions on Networking 11(1): 33-46, 2003.

   [ZOX05]   Zhang, T., P. Osterberg, and Youzhi Xu, Multicast-
             favorable Max-Min Fairness - a General Definition of
             Multicast Fairness, Distributed Frameworks for Multimedia
             Applications, February 2005.

Authors' Addresses

   Sally Floyd
   ICSI Center for Internet Research
   1947 Center Street, Suite 600
   Berkeley, CA 94704
   USA
   EMail: floyd@icir.org
   URL: http:/www.icir.org/floyd/

   Mark Allman
   International Computer Science Institute
   1947 Center Street, Suite 600
   Berkeley, CA 94704-1198
   Phone: (440) 235-1792
   EMail: mallman@icir.org
   URL: http://www.icir.org/mallman/

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