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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.


 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.


 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.

Floyd & Allman Informational [Page 1] RFC 5290 Simple Best-Effort Traffic July 2008

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
 "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.

Floyd & Allman Informational [Page 2] RFC 5290 Simple Best-Effort Traffic July 2008

 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.

Floyd & Allman Informational [Page 3] RFC 5290 Simple Best-Effort Traffic July 2008

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
    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

Floyd & Allman Informational [Page 4] RFC 5290 Simple Best-Effort Traffic July 2008

 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

Floyd & Allman Informational [Page 5] RFC 5290 Simple Best-Effort Traffic July 2008

 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

 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
 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

Floyd & Allman Informational [Page 6] RFC 5290 Simple Best-Effort Traffic July 2008

 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"
 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

Floyd & Allman Informational [Page 7] RFC 5290 Simple Best-Effort Traffic July 2008

 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.

Floyd & Allman Informational [Page 8] RFC 5290 Simple Best-Effort Traffic July 2008

 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].

Floyd & Allman Informational [Page 9] RFC 5290 Simple Best-Effort Traffic July 2008

  • 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
  • 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

Floyd & Allman Informational [Page 10] RFC 5290 Simple Best-Effort Traffic July 2008

    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
 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.

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 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
 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

Floyd & Allman Informational [Page 12] RFC 5290 Simple Best-Effort Traffic July 2008

 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
 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.

Floyd & Allman Informational [Page 13] RFC 5290 Simple Best-Effort Traffic July 2008

 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
 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
           "" or

Floyd & Allman Informational [Page 14] RFC 5290 Simple Best-Effort Traffic July 2008

 [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
 [B07]     B. Briscoe, Flow Rate Fairness: Dismantling a Religion, ACM
           SIGCOMM Computer Communication Review, V.37 N.2, April
 [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
 [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
 [C47]     Churchill, W., speech, House of Commons, November 11, 1947.
 [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.

Floyd & Allman Informational [Page 15] RFC 5290 Simple Best-Effort Traffic July 2008

 [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.
           Internet Society, An Internet 2020 Initiative: The Internet
           is (still) for Everyone, 2007.  URL "http://
 [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,
 [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
 [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.
           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

Floyd & Allman Informational [Page 16] RFC 5290 Simple Best-Effort Traffic July 2008

 [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.
           Network Neutrality, Wikipedia.  URL
 [P2P]     "Maximum Number of Peer-to-Peer Connections", MAC OS X
           Hints web site, February 2007, URL
           Kelly, F., papers on Proportional Fairness.  URL
 [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
 [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.

Floyd & Allman Informational [Page 17] RFC 5290 Simple Best-Effort Traffic July 2008

 [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.

Floyd & Allman Informational [Page 18] RFC 5290 Simple Best-Effort Traffic July 2008

Authors' Addresses

 Sally Floyd
 ICSI Center for Internet Research
 1947 Center Street, Suite 600
 Berkeley, CA 94704
 URL: http:/
 Mark Allman
 International Computer Science Institute
 1947 Center Street, Suite 600
 Berkeley, CA 94704-1198
 Phone: (440) 235-1792

Floyd & Allman Informational [Page 19] RFC 5290 Simple Best-Effort Traffic July 2008

Full Copyright Statement

 Copyright (C) The IETF Trust (2008).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78 and at,
 and except as set forth therein, the authors retain all their rights.
 This document and the information contained herein are provided on an

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 this document or the extent to which any license under such rights
 might or might not be available; nor does it represent that it has
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Floyd & Allman Informational [Page 20]

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