path: root/doc/rfc/rfc5783.txt

Internet Research Task Force (IRTF)                             M. Welzl
Request for Comments: 5783                            University of Oslo
Category: Informational                                          W. Eddy
ISSN: 2070-1721                                              MTI Systems
                                                           February 2010


                  Congestion Control in the RFC Series

Abstract

   This document is an informational snapshot taken by the IRTF's
   Internet Congestion Control Research Group (ICCRG) in October 2008.
   It provides a survey of congestion control topics described by
   documents in the RFC series.  This does not modify or update the
   specifications or status of the RFC documents that are discussed.  It
   may be used as a reference or starting point for the future work of
   the research group, especially in noting gaps or open issues in the
   current IETF standards.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Research Task Force
   (IRTF).  The IRTF publishes the results of Internet-related research
   and development activities.  These results might not be suitable for
   deployment.  This RFC represents the consensus of the Internet
   Congestion Control Research Group of the Internet Research Task Force
   (IRTF).  Documents approved for publication by the IRSG are not a
   candidate for any level of Internet Standard; see Section 2 of RFC
   5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5783.














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Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Architectural Documents  . . . . . . . . . . . . . . . . . . .  5
   3.  TCP Congestion Control . . . . . . . . . . . . . . . . . . . .  9
   4.  Challenging Link and Path Characteristics  . . . . . . . . . . 10
   5.  End-Host and Router Cooperative Signaling  . . . . . . . . . . 12
     5.1.  Explicit Congestion Notification . . . . . . . . . . . . . 13
     5.2.  Quick-Start  . . . . . . . . . . . . . . . . . . . . . . . 15
   6.  Non-TCP Unicast Congestion Control . . . . . . . . . . . . . . 15
   7.  Multicast Congestion Control . . . . . . . . . . . . . . . . . 18
   8.  Guidance for Developing and Analyzing Congestion Control
       Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . 20
   9.  Historic Interest  . . . . . . . . . . . . . . . . . . . . . . 21
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 22
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
   12. Informative References . . . . . . . . . . . . . . . . . . . . 22






















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1.  Introduction

   In this document, we define congestion control as the feedback-based
   adjustment of the rate at which data is sent into the network.
   Congestion control is an indispensable set of principles and
   mechanisms for maintaining the stability of the Internet.  Congestion
   control has been closely associated with TCP since 1988 [Jac88], but
   there has also been a great deal of congestion control work outside
   of TCP (e.g., for real-time multimedia applications, multicast, and
   router-based mechanisms).  Several such proposals have been produced
   within the IETF and published as RFCs, along with RFCs that give
   architectural guidance (e.g., by pointing out the importance of
   performing some form of congestion control).  Several of these
   mechanisms are in use within the Internet.

   When designing a new Internet transport protocol, it is therefore
   important to not only understand how congestion control works in TCP
   but also have a broader understanding of the other congestion control
   RFCs -- some give guidance, some of them describe mechanisms that may
   have a direct influence on a newly designed protocol, and some of
   them may only be "related work" worth knowing about.  The purpose of
   this document is to facilitate and encourage this search for
   knowledge by providing an overview of RFCs related to congestion
   control that have been published thus far.  This document is a
   product of the IRTF's Internet Congestion Control Research Group
   (ICCRG).  It was developed because a strong grasp of the existing
   literature should benefit further ICCRG work.  The ICCRG developed
   consensus on the content of this document during a two-year
   development period based on review comments and ICCRG mailing list
   discussions.  A list of the main review contributors is contained in
   the Acknowledgements section of this document.

   While the ICCRG agreed to the document's production, any opinions
   expressed are the authors' own, and as this document is not an IETF
   publication, it does not update or modify the status of any published
   RFCs.  The format of this document is similar to an annotated
   bibliography.  Although host and router requirements for congestion
   control functions are discussed, this is only an informational
   document and does not contain any formal standards bearing of its
   own.

   Congestion control is a large and active topic, and so the scope of
   this document is limited to published RFCs and a small number of
   current working group drafts.  This allows the document to focus on
   congestion control principles and mechanisms that are among the most
   well-supported, well-accepted, or widely used.  Significant
   contributions to this subject also exist in both the academic
   literature and in the form of Internet-Drafts; however, we exclude



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   these from this study.  In many cases the RFC describing some
   mechanism will contain references to relevant academic publications
   in journals or conference proceedings that presented the research and
   validation of the mechanism.  For instance, RFC 2581 cites Jacobson's
   1988 SIGCOMM paper that has a less standards-oriented but more
   illustrative treatment and explanation of some of the mechanisms in
   RFC 2581.

   The majority of the documents discussed here pertain to end-host-
   based congestion control.  Many network-based mechanisms, such as a
   number of queue management algorithms, do not require any protocol
   exchanges between elements, but merely operate within a single host
   or router.  Thus, network-based congestion control mechanisms have
   often not been described in any RFC, as they generally fall under the
   domain of implementation details that do not influence
   interoperability.

   There are many RFCs related to Quality of Service (QoS), especially
   within the Integrated Services and Differentiated Services frameworks
   [RFC1633] [RFC2475] [RFC2998].  These QoS RFCs themselves deserve a
   similar bibliography to the one that this document provides for
   congestion control.  We specifically do not include the vast amount
   of QoS work into the scope of this document, as it is a full field in
   its own right, and deals with issues that are mostly orthogonal to
   end-host congestion control and router queue management.  Although
   there can certainly be interactions between QoS and congestion
   control mechanisms, scheduling mechanisms used to implement QoS (on
   either a per-flow or an aggregate basis), for instance, can be used
   independently of the end-host congestion control and queue management
   functions also in use.  Similar arguments can be made for traffic-
   shaping, admission control, and other functions that are intended for
   QoS and are only side-notes for congestion control.

   A similar argument can be made for excluding consideration of the
   media access control (MAC) layer protocols used by the links
   throughout a path.  Although the MAC protocols implement various
   forms of resolving contention for shared links (and sometimes offer
   QoS services), these are also distinct from end-to-end congestion
   control.  Furthermore, MAC protocols are not typically discussed in
   the RFC series, but they are defined in outside documents (e.g., IEEE
   standards), since the IETF does not generally work on link layers
   themselves.  Few, if any, of the RFCs that describe mappings of IP
   onto various link layers directly discuss congestion control.

   To organize the subject matter in this document, the content is
   classified into several broad categories.  First, we list documents
   relating to Internet architecture and general architectural concepts
   in Section 2.  Next, the congestion control algorithms used in the



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   TCP transport protocol are discussed in Section 3.  Interactions
   between link properties and mechanisms with the kinds of algorithms
   and heuristics used within end-to-end congestion control are covered
   in Section 4.  One method that has been developed by the IETF (and
   deployed to some extent) for allowing network-based and host-based
   congestion control to interact without dropping packets is the
   subject of Section 5.1.  The congestion control algorithms used by
   unicast transport protocols other than TCP are described in
   Section 6.  Work on congestion control for multicast transports and
   applications is listed in Section 7.  RFCs that give guidance to
   developers of new algorithms are discussed in Section 8.  Finally,
   documents that have historic significance, but perhaps not current
   direct technical application, have been classified into Section 9.
   Note that the use of the term "historic" here has nothing to do with
   the IETF's formal classification of documents as having "Historic"
   status.

2.  Architectural Documents

   Some documents in this section contain architectural guidance and
   concerns, while others specify congestion-control-related mechanisms
   that are broadly applicable and have impacts on more than a single
   class of congestion control techniques.  Some of these documents are
   direct products of the Internet Architecture Board (IAB), giving
   their guidance on specific aspects of congestion control in the
   Internet.

   RFC 1122: "Requirements for Internet Hosts -- Communication Layers"
      (October 1989)

      [RFC1122] formally mandates that hosts perform congestion control.
      For TCP, several congestion control features are described and
      listed as required elements of conforming implementations, and for
      UDP, RFC 1122 leaves congestion control as an issue for higher-
      layered protocols.  Although sending and reacting to ICMP Source
      Quench packets is no longer recommended [RFC1812] [Gont10], the
      rest of the congestion control guidance in this RFC is still a
      basis for several current practices in TCP implementations.


   RFC 1812: "Requirements for IP Version 4 Routers" (June 1995)

      Numerous issues relevant to router behavior are discussed in
      [RFC1812], and requirements for routers to support are prescribed
      within the document.  Portions of RFC 1812 that are particularly
      relevant to congestion control include the directive that routers
      SHOULD NOT originate ICMP Source Quench messages, discussion of
      precedence in queueing, and Section 5.3.6 titled "Congestion



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      Control" that recommends sizing buffers as a function of the
      product of the bandwidth of the link times the path delay of the
      flows using the link, and advises on the implementation of active
      queue management techniques.


   RFC 1958: "Architectural Principles of the Internet" (June 1996)

      Several guidelines for network systems design that have proven
      useful in the evolution of the Internet are sketched in [RFC1958].
      Congestion control is not specifically mentioned or alluded to,
      but the general principles apply to congestion control.  For
      instance, performing end-to-end functions at end nodes, lack of
      centralized control, heterogeneity, scalability, simplicity,
      avoiding options and parameters, etc., are all valid concerns in
      the design and assessment of congestion control schemes for the
      Internet.


   RFC 2140: "TCP Control Block Interdependence" (April 1997)

      [RFC2140] suggests that TCP connections between the same endpoints
      might share some information, including their congestion control
      state.  To some degree, this is done in practice by a few current
      operating systems; for example, Linux currently has a destination
      cache with this information, but this behavior is not yet formally
      standardized or recognized as a best practice by the IETF.


   RFC 2309: "Recommendations on Queue Management and Congestion
      Avoidance in the Internet" (April 1998)

      [RFC2309] briefly discusses the history of congestion and the
      origin of congestion control in the Internet.  The focus is mainly
      on network- or router-based queue management algorithms.  This RFC
      recommends to test, standardize, and deploy Active Queue
      Management (AQM) in routers; it provides an overview of one such
      mechanism, Random Early Detection (RED), and explains how and why
      AQM mechanisms can improve the performance of the Internet.
      Finally, this document explains the danger of a possible
      "congestion collapse" from unresponsive flows and makes a strong
      recommendation to develop and eventually deploy router mechanisms
      to protect the Internet from such traffic.

      Today, the advice in this document has been followed to some
      extent.  Hardware and software vendors have been receptive, and
      AQM techniques are widely available in many popular dedicated
      commercial router products and even in more general operating



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      systems that are sometimes used as routers.  However, AQM
      techniques may not be enabled in default configurations of these
      systems, and it is often left to users and network engineers to
      enable and configure AQM mechanisms when desired.  In some cases,
      enabling QoS mechanisms on a device also enables AQM mechanisms by
      default.  The number of production routers that actually have
      these AQM features enabled is an open question.


   RFC 2914 (BCP 41): "Congestion Control Principles" (September 2000)

      [RFC2914] is an explanation of the principles of congestion
      control, and the IETF's Best Current Practice for congestion
      control design.  It points out that there are an increasing number
      of applications that do not use TCP, and elaborates on the
      importance of performing congestion control for such traffic in
      order to prevent congestion collapse.  The TCP Reno congestion
      control mechanisms are described as an example of end-to-end
      congestion control within transport protocols.

      SCTP is one example of a non-TCP transport protocol that
      implements congestion control based on these principles.  The
      developments of TFRC [RFC3448] and DCCP [RFC4340] are attempts to
      provide useful tools implementing those principles for
      applications with needs similar to streaming media, where TCP's
      reactions are too fast.  It would be beneficial for users and the
      Internet itself if these carefully designed tools become widely
      deployed in place of other ad hoc schemes that may not be well-
      grounded in the congestion control principles.  This replacement
      process is ongoing and not yet complete.  Appropriate and usable
      congestion control schemes for non-TCP flows continue to be an
      open research area.


   RFC 3124: "The Congestion Manager" (June 2001)

      [RFC3124] specifies the Congestion Manager, an end-system service
      that realizes congestion control on a per-host-pair rather than a
      per-connection basis, which may be a more appropriate way to carry
      out congestion control.  Using the Congestion Manager, multiple
      streams between two hosts (which may include TCP flows) can adapt
      to network congestion in a unified fashion.

      This proposal is related to RFC 2140, discussed above, but with a
      wider scope than TCP.  Because some pieces of its supporting
      architecture have not yet been specified, the Congestion Manager's
      techniques are not commonly used today and have not been widely
      implemented and deployed yet beyond experimental stacks.  Sharing



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      of congestion and path information between individual connections
      continues to be an open research area with branches in detecting
      shared bottlenecks when using multiple paths, caching of old state
      for faster startup, and sharing of current state and feedback.


   RFC 3426: "General Architectural and Policy Considerations" (November
      2002)

      [RFC3426] lists a number of questions that can be answered for a
      particular technical solution to determine its architectural
      impact and desirability.  These are valid for congestion control
      mechanisms, and end-point congestion management is used as an
      example case-study several times in RFC 3426.  Two salient
      questions that RFC 3426 advises asking about proposed mechanisms
      are why they are needed in addition to existing protocols, and why
      they are needed at a certain layer rather than at other layers.
      These are particularly relevant for congestion control mechanisms
      since several already exist and since they can span network,
      transport, and application layers.


   RFC 3439: "Some Internet Architectural Guidelines and Philosophy"
      (December 2002)

      [RFC3439] supplements RFC 1958.  Simplicity is stressed, as the
      unpredictable results of complexity (due to amplification and
      coupling) are described.  Congestion control issues stemming from
      layering interactions between transport and lower protocols are
      presented, as well as other items relevant to congestion control,
      including asymmetry and the "myth of over-provisioning".


   RFC 3714: "IAB Concerns Regarding Congestion Control for Voice
      Traffic in the Internet" (March 2004)

      [RFC3714] can be seen as a follow-up to the concerns that were
      discussed in RFC 2914.  It expresses the IAB's concern over the
      lack of effective end-to-end congestion control for best-effort
      voice traffic, which is noted as being a current service with
      growing demand.  An example of a VoIP connection between Atlanta,
      Georgia, USA, and Nairobi, Kenya, is given, where a single VoIP
      call consumed more than half of the access link capacity (which is
      normally shared across several different users).  This example is
      used as the basis for further discussion, making it clear that
      using some form of congestion control for VoIP traffic is highly
      recommended.




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3.  TCP Congestion Control

   The TCP specifications found in RFC 793 and its predecessors did not
   contain any discussion of using or managing a congestion window.
   Other than a simple retransmission timeout and flow control through
   the advertised receive window, TCP implementations based only on RFC
   793 do not contain congestion control.  As several congestion
   collapse events occurred on the Internet, it was later realized that
   congestion control was needed.  The host requirements in RFC 1122
   require conforming TCP implementations to implement Jacobson's slow
   start and congestion avoidance algorithms (later specified in RFC
   2001 and then RFC 2581).  RFC 1122 also recommends several other
   behaviors that influence congestion control like the Nagle algorithm,
   delayed acknowledgements, Jacobson's retransmission timeout (RTO)
   estimation algorithm, and exponential backoff of the retransmission
   timer.

   Basic TCP congestion control is defined in RFC 2581, with many other
   RFCs that specify ancillary modifications and enhancements.  RFC 2581
   obsoletes the first proposed standard for TCP congestion control in
   RFC 2001.  These two RFCs document the mechanisms that had already
   been in common use by TCP implementations for many years.  The reader
   may refer to the TCP Roadmap [RFC4614] for more information on the
   RFCs that specifically describe TCP congestion control, as this
   material is not replicated here.

   Recently, significant effort has been put into experimental TCP
   congestion control modifications for obtaining high throughput with
   reduced startup and recovery times.  RFCs have been published on some
   of these modifications, including HighSpeed TCP [RFC3649], and
   Limited Slow-Start [RFC3742], but high-rate congestion control
   mechanisms are still considered an open issue in congestion control
   research.  Other schemes have been published as Internet-Drafts or
   have been discussed a little by the IETF, but much of the work in
   this area has not been adopted within the IETF yet, so the majority
   of this work is outside the RFC series and may be discussed in other
   products of the ICCRG.

   At the time of writing, the IETF's TCP Maintenance and Minor
   Extensions (TCPM) Working Group was developing an update to RFC 2581
   to incorporate small changes from other documents and advance TCP
   congestion control mechanisms on the IETF Standards Track.  The
   update also clarifies and revises some points.  These include the
   definition of a duplicate ACK, initial congestion window and slow
   start threshold values, behavior in response to retransmission
   timeouts, the use of the limited transmit mechanism, and security
   with regards to misbehaving receivers that practice ACK division.




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4.  Challenging Link and Path Characteristics

   Links with large and/or variable bandwidth-delay products have
   traditionally been problematic for congestion control schemes because
   they can distort the properties of the feedback loop.  Links that
   either expose a high rate of packet losses to the upper layers, or
   use highly-persistent retransmission mechanisms to prevent losses
   also cause problems with some of the standard congestion control
   mechanisms.  The documents in this section discuss challenging link
   characteristics; many of them were written by the Performance
   Implications of Link Characteristics (PILC) Working Group.

   While these documents often refer to specific problems with TCP, the
   link characteristics that they describe can be expected to affect
   other congestion control mechanisms too.  In particular, interactions
   between link properties and TCP congestion control will be shared by
   other protocols that use the similar congestion control behavior,
   such as SCTP [RFC4960] and DCCP with CCID 2 [RFC4341] (see
   Section 6), and should be taken into consideration by designers of
   congestion control mechanisms that utilize the same kind of feedback
   as TCP.

   Some RFCs only make recommendations regarding the implementation and
   configuration of TCP based upon characteristics of special links.  As
   these RFCs are so closely connected to the specification of TCP
   itself, they are not included in this document, but are listed in the
   TCP Roadmap [RFC4614].

   RFC 2488 (BCP 28): "Enhancing TCP Over Satellite Channels using
      Standard Mechanisms" (January 1999)

      The summary of recommendations in [RFC2488] came from the TCP over
      Satellite (TCPSAT) Working Group, whose goal was to identify the
      performance problems that TCP may have over satellite links and
      suggest mitigations.  The document explains several ways that
      existing standards can be applied to improve the performance of
      basic TCP congestion control over paths with characteristics
      similar to those involving satellite links.


   RFC 3135: "Performance Enhancing Proxies Intended to Mitigate Link-
      Related Degradations" (June 2001)

      [RFC3135] is a survey of Performance Enhancing Proxies (PEPs)
      often employed to improve degraded TCP performance caused by
      characteristics of specific link environments, for example, in
      satellite, wireless WAN, and wireless LAN environments.  Different
      types of PEPs are described as well as the mechanisms used to



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      improve performance.  While there is a specific focus on TCP in
      this document, PEPs can operate on any protocol, and the
      performance enhancements that PEPs achieve are often closely
      related to congestion control.

      The use of PEPs has architectural implications as they sometimes
      violate end-to-end assumptions and can add complexity to the inner
      portions of a network.  Certain types of PEPs are commonly used
      today in satellite or long-distance networking because it is
      easier to insert a small number of PEPs near problematic links
      than to upgrade the TCP implementations on all the end hosts that
      might use those links.  One down-side is that their deployment
      raises some issues when introducing new or updated congestion
      control (CC) methods into these deployed networks, since the PEPs
      may be operating with undocumented algorithms, making assumptions
      about the end-host CC behavior, and/or altering packet fields that
      will affect the end-host CC behavior.


   RFC 3150 (BCP 48): "End-to-end Performance Implications of Slow
      Links" (July 2001)

      [RFC3150] makes performance-related recommendations for users of
      network paths that traverse "very low bit-rate" links.  It
      includes a discussion of interactions between such links and TCP
      congestion control.


   RFC 3155 (BCP 50): "End-to-end Performance Implications of Links with
      Errors" (August 2001)

      Under the premise that several types of PEP have undesirable
      implications, [RFC3155] recommends end-to-end alternatives for
      improving TCP performance over paths with error-prone links.


   RFC 3366 (BCP 62): "Advice to link designers on link Automatic Repeat
      reQuest (ARQ)" (August 2002)

      Link-layer ARQ techniques are a popular means to increase the
      robustness of particular links to transmission errors via
      retransmission and acknowledgement mechanisms.  As [RFC3366]
      explains, ARQ techniques on a link can interact poorly with TCP's
      end-to-end congestion control if they lead to additional delay
      variation or reordering.  This RFC gives some advice on limiting
      the extent of these types of problematic interactions.  The proper





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      balance between end-to-end and link-layer reliability mechanisms
      is still an open research issue that has been explored in many
      academic papers outside the IETF.


   RFC 3449 (BCP 69): "TCP Performance Implications of Network Path
      Asymmetry" (December 2002)

      [RFC3449] describes performance limitations of TCP when the
      capacity of the ACK path is limited.  Several techniques to aid
      TCP in these circumstances are recommended as Best Current
      Practices, particularly ACK congestion control and sender pacing
      are relevant to other non-TCP congestion control schemes, outside
      the scope of this document.  For instance, in the design of the
      Reliable Multicast Transport (RMT) protocols for multicast,
      preventing ACK-implosion at multicast sources can be seen as a
      form of ACK congestion control.


   RFC 3481: "TCP over Second (2.5G) and Third (3G) Generation Wireless
      Networks" (February 2003)

      Among other issues, some mobile data systems exhibit delay spikes,
      handovers, and bandwidth oscillation.  [RFC3481] describes the
      problems that these conditions cause for TCP congestion control
      and how some TCP extensions can be used to mitigate them.


   RFC 3819 (BCP 89): "Advice for Internet Subnetwork Designers" (July
      2004)

      Several issues in link design and optimization for carrying IP
      traffic are discussed in [RFC3819], which recommends Best Current
      Practices.  Many of these principles are motivated by properties
      of TCP, but most of them also apply to other transport-layer
      congestion control techniques as well.

5.  End-Host and Router Cooperative Signaling

   Some RFCs define mechanisms that allow routers to add signaling
   information to packets that makes the network's congestion state less
   of a mystery to end-host congestion controllers.  Routers supporting
   these can signal information about the current congestion state to
   flows in-band, providing faster and finer-grained information than
   inference-based methods.  Two examples of this are discussed in this
   section; the first directs sources to slow down in order to avoid
   losses, and the other assists in determining an appropriate starting
   rate for new flows.



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5.1.  Explicit Congestion Notification

   Traditionally, under congestion, IP routers enqueue packets until
   some limit is reached, at which point packets are dropped.  TCP, and
   other IETF transport protocols, use a stream of acknowledgements to
   infer these losses and take congestion control action.  This section
   describes a more advanced way to signal congestion to sources before
   packet-dropping is required.

   There are two Explicit Congestion Notification (ECN) bits in the IP
   header that enable an AQM mechanism (see [RFC2309] or Section 2) to
   convey congestion information to endpoints without dropping packets.
   This can significantly reduce the losses experienced by transport
   endpoints if they are responsive to ECN.  While ECN is most
   frequently discussed in the context of TCP (and therefore included in
   the TCP Roadmap [RFC4614]), its applicability is broader, and ECN use
   has also been specified for protocols such as DCCP and SCTP.

   RFC 2481: "A Proposal to add Explicit Congestion Notification (ECN)
      to IP" (January 1999) - Obsoleted by RFC 3168

      [RFC2481] introduced ECN into the RFC series, describing when the
      Congestion Experienced (CE) bit in the IP header should be set in
      routers, and what modifications are needed to TCP to make it ECN-
      capable.  It includes a discussion of issues related to nodes and
      routers that are non-compliant, IPsec tunnels, and dropped or
      corrupted packets, as well as a summary of related work.  Many of
      these issues will also be faced by operators trying to deploy
      other network-based congestion control methods.  RFC 2481 has been
      obsoleted by RFC 3168.


   RFC 2884: "Performance Evaluation of Explicit Congestion Notification
      (ECN) in IP Networks" (July 2000)

      [RFC2884] presents a performance study of ECN as specified in
      [RFC2481] using an implementation on the Linux operating system.
      The experiments focused on ECN for both bulk and transactional
      transfers, showing that there is improvement in throughput over
      TCP without ECN in the case of bulk transfers and substantial
      improvement for transactional transfers.  Studies like this help
      to build the community's confidence that extensions like ECN are
      both safe and valuable.  Similar RFCs helped the community accept
      larger initial windows for TCP [RFC2414] [RFC2415] [RFC2416].







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   RFC 3168: "The Addition of Explicit Congestion Notification (ECN) to
      IP" (September 2001)

      [RFC3168], which obsoletes [RFC2481], specifies the incorporation
      of ECN into TCP and IP.  One notable change in this significantly
      extended specification is the definition of a bit combination that
      was not defined in [RFC2481], which can be used to realize a nonce
      that would prevent a receiver from falsely claiming that there was
      no congestion.  Potential issues related to ECN are discussed at
      length, including those already included in [RFC2481] and
      backwards compatibility with implementations that would follow the
      specification in the obsoleted document.

      ECN, as specified in RFC 3168, is implemented in several popular
      router and end-host platforms.  It is in active use, to at least
      some extent.  Problems with ECN "blackholes" (Internet routers
      misconfigured to discard packets with ECN-capable bits set) were
      discovered when ECN was enabled by default in some end-host
      operating systems.  Fears about the persisting presence of these
      blackholes currently may be keeping ECN from being used by default
      in many end-host operating systems even though it is implemented
      as an option within them.  Some measurements on ECN support and
      usability are available [PF01] [MAF04] [MAF05].


   RFC 3540: "Robust Explicit Congestion Notification (ECN) Signaling
      with Nonces" (June 2003)

      [RFC3540] specifies a nonce mechanism that uses an ECN bit
      combination that is not used in [RFC2481], but that is specified
      in [RFC3168] to allow a one-bit ECN nonce.  This nonce mechanism
      includes a Nonce Sum (NS) field in the TCP header so that senders
      can ensure that ACKs that do not indicate congestion are credible.
      The mechanism improves the robustness of congestion control by
      preventing receivers from exploiting ECN to gain an unfair share
      of network bandwidth.

      This nonce technique is not understood to have been widely
      implemented or deployed, and there has been some discussion as to
      whether the mechanism is really effective or is the best use of
      these bits (see emails to the IETF Transport Area Working Group
      (TSVWG) mailing list, in the thread "ECN nonce snag in TCP ESTATS
      MIB" from December 2006 - January 2007, or [MBJ07]).








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5.2.  Quick-Start

   RFC 4782: "Quick-Start for TCP and IP" (January 2007)

      Quick-Start provides a way for hosts to ask routers to help them
      select an initial sending rate, and use this rate rather than the
      traditional small initial congestion window and slow-start
      algorithm.  [RFC4782] describes the Quick-Start mechanism and its
      use with TCP.  In addition to discussing the benefits of Quick-
      Start, the document also discusses several limitations of the
      Quick-Start technique with respect to some types of tunnels in use
      over the Internet today and other potential costs of Quick-Start
      including those related to router design.  Analysis of the effects
      of misbehaving entities and appendices containing design rationale
      and related work are also notably present in this RFC.

      Many of the issues discussed in RFC 4782, including router
      architecture, network design / tunnels, and misbehaving agents are
      all challenges relevant to other proposals that try to add router
      assistance into the network.  The consideration of these issues
      can be illustrative for other protocol designers, even if they are
      not interested in Quick-Start itself.

6.  Non-TCP Unicast Congestion Control

   In the past, TCP dominated Internet traffic, as it was used for many
   of the popular applications (email, web browsing, file transfer,
   remote login, etc.).  The majority of early congestion control work
   focused on TCP, and the introduction of congestion control into TCP
   alone is often credited with saving the Internet from additional
   congestion collapse events.  Today, TCP has been joined by other
   transport protocols (e.g., custom UDP-based protocols, SCTP, DCCP,
   RTP over UDP [RFC3550], etc.), and so having properly functioning
   congestion control within these other protocols is important for the
   Internet's health (as explained in RFC 3714, for instance, or see the
   discussion of the "congestion control arms race" scenario in RFC
   2914).  Documents that describe unicast congestion control methods
   for non-TCP transport protocols have been grouped into this section.


   RFC 4960: "Stream Control Transmission Protocol" (September 2007)

      SCTP congestion control is very similar to TCP with Selective
      Acknowledgements, but there are some differences, as described in
      Section 7.1 of [RFC4960].  The major difference lies in the fact
      that SCTP supports multihoming, whereas TCP does not.  Thus, SCTP





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      keeps a different set of congestion control parameters for each
      destination address within an association, whereas TCP only keeps
      a single set of congestion control parameters per connection.


   RFC 5348: "TCP Friendly Rate Control (TFRC): Protocol Specification"
      (September 2008)

      [RFC5348], which obsoletes [RFC3448], specifies TCP-Friendly Rate
      Control (TFRC), a rate-based congestion control mechanism for
      unicast flows operating in a best-effort Internet environment
      where flows are competing with standard TCP traffic.  TFRC ensures
      conformance with TCP by continuously calculating the rate that a
      TCP sender would obtain under similar circumstances using a
      slightly simplified version of the TCP Reno throughput equation in
      [PFTK98].  Its sending rate is smoother than the rate of TCP,
      making it suitable for multimedia applications.  TFRC is not a
      wire protocol but rather a mechanism that could, for instance, be
      used within a UDP-based application, in a transport protocol such
      as RTP, or in the context of endpoint congestion management
      [RFC3124].


   RFC 3550: "RTP: A Transport Protocol for Real-Time Applications"
      (July 2003)

      [RFC3550] specifies the real-time transport protocol RTP along
      with its control protocol RTCP.  RTP/RTCP does not prescribe a
      specific congestion control behavior, but it is recommended that
      such a behavior be specified in each RTP profile (which is due to
      the fact that the potential for reducing the sending rate is often
      content dependent in the case of real-time streams).
      Specifically, [RFC3550] states: "For some profiles, it may be
      sufficient to include an applicability statement restricting the
      use of that profile to environments where congestion is avoided by
      engineering.  For other profiles, specific methods such as data
      rate adaptation based on RTCP feedback may be required".
      [RFC4585], which discusses RTCP feedback and adaptation
      mechanisms, points out that RTCP feedback may operate on much
      slower timescales than transport layer feedback mechanisms, and
      that additional mechanisms are therefore required to perform
      proper congestion control.  One way to make use of such additional
      mechanisms is to run RTP over DCCP.








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   RFC 4336: "Problem Statement for the Datagram Congestion Control
      Protocol (DCCP)" (March 2006)

      [RFC4336] provides the motivation leading to the design of DCCP.
      In doing so, other possibilities of implementing similar
      functionality are discussed, including unreliable extensions of
      SCTP, RTP-based congestion control, and providing congestion
      control above or below UDP.


   RFC 4340: "Datagram Congestion Control Protocol" (March 2006)

      [RFC4340] specifies DCCP, the Datagram Congestion Control
      Protocol.  This protocol provides bidirectional unicast
      connections of congestion-controlled unreliable datagrams.  It is
      suitable for applications that can benefit from control over the
      tradeoff between timeliness and reliability.  The core DCCP
      specification does not include a specific congestion control
      behavior; rather, it functions as a framework for such mechanisms,
      which can be selected via the Congestion Control Identifier
      (CCID).


   RFC 4341: "Profile for Datagram Congestion Control Protocol (DCCP)
      Congestion Control ID 2: TCP-like Congestion Control" (March 2006)

      [RFC4341] is the specification of TCP-like congestion control
      within DCCP.  This should be used by senders who would like to
      take advantage of the available bandwidth in an environment with
      rapidly changing conditions, and who are able to adapt to the
      abrupt changes in the congestion window typical of TCP's Additive
      Increase Multiplicative Decrease (AIMD) congestion control.  ECN
      is also supported within RFC 4341.


   RFC 4342: "Profile for Datagram Congestion Control Protocol (DCCP)
      Congestion Control ID 3: TCP-Friendly Rate Control (TFRC)" (March
      2006)

      [RFC4342] is the specification of TFRC congestion control as
      described in [RFC3448] for DCCP.  This should be used by senders
      who want a TCP-friendly sending rate, possibly with Explicit
      Congestion Notification (ECN), while minimizing abrupt rate
      changes.







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7.  Multicast Congestion Control

   In the IETF, congestion control for multicast (one-to-many)
   communication has primarily been tackled in the Reliable Multicast
   Transport (RMT) Working Group.  Except for [RFC2357] and [RFC3208],
   all the documents in this section were written by this group.  Since
   a "one size fits all" protocol cannot meet the requirements of all
   possible applications in this space, the approach taken is a modular
   one, consisting of "protocol cores" and "building blocks".  Multiple
   congestion control building blocks have been defined, providing both
   sender-driven and receiver-driven congestion control methods that
   differ widely in their assumptions and behavior.

   RFC 2357: "IETF Criteria for Evaluating Reliable Multicast Transport
      and Application Protocols" (June 1998)

      Some early multicast content dissemination proposals did not
      incorporate proper congestion control; this is pointed out as
      being a severe mistake in [RFC2357], as large-scale multicast
      applications have the potential to do vast congestion-related
      damage.  This document clearly makes the case that congestion
      control mechanisms should be developed and incorporated into
      multicast content dissemination protocols intended for use over
      the Internet.


   RFC 2887: "The Reliable Multicast Design Space for Bulk Data
      Transfer" (August 2000)

      Several classes of potential congestion control schemes for
      single-sender multicast protocols are briefly sketched as
      possibilities, but no specific protocols are developed or selected
      in [RFC2887].


   RFC 3048: "Reliable Multicast Transport Building Blocks for One-to-
      Many Bulk-Data Transfer" (January 2001)

      [RFC3048] discusses the building block approach to RMT protocols
      and mentions that several different congestion control building
      blocks may be required in order to deal with different situations.
      Some of the possible interactions between building blocks for
      congestion control and those for Forward Error Correction (FEC),
      acknowledgement, and group management are also mentioned.







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   RFC 3208: "PGM Reliable Transport Protocol Specification" (December
      2001)

      Pragmatic General Multicast (PGM) is a reliable multicast
      transport protocol for applications that require ordered or
      unordered, duplicate-free, multicast data delivery from multiple
      sources to multiple receivers.  As discussed in [RFC3208]'s
      Appendix B, a PGM protocol source can request congestion control
      feedback from both network elements (routers) and receivers (end
      hosts).  These reports can indicate the load on the worst link in
      a particular path, or the load on the worst path.  The actual
      procedure used in response to this feedback is not part of RFC
      3208, but the notion of using multicast routers to assist in
      congestion control is significant.


   RFC 3450: "Asynchronous Layered Coding (ALC) Protocol Instantiation"
      (December 2002)

      [RFC3450] specifies ALC, a rough header format using the RMT
      building blocks, that can be used by multicast content
      dissemination protocols.  ALC is intended to use a multi-rate
      congestion control building block, where the sender does not
      require any feedback, but where multiple multicast groups with
      different transmission rates are available within and ALC session,
      and receivers control their rates by joining or leaving groups.


   RFC 3738: "Wave and Equation Based Rate Control (WEBRC) Building
      Block" (April 2004)

      The WEBRC mechanism defined in [RFC3738] is a receiver-driven form
      of congestion control, where each receiver in a multicast group
      can determine the individual rate at which packets are delivered
      to it.  WEBRC senders create a base channel for control
      information and several multicast channels for data transmission
      that each send packets at a varying rate in the form of a wave.
      The receivers dynamically join and leave channels at chosen points
      within the wave of sending rates to obtain the desired overall
      receive rate based on an equation using the estimated loss
      probability and round-trip time within an epoch.  WEBRC is
      compatible for use within ALC.









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   RFC 4654: "TCP-Friendly Multicast Congestion Control (TFMCC):
      Protocol Specification" (August 2006)

      TFMCC, as described in [RFC4654], is a sender-driven congestion
      control mechanism, where the received rate for the entire
      multicast group is determined by the worst-connected receiver.
      TFMCC builds upon TFRC, but scales down the feedback to prevent
      ACK-implosion effects by having receivers suppress their feedback
      unless they perceive it to be the worst among the reception group.

8.  Guidance for Developing and Analyzing Congestion Control Techniques

   Some recently published RFCs discuss the properties of congestion
   control protocols that are "safe" for Internet deployment, as well as
   how to measure the properties of congestion control mechanisms and
   transport protocols.  These documents are particularly relevant to
   the ICCRG as some of the group's activities involve reviewing
   congestion control proposals that have been brought to the IETF for
   publication (see
   http://www.ietf.org/iesg/statement/congestion-control.html).

   RFC 5033 (BCP 133): "Specifying New Congestion Control Algorithms"
      (August 2007)

      The concurrent development of multiple TCP modifications for high-
      rate use and the deployments of these modifications on the
      Internet prompted [RFC5033] to be written.  RFC 5033 comes from
      the Transport Area Working Group (TSVWG), and gives guidance on
      the classes of Experimental RFC that can be published to document
      algorithms that are either encouraged for investigation on the
      Internet, and those that are only encouraged for experimentation
      in less-critical environments.  It has been described as a list of
      things for people to think about when creating new congestion
      control techniques that they are planning to widely deploy.


   RFC 5166: "Metrics for the Evaluation of Congestion Control
      Mechanisms" (March 2008)

      The IRTF Transport Modeling Research Group (TMRG) produced
      [RFC5166] to describe the set of metrics and related tradeoffs
      between metrics that can be used to compare, contrast, and
      evaluate congestion control techniques.  This RFC gives an
      overview of many such metrics, and gives references to their
      detailed descriptions.






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9.  Historic Interest

   Early in the RFC series, there are many documents that represent an
   author's thoughts on a subject or brief summaries from measurement
   and experimentation, rather than the result of a long formal IETF
   process.  Some of the RFCs listed in this section have this
   distinction.

   RFC 889: "Internet Delay Experiments" (December 1983)

      Based on reported measurement experiments, changes to the TCP
      retransmission timeout (RTO) calculation are suggested in
      [RFC0889].  It is noted that the original TCP RTO calculation
      leads to congestion when a delay spike occurs because it takes too
      long for the RTO to adapt, leading to superfluous retransmissions.


   RFC 896: "Congestion Control in IP/TCP Internetworks" (January 1984)

      [RFC0896] is the first document known to the authors where the
      term "congestion collapse" was used.  Here, it refers to the
      stable state that was observed when a sudden load on the net
      caused the round-trip time to rise faster than the sending hosts
      measured round-trip time could be updated.  Two problems are
      discussed: the "small-packet problem" (now commonly known by the
      name "silly window syndrome") and the "source-quench problem",
      which is about inappropriately deciding when to send and how to
      react to ICMP Source Quench messages.  Solutions for these
      problems are presented.


   RFC 970: "On Packet Switches with Infinite Storage" (December 1985)

      Using a thought experiment based on a router with infinite
      buffering capacity, [RFC0970] develops a different kind of
      congestion collapse scenario, where few useful packet
      transmissions occur due to the queue being longer than the time-
      to-live of the packets within it.  As described in RFC 970, this
      scenario was also demonstrated using real equipment by the author.

      The document also includes discussion of game-theoretic analysis
      of congestion control and obtaining fairness between behaving and
      non-behaving flows, by focusing on the order of scheduling packets
      within the buffer rather than the actual allocation of buffer
      space between flows.






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   RFC 1016: "Something a Host Could Do with Source Quench: The Source
      Quench Introduced Delay (SQuID)" (July 1987)

      [RFC1016] outlines a rate-based congestion control mechanism where
      end-hosts use Source Quench packets from routers to adjust their
      sending rates.  RFC 1016 also suggests sending congestion
      notifications before queues are actually full, at a rate that
      increases with the current queue occupancy.  This strategy has
      been used in several other AQM mechanisms, notably RED [FJ93].


   RFC 1254: "Gateway Congestion Control Survey" (August 1991)

      [RFC1254] is a survey of congestion control approaches in routers
      that first discusses general congestion control performance goals
      (such as fairness), and then elaborates on the use of Source
      Quench messages (which are now discouraged, as they have been
      found ineffective), Random Drop (which would now be called "Active
      Queue Management"), Congestion Indication (DEC Bit; an early form
      of ECN), "Selective Feedback Congestion Indication" (one
      particular method for applying ECN), and Fair Queuing.  Finally,
      end-system congestion control policies are discussed, including
      Jacobson's well-known algorithms [Jac88] and their predecessor --
      "CUTE" [Jain86].


10.  Security Considerations

   This document introduces no new security considerations.  Each RFC
   listed in this document discusses the security considerations of the
   specification it contains.

11.  Acknowledgements

   Several participants in the ICCRG contributed useful comments in the
   development of this document, including Rex Buddenberg, Mitchell
   Erblichs, Lachlan Andrew, Sally Floyd, Stephen Farrell, Gorry
   Fairhurst, Lars Eggert, Mark Allman, and Juergen Schoenwaelder.

12.  Informative References

   [FJ93]     Floyd, S. and V. Jacobson, "Random Early Detection
              Gateways for Congestion Avoidance", IEEE/ACM Transactions
              on Networking, volume 1, number 4, August 1993.

   [Gont10]   Gont, F., "ICMP attacks against TCP", Work in Progress,
              January 2010.




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   [Jac88]    Jacobson, V., "Congestion Avoidance and Control",
              Proceedings of ACM SIGCOMM 1988, in ACM Computer
              Communication Review, 18 (4), pp. 314-329.

   [Jain86]   Jain, R., "A Timeout-Based Congestion Control Scheme for
              Window Flow-Controlled Networks", IEEE Journal on Selected
              Areas in Communications, volume 4, number 7, October 1986.

   [MAF04]    Medina, A., Allman, M., and S. Floyd, "Measuring
              Interactions Between Transport Protocols and Middleboxes",
              Proceedings of the Internet Measurement Conference 2004,
              August 2004.

   [MAF05]    Medina, A., Allman, M., and S. Floyd, "Measuring the
              Evolution of Transport Protocols in the Internet", ACM
              Computer Communications Review, volume 35, issue 2,
              April 2005.

   [MBJ07]    Moncaster, T., Briscoe, B., and A. Jacquet, "A TCP Test to
              Allow Senders to Identify Receiver Non-Compliance", Work
              in Progress, November 2007.

   [PF01]     Padhye, J. and S. Floyd, "On Inferring TCP Behavior",
              Proceedings of ACM SIGCOMM 2001, August 2001.

   [PFTK98]   Padhye, J., Firoiu, V., Towsley, D., and J. Kurose,
              "Modeling TCP Throughput: A Simple Model and its Empirical
              Validation", Proceedings of ACM SIGCOMM 1998.

   [RFC0889]  Mills, D., "Internet delay experiments", RFC 889,
              December 1983.

   [RFC0896]  Nagle, J., "Congestion control in IP/TCP internetworks",
              RFC 896, January 1984.

   [RFC0970]  Nagle, J., "On packet switches with infinite storage",
              RFC 970, December 1985.

   [RFC1016]  Prue, W. and J. Postel, "Something a host could do with
              source quench: The Source Quench Introduced Delay
              (SQuID)", RFC 1016, July 1987.

   [RFC1122]  Braden, R., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC1254]  Mankin, A. and K. Ramakrishnan, "Gateway Congestion
              Control Survey", RFC 1254, August 1991.




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   [RFC1633]  Braden, B., Clark, D., and S. Shenker, "Integrated
              Services in the Internet Architecture: an Overview",
              RFC 1633, June 1994.

   [RFC1812]  Baker, F., "Requirements for IP Version 4 Routers",
              RFC 1812, June 1995.

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

   [RFC2001]  Stevens, W., "TCP Slow Start, Congestion Avoidance, Fast
              Retransmit, and Fast Recovery Algorithms", RFC 2001,
              January 1997.

   [RFC2140]  Touch, J., "TCP Control Block Interdependence", RFC 2140,
              April 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.

   [RFC2357]  Mankin, A., Romanov, A., Bradner, S., and V. Paxson, "IETF
              Criteria for Evaluating Reliable Multicast Transport and
              Application Protocols", RFC 2357, June 1998.

   [RFC2414]  Allman, M., Floyd, S., and C. Partridge, "Increasing TCP's
              Initial Window", RFC 2414, September 1998.

   [RFC2415]  Poduri, K., "Simulation Studies of Increased Initial TCP
              Window Size", RFC 2415, September 1998.

   [RFC2416]  Shepard, T. and C. Partridge, "When TCP Starts Up With
              Four Packets Into Only Three Buffers", RFC 2416,
              September 1998.

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

   [RFC2481]  Ramakrishnan, K. and S. Floyd, "A Proposal to add Explicit
              Congestion Notification (ECN) to IP", RFC 2481,
              January 1999.






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   [RFC2488]  Allman, M., Glover, D., and L. Sanchez, "Enhancing TCP
              Over Satellite Channels using Standard Mechanisms",
              BCP 28, RFC 2488, January 1999.

   [RFC2581]  Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
              Control", RFC 2581, April 1999.

   [RFC2884]  Hadi Salim, J. and U. Ahmed, "Performance Evaluation of
              Explicit Congestion Notification (ECN) in IP Networks",
              RFC 2884, July 2000.

   [RFC2887]  Handley, M., Floyd, S., Whetten, B., Kermode, R.,
              Vicisano, L., and M. Luby, "The Reliable Multicast Design
              Space for Bulk Data Transfer", RFC 2887, August 2000.

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

   [RFC2998]  Bernet, Y., Ford, P., Yavatkar, R., Baker, F., Zhang, L.,
              Speer, M., Braden, R., Davie, B., Wroclawski, J., and E.
              Felstaine, "A Framework for Integrated Services Operation
              over Diffserv Networks", RFC 2998, November 2000.

   [RFC3048]  Whetten, B., Vicisano, L., Kermode, R., Handley, M.,
              Floyd, S., and M. Luby, "Reliable Multicast Transport
              Building Blocks for One-to-Many Bulk-Data Transfer",
              RFC 3048, January 2001.

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

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

   [RFC3150]  Dawkins, S., Montenegro, G., Kojo, M., and V. Magret,
              "End-to-end Performance Implications of Slow Links",
              BCP 48, RFC 3150, July 2001.

   [RFC3155]  Dawkins, S., Montenegro, G., Kojo, M., Magret, V., and N.
              Vaidya, "End-to-end Performance Implications of Links with
              Errors", BCP 50, RFC 3155, August 2001.

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





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   [RFC3208]  Speakman, T., Crowcroft, J., Gemmell, J., Farinacci, D.,
              Lin, S., Leshchiner, D., Luby, M., Montgomery, T., Rizzo,
              L., Tweedly, A., Bhaskar, N., Edmonstone, R.,
              Sumanasekera, R., and L. Vicisano, "PGM Reliable Transport
              Protocol Specification", RFC 3208, December 2001.

   [RFC3366]  Fairhurst, G. and L. Wood, "Advice to link designers on
              link Automatic Repeat reQuest (ARQ)", BCP 62, RFC 3366,
              August 2002.

   [RFC3426]  Floyd, S., "General Architectural and Policy
              Considerations", RFC 3426, November 2002.

   [RFC3439]  Bush, R. and D. Meyer, "Some Internet Architectural
              Guidelines and Philosophy", RFC 3439, December 2002.

   [RFC3448]  Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP
              Friendly Rate Control (TFRC): Protocol Specification",
              RFC 3448, January 2003.

   [RFC3449]  Balakrishnan, H., Padmanabhan, V., Fairhurst, G., and M.
              Sooriyabandara, "TCP Performance Implications of Network
              Path Asymmetry", BCP 69, RFC 3449, December 2002.

   [RFC3450]  Luby, M., Gemmell, J., Vicisano, L., Rizzo, L., and J.
              Crowcroft, "Asynchronous Layered Coding (ALC) Protocol
              Instantiation", RFC 3450, December 2002.

   [RFC3481]  Inamura, H., Montenegro, G., Ludwig, R., Gurtov, A., and
              F. Khafizov, "TCP over Second (2.5G) and Third (3G)
              Generation Wireless Networks", BCP 71, RFC 3481,
              February 2003.

   [RFC3540]  Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
              Congestion Notification (ECN) Signaling with Nonces",
              RFC 3540, June 2003.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC3649]  Floyd, S., "HighSpeed TCP for Large Congestion Windows",
              RFC 3649, December 2003.

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




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   [RFC3738]  Luby, M. and V. Goyal, "Wave and Equation Based Rate
              Control (WEBRC) Building Block", RFC 3738, April 2004.

   [RFC3742]  Floyd, S., "Limited Slow-Start for TCP with Large
              Congestion Windows", RFC 3742, March 2004.

   [RFC3819]  Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
              Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
              Wood, "Advice for Internet Subnetwork Designers", BCP 89,
              RFC 3819, July 2004.

   [RFC4336]  Floyd, S., Handley, M., and E. Kohler, "Problem Statement
              for the Datagram Congestion Control Protocol (DCCP)",
              RFC 4336, March 2006.

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340, March 2006.

   [RFC4341]  Floyd, S. and E. Kohler, "Profile for Datagram Congestion
              Control Protocol (DCCP) Congestion Control ID 2: TCP-like
              Congestion Control", RFC 4341, March 2006.

   [RFC4342]  Floyd, S., Kohler, E., and J. Padhye, "Profile for
              Datagram Congestion Control Protocol (DCCP) Congestion
              Control ID 3: TCP-Friendly Rate Control (TFRC)", RFC 4342,
              March 2006.

   [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
              "Extended RTP Profile for Real-time Transport Control
              Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
              July 2006.

   [RFC4614]  Duke, M., Braden, R., Eddy, W., and E. Blanton, "A Roadmap
              for Transmission Control Protocol (TCP) Specification
              Documents", RFC 4614, September 2006.

   [RFC4654]  Widmer, J. and M. Handley, "TCP-Friendly Multicast
              Congestion Control (TFMCC): Protocol Specification",
              RFC 4654, August 2006.

   [RFC4782]  Floyd, S., Allman, M., Jain, A., and P. Sarolahti, "Quick-
              Start for TCP and IP", RFC 4782, January 2007.

   [RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, September 2007.

   [RFC5033]  Floyd, S. and M. Allman, "Specifying New Congestion
              Control Algorithms", BCP 133, RFC 5033, August 2007.



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

   [RFC5348]  Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
              Friendly Rate Control (TFRC): Protocol Specification",
              RFC 5348, September 2008.

Authors' Addresses

   Michael Welzl
   University of Oslo
   Department of Informatics
   PO Box 1080 Blindern
   N-0316 Oslo, Norway

   Phone: +47 22 85 24 20
   EMail: michawe@ifi.uio.no


   Wesley M. Eddy
   MTI Systems
   NASA Glenn Research Center
   21000 Brookpark Rd, MS 500-ASRC
   Cleveland, OH  44135

   Phone: (216) 433-6682
   EMail: wes@mti-systems.com
























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