RFC5783

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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8. Guidance for Developing and Analyzing Congestion Control

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

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

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.

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

  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

  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

  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.

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.

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

  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

  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.

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.

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.

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]).

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.

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

  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.

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.

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.

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.

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.

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.

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.

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.

[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.

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.

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.

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.

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.

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: [email protected]

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

Phone: (216) 433-6682 EMail: [email protected]