A Roadmap for Transmission Control Protocol (TCP) Specification Documents

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Document Type Active Internet-Draft (tcpm WG)
Authors Martin Duke  , Robert Braden  , Wesley Eddy  , Ethan Blanton  , Alexander Zimmermann 
Last updated 2013-11-21
Replaces draft-zimmermann-tcpm-tcp-rfc4614bis
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TCP Maintenance and Minor Extensions                             M. Duke
(TCPM) WG                                                             F5
Internet-Draft                                                 R. Braden
Obsoletes: 4614 (if approved)                                        ISI
Intended status: Informational                                   W. Eddy
Expires: May 25, 2014                                        MTI Systems
                                                              E. Blanton

                                                           A. Zimmermann
                                                            NetApp, Inc.
                                                       November 21, 2013

    A Roadmap for Transmission Control Protocol (TCP) Specification


   This document contains a "roadmap" to the Requests for Comments (RFC)
   documents relating to the Internet's Transmission Control Protocol
   (TCP).  This roadmap provides a brief summary of the documents
   defining TCP and various TCP extensions that have accumulated in the
   RFC series.  This serves as a guide and quick reference for both TCP
   implementers and other parties who desire information contained in
   the TCP-related RFCs.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 25, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Core Functionality . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Recommended Enhancements . . . . . . . . . . . . . . . . . . .  8
     3.1.  Fundamental Changes  . . . . . . . . . . . . . . . . . . .  8
     3.2.  Congestion Control Extensions  . . . . . . . . . . . . . .  9
     3.3.  Loss Recovery Extensions . . . . . . . . . . . . . . . . . 10
     3.4.  Detection and Prevention of Spurious Retransmissions . . . 12
     3.5.  TCP Timeouts . . . . . . . . . . . . . . . . . . . . . . . 13
     3.6.  Path MTU Discovery . . . . . . . . . . . . . . . . . . . . 13
     3.7.  Header Compression . . . . . . . . . . . . . . . . . . . . 14
     3.8.  Defending Spoofing and Flooding Attacks  . . . . . . . . . 15
   4.  Experimental Extensions  . . . . . . . . . . . . . . . . . . . 17
     4.1.  Architectural Guidelines . . . . . . . . . . . . . . . . . 17
     4.2.  Congestion Control Extensions  . . . . . . . . . . . . . . 18
     4.3.  Loss Recovery Extensions . . . . . . . . . . . . . . . . . 19
     4.4.  Detection and Prevention of Spurious Retransmissions . . . 20
     4.5.  Multipath TCP  . . . . . . . . . . . . . . . . . . . . . . 21
   5.  TCP Parameters at IANA . . . . . . . . . . . . . . . . . . . . 21
   6.  Historic and Undeployed Extensions . . . . . . . . . . . . . . 22
   7.  Support Documents  . . . . . . . . . . . . . . . . . . . . . . 25
     7.1.  Foundational Works . . . . . . . . . . . . . . . . . . . . 25
     7.2.  Architectural Guidelines . . . . . . . . . . . . . . . . . 27
     7.3.  Difficult Network Environments . . . . . . . . . . . . . . 28
     7.4.  Guidance for Developing, Analyzing, and Evaluating TCP . . 30
     7.5.  Implementation Advice  . . . . . . . . . . . . . . . . . . 31
     7.6.  Tools and Tutorials  . . . . . . . . . . . . . . . . . . . 34
     7.7.  Management Information Bases . . . . . . . . . . . . . . . 34
     7.8.  Case Studies . . . . . . . . . . . . . . . . . . . . . . . 36
   8.  Undocumented TCP Features  . . . . . . . . . . . . . . . . . . 37
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 38
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 38
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 39
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 39
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 39
     12.2. Informative References . . . . . . . . . . . . . . . . . . 48

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   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 50

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1.  Introduction

   A correct and efficient implementation of the Transmission Control
   Protocol (TCP) is a critical part of the software of most Internet
   hosts.  As TCP has evolved over the years, many distinct documents
   have become part of the accepted standard for TCP.  At the same time,
   a large number of experimental modifications to TCP have also been
   published in the RFC series, along with informational notes, case
   studies, and other advice.

   As an introduction to newcomers and an attempt to organize the
   plethora of information for old hands, this document contains a
   "roadmap" to the TCP-related RFCs.  It provides a brief summary of
   the RFC documents that define TCP.  This should provide guidance to
   implementers on the relevance and significance of the standards-track
   extensions, informational notes, and best current practices that
   relate to TCP.

   This document is not an update of RFC 1122 and is not a rigorous
   standard for what needs to be implemented in TCP.  This document is
   merely an informational roadmap that captures, organizes, and
   summarizes most of the RFC documents that a TCP implementer,
   experimenter, or student should be aware of.  Particular comments or
   broad categorizations that this document makes about individual
   mechanisms and behaviors are not to be taken as definitive, nor
   should the content of this document alone influence implementation

   This roadmap includes a brief description of the contents of each
   TCP-related RFC.  In some cases, we simply supply the abstract or a
   key summary sentence from the text as a terse description.  In
   addition, a letter code after an RFC number indicates its category in
   the RFC series (see BCP 9 [RFC2026] for explanation of these

      S - Standards Track (Proposed Standard, Draft Standard, or
      Internet Standard)

      E - Experimental

      I - Informational

      H - Historic

      B - Best Current Practice

      U - Unknown (not formally defined)

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   Note that the category of an RFC does not necessarily reflect its
   current relevance.  For instance, RFC 5681 is nearly universally
   deployed although it is only a Draft Standard.  Similarly, some
   Informational RFCs contain significant technical proposals for
   changing TCP.

   Finally, if an error in the technical content has been found after
   publication of an RFC, this fact is indicated by the term "(Errata)"
   in the headline of the RFC's description.  The contents of the errata
   can be found at the RFC editor home page [Errata].

   This roadmap is divided into three main sections.  Section 2 lists
   the RFCs that describe absolutely required TCP behaviors for proper
   functioning and interoperability.  Further RFCs that describe
   strongly encouraged, but non-essential, behaviors are listed in
   Section 3.  Experimental extensions that are not yet standard
   practices, but that potentially could be in the future, are described
   in Section 4.

   The reader will probably notice that these three sections are broadly
   equivalent to MUST/SHOULD/MAY specifications (per RFC 2119), and
   although the authors support this intuition, this document is merely
   descriptive; it does not represent a binding standards-track
   position.  Individual implementers still need to examine the
   standards documents themselves to evaluate specific requirement

   Section 5 describes both the procedures that the Internet Assigned
   Numbers Authority (IANA) uses and an RFC author should follow when
   new TCP parameters are requested and finally assigned.

   A small number of older experimental extensions that have not been
   widely implemented, deployed, and used are noted in Section 6.  Many
   other supporting documents that are relevant to the development,
   implementation, and deployment of TCP are described in Section 7.

   A small number of fairly ubiquitous important implementation
   practices that are not currently documented in the RFC series is
   listed in Section 8.

   Within each section, RFCs are listed in the chronological order of
   their publication dates.

2.  Core Functionality

   A small number of documents compose the core specification of TCP.
   These define the required core functionalities of TCP's header

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   parsing, state machine, congestion control, and retransmission
   timeout computation.  These base specifications must be correctly
   followed for interoperability.

   RFC 793 S: "Transmission Control Protocol", STD 7 (September 1981)

      This is the fundamental TCP specification document [RFC0793].
      Written by Jon Postel as part of the Internet protocol suite's
      core, it describes the TCP packet format, the TCP state machine
      and event processing, and TCP's semantics for data transmission,
      reliability, flow control, multiplexing, and acknowledgment.

      Section 3.6 of RFC 793, describing TCP's handling of the IP
      precedence and security compartment, is mostly irrelevant today.
      RFC 2873 changed the IP precedence handling, and the security
      compartment portion of the API is no longer implemented or used.
      In addition, RFC 793 did not describe any congestion control
      mechanism.  Otherwise, however, the majority of this document
      still accurately describes modern TCPs.  RFC 793 is the last of a
      series of developmental TCP specifications, starting in the
      Internet Experimental Notes (IENs) and continuing in the RFC

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

      This document [RFC1122] updates and clarifies RFC 793, fixing some
      specification bugs and oversights.  It also explains some features
      such as keep-alives and Karn's and Jacobson's RTO estimation
      algorithms [KP87][Jac88][JK92].  ICMP interactions are mentioned,
      and some tips are given for efficient implementation.  RFC 1122 is
      an Applicability Statement, listing the various features that
      MUST, SHOULD, MAY, SHOULD NOT, and MUST NOT be present in
      standards-conforming TCP implementations.  Unlike a purely
      informational "roadmap", this Applicability Statement is a
      standards document and gives formal rules for implementation.

   RFC 2460 S: "Internet Protocol, Version 6 (IPv6) Specification"
   (December 1998) (Errata)

      This document [RFC2460] is of relevance to TCP because it defines
      how the pseudo-header for TCP's checksum computation is derived
      when 128-bit IPv6 addresses are used instead of 32-bit IPv4
      addresses.  Additionally, RFC 2675 describes TCP changes required
      to support IPv6 jumbograms.

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   RFC 2873 S: "TCP Processing of the IPv4 Precedence Field" (June 2000)

      This document [RFC2873] removes from the TCP specification all
      processing of the precedence bits of the TOS byte of the IP
      header.  This resolves a conflict over the use of these bits
      between RFC 793 and Differentiated Services [RFC2474].

   RFC 5681 S: "TCP Congestion Control" (August 2009)

      Although RFC 793 did not contain any congestion control
      mechanisms, today congestion control is a required component of
      TCP implementations.  This document [RFC5681] defines the current
      versions of Van Jacobson's congestion avoidance and control
      mechanisms for TCP, based on his 1988 SIGCOMM paper [Jac88].

      A number of behaviors that together constitute what the community
      refers to as "Reno TCP" are described in RFC 5681.  The name
      "Reno" comes from the Net/2 release of the 4.3 BSD operating
      system.  This is generally regarded as the least common
      denominator among TCP flavors currently found running on Internet
      hosts.  Reno TCP includes the congestion control features of slow
      start, congestion avoidance, fast retransmit, and fast recovery.

      RFC 1122 [RFC1122] mandates the implementation of a congestion
      control mechanism, and RFC 5681 [RFC5681] details the currently
      accepted mechanism.  RFC 5681 differs slightly from the other
      documents listed in this section, as it does not affect the
      ability of two TCP endpoints to communicate; however, congestion
      control remains a critical component of any widely deployed TCP
      implementation and is required for the avoidance of congestion
      collapse and to ensure fairness among competing flows.

      RFC 2001 and RFC 2581 are the conceptual precursors of RFC 5681.
      The most important changes relative to RFC 2581 are:
      (a)  The initial window requirements were changed to allow larger
           Initial Windows as standardized in [RFC3390].
      (b)  During slow start and congestion avoidance, the usage of
           Appropriate Byte Counting [RFC3465] is explicitly
      (c)  The use of Limited Transmit [RFC3042] is now recommended.

   RFC 6093 S: "On the Implementation of the TCP Urgent Mechanism"
   (January 2011)

      This document [RFC6093] analyzes how current TCP stacks process
      TCP urgent indications, and how the behavior of widely deployed
      middleboxes affects the urgent indications processing.  The

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      document updates the relevant specifications such that it
      accommodates current practice in processing TCP urgent
      indications.  Finally, the document raises awareness about the
      reliability of TCP urgent indications in the Internet, and
      recommends against the use of urgent mechanism.

   RFC 6298 S: "Computing TCP's Retransmission Timer" (June 2011)

      Abstract: "This document defines the standard algorithm that
      Transmission Control Protocol (TCP) senders are required to use to
      compute and manage their retransmission timer.  It expands on the
      discussion in section of RFC 1122 and upgrades the
      requirement of supporting the algorithm from a SHOULD to a MUST."
      [RFC6298].  RFC 6298 updates RFC 2988 by changing the initial RTO
      from 3s to 1s

   RFC 6691 I: "TCP Options and Maximum Segment Size (MSS)" (July 2012)

      This document [RFC6691] clarifies what value to use with the TCP
      Maximum Segment Size (MSS) option when IP and TCP options are in

3.  Recommended Enhancements

   This section describes recommended TCP modifications that improve
   performance and security.  Section 3.1 represents fundamental changes
   to the protocol.  Section 3.2 and Section 3.3 list improvements over
   the congestion control and loss recovery mechanisms as specified in
   RFC 5681.  Section 3.4 describes algorithms that allow a TCP sender
   to detect whether it has entered loss recovery spuriously.
   Section 3.5 lists documents that revolve around the various TCP
   timers.  Section 3.6 comprises Path MTU Discovery mechanisms.
   Schemes for TCP/IP header compression are listed in Section 3.7.
   Finally, Section 3.8 deals with the problem of preventing preventing
   acceptance of forged segments and flooding attacks.

3.1.  Fundamental Changes

   RFC 1323 allows better utilization of high bandwidth-delay product
   paths by providing some needed mechanisms for high-rate transfers.
   RFC 2675 describes changes to TCP's semantic for using IPv6

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   RFC 1323 S: "TCP Extensions for High Performance" (May 1992)

      This document [RFC1323] defines TCP extensions for window scaling,
      timestamps, and protection against wrapped sequence numbers, for
      efficient and safe operation over paths with large bandwidth-delay
      products.  These extensions are commonly found in currently used
      systems; however, they may require manual tuning and
      configuration.  One issue in this specification that is still
      under discussion concerns a modification to the algorithm for
      estimating the mean RTT when timestamps are used.  RFC 1072 and
      RFC 1185 are the conceptual precursors of RFC 1323.

   RFC 2675 S: "IPv6 Jumbograms" (August 1999) (Errata)

      IPv6 supports longer datagrams than were allowed in IPv4.  These
      are known as Jumbograms, and use with TCP has necessitated changes
      to the handling of TCP's MSS and Urgent fields (both 16 bits).
      This document [RFC2675] explains those changes.  Although it
      describes changes to basic header semantics, these changes should
      only affect the use of very large segments, such as IPv6
      jumbograms, which are currently rarely used in the general

      Supporting the behavior described in this document does not affect
      interoperability with other TCP implementations when IPv4 or non-
      jumbogram IPv6 is used.  This document states that jumbograms are
      to only be used when it can be guaranteed that all receiving
      nodes, including each router in the end-to-end path, will support
      jumbograms.  If even a single node that does not support
      jumbograms is attached to a local network, then no host on that
      network may use jumbograms.  This explains why jumbogram use has
      been rare, and why this document is considered a performance
      optimization and not part of TCP over IPv6's basic functionality.

3.2.  Congestion Control Extensions

   Two of the most important aspects of TCP are its congestion control
   and loss recovery features.  TCP traditionally treats lost packets as
   indicating congestion-related loss, and cannot distinguish between
   congestion-related loss and loss due to transmission errors.  Even
   when ECN is in use, there is a rather intimate coupling between
   congestion control and loss recovery mechanisms.  There are several
   extensions to both features, and more often than not, a particular
   extension applies to both.  In this two sub-sections, we group
   enhancements to TCP's congestion control, while the next sub-section
   focus on TCP's loss recovery.

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

      This document [RFC3168] defines a means for end hosts to detect
      congestion before congested routers are forced to discard packets.
      Although congestion notification takes place at the IP level, ECN
      requires support at the transport level (e.g., in TCP) to echo the
      bits and adapt the sending rate.  This document updates RFC 793 to
      define two previously unused flag bits in the TCP header for ECN
      support.  RFC 3540 provides a supplementary (experimental) means
      for more secure use of ECN, and RFC 2884 provides some sample
      results from using ECN.

   RFC 3465 E: "TCP Congestion Control with Appropriate Byte Counting
   (ABC)" (February 2003)

      This document [RFC3465] suggests that congestion control use the
      number of bytes acknowledged instead of the number of
      acknowledgments received.  The ABC mechanism behaves differently
      than the standard method when there is not a one-to-one
      relationship between data segments and acknowledgments.  ABC still
      operates within the accepted guidelines, but is more robust to
      delayed ACKs and ACK-division [SCWA99][RFC3449].  [RFC3465] is
      recommended by [RFC5681].

   RFC 3390 S: "Increasing TCP's Initial Window" (October 2002)

      This document [RFC3390] specifies an increase in the permitted
      initial window for TCP from one segment to three or four segments
      during the slow start phase, depending on the segment size.

   RFC 6633 S: "Deprecation of ICMP Source Quench Messages" (May 2012)

      This document [RFC6633] formally deprecates the use of ICMP Source
      Quench messages by transport protocols and provides a
      recommendation against the implementation of [RFC1016].

3.3.  Loss Recovery Extensions

   For the typical implementation of the TCP fast recovery algorithm
   described in [RFC5681], a TCP sender only retransmits a segment after
   a retransmit timeout has occurred, or after three duplicate ACKs have
   arrived triggering the fast retransmit.  A single RTO might result in
   the retransmission of several segments, while the fast retransmit
   algorithm in RFC 5681 leads only to a single retransmission.  Hence,
   multiple losses from a single window of data can lead to a
   performance degradation.  Documents listed in this section aim to
   improve the overall performance of TCP's standard loss recovery

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   algorithms.  In particular, some of them allows TCP senders to
   recover more effectively when multiple segments are lost from a
   single flight of data.

   RFC 2018 S: "TCP Selective Acknowledgment Options" (October 1996)

      When more than one packet is lost during one round trip time TCP
      may experience poor performance since a TCP sender can only learn
      about a single lost packet per round trip time from cumulative
      acknowledgments.  This document [RFC2018] defines the basic
      selective acknowledgment (SACK) mechanism for TCP, which can help
      to overcome these limitations.  The receiving TCP returns SACK
      blocks to inform the sender which data has been received.  The
      sender can then retransmit only the missing data segments.

   RFC 3042 S: "Enhancing TCP's Loss Recovery Using Limited Transmit"
   (January 2001)

      Abstract: "This document proposes Limited Transmit, a new
      Transmission Control Protocol (TCP) mechanism that can be used to
      more effectively recover lost segments when a connection's
      congestion window is small, or when a large number of segments are
      lost in a single transmission window."  [RFC3042] Tests from 2004
      showed that Limited Transmit was deployed in roughly one third of
      the web servers tested [MAF04].  [RFC3042] is recommended by

   RFC 6582 S: "The NewReno Modification to TCP's Fast Recovery
   Algorithm" (April 2012)

      This document [RFC6582] specifies a modification to the standard
      Reno fast recovery algorithm, whereby a TCP sender can use partial
      acknowledgments to make inferences determining the next segment to
      send in situations where SACK would be helpful but isn't
      available.  Although it is only a slight modification, the NewReno
      behavior can make a significant difference in performance when
      multiple segments are lost from a single window of data.

      RFC 2582 and RFC 3782 are the conceptual precursors of RFC 6582.
      The main change in RFC 3782 relative to RFC 2582 was to specify
      the Careful variant of NewReno's Fast Retransmit and Fast Recovery
      algorithms and advance those two algorithms from Experimental to
      Standards Track status.  The main change in RFC 6582 relative to
      RFC 3782 was to solve a performance degradation that could occur
      if FlightSize on Full ACK reception is zero.

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   RFC 6675 S: "A Conservative Loss Recovery Algorithm Based on
   Selective Acknowledgment (SACK) for TCP" (August 2012)

      This document [RFC6675] describes a conservative loss recovery
      algorithm for TCP that is based on the use of the selective
      acknowledgment (SACK) TCP option [RFC2018].  The algorithm
      conforms to the spirit of the congestion control specification in
      RFC 5681, but allows TCP senders to recover more effectively when
      multiple segments are lost from a single flight of data.

      RFC 6675 is a revision of RFC 3517 to address several situations
      that are not handled explicitly before.  In particular
      (a)  it improves the loss detection in the event that the sender
           has outstanding segments that are smaller than SMSS.
      (b)  it modifies the definition of a "duplicate acknowledgment" to
           utilize the SACK information in detecting loss.
      (c)  it maintains the ACK clock under certain circumstances
           involving loss at the end of the window.

3.4.  Detection and Prevention of Spurious Retransmissions

   Spurious retransmission timeouts are harmful to TCP performance and
   multiple algorithms have been defined for detecting when spurious
   retransmissions have occurred, and then responding differently in
   order to recover performance.  The IETF defined multiple algorithms
   because there are tradeoffs in whether or not certain TCP options
   need to be implemented, and concerns about IPR status.  The Standards
   Track documents in this section are closely related to the
   Experimental documents in Section 4.4 also addressing this topic.

   RFC 2883 S: "An Extension to the Selective Acknowledgement (SACK)
   Option for TCP" (July 2000)

      This document [RFC2883] extends RFC 2018.  It enables use of the
      SACK option to acknowledge duplicate packets.  With this
      extension, called DSACK, the sender is able to infer the order of
      packets received at the receiver, and therefore to infer when it
      has unnecessarily retransmitted a packet.

   RFC 4015 S: "The Eifel Response Algorithm for TCP" (February 2005)

      This document [RFC4015] describes the response portion of the
      Eifel algorithm, which can be used in conjunction with one of
      several methods of detecting when loss recovery has been
      spuriously entered, such as the Eifel detection algorithm in RFC
      3522, the algorithm in RFC 3708, or F-RTO in RFC 5682.

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      Abstract: "Based on an appropriate detection algorithm, the Eifel
      response algorithm provides a way for a TCP sender to respond to a
      detected spurious timeout.  It adapts the retransmission timer to
      avoid further spurious timeouts, and can avoid - depending on the
      detection algorithm - the often unnecessary go-back-N retransmits
      that would otherwise be sent.  In addition, the Eifel response
      algorithm restores the congestion control state in such a way that
      packet bursts are avoided."

   RFC 5682 S: "Forward RTO-Recovery (F-RTO): An Algorithm for Detecting
   Spurious Retransmission Timeouts with TCP" (September 2009)

      The F-RTO detection algorithm [RFC5682], originally described in
      RFC 4138, provides an option for inferring spurious retransmission
      timeouts.  Unlike some similar detection methods (e.g.  RFC 3522
      and RFC 3708), F-RTO does not rely on the use of any TCP options.
      The basic idea is to send previously unsent data after the first
      retransmission after a RTO.  If the ACKs advance the window, the
      RTO may be declared spurious.

3.5.  TCP Timeouts


   RFC 5482 S: "TCP User Timeout Option" (June 2009)

      As a local per-connection parameter the TCP user timeout controls
      how long transmitted data may remain unacknowledged before a
      connection is forcefully closed.  This document [RFC5482]
      specifies the TCP User Timeout Option that allows one end of a TCP
      connection to advertise its current user timeout value.  This
      information provides advice to the other end of the TCP connection
      to adapt its user timeout accordingly.

3.6.  Path MTU Discovery

   The MTUs supported by different links and tunnels within the Internet
   can vary widely.  Fragmentation of packets larger than the supported
   MTU on a hop is undesirable.  As TCP is the segmentation layer for
   dividing an application's bytestream into IP packet payloads, TCP
   implementations generally include Path MTU Discovery (PMTUD)
   mechanisms in order to maximize the size of segments they send,
   without causing fragmentation within the network.  Some algorithms
   may utilize signaling from routers on the path that the MTU has been

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   RFC 1191 S: "Path MTU Discovery" (November 1990)

      Abstract: "This memo describes a technique for dynamically
      discovering the MTU of an arbitrary Internet path.  It specifies a
      small change to the way routers generate one type of ICMP message.
      For a path that passes through a router that has not been so
      changed, this technique might not discover the correct path MTU,
      but it will always choose a path MTU as accurate as, and in many
      cases more accurate than, the path MTU that would be chosen by
      current practice."  [RFC1191]

   RFC 1981 S: "Path MTU Discovery for IP version 6" (August 1996)

      Abstract: "This document describes Path MTU Discovery for IP
      version 6.  It is largely derived from RFC 1191, which describes
      Path MTU Discovery for IP version 4."  [RFC1981]

   RFC 4821 S: "Packetization Layer Path MTU Discovery" (March 2007)

      Abstract: "This document describes a robust method for Path MTU
      Discovery (PMTUD) that relies on TCP or some other Packetization
      Layer to probe an Internet path with progressively larger packets.
      This method is described as an extension to RFC 1191 and RFC 1981,
      which specify ICMP-based Path MTU Discovery for IP versions 4 and
      6, respectively."  [RFC4821]

3.7.  Header Compression

   Especially in streaming applications, the overhead of TCP/IP headers
   could correspond to more then 50% of the total amount of data sent.
   Such large overheads may be tolerable in wired LANs where capacity is
   often not an issue, but are excessive for WANs and wireless systems
   where bandwidth is scarce.  Header compression schemes for TCP/IP
   like "RObust Header Compression (ROHC) can significantly compress
   this overhead.  It performs well over links with significant error
   rates and long round-trip times.

   RFC 1144 S: "Compressing TCP/IP Headers for Low-Speed Serial Links"
   (February 1990)

      This document [RFC1144] describes a method for compressing the
      headers of TCP/IP datagrams to improve performance over low speed
      serial links.  The method described in this document is limited in
      its handling of TCP options and cannot compress the headers of
      SYNs and FINs.

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   RFC 6846 S: "RObust Header Compression (ROHC): A Profile for TCP/IP
   (ROHC-TCP)" January 2013)

      From abstract: "This document specifies a RObust Header
      Compression (ROHC) profile for compression of TCP/IP packets.  The
      profile, called ROHC-TCP, provides efficient and robust
      compression of TCP headers, including frequently used TCP options
      such as selective acknowledgments (SACKs) and Timestamps."
      [RFC6846] RFC 6846 is the successor of RFC 4996.  It fixes a
      technical issue with the SACK compression and clarifies other
      compression methods used.

3.8.  Defending Spoofing and Flooding Attacks

   By default, TCP lacks any cryptographic structures to differentiate
   legitimate segments and those spoofed from malicious hosts.  Spoofing
   valid segments requires correctly guessing a number of fields.  The
   documents in this sub-section describe ways to make that guessing
   harder, or to prevent it from being able to affect a connection

   RFC 4953 I: "Defending TCP Against Spoofing Attacks" (July 2007)

      This document [RFC4953] discusses the recently increased
      vulnerability of long-lived TCP connections, such as BGP
      connections, to reset (RST) spoofing attacks.  The document
      analyzes the vulnerability, discussing proposed solutions at the
      transport level and their inherent challenges, as well as existing
      network level solutions and the feasibility of their deployment.

   RFC 5461 I: "TCP's Reaction to Soft Errors" (February 2009)

      This document [RFC5461] describes a non-standard but widely
      implemented modification to TCP's handling of ICMP soft error
      messages that rejects pending connection-requests when such error
      messages are received.  This behavior reduces the likelihood of
      long delays between connection-establishment attempts that may
      arise in some scenarios.

   RFC 4987 I: "TCP SYN Flooding Attacks and Common Mitigations" (August

      This document [RFC4987] describes the well-known TCP SYN flooding
      attack.  It analyzes and discusses various countermeasures against
      these attacks, including their use and trade-offs.

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   RFC 5925 S: "The TCP Authentication Option" (May 2010)

      This document [RFC5925] describes the TCP Authentication Option
      (TCP-AO), which is used to authenticate TCP segments.  TCP-AO
      obsoletes the TCP MD5 Signature option of RFC 2385.  It supports
      the use of stronger hash functions, protects against replays for
      long-lived TCP connections (as used, e.g., in BGP and LDP),
      coordinates key exchanges between endpoints, and provides a more
      explicit recommendation for external key management.
      Cryptographic algorithms for TCP-AO are defined in [RFC5926].

   RFC 5926 S: "Cryptographic Algorithms for the TCP Authentication
   Option (TCP-AO)" (May 2010)

      This document [RFC5926] specifies the algorithms and attributes
      that can be used in TCP Authentication Option's (TCP-AO) current
      manual keying mechanism and provides the interface for future
      message authentication codes (MACs).

   RFC 5927 I: "ICMP attacks against TCP" (July 2010)

      Abstract: "This document discusses the use of the Internet Control
      Message Protocol (ICMP) to perform a variety of attacks against
      the Transmission Control Protocol (TCP).  Additionally, this
      document describes a number of widely implemented modifications to
      TCP's handling of ICMP error messages that help to mitigate these
      issues."  [RFC5927]

   RFC 5961 S: "Improving TCP's Robustness to Blind In-Window Attacks"
   (August 2010)

      This document [RFC5961] describes minor modifications to how TCP
      handles inbound segments.  This renders TCP connections,
      especially long-lived connections such as H-323 or BGP, less
      vulnerable to spoofed packet injection attacks where the 4-tuple
      (the source and destination IP addresses and the source and
      destination ports) has been guessed.

   RFC 6528 S: "Defending Against Sequence Number Attacks" (February

      Abstract: "This document [RFC6528] specifies an algorithm for the
      generation of TCP Initial Sequence Numbers (ISNs), such that the
      chances of an off-path attacker guessing the sequence numbers in
      use by a target connection are reduced.  This document revises
      (and formally obsoletes) RFC 1948, and takes the ISN generation
      algorithm originally proposed in that document to Standards Track,
      formally updating RFC 793.

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4.  Experimental Extensions

   The RFCs in this section are still experimental, but they may become
   proposed standards in the future.  At least part of the reason that
   they are still experimental is to gain more wide-scale experience
   with them before a standards track decision is made.

   At this point is worth mentioning that if the experimental RFC is a
   proposal for a new protocol capability or service, i.e., it requires
   a new TCP option code point, the implementation and experimentation
   should follows [RFC6994] (see Section 5), which describes how the
   experimental TCP option code points can concurrently support multiple
   TCP extensions.

   By their publication as experimental RFCs, it is hoped that the
   community of TCP researchers will analyze and test the contents of
   these RFCs.  Although experimentation is encouraged, there is not yet
   formal consensus that these are fully logical and safe behaviors.
   Wide-scale deployment of implementations that use these features
   should be well thought-out in terms of consequences.

4.1.  Architectural Guidelines

   As multiple flows may share the same paths, sections of paths, or
   other resources, the TCP implementation may benefit from sharing
   information across TCP connections or other flows.  Some Experimental
   proposals have been documented and some implementations have included
   the concepts.

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

      This document [RFC2140] suggests how TCP connections between the
      same endpoints might share information, such as their congestion
      control state.  To some degree, this is done in practice by a few
      operating systems; for example, Linux currently has a destination
      cache.  Although this RFC is technically informational, the
      concepts it describes are in experimental use, so we include it in
      this section.

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

      This document [RFC3124], the Congestion Manager, is a related
      proposal to RFC 2140.  The idea behind the Congestion Manager,
      moving congestion control outside of individual TCP connections,
      represents a modification to the core of TCP, which supports
      sharing information among TCP connections.  Although a Proposed
      Standard, some pieces of the Congestion Manager support
      architecture have not been specified yet, and it has not achieved

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      use or implementation beyond experimental stacks, so it is not
      listed among the standard TCP enhancements in this roadmap.

4.2.  Congestion Control Extensions

   TCP congestion control has been an extremely active research area for
   many years (see [RFC5783], as it determines the performance of many
   applications that use TCP.  A number of experimental RFCs address
   issues with flow start-up, overshoot, and steady-state behavior in
   the basic RFC 5681 algorithms.  In this sub-sections, enhancements to
   TCP's congestion control are listed.  The next sub-section focus on
   TCP's loss recovery.

   RFC 2861 E: "TCP Congestion Window Validation" (June 2000)

      This document [RFC2861] suggests reducing the congestion window
      over time when no packets are flowing.  This behavior is more
      aggressive than that specified in RFC 5681, which says that a TCP
      sender SHOULD set its congestion window to the initial window
      after an idle period of an RTO or greater.

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

      This document [RFC3540] describes an optional addition to ECN that
      protects against accidental or malicious concealment of marked
      packets from the TCP sender.

   RFC 3649 E: "HighSpeed TCP for Large Congestion Windows" (December

      This document [RFC3649] proposes a modification to TCP's
      congestion control mechanism for use with TCP connections with
      large congestion windows, to allow TCP to achieve a higher
      throughput in high-bandwidth environments.

   RFC 3742 E: "Limited Slow-Start for TCP with Large Congestion
   Windows" (March 2004)

      This document [RFC3742] describes a more conservative slow-start
      behavior to prevent massive packet losses when a connection uses a
      very large congestion window.

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

      This document [RFC4782] specifies the optional Quick-Start
      mechanism for TCP.  This mechanism allows connections to use
      higher sending rates at the beginning of the data transfer or

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      after an idle period, provided that there is significant unused
      bandwidth along the path, and the sender and all of the routers
      along the path approve this higher rate.

   RFC 5562 E: "Adding Explicit Congestion Notification (ECN) Capability
   to TCP's SYN/ACK Packets" (June 2009)

      This document [RFC5562] describes an experimental modification to
      ECN [RFC3168] for the use of ECN in TCP SYN/ACK packets.  This
      would allow to ECN-mark rather than drop the TCP SYN/ACK packet at
      an ECN-capable router, and to avoid the severe penalty of a
      retransmission timeout for a connection when the SYN/ACK packet is

   RFC 5690 I: "Adding Acknowledgement Congestion Control to TCP"
   (February 2010)

      This document [RFC5690] describes a congestion control mechanism
      for acknowledgment (ACKs) traffic in TCP.  The mechanism is based
      on the acknowledgment congestion control of the Datagram
      Congestion Control Protocol's (DCCP's) [RFC4340] Congestion
      Control Identifier (CCID) 2 [RFC4341].

   RFC 6928 E: "Increasing TCP's Initial Window" (April 2013)

      This document [RFC6928] proposes to increase the TCP initial
      window from between 2 and 4 segments, as specified in RFC 3390, to
      10 segments with a fallback to the existing recommendation when
      performance issues are detected.

4.3.  Loss Recovery Extensions

   RFC 5827 E: "Early Retransmit for TCP and SCTP" (April 2010)

      This document [RFC5827] proposes the "Early Retransmit" mechanism
      for TCP (and SCTP) that can be used to recover lost segments when
      a connection's congestion window is small.  In certain special
      circumstances, Early Retransmit reduces the number of duplicate
      acknowledgments required to trigger fast retransmit to recover
      segment losses without waiting for a lengthy retransmission

   RFC 6069 E: "Making TCP more Robust to Long Connectivity Disruptions
   (TCP-LCD)" (December 2010)

      This document [RFC6069] describes how standard ICMP messages can
      be used to disambiguate true congestion loss from non-congestion
      loss caused by connectivity disruptions.  It proposes a reversion

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      strategy of TCP's retransmission timer that enables a more prompt
      detection of whether or not the connectivity has been restored.

   RFC 6937 E: "Proportional Rate Reduction for TCP" (May 2013)

      This document [RFC6937] describes an experimental Proportional
      Rate Reduction (PRR) algorithm as an alternative to the widely
      deployed Fast Recovery algorithm, to improve the accuracy of the
      amount of data sent by TCP during loss recovery.

4.4.  Detection and Prevention of Spurious Retransmissions

   In addition to the Standards Track extensions to deal with spurious
   retransmissions in Section 3.4, Experimental proposals have also been

   RFC 3522 E: "The Eifel Detection Algorithm for TCP" (April 2003)

      The Eifel detection algorithm [RFC3522] allows a TCP sender to
      detect a posteriori whether it has entered loss recovery
      unnecessarily by using the TCP timestamp option to solve the ACK

   RFC 3708 E: "Using TCP Duplicate Selective Acknowledgement (DSACKs)
   and Stream Control Transmission Protocol (SCTP) Duplicate
   Transmission Sequence Numbers (TSNs) to Detect Spurious
   Retransmissions" (February 2004)

      Abstract: "TCP and Stream Control Transmission Protocol (SCTP)
      provide notification of duplicate segment receipt through
      Duplicate Selective Acknowledgement (DSACKs) and Duplicate
      Transmission Sequence Number (TSN) notification, respectively.
      This document presents conservative methods of using this
      information to identify unnecessary retransmissions for various
      applications."  [RFC3708]

   RFC 4653 E: "Improving the Robustness of TCP to Non-Congestion
   Events" (August 2008)

      In the presence of non-congestion events, such as reordering an
      out-of-order segment does not necessarily indicates a lost segment
      and congestion.  This document [RFC4653] proposes to increase the
      threshold used to trigger a fast retransmission from the fixed
      value of three duplicate ACKs to about one congestion window of
      data in order to disambiguate true segment loss from segment

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4.5.  Multipath TCP

   MultiPath TCP (MPTCP) is an ongoing effort within the IETF that
   allows a TCP connection to simultaneously use multiple IP-addresses/
   interfaces to spread their data across several subflows, while
   presenting a regular TCP interface to applications.  Benefits of this
   include better resource utilization, better throughput and smoother
   reaction to failures.  The documents listed in this section specify
   the Multipath TCP scheme, while the documents in Sections 7.2, 7.4,
   and 7.5 provide some additional background information.

   RFC 6356 E: "Coupled Congestion Control for Multipath Transport
   Protocols" (August 2011)

      This document [RFC6356] presents a congestion control algorithm
      for multipath transport protocols such as Multipath TCP.  It
      couples the congestion control algorithms running on different
      subflows by linking their increase functions, and dynamically
      controls the overall aggressiveness of the multipath flow.  The
      result is an algorithm that is fair to TCP at bottlenecks while
      moving traffic away from congested links.

   RFC 6824 E: "TCP Extensions for Multipath Operation with Multiple
   Addresses" (January 2013) (Errata)

      This document [RFC6824] presents protocol changes required to add
      multipath capability to TCP; specifically, those for signaling and
      setting up multiple paths ("subflows"), managing these subflows,
      reassembly of data, and termination of sessions.

5.  TCP Parameters at IANA

   RFCs listed here describes both the procedures that the Internet
   Assigned Numbers Authority (IANA) uses when handling assignments and
   the procedures an RFC author should follow when requesting new TCP
   option codepoints.

   RFC 2780 B: "IANA Allocation Guidelines For Values In the Internet
   Protocol and Related Headers" (March 2000)

      Abstract: "This memo provides guidance for the IANA to use in
      assigning parameters for fields in the IPv4, IPv6, ICMP, UDP and
      TCP protocol headers."[RFC2780]

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   RFC 4727 S: "Experimental Values" (November 2006)

      This document [RFC4727] reserves both TCP options 253 and 254 for
      experimentation purposes.  When such experiments are deployed in
      the Internet, they should follow the additional requirements in
      RFC 6994.

   RFC 6335 B: "Internet Assigned Numbers Authority (IANA) Procedures
   for the Management of the Service Name and Transport Protocol Port
   Number Registry (August 2011)

      From abstract: "This document defines the procedures that the
      Internet Assigned Numbers Authority (IANA) uses when handling
      assignment and other requests related to the Service Name and
      Transport Protocol Port Number registry."  [RFC6335]

   RFC 6994 S: "Shared Use of Experimental TCP Options (August 2013)

      This document [RFC6994] describes how the experimental TCP option
      code points can concurrently support multiple TCP extensions, even
      within the same connection.  It creates an IANA registry for
      extensions to the experimental code points.

6.  Historic and Undeployed Extensions

   The RFCs listed here define extensions that have thus far failed to
   arouse substantial interest from implementers and have never seen
   widespread deployment, or were found to be defective for general use.
   Most of them are reclassified by RFC 6247 [RFC6247] to Historic

   RFC 721 U: "Out-of-Band Control Signals in a Host-to-Host Protocol"
   (September 1976): lack of interest

      RFC 721 [RFC0721] addresses the problem of implementing a reliable
      out-of-band signal (interrupts) for use in a host-to-host
      protocol.  The proposal was not included in the final TCP

   RFC 1078 U: "TCP Port Service Multiplexer (TCPMUX)" (November 1988):
   lack of interest

      This document [RFC1078] proposes a protocol to contact multiple
      services on a single well-known TCP port using a service name
      instead of a well-known number.

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   RFC 1106 H: "TCP Big Window and NAK Options" (June 1989): found

      This RFC [RFC1106] defined an alternative to the Window Scale
      option for using large windows and described the "negative
      acknowledgment" or NAK option.  There is a comparison of NAK and
      SACK methods, and early discussion of TCP over satellite issues.
      RFC 1110 explains some problems with the approaches described in
      RFC 1106.  The options described in this document have not been
      adopted by the larger community, although NAKs are used in the
      SCPS-TP adaptation of TCP for satellite and spacecraft use,
      developed by the Consultative Committee for Space Data Systems

   RFC 1110 H: "A Problem with the TCP Big Window Option" (August 1989):
   deprecates RFC 1106

      Abstract: "The TCP Big Window option discussed in RFC 1106 will
      not work properly in an Internet environment which has both a high
      bandwidth * delay product and the possibility of disordering and
      duplicating packets.  In such networks, the window size must not
      be increased without a similar increase in the sequence number
      space.  Therefore, a different approach to big windows should be
      taken in the Internet."  [RFC1110]

   RFC 1146 H: "TCP Alternate Checksum Options" (March 1990): lack of

      This document [RFC1146] defined more robust TCP checksums than the
      16-bit ones-complement in use today.  A typographical error in RFC
      1145 is fixed in RFC 1146; otherwise, the documents are the same.

   RFC 1263 I: "TCP Extensions Considered Harmful" (October 1991): lack
   of interest

      This document [RFC1263] argues against "backwards compatible" TCP
      extensions.  Specifically mentioned are several TCP enhancements
      that have been successful, including timestamps, window scaling,
      PAWS, and SACK.  RFC 1263 presents an alternative approach called
      "protocol evolution", whereby several evolutionary versions of TCP
      would exist on hosts.  These distinct TCP versions would represent
      upgrades to each other and could be header-incompatible.
      Interoperability would be provided by having a virtualization
      layer select the right TCP version for a particular connection.
      This idea did not catch on with the community, while the type of
      extensions RFC 1263 specifically targeted as harmful did become

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   RFC 1379 H: "Extending TCP for Transactions -- Concepts" (November
   1992): found defective

      See RFC 1644.

   RFC 1644 H: "T/TCP -- TCP Extensions for Transactions Functional
   Specification" (July 1994): found defective

      The inventors of TCP believed that cached connection state could
      have been used to eliminate TCP's 3-way handshake, to support two-
      packet request/response exchanges.  RFCs 1379 [RFC1379] and 1644
      [RFC1644] show that this is far from simple.  Furthermore, T/TCP
      floundered on the ease of denial-of-service attacks that can
      result.  One idea pioneered by T/TCP lives on in RFC 2140, in the
      sharing of state across connections.

   RFC 1693 H: "An Extension to TCP: Partial Order Service" (November
   1994): lack of interest

      This document [RFC1693] defines a TCP extension for applications
      that do not care about the order in which application-layer
      objects are received.  Examples are multimedia and database
      applications.  In practice, these applications either accept the
      possible performance loss because of TCP's strict ordering or they
      use more specialized transport protocols.

   RFC 1705 I: "Six Virtual Inches to the Left: The Problem with IPng"
   (October 1994): lack of interest

      To overcome the exhaustion of the IP class B address space,
      suggest this document [RFC1705] that a new version of TCP (TCPng)
      needs to be developed and deployed.  It proposes that a globally
      unique address be assigned to Transport layer to uniquely identify
      an Internet host without specifying any routing information.

   RFC 6013 E: "TCP Cookie Transactions (TCPCT)" (January 2011): lack of

      This document [RFC6013] describes a method to exchange a cookie
      (nonce) during the connection establishment to negotiate
      elimination of receiver state.  These cookies are later used to
      inhibit premature closing of connections, and reduce retention of
      state after the connection has terminated.

      Since the cookie pair is too large to fit with the other TCP
      options in the 40 bytes of TCP option space, the document further
      describes a method to extent the option space after the connection

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      Although RFC 6013 was published in 2011, the authors of this
      document places it in this section of the roadmap document due to
      two factors.
      (a)  The authors are not aware of any wide deployment and use of
           RFC 6013.
      (b)  RFC 6013 uses experimental TCP option codepoints, which
           prohibits a large scale deployment.

7.  Support Documents

   This section contains several classes of documents that do not
   necessarily define current protocol behaviors, but that are
   nevertheless of interest to TCP implementers.  Section 7.1 describes
   several foundational RFCs that give modern readers a better
   understanding of the principles underlying TCP's behaviors and
   development over the years.  Section 7.2 contains architectural
   guidelines and principles for TCP architects and designers.  The
   documents listed in Section 7.3 provide advice on using TCP in
   various types of network situations that pose challenges above those
   of typical wired links.  Guidance for developing, analyzing, and
   evaluating TCP is given in Section 7.4.  Some implementation notes
   and implementation advice can be found in Section 7.5.  RFCs that
   describe tools for testing and debugging TCP implementations or that
   contain high-level tutorials on the protocol are listed Section 7.6.
   The TCP Management Information Bases are described in Section 7.7,
   and Section 7.8 lists a number of case studies that have explored TCP

7.1.  Foundational Works

   The documents listed in this section contain information that is
   largely duplicated by the standards documents previously discussed.
   However, some of them contain a greater depth of problem statement
   explanation or other context.  Particularly, RFCs 813 - 817 (known as
   the "Dave Clark Five") describe some early problems and solutions
   (RFC 815 only describes the reassembly of IP fragments and is not
   included in this TCP roadmap).

   RFC 675 U: "Specification of Internet Transmission Control Program"
   (December 1974)

      This document [RFC0675] is a very early precursor of the
      fundamental RFC 793 which already contained the three-way
      handshake in its final form and the concept of sliding windows for
      reliable data transmission.  Apart from that the segment layout is
      totally different and the specified API differs from the latter
      RFC 793.

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   RFC 761 H: "DoD standard Transmission Control Protocol" (Januar

      This document [RFC0761] is the immediate precursor of RFC 793.
      The header format, the connection establishment including the
      different connection states, and the overall API correspond mostly
      the final Standard RFC 793.

   RFC 813 U: "Window and Acknowledgement Strategy in TCP" (July 1982)

      This document [RFC0813] contains an early discussion of Silly
      Window Syndrome and its avoidance and motivates and describes the
      use of delayed acknowledgments.

   RFC 814 U: "Name, Addresses, Ports, and Routes" (July 1982)

      Suggestions and guidance for the design of tables and algorithms
      to keep track of various identifiers within a TCP/IP
      implementation are provided by this document [RFC0814].

   RFC 816 U: "Fault Isolation and Recovery" (July 1982)

      In this document [RFC0816], TCP's response to indications of
      network error conditions such as timeouts or received ICMP
      messages is discussed.

   RFC 817 U: "Modularity and Efficiency in Protocol Implementation"
   (July 1982)

      This document [RFC0817] contains implementation suggestions that
      are general and not TCP specific.  However, they have been used to
      develop TCP implementations and describe some performance
      implications of the interactions between various layers in the
      Internet stack.

   RFC 872 U: "TCP-ON-A-LAN" (September 1982)

      Conclusion: "The sometimes-expressed fear that using TCP on a
      local net is a bad idea is unfounded."  [RFC0872]

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

      This document [RFC0896] contains some early experiences with
      congestion collapse and some initial thoughts on how to avoid it
      using congestion control in TCP.

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   RFC 964 U: "Some Problems with the Specification of the Military
   Standard Transmission Control Protocol" (November 1985)

      This document [RFC0964] points out several specification bugs in
      the US Military's MIL-STD-1778 document, which was intended as a
      successor to RFC 793.  This serves to remind us of the difficulty
      in specification writing (even when we work from existing

7.2.  Architectural Guidelines

   Some documents in this section contain architectural guidance and
   concerns, while others specify TCP- and congestion-control-related
   mechanisms that are broadly applicable and have impacts on TCP's
   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 1958 I: "Architectural Principles of the Internet" (June 1996)

      This document [RFC1958] describes the underlying principles of the
      Internet architecture.  It provides guidelines for network systems
      design that have proven useful in the evolution of the Internet.

   RFC 2914 B: "Congestion Control Principles" (September 2000)

      This document [RFC2914] motivates the use of end-to-end congestion
      control for preventing congestion collapse and providing fairness
      to TCP.

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

      This document [RFC3439] extends RFC 1958 by outlining some
      philosophical guidelines for architects and designers of Internet
      backbone networks.  The document describes the Simplicity
      Principle, which states that complexity is the primary mechanism
      that impedes efficient scaling.

   RFC 6182 I: "Architectural Guidelines for Multipath TCP Development"
   (March 2011)

      Abstract: "This document outlines architectural guidelines for the
      development of a Multipath Transport Protocol, with references to
      how these architectural components come together in the
      development of a Multipath TCP (MPTCP).  This document lists
      certain high-level design decisions that provide foundations for
      the design of the MPTCP protocol, based upon these architectural

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      requirements" [RFC6182]

7.3.  Difficult Network Environments

   As the internetworking field has explored wireless, satellite,
   cellular telephone, and other kinds of link-layer technologies, a
   large body of work has built up on enhancing TCP performance for such
   links.  The RFCs listed in this section describe some of these more
   challenging network environments and how TCP interacts with them.

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

      From abstract: "While TCP works over satellite channels there are
      several IETF standardized mechanisms that enable TCP to more
      effectively utilize the available capacity of the network path.
      This document outlines some of these TCP mitigations.  At this
      time, all mitigations discussed in this document are IETF
      standards track mechanisms (or are compliant with IETF
      standards)."  [RFC2488]

   RFC 2757 I: "Long Thin Networks" (January 2000)

      Several methods of improving TCP performance over long thin
      networks, such as geosynchronous satellite links, are discussed in
      this document [RFC2757].  A particular set of TCP options is
      developed that should work well in such environments and be safe
      to use in the global Internet.  The implications of such
      environments have been further discussed in RFC 3150 and RFC 3155,
      and these documents should be preferred where there is overlap
      between them and RFC 2757.

   RFC 2760 I: "Ongoing TCP Research Related to Satellites" (February

      This document [RFC2760] discusses the advantages and disadvantages
      of several different experimental means of improving TCP
      performance over long-delay or error-prone paths.  These include
      T/TCP, larger initial windows, byte counting, delayed
      acknowledgments, slow start thresholds, NewReno and SACK-based
      loss recovery, FACK [MM96], ECN, various corruption-detection
      mechanisms, congestion avoidance changes for fairness, use of
      multiple parallel flows, pacing, header compression, state
      sharing, and ACK congestion control, filtering, and
      reconstruction.  Although RFC 2488 looks at standard extensions,
      this document focuses on more experimental means of performance

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

      From abstract: "This document 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 Performance Enhancing Proxies
      are described as well as the mechanisms used to improve
      performance."  [RFC3135]

   RFC 3150 B: "End-to-end Performance Implications of Slow Links" (July

      From abstract: "This document makes performance-related
      recommendations for users of network paths that traverse "very low
      bit-rate" links....This recommendation may be useful in any
      network where hosts can saturate available bandwidth, but the
      design space for this recommendation explicitly includes
      connections that traverse 56 Kb/second modem links or 4.8 Kb/
      second wireless access links - both of which are widely deployed."

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

      From abstract: "This document discusses the specific TCP
      mechanisms that are problematic in environments with high
      uncorrected error rates, and discusses what can be done to
      mitigate the problems without introducing intermediate devices
      into the connection."  [RFC3155]

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

      From abstract: "This document provides advice to the designers of
      digital communication equipment and link-layer protocols employing
      link-layer Automatic Repeat reQuest (ARQ) techniques.  This
      document presumes that the designers wish to support Internet
      protocols, but may be unfamiliar with the architecture of the
      Internet and with the implications of their design choices for the
      performance and efficiency of Internet traffic carried over their
      links."  [RFC3366]

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

      From abstract: "This document describes TCP performance problems

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      that arise because of asymmetric effects.  These problems arise in
      several access networks, including bandwidth-asymmetric networks
      and packet radio subnetworks, for different underlying reasons.
      However, the end result on TCP performance is the same in both
      cases: performance often degrades significantly because of
      imperfection and variability in the ACK feedback from the receiver
      to the sender.

      The document details several mitigations to these effects, which
      have either been proposed or evaluated in the literature, or are
      currently deployed in networks."  [RFC3449]

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

      From abstract: "This document describes a profile for optimizing
      TCP to adapt so that it handles paths including second (2.5G) and
      third (3G) generation wireless networks."  [RFC3481]

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

      This document [RFC3819] describes how TCP performance can be
      negatively affected by some particular lower-layer behaviors and
      provides guidance in designing lower-layer networks and protocols
      to be amicable to TCP.

7.4.  Guidance for Developing, Analyzing, and Evaluating TCP

   Documents in this section give general guidance for developing,
   analyzing, and evaluating TCP.  Some of the documents discuss for
   example 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.

   RFC 4774 B: "Specifying Alternate Semantics for the Explicit
   Congestion Notification (ECN) Field" (November 2006)

      This document [RFC4774] discusses some of the issues in defining
      alternate semantics for the ECN field, and specifies requirements
      for a safe co- existence in an Internet that may include routers
      that do not understand the defined alternate semantics.

   RFC 5033 B: "Specifying New Congestion Control Algorithms" (August

      This document [RFC5033] considers the evaluation of suggested
      congestion control algorithms that differ from the principles
      outlined in RFC 2914.  It is useful for authors of such algorithms

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      as well as for IETF members reviewing the associated documents.

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

      This document [RFC5166] discusses metrics that needs to be
      considered when evaluating new or modified congestion control
      mechanisms for the Internet.  Among others topics, the document
      discusses throughput, delay, loss rates, response times, fairness
      and robustness for challenging environments.

   RFC 6077 I: "Open Research Issues in Internet Congestion Control"
   (January 2011)

      This RFC [RFC6077] summarizes the main open problems in the domain
      of Internet congestion control.  As a good starting point for
      newcomers, the document describes several new challenges that are
      becoming important as the network grows, as well as some issues
      that have been known for many years.

   RFC 6181 I: "Threat Analysis for TCP Extensions for Multipath
   Operation with Multiple Addresses" (March 2011)

      This document [RFC6181] describes a threat analysis for Multipath
      TCP (MPTCP).  The document discusses several types of attacks and
      provides recommendations for MPTCP designers how to create an
      MPTCP specification that is as secure as the current (single-path)

   RFC 6349 I: "Framework for TCP Throughput Testing" (August 2011)

      From abstract: "This document describes a practical methodology
      for measuring end-to-end TCP throughput in a managed IP network.
      The goal is to provide a better indication in regard to user
      experience.  In this framework, TCP and IP parameters are
      specified to optimize TCP throughput."  [RFC6349]

7.5.  Implementation Advice

   RFC 794 U: "PRE-EMPTION" (September 1981)

      This document [RFC0794] discusses on a high-level the realization
      of pre-emption in TCP.

   RFC 879 U: "The TCP Maximum Segment Size and Related Topics"
   (November 1983)

      Abstract: "This memo discusses the TCP Maximum Segment Size Option

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      and related topics.  The purposes is to clarify some aspects of
      TCP and its interaction with IP.  This memo is a clarification to
      the TCP specification, and contains information that may be
      considered as 'advice to implementers'."  [RFC0879]

   RFC 1071 U: "Computing the Internet Checksum" (September 1988)

      This document [RFC1071] lists a number of implementation
      techniques for efficiently computing the Internet checksum (used
      by TCP).

   RFC 1624 I: "Computation of the Internet Checksum via Incremental
   Update" (May 1994)

      Incrementally updating the Internet checksum is useful to routers
      in updating IP checksums.  Some middleboxes that alter TCP headers
      may also be able to update the TCP checksum incrementally.  This
      document [RFC1624] expands upon the explanation of the incremental
      update procedure in RFC 1071.

   RFC 1936 I: "Implementing the Internet Checksum in Hardware" (April

      This document [RFC1936] describes the motivation for implementing
      the Internet checksum in hardware, rather than in software, and
      provides an implementation example.

   RFC 2525 I: "Known TCP Implementation Problems" (March 1999)

      From abstract: "This memo catalogs a number of known TCP
      implementation problems.  The goal in doing so is to improve
      conditions in the existing Internet by enhancing the quality of
      current TCP/IP implementations."  [RFC2525]

   RFC 2923 I: "TCP Problems with Path MTU Discovery" (September 2000)

      From abstract: "This memo catalogs several known Transmission
      Control Protocol (TCP) implementation problems dealing with Path
      Maximum Transmission Unit Discovery (PMTUD), including the long-
      standing black hole problem, stretch acknowledgments (ACKs) due to
      confusion between Maximum Segment Size (MSS) and segment size, and
      MSS advertisement based on PMTU."  [RFC2923]

   RFC 3360 B: "Inappropriate TCP Resets Considered Harmful" (August

      This document [RFC3360] is a plea that firewall vendors not send

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      gratuitous TCP RST (Reset) packets when unassigned TCP header bits
      are used.  This practice prevents desirable extension and
      evolution of the protocol and thus is potentially harmful to the
      future of the Internet.

   RFC 3493 I: "Basic Socket Interface Extensions for IPv6" (February

      This document [RFC3493] describes the de facto standard sockets
      API for programming with TCP.  This API is implemented nearly
      ubiquitously in modern operating systems and programming

   RFC 6056 B: "Recommendations for Transport-Protocol Port
   Randomization" (December 2010)

      This document [RFC6056] describes a number of simple and efficient
      methods for the selection of the client port number.  It reduces
      the possibility of an attacker guessing the correct five-tuple
      (Protocol, Source/Destination Address, Source/Destination Port).

   RFC 6191 B: "Reducing the TIME-WAIT State Using TCP timestamps"
   (April 2011)

      This document [RFC6191] describes the usage of the TCP Timestamps
      option [JBB92] to perform heuristics to determine whether or not
      to allow the creation of a new incarnation of a connection that is
      in the TIME-WAIT state.

   RFC 6429 I: "TCP Sender Clarification for Persist Condition"
   (December 2011)

      This document [RFC6429] clarifies the actions that a TCP can be
      taken on connections that are experiencing the Zero Window Probe
      (ZWP) condition.

   RFC 6897 I: "Multipath TCP (MPTCP) Application Interface
   Considerations" (March 2013)

      This document [RFC6897] characterizes the impact that Multipath
      TCP (MPTCP) may have on applications.  It further discusses
      compatibility issues of MPTCP in combination with non-MPTCP-aware
      applications.  Finally, it describes a basic API that is a simple
      extension of TCP's interface for MPTCP-aware applications.

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7.6.  Tools and Tutorials

   RFC 1180 I: "TCP/IP Tutorial" (January 1991) (Errata)

      This document [RFC1180] is an extremely brief overview of the
      TCP/IP protocol suite as a whole.  It gives some explanation as to
      how and where TCP fits in.

   RFC 1470 I: "FYI on a Network Management Tool Catalog: Tools for
   Monitoring and Debugging TCP/IP Internets and Interconnected Devices"
   (June 1993)

      A few of the tools that this document [RFC1470] describes are
      still maintained and in use today; for example, ttcp and tcpdump.
      However, many of the tools described do not relate specifically to
      TCP and are no longer used or easily available.

   RFC 2398 I: "Some Testing Tools for TCP Implementors" (August 1998)

      This document [RFC2398] describes a number of TCP packet
      generation and analysis tools.  Although some of these tools are
      no longer readily available or widely used, for the most part they
      are still relevant and usable.

   RFC 5783 I: "Congestion Control in the RFC Series" (February 2010)

      This document [RFC5783] provides an overview of RFCs related to
      congestion control that have been published so far.  The focus of
      the document is on end-host-based congestion control.

7.7.  Management Information Bases

   The first MIB module defined for use with Simple Network Management
   Protocol (SNMP) (in RFC 1066 and its update, RFC 1156) was a single
   monolithic MIB module, called MIB-I.  This evolved over time to be
   MIB-II (RFC 1213).  It then became apparent that having a single
   monolithic MIB module was not scalable, given the number and breadth
   of MIB data definitions that needed to be included.  Thus, additional
   MIB modules were defined, and those parts of MIB-II that needed to
   evolve were split off.  Eventually, the remaining parts of MIB-II
   were also split off, the TCP-specific part being documented in RFC

   RFC 2012 was obsoleted by RFC 4022, which is the primary TCP MIB
   document today.  MIB-I, defined in RFC 1156, has been obsoleted by
   the MIB-II specification in RFC 1213.  For current TCP implementers,
   RFC 4022 should be supported.

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   RFC 1066 H: "Management Information Base for Network Management of
   TCP/IP-based Internets" (August 1988)

      This document [RFC1066] was the description of the TCP MIB.  It
      was obsoleted by RFC 1156.

   RFC 1156 S: "Management Information Base for Network Management of
   TCP/IP-based Internets" (May 1990)

      This document [RFC1156] describes the required MIB fields for TCP
      implementations, with minor corrections and no technical changes
      from RFC 1066, which it obsoletes.  This is the standards track
      document for MIB-I.

   RFC 1213 S: "Management Information Base for Network Management of
   TCP/IP-based Internets: MIB-II" (March 1991)

      This document [RFC1213] describes the second version of the MIB in
      a monolithic form.  RFC 2012 updates this document by splitting
      out the TCP-specific portions.

   RFC 2012 S: "SNMPv2 Management Information Base for the Transmission
   Control Protocol using SMIv2" (November 1996)

      This document [RFC2012] defined the TCP MIB, in an update to RFC
      1213.  It is now obsoleted by RFC 4022.

   RFC 2452 S: "IP Version 6 Management Information Base for the
   Transmission Control Protocol" (December 1998)

      This document [RFC2452] augments RFC 2012 by adding an IPv6-
      specific connection table.  The rest of 2012 holds for any IP
      version.  RFC 2012 is now obsoleted by RFC 4022.

      Although it is a standards track document, RFC 2452 is considered
      a historic mistake by the MIB community, as it is based on the
      idea of parallel IPv4 and IPv6 structures.  Although IPv6 requires
      new structures, the community has decided to define a single
      generic structure for both IPv4 and IPv6.  This will aid in
      definition, implementation, and transition between IPv4 and IPv6.

   RFC 4022 S: "Management Information Base for the Transmission Control
   Protocol (TCP)" (March 2005)

      This document [RFC4022] obsoletes RFC 2012 and RFC 2452 and
      specifies the current standard for the TCP MIB that should be

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7.8.  Case Studies

   RFC 700 U: "A Protocol Experiment" (August 1974)

      This document [RFC0700] presents a field report about the
      deployment of a very early version of TCP, the so-called INWN #39
      protocol, which is originally described by Cerf and Kahn in INWG
      Note #39 [CK73] to use a PDP-11 line printer via the ARPANET.

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

      This document [RFC0889] is a status report about experiments
      concerning the TCP retransmission timeout calculation and also
      provides advices for implementers.

   RFC 1337 I: "TIME-WAIT Assassination Hazards in TCP" (May 1992)

      This document [RFC1337] points out a problem with acting on
      received reset segments while one is in the TIME-WAIT state.  The
      main recommendation is that hosts in TIME-WAIT ignore resets.
      This recommendation might not currently be widely implemented.

   RFC 2415 I: "Simulation Studies of Increased Initial TCP Window Size"
   (September 1998)

      This document [RFC2415] presents results of some simulations using
      TCP initial windows greater than 1 segment.  The analysis
      indicates that user-perceived performance can be improved by
      increasing the initial window to 3 segments.

   RFC 2416 I: "When TCP Starts Up With Four Packets Into Only Three
   Buffers" (September 1998)

      This document [RFC2416] uses simulation results to clear up some
      concerns about using an initial window of 4 segments when the
      network path has less provisioning.

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

      This document [RFC2884] describes experimental results that show
      some improvements to the performance of both short- and long-lived
      connections due to ECN.

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8.  Undocumented TCP Features

   There are a few important implementation tactics for the TCP that
   have not yet been described in any RFC.  Although this roadmap is
   primarily concerned with mapping the TCP RFCs, this section is
   included because an implementer needs to be aware of these important

   Header Prediction

      Header prediction is a trick to speed up the processing of
      segments.  Van Jacobson and Mike Karels developed the technique in
      the late 1980s.  The basic idea is that some processing time can
      be saved when most of a segment's fields can be predicted from
      previous segments.  A good description of this was sent to the
      TCP-IP mailing list by Van Jacobson on March 9, 1988:

      "Quite a bit of the speedup comes from an algorithm that we ('we'
      refers to collaborator Mike Karels and myself) are calling "header
      prediction".  The idea is that if you're in the middle of a bulk
      data transfer and have just seen acpacket, you know what the next
      packet is going to look like: It will look just like the current
      packet with either the sequence number or ack number updated
      (depending on whether you're the sender or receiver).  Combining
      this with the "Use hints" epigram from Butler Lampson's classic
      "Epigrams for System Designers", you start to think of the tcp
      state (rcv.nxt, snd.una, etc.) as "hints" about what the next
      packet should look like.

      If you arrange those "hints" so they match the layout of a tcp
      packet header, it takes a single 14-byte compare to see if your
      prediction is correct (3 longword compares to pick up the send &
      ack sequence numbers, header length, flags and window, plus a
      short compare on the length).  If the prediction is correct,
      there's a single test on the length to see if you're the sender or
      receiver followed by the appropriate processing.  E.g., if the
      length is non-zero (you're the receiver), checksum and append the
      data to the socket buffer then wake any process that's sleeping on
      the buffer.  Update rcv.nxt by the length of this packet (this
      updates your "prediction" of the next packet).  Check if you can
      handle another packet the same size as the current one.  If not,
      set one of the unused flag bits in your header prediction to
      guarantee that the prediction will fail on the next packet and
      force you to go through full protocol processing.  Otherwise,
      you're done with this packet.  So, the *total* tcp protocol
      processing, exclusive of checksumming, is on the order of 6
      compares and an add."

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   Forward Acknowledgement (FACK)

      FACK [MM96] includes an alternate algorithm for triggering fast
      retransmit, based on the extent of the SACK scoreboard.  Its goal
      is to trigger fast retransmit as soon as the receiver's reassembly
      queue is larger than the DUPACK threshold, as indicated by the
      difference between the forward most SACK block edge and SND.UNA.
      This algorithm quickly and reliably triggers fast retransmit in
      the presence of burst losses -- often on the first SACK following
      such a loss.  Such a threshold based algorithm also triggers fast
      retransmit immediately in the presence of any reordering with
      extent greater than the DUPACK threshold.  FACK is implemented in
      Linux and turned on per default.

   Highspeed Congestion Control

      In the last decade significant research effort has been put into
      experimental TCP congestion control modifications for obtaining
      high throughput with reduced startup and recovery times.  Only few
      RFCs have been published on some of these modifications, including
      HighSpeed TCP [RFC3649], Limited Slow-Start [RFC3742], and Quick-
      Start [RFC4782] (see Section 4.2), but high-rate congestion
      control mechanisms are still considered an open issue in
      congestion control research.  Some other schemes have been
      published as Internet-Drafts, e.g.  CUBIC [I-D.rhee-tcpm-cubic]
      (the standard TCP congestion control algorithm in Linux), Compound
      TCP [I-D.sridharan-tcpm-ctcp], and H-TCP [I-D.leith-tcp-htcp] 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 IRTF Internet Congestion
      Control Research Group (ICCRG).

9.  Security Considerations

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

10.  IANA Considerations

   This document contains no IANA considerations.

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11.  Acknowledgments

   This document grew out of a discussion on the end2end-interest
   mailing list, the public list of the End-to-End Research Group of the
   IRTF, and continued development under the IETF's TCP Maintenance and
   Minor Extensions (TCPM) working group.  We thank Mark Allman, Yuchung
   Cheng, Ted Faber, Fairhurst, Sally Floyd, Janardhan Iyengar, Reiner
   Ludwig, Pekka Savola, and Joe Touch for their contributions, in
   particular.  Keith McCloghrie provided some useful notes and
   clarification on the various MIB-related RFCs.

12.  References

12.1.  Normative References

   [RFC0675]  Cerf, V., Dalal, Y., and C. Sunshine, "Specification of
              Internet Transmission Control Program", RFC 675,
              December 1974.

   [RFC0700]  Mader, E., Plummer, W., and R. Tomlinson, "Protocol
              experiment", RFC 700, August 1974.

   [RFC0721]  Garlick, L., "Out-of-Band Control Signals in a Host-to-
              Host Protocol", RFC 721, September 1976.

   [RFC0761]  Postel, J., "DoD standard Transmission Control Protocol",
              RFC 761, January 1980.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

   [RFC0794]  Cerf, V., "Pre-emption", RFC 794, September 1981.

   [RFC0813]  Clark, D., "Window and Acknowledgement Strategy in TCP",
              RFC 813, July 1982.

   [RFC0814]  Clark, D., "Name, addresses, ports, and routes", RFC 814,
              July 1982.

   [RFC0816]  Clark, D., "Fault isolation and recovery", RFC 816,
              July 1982.

   [RFC0817]  Clark, D., "Modularity and efficiency in protocol
              implementation", RFC 817, July 1982.

   [RFC0872]  Padlipsky, M., "TCP-on-a-LAN", RFC 872, September 1982.

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   [RFC0879]  Postel, J., "TCP maximum segment size and related topics",
              RFC 879, November 1983.

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

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

   [RFC0964]  Sidhu, D. and T. Blumer, "Some problems with the
              specification of the Military Standard Transmission
              Control Protocol", RFC 964, November 1985.

   [RFC1066]  McCloghrie, K. and M. Rose, "Management Information Base
              for network management of TCP/IP-based internets",
              RFC 1066, August 1988.

   [RFC1071]  Braden, R., Borman, D., Partridge, C., and W. Plummer,
              "Computing the Internet checksum", RFC 1071,
              September 1988.

   [RFC1078]  Lottor, M., "TCP port service Multiplexer (TCPMUX)",
              RFC 1078, November 1988.

   [RFC1106]  Fox, R., "TCP big window and NAK options", RFC 1106,
              June 1989.

   [RFC1110]  McKenzie, A., "Problem with the TCP big window option",
              RFC 1110, August 1989.

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

   [RFC1144]  Jacobson, V., "Compressing TCP/IP headers for low-speed
              serial links", RFC 1144, February 1990.

   [RFC1146]  Zweig, J. and C. Partridge, "TCP alternate checksum
              options", RFC 1146, March 1990.

   [RFC1156]  McCloghrie, K. and M. Rose, "Management Information Base
              for network management of TCP/IP-based internets",
              RFC 1156, May 1990.

   [RFC1180]  Socolofsky, T. and C. Kale, "TCP/IP tutorial", RFC 1180,
              January 1991.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              November 1990.

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   [RFC1213]  McCloghrie, K. and M. Rose, "Management Information Base
              for Network Management of TCP/IP-based internets:MIB-II",
              STD 17, RFC 1213, March 1991.

   [RFC1263]  O'Malley, S. and L. Peterson, "TCP Extensions Considered
              Harmful", RFC 1263, October 1991.

   [RFC1323]  Jacobson, V., Braden, B., and D. Borman, "TCP Extensions
              for High Performance", RFC 1323, May 1992.

   [RFC1337]  Braden, B., "TIME-WAIT Assassination Hazards in TCP",
              RFC 1337, May 1992.

   [RFC1379]  Braden, B., "Extending TCP for Transactions -- Concepts",
              RFC 1379, November 1992.

   [RFC1470]  Enger, R. and J. Reynolds, "FYI on a Network Management
              Tool Catalog: Tools for Monitoring and Debugging TCP/IP
              Internets and Interconnected Devices", RFC 1470,
              June 1993.

   [RFC1624]  Rijsinghani, A., "Computation of the Internet Checksum via
              Incremental Update", RFC 1624, May 1994.

   [RFC1644]  Braden, B., "T/TCP -- TCP Extensions for Transactions
              Functional Specification", RFC 1644, July 1994.

   [RFC1693]  Connolly, T., Amer, P., and P. Conrad, "An Extension to
              TCP : Partial Order Service", RFC 1693, November 1994.

   [RFC1705]  Carlson, R. and D. Ficarella, "Six Virtual Inches to the
              Left: The Problem with IPng", RFC 1705, October 1994.

   [RFC1936]  Touch, J. and B. Parham, "Implementing the Internet
              Checksum in Hardware", RFC 1936, April 1996.

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

   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, August 1996.

   [RFC2012]  McCloghrie, K., "SNMPv2 Management Information Base for
              the Transmission Control Protocol using SMIv2", RFC 2012,
              November 1996.

   [RFC2018]  Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
              Selective Acknowledgment Options", RFC 2018, October 1996.

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   [RFC2140]  Touch, J., "TCP Control Block Interdependence", RFC 2140,
              April 1997.

   [RFC2398]  Parker, S. and C. Schmechel, "Some Testing Tools for TCP
              Implementors", RFC 2398, August 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.

   [RFC2452]  Daniele, M., "IP Version 6 Management Information Base for
              the Transmission Control Protocol", RFC 2452,
              December 1998.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

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

   [RFC2525]  Paxson, V., Dawson, S., Fenner, W., Griner, J., Heavens,
              I., Lahey, K., Semke, J., and B. Volz, "Known TCP
              Implementation Problems", RFC 2525, March 1999.

   [RFC2675]  Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
              RFC 2675, August 1999.

   [RFC2757]  Montenegro, G., Dawkins, S., Kojo, M., Magret, V., and N.
              Vaidya, "Long Thin Networks", RFC 2757, January 2000.

   [RFC2760]  Allman, M., Dawkins, S., Glover, D., Griner, J., Tran, D.,
              Henderson, T., Heidemann, J., Touch, J., Kruse, H.,
              Ostermann, S., Scott, K., and J. Semke, "Ongoing TCP
              Research Related to Satellites", RFC 2760, February 2000.

   [RFC2780]  Bradner, S. and V. Paxson, "IANA Allocation Guidelines For
              Values In the Internet Protocol and Related Headers",
              BCP 37, RFC 2780, March 2000.

   [RFC2861]  Handley, M., Padhye, J., and S. Floyd, "TCP Congestion
              Window Validation", RFC 2861, June 2000.

   [RFC2873]  Xiao, X., Hannan, A., Paxson, V., and E. Crabbe, "TCP
              Processing of the IPv4 Precedence Field", RFC 2873,

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              June 2000.

   [RFC2883]  Floyd, S., Mahdavi, J., Mathis, M., and M. Podolsky, "An
              Extension to the Selective Acknowledgement (SACK) Option
              for TCP", RFC 2883, July 2000.

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

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

   [RFC2923]  Lahey, K., "TCP Problems with Path MTU Discovery",
              RFC 2923, September 2000.

   [RFC3042]  Allman, M., Balakrishnan, H., and S. Floyd, "Enhancing
              TCP's Loss Recovery Using Limited Transmit", RFC 3042,
              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.

   [RFC3360]  Floyd, S., "Inappropriate TCP Resets Considered Harmful",
              BCP 60, RFC 3360, August 2002.

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

   [RFC3390]  Allman, M., Floyd, S., and C. Partridge, "Increasing TCP's
              Initial Window", RFC 3390, October 2002.

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

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

   [RFC3465]  Allman, M., "TCP Congestion Control with Appropriate Byte
              Counting (ABC)", RFC 3465, February 2003.

   [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.

   [RFC3493]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
              Stevens, "Basic Socket Interface Extensions for IPv6",
              RFC 3493, February 2003.

   [RFC3522]  Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm
              for TCP", RFC 3522, April 2003.

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

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

   [RFC3708]  Blanton, E. and M. Allman, "Using TCP Duplicate Selective
              Acknowledgement (DSACKs) and Stream Control Transmission
              Protocol (SCTP) Duplicate Transmission Sequence Numbers
              (TSNs) to Detect Spurious Retransmissions", RFC 3708,
              February 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.

   [RFC4015]  Ludwig, R. and A. Gurtov, "The Eifel Response Algorithm
              for TCP", RFC 4015, February 2005.

   [RFC4022]  Raghunarayan, R., "Management Information Base for the
              Transmission Control Protocol (TCP)", RFC 4022,

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              March 2005.

   [RFC4653]  Bhandarkar, S., Reddy, A., Allman, M., and E. Blanton,
              "Improving the Robustness of TCP to Non-Congestion
              Events", RFC 4653, August 2006.

   [RFC4727]  Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4,
              ICMPv6, UDP, and TCP Headers", RFC 4727, November 2006.

   [RFC4774]  Floyd, S., "Specifying Alternate Semantics for the
              Explicit Congestion Notification (ECN) Field", BCP 124,
              RFC 4774, November 2006.

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

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, March 2007.

   [RFC4953]  Touch, J., "Defending TCP Against Spoofing Attacks",
              RFC 4953, July 2007.

   [RFC4987]  Eddy, W., "TCP SYN Flooding Attacks and Common
              Mitigations", RFC 4987, August 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.

   [RFC5461]  Gont, F., "TCP's Reaction to Soft Errors", RFC 5461,
              February 2009.

   [RFC5482]  Eggert, L. and F. Gont, "TCP User Timeout Option",
              RFC 5482, March 2009.

   [RFC5562]  Kuzmanovic, A., Mondal, A., Floyd, S., and K.
              Ramakrishnan, "Adding Explicit Congestion Notification
              (ECN) Capability to TCP's SYN/ACK Packets", RFC 5562,
              June 2009.

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, September 2009.

   [RFC5682]  Sarolahti, P., Kojo, M., Yamamoto, K., and M. Hata,
              "Forward RTO-Recovery (F-RTO): An Algorithm for Detecting
              Spurious Retransmission Timeouts with TCP", RFC 5682,

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              September 2009.

   [RFC5690]  Floyd, S., Arcia, A., Ros, D., and J. Iyengar, "Adding
              Acknowledgement Congestion Control to TCP", RFC 5690,
              February 2010.

   [RFC5783]  Welzl, M. and W. Eddy, "Congestion Control in the RFC
              Series", RFC 5783, February 2010.

   [RFC5827]  Allman, M., Avrachenkov, K., Ayesta, U., Blanton, J., and
              P. Hurtig, "Early Retransmit for TCP and Stream Control
              Transmission Protocol (SCTP)", RFC 5827, May 2010.

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, June 2010.

   [RFC5926]  Lebovitz, G. and E. Rescorla, "Cryptographic Algorithms
              for the TCP Authentication Option (TCP-AO)", RFC 5926,
              June 2010.

   [RFC5927]  Gont, F., "ICMP Attacks against TCP", RFC 5927, July 2010.

   [RFC5961]  Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
              Robustness to Blind In-Window Attacks", RFC 5961,
              August 2010.

   [RFC6013]  Simpson, W., "TCP Cookie Transactions (TCPCT)", RFC 6013,
              January 2011.

   [RFC6056]  Larsen, M. and F. Gont, "Recommendations for Transport-
              Protocol Port Randomization", BCP 156, RFC 6056,
              January 2011.

   [RFC6069]  Zimmermann, A. and A. Hannemann, "Making TCP More Robust
              to Long Connectivity Disruptions (TCP-LCD)", RFC 6069,
              December 2010.

   [RFC6077]  Papadimitriou, D., Welzl, M., Scharf, M., and B. Briscoe,
              "Open Research Issues in Internet Congestion Control",
              RFC 6077, February 2011.

   [RFC6093]  Gont, F. and A. Yourtchenko, "On the Implementation of the
              TCP Urgent Mechanism", RFC 6093, January 2011.

   [RFC6181]  Bagnulo, M., "Threat Analysis for TCP Extensions for
              Multipath Operation with Multiple Addresses", RFC 6181,
              March 2011.

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   [RFC6182]  Ford, A., Raiciu, C., Handley, M., Barre, S., and J.
              Iyengar, "Architectural Guidelines for Multipath TCP
              Development", RFC 6182, March 2011.

   [RFC6191]  Gont, F., "Reducing the TIME-WAIT State Using TCP
              Timestamps", BCP 159, RFC 6191, April 2011.

   [RFC6247]  Eggert, L., "Moving the Undeployed TCP Extensions RFC
              1072, RFC 1106, RFC 1110, RFC 1145, RFC 1146, RFC 1379,
              RFC 1644, and RFC 1693 to Historic Status", RFC 6247,
              May 2011.

   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298,
              June 2011.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, August 2011.

   [RFC6349]  Constantine, B., Forget, G., Geib, R., and R. Schrage,
              "Framework for TCP Throughput Testing", RFC 6349,
              August 2011.

   [RFC6356]  Raiciu, C., Handley, M., and D. Wischik, "Coupled
              Congestion Control for Multipath Transport Protocols",
              RFC 6356, October 2011.

   [RFC6429]  Bashyam, M., Jethanandani, M., and A. Ramaiah, "TCP Sender
              Clarification for Persist Condition", RFC 6429,
              December 2011.

   [RFC6528]  Gont, F. and S. Bellovin, "Defending against Sequence
              Number Attacks", RFC 6528, February 2012.

   [RFC6582]  Henderson, T., Floyd, S., Gurtov, A., and Y. Nishida, "The
              NewReno Modification to TCP's Fast Recovery Algorithm",
              RFC 6582, April 2012.

   [RFC6633]  Gont, F., "Deprecation of ICMP Source Quench Messages",
              RFC 6633, May 2012.

   [RFC6675]  Blanton, E., Allman, M., Wang, L., Jarvinen, I., Kojo, M.,
              and Y. Nishida, "A Conservative Loss Recovery Algorithm
              Based on Selective Acknowledgment (SACK) for TCP",
              RFC 6675, August 2012.

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   [RFC6691]  Borman, D., "TCP Options and Maximum Segment Size (MSS)",
              RFC 6691, July 2012.

   [RFC6824]  Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
              "TCP Extensions for Multipath Operation with Multiple
              Addresses", RFC 6824, January 2013.

   [RFC6846]  Pelletier, G., Sandlund, K., Jonsson, L-E., and M. West,
              "RObust Header Compression (ROHC): A Profile for TCP/IP
              (ROHC-TCP)", RFC 6846, January 2013.

   [RFC6897]  Scharf, M. and A. Ford, "Multipath TCP (MPTCP) Application
              Interface Considerations", RFC 6897, March 2013.

   [RFC6928]  Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
              "Increasing TCP's Initial Window", RFC 6928, April 2013.

   [RFC6937]  Mathis, M., Dukkipati, N., and Y. Cheng, "Proportional
              Rate Reduction for TCP", RFC 6937, May 2013.

   [RFC6994]  Touch, J., "Shared Use of Experimental TCP Options",
              RFC 6994, August 2013.

12.2.  Informative References

   [CK73]     Cerf, V. and R. Kahn, "Towards Protocols for Internetwork
              Communication", IFIP/TC6.1, NIC 18764, INWG 39,
              September 1973.

   [Errata]   "RFC Editor - RFC Errata",

              Leith, D., "H-TCP: TCP Congestion Control for High
              Bandwidth-Delay Product Paths", draft-leith-tcp-htcp-06
              (work in progress), April 2008.

              Rhee, I., Xu, L., and S. Ha, "CUBIC for Fast Long-Distance
              Networks", draft-rhee-tcpm-cubic-02 (work in progress),
              August 2008.

              Sridharan, M., Tan, K., Bansal, D., and D. Thaler,
              "Compound TCP: A New TCP Congestion Control for High-Speed
              and Long Distance  Networks", draft-sridharan-tcpm-ctcp-02
              (work in progress), November 2008.

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   [JK92]     Jacobson, V. and M. Karels, "Congestion Avoidance and
              Control", This paper is a revised version of [Jac88], that
              includes an additional appendix. This paper has not been
              traditionally published, but is currently available at
              ftp://ftp.ee.lbl.gov/papers/congavoid.ps.Z. 1992.

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

   [KP87]     Karn, P. and C. Partridge, "Round Trip Time Estimation",
              ACM SIGCOMM 1987 Proceedings, in ACM Computer
              Communication Review, 17 (5), pp. 2-7, August 1987.

   [MAF04]    Medina, A., Allman, M., and S. Floyd, "Measuring the
              Evolution of Transport Protocols in the Internet", ACM
              Computer Communication Review, 35 (2), April 2005.

   [MM96]     Mathis, M. and J. Mahdavi, "Forward Acknowledgement:
              Refining TCP Congestion Control", ACM SIGCOMM 1996
              Proceedings, in ACM Computer Communication Review 26 (4),
              pp. 281-292, October 1996.

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

   [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, October 1996.

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              December 1998.

   [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.

   [SCWA99]   Savage, S., Cardwell, N., Wetherall, D., and T. Anderson,
              "TCP Congestion Control with a Misbehaving Receiver", ACM
              Computer Communication Review, 29 (5), pp. 71-78,
              October 1999.

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Authors' Addresses

   Martin Duke
   F5 Networks
   401 Elliott Ave W
   Seattle, WA  98119

   Phone: 206-272-7537
   Email: m.duke@f5.com

   Robert Braden
   USC Information Sciences Institute
   Marina del Rey, CA  90292-6695

   Phone: 310-448-9173
   Email: braden@isi.edu

   Wesley M. Eddy
   MTI Systems
   MS 500-ASRC; 21000 Brookpark Rd
   Cleveland, OH  44135

   Phone: 216-433-6682
   Email: wes@mti-systems.com

   Ethan Blanton

   Email: elb@psg.com

   Alexander Zimmermann
   NetApp, Inc.
   Sonnenallee 1
   Kirchheim  85551

   Phone: +49 89 900594712
   Email: alexander.zimmermann@netapp.com

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