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rfc:rfc7414

Internet Engineering Task Force (IETF) M. Duke Request for Comments: 7414 F5 Obsoletes: 4614 R. Braden Category: Informational ISI ISSN: 2070-1721 W. Eddy

                                                           MTI Systems
                                                            E. Blanton
                                                    Interrupt Sciences
                                                         A. Zimmermann
                                                          NetApp, Inc.
                                                         February 2015
         A Roadmap for Transmission Control Protocol (TCP)
                      Specification Documents

Abstract

 This document contains a roadmap to the Request 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.
 This document obsoletes RFC 4614.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Not all documents
 approved by the IESG are a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7414.

Duke, et al. Informational [Page 1] RFC 7414 TCP Roadmap February 2015

Copyright Notice

 Copyright (c) 2015 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  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.

Duke, et al. Informational [Page 2] RFC 7414 TCP Roadmap February 2015

Table of Contents

 1. Introduction ....................................................4
 2. Core Functionality ..............................................6
 3. Strongly Encouraged Enhancements ................................8
    3.1. Fundamental Changes ........................................9
    3.2. Congestion Control Extensions .............................10
    3.3. Loss Recovery Extensions ..................................11
    3.4. Detection and Prevention of Spurious Retransmissions ......13
    3.5. Path MTU Discovery ........................................14
    3.6. Header Compression ........................................15
    3.7. Defending Spoofing and Flooding Attacks ...................15
 4. Experimental Extensions ........................................17
    4.1. Architectural Guidelines ..................................18
    4.2. Fundamental Changes .......................................18
    4.3. Congestion Control Extensions .............................19
    4.4. Loss Recovery Extensions ..................................20
    4.5. Detection and Prevention of Spurious Retransmissions ......21
    4.6. TCP Timeouts ..............................................22
    4.7. Multipath TCP .............................................22
 5. TCP Parameters at IANA .........................................23
 6. Historic and Undeployed Extensions .............................24
 7. Support Documents ..............................................27
    7.1. Foundational Works ........................................27
    7.2. Architectural Guidelines ..................................29
    7.3. Difficult Network Environments ............................30
    7.4. Guidance for Developing, Analyzing, and Evaluating TCP ....33
    7.5. Implementation Advice .....................................34
    7.6. Tools and Tutorials .......................................36
    7.7. MIB Modules ...............................................37
    7.8. Case Studies ..............................................39
 8. Undocumented TCP Features ......................................40
 9. Security Considerations ........................................41
 10. References ....................................................42
    10.1. Normative References .....................................42
    10.2. Informative References ...................................53
 Acknowledgments ...................................................56
 Authors' Addresses ................................................57

Duke, et al. Informational [Page 3] RFC 7414 TCP Roadmap February 2015

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 [RFC1122] 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
 decisions.
 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
 categories):
 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)

Duke, et al. Informational [Page 4] RFC 7414 TCP Roadmap February 2015

 Note that the category of an RFC does not necessarily reflect its
 current relevance.  For instance, RFC 5681 [RFC5681] is considered
 part of the required core functionality of TCP, although the RFC 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 (at the time of this writing), this fact is
 indicated by the term "(Errata)" in the headline of the RFC's
 description.  The contents of the errata can be found through the RFC
 Errata 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 nonessential, 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
 [RFC2119]), 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 Track RFCs themselves to evaluate specific
 requirement levels.
 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 are
 listed in Section 8.
 Within each section, RFCs are listed in the chronological order of
 their publication dates.

Duke, et al. Informational [Page 5] RFC 7414 TCP Roadmap February 2015

2. Core Functionality

 A small number of documents compose the core specification of TCP.
 These define the required core functionalities of TCP's header
 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)
            (Errata)
    This is the fundamental TCP specification document [RFC793].
    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 (discussed later in Section 2 below) 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 series.
 RFC 1122 S: "Requirements for Internet Hosts - Communication Layers"
             (October 1989)
    This document [RFC1122] updates and clarifies RFC 793 (see above
    in Section 2), 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.

Duke, et al. Informational [Page 6] RFC 7414 TCP Roadmap February 2015

 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 (see Section 3.1 of this
    document) describes TCP changes required to support IPv6
    jumbograms.
 RFC 2873 S: "TCP Processing of the IPv4 Precedence Field" (June 2000)
             (Errata)
    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 (see above in Section 2) and Differentiated
    Services [RFC2474].
 RFC 5681 S: "TCP Congestion Control" (August 2009)
    Although RFC 793 (see above in Section 2) did not contain any
    congestion control mechanisms, today congestion control is a
    required component of TCP implementations.  This document
    [RFC5681] defines congestion avoidance and control mechanism for
    TCP, based on Van Jacobson's 1988 SIGCOMM paper [Jac88].
    A number of behaviors that together constitute what the community
    refers to as "Reno TCP" is 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 5681 details the currently accepted congestion control
    mechanism, while RFC 1122, (see above in Section 2) mandates that
    such a congestion control mechanism must be implemented.  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.

Duke, et al. Informational [Page 7] RFC 7414 TCP Roadmap February 2015

    RFCs 2001 and 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] (see Section 3.2
         of this document).
    (b)  During slow start and congestion avoidance, the usage of
         Appropriate Byte Counting [RFC3465] (see Section 3.2 of this
         document) is explicitly recommended.
    (c)  The use of Limited Transmit [RFC3042] (see Section 3.3 of
         this document) 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
    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 of RFC 6298 [RFC6298]: "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 4.2.3.1 of RFC 1122 and upgrades the requirement of
    supporting the algorithm from a SHOULD to a MUST."  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
    use.

3. Strongly Encouraged Enhancements

 This section describes recommended TCP modifications that improve
 performance and security.  Section 3.1 represents fundamental changes
 to the protocol.  Sections 3.2 and 3.3 list improvements over the
 congestion control and loss recovery mechanisms as specified in RFC
 5681 (see Section 2).  Section 3.4 describes algorithms that allow a
 TCP sender to detect whether it has entered loss recovery spuriously.

Duke, et al. Informational [Page 8] RFC 7414 TCP Roadmap February 2015

 Section 3.5 comprises Path MTU Discovery mechanisms.  Schemes for
 TCP/IP header compression are listed in Section 3.6.  Finally,
 Section 3.7 deals with the problem of preventing acceptance of forged
 segments and flooding attacks.

3.1. Fundamental Changes

 RFCs 2675 and 7323 represent fundamental changes to TCP by redefining
 how parts of the basic TCP header and options are interpreted.  RFC
 7323 defines the Window Scale option, which reinterprets the
 advertised receive window.  RFC 2675 specifies that MSS option and
 urgent pointer fields with a value of 65,535 are to be treated
 specially.
 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
    Internet.
    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.
 RFC 7323 S: "TCP Extensions for High Performance" (September 2014)
    This document [RFC7323] 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.  The predecessor of this document, RFC 1323, was
    published in 1992, and is deployed in most TCP implementations.
    This document includes fixes and clarifications based on the
    gained deployment experience.  One specific issued addressed in

Duke, et al. Informational [Page 9] RFC 7414 TCP Roadmap February 2015

    this specification is a recommendation how to modify the algorithm
    for estimating the mean RTT when timestamps are used.  RFCs 1072,
    1185, and 1323 are the conceptual precursors of RFC 7323.

3.2. Congestion Control Extensions

 Two of the most important aspects of TCP are its congestion control
 and loss recovery features.  TCP 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 these two subsections, we group enhancements to
 TCP's congestion control, while the next subsection focus on TCP's
 loss recovery.
 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
    (see Section 2 of this document) to define two previously unused
    flag bits in the TCP header for ECN support.  RFC 3540 (see
    Section 4.3 of this document) provides a supplementary
    (experimental) means for more secure use of ECN, and RFC 2884 (see
    Section 7.8 of this document) provides some sample results from
    using ECN.
 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 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.  This change improves the performance of
    TCP in situations where there is no one-to-one relationship
    between data segments and acknowledgments (e.g., delayed ACKs or
    ACK loss) and closes a security hole TCP receivers can use to

Duke, et al. Informational [Page 10] RFC 7414 TCP Roadmap February 2015

    induce the sender into increasing the sending rate too rapidly
    (ACK-division [SCWA99] [RFC3449]).  ABC is recommended by RFC 5681
    (see Section 2 of this document).
 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 recommends against the
    implementation of [RFC1016].

3.3. Loss Recovery Extensions

 For the typical implementation of the TCP fast recovery algorithm
 described in RFC 5681 (see Section 2 of this document), 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 algorithms.  In
 particular, some of them allow 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)
             (Errata)
    When more than one packet is lost during one RTT, TCP may
    experience poor performance since a TCP sender can only learn
    about a single lost packet per RTT 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 of RFC 3042 [RFC3042]: "This document proposes 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."  This algorithm described
    in RFC 3042 is called "Limited Transmit".  Tests from 2004 showed

Duke, et al. Informational [Page 11] RFC 7414 TCP Roadmap February 2015

    that Limited Transmit was deployed in roughly one third of the web
    servers tested [MAF04].  Limited Transmit is recommended by RFC
    5681 (see Section 2 of this document).
 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.
    RFCs 2582 and 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.
 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] (see above in
    Section 3.3).  The algorithm conforms to the spirit of the
    congestion control specification in RFC 5681 (see Section 2 of
    this document), 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 Sender Maximum
         Segment Size (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.

Duke, et al. Informational [Page 12] RFC 7414 TCP Roadmap February 2015

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, but they respond differently with
 regard to their manners of recovering performance.  The IETF defined
 multiple algorithms because there are trade-offs in whether or not
 certain TCP options need to be implemented and concerns about IPR
 status.  The Standards Track RFCs in this section are closely related
 to the Experimental RFCs in Section 4.5 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 (see Section 3.3 of this
    document).  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.  A TCP sender could then use this information to detect
    spurious retransmissions (see [RFC3708]).
 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 (see Section 4.5), the algorithm in RFC 3708 (see Section 4.5
    of this document), or F-RTO in RFC 5682 (see below in
    Section 3.4).
    Abstract of RFC 4015 [RFC4015]: "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
    (depending on the detection algorithm) can avoid 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., RFCs 3522

Duke, et al. Informational [Page 13] RFC 7414 TCP Roadmap February 2015

    and 3708, both listed in Section 4.5 of this document), 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. 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 byte stream 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 to determine that the
 MTU on some part of the path has been exceeded.
 RFC 1191 S: "Path MTU Discovery" (November 1990)
    Abstract of RFC 1191 [RFC1191]: "This memo describes a technique
    for dynamically discovering the maximum transmission unit (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."
 RFC 1981 S: "Path MTU Discovery for IP version 6" (August 1996)
    Abstract of RFC 1981 [RFC1981]: "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."
 RFC 4821 S: "Packetization Layer Path MTU Discovery" (March 2007)
    Abstract of RFC 4821 [RFC4821]: "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."

Duke, et al. Informational [Page 14] RFC 7414 TCP Roadmap February 2015

3.6. Header Compression

 Especially in streaming applications, the overhead of TCP/IP headers
 could correspond to more than 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.
 RFC 6846 S: "RObust Header Compression (ROHC): A Profile for TCP/IP
             (ROHC-TCP)" (January 2013)
    From the Abstract of RFC 6846 [RFC6846]: "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."
    RFC 6846 is the successor of RFC 4996.  It fixes a technical issue
    with the SACK compression and clarifies other compression methods
    used.

3.7. Defending Spoofing and Flooding Attacks

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

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 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 (send 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 nonstandard 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
             2007)
    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.
 RFC 5925 S: "The TCP Authentication Option" (June 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] (see
    below in Section 3.7).
 RFC 5926 S: "Cryptographic Algorithms for the TCP Authentication
             Option (TCP-AO)" (June 2010)
    This document [RFC5926] specifies the algorithms and attributes
    that can be used in TCP Authentication Option's (TCP-AO) [RFC5925]
    (see above in Section 3.7) current manual keying mechanism and
    provides the interface for future message authentication codes
    (MACs).

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 RFC 5927 I: "ICMP Attacks against TCP" (July 2010)
    Abstract of RFC 5927 [RFC5927]: "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."
 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
             2012)
    Abstract of RFC 6528 [RFC6528]: "This document 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"

4. Experimental Extensions

 The RFCs in this section are either Experimental and may become
 Proposed Standards in the future or are Proposed Standards (or
 Informational), but can be considered experimental due to lack of
 wide deployment.  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.
 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 follow [RFC6994] (see
 Section 5 of this document), 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

Duke, et al. Informational [Page 17] RFC 7414 TCP Roadmap February 2015

 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] is a related proposal to RFC 2140 (see
    above in Section 4.1).  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
    use or implementation beyond experimental stacks, so it is not
    listed among the standard TCP enhancements in this roadmap.

4.2. Fundamental Changes

 Like the Standards Track documents listed in Section 3.1, there also
 exist new Experimental RFCs that specify fundamental changes to TCP.
 At the time of writing, the only example so far is TCP Fast Open that
 deviates from the standard TCP semantics of [RFC793].
 RFC 7413 E: "TCP Fast Open" (December 2014)
    This document [RFC7413] describes TCP Fast Open that allows data
    to be carried in the SYN and SYN-ACK packets and consumed by the
    receiver during the initial connection handshake.  It saves up to
    one RTT compared to the standard TCP, which requires a three-way
    handshake to complete before data can be exchanged.

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4.3. Congestion Control Extensions

 TCP congestion control has been an extremely active research area for
 many years (see RFC 5783 discussed in Section 7.6 of this document),
 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 algorithms of RFC
 5681 (see Section 2 of this document).  In these subsections,
 enhancements to TCP's congestion control are listed.  The next
 subsection focuses 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 (see Section 2 of this
    document), 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
             2003)
    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.

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 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
    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] (see Section 3.2 of this document) 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 dropped.
 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
    (see Section 3.2 of this document), to 10 segments with a fallback
    to the existing recommendation when performance issues are
    detected.

4.4. Loss Recovery Extensions

 RFC 5827 E: "Early Retransmit for TCP and Stream Control Transmission
             Protocol (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
    timeout.

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 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
    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.

4.5. 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
 documented.
 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
    ambiguity.
 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."
 RFC 4653 E: "Improving the Robustness of TCP to Non-Congestion
             Events" (August 2006)
    In the presence of non-congestion events, such as packet
    reordering, an out-of-order segment does not necessarily indicate
    a lost segment and congestion.  This document [RFC4653] proposes

Duke, et al. Informational [Page 21] RFC 7414 TCP Roadmap February 2015

    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 reordering.

4.6. TCP Timeouts

 Besides the well-known retransmission timeout the TCP standard
 [RFC793] defines other timeouts.  This section lists documents that
 deal with TCP's various timeouts.
 RFC 5482 S: "TCP User Timeout Option" (March 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.

4.7. 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" (October 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.

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 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 code points.
 RFC 2780 B: "IANA Allocation Guidelines For Values In the Internet
             Protocol and Related Headers" (March 2000)
    Abstract of RFC 2780 [RFC2780]: "This memo provides guidance for
    the IANA to use in assigning parameters for fields in the IPv4,
    IPv6, ICMP, UDP and TCP protocol headers."
 RFC 4727 S: "Experimental Values in IPv4, IPv6, ICMPv4, ICMPv6, UDP,
             and TCP Headers" (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 (see below in Section 5).
 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 the Abstract of RFC 6335 [RFC6335]: "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."
 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.

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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 were reclassified by [RFC6247] to Historic status.
 RFC 721 U: "Out-of-Band Control Signals in a Host-to-Host Protocol"
             (September 1976): lack of interest
    RFC 721 [RFC721] 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
    specification.
 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.
 RFC 1106 H: "TCP Big Window and Nak Options" (June 1989): found
             defective
    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 (see below in Section 6) 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 (CCSDS).
 RFC 1110 H: "A Problem with the TCP Big Window Option" (August 1989):
             deprecates RFC 1106
    Abstract of RFC 1110 [RFC1110]: "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."

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 RFC 1146 H: "TCP Alternate Checksum Options" (March 1990): lack of
             interest
    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
    popular.
 RFC 1379 H: "Extending TCP for Transactions -- Concepts" (November
             1992): found defective
    See RFC 1644, in Section 6 below.
 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 three-way handshake, to support
    two-packet request/response exchanges.  RFC 1379 [RFC1379] (see
    above in Section 6) and RFC 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 (see Section 4.1 of this document), in the
    sharing of state across connections.

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 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 use
    specialized transport protocols other than TCP, such as PR-SCTP
    [RFC3758].
 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, this
    document [RFC1705] suggests that a new version of TCP (TCPng)
    needs to be developed and deployed.  It proposes that a globally
    unique address be assigned to the transport layer to uniquely
    identify an Internet host without specifying any routing
    information.  Later work on splitting locator and identifier
    values is summarized well in [RFC6115], but no resulting changes
    to TCP have occurred.
 RFC 6013 E: "TCP Cookie Transactions (TCPCT)" (January 2011): lack of
             interest
    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
    establishment.
    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 code points, which
         prohibits a large-scale deployment.

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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
 performance.

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 [RFC675] is a very early precursor of the
    fundamental RFC 793 (see Section 2 of this document), 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 (see Section 2 of
    this document).

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 RFC 761 U: "DoD Standard Transmission Control Protocol" (January
             1980)
    This document [RFC761] is the immediate precursor of RFC 793 (see
    Section 2 of this document).  The header format, the connection
    establishment (including the different connection states), and the
    overall API correspond mostly to the final Standard RFC 793 (see
    Section 2 of this document).
 RFC 813 U: "Window and Acknowledgement Strategy in TCP" (July 1982)
    This document [RFC813] 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 [RFC814].
 RFC 816 U: "Fault Isolation and Recovery" (July 1982)
    In this document [RFC816], 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 [RFC817] 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 of RFC 872 [RFC872]: "The sometimes-expressed fear that
    using TCP on a local net is a bad idea is unfounded."

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 RFC 896 U: "Congestion Control in IP/TCP Internetworks" (January
             1984)
    This document [RFC896] contains some early experiences with
    congestion collapse and some initial thoughts on how to avoid it
    using congestion control in TCP.  Furthermore, it defined an
    algorithm for efficient transmission of small packets that is
    today known as the Nagle algorithm.
 RFC 964 U: "Some Problems with the Specification of the Military
             Standard Transmission Control Protocol" (November 1985)
    This document [RFC964] points out several specification bugs in
    the US Military's MIL-STD-1778 document, which was intended as a
    successor to RFC 793 (see Section 2 of this document).  This
    serves to remind us of the difficulty in specification writing
    (even when we work from existing documents!).

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
    designs 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.  Later work on TCP has included several more aggressive
    mechanisms than Reno TCP includes, and RFC 5033 (see Section 7.4
    of this document) provides additional guidance on use of such
    algorithms.  The fundamental architectural discussion in RFC 2914
    remains valid, regarding the standards process role in defining
    protocol aspects that are critical to performance and avoiding
    congestion collapse scenarios.

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 RFC 3360 B: "Inappropriate TCP Resets Considered Harmful" (August
             2002)
    This document [RFC3360] is a plea that firewall vendors not send
    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 3439 I: "Some Internet Architectural Guidelines and Philosophy"
             (December 2002)
    This document [RFC3439] updates RFC 1958 (see above in
    Section 7.2) 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 impediment to efficient scaling.
 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 coexistence with routers that do not understand the
    defined alternate semantics.
 RFC 6182 I: "Architectural Guidelines for Multipath TCP Development"
             (March 2011)
    Abstract of RFC 6182 [RFC6182]: "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) (see Section 4.7 of this document).  This document lists
    certain high-level design decisions that provide foundations for
    the design of the MPTCP protocol, based upon these architectural
    requirements"

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.

Duke, et al. Informational [Page 30] RFC 7414 TCP Roadmap February 2015

 RFC 2488 B: "Enhancing TCP Over Satellite Channels using Standard
             Mechanisms" (January 1999)
    From the Abstract of RFC 2488 [RFC2488]: "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)."
 RFC 2757 I: "Long Thin Networks" (January 2000)
    Several methods of improving TCP performance over long thin
    networks (i.e., networks with low bandwidth and high delay), 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 RFCs 3150 and 3155 (see below in
    Section 7.3), and these documents should be preferred where there
    is overlap between them and RFC 2757 (see Section 7.3 of this
    document).
 RFC 2760 I: "Ongoing TCP Research Related to Satellites" (February
             2000)
    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 (see above in Section 7.3)
    looks at standard extensions, this document focuses on more
    experimental means of performance enhancement.
 RFC 3135 I: "Performance Enhancing Proxies Intended to Mitigate Link-
             Related Degradations" (June 2001)
    From the Abstract of RFC 3135 [RFC3135]: "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

Duke, et al. Informational [Page 31] RFC 7414 TCP Roadmap February 2015

    WAN, and wireless LAN environments.  Different types of
    Performance Enhancing Proxies are described as well as the
    mechanisms used to improve performance."
 RFC 3150 B: "End-to-end Performance Implications of Slow Links" (July
             2001)
    From the Abstract of RFC 3150 [RFC3150]: "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 the Abstract of RFC 3155 [RFC3155]: "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."
 RFC 3366 B: "Advice to link designers on link Automatic Repeat
             reQuest (ARQ)" (August 2002)
    From the Abstract of RFC 3366 [RFC3366]: "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."
 RFC 3449 B: "TCP Performance Implications of Network Path Asymmetry"
             (December 2002)
    From the Abstract of RFC 3449 [RFC3449]: "This document describes
    TCP performance problems 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.

Duke, et al. Informational [Page 32] RFC 7414 TCP Roadmap February 2015

    The document details several mitigations to these effects, which
    have either been proposed or evaluated in the literature, or are
    currently deployed in networks.
 RFC 3481 B: "TCP over Second (2.5G) and Third (3G) Generation
             Wireless Networks" (February 2003)
    From the Abstract of RFC 3481 [RFC3481]: "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."
 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.  RFC 3366 (see above in Section 7.3)
    specifically focuses on ARQ mechanisms, while RFC 3819 more widely
    covers additional aspects of the underlying layers

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 5033 B: "Specifying New Congestion Control Algorithms" (August
             2007)
    This document [RFC5033] considers the evaluation of suggested
    congestion control algorithms that differ from the principles
    outlined in RFC 2914 (see Section 7.2 of this document).  It is
    useful for authors of such algorithms 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 need to be
    considered when evaluating new or modified congestion control
    mechanisms for the Internet.  Among other topics, the document
    discusses throughput, delay, loss rates, response times, fairness,
    and robustness for challenging environments.

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 RFC 6077 I: "Open Research Issues in Internet Congestion Control"
             (February 2011)
    This document [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) (see Section 4.7 of this document).  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) TCP.
 RFC 6349 I: "Framework for TCP Throughput Testing" (August 2011)
    From the Abstract of RFC 6349 [RFC6349]: "This framework 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."

7.5. Implementation Advice

 RFC 794 U: "PRE-EMPTION" (September 1981)
    This document [RFC794] clarifies that operating systems need to
    manage their limited resources, which may include TCP connection
    state, and that these decisions can be made with application
    input, but they do not need to be part of the TCP protocol
    specification itself.
 RFC 879 U: "The TCP Maximum Segment Size and Related Topics"
             (November 1983)
    Abstract of RFC 879 [RFC879]: "This memo discusses the TCP Maximum
    Segment Size Option and related topics.  The purposes [sic] 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'."

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 RFC 1071 U: "Computing the Internet Checksum" (September 1988)
             (Errata)
    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 (see above in Section 7.5).
 RFC 1936 I: "Implementing the Internet Checksum in Hardware" (April
             1996)
    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 the Abstract of RFC 2525 [RFC2525]: "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."
 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 3493 I: "Basic Socket Interface Extensions for IPv6" (February
             2003)
    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
    languages.

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 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 (RFC 7323, see Section 3.1 of this document) 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 take
    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) (see Section 4.7 of this document) 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.

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.

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 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 had been published at the time.  The focus
    of the document is on end-host-based congestion control.

7.7. MIB Modules

 The first MIB module defined for use with Simple Network Management
 Protocol (SNMP) was a single monolithic MIB module, called MIB-I,
 defined in RFC 1156.  This evolved over time to the MIB-II
 specification in RFC 1213, which obsoletes RFC 1156.  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 2012.  RFC 2012 was obsoleted by RFC
 4022, which is the primary TCP MIB document at the time of writing.
 For current TCP implementers, RFC 4022 should be supported.
 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
    RFC for MIB-I.

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 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.  It is the immediate successor of RFC 1158,
    with minor modifications.  It obsoletes the MIB-I, defined in RFC
    1156 (see above in Section 7.7).
 RFC 2012 S: "SNMPv2 Management Information Base for the Transmission
             Control Protocol using SMIv2" (November 1996)
    In an update to RFC 1213 (see Section 7.7 of this document), this
    document [RFC2012] defines the TCP MIB by splitting out the TCP-
    specific portions.  It is now obsoleted by RFC 4022 (see below in
    Section 7.7).
 RFC 2452 S: "IP Version 6 Management Information Base for the
             Transmission Control Protocol" (December 1998)
    This document [RFC2452] augments RFC 2012 (see Section 7.7 of this
    document) by adding an IPv6-specific connection table.  The rest
    of RFC 2012 holds for any IP version.  RFC 2452 is now obsoleted
    by RFC 4022 (see below in Section 7.7).
    Although it is a Standards Track RFC, 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 RFCs 2012 and 2452 (see above in
    Section 7.7) and specifies the current standard for the TCP MIB
    that should be deployed.
 RFC 4898 S: "TCP Extended Statistics MIB" (May 2007)
    This document [RFC4898] describes extended performance statistics
    for TCP.  They are designed to use TCP's ideal vantage point to
    diagnose performance problems in both the network and the
    application.

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7.8. Case Studies

 RFC 700 U: "A Protocol Experiment" (August 1974)
    This document [RFC700] 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 [RFC889] is a status report about experiments
    concerning the TCP retransmission timeout calculation and also
    provides advice 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
 issues.
 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 (see
    [Jacobson] for the full message):
       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 a packet, 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 [RFC5681], 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 duplicate ACK 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 duplicate
    ACK threshold.  FACK is implemented in Linux and turned on per
    default.
 Congestion Control for High Rate Flows
    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 a
    few RFCs have been published on some of these modifications,
    including HighSpeed TCP [RFC3649], Limited Slow-Start [RFC3742],
    and Quick-Start [RFC4782] (see Section 4.3 of this document for
    more information on each), 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 [CUBIC] (the standard TCP congestion
    control algorithm in Linux), Compound TCP [CTCP], and H-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.

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10. References

10.1. Normative References

 [RFC675]   Cerf, V., Dalal, Y., and C. Sunshine, "Specification of
            Internet Transmission Control Program", RFC 675, December
            1974, <http://www.rfc-editor.org/info/rfc675>.
 [RFC700]   Mader, E., Plummer, W., and R. Tomlinson, "Protocol
            experiment", RFC 700, August 1974,
            <http://www.rfc-editor.org/info/rfc700>.
 [RFC721]   Garlick, L., "Out-of-Band Control Signals in a Host-to-
            Host Protocol", RFC 721, September 1976,
            <http://www.rfc-editor.org/info/rfc721>.
 [RFC761]   Postel, J., "DoD standard Transmission Control Protocol",
            RFC 761, January 1980,
            <http://www.rfc-editor.org/info/rfc761>.
 [RFC793]   Postel, J., "Transmission Control Protocol", STD 7, RFC
            793, September 1981,
            <http://www.rfc-editor.org/info/rfc793>.
 [RFC794]   Cerf, V., "Pre-emption", RFC 794, September 1981,
            <http://www.rfc-editor.org/info/rfc794>.
 [RFC813]   Clark, D., "Window and Acknowledgement Strategy in TCP",
            RFC 813, July 1982,
            <http://www.rfc-editor.org/info/rfc813>.
 [RFC814]   Clark, D., "Name, addresses, ports, and routes", RFC 814,
            July 1982, <http://www.rfc-editor.org/info/rfc814>.
 [RFC816]   Clark, D., "Fault isolation and recovery", RFC 816, July
            1982, <http://www.rfc-editor.org/info/rfc816>.
 [RFC817]   Clark, D., "Modularity and efficiency in protocol
            implementation", RFC 817, July 1982,
            <http://www.rfc-editor.org/info/rfc817>.
 [RFC872]   Padlipsky, M., "TCP-on-a-LAN", RFC 872, September 1982,
            <http://www.rfc-editor.org/info/rfc872>.
 [RFC879]   Postel, J., "TCP maximum segment size and related topics",
            RFC 879, November 1983,
            <http://www.rfc-editor.org/info/rfc879>.

Duke, et al. Informational [Page 42] RFC 7414 TCP Roadmap February 2015

 [RFC889]   Mills, D., "Internet delay experiments", RFC 889, December
            1983, <http://www.rfc-editor.org/info/rfc889>.
 [RFC896]   Nagle, J., "Congestion control in IP/TCP internetworks",
            RFC 896, January 1984,
            <http://www.rfc-editor.org/info/rfc896>.
 [RFC964]   Sidhu, D. and T. Blumer, "Some problems with the
            specification of the Military Standard Transmission
            Control Protocol", RFC 964, November 1985,
            <http://www.rfc-editor.org/info/rfc964>.
 [RFC1071]  Braden, R., Borman, D., Partridge, C., and W. Plummer,
            "Computing the Internet checksum", RFC 1071, September
            1988, <http://www.rfc-editor.org/info/rfc1071>.
 [RFC1078]  Lottor, M., "TCP port service Multiplexer (TCPMUX)", RFC
            1078, November 1988,
            <http://www.rfc-editor.org/info/rfc1078>.
 [RFC1106]  Fox, R., "TCP big window and NAK options", RFC 1106, June
            1989, <http://www.rfc-editor.org/info/rfc1106>.
 [RFC1110]  McKenzie, A., "Problem with the TCP big window option",
            RFC 1110, August 1989,
            <http://www.rfc-editor.org/info/rfc1110>.
 [RFC1122]  Braden, R., "Requirements for Internet Hosts -
            Communication Layers", STD 3, RFC 1122, October 1989,
            <http://www.rfc-editor.org/info/rfc1122>.
 [RFC1144]  Jacobson, V., "Compressing TCP/IP headers for low-speed
            serial links", RFC 1144, February 1990,
            <http://www.rfc-editor.org/info/rfc1144>.
 [RFC1146]  Zweig, J. and C. Partridge, "TCP alternate checksum
            options", RFC 1146, March 1990,
            <http://www.rfc-editor.org/info/rfc1146>.
 [RFC1156]  McCloghrie, K. and M. Rose, "Management Information Base
            for network management of TCP/IP-based internets", RFC
            1156, May 1990, <http://www.rfc-editor.org/info/rfc1156>.
 [RFC1180]  Socolofsky, T. and C. Kale, "TCP/IP tutorial", RFC 1180,
            January 1991, <http://www.rfc-editor.org/info/rfc1180>.
 [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
            November 1990, <http://www.rfc-editor.org/info/rfc1191>.

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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,
            <http://www.rfc-editor.org/info/rfc1213>.
 [RFC1263]  O'Malley, S. and L. Peterson, "TCP Extensions Considered
            Harmful", RFC 1263, October 1991,
            <http://www.rfc-editor.org/info/rfc1263>.
 [RFC1337]  Braden, B., "TIME-WAIT Assassination Hazards in TCP", RFC
            1337, May 1992, <http://www.rfc-editor.org/info/rfc1337>.
 [RFC1379]  Braden, B., "Extending TCP for Transactions -- Concepts",
            RFC 1379, November 1992,
            <http://www.rfc-editor.org/info/rfc1379>.
 [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, <http://www.rfc-editor.org/info/rfc1470>.
 [RFC1624]  Rijsinghani, A., "Computation of the Internet Checksum via
            Incremental Update", RFC 1624, May 1994,
            <http://www.rfc-editor.org/info/rfc1624>.
 [RFC1644]  Braden, B., "T/TCP -- TCP Extensions for Transactions
            Functional Specification", RFC 1644, July 1994,
            <http://www.rfc-editor.org/info/rfc1644>.
 [RFC1693]  Connolly, T., Amer, P., and P. Conrad, "An Extension to
            TCP : Partial Order Service", RFC 1693, November 1994,
            <http://www.rfc-editor.org/info/rfc1693>.
 [RFC1705]  Carlson, R. and D. Ficarella, "Six Virtual Inches to the
            Left: The Problem with IPng", RFC 1705, October 1994,
            <http://www.rfc-editor.org/info/rfc1705>.
 [RFC1936]  Touch, J. and B. Parham, "Implementing the Internet
            Checksum in Hardware", RFC 1936, April 1996,
            <http://www.rfc-editor.org/info/rfc1936>.
 [RFC1958]  Carpenter, B., "Architectural Principles of the Internet",
            RFC 1958, June 1996,
            <http://www.rfc-editor.org/info/rfc1958>.
 [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
            for IP version 6", RFC 1981, August 1996,
            <http://www.rfc-editor.org/info/rfc1981>.

Duke, et al. Informational [Page 44] RFC 7414 TCP Roadmap February 2015

 [RFC2012]  McCloghrie, K., "SNMPv2 Management Information Base for
            the Transmission Control Protocol using SMIv2", RFC 2012,
            November 1996, <http://www.rfc-editor.org/info/rfc2012>.
 [RFC2018]  Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
            Selective Acknowledgment Options", RFC 2018, October 1996,
            <http://www.rfc-editor.org/info/rfc2018>.
 [RFC2140]  Touch, J., "TCP Control Block Interdependence", RFC 2140,
            April 1997, <http://www.rfc-editor.org/info/rfc2140>.
 [RFC2398]  Parker, S. and C. Schmechel, "Some Testing Tools for TCP
            Implementors", RFC 2398, August 1998,
            <http://www.rfc-editor.org/info/rfc2398>.
 [RFC2415]  Poduri, K., "Simulation Studies of Increased Initial TCP
            Window Size", RFC 2415, September 1998,
            <http://www.rfc-editor.org/info/rfc2415>.
 [RFC2416]  Shepard, T. and C. Partridge, "When TCP Starts Up With
            Four Packets Into Only Three Buffers", RFC 2416, September
            1998, <http://www.rfc-editor.org/info/rfc2416>.
 [RFC2452]  Daniele, M., "IP Version 6 Management Information Base for
            the Transmission Control Protocol", RFC 2452, December
            1998, <http://www.rfc-editor.org/info/rfc2452>.
 [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
            (IPv6) Specification", RFC 2460, December 1998,
            <http://www.rfc-editor.org/info/rfc2460>.
 [RFC2488]  Allman, M., Glover, D., and L. Sanchez, "Enhancing TCP
            Over Satellite Channels using Standard Mechanisms", BCP
            28, RFC 2488, January 1999,
            <http://www.rfc-editor.org/info/rfc2488>.
 [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,
            <http://www.rfc-editor.org/info/rfc2525>.
 [RFC2675]  Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
            RFC 2675, August 1999,
            <http://www.rfc-editor.org/info/rfc2675>.
 [RFC2757]  Montenegro, G., Dawkins, S., Kojo, M., Magret, V., and N.
            Vaidya, "Long Thin Networks", RFC 2757, January 2000,
            <http://www.rfc-editor.org/info/rfc2757>.

Duke, et al. Informational [Page 45] RFC 7414 TCP Roadmap February 2015

 [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,
            <http://www.rfc-editor.org/info/rfc2760>.
 [RFC2780]  Bradner, S. and V. Paxson, "IANA Allocation Guidelines For
            Values In the Internet Protocol and Related Headers", BCP
            37, RFC 2780, March 2000,
            <http://www.rfc-editor.org/info/rfc2780>.
 [RFC2861]  Handley, M., Padhye, J., and S. Floyd, "TCP Congestion
            Window Validation", RFC 2861, June 2000,
            <http://www.rfc-editor.org/info/rfc2861>.
 [RFC2873]  Xiao, X., Hannan, A., Paxson, V., and E. Crabbe, "TCP
            Processing of the IPv4 Precedence Field", RFC 2873, June
            2000, <http://www.rfc-editor.org/info/rfc2873>.
 [RFC2883]  Floyd, S., Mahdavi, J., Mathis, M., and M. Podolsky, "An
            Extension to the Selective Acknowledgement (SACK) Option
            for TCP", RFC 2883, July 2000,
            <http://www.rfc-editor.org/info/rfc2883>.
 [RFC2884]  Hadi Salim, J. and U. Ahmed, "Performance Evaluation of
            Explicit Congestion Notification (ECN) in IP Networks",
            RFC 2884, July 2000,
            <http://www.rfc-editor.org/info/rfc2884>.
 [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41, RFC
            2914, September 2000,
            <http://www.rfc-editor.org/info/rfc2914>.
 [RFC2923]  Lahey, K., "TCP Problems with Path MTU Discovery", RFC
            2923, September 2000,
            <http://www.rfc-editor.org/info/rfc2923>.
 [RFC3042]  Allman, M., Balakrishnan, H., and S. Floyd, "Enhancing
            TCP's Loss Recovery Using Limited Transmit", RFC 3042,
            January 2001, <http://www.rfc-editor.org/info/rfc3042>.
 [RFC3124]  Balakrishnan, H. and S. Seshan, "The Congestion Manager",
            RFC 3124, June 2001,
            <http://www.rfc-editor.org/info/rfc3124>.

Duke, et al. Informational [Page 46] RFC 7414 TCP Roadmap February 2015

 [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,
            <http://www.rfc-editor.org/info/rfc3135>.
 [RFC3150]  Dawkins, S., Montenegro, G., Kojo, M., and V. Magret,
            "End-to-end Performance Implications of Slow Links", BCP
            48, RFC 3150, July 2001,
            <http://www.rfc-editor.org/info/rfc3150>.
 [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,
            <http://www.rfc-editor.org/info/rfc3155>.
 [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
            of Explicit Congestion Notification (ECN) to IP", RFC
            3168, September 2001,
            <http://www.rfc-editor.org/info/rfc3168>.
 [RFC3360]  Floyd, S., "Inappropriate TCP Resets Considered Harmful",
            BCP 60, RFC 3360, August 2002,
            <http://www.rfc-editor.org/info/rfc3360>.
 [RFC3366]  Fairhurst, G. and L. Wood, "Advice to link designers on
            link Automatic Repeat reQuest (ARQ)", BCP 62, RFC 3366,
            August 2002, <http://www.rfc-editor.org/info/rfc3366>.
 [RFC3390]  Allman, M., Floyd, S., and C. Partridge, "Increasing TCP's
            Initial Window", RFC 3390, October 2002,
            <http://www.rfc-editor.org/info/rfc3390>.
 [RFC3439]  Bush, R. and D. Meyer, "Some Internet Architectural
            Guidelines and Philosophy", RFC 3439, December 2002,
            <http://www.rfc-editor.org/info/rfc3439>.
 [RFC3449]  Balakrishnan, H., Padmanabhan, V., Fairhurst, G., and M.
            Sooriyabandara, "TCP Performance Implications of Network
            Path Asymmetry", BCP 69, RFC 3449, December 2002,
            <http://www.rfc-editor.org/info/rfc3449>.
 [RFC3465]  Allman, M., "TCP Congestion Control with Appropriate Byte
            Counting (ABC)", RFC 3465, February 2003,
            <http://www.rfc-editor.org/info/rfc3465>.

Duke, et al. Informational [Page 47] RFC 7414 TCP Roadmap February 2015

 [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, <http://www.rfc-editor.org/info/rfc3481>.
 [RFC3493]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
            Stevens, "Basic Socket Interface Extensions for IPv6", RFC
            3493, February 2003,
            <http://www.rfc-editor.org/info/rfc3493>.
 [RFC3522]  Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm
            for TCP", RFC 3522, April 2003,
            <http://www.rfc-editor.org/info/rfc3522>.
 [RFC3540]  Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
            Congestion Notification (ECN) Signaling with Nonces", RFC
            3540, June 2003, <http://www.rfc-editor.org/info/rfc3540>.
 [RFC3649]  Floyd, S., "HighSpeed TCP for Large Congestion Windows",
            RFC 3649, December 2003,
            <http://www.rfc-editor.org/info/rfc3649>.
 [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, <http://www.rfc-editor.org/info/rfc3708>.
 [RFC3742]  Floyd, S., "Limited Slow-Start for TCP with Large
            Congestion Windows", RFC 3742, March 2004,
            <http://www.rfc-editor.org/info/rfc3742>.
 [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,
            <http://www.rfc-editor.org/info/rfc3819>.
 [RFC4015]  Ludwig, R. and A. Gurtov, "The Eifel Response Algorithm
            for TCP", RFC 4015, February 2005,
            <http://www.rfc-editor.org/info/rfc4015>.
 [RFC4022]  Raghunarayan, R., "Management Information Base for the
            Transmission Control Protocol (TCP)", RFC 4022, March
            2005, <http://www.rfc-editor.org/info/rfc4022>.

Duke, et al. Informational [Page 48] RFC 7414 TCP Roadmap February 2015

 [RFC4653]  Bhandarkar, S., Reddy, A., Allman, M., and E. Blanton,
            "Improving the Robustness of TCP to Non-Congestion
            Events", RFC 4653, August 2006,
            <http://www.rfc-editor.org/info/rfc4653>.
 [RFC4727]  Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4,
            ICMPv6, UDP, and TCP Headers", RFC 4727, November 2006,
            <http://www.rfc-editor.org/info/rfc4727>.
 [RFC4774]  Floyd, S., "Specifying Alternate Semantics for the
            Explicit Congestion Notification (ECN) Field", BCP 124,
            RFC 4774, November 2006,
            <http://www.rfc-editor.org/info/rfc4774>.
 [RFC4782]  Floyd, S., Allman, M., Jain, A., and P. Sarolahti, "Quick-
            Start for TCP and IP", RFC 4782, January 2007,
            <http://www.rfc-editor.org/info/rfc4782>.
 [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
            Discovery", RFC 4821, March 2007,
            <http://www.rfc-editor.org/info/rfc4821>.
 [RFC4898]  Mathis, M., Heffner, J., and R. Raghunarayan, "TCP
            Extended Statistics MIB", RFC 4898, May 2007,
            <http://www.rfc-editor.org/info/rfc4898>.
 [RFC4953]  Touch, J., "Defending TCP Against Spoofing Attacks", RFC
            4953, July 2007, <http://www.rfc-editor.org/info/rfc4953>.
 [RFC4987]  Eddy, W., "TCP SYN Flooding Attacks and Common
            Mitigations", RFC 4987, August 2007,
            <http://www.rfc-editor.org/info/rfc4987>.
 [RFC5033]  Floyd, S. and M. Allman, "Specifying New Congestion
            Control Algorithms", BCP 133, RFC 5033, August 2007,
            <http://www.rfc-editor.org/info/rfc5033>.
 [RFC5166]  Floyd, S., "Metrics for the Evaluation of Congestion
            Control Mechanisms", RFC 5166, March 2008,
            <http://www.rfc-editor.org/info/rfc5166>.
 [RFC5461]  Gont, F., "TCP's Reaction to Soft Errors", RFC 5461,
            February 2009, <http://www.rfc-editor.org/info/rfc5461>.
 [RFC5482]  Eggert, L. and F. Gont, "TCP User Timeout Option", RFC
            5482, March 2009,
            <http://www.rfc-editor.org/info/rfc5482>.

Duke, et al. Informational [Page 49] RFC 7414 TCP Roadmap February 2015

 [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, <http://www.rfc-editor.org/info/rfc5562>.
 [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
            Control", RFC 5681, September 2009,
            <http://www.rfc-editor.org/info/rfc5681>.
 [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,
            September 2009, <http://www.rfc-editor.org/info/rfc5682>.
 [RFC5690]  Floyd, S., Arcia, A., Ros, D., and J. Iyengar, "Adding
            Acknowledgement Congestion Control to TCP", RFC 5690,
            February 2010, <http://www.rfc-editor.org/info/rfc5690>.
 [RFC5783]  Welzl, M. and W. Eddy, "Congestion Control in the RFC
            Series", RFC 5783, February 2010,
            <http://www.rfc-editor.org/info/rfc5783>.
 [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,
            <http://www.rfc-editor.org/info/rfc5827>.
 [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
            Authentication Option", RFC 5925, June 2010,
            <http://www.rfc-editor.org/info/rfc5925>.
 [RFC5926]  Lebovitz, G. and E. Rescorla, "Cryptographic Algorithms
            for the TCP Authentication Option (TCP-AO)", RFC 5926,
            June 2010, <http://www.rfc-editor.org/info/rfc5926>.
 [RFC5927]  Gont, F., "ICMP Attacks against TCP", RFC 5927, July 2010,
            <http://www.rfc-editor.org/info/rfc5927>.
 [RFC5961]  Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
            Robustness to Blind In-Window Attacks", RFC 5961, August
            2010, <http://www.rfc-editor.org/info/rfc5961>.
 [RFC6013]  Simpson, W., "TCP Cookie Transactions (TCPCT)", RFC 6013,
            January 2011, <http://www.rfc-editor.org/info/rfc6013>.
 [RFC6056]  Larsen, M. and F. Gont, "Recommendations for Transport-
            Protocol Port Randomization", BCP 156, RFC 6056, January
            2011, <http://www.rfc-editor.org/info/rfc6056>.

Duke, et al. Informational [Page 50] RFC 7414 TCP Roadmap February 2015

 [RFC6069]  Zimmermann, A. and A. Hannemann, "Making TCP More Robust
            to Long Connectivity Disruptions (TCP-LCD)", RFC 6069,
            December 2010, <http://www.rfc-editor.org/info/rfc6069>.
 [RFC6077]  Papadimitriou, D., Welzl, M., Scharf, M., and B. Briscoe,
            "Open Research Issues in Internet Congestion Control", RFC
            6077, February 2011,
            <http://www.rfc-editor.org/info/rfc6077>.
 [RFC6093]  Gont, F. and A. Yourtchenko, "On the Implementation of the
            TCP Urgent Mechanism", RFC 6093, January 2011,
            <http://www.rfc-editor.org/info/rfc6093>.
 [RFC6181]  Bagnulo, M., "Threat Analysis for TCP Extensions for
            Multipath Operation with Multiple Addresses", RFC 6181,
            March 2011, <http://www.rfc-editor.org/info/rfc6181>.
 [RFC6182]  Ford, A., Raiciu, C., Handley, M., Barre, S., and J.
            Iyengar, "Architectural Guidelines for Multipath TCP
            Development", RFC 6182, March 2011,
            <http://www.rfc-editor.org/info/rfc6182>.
 [RFC6191]  Gont, F., "Reducing the TIME-WAIT State Using TCP
            Timestamps", BCP 159, RFC 6191, April 2011,
            <http://www.rfc-editor.org/info/rfc6191>.
 [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, <http://www.rfc-editor.org/info/rfc6247>.
 [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
            "Computing TCP's Retransmission Timer", RFC 6298, June
            2011, <http://www.rfc-editor.org/info/rfc6298>.
 [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,
            <http://www.rfc-editor.org/info/rfc6335>.
 [RFC6349]  Constantine, B., Forget, G., Geib, R., and R. Schrage,
            "Framework for TCP Throughput Testing", RFC 6349, August
            2011, <http://www.rfc-editor.org/info/rfc6349>.

Duke, et al. Informational [Page 51] RFC 7414 TCP Roadmap February 2015

 [RFC6356]  Raiciu, C., Handley, M., and D. Wischik, "Coupled
            Congestion Control for Multipath Transport Protocols", RFC
            6356, October 2011,
            <http://www.rfc-editor.org/info/rfc6356>.
 [RFC6429]  Bashyam, M., Jethanandani, M., and A. Ramaiah, "TCP Sender
            Clarification for Persist Condition", RFC 6429, December
            2011, <http://www.rfc-editor.org/info/rfc6429>.
 [RFC6528]  Gont, F. and S. Bellovin, "Defending against Sequence
            Number Attacks", RFC 6528, February 2012,
            <http://www.rfc-editor.org/info/rfc6528>.
 [RFC6582]  Henderson, T., Floyd, S., Gurtov, A., and Y. Nishida, "The
            NewReno Modification to TCP's Fast Recovery Algorithm",
            RFC 6582, April 2012,
            <http://www.rfc-editor.org/info/rfc6582>.
 [RFC6633]  Gont, F., "Deprecation of ICMP Source Quench Messages",
            RFC 6633, May 2012,
            <http://www.rfc-editor.org/info/rfc6633>.
 [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,
            <http://www.rfc-editor.org/info/rfc6675>.
 [RFC6691]  Borman, D., "TCP Options and Maximum Segment Size (MSS)",
            RFC 6691, July 2012,
            <http://www.rfc-editor.org/info/rfc6691>.
 [RFC6824]  Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
            "TCP Extensions for Multipath Operation with Multiple
            Addresses", RFC 6824, January 2013,
            <http://www.rfc-editor.org/info/rfc6824>.
 [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,
            <http://www.rfc-editor.org/info/rfc6846>.
 [RFC6897]  Scharf, M. and A. Ford, "Multipath TCP (MPTCP) Application
            Interface Considerations", RFC 6897, March 2013,
            <http://www.rfc-editor.org/info/rfc6897>.

Duke, et al. Informational [Page 52] RFC 7414 TCP Roadmap February 2015

 [RFC6928]  Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
            "Increasing TCP's Initial Window", RFC 6928, April 2013,
            <http://www.rfc-editor.org/info/rfc6928>.
 [RFC6937]  Mathis, M., Dukkipati, N., and Y. Cheng, "Proportional
            Rate Reduction for TCP", RFC 6937, May 2013,
            <http://www.rfc-editor.org/info/rfc6937>.
 [RFC6994]  Touch, J., "Shared Use of Experimental TCP Options", RFC
            6994, August 2013,
            <http://www.rfc-editor.org/info/rfc6994>.
 [RFC7323]  Borman, D., Braden, B., Jacobson, V., and R.
            Scheffenegger, "TCP Extensions for High Performance", RFC
            7323, September 2014,
            <http://www.rfc-editor.org/info/rfc7323>.
 [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
            Fast Open", RFC 7413, December 2014,
            <http://www.rfc-editor.org/info/rfc7413>.

10.2. Informative References

 [CK73]     Cerf, V. and R. Kahn, "Towards Protocols for Internetwork
            Communication", IFIP/TC6.1, NIC 18764, INWG 39, September
            1973.
 [CTCP]     Sridharan, M., Tan, K., Bansal, D., and D. Thaler,
            "Compound TCP: A New TCP Congestion Control for High-Speed
            and Long Distance Networks", Work in Progress,
            draft-sridharan-tcpm-ctcp-02, November 2008.
 [CUBIC]    Rhee, I., Xu, L., and S. Ha, "CUBIC for Fast Long-Distance
            Networks", Work in Progress, draft-rhee-tcpm-cubic-02,
            August 2008.
 [Errata]   RFC Editor, "RFC Errata",
            <http://www.rfc-editor.org/errata.php>.
 [HTCP]     Leith, D., "H-TCP: TCP Congestion Control for High
            Bandwidth-Delay Product Paths", Work in Progress,
            draft-leith-tcp-htcp-06, April 2008.
 [JK92]     Jacobson, V. and M. Karels, "Congestion Avoidance and
            Control", November 1992,
            <ftp://ftp.ee.lbl.gov/papers/congavoid.ps.Z>.

Duke, et al. Informational [Page 53] RFC 7414 TCP Roadmap February 2015

 [Jac88]    Jacobson, V., "Congestion Avoidance and Control", ACM
            SIGCOMM 1988 Proceedings, in ACM Computer Communication
            Review, 18 (4), pp. 314-329, August 1988.
 [Jacobson] Jacobson, V., "TCP-IP Mailing List", Article 167 of
            comp.protocols.tcp-ip, March 1988,
            <ftp://ftp.ee.lbl.gov/email/vanj.88mar10.txt>.
 [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,
            <http://www.rfc-editor.org/info/rfc1016>.
 [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
            3", BCP 9, RFC 2026, October 1996,
            <http://www.rfc-editor.org/info/rfc2026>.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [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, <http://www.rfc-editor.org/info/rfc2474>.
 [RFC3758]  Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P.
            Conrad, "Stream Control Transmission Protocol (SCTP)
            Partial Reliability Extension", RFC 3758, May 2004,
            <http://www.rfc-editor.org/info/rfc3758>.
 [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
            Congestion Control Protocol (DCCP)", RFC 4340, March 2006,
            <http://www.rfc-editor.org/info/rfc4340>.

Duke, et al. Informational [Page 54] RFC 7414 TCP Roadmap February 2015

 [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,
            <http://www.rfc-editor.org/info/rfc4341>.
 [RFC6115]  Li, T., "Recommendation for a Routing Architecture", RFC
            6115, February 2011,
            <http://www.rfc-editor.org/info/rfc6115>.
 [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.

Duke, et al. Informational [Page 55] RFC 7414 TCP Roadmap February 2015

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, Gorry 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.

Duke, et al. Informational [Page 56] RFC 7414 TCP Roadmap February 2015

Authors' Addresses

 Martin Duke
 F5 Networks
 401 Elliott Ave W
 Seattle, WA  98119
 United States
 Phone: 206-272-7537
 EMail: m.duke@f5.com
 Robert Braden
 USC Information Sciences Institute
 Marina del Rey, CA  90292-6695
 United States
 Phone: 310-448-9173
 EMail: braden@isi.edu
 Wesley M. Eddy
 MTI Systems
 18013 Cleveland Parkway
 Suite 170
 Cleveland, OH  44135
 United States
 Phone: 216-433-6682
 EMail: wes@mti-systems.com
 Ethan Blanton
 Interrupt Sciences
 EMail: elb@interruptsciences.com
 Alexander Zimmermann
 NetApp, Inc.
 Sonnenallee 1
 Kirchheim  85551
 Germany
 Phone: +49 89 900594712
 EMail: alexander.zimmermann@netapp.com

Duke, et al. Informational [Page 57]

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