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RFC 7414

Informational
Pages: 57
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A Roadmap for Transmission Control Protocol (TCP) Specification Documents

Part 1 of 3, p. 1 to 17
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Obsoletes:    4614
Updated by:    7805


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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.

Page 2 
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.

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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

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1.  Introduction

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

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

   This document is not an update of RFC 1122 [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)

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   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.

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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.

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   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.

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      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.

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   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

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      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

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      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

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      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.

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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

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      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."

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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"



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