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Versions: 00 01 02 03 04 05 06 rfc4614                                  
Network Working Group                                            M. Duke
Internet-Draft                                      Boeing Phantom Works
Expires: July 21, 2005                                         R. Braden
                                      USC Information Sciences Institute
                                                                 W. Eddy
                                                    NASA GRC/Verizon FNS
                                                              E. Blanton
                                                       Purdue University
                                                        January 20, 2005

               A Roadmap for TCP Specification Documents

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as

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

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on July 21, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).


   This document contains a "roadmap" to the Requests for Comments (RFC)
   documents relating to the Internet's Transmission Control Protocol

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   (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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Basic Functionality  . . . . . . . . . . . . . . . . . . . . .  5
   3.  Standard Enhancements  . . . . . . . . . . . . . . . . . . . .  7
     3.1   Congestion Control and Loss Recovery Extensions  . . . . .  7
     3.2   SACK-based Loss Recovery and Congestion Control  . . . . .  9
     3.3   Dealing with Forged Segments . . . . . . . . . . . . . . .  9
   4.  Experimental Extensions  . . . . . . . . . . . . . . . . . . . 11
   5.  Historic Extensions  . . . . . . . . . . . . . . . . . . . . . 13
   6.  Support Documents  . . . . . . . . . . . . . . . . . . . . . . 15
     6.1   Foundational Works . . . . . . . . . . . . . . . . . . . . 15
     6.2   Difficult Network Environments . . . . . . . . . . . . . . 16
     6.3   Implementation Advice  . . . . . . . . . . . . . . . . . . 18
     6.4   Management Information Bases . . . . . . . . . . . . . . . 19
     6.5   Tools and Tutorials  . . . . . . . . . . . . . . . . . . . 20
     6.6   Case Studies . . . . . . . . . . . . . . . . . . . . . . . 21
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 23
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 24
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
     9.1   Basic Functionality  . . . . . . . . . . . . . . . . . . . 25
     9.2   Standard Enhancements  . . . . . . . . . . . . . . . . . . 25
     9.3   Experimental Extensions  . . . . . . . . . . . . . . . . . 26
     9.4   Historic Extensions  . . . . . . . . . . . . . . . . . . . 27
     9.5   Support Documents  . . . . . . . . . . . . . . . . . . . . 27
     9.6   Informative References Outside the RFC Series  . . . . . . 30
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 30
       Intellectual Property and Copyright Statements . . . . . . . . 32

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1.  Introduction

   A correct and efficient implementation of the Transmission Control
   Protocol (TCP) [RFC0793] 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 more 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 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 each RFC number indicates its category
   in the RFC series:

      S - Standards Track (Proposed Standard, Draft Standard, or
      E - Experimental
      B - Best Current Practice
      I - Informational

   Note that the category of an RFC does not necessarily reflect its
   current relevance.  For instance, RFC 2581 is nearly universally
   deployed although it is only a Proposed Standard.  Similarly, some
   Informational RFCs contain significant technical proposals for
   changing TCP.

   This roadmap is divided into four 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 not essential, behaviors are listed in
   Section 3.  Experimental extensions which are not yet standard
   practices, but potentially could be in the future, are described in
   Section 4.

   The reader probably notices that these three sections are broadly
   equivalent to MUST/SHOULD/MAY specifications, and while the authors
   support this intuition, this document is merely descriptive; it does
   not represent a binding standards track position.  An individual

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   implementor still needs to examine the standards documents themselves
   to evaluate specific requirement levels.

   A small number of older experimental extensions which have not caught
   on are noted in Section 5.  Many other supporting documents that are
   relevant to the development, implementation, and deployment of TCP
   are described in Section 6.  Within each section, RFCs are listed in
   chronological order.

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2.  Basic Functionality

   A small number of documents compose the core specification of TCP.
   These define the required basic 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)

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

      Section 3.6 of RFC 793, describing TCP's handling of the IP
      precedence and security compartment, is mostly irrelevant today.
      RFC 2873 changed the IP precedence handling, and the security
      compartment portion of the API is no longer implemented or used.
      In addition, RFC 793 did not describe any congestion control
      mechanism.  Otherwise, however, the majority of this document
      still acurately describes modern TCPs.  RFC 793 is the last of a
      series of developmental TCP specifications, starting from 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, fixing some
      specification bugs and oversights.  It also explains some features
      such as keep-alives and Karn's and Jacobson's RTO estimation
      algorithms [Karn][VJ88].  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.

   RFC 2147 S: "TCP and UDP over IPv6 Jumbograms" (May 1997)

      IPv6's support for longer datagrams than were allowed in IPv4,
      necessitated some changes to the way that TCP's MSS and Urgent
      fields (both 16 bits) are treated.

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

      This document [RFC2460] makes a slight update to the way the

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      pseudo-header for checksum computation is derived, defining the
      process for IPv6 in addition to the previous practice for IPv4.

   RFC 2581 S: "TCP Congestion Control" (April 1999)

      Although RFC 793 did not contain any congestion control
      mechanisms, today congestion control is a required component of
      TCP implementations.  This document [RFC2581] defines the current
      versions of Van Jacobson's congestion avoidance and control
      mechanisms for TCP, based on his 1988 SIGCOMM paper [VJ88].  RFC
      2001 was a conceptual precursor that was obsoleted by RFC 2581.

      A number of behaviors that together comprise what the community
      refers to as "Reno TCP", are described in RFC 2581.  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 2873 S: "TCP Processing of the IPv4 Precendence Field" (June

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

   RFC 2988 S: "Computing TCP's Retransmission Timer" (November 2000)

      Abstract: "This document defines the standard algorithm that
      Transmission Control Protocol (TCP) senders are required to use to
      compute and manage their retransmission timer.  It expands on the
      discussion in section of RFC 1122 and upgrades the
      requirement of supporting the algorithm from a SHOULD to a MUST."

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3.  Standard Enhancements

   This section describes recommended TCP modifications that improve
   performance and security.  RFCs 1323 and 3168 represent fundamental
   changes to the protocol.  RFC 1323, based on RFCs 1072 and 1185,
   allows better utilization of high bandwidth-delay product paths by
   providing some needed mechanisms for high-rate transfers.  RFC 3168
   describes a change to the Internet's architecture, where routers
   signal end-hosts of growing congestion levels, and can do so before
   packet losses are forced.  Section 3.1 lists improvements in the
   congestion control and loss recovery mechanisms specified in RFC
   2581.  Section 3.2 describes further refinements that make use of
   selective acknowledgements.  Section 3.3 deals with the problem of
   preventing forged segments.

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

      This document [RFC1323] defines TCP extensions for window scaling,
      timestamps, and protection against wrapped sequence numbers, for
      efficient and safe operation over paths with large bandwidth-delay
      products.  These extensions are commonly found in currently-used
      systems; however, they may require manual tuning and
      configuration.  Some "corner cases" in this specification are
      still under discussion.

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

      This document [RFC3168] defines a means of detecting congestion
      without resorting to packet loss.  Although congestion
      notification takes place at the IP level, ECN requires support at
      the transport level (e.g., in TCP) to echo the bits and adapt the
      sending rate.  This document updates RFC 793 to define two
      previously-unused flag bits in the TCP header for ECN support.
      RFC 3540 provides a supplementary (experimental) means for making
      ECN use more secure, and RFC 2884 provides some sample results
      from using ECN.

3.1  Congestion Control and Loss Recovery Extensions

   Two of the most important aspects of TCP are its congestion control
   and loss recovery features.  Since TCP traditionally (in the absence
   of ECN) uses losses to infer congestion, there is a rather intimate
   coupling between congestion control and loss recovery mechanisms.
   There are several extensions to both features, and more often than
   not, a particular extension applies to both.  In this sub-section, we
   group enhancements to either congestion control, loss recovery, or

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   both, which can be performed unilaterally - without negotiating
   support between endpoints.  In the next sub-section, we group the
   extensions which specify or rely on the SACK option, whose use must
   be negotiated bilaterally.  TCP implementations should include the
   enhancements from both sub-sections so that they can perform well
   without regard to the feature sets of other hosts they connect to.
   For example, if SACK use is not successfully negotiated, a TCP should
   use the NewReno behavior as a fall-back.

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

      Abstract: "This document proposes [Limited Transmit,] a new
      Transmission Control Protocol (TCP) mechanism that can be used to
      more effectively recover lost segments when a connection's
      congestion window is small, or when a large number of segments are
      lost in a single transmission window." [RFC3042]

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

      This document [RFC3390] updates RFC 2581 to permit an initial TCP
      window larger that one packet during in the slow-start phase.

   RFC 3782 S: "The NewReno Modification to TCP's Fast Recovery
   Algorithm" (April 2004)

      This document [RFC3782] specifies a slight modification to the
      standard Reno fast recovery algorithm, whereby a TCP sender can
      use partial acknowledgements to make inferences determining the
      next segment to send in situations where SACK would be helpful,
      but isn't available.

   Work in progress: The Eifel Response Algorithm for TCP (Internet
   Draft name: draft-ietf-tsvwg-tcp-eifel-response)

      At the time of this writing, work on this document (from authors
      Reiner Ludwig and Andrei Gurtov) had stabilized within the
      Transport Area Working Group, and the document was planned to
      become a Proposed Standard, pending IESG review, but was not yet a
      part of the RFC series.  This document describes the response
      portion of the Eifel algorithm, which can be used in conjunction
      with one of several methods of detecting when loss recovery has
      been spuriously entered, such as the Eifel detection algorithm in
      RFC 3522, the algorithm in RFC 3708, or F-RTO.

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

3.2  SACK-based Loss Recovery and Congestion Control

   The base TCP specification in RFC 793 provided only a simple
   cumulative acknowledgment mechanism.  However, a selective
   acknowledgment (SACK) mechanism provides significant performance
   improvement in the presence of packet losses, more than outweighing
   the modest increase in complexity.  A TCP should be expected to
   implement SACK, however SACK is a negotiated option and is only used
   if support is advertised by both sides of a connection.

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

      This document [RFC2018] defines the basic selective
      acknowledgement (SACK) mechanism for TCP.

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

      This document [RFC2883] extends RFC 2018 to cover the case of
      acknowledging duplicate packets.

   RFC 3517 S: "A Conservative Selective Acknowledgement (SACK)-based
   Loss Recovery Algorithm for TCP" (April 2003)

      This document [RFC3517] describes a relatively sophisticated
      algorithm that a TCP sender can use for loss recovery when SACK
      reports more than one segment lost from a single flight of data.
      While support for the exchange of SACK information is widely
      implemented, not all implementations use an algorithm as
      sophisticated as that described in RFC 3517.

3.3  Dealing with Forged Segments

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

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   harder, or prevent it from being able to negatively impact a

   RFC 1948 I: "Defending Against Sequence Number Attacks" (May 1996)

      This document [RFC1948] describes the TCP vulnerability based upon
      guessing sequence numbers and as well as defenses against this
      exploit.  Some variation is implemented in most currently-used
      operating systems.

   RFC 2385 S: "Protection of BGP Sessions via the TCP MD5 Signature
   Option" (August 1998)

      From document: "This document describes currrent existing practice
      for securing BGP against certain simple attacks.  It is understood
      to have security weaknesses against concerted attacks.

      This memo describes a TCP extension to enhance security for BGP.
      It defines a new TCP option for carrying an MD5 [RFC1321] digest
      in a TCP segment.  This digest acts like a signature for that
      segment, incorporating information known only to the connection
      end points.  Since BGP uses TCP as its transport, using this
      option in the way described in this paper significantly reduces
      the danger from certain security attacks on BGP." [RFC2385]

      TCP MD5 options are currently only used in very limited contexts,
      primarily for defending BGP exchanges between routers.  Some
      deployment notes for those using TCP MD5 are found in the later
      RFC 3562, "Key Management Considerations for the TCP MD5 Signature
      Option" [RFC3562].

   Work in progress: Transmission Control Protocol Security
   Considerations (Internet Draft name: draft-ietf-tcpm-tcpsecure)

      At the time of this writing, the TCP Maintenance and Minor
      Extensions Working Group is producing a document (edited by Mitesh
      Dalal) which describes a challenge-response mechanism for securing
      TCP against spoofed control segments.  This document is expected
      to become an RFC in the near future.

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4.  Experimental Extensions

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

   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 has a destination cache.

      A related proposal, the Congestion Manager, is specified in RFC
      3124 [RFC3124].  The idea behind the Congestion Manager, moving
      congestion control outside of individual TCP connections,
      represents a modification to the core of TCP.  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.

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

      This document [RFC2861] suggests reducing the congestion window
      over time when no packets are flowing.

   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 rather than the number of
      acknowledgements received.  This has been implemented in Linux.
      The ABC mechanism behaves differently than the standard means when
      there is not a one-to-one relationship between data segments and
      acknowledgements.  ABC still operates within the accepted
      guidelines, but is more robust to delayed ACKs and ACK-division

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

      This document [RFC3522] suggests using timestamps to detect
      spurious timeouts.

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

      This document [RFC3540] suggests a modified ECN to address

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      security concerns, and updates RFC 3168.

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

      This document [RFC3649] suggests a modification to TCP's
      steady-state behavior to efficiently use very large windows.

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

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

   RFC 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 window.

   Work in progress: Forward RTO-Recovery (F-RTO): An Algorithm for
   Detecting Spurious Retransmission Timeouts with TCP and SCTP
   (Internet Draft name: draft-ietf-tcpm-frto)

      The F-RTO detection algorithm provides another option for
      inferring spurious retransmission timeouts.  At the time of this
      writing, the TCP Maintenance and Minor Extensions Working Group
      had completed a document describing F-RTO (by Pasi Sarolahti and
      Markku Kojo), and planned to make this an Experimental part of the
      RFC series, pending IESG review.

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5.  Historic Extensions

   The RFCs listed here define extensions that have thus far failed to
   arouse substantial interest, or were found to be defective.

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

      This RFC [RFC1106] defined an alternative to the Window Scale
      option for using large windows, and described the "negative
      acknowledgement" or NAK option.  There is a comparison of NAK and
      SACK methods, and early discussion of TCP over satellite issues.
      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, developed by the Consultive Committee for Space
      Data Systems (CCSDS).

   RFC 1110 "A Problem with the TCP Big Window Option" (August 1989)

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

   RFC 1146 E "TCP Alternate Checksum Options" (March 1990)

      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 "TCP Extensions Considered Harmful" (October 1991)

      This interesting 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.

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   RFC 1379 I "Extending TCP for Transactions -- Concepts" (November

      See RFC 1644.

   RFC 1644 E "T/TCP -- TCP Extensions for Transactions Functional
   Specification" (July 1994)

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

   RFC 1693 E "An Extension to TCP: Partial Order Service" (November

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

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6.  Support Documents

   This section contains several classes of documents that do not
   necessarily define current protocol behaviors, but are nevertheless
   of interest to TCP implementors.  Section 6.1 describes several
   foundational RFCs that give modern readers a better understanding of
   the principles underlying TCP's behaviors and development over the
   years.  The documents listed in Section 6.2 provide advice on using
   TCP in various types of network situations that pose challenges above
   those of typical wired links.  Some implementation notes can be found
   in Section 6.3.  The TCP Management Information Bases are described
   in Section 6.4.  RFCs that describe tools for testing and debugging
   TCP implementations or contain high-level tutorials on the protocol
   are listed Section 6.5, while Section 6.6 lists a number of case
   studies that have explored TCP performance.

6.1  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 here).

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

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

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

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

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

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

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

      This document [RFC0817] contains implementation suggestions that

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      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: "TCP-ON-A-LAN" (September 1982)

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

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

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

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

      This document [RFC0964] was prepared by the US Military to define
      TCP in greater detail than RFC 793.  A few serious specification
      bugs are detailed in RFC 964, reminding us of the difficulty in
      specification writing (even when working from existing

   RFC 1072: "TCP Extensions for Long-Delay Paths" (October 1988)

      This document [RFC1072] contains early explanations of the
      mechanisms that were later described by RFCs 1323 and 2018, which
      obsolete it.

   RFC 1185: "TCP Extension for High-Speed Paths" (October 1990)

      This document [RFC1185] builds on RFC 1072 to describe more
      advanced strategies for dealing with sequence number wrapping and
      detecting duplicates from earlier connections.  This document was
      obsoleted by RFC 1323.

   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.

6.2  Difficult Network Environments

   As the internetworking field has explored wireless, satellite,

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

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

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

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

      Several methods of improving TCP performance over long thin
      networks, such as geosynchronous satellite links, are discussed in
      this document [RFC2757].  A particular set of TCP options is
      developed that should work well in such environments, and be safe
      to use in the global Internet.

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

      This document [RFC2760] discusses the advantages and disadvantages
      of several different experimental means of improving TCP
      performance over long-delay or error-prone paths.  These include:
      T/TCP, larger initial windows, byte counting, delayed
      acknowledgements, slow start thresholds, NewReno and SACK-based
      loss recovery, FACK [FACK], 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.  While RFC 2488 looks at standard extensions, this
      document focuses on more experimental means of performance

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

      From abstract: "This document is a survey of Performance Enhancing
      Proxies (PEPs) often employed to improve degraded TCP performance
      caused by characteristics of specific link environments, for
      example, in satellite, wireless WAN, and wireless LAN
      environments.  Different types of Performance Enhancing Proxies

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      are described as well as the mechanisms used to improve
      performance."  [RFC3135]

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

      From abstract: "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.

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

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

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

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

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

6.3  Implementation Advice

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

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

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

      From abstract: "This memo catalogs a number of known TCP
      implementation problems.  The goal in doing so is to improve

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      conditions in the existing Internet by enhancing the quality of
      current TCP/IP implementations." [RFC2525]

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

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

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

      This document [RFC3360] is a plea that firewall vendors not send
      gratuitous TCP RST (Reset) packets when unassigned TCP header bits
      are used.  This practice prevents desirable extension and
      evolution of the protocol and hence is inimical to the future of
      the Internet.

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

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

6.4  Management Information Bases

   The first MIB module defined for use with SNMP (in RFC 1066 and its
   update, RFC 1156) was a single monolithic MIB module, called MIB-I.
   This evolved over time to be MIB-II (RFC 1213).  It then became
   apparent that having a single monolithic MIB module was not scalable,
   given the number and breadth of MIB data definitions that needed to
   be included.  Thus, additional MIB modules were defined, and those
   parts of MIB-II which needed to evolve were split off.  Eventually,
   the remaining parts of MIB-II were also split off, with the
   TCP-specific part being documented in RFC 2012.

   RFC 2012 is the primary document for MIB-II.  MIB-I, defined in RFC
   1156, has been obsoleted by the MIB-II specification in RFC 1213
   (updated by 2012).  Work is in progress, at the time of this writing,
   on a document that incorporates IPv6 and updates and obsoletes RFC
   2012 (currently in the form of draft-ietf-ipv6-rfc2012-update, edited
   by Rajiv Raghunarayan, under submission to the IESG as a Proposed

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

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

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

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

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

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

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

      This document [RFC2012] defines the TCP MIB, updating RFC 1213.

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

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

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

6.5  Tools and Tutorials

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

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

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

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

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

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

6.6  Case Studies

   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 in the TIME-WAIT state.  The main
      recemmendation is that hosts in TIME-WAIT ignore resets.

   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.

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   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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7.  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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8.  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.  We thank Joe Touch, Reiner Ludwig, and Pekka Savola for their
   contributions, in particular.  The chairs of the TCPM working group,
   Mark Allman and Ted Faber, have been instrumental in the development
   of this document.  Keith McCloghrie provided some useful notes and
   clarification on the various MIB-related RFCs.

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9.  References

9.1  Basic Functionality

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

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

   [RFC2147]  Borman, D., "TCP and UDP over IPv6 Jumbograms", RFC 2147,
              May 1997.

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

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

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

   [RFC2988]  Paxson, V. and M. Allman, "Computing TCP's Retransmission
              Timer", RFC 2988, November 2000.

9.2  Standard Enhancements

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

   [RFC1948]  Bellovin, S., "Defending Against Sequence Number Attacks",
              RFC 1948, May 1996.

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

   [RFC2385]  Heffernan, A., "Protection of BGP Sessions via the TCP MD5
              Signature Option", RFC 2385, August 1998.

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

   [RFC3042]  Allman, M., Balakrishnan, H. and S. Floyd, "Enhancing
              TCP's Loss Recovery Using Limited Transmit", RFC 3042,
              January 2001.

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

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

   [RFC3517]  Blanton, E., Allman, M., Fall, K. and L. Wang, "A
              Conservative Selective Acknowledgment (SACK)-based Loss
              Recovery Algorithm for TCP", RFC 3517, April 2003.

   [RFC3562]  Leech, M., "Key Management Considerations for the TCP MD5
              Signature Option", RFC 3562, July 2003.

   [RFC3782]  Floyd, S., Henderson, T. and A. Gurtov, "The NewReno
              Modification to TCP's Fast Recovery Algorithm", RFC 3782,
              April 2004.

9.3  Experimental Extensions

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

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

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

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

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

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

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

   [RFC3708]  Blanton, E. and M. Allman, "Using TCP Duplicate Selective
              Acknowledgement (DSACKs) and Stream Control Transmission
              Protocol (SCTP) Duplicate Transmission Sequence Numbers
              (TSNs) to Detect Spurious Retransmissions", RFC 3708,
              February 2004.

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

9.4  Historic Extensions

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

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

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

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

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

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

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

9.5  Support Documents

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

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

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

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

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

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

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

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

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

   [RFC1072]  Jacobson, V. and R. Braden, "TCP extensions for long-delay
              paths", RFC 1072, October 1988.

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

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

   [RFC1185]  Jacobson, V., Braden, B. and L. Zhang, "TCP Extension for
              High-Speed Paths", RFC 1185, October 1990.

   [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.

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

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

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

   [RFC2398]  Parker, S. and C. Schmechel, "Some Testing Tools for TCP
              Implementors", RFC 2398, August 1998.

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

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

   [RFC2452]  Daniele, M., "IP Version 6 Management Information Base for

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              the Transmission Control Protocol", RFC 2452, December

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

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

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

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

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

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

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

   [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.

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

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

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

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

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   [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.

9.6  Informative References Outside the RFC Series

   [FACK]    Mathis, M. and J. Mahdavi, "Forward Acknowledgement:
             Refining TCP Congestion Control", ACM SIGCOMM, August 1996.

   [Karn]    Karn, P. and C. Partridge, "Round Trip Time Estimation",
             ACM SIGCOMM, August 1987.

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

   [VJ88]    Jacobson, V., "Congestion Avoidance and Control", ACM
             SIGCOMM, August 1988.

Authors' Addresses

   Martin Duke
   Boeing Phantom Works
   PO Box 3707, MC 3W-51
   Seattle, WA  98124-2207

   Phone: 253-657-8203
   EMail: mduke26@comcast.net

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

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

   Wesley M. Eddy
   NASA GRC/Verizon FNS

   EMail: weddy@grc.nasa.gov

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   Ethan Blanton
   Purdue University

   EMail: eblanton@cs.purdue.edu

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Copyright Statement

   Copyright (C) The Internet Society (2005).  This document is subject
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   Funding for the RFC Editor function is currently provided by the
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