4.  Experimental Extensions

   The RFCs in this section are either Experimental and may become
   Proposed Standards in the future or are Proposed Standards (or
   Informational), but can be considered experimental due to lack of
   wide deployment.  At least part of the reason that they are still
   experimental is to gain more wide-scale experience with them before a
   standards track decision is made.

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

   By their publication as Experimental RFCs, it is hoped that the
   community of TCP researchers will analyze and test the contents of
   these RFCs.  Although experimentation is encouraged, there is not yet

   formal consensus that these are fully logical and safe behaviors.
   Wide-scale deployment of implementations that use these features
   should be well thought out in terms of consequences.

4.1.  Architectural Guidelines

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

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

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

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

      This document [RFC3124] is a related proposal to RFC 2140 (see
      above in Section 4.1).  The idea behind the Congestion Manager,
      moving congestion control outside of individual TCP connections,
      represents a modification to the core of TCP, which supports
      sharing information among TCP connections.  Although a Proposed
      Standard, some pieces of the Congestion Manager support
      architecture have not been specified yet, and it has not achieved
      use or implementation beyond experimental stacks, so it is not
      listed among the standard TCP enhancements in this roadmap.

4.2.  Fundamental Changes

   Like the Standards Track documents listed in Section 3.1, there also
   exist new Experimental RFCs that specify fundamental changes to TCP.
   At the time of writing, the only example so far is TCP Fast Open that
   deviates from the standard TCP semantics of [RFC793].

   RFC 7413 E: "TCP Fast Open" (December 2014)

      This document [RFC7413] describes TCP Fast Open that allows data
      to be carried in the SYN and SYN-ACK packets and consumed by the
      receiver during the initial connection handshake.  It saves up to
      one RTT compared to the standard TCP, which requires a three-way
      handshake to complete before data can be exchanged.

4.3.  Congestion Control Extensions

   TCP congestion control has been an extremely active research area for
   many years (see RFC 5783 discussed in Section 7.6 of this document),
   as it determines the performance of many applications that use TCP.
   A number of Experimental RFCs address issues with flow start up,
   overshoot, and steady-state behavior in the basic algorithms of RFC
   5681 (see Section 2 of this document).  In these subsections,
   enhancements to TCP's congestion control are listed.  The next
   subsection focuses on TCP's loss recovery.

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

      This document [RFC2861] suggests reducing the congestion window
      over time when no packets are flowing.  This behavior is more
      aggressive than that specified in RFC 5681 (see Section 2 of this
      document), which says that a TCP sender SHOULD set its congestion
      window to the initial window after an idle period of an RTO or
      greater.

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

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

   RFC 3649 E: "HighSpeed TCP for Large Congestion Windows" (December
               2003)

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

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

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

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

      This document [RFC4782] specifies the optional Quick-Start
      mechanism for TCP.  This mechanism allows connections to use
      higher sending rates at the beginning of the data transfer or
      after an idle period, provided that there is significant unused
      bandwidth along the path, and the sender and all of the routers
      along the path approve this higher rate.

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

      This document [RFC5562] describes an experimental modification to
      ECN [RFC3168] (see Section 3.2 of this document) for the use of
      ECN in TCP SYN/ACK packets.  This would allow to ECN-mark rather
      than drop the TCP SYN/ACK packet at an ECN-capable router, and to
      avoid the severe penalty of a retransmission timeout for a
      connection when the SYN/ACK packet is dropped.

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

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

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

      This document [RFC6928] proposes to increase the TCP initial
      window from between 2 and 4 segments, as specified in RFC 3390
      (see Section 3.2 of this document), to 10 segments with a fallback
      to the existing recommendation when performance issues are
      detected.

4.4.  Loss Recovery Extensions

   RFC 5827 E: "Early Retransmit for TCP and Stream Control Transmission
               Protocol (SCTP)" (April 2010)

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

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

      This document [RFC6069] describes how standard ICMP messages can
      be used to disambiguate true congestion loss from non-congestion
      loss caused by connectivity disruptions.  It proposes a reversion
      strategy of TCP's retransmission timer that enables a more prompt
      detection of whether or not the connectivity has been restored.

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

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

4.5.  Detection and Prevention of Spurious Retransmissions

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

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

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

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

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

   RFC 4653 E: "Improving the Robustness of TCP to Non-Congestion
               Events" (August 2006)

      In the presence of non-congestion events, such as packet
      reordering, an out-of-order segment does not necessarily indicate
      a lost segment and congestion.  This document [RFC4653] proposes

      to increase the threshold used to trigger a fast retransmission
      from the fixed value of three duplicate ACKs to about one
      congestion window of data in order to disambiguate true segment
      loss from segment reordering.

4.6.  TCP Timeouts

   Besides the well-known retransmission timeout the TCP standard
   [RFC793] defines other timeouts.  This section lists documents that
   deal with TCP's various timeouts.

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

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

4.7.  Multipath TCP

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

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

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

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

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

5.  TCP Parameters at IANA

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

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

      Abstract of RFC 2780 [RFC2780]: "This memo provides guidance for
      the IANA to use in assigning parameters for fields in the IPv4,
      IPv6, ICMP, UDP and TCP protocol headers."

   RFC 4727 S: "Experimental Values in IPv4, IPv6, ICMPv4, ICMPv6, UDP,
               and TCP Headers" (November 2006)

      This document [RFC4727] reserves both TCP options 253 and 254 for
      experimentation purposes.  When such experiments are deployed in
      the Internet, they should follow the additional requirements in
      RFC 6994 (see below in Section 5).

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

      From the Abstract of RFC 6335 [RFC6335]: "This document defines
      the procedures that the Internet Assigned Numbers Authority (IANA)
      uses when handling assignment and other requests related to the
      Service Name and Transport Protocol Port Number registry."

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

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

6.  Historic and Undeployed Extensions

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

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

      RFC 721 [RFC721] addresses the problem of implementing a reliable
      out-of-band signal (interrupts) for use in a host-to-host
      protocol.  The proposal was not included in the final TCP
      specification.

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

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

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

      This RFC [RFC1106] defined an alternative to the Window Scale
      option for using large windows and described the "negative
      acknowledgment" or NAK option.  There is a comparison of NAK and
      SACK methods and early discussion of TCP over satellite issues.
      RFC 1110 (see below in Section 6) explains some problems with the
      approaches described in RFC 1106.  The options described in this
      document have not been adopted by the larger community, although
      NAKs are used in the SCPS-TP adaptation of TCP for satellite and
      spacecraft use, developed by the Consultative Committee for Space
      Data Systems (CCSDS).

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

      Abstract of RFC 1110 [RFC1110]: "The TCP Big Window option
      discussed in RFC 1106 will not work properly in an Internet
      environment which has both a high bandwidth * delay product and
      the possibility of disordering and duplicating packets.  In such
      networks, the window size must not be increased without a similar
      increase in the sequence number space.  Therefore, a different
      approach to big windows should be taken in the Internet."

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

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

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

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

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

      See RFC 1644, in Section 6 below.

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

      The inventors of TCP believed that cached connection state could
      have been used to eliminate TCP's three-way handshake, to support
      two-packet request/response exchanges.  RFC 1379 [RFC1379] (see
      above in Section 6) and RFC 1644 [RFC1644] show that this is far
      from simple.  Furthermore, T/TCP floundered on the ease of denial-
      of-service attacks that can result.  One idea pioneered by T/TCP
      lives on in RFC 2140 (see Section 4.1 of this document), in the
      sharing of state across connections.

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

      This document [RFC1693] defines a TCP extension for applications
      that do not care about the order in which application-layer
      objects are received.  Examples are multimedia and database
      applications.  In practice, these applications either accept the
      possible performance loss because of TCP's strict ordering or use
      specialized transport protocols other than TCP, such as PR-SCTP
      [RFC3758].

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

      To overcome the exhaustion of the IP class B address space, this
      document [RFC1705] suggests that a new version of TCP (TCPng)
      needs to be developed and deployed.  It proposes that a globally
      unique address be assigned to the transport layer to uniquely
      identify an Internet host without specifying any routing
      information.  Later work on splitting locator and identifier
      values is summarized well in [RFC6115], but no resulting changes
      to TCP have occurred.

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

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

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

      Although RFC 6013 was published in 2011, the authors of this
      document places it in this section of the roadmap document due to
      two factors.

      (a)  The authors are not aware of any wide deployment and use of
           RFC 6013.
      (b)  RFC 6013 uses experimental TCP option code points, which
           prohibits a large-scale deployment.

7.  Support Documents

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

7.1.  Foundational Works

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

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

      This document [RFC675] is a very early precursor of the
      fundamental RFC 793 (see Section 2 of this document), which
      already contained the three-way handshake in its final form and
      the concept of sliding windows for reliable data transmission.
      Apart from that, the segment layout is totally different and the
      specified API differs from the latter RFC 793 (see Section 2 of
      this document).

   RFC 761 U: "DoD Standard Transmission Control Protocol" (January
               1980)

      This document [RFC761] is the immediate precursor of RFC 793 (see
      Section 2 of this document).  The header format, the connection
      establishment (including the different connection states), and the
      overall API correspond mostly to the final Standard RFC 793 (see
      Section 2 of this document).

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

      This document [RFC813] contains an early discussion of Silly
      Window Syndrome and its avoidance and motivates and describes the
      use of delayed acknowledgments.

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

      Suggestions and guidance for the design of tables and algorithms
      to keep track of various identifiers within a TCP/IP
      implementation are provided by this document [RFC814].

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

      In this document [RFC816], TCP's response to indications of
      network error conditions such as timeouts or received ICMP
      messages is discussed.

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

      This document [RFC817] contains implementation suggestions that
      are general and not TCP specific.  However, they have been used to
      develop TCP implementations and describe some performance
      implications of the interactions between various layers in the
      Internet stack.

   RFC 872 U: "TCP-on-a-LAN" (September 1982)

      Conclusion of RFC 872 [RFC872]: "The sometimes-expressed fear that
      using TCP on a local net is a bad idea is unfounded."

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

      This document [RFC896] contains some early experiences with
      congestion collapse and some initial thoughts on how to avoid it
      using congestion control in TCP.  Furthermore, it defined an
      algorithm for efficient transmission of small packets that is
      today known as the Nagle algorithm.

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

      This document [RFC964] points out several specification bugs in
      the US Military's MIL-STD-1778 document, which was intended as a
      successor to RFC 793 (see Section 2 of this document).  This
      serves to remind us of the difficulty in specification writing
      (even when we work from existing documents!).

7.2.  Architectural Guidelines

   Some documents in this section contain architectural guidance and
   concerns, while others specify TCP- and congestion-control-related
   mechanisms that are broadly applicable and have impacts on TCP's
   congestion control techniques.  Some of these documents are direct
   products of the Internet Architecture Board (IAB) giving their
   guidance on specific aspects of congestion control in the Internet.

   RFC 1958 I: "Architectural Principles of the Internet" (June 1996)

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

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

      This document [RFC2914] motivates the use of end-to-end congestion
      control for preventing congestion collapse and providing fairness
      to TCP.  Later work on TCP has included several more aggressive
      mechanisms than Reno TCP includes, and RFC 5033 (see Section 7.4
      of this document) provides additional guidance on use of such
      algorithms.  The fundamental architectural discussion in RFC 2914
      remains valid, regarding the standards process role in defining
      protocol aspects that are critical to performance and avoiding
      congestion collapse scenarios.

   RFC 3360 B: "Inappropriate TCP Resets Considered Harmful" (August
               2002)

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

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

      This document [RFC3439] updates RFC 1958 (see above in
      Section 7.2) by outlining some philosophical guidelines for
      architects and designers of Internet backbone networks.  The
      document describes the Simplicity Principle, which states that
      complexity is the primary impediment to efficient scaling.

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

      This document [RFC4774] discusses some of the issues in defining
      alternate semantics for the ECN field and specifies requirements
      for a safe coexistence with routers that do not understand the
      defined alternate semantics.

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

      Abstract of RFC 6182 [RFC6182]: "This document outlines
      architectural guidelines for the development of a Multipath
      Transport Protocol, with references to how these architectural
      components come together in the development of a Multipath TCP
      (MPTCP) (see Section 4.7 of this document).  This document lists
      certain high-level design decisions that provide foundations for
      the design of the MPTCP protocol, based upon these architectural
      requirements"

7.3.  Difficult Network Environments

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

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

      From the Abstract of RFC 2488 [RFC2488]: "While TCP works over
      satellite channels there are several IETF standardized mechanisms
      that enable TCP to more effectively utilize the available capacity
      of the network path.  This document outlines some of these TCP
      mitigations.  At this time, all mitigations discussed in this
      document are IETF standards track mechanisms (or are compliant
      with IETF standards)."

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

      Several methods of improving TCP performance over long thin
      networks (i.e., networks with low bandwidth and high delay), such
      as geosynchronous satellite links, are discussed in this document
      [RFC2757].  A particular set of TCP options is developed that
      should work well in such environments and be safe to use in the
      global Internet.  The implications of such environments have been
      further discussed in RFCs 3150 and 3155 (see below in
      Section 7.3), and these documents should be preferred where there
      is overlap between them and RFC 2757 (see Section 7.3 of this
      document).

   RFC 2760 I: "Ongoing TCP Research Related to Satellites" (February
               2000)

      This document [RFC2760] discusses the advantages and disadvantages
      of several different experimental means of improving TCP
      performance over long-delay or error-prone paths.  These include
      T/TCP, larger initial windows, byte counting, delayed
      acknowledgments, slow start thresholds, NewReno and SACK-based
      loss recovery, FACK [MM96], ECN, various corruption-detection
      mechanisms, congestion avoidance changes for fairness, use of
      multiple parallel flows, pacing, header compression, state
      sharing, and ACK congestion control, filtering, and
      reconstruction.  Although RFC 2488 (see above in Section 7.3)
      looks at standard extensions, this document focuses on more
      experimental means of performance enhancement.

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

      From the Abstract of RFC 3135 [RFC3135]: "This document is a
      survey of Performance Enhancing Proxies (PEPs) often employed to
      improve degraded TCP performance caused by characteristics of
      specific link environments, for example, in satellite, wireless

      WAN, and wireless LAN environments.  Different types of
      Performance Enhancing Proxies are described as well as the
      mechanisms used to improve performance."

   RFC 3150 B: "End-to-end Performance Implications of Slow Links" (July
               2001)

      From the Abstract of RFC 3150 [RFC3150]: "This document makes
      performance-related recommendations for users of network paths
      that traverse "very low bit-rate" links....This recommendation may
      be useful in any network where hosts can saturate available
      bandwidth, but the design space for this recommendation explicitly
      includes connections that traverse 56 Kb/second modem links or 4.8
      Kb/second wireless access links - both of which are widely
      deployed."

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

      From the Abstract of RFC 3155 [RFC3155]: "This document discusses
      the specific TCP mechanisms that are problematic in environments
      with high uncorrected error rates, and discusses what can be done
      to mitigate the problems without introducing intermediate devices
      into the connection."

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

      From the Abstract of RFC 3366 [RFC3366]: "This document provides
      advice to the designers of digital communication equipment and
      link-layer protocols employing link-layer Automatic Repeat reQuest
      (ARQ) techniques.  This document presumes that the designers wish
      to support Internet protocols, but may be unfamiliar with the
      architecture of the Internet and with the implications of their
      design choices for the performance and efficiency of Internet
      traffic carried over their links."

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

      From the Abstract of RFC 3449 [RFC3449]: "This document describes
      TCP performance problems that arise because of asymmetric effects.
      These problems arise in several access networks, including
      bandwidth-asymmetric networks and packet radio subnetworks, for
      different underlying reasons.  However, the end result on TCP
      performance is the same in both cases: performance often degrades
      significantly because of imperfection and variability in the ACK
      feedback from the receiver to the sender.

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

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

      From the Abstract of RFC 3481 [RFC3481]: "This document describes
      a profile for optimizing TCP to adapt so that it handles paths
      including second (2.5G) and third (3G) generation wireless
      networks."

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

      This document [RFC3819] describes how TCP performance can be
      negatively affected by some particular lower-layer behaviors and
      provides guidance in designing lower-layer networks and protocols
      to be amicable to TCP.  RFC 3366 (see above in Section 7.3)
      specifically focuses on ARQ mechanisms, while RFC 3819 more widely
      covers additional aspects of the underlying layers

7.4.  Guidance for Developing, Analyzing, and Evaluating TCP

   Documents in this section give general guidance for developing,
   analyzing, and evaluating TCP.  Some of the documents discuss, for
   example, the properties of congestion control protocols that are
   "safe" for Internet deployment as well as how to measure the
   properties of congestion control mechanisms and transport protocols.

   RFC 5033 B: "Specifying New Congestion Control Algorithms" (August
               2007)

      This document [RFC5033] considers the evaluation of suggested
      congestion control algorithms that differ from the principles
      outlined in RFC 2914 (see Section 7.2 of this document).  It is
      useful for authors of such algorithms as well as for IETF members
      reviewing the associated documents.

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

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

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

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

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

      This document [RFC6181] describes a threat analysis for Multipath
      TCP (MPTCP) (see Section 4.7 of this document).  The document
      discusses several types of attacks and provides recommendations
      for MPTCP designers how to create an MPTCP specification that is
      as secure as the current (single-path) TCP.

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

      From the Abstract of RFC 6349 [RFC6349]: "This framework describes
      a practical methodology for measuring end-to-end TCP Throughput in
      a managed IP network.  The goal is to provide a better indication
      in regard to user experience.  In this framework, TCP and IP
      parameters are specified to optimize TCP Throughput."

7.5.  Implementation Advice

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

      This document [RFC794] clarifies that operating systems need to
      manage their limited resources, which may include TCP connection
      state, and that these decisions can be made with application
      input, but they do not need to be part of the TCP protocol
      specification itself.

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

      Abstract of RFC 879 [RFC879]: "This memo discusses the TCP Maximum
      Segment Size Option and related topics.  The purposes [sic] is to
      clarify some aspects of TCP and its interaction with IP.  This
      memo is a clarification to the TCP specification, and contains
      information that may be considered as 'advice to implementers'."

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

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

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

      Incrementally updating the Internet checksum is useful to routers
      in updating IP checksums.  Some middleboxes that alter TCP headers
      may also be able to update the TCP checksum incrementally.  This
      document [RFC1624] expands upon the explanation of the incremental
      update procedure in RFC 1071 (see above in Section 7.5).

   RFC 1936 I: "Implementing the Internet Checksum in Hardware" (April
               1996)

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

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

      From the Abstract of RFC 2525 [RFC2525]: "This memo catalogs a
      number of known TCP implementation problems.  The goal in doing so
      is to improve conditions in the existing Internet by enhancing the
      quality of current TCP/IP implementations."

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

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

   RFC 3493 I: "Basic Socket Interface Extensions for IPv6" (February
               2003)

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

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

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

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

      This document [RFC6191] describes the usage of the TCP Timestamps
      option (RFC 7323, see Section 3.1 of this document) to perform
      heuristics to determine whether or not to allow the creation of a
      new incarnation of a connection that is in the TIME-WAIT state.

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

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

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

      This document [RFC6897] characterizes the impact that Multipath
      TCP (MPTCP) (see Section 4.7 of this document) may have on
      applications.  It further discusses compatibility issues of MPTCP
      in combination with non-MPTCP-aware applications.  Finally, it
      describes a basic API that is a simple extension of TCP's
      interface for MPTCP-aware applications.

7.6.  Tools and Tutorials

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

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

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

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

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

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

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

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

7.7.  MIB Modules

   The first MIB module defined for use with Simple Network Management
   Protocol (SNMP) was a single monolithic MIB module, called MIB-I,
   defined in RFC 1156.  This evolved over time to the MIB-II
   specification in RFC 1213, which obsoletes RFC 1156.  It then became
   apparent that having a single monolithic MIB module was not scalable,
   given the number and breadth of MIB data definitions that needed to
   be included.  Thus, additional MIB modules were defined, and those
   parts of MIB-II that needed to evolve were split off.  Eventually,
   the remaining parts of MIB-II were also split off, the TCP-specific
   part being documented in RFC 2012.  RFC 2012 was obsoleted by RFC
   4022, which is the primary TCP MIB document at the time of writing.
   For current TCP implementers, RFC 4022 should be supported.

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

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

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

      This document [RFC1213] describes the second version of the MIB in
      a monolithic form.  It is the immediate successor of RFC 1158,
      with minor modifications.  It obsoletes the MIB-I, defined in RFC
      1156 (see above in Section 7.7).

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

      In an update to RFC 1213 (see Section 7.7 of this document), this
      document [RFC2012] defines the TCP MIB by splitting out the TCP-
      specific portions.  It is now obsoleted by RFC 4022 (see below in
      Section 7.7).

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

      This document [RFC2452] augments RFC 2012 (see Section 7.7 of this
      document) by adding an IPv6-specific connection table.  The rest
      of RFC 2012 holds for any IP version.  RFC 2452 is now obsoleted
      by RFC 4022 (see below in Section 7.7).

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

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

      This document [RFC4022] obsoletes RFCs 2012 and 2452 (see above in
      Section 7.7) and specifies the current standard for the TCP MIB
      that should be deployed.

   RFC 4898 S: "TCP Extended Statistics MIB" (May 2007)

      This document [RFC4898] describes extended performance statistics
      for TCP.  They are designed to use TCP's ideal vantage point to
      diagnose performance problems in both the network and the
      application.

7.8.  Case Studies

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

      This document [RFC700] presents a field report about the
      deployment of a very early version of TCP, the so-called INWN #39
      protocol, which is originally described by Cerf and Kahn in INWG
      Note #39 [CK73] to use a PDP-11 line printer via the ARPANET.

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

      This document [RFC889] is a status report about experiments
      concerning the TCP retransmission timeout calculation and also
      provides advice for implementers.

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

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

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

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

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

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


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

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