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Internet Engineering Task Force                             G. Fairhurst
Internet-Draft                                    University of Aberdeen
Intended status: Standards Track                           July 26, 2019
Expires: January 27, 2020


        Guidelines for Internet Congestion Control at Endpoints
                      draft-fairhurst-tsvwg-cc-02

Abstract

   This document provides guidance on the design of methods to avoid
   congestion collapse and to provide congestion control.
   Recommendations and requirements on this topic are distributed across
   many documents in the RFC series.  This therefore seeks to gather and
   consolidate these recommendations.  This is intended to provide input
   to the design of new congestion control methods in protocols, such as
   IETF QUIC.

   The present document is for discussion and comment by the IETF.

Status of This Memo

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

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

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

   This Internet-Draft will expire on January 27, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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



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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Principles of Congestion Control  . . . . . . . . . . . . . .   3
     3.1.  A Diversity of Path Characteristics . . . . . . . . . . .   3
     3.2.  Flow Multiplexing and Congestion  . . . . . . . . . . . .   4
     3.3.  Avoiding Congestion Collapse  . . . . . . . . . . . . . .   5
   4.  Guidelines for Performing Congestion Control  . . . . . . . .   6
     4.1.  Connection Initialization . . . . . . . . . . . . . . . .   6
     4.2.  Using Path Capacity . . . . . . . . . . . . . . . . . . .   8
     4.3.  Timers and Retransmission . . . . . . . . . . . . . . . .   9
     4.4.  Responding to Potential Congestion  . . . . . . . . . . .  10
     4.5.  Using More Capacity . . . . . . . . . . . . . . . . . . .  12
     4.6.  Network Signals . . . . . . . . . . . . . . . . . . . . .  13
     4.7.  Protection of Protocol Mechanisms . . . . . . . . . . . .  13
   5.  IETF Guidelines on Evaluation of Congestion Control . . . . .  13
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Appendix A.  Revision Notes . . . . . . . . . . . . . . . . . . .  17
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   The IETF has specified Internet transports (e.g., TCP [ID.ietf-tcpm-
   rfc793bis], UDP [RFC0768], UDP-Lite [RFC3828], SCTP [RFC4960], and
   DCCP [RFC4340]) as well as protocols layered on top of these
   transports (e.g., RTP, QUIC [I-D.ietf-quic-transport], SCTP/UDP
   [RFC6951], DCCP/UDP) and transports that work directly over the IP
   network layer.  These transports are implemented in endpoints
   (Internet hosts or routers acting as endpoints) and are designed to
   detect and react to network congestion.

   Recommendations and requirements on this topic are distributed across
   many documents in the RFC series.  This document therefore seeks to
   gather and consolidate these recommendations.  This is intended to
   provide input to the design of new congestion control methods in
   protocols.




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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   The path between endpoints (sometimes called "Internet Hosts")
   consists of the endpoint protocol stack at the sender and receiver
   (which implements the transport service), and a succession of links
   and network devices (routers or middleboxes) that provide
   connectivity across the network.  The set of network devices forming
   the path is not usually fixed, and it should generally be assumed
   that this set can change over arbitrary lengths of time.

   Other terminology is directly copied from the cited RFCs.

3.  Principles of Congestion Control

   This section summarises the principles for providing congestion
   control, and provides the background for section Section 4.

3.1.  A Diversity of Path Characteristics

   Internet transports do not usually rely upon prior reservation of
   capacity along the path they use.  In the absence of such a resource
   reservation, endpoints are unable to determine a safe rate at which
   to start or continue their transmission.  The use of an Internet path
   therefore requires a combination of end-to-end transport mechanisms
   to detect and then respond to changes in the capacity that it
   discovers is available across the network path.  Buffering (an
   increase in latency) or loss (discard of a packet) arises when the
   traffic arriving at a link or network exceeds the resources
   available.

   A network device that does not support Active Queue Management (AQM)
   [RFC7567] typically uses a drop-tail policy to drop excess IP packets
   when its queue becomes full.  Although losses are not always due to
   congestion (loss may be due to link corruption, receiver overrun,
   etc.  [RFC3819]), endpoint congestion control has to conservatively
   assume that any loss is potentially due to congestion and then reduce
   the sending rate of their flows to reflect the available capacity.

   The use of a path to send packets impacts any flows (possibly from or
   to other endpoints) that share the capacity of a common network
   device or link (i.e., are (i.e., multiplexed) . As with loss, latency
   can also be incurred for other reasons [RFC3819] (Quality of Service
   link scheduling, link radio resource management/bandwidth on demand,




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   transient outages, link retransmission, and connection/resource setup
   below the IP layer, etc).

   Principles include:

   o  A design is REQUIRED be robust to a change in the set of devices
      forming the network path.  A reconfiguration, reset or other event
      could interrupt this path or trigger a change in the set of
      network devices forming the path.

   o  Transports are REQUIRED to operate safely over the wide range of
      path characteristics presented by Internet paths.

   o  The path characteristics can change over relatively short
      intervals of time (i.e., characteristics discovered do not
      necessarily remain valid for multiple Round Trip Times, RTTs).  In
      particular, a sender SHOULD measure and adapt to the
      characteristics of the path(s) they use.

3.2.  Flow Multiplexing and Congestion

   It is normal to observe some perturbation in latency and/or loss when
   flows shares a common network bottleneck with other traffic.  This
   impact needs to be considered and Internet flows ought to implement
   appropriate safeguards to avoid inappropriate impact on other flows
   that share the resources along a path.  Congestion control methods
   satisfy this requirement and therefore can help avoid congestion
   collapse [@ARTICLE{author = {Bob Briscoe}, title = {Flow Rate
   Fairness: Dismantling a Religion}, journal = {ACM CCR}, year = {2007}
   }].

   An endpoint can become aware of congestion by various means.  A
   signal that indicates congestion on the end-to-end network path,
   needs to result in a congestion control reaction by the transport to
   reduce the maximum rate permitted by the sending endpoint [RFC8087]].
   Internet transports should react to avoid congestion that impacts
   other flows sharing a path, and need to be designed to avoid starving
   other flows of capacity.  This could include methods seeking to
   equally distribute resources between sharing flows, but this is
   explicitly not a requirement for a network device.

   The Requirements for Internet Hosts [RFC1122] formally mandates that
   endpoints perform congestion control.  "Because congestion control is
   critical to the stable operation of the Internet, applications and
   other protocols that choose to use UDP as an Internet transport must
   employ mechanisms to prevent congestion collapse and to establish
   some degree of fairness with concurrent traffic [RFC2914].  They may
   also need to implement additional mechanisms, depending on how they



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   use UDP" [RFC8085].  [RFC2309] also discussed the dangers of
   congestion-unresponsive flows, and states that "all UDP-based
   streaming applications should incorporate effective congestion
   avoidance mechanisms."  [RFC7567] and [RFC8085] reaffirm this.

   The general recommendation in the UDP Guidelines [RFC8085] is that
   applications SHOULD leverage existing congestion control techniques,
   such as those defined for TCP [RFC5681], TFRC [RFC5348], SCTP
   [RFC4960], and other IETF-defined transports.  This is because there
   are many trade offs and details that can have a serious impact on the
   performance of congestion control for the application they support
   and other traffic that seeks to share the resources along the path
   over which they communicate.

   Experience has shown that successful protocols developed in a
   specific context or for a particular application tend to also become
   used in a wider range of contexts.  Therefore, IETF specifications by
   default target deployment on the general Internet, or need to be
   defined for use only within a controlled environment.

   Principles include:

   o  Endpoints MUST perform congestion control [RFC1122] .

   o  Transports MUST avoid inducing flow starvation to other flows that
      share resources along the path they use.

   o  "If an application or protocol chooses not to use a congestion-
      controlled transport protocol, it SHOULD control the rate at which
      it sends UDP datagrams to a destination host, in order to fulfil
      the requirements of [RFC2914]", as stated in [RFC8085].

   o  Transports that do not target Internet deployment need to be
      constrained to only operate in a controlled environment (e.g. see
      Section 3.6 of [RFC8085]) and provide appropriate mechanisms to
      prevent traffic accidentally leaving the controlled environment
      [RFC8084].

3.3.  Avoiding Congestion Collapse

   A significant pathology can arise when a poorly designed transport
   creates congestion.  This can result in severe service degradation or
   "Internet meltdown".  This phenomenon was first observed during the
   early growth phase of the Internet in the mid 1980s [RFC896]
   [RFC970]; it is technically called "congestion collapse" and was a
   key focus of [RFC2309].

   o  Endpoints MUST control their flows to avoid Congestion Collapse.



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   o  A sender SHOULD measure and adapt protocol timers to the measured
      the path RTT.

   o  When a endpoint detects persitent congestion, it MUST employ
      exponential backoff to the maximum rate (i.e. reduce its
      congestion window).

   o  Endpoints MUST treat a loss of all feedback (e.g., RTO expiry) as
      an indication of persisent congestions (an indication of potential
      congestion collapse), until the path characteristics can again be
      confirmed.

   o  Network devices can provide mechanisms to mitigate the impact of
      congestion collapse by transport flows (e.g., priority forwarding
      of control information, and starvation detection) to mitigate the
      impact of non-conformant and malicious flows [RFC7567]).

4.  Guidelines for Performing Congestion Control

   This section provides guidance for designers of a new transport
   protocol that decide to implement congestion control and its
   associated mechanisms.

4.1.  Connection Initialization

   When a connection or flow to a new destination is established, the
   endpoints have little information about the characteristics of the
   network path.  This section describes how a flow starts transmission
   over such a path.

   Flow Start:  A new flow between two endpoints cannot assume that
      capacity is available at the start of the flow, unless it uses a
      mechanism to explicitly reserve capacity.  In the absence of a
      capacity signal, a flow MUST therefore start slowly.

      The slow-start algorithm is the accepted standard for flow startup
      [RFC5681].  TCP uses the notion of an Initial Window (IW [RFC3390]
      updated by [RFC6928]) to define the initial volume of data that
      can be sent on a path.  This is not the smallest burst, or the
      smallest window - but it is considered a safe starting point for a
      network that is not suffering persistent congestion, and
      applicable until feedback about the path is received.  This
      initial sending rate needs to be viewed as tentative until the
      capacity is confirmed to be available.

   Initial RTO Interval:  When a flow sends the first packet it
      typically has no way to know the actual RTT of the path it uses.
      The initial value used to the principal retransmission timer, used



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      to detect lack of responsiveness from the remote endpoint.  In TCP
      this is the starting value of the Retransmission Timeout (RTO), or
      corresponding timer in another protocol.  The initial value is
      therefore a trade off that has important consequences on the
      overall Internet stability [RFC6928] [RFC8085].  In the absence of
      any knowledge about the latency of a path, the RTO MUST be
      conservatively set to no less than 1 second.  Values shorter than
      1 second can be problematic (see the appendix of [RFC6298]).
      (Note: Linux TCP has deployed a smaller initiaL RTO value)

   Initial RTO Expiry:  If the RTO timer expires while awaiting
      completion of the connection setup (in TCP, the ACK of a SYN
      segment), and the implementation is using an RTO less than 3
      seconds, the local endpoint can resend the connection setup.  The
      RTO MUST then be re-initialized to increase it to 3 seconds when
      data transmission begins (i.e., after the three-way handshake
      completes) [RFC6298] [RFC8085].  This conservative increase is
      necessary to avoid congestion collapse when many flows retransmit
      across a shared bottleneck with restricted capacity.

   Initial Measured RTO:  Once an RTT measurement is available (e.g.,
      through reception of an acknowledgement), this value must be
      adjusted, and MUST take into account the RTT variance.  For the
      first sample, this variance cannot be determined, and a local
      endpoint must therefore initialise the variance to RTT/2 (see
      equation 2.2 of [RFC6928] and related text for UDP in section
      3.1.1 of [RFC8085]).

   Current State:  A congestion controller MAY assume that recently used
      capacity between a pair of endpoints is an indication of future
      capacity available in the next RTT between the same endpoints.  It
      must react (reduce its rate) if this is not confirmed to be true.

   Cached State:  A congestion controller that recently used a specific
      path could use additional state that lets a flow take-over the
      capacity that was previously consumed by another flow (e.g., in
      the last RTT) which it understands is using the same path, or
      which was recently using that path.  In TCP, this mechanism is
      referred to as TCP Control Block (TCB) sharing [RFC2140] [ID.ietf-
      tcpm-2140bis].  The capacity and other information can be used to
      suggest a faster initial sending rate, but this information MUST
      be viewed as tentative until it is confirmed by receiving
      confirmation that actual traffic has been sent across the path.  A
      sender MUST reduce its rate if this capacity is not confirmed
      within the current RTO interval.






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4.2.  Using Path Capacity

   This section describes how a sender needs to regulate the maximum
   volume of data in flight over the interval of the current RTT, and
   how it manages transmission of the capacity that it perceives is
   available.

   Congestion Management:  The capacity available to a flow could be
      expressed as the number of bytes in flight, the sending rate or a
      limit on the number of unacknowledged segments.  In steady-state
      this congestion window reflects a safe limit to the sending rate
      that has not resulted in persistent congestion.  A sender
      performing congestion management will usually optimise performance
      for its application by avoiding excessive loss or delay.

      One common model views the path between two endpoints as a pipe.
      New packets enter the pipe at the sending endpoint, older ones
      leave at the receiving endpoint.  Received data (leaviung the
      network path) is usually acknowledged to the sender.  The rate
      that data leaves the pipe indicates the share of the capacity that
      has been utilised by the flow.  If, on average (over an RTT), the
      sending rate equals the receiving rate, this indicates that this
      capacity can be safely used again in the next RTT.  If the average
      receiving rate is less than the sending rate, then the path is
      either queuing packets, the RTT/path has changed, or there is
      packet loss.

   Transient Path:  Path capacity information is transient.  A sender
      that fails to use capacity has no understanding whether that
      previously used capacity remains available to use - or whether it
      has disappeared (e.g., to a change to a path with a smaller
      bottleneck, or more traffic has emerged that has consumed the
      previously available capacity).  For this reason, a sender that is
      limited by the volume of data available to send MUST NOT continue
      to grow the congestion window [RFC5681].

      Standard TCP states that a TCP sender SHOULD set the congestion
      window to no more than the Restart Window (R) before beginning
      transmission if the TCP sender has not sent data in an interval
      that exceeds the current retransmission timeout, i.e., when an
      application becomes idle [RFC5681].  Experimental specifications
      permit TCP senders to tentatively maintain a congestion window
      when application-limited, provided that they appropriately and
      rapidly collapse the window when potential congestion is detected
      [RFC7661].  This mechanism is called Congestion Window Validation
      (CWV).





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   Burst Mitigation:  Even in the absence of congestion, statistical
      multiplexing of flows can result in transient effects for flows
      sharing common resources.  A sender therefore SHOULD avoid
      inducing excessive congestion to other flows (collateral damage).

      While a congestion controller ought to limit sending at the
      granularity of the current RTT, this can be insufficient to
      satisfy the goals of preventing starvation and mitigating
      collateral damage.  This requires moderating the burst rate of the
      sender to avoid significant periods where a flow(s) consume all
      buffer capacity at the path bottleneck, which would otherwise
      prevent other flows from gaining a reasonable share.

      Endpoints SHOULD provide mechanisms to regulate the bursts of
      transmission that the application/protocol sends to the network
      (section 3.1.6 of [RFC8085]).  ACK-Clocking [RFC5681] can help
      mitigate bursts for protocols that receive continuous feedback of
      reception (such as TCP).  Sender pacing can mitigate this
      [RFC8085], (See Section 4.6 of [RFC3449]), and has been
      recommended for TCP in conditions where ACK-Clocking is not
      effective, (e.g., [RFC3742], [RFC7661]).  SCTP [RFC4960] defines a
      maximum burst length (Max.Burst) with a recommended value of 4
      segments to limit the SCTP burst size.

4.3.  Timers and Retransmission

   This section describes mechanisms to detect and provide
   retransmission, and to protect the network in the absence of timely
   feedback.

   Loss Detection:  Loss detection occurs after a sender determines
      there is no delivery confirmation within an expected period of
      time (e.g. by observing the time-ordering of the reception of
      ACKs, as in TCP DupACK) or by utilising a timer to detect loss
      (e.g. a transmission timer with a period less than the RTO,
      [RFC8085] [ID.ietf-tcpm-rack]) or a combination of using a timer
      and ordering information to trigger retransmission of data.

   Retransmission:  Retransmission of lost packets or messages is a
      common reliability mechanism.  When loss is detected, the sender
      can choose to retransmit the lost data, ignore the loss, or send
      other data.  Any transmission consumes network capacity, therefore
      retransmissions MUST NOT increase the network load in response to
      congestion loss (which worsens that congestion) [RFC8085].  Any
      method that sends additional data following loss is responsible
      for congestion control of the retransmissions (and any other
      packets sent) as well as the original traffic.




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   Measuring the RTT:  Once an endpoint has started communicating with
      its peer, the RTT be MUST adjusted by measuring the actual path
      RTT and its variance (see equation 2.3 of [RFC6928]).

   Maintaining the RTO:  The RTO SHOULD be set based on recent RTT
      observations [RFC8530].

   RTO Expiry:  Persistent lack of feedback (e.g.  detected by the RTO
      timer, or other means) MUST be treated an indication of potential
      congestion collapse.  A failure to receive any specific response
      within a RTO interval could potentially be a result of a RTT
      change, change of path, excessive loss, or even congestion
      collapse.  If there is no response within the RTO interval, TCP
      collapses the congestion window to one segment [RFC5681].  Other
      transports must similarly respond when they detect loss of
      feedback.

      An endpoint needs to exponentially backoff the RTO interval
      [RFC8085] each time the RTO expires.  That is the RTO interval
      MUST be set to the RTO * 2 [RFC6298] [RFC8085].

   Maximum RTO:  A maximum value MAY be placed on the RTO interval.  The
      maximum limit to the RTO interval MUST NOT be less than 60 seconds
      [RFC6298].

4.4.  Responding to Potential Congestion

   Internet flows SHOULD implement appropriate safeguards to avoid
   inappropriate impact on other flows that share the resources along a
   path.  The safety and responsiveness of new proposals need to be
   evaluated [RFC5166].  In determining an appropriate congestion
   response, designs could take into consideration the size of the
   packets that experience congestion [RFC4828].

   Congestion Response:  An endpoint MUST promptly reduce the rate of
      transmission when it receive or detects an indication of
      congestion (e.g. loss) [RFC2914].

      TCP Reno established a method that relies on multiplicative-
      decrease to halve the sending rate while congestion is detected.
      This response to congestion indications is considered sufficient
      for safe Internet operation, but other decrease factors have also
      been published in the RFC Series [RFC8312].

   ECN Response:  A congestion control design should provide the
      necessary mechanisms to support Explicit Congestion Notification
      (ECN) [RFC3168] [RFC6679], as described in section 3.1.7. of
      [RFC8085].  This can help determine an appropriate congestion



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      window when supported by routers on the path [RFC7567] to enable
      rapid early indication of incipient congestion.

      The early detection of incipient congestion justifies a different
      reaction to packet loss to that for the congestion signal
      [RFC8311] [RFC8087]].  Simple feedback of received Congestion
      Experienced (CE) marks [RFC3168], relies only on an indication
      that congestion has been experienced within the last RTT.  This
      style of response is appropriate when a flow uses ECT(0).  The
      reaction to reception of this indication was modified in TCP ABE
      [RFC8511].  Further detail about the received CE-marking can be
      obtained by using more accurate receiver feedback (e.g.  [ID.-
      ietf-tcpm-accurate-ecn] and extended RTP feedback).  The more
      detailed feedback provides an opportunity for a finer-granularity
      of congestion response.

      Current work-in-progress [ID.ietf-tsvwg-l4s-arch] defines a
      reaction for packets marked with ECT(1), building on the style of
      detailed feedback provided by [ID.-ietf-tcpm-accurate-ecn] and a
      modified marking system [ID.ietf-tsvwg-aqm-dualq-coupled].

   Robustness to Path Change:  The detection of congestion and the
      resulting reduction MUST NOT solely depend upon reception of a
      signal from the remote endpoint, because congestion indications
      could themselves be lost under persistent congestion.

      The only way to surely confirm that a sending endpoint has
      successfully communicated with a remote endpoint is to utilise a
      timer (see (Section 4.3)) to detect a lack of response that could
      result from a change in the path or the path characteristics
      (usually called the RTO).  Congestion controllers that are unable
      to react after one (or at most a few) RTTs after receiving a
      congestion indication should observe the guidance in section 3.3
      of the UDP Guidelines [RFC8085].

   Persistent Congestion:  Persistent congestion can result in
      congestion collapse, which MUST be aggressively avoided [RFC2914].
      Endpoints that experience persistent congestion and have already
      exponentially reduced their congestion window to the restart
      window (e.g., 1 packet), MUST further reduce the rate if the RTO
      timer continues to expire.  For example, TCP-Friendly Rate Control
      (TFRC) [RFC5348] continues to reduce its sending rate under
      persistent congestion to one packet per RT, and then exponentially
      backs off the time between single packet transmissions if the
      congestion continues to persist [RFC2914].

      [RFC8085] provides guidelines for a sender that does not, or is
      unable to, adapt the congestion window.



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4.5.  Using More Capacity

   In the absence of persistent congestion, an endpoint is permitted to
   increase its congestion window and hence the sending rate.  An
   increase should only occur when there is additional data available to
   send across the path (i.e., the sender will utilise the additional
   capacity in the next RTT).

   TCP Reno [RFC5681] defines an algorithm, known as the AIMD (additive-
   increase/ multiplicative-decrease) that allows a sender to
   exponentially increase the congestion window each RTT from the
   initial window to the first detected congestion event.  This is
   designed to allow new flows to rapidly acquire a suitable congestion
   window.  Where the bandwidth delay product (BDP) is large, it can
   take many RTTs to determine a suitable share of the path capacity.
   Such high BDP paths benefit from methods that more rapidly increase
   the congestion window, but in compensation these need to be designed
   to also react rapidly to any detected congestion (e.g., TCP Cubic
   [RFC8312]).

   Increasing Congestion Window:  A sender MUST NOT continue to increase
      its rate for more than an RTT after a congestion indication is
      received.  It SHOULD stop increasing its congestion window as soon
      as it receives indication of congestion to avoid excessive
      "overshoot".

      While the sender is increasing the congestion window, a sender
      will transmit faster than the last known safe rate.  Any increase
      above the last confirmed rate needs to be regarded as tentative
      and the sender reduce their rate below the last confirmed safe
      rate when congestion is experienced (a congestion event).

   Congestion:  An endpoint MUST utilise a method that assures the
      sender will keep the rate below the previously confirmed safe rate
      for multiple RTTs after an observed congestion event.  In TCP,
      this is performed by using a linear increase from a slow start
      threshold that is re-initialised when congestion is experienced.

   Avoiding Overshoot:  Overshoot of the congestion window beyond the
      point of congestion can significantly impact other flows sharing
      resources along a path.  It is important to note that as endpoints
      experience more paths with a large BDP and a wider range of
      potential path RTT, that variability or changes in the path can
      have very significant constraints on appropriate dynamics for
      increasing the congestion window (see also burst mitigation
      Section 4.2).





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4.6.  Network Signals

   An endpoint can utilise signals from the network to help determine
   how to regulate the traffic it sends.

   Network Signals:  Mechanisms MUST NOT solely rely on messages or
      other specific signalling messages to perform safely.  (See
      section 5.2 of [RFC8085] describing use of ICMP messages).  The
      path characteristics can change at any time.  Transport mechanisms
      need to be robust to potential black-holing of any signals (i.e.,
      need to be robust to loss or modification of packets).

      A mechanism that utilises signals originating in the network (e.g.
      RSVP, NSIS, Quick-Start, ECN), must assume that the set of network
      devices on the path can change.  This motivates the use of soft-
      state when designing protocols that interact with signals
      originating from network devices [panrg] (e.g., ECN).  This can
      include context-sensitive treatment of "soft" signals provided to
      the endpoint [RFC5461].

4.7.  Protection of Protocol Mechanisms

   An endpoint needs to provide protection from attacks on the traffic
   it generates, or attacks that seek to increase the capacity it
   consumes (impacting other traffic that shared a bottleneck).

   Off Path Attack:   A design MUST protect from off-path attack to the
      protocol [RFC8085].  An attack on the congestion control can lead
      to a DoS vulnerability for the flow being controlled and/or other
      flows that share network resources along the path.

   Validation of Signals:  Network signalling and control messages
      (e.g., ICMP [RFC0792]) MUST be validated before they are used to
      protect from malicious abuse.  This MUST at least include
      protection from off-path attack [RFC8085].

   On Path Attack:   A protocol can be designed to protect from on-path
      attacks, but this requires more complexity and the use of
      encryption/authentication mechanisms (e.g., IPsec [RFC4301], QUIC
      [I-D.ietf-quic-transport]).

5.  IETF Guidelines on Evaluation of Congestion Control

   The IETF has provided guidance [RFC5033] for considering alternate
   congestion control algorithms.  The IRTF has described a set of
   metrics and related trade-off between metrics that can be used to
   compare, contrast, and evaluate congestion control techniques
   [RFC5166].



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6.  Acknowledgements

   Nicholas Kuhn helped develop the first draft of these guidelines.
   Tom Jones reviewed the first version of this draft.  Gorry Fairhurst
   and Tom Jones were funded at the University of Aberdeen by the
   European Space Agency.  Ana Custura helped review the text.

   The views expressed are solely those of the author(s).

7.  IANA Considerations

   This memo includes no request to IANA.

   RFC Editor Note: If there are no requirements for IANA, the section
   will be removed during conversion into an RFC by the RFC Editor.

8.  Security Considerations

   The security considerations for the use of transports are provided in
   the references section of the cited RFCs.  Security guidance for
   applications using UDP is provided in the UDP Usage Guidelines
   [RFC8085].

   Section Section 4.6 supports current best practice to validate ICMP
   messages prior to use.  Section Section 4.7 describes general
   requirements relating to the design of safe protocols and their
   protection from on and off path attack.

9.  References

9.1.  Normative References

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <https://www.rfc-editor.org/info/rfc1122>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41,
              RFC 2914, DOI 10.17487/RFC2914, September 2000,
              <https://www.rfc-editor.org/info/rfc2914>.






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   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <https://www.rfc-editor.org/info/rfc3168>.

   [RFC3742]  Floyd, S., "Limited Slow-Start for TCP with Large
              Congestion Windows", RFC 3742, DOI 10.17487/RFC3742, March
              2004, <https://www.rfc-editor.org/info/rfc3742>.

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
              <https://www.rfc-editor.org/info/rfc5681>.

   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298,
              DOI 10.17487/RFC6298, June 2011,
              <https://www.rfc-editor.org/info/rfc6298>.

   [RFC6928]  Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
              "Increasing TCP's Initial Window", RFC 6928,
              DOI 10.17487/RFC6928, April 2013,
              <https://www.rfc-editor.org/info/rfc6928>.

   [RFC7567]  Baker, F., Ed. and G. Fairhurst, Ed., "IETF
              Recommendations Regarding Active Queue Management",
              BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
              <https://www.rfc-editor.org/info/rfc7567>.

   [RFC7661]  Fairhurst, G., Sathiaseelan, A., and R. Secchi, "Updating
              TCP to Support Rate-Limited Traffic", RFC 7661,
              DOI 10.17487/RFC7661, October 2015,
              <https://www.rfc-editor.org/info/rfc7661>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <https://www.rfc-editor.org/info/rfc8085>.

9.2.  Informative References

   [I-D.ietf-quic-transport]
              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-20 (work
              in progress), April 2019.

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,
              <https://www.rfc-editor.org/info/rfc768>.




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   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,
              <https://www.rfc-editor.org/info/rfc792>.

   [RFC2309]  Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
              S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
              Partridge, C., Peterson, L., Ramakrishnan, K., Shenker,
              S., Wroclawski, J., and L. Zhang, "Recommendations on
              Queue Management and Congestion Avoidance in the
              Internet", RFC 2309, DOI 10.17487/RFC2309, April 1998,
              <https://www.rfc-editor.org/info/rfc2309>.

   [RFC3449]  Balakrishnan, H., Padmanabhan, V., Fairhurst, G., and M.
              Sooriyabandara, "TCP Performance Implications of Network
              Path Asymmetry", BCP 69, RFC 3449, DOI 10.17487/RFC3449,
              December 2002, <https://www.rfc-editor.org/info/rfc3449>.

   [RFC3819]  Karn, P., Ed., 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, DOI 10.17487/RFC3819, July 2004,
              <https://www.rfc-editor.org/info/rfc3819>.

   [RFC3828]  Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., Ed.,
              and G. Fairhurst, Ed., "The Lightweight User Datagram
              Protocol (UDP-Lite)", RFC 3828, DOI 10.17487/RFC3828, July
              2004, <https://www.rfc-editor.org/info/rfc3828>.

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340,
              DOI 10.17487/RFC4340, March 2006,
              <https://www.rfc-editor.org/info/rfc4340>.

   [RFC4828]  Floyd, S. and E. Kohler, "TCP Friendly Rate Control
              (TFRC): The Small-Packet (SP) Variant", RFC 4828,
              DOI 10.17487/RFC4828, April 2007,
              <https://www.rfc-editor.org/info/rfc4828>.

   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, DOI 10.17487/RFC4960, September 2007,
              <https://www.rfc-editor.org/info/rfc4960>.

   [RFC5033]  Floyd, S. and M. Allman, "Specifying New Congestion
              Control Algorithms", BCP 133, RFC 5033,
              DOI 10.17487/RFC5033, August 2007,
              <https://www.rfc-editor.org/info/rfc5033>.





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   [RFC6951]  Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream
              Control Transmission Protocol (SCTP) Packets for End-Host
              to End-Host Communication", RFC 6951,
              DOI 10.17487/RFC6951, May 2013,
              <https://www.rfc-editor.org/info/rfc6951>.

   [RFC8087]  Fairhurst, G. and M. Welzl, "The Benefits of Using
              Explicit Congestion Notification (ECN)", RFC 8087,
              DOI 10.17487/RFC8087, March 2017,
              <https://www.rfc-editor.org/info/rfc8087>.

   [RFC8311]  Black, D., "Relaxing Restrictions on Explicit Congestion
              Notification (ECN) Experimentation", RFC 8311,
              DOI 10.17487/RFC8311, January 2018,
              <https://www.rfc-editor.org/info/rfc8311>.

   [RFC8511]  Khademi, N., Welzl, M., Armitage, G., and G. Fairhurst,
              "TCP Alternative Backoff with ECN (ABE)", RFC 8511,
              DOI 10.17487/RFC8511, December 2018,
              <https://www.rfc-editor.org/info/rfc8511>.

Appendix A.  Revision Notes

   Note to RFC-Editor: please remove this entire section prior to
   publication.

   Individual draft -00:

   o  Comments and corrections are welcome directly to the authors or
      via the IETF TSVWG, working group mailing list.

   Individual draft -01:

   o

   o  This update is proposed for initial WG comments.

   o  If there is interest in progressing this document, the next
      version will include more complee referencing to citred material.

   Individual draft -02:

   o  Correction of typos.








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Author's Address

   Godred Fairhurst
   University of Aberdeen
   School of Engineering
   Fraser Noble Building
   Aberdeen  AB24 3U
   UK

   Email: gorry@erg.abdn.ac.uk









































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