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Versions: (draft-black-tsvwg-ecn-experimentation) 00 01 02 03 04 05 06 07

Transport Area Working Group                                    D. Black
Internet-Draft                                                  Dell EMC
Updates: 3168, 4341, 4342, 5622, 6679                   October 20, 2017
         (if approved)
Intended status: Standards Track
Expires: April 23, 2018


    Relaxing Restrictions on Explicit Congestion Notification (ECN)
                            Experimentation
                draft-ietf-tsvwg-ecn-experimentation-07

Abstract

   This memo updates RFC 3168, which specifies Explicit Congestion
   Notification (ECN) as an alternative to packet drops for indicating
   network congestion to endpoints.  It relaxes restrictions in RFC 3168
   that hinder experimentation towards benefits beyond just removal of
   loss.  This memo summarizes the anticipated areas of experimentation
   and updates RFC 3168 to enable experimentation in these areas.  An
   Experimental RFC in the IETF document stream is required to take
   advantage of any of these enabling updates.  In addition, this memo
   makes related updates to the ECN specifications for RTP in RFC 6679
   and for DCCP in RFC 4341, RFC 4342 and RFC 5622.  This memo also
   records the conclusion of the ECN nonce experiment in RFC 3540, and
   provides the rationale for reclassification of RFC 3540 as Historic;
   this reclassification enables new experimental use of the ECT(1)
   codepoint.

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 April 23, 2018.






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Copyright Notice

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  ECN Terminology . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  ECN Experimentation: Overview . . . . . . . . . . . . . . . .   4
     2.1.  Effective Congestion Control is Required  . . . . . . . .   5
     2.2.  Considerations for Other Protocols  . . . . . . . . . . .   5
     2.3.  Operational and Management Considerations . . . . . . . .   6
   3.  ECN Nonce and RFC 3540  . . . . . . . . . . . . . . . . . . .   7
   4.  Updates to RFC 3168 . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Congestion Response Differences . . . . . . . . . . . . .   8
     4.2.  Congestion Marking Differences  . . . . . . . . . . . . .   9
     4.3.  TCP Control Packets and Retransmissions . . . . . . . . .  12
   5.  ECN for RTP Updates to RFC 6679 . . . . . . . . . . . . . . .  13
   6.  ECN for DCCP Updates to RFCs 4341, 4342 and 5622  . . . . . .  14
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  16



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     10.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   This memo updates RFC 3168 [RFC3168] which specifies Explicit
   Congestion Notification (ECN) as an alternative to packet drops for
   indicating network congestion to endpoints.  It relaxes restrictions
   in RFC 3168 that hinder experimentation towards benefits beyond just
   removal of loss.  This memo summarizes the proposed areas of
   experimentation and updates RFC 3168 to enable experimentation in
   these areas.  An Experimental RFC in the IETF document stream
   [RFC4844] is required to take advantage of any of these enabling
   updates.  Putting all of these updates into a single document enables
   experimentation to proceed without requiring a standards process
   exception for each Experimental RFC that needs changes to RFC 3168, a
   Proposed Standard RFC.

   There is no need for this memo to update RFC 3168 to simplify
   standardization of protocols and mechanisms that are documented in
   Standards Track RFCs, as any Standards Track RFC can update RFC 3168
   directly without either relying on updates in this memo or using a
   standards process exception.

   In addition, this memo makes related updates to the ECN specification
   for RTP [RFC6679] and for three DCCP profiles ([RFC4341], [RFC4342]
   and [RFC5622]) for the same reason.  Each experiment is still
   required to be documented in one or more separate RFCs, but use of
   Experimental RFCs for this purpose does not require a process
   exception to modify any of these Proposed Standard RFCs when the
   modification falls within the bounds established by this memo (RFC
   5622 is an Experimental RFC; it is modified by this memo for
   consistency with modifications to the other two DCCP RFCs).

   Some of the anticipated experimentation includes use of the ECT(1)
   codepoint that was dedicated to the ECN nonce experiment in RFC 3540
   [RFC3540].  This memo records the conclusion of the ECN nonce
   experiment and provides the explanation for reclassification of RFC
   3540 as Historic in order to enable new experimental use of the
   ECT(1) codepoint.

1.1.  ECN Terminology

   ECT: ECN-Capable Transport.  One of the two codepoints ECT(0) or
   ECT(1) in the ECN field [RFC3168] of the IP header (v4 or v6).  An
   ECN-capable sender sets one of these to indicate that both transport
   end-points support ECN.




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   Not-ECT: The ECN codepoint set by senders that indicates that the
   transport is not ECN-capable.

   CE: Congestion Experienced.  The ECN codepoint that an intermediate
   node sets to indicate congestion.  A node sets an increasing
   proportion of ECT packets to CE as the level of congestion increases.

1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in RFC
   2119 [RFC2119].

2.  ECN Experimentation: Overview

   Three areas of ECN experimentation are covered by this memo; the
   cited Internet-Drafts should be consulted for the detailed goals and
   rationale of each proposed experiment:

   Congestion Response Differences:  An ECN congestion indication
      communicates a higher likelihood that a shorter queue exists at
      the network bottleneck node by comparison to a longer queue that
      is more likely when a packet drop occurs that indicates congestion
      [I-D.ietf-tcpm-alternativebackoff-ecn].  This difference suggests
      that for congestion indicated by ECN, a different sender
      congestion response (e.g., sender backs off by a smaller amount)
      may be appropriate by comparison to the sender response to
      congestion indicated by loss.  Two examples of proposed sender
      congestion response changes are described in
      [I-D.ietf-tcpm-alternativebackoff-ecn] and
      [I-D.ietf-tsvwg-ecn-l4s-id] - the proposal in the latter draft
      couples the sender congestion response change to Congestion
      Marking Differences changes (see next paragraph).  This is at
      variance with RFC 3168's requirement that a sender's congestion
      control response to ECN congestion indications be the same as to
      drops.  IETF approval, e.g., via an Experimental RFC in the IETF
      document stream, is required for any sender congestion response
      used in this area of experimentation.  See Section 4.1 for further
      discussion.

   Congestion Marking Differences:  Congestion marking at network nodes
      can be configured to maintain very shallow queues in conjunction
      with a different sender response to congestion indications (CE
      marks), e.g., as proposed in [I-D.ietf-tsvwg-ecn-l4s-id].  The
      traffic involved needs to be identified by the senders to the
      network nodes in order to avoid damage to other network traffic
      whose senders do not expect the more frequent congestion marking



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      used to maintain very shallow queues.  Use of different ECN
      codepoints, specifically ECT(0) and ECT(1), is a promising means
      of traffic identification for this purpose, but that technique is
      at variance with RFC 3168's requirement that ECT(0)-marked traffic
      and ECT(1)-marked traffic not receive different treatment in the
      network.  IETF approval, e.g., via an Experimental RFC in the IETF
      document stream, is required for any sender congestion response
      used in this area of experimentation.  See Section 4.2 for further
      discussion.

   TCP Control Packets and Retransmissions:  RFC 3168 limits the use of
      ECN with TCP to data packets, excluding retransmissions.  With the
      successful deployment of ECN in large portions of the Internet,
      there is interest in extending the benefits of ECN to TCP control
      packets (e.g., SYNs) and retransmitted packets, e.g., as proposed
      in [I-D.bagnulo-tcpm-generalized-ecn].  This is at variance with
      RFC 3168's prohibition of use of ECN for TCP control packets and
      retransmitted packets.  See Section 4.3 for further discussion.

   The scope of this memo is limited to these three areas of
   experimentation.  This memo expresses no view on the likely outcomes
   of the proposed experiments and does not specify the experiments in
   detail.  Additional experiments in these areas are possible, e.g., on
   use of ECN to support deployment of a protocol similar to DCTCP
   [I-D.ietf-tcpm-dctcp] beyond DCTCP's current applicability that is
   limited to data center environments.  The purpose of this memo is to
   remove constraints in standards track RFCs that stand in the way of
   these areas of experimentation.

2.1.  Effective Congestion Control is Required

   Congestion control remains an important aspect of the Internet
   architecture [RFC2914].  Any Experimental RFC in the IETF document
   stream that takes advantage of this memo's updates to any RFC is
   required to discuss the congestion control implications of the
   experiment(s) in order to provide assurance that deployment of the
   experiment(s) does not pose a congestion-based threat to the
   operation of the Internet.

2.2.  Considerations for Other Protocols

   ECN is widely deployed in the Internet and is being designed into
   additional protocols such as TRILL [I-D.ietf-trill-ecn-support].
   While the responsibility for coexistence with other protocols and
   transition from current ECN functionality falls primary upon the
   designers of experimental changes to ECN, this subsection provides
   some general guidelines for designers and users of other protocols




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   that minimize the likelihood of interaction with the areas of ECN
   experimentation enabled by this memo.

   1.  RFC 3168's forwarding behavior remains the preferred approach for
       routers that are not involved in ECN experiments, in particular
       continuing to treat the ECT(0) and ECT(1) codepoints as
       equivalent, as specified in Section 4.2 below.

   2.  The ECN CE codepoint SHOULD NOT be assumed to indicate that the
       packet would have been dropped if ECN were not in use, as that is
       not the case for either Congestion Response Differences
       experiments (see Section 4.1 below) or Congestion Marking
       Differences experiments (see Section 4.2 below).  This is already
       the case when the ECN field is used for Pre-Congestion
       Notification (PCN) [RFC6660].

   3.  Traffic marked with ECT(1) MUST NOT be originated, as specified
       in Section 4.2 below.

   4.  ECN may now be used on packets where it has not been used
       previously, specifically TCP control packets and retransmissions,
       see Section 4.3 below, and in particular its new requirements for
       middlebox behavior.  In general, any system or protocol that
       inspects or monitors network traffic SHOULD be prepared to
       encounter ECN usage on packets and traffic that currently do not
       use ECN.

   5.  Requirements for handling of the ECN field by tunnel
       encapsulation and decapsulation are specified in [RFC6040].
       Additional related guidance can be found in
       [I-D.ietf-tsvwg-ecn-encap-guidelines] and
       [I-D.ietf-tsvwg-rfc6040update-shim].

2.3.  Operational and Management Considerations

   Changes in network traffic behavior that result from ECN
   experimentation are likely to impact network operations and
   management.  Designers of ECN experiments are expected to anticipate
   possible impacts and consider how they may be dealt with.  Specific
   topics to consider include possible network management changes or
   extensions, monitoring of the experimental deployment, collection of
   data for evaluation of the experiment and possible interactions with
   other protocols, particularly protocols that encapsulate network
   traffic.

   For further discussion, see [RFC5706]; the questions in Appendix A
   provide a concise survey of some important aspects to consider.




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3.  ECN Nonce and RFC 3540

   As specified in RFC 3168, ECN uses two ECN Capable Transport (ECT)
   codepoints to indicate that a packet supports ECN, ECT(0) and ECT(1).
   The second codepoint, ECT(1), is used to support ECN nonce
   functionality that discourages receivers from exploiting ECN to
   improve their throughput at the expense of other network users, as
   specified in Experimental RFC 3540 [RFC3540].  This section explains
   why RFC 3540 is being reclassified as Historic and makes associated
   updates to RFC 3168.

   While the ECN nonce works as specified, and has been deployed in
   limited environments, widespread usage in the Internet has not
   materialized.  A study of the ECN behaviour of the top one million
   web servers using 2014 data [Trammell15] found that after ECN was
   negotiated, none of the 581,711 IPv4 servers tested were using both
   ECT codepoints, which would have been a possible sign of ECN nonce
   usage.  Of the 17,028 IPv6 servers tested, 4 set both ECT(0) and
   ECT(1) on data packets.  This might have been evidence of use of the
   ECN nonce by these 4 servers, but might equally have been due to
   erroneous re-marking of the ECN field by a middlebox or router.

   With the emergence of new experimental functionality that depends on
   use of the ECT(1) codepoint for other purposes, continuing to reserve
   that codepoint for the ECN nonce experiment is no longer justified.
   In addition, other approaches to discouraging receivers from
   exploiting ECN have emerged, see Appendix B.1 of
   [I-D.ietf-tsvwg-ecn-l4s-id].  Therefore, in support of ECN
   experimentation with the ECT(1) codepoint, this memo:

   o  Declares that the ECN nonce experiment [RFC3540] has concluded,
      and notes the absence of widespread deployment.

   o  Updates RFC 3168 [RFC3168] to remove discussion of the ECN nonce
      and use of ECT(1) for that nonce.

   The four primary updates to RFC 3168 that remove discussion of the
   ECN nonce and use of ECT(1) for that nonce are:

   1.  Remove the paragraph in Section 5 that immediately follows
       Figure 1; this paragraph discusses the ECN nonce as the
       motivation for two ECT codepoints.

   2.  Remove Section 11.2 "A Discussion of the ECN nonce." in its
       entirety.

   3.  Remove the last paragraph of Section 12, which states that ECT(1)
       may be used as part of the implementation of the ECN nonce.



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   4.  Remove the first two paragraphs of Section 20.2, which discuss
       the ECN nonce and alternatives.  No changes are made to the rest
       of Section 20.2, which discusses alternate uses for the fourth
       ECN codepoint.

   In addition, other less substantive RFC 3168 changes are required to
   remove all other mentions of the ECN nonce and to remove implications
   that ECT(1) is intended for use by the ECN nonce; these specific text
   updates are omitted for brevity.

4.  Updates to RFC 3168

   The following subsections specify updates to RFC 3168 to enable the
   three areas of experimentation summarized in Section 2.

4.1.  Congestion Response Differences

   RFC 3168 specifies that senders respond identically to packet drops
   and ECN congestion indications.  ECN congestion indications are
   predominately originated by Active Queue Management (AQM) mechanisms
   in intermediate buffers.  AQM mechanisms are usually configured to
   maintain shorter queue lengths than non-AQM based mechanisms,
   particularly non-AQM drop-based mechanisms such as tail-drop, as AQM
   mechanisms indicate congestion before the queue overflows.  While the
   occurrence of loss does not easily enable the receiver to determine
   if AQM is used, the receipt of an ECN Congestion Experienced (CE)
   mark conveys a strong likelihood that AQM was used to manage the
   bottleneck queue.  Hence an ECN congestion indication communicates a
   higher likelihood that a shorter queue exists at the network
   bottleneck node by comparison to a packet drop that indicates
   congestion [I-D.ietf-tcpm-alternativebackoff-ecn].  This difference
   suggests that for congestion indicated by ECN, a different sender
   congestion response (e.g., sender backs off by a smaller amount) may
   be appropriate by comparison to the sender response to congestion
   indicated by loss.  However, section 5 of RFC 3168 specifies that:

      Upon the receipt by an ECN-Capable transport of a single CE
      packet, the congestion control algorithms followed at the end-
      systems MUST be essentially the same as the congestion control
      response to a *single* dropped packet.

   This memo updates this RFC 3168 text to allow the congestion control
   response (including the TCP Sender's congestion control response) to
   a CE-marked packet to differ from the response to a dropped packet,
   provided that the changes from RFC 3168 are documented in an
   Experimental RFC in the IETF document stream.  The specific change to
   RFC 3168 is to insert the words "unless otherwise specified by an




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   Experimental RFC in the IETF document stream" at the end of the
   sentence quoted above.

   RFC 4774 [RFC4774] quotes the above text from RFC 3168 as background,
   but does not impose requirements based on that text.  Therefore no
   update to RFC 4774 is required to enable this area of
   experimentation.

   Section 6.1.2 of RFC 3168 specifies that:

      If the sender receives an ECN-Echo (ECE) ACK packet (that is, an
      ACK packet with the ECN-Echo flag set in the TCP header), then the
      sender knows that congestion was encountered in the network on the
      path from the sender to the receiver.  The indication of
      congestion should be treated just as a congestion loss in non-
      ECN-Capable TCP.  That is, the TCP source halves the congestion
      window "cwnd" and reduces the slow start threshold "ssthresh".

   This memo also updates this RFC 3168 text to allow the congestion
   control response (including the TCP Sender's congestion control
   response) to a CE-marked packet to differ from the response to a
   dropped packet, provided that the changes from RFC 3168 are
   documented in an Experimental RFC in the IETF document stream.  The
   specific change to RFC 3168 is to insert the words "Unless otherwise
   specified by an Experimental RFC in the IETF document stream" at the
   beginning of the second sentence quoted above.

4.2.  Congestion Marking Differences

   Taken to its limit, an AQM algorithm that uses ECN congestion
   indications can be configured to maintain very shallow queues,
   thereby reducing network latency by comparison to maintaining a
   larger queue.  Significantly more aggressive sender responses to ECN
   are needed to make effective use of such very shallow queues;
   Datacenter TCP (DCTCP) [I-D.ietf-tcpm-dctcp] provides an example.  In
   this case, separate network node treatments are essential, both to
   prevent the aggressive low latency traffic from starving conventional
   traffic (if present) and to prevent any conventional traffic
   disruption to any lower latency service that uses the very shallow
   queues.  Use of different ECN codepoints is a promising means of
   identifying these two classes of traffic to network nodes, and hence
   this area of experimentation is based on the use of the ECT(1)
   codepoint to request ECN congestion marking behavior in the network
   that differs from ECT(0) counterbalanced by use of a different IETF-
   approved congestion response to CE marks at the sender, e.g., as
   proposed in [I-D.ietf-tsvwg-ecn-l4s-id].

   Section 5 of RFC 3168 specifies that:



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      Routers treat the ECT(0) and ECT(1) codepoints as equivalent.

   This memo updates RFC 3168 to allow routers to treat the ECT(0) and
   ECT(1) codepoints differently, provided that the changes from RFC
   3168 are documented in an Experimental RFC in the IETF document
   stream.  The specific change to RFC 3168 is to insert the words
   "unless otherwise specified by an Experimental RFC in the IETF
   document stream" at the end of the above sentence.

   When an AQM is configured to use ECN congestion indications to
   maintain a very shallow queue, congestion indications are marked on
   packets that would not have been dropped if ECN was not in use.
   Section 5 of RFC 3168 specifies that:

      For a router, the CE codepoint of an ECN-Capable packet SHOULD
      only be set if the router would otherwise have dropped the packet
      as an indication of congestion to the end nodes.  When the
      router's buffer is not yet full and the router is prepared to drop
      a packet to inform end nodes of incipient congestion, the router
      should first check to see if the ECT codepoint is set in that
      packet's IP header.  If so, then instead of dropping the packet,
      the router MAY instead set the CE codepoint in the IP header.

   This memo updates RFC 3168 to allow congestion indications that are
   not equivalent to drops, provided that the changes from RFC 3168 are
   documented in an Experimental RFC in the IETF document stream.  The
   specific change is to change "For a router," to "Unless otherwise
   specified by an Experimental RFC in the IETF document stream" at the
   beginning of the first sentence of the above paragraph.

   A larger update to RFC 3168 is necessary to enable sender usage of
   ECT(1) to request network congestion marking behavior that maintains
   very shallow queues at network nodes.  When using loss as a
   congestion signal, the number of signals provided should be reduced
   to a minimum and hence only presence or absence of congestion is
   communicated.  In contrast, ECN can provide a richer signal, e.g., to
   indicate the current level of congestion, without the disadvantage of
   a larger number of packet losses.  A proposed experiment in this
   area, Low Latency Low Loss Scalable throughput (L4S)
   [I-D.ietf-tsvwg-ecn-l4s-id] significantly increases the CE marking
   probability for ECT(1)-marked traffic in a fashion that would
   interact badly with existing sender congestion response functionality
   because that functionality assumes that the network marks ECT packets
   as frequently as it would drop Not-ECT packets.  If network traffic
   that uses such a conventional sender congestion response were to
   encounter L4S's increased marking probability (and hence rate) at a
   network bottleneck queue, the resulting traffic throughput is likely




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   to be much less than intended for the level of congestion at the
   bottleneck queue.

   This memo updates RFC 3168 to remove that interaction for ECT(1).
   The specific update to Section 5 of RFC 3168 is to replace the
   following two paragraphs:

      Senders are free to use either the ECT(0) or the ECT(1) codepoint
      to indicate ECT, on a packet-by-packet basis.

      The use of both the two codepoints for ECT, ECT(0) and ECT(1), is
      motivated primarily by the desire to allow mechanisms for the data
      sender to verify that network elements are not erasing the CE
      codepoint, and that data receivers are properly reporting to the
      sender the receipt of packets with the CE codepoint set, as
      required by the transport protocol.  Guidelines for the senders
      and receivers to differentiate between the ECT(0) and ECT(1)
      codepoints will be addressed in separate documents, for each
      transport protocol.  In particular, this document does not address
      mechanisms for TCP end-nodes to differentiate between the ECT(0)
      and ECT(1) codepoints.  Protocols and senders that only require a
      single ECT codepoint SHOULD use ECT(0).

   with this paragraph:

      Protocols and senders MUST use the ECT(0) codepoint to indicate
      ECT unless otherwise specified by an Experimental RFC in the IETF
      document stream.  Protocols and senders MUST NOT use the ECT(1)
      codepoint to indicate ECT unless otherwise specified by an
      Experimental RFC in the IETF document stream.  Guidelines for
      senders and receivers to differentiate between the ECT(0) and
      ECT(1) codepoints will be addressed in separate documents, for
      each transport protocol.  In particular, this document does not
      address mechanisms for TCP end-nodes to differentiate between the
      ECT(0) and ECT(1) codepoints.

   Congestion Marking Differences experiments SHOULD modify the network
   behavior for ECT(1)-marked traffic rather than ECT(0)-marked traffic
   if network behavior for only one ECT codepoint is modified.
   Congestion Marking Differences experiments MUST NOT modify the
   network behavior for ECT(0)-marked traffic in a fashion that requires
   changes to sender congestion response to obtain desired network
   behavior.  If a Congestion Marking Differences experiment modifies
   the network behavior for ECT(1)-marked traffic, e.g., CE-marking
   behavior, in a fashion that requires changes to sender congestion
   response to obtain desired network behavior, then the Experimental
   RFC in the IETF document stream for that experiment MUST specify:




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   o  The sender congestion response to CE marking in the network, and

   o  Router behavior changes, or the absence thereof, in forwarding CE-
      marked packets that are part of the experiment.

   In addition, this memo updates RFC 3168 to remove discussion of the
   ECN nonce, as noted in Section 3 above.

4.3.  TCP Control Packets and Retransmissions

   With the successful use of ECN for traffic in large portions of the
   Internet, there is interest in extending the benefits of ECN to TCP
   control packets (e.g., SYNs) and retransmitted packets, e.g., as
   proposed by ECN++ [I-D.bagnulo-tcpm-generalized-ecn].

   RFC 3168 prohibits use of ECN for TCP control packets and
   retransmitted packets in a number of places:

   o  "To ensure the reliable delivery of the congestion indication of
      the CE codepoint, an ECT codepoint MUST NOT be set in a packet
      unless the loss of that packet in the network would be detected by
      the end nodes and interpreted as an indication of congestion."
      (Section 5.2)

   o  "A host MUST NOT set ECT on SYN or SYN-ACK packets."
      (Section 6.1.1)

   o  "pure acknowledgement packets (e.g., packets that do not contain
      any accompanying data) MUST be sent with the not-ECT codepoint."
      (Section 6.1.4)

   o  "This document specifies ECN-capable TCP implementations MUST NOT
      set either ECT codepoint (ECT(0) or ECT(1)) in the IP header for
      retransmitted data packets, and that the TCP data receiver SHOULD
      ignore the ECN field on arriving data packets that are outside of
      the receiver's current window."  (Section 6.1.5)

   o  "the TCP data sender MUST NOT set either an ECT codepoint or the
      CWR bit on window probe packets."  (Section 6.1.6)

   This memo updates RFC 3168 to allow the use of ECT codepoints on SYN
   and SYN-ACK packets, pure acknowledgement packets, window probe
   packets and retransmissions of packets that were originally sent with
   an ECT codepoint, provided that the changes from RFC 3168 are
   documented in an Experimental RFC in the IETF document stream.  The
   specific change to RFC 3168 is to insert the words "unless otherwise
   specified by an Experimental RFC in the IETF document stream" at the
   end of each sentence quoted above.



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   In addition, beyond requiring TCP senders not to set ECT on TCP
   control packets and retransmitted packets, RFC 3168 is silent on
   whether it is appropriate for a network element, e.g. a firewall, to
   discard such a packet as invalid.  For this area of ECN
   experimentation to be useful, middleboxes ought not to do that,
   therefore RFC 3168 is updated by adding the following text to the end
   of Section 6.1.1.1 on Middlebox Issues:

      Unless otherwise specified by an Experimental RFC in the IETF
      document stream, middleboxes SHOULD NOT discard TCP control
      packets and retransmitted TCP packets solely because the ECN field
      in the IP header does not contain Not-ECT.  An exception to this
      requirement occurs in responding to an attack that uses ECN
      codepoints other than Not-ECT.  For example, as part of the
      response, it may be appropriate to drop ECT-marked TCP SYN packets
      with higher probability than TCP SYN packets marked with not-ECT.
      Any such exceptional discarding of TCP control packets and
      retransmitted TCP packets in response to an attack MUST NOT be
      done routinely in the absence of an attack and SHOULD only be done
      if it is determined that the use of ECN is contributing to the
      attack.

5.  ECN for RTP Updates to RFC 6679

   RFC 6679 [RFC6679] specifies use of ECN for RTP traffic; it allows
   use of both the ECT(0) and ECT(1) codepoints, and provides the
   following guidance on use of these codepoints in section 7.3.1 :

      The sender SHOULD mark packets as ECT(0) unless the receiver
      expresses a preference for ECT(1) or for a random ECT value using
      the "ect" parameter in the "a=ecn-capable-rtp:" attribute.

   The Congestion Marking Differences area of experimentation increases
   the potential consequences of using ECT(1) instead of ECT(0), and
   hence the above guidance is updated by adding the following two
   sentences:

      Random ECT values MUST NOT be used, as that may expose RTP to
      differences in network treatment of traffic marked with ECT(1) and
      ECT(0) and differences in associated endpoint congestion
      responses.  In addition, ECT(0) MUST be used unless otherwise
      specified in an Experimental RFC in the IETF document stream.

   Section 7.3.3 of RFC 6679 specifies RTP's response to receipt of CE
   marked packets as being identical to the response to dropped packets:

      The reception of RTP packets with ECN-CE marks in the IP header is
      a notification that congestion is being experienced.  The default



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      reaction on the reception of these ECN-CE-marked packets MUST be
      to provide the congestion control algorithm with a congestion
      notification that triggers the algorithm to react as if packet
      loss had occurred.  There should be no difference in congestion
      response if ECN-CE marks or packet drops are detected.

   In support of Congestion Response Differences experimentation, this
   memo updates this text in a fashion similar to RFC 3168 to allow the
   RTP congestion control response to a CE-marked packet to differ from
   the response to a dropped packet, provided that the changes from RFC
   6679 are documented in an Experimental RFC in the IETF document
   stream.  The specific change to RFC 6679 is to insert the words
   "Unless otherwise specified by an Experimental RFC in the IETF
   document stream" and reformat the last two sentences to be subject to
   that condition, i.e.:

      The reception of RTP packets with ECN-CE marks in the IP header is
      a notification that congestion is being experienced.  Unless
      otherwise specified by an Experimental RFC in the IETF document
      stream:

      *  The default reaction on the reception of these ECN-CE-marked
         packets MUST be to provide the congestion control algorithm
         with a congestion notification that triggers the algorithm to
         react as if packet loss had occurred.

      *  There should be no difference in congestion response if ECN-CE
         marks or packet drops are detected.

   The second sentence of the immediately following paragraph in RFC
   6679 requires a related update:

      Other reactions to ECN-CE may be specified in the future,
      following IETF Review.  Detailed designs of such additional
      reactions MUST be specified in a Standards Track RFC and be
      reviewed to ensure they are safe for deployment under any
      restrictions specified.

   The update is to change "Standards Track RFC" to "Standards Track RFC
   or Experimental RFC in the IETF document stream" for consistency with
   the first update.

6.  ECN for DCCP Updates to RFCs 4341, 4342 and 5622

   The specifications of the three DCCP Congestion Control IDs (CCIDs) 2
   [RFC4341], 3 [RFC4342] and 4 [RFC5622] contain broadly the same
   wording as follows:




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      each DCCP-Data and DCCP-DataAck packet is sent as ECN Capable with
      either the ECT(0) or the ECT(1) codepoint set.

   This memo updates these sentences in each of the three RFCs as
   follows:

      each DCCP-Data and DCCP-DataAck packet is sent as ECN Capable.
      Unless otherwise specified by an Experimental RFC in the IETF
      document stream, such DCCP senders MUST set the ECT(0) codepoint.

   In support of Congestion Marking Differences experimentation (as
   noted in Section 3), this memo also updates all three of these RFCs
   to remove discussion of the ECN nonce.  The specific text updates are
   omitted for brevity.

7.  Acknowledgements

   The content of this draft, including the specific portions of RFC
   3168 that are updated draws heavily from
   [I-D.khademi-tsvwg-ecn-response], whose authors are gratefully
   acknowledged.  The authors of the Internet Drafts describing the
   experiments have motivated the production of this memo - their
   interest in innovation is welcome and heartily acknowledged.  Colin
   Perkins suggested updating RFC 6679 on RTP and provided guidance on
   where to make the updates.

   The draft has been improved as a result of comments from a number of
   reviewers, including Ben Campbell, Brian Carpenter, Benoit Claise,
   Spencer Dawkins, Gorry Fairhurst, Sue Hares, Ingemar Johansson, Naeem
   Khademi, Mirja Kuehlewind, Karen Nielsen, Hilarie Orman, Eric
   Rescorla, Adam Roach and Michael Welzl.  Bob Briscoe's thorough
   review of an early version of this memo resulted in numerous
   improvements including addition of the updates to the DCCP RFCs.

8.  IANA Considerations

   To reflect the reclassification of RFC 3540 as Historic, IANA is
   requested to update the Transmission Control Protocol (TCP) Header
   Flags registry (https://www.iana.org/assignments/tcp-header-flags/
   tcp-header-flags.xhtml#tcp-header-flags-1) to remove the registration
   of bit 7 as the NS (Nonce Sum) bit and add an annotation to the
   registry to state that bit 7 was used by Historic RFC 3540 as the NS
   (Nonce Sum) bit.








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9.  Security Considerations

   As a process memo that only relaxes restrictions on experimentation,
   there are no protocol security considerations, as security
   considerations for any experiments that take advantage of the relaxed
   restrictions are discussed in the Internet-Drafts that propose the
   experiments.

   However, effective congestion control is crucial to the continued
   operation of the Internet, and hence this memo places the
   responsibility for not breaking Internet congestion control on the
   experiments and the experimenters who propose them.  This
   responsibility includes the requirement to discuss congestion control
   implications in an IETF document stream Experimental RFC for each
   experiment, as stated in Section 2.1; review of that discussion by
   the IETF community and the IESG prior to RFC publication is intended
   to provide assurance that each experiment does not break Internet
   congestion control.

   See Appendix C.1 of [I-D.ietf-tsvwg-ecn-l4s-id] for discussion of
   alternatives to the ECN nonce.

10.  References

10.1.  Normative References

   [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>.

   [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>.

   [RFC3540]  Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
              Congestion Notification (ECN) Signaling with Nonces",
              RFC 3540, DOI 10.17487/RFC3540, June 2003,
              <https://www.rfc-editor.org/info/rfc3540>.







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   [RFC4341]  Floyd, S. and E. Kohler, "Profile for Datagram Congestion
              Control Protocol (DCCP) Congestion Control ID 2: TCP-like
              Congestion Control", RFC 4341, DOI 10.17487/RFC4341, March
              2006, <https://www.rfc-editor.org/info/rfc4341>.

   [RFC4342]  Floyd, S., Kohler, E., and J. Padhye, "Profile for
              Datagram Congestion Control Protocol (DCCP) Congestion
              Control ID 3: TCP-Friendly Rate Control (TFRC)", RFC 4342,
              DOI 10.17487/RFC4342, March 2006,
              <https://www.rfc-editor.org/info/rfc4342>.

   [RFC5622]  Floyd, S. and E. Kohler, "Profile for Datagram Congestion
              Control Protocol (DCCP) Congestion ID 4: TCP-Friendly Rate
              Control for Small Packets (TFRC-SP)", RFC 5622,
              DOI 10.17487/RFC5622, August 2009,
              <https://www.rfc-editor.org/info/rfc5622>.

   [RFC6679]  Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
              and K. Carlberg, "Explicit Congestion Notification (ECN)
              for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August
              2012, <https://www.rfc-editor.org/info/rfc6679>.

10.2.  Informative References

   [I-D.bagnulo-tcpm-generalized-ecn]
              Bagnulo, M. and B. Briscoe, "ECN++: Adding Explicit
              Congestion Notification (ECN) to TCP Control Packets",
              draft-bagnulo-tcpm-generalized-ecn-04 (work in progress),
              May 2017.

   [I-D.ietf-tcpm-alternativebackoff-ecn]
              Khademi, N., Welzl, M., Armitage, G., and G. Fairhurst,
              "TCP Alternative Backoff with ECN (ABE)", draft-ietf-tcpm-
              alternativebackoff-ecn-01 (work in progress), May 2017.

   [I-D.ietf-tcpm-dctcp]
              Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.,
              and G. Judd, "Datacenter TCP (DCTCP): TCP Congestion
              Control for Datacenters", draft-ietf-tcpm-dctcp-10 (work
              in progress), August 2017.

   [I-D.ietf-trill-ecn-support]
              Eastlake, D. and B. Briscoe, "TRILL: ECN (Explicit
              Congestion Notification) Support", draft-ietf-trill-ecn-
              support-03 (work in progress), May 2017.






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   [I-D.ietf-tsvwg-ecn-encap-guidelines]
              Briscoe, B., Kaippallimalil, J., and P. Thaler,
              "Guidelines for Adding Congestion Notification to
              Protocols that Encapsulate IP", draft-ietf-tsvwg-ecn-
              encap-guidelines-09 (work in progress), July 2017.

   [I-D.ietf-tsvwg-ecn-l4s-id]
              Schepper, K. and B. Briscoe, "Identifying Modified
              Explicit Congestion Notification (ECN) Semantics for
              Ultra-Low Queuing Delay", draft-ietf-tsvwg-ecn-l4s-id-00
              (work in progress), May 2017.

   [I-D.ietf-tsvwg-rfc6040update-shim]
              Briscoe, B., "Propagating Explicit Congestion Notification
              Across IP Tunnel Headers Separated by a Shim", draft-ietf-
              tsvwg-rfc6040update-shim-04 (work in progress), July 2017.

   [I-D.khademi-tsvwg-ecn-response]
              Khademi, N., Welzl, M., Armitage, G., and G. Fairhurst,
              "Updating the Explicit Congestion Notification (ECN)
              Specification to Allow IETF Experimentation", draft-
              khademi-tsvwg-ecn-response-01 (work in progress), July
              2016.

   [RFC4774]  Floyd, S., "Specifying Alternate Semantics for the
              Explicit Congestion Notification (ECN) Field", BCP 124,
              RFC 4774, DOI 10.17487/RFC4774, November 2006,
              <https://www.rfc-editor.org/info/rfc4774>.

   [RFC4844]  Daigle, L., Ed. and Internet Architecture Board, "The RFC
              Series and RFC Editor", RFC 4844, DOI 10.17487/RFC4844,
              July 2007, <https://www.rfc-editor.org/info/rfc4844>.

   [RFC5706]  Harrington, D., "Guidelines for Considering Operations and
              Management of New Protocols and Protocol Extensions",
              RFC 5706, DOI 10.17487/RFC5706, November 2009,
              <https://www.rfc-editor.org/info/rfc5706>.

   [RFC6040]  Briscoe, B., "Tunnelling of Explicit Congestion
              Notification", RFC 6040, DOI 10.17487/RFC6040, November
              2010, <https://www.rfc-editor.org/info/rfc6040>.

   [RFC6660]  Briscoe, B., Moncaster, T., and M. Menth, "Encoding Three
              Pre-Congestion Notification (PCN) States in the IP Header
              Using a Single Diffserv Codepoint (DSCP)", RFC 6660,
              DOI 10.17487/RFC6660, July 2012,
              <https://www.rfc-editor.org/info/rfc6660>.




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   [Trammell15]
              Trammell, B., Kuehlewind, M., Boppart, D., Learmonth, I.,
              Fairhurst, G., and R. Scheffenegger, "Enabling Internet-
              Wide Deployment of Explicit Congestion Notification".

              In Proc Passive & Active Measurement (PAM'15) Conference
              (2015)

Appendix A.  Change History

   [To be removed before RFC publication.]

   Changes from draft-ietf-tsvwg-ecn-experimentation-00 to -01:

   o  Add mention of DCTCP as another protocol that could benefit from
      ECN experimentation (near end of Section 2).

   Changes from draft-ietf-tsvwg-ecn-experimentation-01 to -02:

   o  Generalize to describe rationale for areas of experimentation,
      with less focus on individual experiments

   o  Add ECN terminology section

   o  Change name of "ECT Differences" experimentation area to
      "Congestion Marking Differences"

   o  Add overlooked RFC 3168 modification to Section 4.1

   o  Clarify text for Experimental RFC exception to ECT(1) non-usage
      requirement

   o  Add explanation of exception to "SHOULD NOT drop" requirement in
      4.3

   o  Rework RFC 3540 status change text to provide rationale for a
      separate status change document that makes RFC 3540 Historic.
      Don't obsolete RFC 3540.

   o  Significant editorial changes based on reviews by Mirja
      Kuehlewind, Michael Welzl and Bob Briscoe.

   Changes from draft-ietf-tsvwg-ecn-experimentation-02 to -03:

   o  Remove change history prior to WG adoption.

   o  Update L4S draft reference to reflect TSVWG adoption of draft.




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   o  Change the "SHOULD" for DCCP sender use of ECT(0) to a "MUST"
      (overlooked in earlier editing).

   o  Other minor edits.

   Changes from draft-ietf-tsvwg-ecn-experimentation-03 to -04:

   o  Change name of "Generalized ECN" experimentation area to "TCP
      Control Packets and Retransmissions."

   o  Add IANA Considerations text to request removal of the
      registration of the NS bit in the TCP header.

   Changes from draft-ietf-tsvwg-ecn-experimentation-04 to -05:

   o  Minor editorial changes from Area Director review

   Changes from draft-ietf-tsvwg-ecn-experimentation-05 to -06:

   o  Add summary of RFC 3168 changes to remove the ECN nonce, and use
      lower-case "nonce" instead of "Nonce" to match RFC 3168 usage.

   o  Add security considerations sentence to indicate that review of
      Experimental RFCs prior to publication approval is the means to
      ensure that congestion control is not broken by experiments.

   o  Other minor editorial changes from IETF Last Call

   Changes from draft-ietf-tsvwg-ecn-experimentation-06 to -07:

   o  Change draft title to make scope clear - this only covers relaxing
      of restrictions on ECN experimentation.

   o  Any Experimental RFC that takes advantage of this memo has to be
      in the IETF document stream.

   o  Added sections 2.2 and 2.3 on considerations for other protocols
      and O&M, relocated discussion of congestion control requirement to
      section 2.1 from section 4.4

   o  Remove text indicating that ECT(1) may be assigned to L4S - the
      requirement for an Experimental RFC suffices to ensure that
      coordination with L4S will occur.

   o  Improve explanation of attack response exception to not dropping
      packets "solely because the ECN field in the IP header does not
      contain Not-ECT" in Section 4.3




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   o  Fix L4S draft reference for discussion of ECN Nonce alternatives -
      it's Appendix C.1, not B.1.

   o  Numerous additional editorial changes from IESG Evaluation

Author's Address

   David Black
   Dell EMC
   176 South Street
   Hopkinton, MA  01748
   USA

   Email: david.black@dell.com





































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