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Versions: 00 01 02 03 04 draft-ietf-tsvwg-ecn-experimentation

Transport Area Working Group                                    D. Black
Internet-Draft                                                  Dell EMC
Obsoletes: 3540 (if approved)                           October 28, 2016
Updates: 3168, 4341, 4342, 5622, 6679
         (if approved)
Intended status: Standards Track
Expires: May 1, 2017


         Explicit Congestion Notification (ECN) Experimentation
                draft-black-tsvwg-ecn-experimentation-02

Abstract

   Multiple protocol experiments have been proposed that involve changes
   to Explicit Congestion Notification (ECN) as specified in RFC 3168.
   This memo summarizes the proposed areas of experimentation to provide
   an overview to the Internet community and updates RFC 3168, a
   Proposed Standard RFC, to allow the experiments to proceed without
   requiring a standards process exception for each Experimental RFC to
   update RFC 3168.  Each experiment is still required to be documented
   in an Experimental RFC.  In addition, this memo makes related updates
   to the ECN specifications for RTP in RFC 6679 and to the ECN
   specifications for DCCP in RFC 4341, RFC 4342 and RFC 5622.  This
   memo also records the conclusion of the ECN Nonce experiment in RFC
   3540, obsoletes RFC 3540 and reclassifies it as Historic to enable
   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 http://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 May 1, 2017.







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Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Scope of ECN Experiments  . . . . . . . . . . . . . . . . . .   3
   3.  ECN Nonce and RFC 3540  . . . . . . . . . . . . . . . . . . .   4
   4.  Updates to RFC 3168 . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Alternative Backoff . . . . . . . . . . . . . . . . . . .   5
     4.2.  ECT Differences . . . . . . . . . . . . . . . . . . . . .   5
     4.3.  Generalized ECN . . . . . . . . . . . . . . . . . . . . .   6
     4.4.  Effective Congestion Control is Required  . . . . . . . .   7
   5.  ECN for RTP Updates to RFC 6679 . . . . . . . . . . . . . . .   7
   6.  ECN for DCCP Updates to RFCs 4341, 4342 and 5622  . . . . . .   8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Multiple protocol experiments have been proposed that involve changes
   to Explicit Congestion Notification (ECN) as specified in RFC 3168
   [RFC3168].  This memo summarizes the proposed areas of
   experimentation to provide an overview to the Internet community and
   updates RFC 3168 to allow the experiments to proceed without
   requiring a standards process exception for each Experimental RFC to
   update RFC 3168, a Proposed Standard RFC.  This memo also makes
   related updates to the ECN specification for RTP in RFC 6679
   [RFC6679] for the same reason.  Each experiment is still required to
   be documented in one or more separate RFCs, but use of Experimental



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   RFCs for this purpose does not require a process exception to modify
   RFC 3168 or RFC 6679 when the modification falls within the bounds
   established by this memo.

   One of these areas of experimentation involves use of the ECT(1)
   codepoint that was dedicated to the ECN Nonce experiment as described
   in RFC 3540 [RFC3540].  This memo records the conclusion of the ECN
   Nonce experiment, obsoletes RFC 3540 and reclassifies it as Historic.

1.1.  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.  Scope of ECN Experiments

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

   Alternative Backoff:  For congestion indicated by ECN, use a
      different IETF-approved TCP sender response (e.g., reduce the
      response so that the sender backs off by a smaller amount) by
      comparison to congestion indicated by loss, e.g., as proposed in
      [I-D.khademi-tcpm-alternativebackoff-ecn] and
      [I-D.briscoe-tsvwg-ecn-l4s-id] - the experiment in the latter
      draft couples the backoff change to ECT Differences functionality
      (next bullet).  This is at variance with RFC 3168's requirement
      that a TCP sender's congestion control response to ECN congestion
      indications be the same as to drops.

   ECT Differences:  Use ECT(1) to request ECN congestion marking
      behavior in the network that differs from ECT(0) counterbalanced
      by a changed response to each mark at the sender, e.g., as
      proposed in [I-D.briscoe-tsvwg-ecn-l4s-id].  This 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.

   Generalized ECN:  Use ECN for TCP control packets (i.e., send control
      packets such as SYN with ECT marking) and for retransmitted
      packets, e.g., as proposed in [I-D.bagnulo-tsvwg-generalized-ecn].
      This is at variance with RFC 3168's prohibition of use of ECN for
      TCP control packets and retransmitted packets

   The scope of this memo is limited to these three areas of
   experimentation.  This memo neither prejudges the outcomes of the



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   proposed experiments nor specifies the experiments in detail.  The
   purpose of this memo is to remove constraints in standards track RFCs
   that serve to prohibit these areas of experimentation.

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),
   with the second codepoint used to support ECN nonce functionality to
   discourage receivers from exploiting ECN to improve their throughput
   at the expense of other network users, as specified in experimental
   RFC 3540 [RFC3540].

   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 Alexa top 1M 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 equally it might have been due to
   re-marking of the ECN field by an erroneous 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, alternative approaches to discouraging receivers from
   exploiting ECN have emerged, see Appendix B.1 of
   [I-D.briscoe-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  Obsoletes RFC 3540 in order to facilitate experimental use of the
      ECT(1) codepoint.

   o  Reclassifies RFC 3540 as Historic to document the ECN Nonce
      experiment and discourage further implementation of the ECN Nonce.

   o  Updates RFC 3168 [RFC3168] to remove discussion of the ECN Nonce
      and use of ECT(1) for that Nonce.  The specific text updates are
      omitted for brevity.

   The following guidance on ECT codepoint usage in Section 5 of RFC
   3168 is relevant when the ECN Nonce is not implemented:




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      Protocols and senders that only require a single ECT codepoint
      SHOULD use ECT(0).

   OPEN ISSUE: Change the above requirement in RFC 3168 from SHOULD to
   MUST towards reserving ECT(1) for experimentation?

4.  Updates to RFC 3168

   RFC 3168 can only be updated directly by another standards track RFC
   unless a standards process exception is approved by the IESG.  In
   support of the above areas of experimentation, and specifically to
   avoid multiple uncoordinated requests to the IESG for process
   exceptions, this memo updates RFC 3168 [RFC3168] ito allow changes in
   the following areas, provided that the changes are documented by an
   Experimental RFC.  It is also possible to change RFC 3168 via
   publication of another standards track RFC.

4.1.  Alternative Backoff

   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."

   In support of Alternative Backoff experimentation, this memo updates
   RFC 3168 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.  The specific change
   to RFC 3168 is to insert the words "unless otherwise specified by an
   Experimental RFC" 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.

4.2.  ECT Differences

   Section 5 of RFC 3168 specifies that:

      "Routers treat the ECT(0) and ECT(1) codepoints as equivalent.
      Senders are free to use either the ECT(0) or the ECT(1) codepoint
      to indicate ECT, on a packet-by-packet basis."





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   In support of ECT Differences experimentation, this memo updates RFC
   3168 to allow routers to treat the ECT(0) and ECT(1) codepoints
   differently, and allow requirements to be imposed on sender usage of
   ECT(0) and ECT(1), provided that the changes from RFC 3168 are
   documented in an Experimental RFC.  That change makes the second
   sentence quoted above misleading, so RFC 3168 is also updated to
   remove that sentence.  The specific change to RFC 3168 is to insert
   the words "unless otherwise specified by an Experimental RFC" at the
   end of the first sentence, and remove the second sentence with this
   result:

      "Routers treat the ECT(0) and ECT(1) codepoints as equivalent
      unless otherwise specified by an Experimental RFC."

   As ECT(0) was the original codepoint used to signal ECN capability,
   ECT Differences experiments SHOULD modify the network behavior for
   ECT(1) rather than ECT(0) if network behavior for only one ECT
   codepoint is modified.

   In support of ECT Differences experimentation, this memo also updates
   RFC 3168 to remove discussion of the ECN Nonce, as noted in Section 3
   above.

4.3.  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)





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

   In support of Generalized ECN experimentation, 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.  The specific change to RFC 3168 is to insert
   the words "unless otherwise specified by an Experimental RFC" at the
   end of each sentence quoted above.

4.4.  Effective Congestion Control is Required

   Congestion control remains an important aspect of the Internet
   architecture [RFC2914].  Any Experimental RFC that takes advantage of
   this memo's updates to RFC 3168 or RFC 6679 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.

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 ECT 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:

      Use of random ECT values is NOT RECOMMENDED, 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, e.g., as proposed in
      [I-D.briscoe-tsvwg-ecn-l4s-id].  ECT(0) SHOULD be used unless
      there is a need for more than one ECT codepoint or unless
      otherwise specified in an Experimental RFC.

   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 Alternative Backoff 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.  The specific change to RFC
   6679 is to insert the words "Unless otherwise specified by an
   Experimental RFC" 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:

      *  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 alternative
      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" 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:

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




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   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, such DCCP
      senders SHOULD set the ECT(0) codepoint.

   In support of ECT 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 Spencer Dawkins, Gorry Fairhurst, Ingemar
   Johansson, Naeem Khademi, Mirja Kuehlewind and Michael Welzl.  Bob
   Briscoe's thorough review of an early version of this draft resulted
   in numerous improvments including addition of the updates to the DCCP
   RFCs.

8.  IANA Considerations

   This memo includes no request to IANA.

9.  Security Considerations

   As a process memo that makes no changes to existing protocols, there
   are no protocol security considerations.

   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, as specified in
   Section 4.4.

   Security considerations for the proposed experiments are dicussed in
   the Internet-Drafts that propose them.




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   See Appendix B.1 of [I-D.briscoe-tsvwg-ecn-l4s-id] for discussion of
   alteratives 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,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41,
              RFC 2914, DOI 10.17487/RFC2914, September 2000,
              <http://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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc3540>.

   [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, <http://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,
              <http://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,
              <http://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, <http://www.rfc-editor.org/info/rfc6679>.




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10.2.  Informative References

   [I-D.bagnulo-tsvwg-generalized-ecn]
              Bagnulo, M. and B. Briscoe, "Adding Explicit Congestion
              Notification (ECN) to TCP control packets", draft-bagnulo-
              tsvwg-generalized-ecn-01 (work in progress), July 2016.

   [I-D.briscoe-tsvwg-ecn-l4s-id]
              Schepper, K., Briscoe, B., and I. Tsang, "Identifying
              Modified Explicit Congestion Notification (ECN) Semantics
              for Ultra-Low Queuing Delay", draft-briscoe-tsvwg-ecn-l4s-
              id-01 (work in progress), March 2016.

   [I-D.khademi-tcpm-alternativebackoff-ecn]
              Khademi, N., Welzl, M., Armitage, G., and G. Fairhurst,
              "TCP Alternative Backoff with ECN (ABE)", draft-khademi-
              tcpm-alternativebackoff-ecn-00 (work in progress), May
              2016.

   [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,
              <http://www.rfc-editor.org/info/rfc4774>.

   [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-black-tsvwg-ecn-experimentation-00 to -01

   o  Section 4.2 - also update RFC 3168 to remove sentence indicating
      that senders are free to use both ECT codepoints.  Add a SHOULD
      for ECT Differences experiments to use ECT(1).



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   o  Section 5 - only discourage use of random ECT values, but use NOT
      RECOMMENDED to do so.  Consistent use of ECT(1) without using
      ECT(0) is ok.  Mention possible changes in endpoint response.

   o  Add more Acknowledgements and Change History

   o  Additional editorial changes.

   Changes from draft-black-tsvwg-ecn-experimentation-01 to -02

   o  Add DCCP RFC updates and one missing RFC 3168 update (probe
      packets).

   o  Discourage RTP usage of ECT(1).

   o  Strengthen text on lack of ECN Nonce deployment.

   o  Cross-reference the L4S draft appendix that discusses ECN Nonce
      alternatives.

   o  Additional editorial changes.

Author's Address

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

   Email: david.black@dell.com




















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