Internet DRAFT - draft-grimes-tcpm-tcpsce

draft-grimes-tcpm-tcpsce







TCP Maintenance and Minor Extensions                         R.W. Grimes
Internet-Draft                                                  P. Heist
Intended status: Experimental                            4 November 2019
Expires: 7 May 2020


                   Some Congestion Experienced in TCP
                      draft-grimes-tcpm-tcpsce-01

Abstract

   This memo classifies a TCP code point ESCE ("Echo Some Congestion
   Experienced") for use in feedback of IP code point SCE ("Some
   Congestion Experienced").

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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 7 May 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
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   Please review these documents carefully, as they describe your rights
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   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.






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Table of Contents

   1.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  TCP Receiver  . . . . . . . . . . . . . . . . . . . . . . . .   3
     4.1.  Single ACK implementation . . . . . . . . . . . . . . . .   3
     4.2.  Simple Delayed ACK implementation . . . . . . . . . . . .   3
     4.3.  Dithered Delayed ACK implementation . . . . . . . . . . .   3
     4.4.  Advanced ACK implementation . . . . . . . . . . . . . . .   4
     4.5.  ACK Thinning  . . . . . . . . . . . . . . . . . . . . . .   4
   5.  TCP Sender  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   6.  Related Work  . . . . . . . . . . . . . . . . . . . . . . . .   4
     6.1.  More Accurate ECN Feedback in TCP . . . . . . . . . . . .   4
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   10. Normative References  . . . . . . . . . . . . . . . . . . . .   5
   11. Informative References  . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Terminology

   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
   [RFC2119] and [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Introduction

   This memo requests a TCP header codepoint for use as ESCE.

   This memo limits its scope to the definition of the TCP codepoint
   ESCE, with a few brief illustrations of how it may be used.

   SCE provides early and proportional feedback to the CC (congestion
   control) algorithms for transport protocols, including but not
   limited to TCP.  The [sce-repo] is a Linux kernel modified to support
   SCE, including:

   *  Enhancements to Linux's [cake] (Common Applications Kept Enhanced)
      AQM to support SCE signaling

   *  Modifications to the TCP receive path to reflect SCE signals back
      to the sender





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   *  The addition of three new TCP CC algorithms that modify the
      originals to add SCE support: Reno-SCE, DCTCP-SCE and Cubic-SCE
      (work in progress as of this writing)

3.  Background

   [I-D.morton-tsvwg-sce] defines the IP SCE codepoint.

4.  TCP Receiver

   The mechanism defined to feed back SCE signals to the sender
   explicitly makes use of the ESCE ("Echo Some Congestion Experienced")
   code point in the TCP header.

4.1.  Single ACK implementation

   Upon receipt of a packet an ACK is immediatly generated, the SCE
   codepoint is copied into the ESCE codepoint of the ACK.  This keeps
   the count of bytes SCE marked or not marked properly reflected in the
   ACK packet(s).  This valid implementation has the downside of
   increasing ACK traffic.  This implementation is NOT RECOMMENDED, but
   useful for experimental work.

4.2.  Simple Delayed ACK implementation

   Upon receipt of a packet without an SCE codepoint traditional delayed
   ACK processing is performed.  Upon receipt of a packet with an SCE
   codepoint immediate ACK processing SHOULD be done, this allows some
   delaying of ACK's, but creates earlier feedback of the congested
   state.  This has the negative effect of over signalling ESCE.

4.3.  Dithered Delayed ACK implementation

   Upon receipt of a packet the SCE codepoint is stored in the TCP
   state.  Multiple packets state may be stored.  Upon generation of an
   ACK, normal or delayed, the stored SCE state is used to set the state
   of ESCE.  If no SCE state is in the TCP state, then the ESCE code
   point MUST NOT be set.  If all of the packets to be ACKed have SCE
   state set then the ESCE code point MUST be set in the ACK.  If some
   of the packets to be ACKed have SCE state set then some proportional
   number of ACK packets SHOULD be sent with the ESCE code point set.
   Though this may defer a ESCE congestion signal when there is not a
   next packet for some time it is generally accepted that such sparse
   flows are not the source of congestion and thus the delayed signal is
   of low impact.  The goal is to have the same number of bytes marked
   with ESCE as arrived with SCE.





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4.4.  Advanced ACK implementation

   The Advanced ACK implementation actually immediately flushes any
   pending ACK's up to the _previous_ segment when the state of the SCE
   marking _changes_, allowing consecutive packets with the same SCE
   state to be coalesced by the normal delayed-ack logic.  The ACK
   volume is then inflated only slightly compared to an unmarked
   connection, and may actually involve fewer acks than a connection
   involving CE marks or losses, during which delayed acks are
   temporarily disabled.

4.5.  ACK Thinning

   Ack thinning is something that has been considered, given that [cake]
   includes an optional ack-filter which does thinning.  We have, for
   example, added consideration of the ESCE bit to Cake's ack-filter.
   Mathematically, the most extreme errors possible in either direction,
   due to ack thinning, are easily corrected during subsequent RTTs.

5.  TCP Sender

   The recommended response to each single segment marked with ESCE is
   to reduce cwnd by an amortised 1/sqrt(cwnd) segments.  If the growth
   rate is greater than that provided by the Reno-linear algorithm - eg.
   slow-start exponential or CUBIC polynomial - then the growth rate
   SHOULD also be reduced.

   Other responses, such as the 1/cwnd from DCTCP, are also acceptable
   but may perform less well.

   There are no changes to the response functions with respect to CE or
   packet loss specificed by this draft, hence [RFC3168] and [RFC8511]
   are still applicable

   This is still an area of continued investigation.

6.  Related Work

6.1.  More Accurate ECN Feedback in TCP [I-D.ietf-tcpm-accurate-ecn]

   AccECN replaces the [RFC3168] definition of the ECE and CWR bits (and
   the former NS bit) with its own three-bit field.  This new
   interpretation is predicated on successfully negotiating AccECN, and
   is not useful to SCE implementations because it provides no
   information about any ECT(1) codepoints received, and SCE does not
   need or use the extra information about CE marks that the three-bit
   field does provide.  Hence SCE may be considered mutually exclusive
   with AccECN on any given connection.



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   AccECN supports a fallback to [RFC3168] style signalling during the
   three-way handshake by recognising the normal requests and responses
   of an [RFC3168] endpoint.  SCE endpoints also exhibit [RFC3168]
   behaviour during the handshake, so this mutual exclusivity occurs
   naturally.  There will therefore be no confusion on the wire between
   the two experiments, even though SCE does not explicitly negotiate
   its upgrade from plain [RFC3168] behaviour.

   The latter is consistent with the (now historic) Nonce Sum
   specification, which also did not explicitly negotiate support, and
   used the same additional ECN codepoint and TCP header bit that SCE is
   now requesting.

7.  IANA Considerations

   This document requests one of the reserved bits in the TCP header,
   with the former TCP NS ("Nonce Sum") bit (bit 7) being suggested due
   to similarities with its previous usage.  [RFC8311] (section 3)
   obsoletes the NS codepoint making it avaliable for use.

8.  Security Considerations

   There are no Security considerations.

9.  Acknowledgements

   TBD

10.  Normative References

   [I-D.morton-tsvwg-sce]
              Morton, J. and R. Grimes, "The Some Congestion Experienced
              ECN Codepoint", draft-morton-tsvwg-sce-00 (work in
              progress), 2 July 2019,
              <https://www.ietf.org/archive/id/draft-morton-tsvwg-sce-
              00>.

   [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>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8311]  Black, D., "Relaxing Restrictions on Explicit Congestion
              Notification (ECN) Experimentation", RFC 8311,



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              DOI 10.17487/RFC8311, January 2018,
              <https://www.rfc-editor.org/info/rfc8311>.

11.  Informative References

   [cake]     "Cake - Common Applications Kept Enhanced", November 2019,
              <http://www.bufferbloat.net/projects/codel/wiki/Cake>.

   [I-D.ietf-tcpm-accurate-ecn]
              Briscoe, B., Kuehlewind, M., and R. Scheffenegger, "More
              Accurate ECN Feedback in TCP", draft-ietf-tcpm-accurate-
              ecn-09 (work in progress), 8 July 2019,
              <https://www.ietf.org/archive/id/draft-ietf-tcpm-accurate-
              ecn-09>.

   [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>.

   [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>.

   [sce-repo] "Some Congestion Experienced Reference Implementation
              GitHub Repository", November 2019,
              <https://github.com/chromi/sce/>.

Authors' Addresses

   Rodney W. Grimes
   Redacted
   Portland, OR 97217
   United States

   Email: rgrimes@freebsd.org


   Peter G. Heist
   Redacted
   463 11 Liberec 30
   Czech Republic

   Email: pete@heistp.net






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