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.
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Please review these documents carefully, as they describe your rights
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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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