Transport Area Working Group D. Black
Internet-Draft Dell EMC
Obsoletes: 3540 (if approved) September 30, 2016
Updates: 3168, 6679 (if approved)
Intended status: Standards Track
Expires: April 3, 2017
Explicit Congestion Notification (ECN) Experimentation
draft-black-tsvwg-ecn-experimentation-00
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. This memo also makes related updates to the ECN
specification for RTP in RFC 6679 for the same reason. Each
experiment is still required to be documented in an Experimental RFC.
This memo also records the conclusion of the ECN Nonce experiment in
RFC 3540, obsoletes RFC 3540 and reclassifies it as Historic.
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 April 3, 2017.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Scope of ECN Experiments . . . . . . . . . . . . . . . . . . 3
3. ECN Nonce and RFC 3540 . . . . . . . . . . . . . . . . . . . 3
4. Updates to RFC 3168 . . . . . . . . . . . . . . . . . . . . . 4
4.1. Alternative Backoff . . . . . . . . . . . . . . . . . . . 4
4.2. ECT Differences . . . . . . . . . . . . . . . . . . . . . 5
4.3. Generalized ECN . . . . . . . . . . . . . . . . . . . . . 5
4.4. Effective Congestion Control is Required . . . . . . . . 6
5. ECN for RTP Updates to RFC 6679 . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
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
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
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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", "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; in each
case, the cited Internet-Draft should be consulted for the goals and
rationale of the proposed experiment:
Alternative Backoff: For congestion indicated by ECN, use a
different TCP sender response (e.g., backoff by a smaller amount)
by comparison to congestion indicated by loss, e.g., as specified
in [I-D.khademi-tcpm-alternativebackoff-ecn]. 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), e.g., as
specified 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 specified 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.
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].
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While the ECN Nonce works as specified, and has been deployed in
limited environments, widespread usage in the Internet has not
materialized, as the potential for this sort of receiver ECN
exploitation has not turned out to be a significant concern in
practice. 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 is no longer justified.
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:
Protocols and senders that only require a single ECT codepoint
SHOULD use ECT(0).
4. Updates to RFC 3168
In support of these areas of experimentation, this memo updates RFC
3168 [RFC3168] to 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 a 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
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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."
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. The specific change to RFC 3168
is to insert the words "Unless otherwise specified by an Experimental
RFC" and combine the two sentences into a single sentence with this
result:
"Unless otherwise specified by an Experimental RFC, routers treat
the ECT(0) and ECT(1) codepoints as equivalent, and senders are
free to use either the ECT(0) or the ECT(1) codepoint to indicate
ECT, on a packet-by-packet basis."
As ECT(0) was the original codepoint used to signal ECN capability,
it is preferable for ECT Differences experiments to modify the
behavior of ECT(1) rather than ECT(0) if behavior of 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:
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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)
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, 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.
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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 sentence:
Use of ECT(1) and random ECT values is discouraged, as that may
expose RTP to differences in network treatment of ECT(1) and
ECT(0), e.g., as proposed in [I-D.briscoe-tsvwg-ecn-l4s-id].
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
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
3168 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.
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The update is to change "Standards Track RFC" to "Standards Track RFC
or Experimental RFC" for consistency with the first update.
6. 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 and provided guidance on where to
make the updates.
7. IANA Considerations
This memo includes no request to IANA.
8. 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.
9. References
9.1. 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>.
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[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>.
[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>.
9.2. 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>.
Author's Address
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David Black
Dell EMC
176 South Street
Hopkinton, MA 01748
USA
Email: david.black@dell.com
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