draft-ietf-tsvwg-ecn-experimentation-08.txt   rfc8311.txt 
Transport Area Working Group D. Black Internet Engineering Task Force (IETF) D. Black
Internet-Draft Dell EMC Request for Comments: 8311 Dell EMC
Updates: 3168, 4341, 4342, 5622, 6679 November 13, 2017 Updates: 3168, 4341, 4342, 5622, 6679 January 2018
(if approved) Category: Standards Track
Intended status: Standards Track ISSN: 2070-1721
Expires: May 17, 2018
Relaxing Restrictions on Explicit Congestion Notification (ECN) Relaxing Restrictions on
Experimentation Explicit Congestion Notification (ECN) Experimentation
draft-ietf-tsvwg-ecn-experimentation-08
Abstract Abstract
This memo updates RFC 3168, which specifies Explicit Congestion This memo updates RFC 3168, which specifies Explicit Congestion
Notification (ECN) as an alternative to packet drops for indicating Notification (ECN) as an alternative to packet drops for indicating
network congestion to endpoints. It relaxes restrictions in RFC 3168 network congestion to endpoints. It relaxes restrictions in RFC 3168
that hinder experimentation towards benefits beyond just removal of that hinder experimentation towards benefits beyond just removal of
loss. This memo summarizes the anticipated areas of experimentation loss. This memo summarizes the anticipated areas of experimentation
and updates RFC 3168 to enable experimentation in these areas. An and updates RFC 3168 to enable experimentation in these areas. An
Experimental RFC in the IETF document stream is required to take Experimental RFC in the IETF document stream is required to take
advantage of any of these enabling updates. In addition, this memo advantage of any of these enabling updates. In addition, this memo
makes related updates to the ECN specifications for RTP in RFC 6679 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 and for the Datagram Congestion Control Protocol (DCCP) in RFCs 4341,
records the conclusion of the ECN nonce experiment in RFC 3540, and 4342, and 5622. This memo also records the conclusion of the ECN
provides the rationale for reclassification of RFC 3540 as Historic; nonce experiment in RFC 3540 and provides the rationale for
this reclassification enables new experimental use of the ECT(1) reclassification of RFC 3540 from Experimental to Historic; this
reclassification enables new experimental use of the ECT(1)
codepoint. codepoint.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This is an Internet Standards Track document.
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-
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Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
This Internet-Draft will expire on May 17, 2018. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8311.
Copyright Notice Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the Copyright (c) 2018 IETF Trust and the persons identified as the
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than English. than English.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. ECN Terminology . . . . . . . . . . . . . . . . . . . . . 3 1.1. ECN Terminology . . . . . . . . . . . . . . . . . . . . . 4
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 4 1.2. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. ECN Experimentation: Overview . . . . . . . . . . . . . . . . 4 2. ECN Experimentation: Overview . . . . . . . . . . . . . . . . 5
2.1. Effective Congestion Control is Required . . . . . . . . 5 2.1. Effective Congestion Control is Required . . . . . . . . 6
2.2. Network Considerations for ECN Experimentation . . . . . 5 2.2. Network Considerations for ECN Experimentation . . . . . 6
2.3. Operational and Management Considerations . . . . . . . . 6 2.3. Operational and Management Considerations . . . . . . . . 7
3. ECN Nonce and RFC 3540 . . . . . . . . . . . . . . . . . . . 7 3. ECN Nonce and RFC 3540 . . . . . . . . . . . . . . . . . . . 8
4. Updates to RFC 3168 . . . . . . . . . . . . . . . . . . . . . 8 4. Updates to RFC 3168 . . . . . . . . . . . . . . . . . . . . . 9
4.1. Congestion Response Differences . . . . . . . . . . . . . 8 4.1. Congestion Response Differences . . . . . . . . . . . . . 9
4.2. Congestion Marking Differences . . . . . . . . . . . . . 9 4.2. Congestion Marking Differences . . . . . . . . . . . . . 10
4.3. TCP Control Packets and Retransmissions . . . . . . . . . 12 4.3. TCP Control Packets and Retransmissions . . . . . . . . . 13
5. ECN for RTP Updates to RFC 6679 . . . . . . . . . . . . . . . 13 5. ECN for RTP Updates to RFC 6679 . . . . . . . . . . . . . . . 14
6. ECN for DCCP Updates to RFCs 4341, 4342 and 5622 . . . . . . 15 6. ECN for DCCP Updates to RFCs 4341, 4342, and 5622 . . . . . . 16
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 8. Security Considerations . . . . . . . . . . . . . . . . . . . 16
9. Security Considerations . . . . . . . . . . . . . . . . . . . 16 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 16 9.1. Normative References . . . . . . . . . . . . . . . . . . 17
10.1. Normative References . . . . . . . . . . . . . . . . . . 16 9.2. Informative References . . . . . . . . . . . . . . . . . 18
10.2. Informative References . . . . . . . . . . . . . . . . . 17 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 20
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 21 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction 1. Introduction
This memo updates RFC 3168 [RFC3168] which specifies Explicit This memo updates RFC 3168 [RFC3168], which specifies Explicit
Congestion Notification (ECN) as an alternative to packet drops for Congestion Notification (ECN) as an alternative to packet drops for
indicating network congestion to endpoints. It relaxes restrictions indicating network congestion to endpoints. It relaxes restrictions
in RFC 3168 that hinder experimentation towards benefits beyond just in RFC 3168 that hinder experimentation towards benefits beyond just
removal of loss. This memo summarizes the proposed areas of removal of loss. This memo summarizes the proposed areas of
experimentation and updates RFC 3168 to enable experimentation in experimentation and updates RFC 3168 to enable experimentation in
these areas. An Experimental RFC in the IETF document stream these areas. An Experimental RFC in the IETF document stream
[RFC4844] is required to take advantage of any of these enabling [RFC4844] is required to take advantage of any of these enabling
updates. Putting all of these updates into a single document enables updates. Putting all of these updates into a single document enables
experimentation to proceed without requiring a standards process experimentation to proceed without requiring a standards process
exception for each Experimental RFC that needs changes to RFC 3168, a exception for each Experimental RFC that needs changes to RFC 3168, a
Proposed Standard RFC. Proposed Standard RFC.
There is no need for this memo to update RFC 3168 to simplify There is no need for this memo to update RFC 3168 to simplify
standardization of protocols and mechanisms that are documented in standardization of protocols and mechanisms that are documented in
Standards Track RFCs, as any Standards Track RFC can update RFC 3168 Standards Track RFCs, as any Standards Track RFC can update RFC 3168
directly without either relying on updates in this memo or using a directly without either relying on updates in this memo or using a
standards process exception. standards process exception.
In addition, this memo makes related updates to the ECN specification In addition, this memo makes related updates to the ECN specification
for RTP [RFC6679] and for three DCCP profiles ([RFC4341], [RFC4342] for RTP [RFC6679] and for three DCCP profiles ([RFC4341], [RFC4342],
and [RFC5622]) for the same reason. Each experiment is still and [RFC5622]) for the same reason. Each experiment is still
required to be documented in one or more separate RFCs, but use of required to be documented in one or more separate RFCs, but use of
Experimental RFCs for this purpose does not require a process Experimental RFCs for this purpose does not require a process
exception to modify any of these Proposed Standard RFCs when the exception to modify any of these Proposed Standard RFCs when the
modification falls within the bounds established by this memo (RFC modification falls within the bounds established by this memo (RFC
5622 is an Experimental RFC; it is modified by this memo for 5622 is an Experimental RFC; it is modified by this memo for
consistency with modifications to the other two DCCP RFCs). consistency with modifications to the other two DCCP RFCs).
Some of the anticipated experimentation includes use of the ECT(1) Some of the anticipated experimentation includes use of the ECT(1)
codepoint that was dedicated to the ECN nonce experiment in RFC 3540 codepoint that was dedicated to the ECN nonce experiment in RFC 3540
[RFC3540]. This memo records the conclusion of the ECN nonce [RFC3540]. This memo records the conclusion of the ECN nonce
experiment and provides the explanation for reclassification of RFC experiment and provides the explanation for reclassification of RFC
3540 as Historic in order to enable new experimental use of the 3540 from Experimental to Historic in order to enable new
ECT(1) codepoint. experimental use of the ECT(1) codepoint.
1.1. ECN Terminology 1.1. ECN Terminology
ECT: ECN-Capable Transport. One of the two codepoints ECT(0) or 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 ECT(1), in the ECN field [RFC3168] of the IP header (v4 or v6).
ECN-capable sender sets one of these to indicate that both transport An ECN-capable sender sets one of these to indicate that both
end-points support ECN. transport endpoints support ECN.
Not-ECT: The ECN codepoint set by senders that indicates that the Not-ECT: The ECN codepoint set by senders that indicates that the
transport is not ECN-capable. transport is not ECN capable.
CE: Congestion Experienced. The ECN codepoint that an intermediate CE: Congestion Experienced. The ECN codepoint that an intermediate
node sets to indicate congestion. A node sets an increasing node sets to indicate congestion. A node sets an increasing
proportion of ECT packets to CE as the level of congestion increases. proportion of ECT packets to Congestion Experienced (CE) as the
level of congestion increases.
1.2. Requirements Language 1.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC "OPTIONAL" in this document are to be interpreted as described in
2119 [RFC2119]. BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. ECN Experimentation: Overview 2. ECN Experimentation: Overview
Three areas of ECN experimentation are covered by this memo; the Three areas of ECN experimentation are covered by this memo; the
cited Internet-Drafts should be consulted for the detailed goals and cited documents should be consulted for the detailed goals and
rationale of each proposed experiment: rationale of each proposed experiment:
Congestion Response Differences: An ECN congestion indication Congestion Response Differences: An ECN congestion indication
communicates a higher likelihood than a dropped packet that a communicates a higher likelihood than a dropped packet that a
short queue exists at the network bottleneck node short queue exists at the network bottleneck node [TCP-ABE]. This
[I-D.ietf-tcpm-alternativebackoff-ecn]. This difference suggests difference suggests that for congestion indicated by ECN, a
that for congestion indicated by ECN, a different sender different sender congestion response (e.g., sender backs off by a
congestion response (e.g., sender backs off by a smaller amount) smaller amount) may be appropriate by comparison to the sender
may be appropriate by comparison to the sender response to response to congestion indicated by loss. Two examples of
congestion indicated by loss. Two examples of proposed sender proposed sender congestion response changes are described in
congestion response changes are described in [TCP-ABE] and [ECN-L4S] -- the proposal in the latter document
[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 couples the sender congestion response change to Congestion
Marking Differences functionality (see next paragraph). These Marking Differences functionality (see next paragraph). These
changes are at variance with RFC 3168's requirement that a changes are at variance with the requirement in RFC 3168 that a
sender's congestion control response to ECN congestion indications sender's congestion control response to ECN congestion indications
be the same as to drops. IETF approval, e.g., via an Experimental be the same as to drops. IETF approval, e.g., via an Experimental
RFC in the IETF document stream, is required for any sender RFC in the IETF document stream, is required for any sender
congestion response used in this area of experimentation. See congestion response used in this area of experimentation. See
Section 4.1 for further discussion. Section 4.1 for further discussion.
Congestion Marking Differences: Congestion marking at network nodes Congestion Marking Differences: Congestion marking at network nodes
can be configured to maintain very shallow queues in conjunction can be configured to maintain very shallow queues in conjunction
with a different sender response to congestion indications (CE with a different sender response to congestion indications (CE
marks), e.g., as proposed in [I-D.ietf-tsvwg-ecn-l4s-id]. The marks), e.g., as proposed in [ECN-L4S]. The traffic involved
traffic involved needs to be identified by the senders to the needs to be identified by the senders to the network nodes in
network nodes in order to avoid damage to other network traffic order to avoid damage to other network traffic whose senders do
whose senders do not expect the more frequent congestion marking not expect the more frequent congestion marking used to maintain
used to maintain very shallow queues. Use of different ECN very shallow queues. Use of different ECN codepoints,
codepoints, specifically ECT(0) and ECT(1), is a promising means specifically ECT(0) and ECT(1), is a promising means of traffic
of traffic identification for this purpose, but that technique is identification for this purpose, but that technique is at variance
at variance with RFC 3168's requirement that ECT(0)-marked traffic with the requirement in RFC 3168 that traffic marked as ECT(0) not
and ECT(1)-marked traffic not receive different treatment in the receive different treatment in the network by comparison to
network. IETF approval, e.g., via an Experimental RFC in the IETF traffic marked as ECT(1). IETF approval, e.g., via an
document stream, is required for any differences in congestion Experimental RFC in the IETF document stream, is required for any
marking or sender congestion response used in this area of differences in congestion marking or sender congestion response
experimentation. See Section 4.2 for further discussion. used in this area of experimentation. See Section 4.2 for further
discussion.
TCP Control Packets and Retransmissions: RFC 3168 limits the use of TCP Control Packets and Retransmissions: RFC 3168 limits the use of
ECN with TCP to data packets, excluding retransmissions. With the ECN with TCP to data packets, excluding retransmissions. With the
successful deployment of ECN in large portions of the Internet, successful deployment of ECN in large portions of the Internet,
there is interest in extending the benefits of ECN to TCP control there is interest in extending the benefits of ECN to TCP control
packets (e.g., SYNs) and retransmitted packets, e.g., as proposed packets (e.g., SYNs) and retransmitted packets, e.g., as proposed
in [I-D.bagnulo-tcpm-generalized-ecn]. This is at variance with in [ECN-TCP]. This is at variance with RFC 3168's prohibition of
RFC 3168's prohibition of use of ECN for TCP control packets and ECN for TCP control packets and retransmitted packets. See
retransmitted packets. See Section 4.3 for further discussion. Section 4.3 for further discussion.
The scope of this memo is limited to these three areas of The scope of this memo is limited to these three areas of
experimentation. This memo expresses no view on the likely outcomes experimentation. This memo expresses no view on the likely outcomes
of the proposed experiments and does not specify the experiments in of the proposed experiments and does not specify the experiments in
detail. Additional experiments in these areas are possible, e.g., on detail. Additional experiments in these areas are possible, e.g., on
use of ECN to support deployment of a protocol similar to DCTCP use of ECN to support deployment of a protocol similar to Data Center
[I-D.ietf-tcpm-dctcp] beyond DCTCP's current applicability that is TCP (DCTCP) [RFC8257] beyond DCTCP's current applicability that is
limited to data center environments. The purpose of this memo is to limited to data center environments. The purpose of this memo is to
remove constraints in standards track RFCs that stand in the way of remove constraints in Standards Track RFCs that stand in the way of
these areas of experimentation. these areas of experimentation.
2.1. Effective Congestion Control is Required 2.1. Effective Congestion Control is Required
Congestion control remains an important aspect of the Internet Congestion control remains an important aspect of the Internet
architecture [RFC2914]. Any Experimental RFC in the IETF document architecture [RFC2914]. Any Experimental RFC in the IETF document
stream that takes advantage of this memo's updates to any RFC is stream that takes advantage of this memo's updates to any RFC is
required to discuss the congestion control implications of the required to discuss the congestion control implications of the
experiment(s) in order to provide assurance that deployment of the experiment(s) in order to provide assurance that deployment of the
experiment(s) does not pose a congestion-based threat to the experiment(s) does not pose a congestion-based threat to the
operation of the Internet. operation of the Internet.
2.2. Network Considerations for ECN Experimentation 2.2. Network Considerations for ECN Experimentation
ECN functionality [RFC3168] is becoming widely deployed in the ECN functionality [RFC3168] is becoming widely deployed in the
Internet and is being designed into additional protocols such as Internet and is being designed into additional protocols such as
TRILL [I-D.ietf-trill-ecn-support]. ECN experiments are expected to Transparent Interconnection of Lots of Links (TRILL) [ECN-TRILL].
coexist with deployed ECN functionality, with the responsibility for ECN experiments are expected to coexist with deployed ECN
that coexistence falling primarily upon designers of experimental functionality, with the responsibility for that coexistence falling
changes to ECN. In addition, protocol designers and implementers, as primarily upon designers of experimental changes to ECN. In
well as network operators, may desire to anticipate and/or support addition, protocol designers and implementers, as well as network
ECN experiments. The following guidelines will help avoid conflicts operators, may desire to anticipate and/or support ECN experiments.
with the areas of ECN experimentation enabled by this memo: The following guidelines will help avoid conflicts with the areas of
ECN experimentation enabled by this memo:
1. RFC 3168's forwarding behavior remains the preferred approach for 1. Forwarding behavior as described in RFC 3168 remains the
routers that are not involved in ECN experiments, in particular preferred approach for routers that are not involved in ECN
continuing to treat the ECT(0) and ECT(1) codepoints as experiments, in particular continuing to treat the ECT(0) and
equivalent, as specified in Section 4.2 below. ECT(1) codepoints as equivalent, as specified in Section 4.2
below.
2. Network nodes that forward packets SHOULD NOT assume that the ECN 2. Network nodes that forward packets SHOULD NOT assume that the ECN
CE codepoint indicates that the packet would have been dropped if CE codepoint indicates that the packet would have been dropped if
ECN were not in use. This is because Congestion Response ECN were not in use. This is because Congestion Response
Differences experiments employ different congestion responses to Differences experiments employ different congestion responses to
dropped packets by comparison to receipt of CE-marked packets dropped packets by comparison to receipt of CE-marked packets
(see Section 4.1 below), so CE-marked packets SHOULD NOT be (see Section 4.1 below), so CE-marked packets SHOULD NOT be
arbitrarily dropped. A corresponding difference in congestion arbitrarily dropped. A corresponding difference in congestion
responses already occurs when the ECN field is used for Pre- responses already occurs when the ECN field is used for
Congestion Notification (PCN) [RFC6660]. Pre-Congestion Notification (PCN) [RFC6660].
3. A network node MUST NOT originate traffic marked with ECT(1) 3. A network node MUST NOT originate traffic marked with ECT(1)
unless the network node is participating in a Congestion Marking unless the network node is participating in a Congestion Marking
Differences experiment that uses ECT(1), as specified in Differences experiment that uses ECT(1), as specified in
Section 4.2 below. Section 4.2 below.
Some ECN experiments use ECN with packets where it has not been used Some ECN experiments use ECN with packets where ECN has not been used
previously, specifically TCP control packets and retransmissions, see previously, specifically TCP control packets and retransmissions; see
Section 4.3 below, and in particular its new requirements for Section 4.3 below. The new middlebox behavior requirements in that
middlebox behavior. In general, any system or protocol that inspects section are of particular importance. In general, any system or
or monitors network traffic SHOULD be prepared to encounter ECN usage protocol that inspects or monitors network traffic SHOULD be prepared
on packets and traffic that currently do not use ECN. to encounter ECN usage on packets and traffic that currently do not
use ECN.
ECN field handling requirements for tunnel encapsulation and ECN field handling requirements for tunnel encapsulation and
decapsulation are specified in [RFC6040] which is in the process of decapsulation are specified in [RFC6040], which is in the process of
being updated by [I-D.ietf-tsvwg-rfc6040update-shim]. Related being updated by [ECN-SHIM]. Related guidance for encapsulations
guidance for encapsulations whose outer headers are not IP headers whose outer headers are not IP headers can be found in [ECN-ENCAP].
can be found in [I-D.ietf-tsvwg-ecn-encap-guidelines]. These These requirements and guidance apply to all traffic, including
requirements and guidance apply to all traffic, including traffic traffic that is part of any ECN experiment.
that is part of any ECN experiment.
2.3. Operational and Management Considerations 2.3. Operational and Management Considerations
Changes in network traffic behavior that result from ECN Changes in network traffic behavior that result from ECN
experimentation are likely to impact network operations and experimentation are likely to impact network operations and
management. Designers of ECN experiments are expected to anticipate management. Designers of ECN experiments are expected to anticipate
possible impacts and consider how they may be dealt with. Specific possible impacts and consider how they may be dealt with. Specific
topics to consider include possible network management changes or topics to consider include possible network management changes or
extensions, monitoring of the experimental deployment, collection of extensions, monitoring of the experimental deployment, collection of
data for evaluation of the experiment and possible interactions with data for evaluation of the experiment, and possible interactions with
other protocols, particularly protocols that encapsulate network other protocols, particularly protocols that encapsulate network
traffic. traffic.
For further discussion, see [RFC5706]; the questions in Appendix A of For further discussion, see [RFC5706]; the questions in Appendix A of
RFC 5706 provide a concise survey of some important aspects to RFC 5706 provide a concise survey of some important aspects to
consider. consider.
3. ECN Nonce and RFC 3540 3. ECN Nonce and RFC 3540
As specified in RFC 3168, ECN uses two ECN Capable Transport (ECT) 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). codepoints, ECT(0) and ECT(1), to indicate that a packet supports
RFC 3168 assigned the second codepoint, ECT(1), to support ECN nonce ECN. RFC 3168 assigned the second codepoint, ECT(1), to support ECN
functionality that discourages receivers from exploiting ECN to nonce functionality that discourages receivers from exploiting ECN to
improve their throughput at the expense of other network users. That improve their throughput at the expense of other network users. That
ECN nonce functionality is fully specified in Experimental RFC 3540 ECN nonce functionality is fully specified in RFC 3540 [RFC3540].
[RFC3540]. This section explains why RFC 3540 is being reclassified This section explains why RFC 3540 has been reclassified from
as Historic and makes associated updates to RFC 3168. Experimental to Historic and makes associated updates to RFC 3168.
While the ECN nonce works as specified, and has been deployed in While the ECN nonce works as specified, and has been deployed in
limited environments, widespread usage in the Internet has not limited environments, widespread usage in the Internet has not
materialized. A study of the ECN behaviour of the top one million materialized. A study of the ECN behavior of the top one million web
web servers using 2014 data [Trammell15] found that after ECN was servers using 2014 data [Trammell15] found that after ECN was
negotiated, none of the 581,711 IPv4 servers tested were using both negotiated, none of the 581,711 IPv4 servers tested were using both
ECT codepoints, which would have been a possible sign of ECN nonce 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 usage. Of the 17,028 IPv6 servers tested, four set both ECT(0) and
ECT(1) on data packets. This might have been evidence of use of the 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 ECN nonce by these four servers, but it might equally have been due
erroneous re-marking of the ECN field by a middlebox or router. to erroneous re-marking of the ECN field by a middlebox or router.
With the emergence of new experimental functionality that depends on With the emergence of new experimental functionality that depends on
use of the ECT(1) codepoint for other purposes, continuing to reserve use of the ECT(1) codepoint for other purposes, continuing to reserve
that codepoint for the ECN nonce experiment is no longer justified. that codepoint for the ECN nonce experiment is no longer justified.
In addition, other approaches to discouraging receivers from In addition, other approaches to discouraging receivers from
exploiting ECN have emerged, see Appendix B.1 of exploiting ECN have emerged; see Appendix B.1 of [ECN-L4S].
[I-D.ietf-tsvwg-ecn-l4s-id]. Therefore, in support of ECN Therefore, in support of ECN experimentation with the ECT(1)
experimentation with the ECT(1) codepoint, this memo: codepoint, this memo:
o Declares that the ECN nonce experiment [RFC3540] has concluded, o Declares that the ECN nonce experiment [RFC3540] has concluded and
and notes the absence of widespread deployment. notes the absence of widespread deployment.
o Updates RFC 3168 [RFC3168] to remove discussion of the ECN nonce o Updates RFC 3168 [RFC3168] to remove discussion of the ECN nonce
and use of ECT(1) for that nonce. and use of ECT(1) for that nonce.
The four primary updates to RFC 3168 that remove discussion of the The four primary updates to RFC 3168 that remove discussion of the
ECN nonce and use of ECT(1) for that nonce are: ECN nonce and use of ECT(1) for that nonce are as follows:
1. Remove the paragraph in Section 5 that immediately follows 1. The removal of the paragraph in Section 5 that immediately
Figure 1; this paragraph discusses the ECN nonce as the follows Figure 1; this paragraph discusses the ECN nonce as the
motivation for two ECT codepoints. motivation for two ECT codepoints.
2. Remove Section 11.2 "A Discussion of the ECN nonce." in its 2. The removal of Section 11.2, "A Discussion of the ECN nonce", in
entirety. its entirety.
3. Remove the last paragraph of Section 12, which states that ECT(1) 3. The removal of the last paragraph of Section 12, which states
may be used as part of the implementation of the ECN nonce. that ECT(1) may be used as part of the implementation of the ECN
nonce.
4. Remove the first two paragraphs of Section 20.2, which discuss 4. The removal of the first two paragraphs of Section 20.2, which
the ECN nonce and alternatives. No changes are made to the rest discuss the ECN nonce and alternatives. No changes are made to
of Section 20.2, which discusses alternative uses for the fourth the rest of Section 20.2, which discusses alternative uses for
ECN codepoint. the fourth ECN codepoint.
In addition, other less substantive RFC 3168 changes are required to In addition, other less-substantive changes to RFC 3168 are required
remove all other mentions of the ECN nonce and to remove implications to remove all other mentions of the ECN nonce and to remove
that ECT(1) is intended for use by the ECN nonce; these specific text implications that ECT(1) is intended for use by the ECN nonce; these
updates are omitted for brevity. specific text updates are omitted for brevity.
4. Updates to RFC 3168 4. Updates to RFC 3168
The following subsections specify updates to RFC 3168 to enable the The following subsections specify updates to RFC 3168 to enable the
three areas of experimentation summarized in Section 2. three areas of experimentation summarized in Section 2.
4.1. Congestion Response Differences 4.1. Congestion Response Differences
RFC 3168 specifies that senders respond identically to packet drops RFC 3168 specifies that senders respond identically to packet drops
and ECN congestion indications. ECN congestion indications are and ECN congestion indications. ECN congestion indications are
predominately originated by Active Queue Management (AQM) mechanisms predominately originated by Active Queue Management (AQM) mechanisms
in intermediate buffers. AQM mechanisms are usually configured to in intermediate buffers. AQM mechanisms are usually configured to
maintain shorter queue lengths than non-AQM based mechanisms, maintain shorter queue lengths than non-AQM-based mechanisms,
particularly non-AQM drop-based mechanisms such as tail-drop, as AQM particularly non-AQM drop-based mechanisms such as tail-drop, as AQM
mechanisms indicate congestion before the queue overflows. While the mechanisms indicate congestion before the queue overflows. While the
occurrence of loss does not easily enable the receiver to determine occurrence of loss does not easily enable the receiver to determine
if AQM is used, the receipt of an ECN Congestion Experienced (CE) if AQM is used, the receipt of an ECN CE mark conveys a strong
mark conveys a strong likelihood that AQM was used to manage the likelihood that AQM was used to manage the bottleneck queue. Hence,
bottleneck queue. Hence an ECN congestion indication communicates a an ECN congestion indication communicates a higher likelihood than a
higher likelihood than a dropped packet that a short queue exists at dropped packet that a short queue exists at the network bottleneck
the network bottleneck node [I-D.ietf-tcpm-alternativebackoff-ecn]. node [TCP-ABE]. This difference suggests that for congestion
This difference suggests that for congestion indicated by ECN, a indicated by ECN, a different sender congestion response (e.g.,
different sender congestion response (e.g., sender backs off by a sender backs off by a smaller amount) may be appropriate by
smaller amount) may be appropriate by comparison to the sender comparison to the sender response to congestion indicated by loss.
response to congestion indicated by loss. However, section 5 of RFC However, Section 5 of RFC 3168 specifies that:
3168 specifies that:
Upon the receipt by an ECN-Capable transport of a single CE Upon the receipt by an ECN-Capable transport of a single CE
packet, the congestion control algorithms followed at the end- packet, the congestion control algorithms followed at the end-
systems MUST be essentially the same as the congestion control systems MUST be essentially the same as the congestion control
response to a *single* dropped packet. response to a *single* dropped packet.
This memo updates this RFC 3168 text to allow the congestion control This memo updates this text from RFC 3168 to allow the congestion
response (including the TCP Sender's congestion control response) to control response (including the TCP Sender's congestion control
a CE-marked packet to differ from the response to a dropped packet, response) to a CE-marked packet to differ from the response to a
provided that the changes from RFC 3168 are documented in an dropped packet, provided that the changes from RFC 3168 are
Experimental RFC in the IETF document stream. The specific change to documented in an Experimental RFC in the IETF document stream. The
RFC 3168 is to insert the words "unless otherwise specified by an specific change to RFC 3168 is to insert the words "unless otherwise
Experimental RFC in the IETF document stream" at the end of the specified by an Experimental RFC in the IETF document stream" at the
sentence quoted above. end of the sentence quoted above.
RFC 4774 [RFC4774] quotes the above text from RFC 3168 as background, RFC 4774 [RFC4774] quotes the above text from RFC 3168 as background,
but does not impose requirements based on that text. Therefore no but it does not impose requirements based on that text. Therefore,
update to RFC 4774 is required to enable this area of no update to RFC 4774 is required to enable this area of
experimentation. experimentation.
Section 6.1.2 of RFC 3168 specifies that: Section 6.1.2 of RFC 3168 specifies that:
If the sender receives an ECN-Echo (ECE) ACK packet (that is, an 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 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 sender knows that congestion was encountered in the network on the
path from the sender to the receiver. The indication of path from the sender to the receiver. The indication of
congestion should be treated just as a congestion loss in non- congestion should be treated just as a congestion loss in
ECN-Capable TCP. That is, the TCP source halves the congestion non-ECN-Capable TCP. That is, the TCP source halves the
window "cwnd" and reduces the slow start threshold "ssthresh". congestion window "cwnd" and reduces the slow start threshold
"ssthresh".
This memo also updates this RFC 3168 text to allow the congestion This memo also updates this text from RFC 3168 to allow the
control response (including the TCP Sender's congestion control congestion control response (including the TCP Sender's congestion
response) to a CE-marked packet to differ from the response to a control response) to a CE-marked packet to differ from the response
dropped packet, provided that the changes from RFC 3168 are to a dropped packet, provided that the changes from RFC 3168 are
documented in an Experimental RFC in the IETF document stream. The documented in an Experimental RFC in the IETF document stream. The
specific change to RFC 3168 is to insert the words "Unless otherwise specific change to RFC 3168 is to insert the words "Unless otherwise
specified by an Experimental RFC in the IETF document stream" at the specified by an Experimental RFC in the IETF document stream" at the
beginning of the second sentence quoted above. beginning of the second sentence quoted above.
4.2. Congestion Marking Differences 4.2. Congestion Marking Differences
Taken to its limit, an AQM algorithm that uses ECN congestion Taken to its limit, an AQM algorithm that uses ECN congestion
indications can be configured to maintain very shallow queues, indications can be configured to maintain very shallow queues,
thereby reducing network latency by comparison to maintaining a thereby reducing network latency by comparison to maintaining a
larger queue. Significantly more aggressive sender responses to ECN larger queue. Significantly more aggressive sender responses to ECN
are needed to make effective use of such very shallow queues; are needed to make effective use of such very shallow queues;
Datacenter TCP (DCTCP) [I-D.ietf-tcpm-dctcp] provides an example. In "Datacenter TCP (DCTCP)" [RFC8257] provides an example. In this
this case, separate network node treatments are essential, both to case, separate network node treatments are essential, both to prevent
prevent the aggressive low latency traffic from starving conventional the aggressive low-latency traffic from starving conventional traffic
traffic (if present) and to prevent any conventional traffic (if present) and to prevent any conventional traffic disruption to
disruption to any lower latency service that uses the very shallow any lower-latency service that uses the very shallow queues. Use of
queues. Use of different ECN codepoints is a promising means of different ECN codepoints is a promising means of identifying these
identifying these two classes of traffic to network nodes, and hence two classes of traffic to network nodes; hence, this area of
this area of experimentation is based on the use of the ECT(1) experimentation is based on the use of the ECT(1) codepoint to
codepoint to request ECN congestion marking behavior in the network request ECN congestion marking behavior in the network that differs
that differs from ECT(0). It is essential that any such change in from ECT(0). It is essential that any such change in ECN congestion
ECN congestion marking behavior be counterbalanced by use of a marking behavior be counterbalanced by use of a different IETF-
different IETF-approved congestion response to CE marks at the approved congestion response to CE marks at the sender, e.g., as
sender, e.g., as proposed in [I-D.ietf-tsvwg-ecn-l4s-id]. proposed in [ECN-L4S].
Section 5 of RFC 3168 specifies that:
Routers treat the ECT(0) and ECT(1) codepoints as equivalent. Section 5 of RFC 3168 specifies that "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 This memo updates RFC 3168 to allow routers to treat the ECT(0) and
ECT(1) codepoints differently, provided that the changes from RFC ECT(1) codepoints differently, provided that the changes from RFC
3168 are documented in an Experimental RFC in the IETF document 3168 are documented in an Experimental RFC in the IETF document
stream. The specific change to RFC 3168 is to insert the words stream. The specific change to RFC 3168 is to insert the words
"unless otherwise specified by an Experimental RFC in the IETF "unless otherwise specified by an Experimental RFC in the IETF
document stream" at the end of the above sentence. document stream" at the end of the above sentence.
When an AQM is configured to use ECN congestion indications to When an AQM is configured to use ECN congestion indications to
maintain a very shallow queue, congestion indications are marked on maintain a very shallow queue, congestion indications are marked on
skipping to change at page 10, line 41 skipping to change at page 11, line 34
as an indication of congestion to the end nodes. When the 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 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 a packet to inform end nodes of incipient congestion, the router
should first check to see if the ECT codepoint is set in that 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, packet's IP header. If so, then instead of dropping the packet,
the router MAY instead set the CE codepoint in the IP header. the router MAY instead set the CE codepoint in the IP header.
This memo updates RFC 3168 to allow congestion indications that are This memo updates RFC 3168 to allow congestion indications that are
not equivalent to drops, provided that the changes from RFC 3168 are not equivalent to drops, provided that the changes from RFC 3168 are
documented in an Experimental RFC in the IETF document stream. The documented in an Experimental RFC in the IETF document stream. The
specific change is to change "For a router," to "Unless otherwise specific change is to change "For a router" to "Unless otherwise
specified by an Experimental RFC in the IETF document stream" at the specified by an Experimental RFC in the IETF document stream" at the
beginning of the first sentence of the above paragraph. beginning of the first sentence of the above paragraph.
A larger update to RFC 3168 is necessary to enable sender usage of A larger update to RFC 3168 is necessary to enable sender usage of
ECT(1) to request network congestion marking behavior that maintains ECT(1) to request network congestion marking behavior that maintains
very shallow queues at network nodes. When using loss as a very shallow queues at network nodes. When using loss as a
congestion signal, the number of signals provided should be reduced congestion signal, the number of signals provided should be reduced
to a minimum and hence only presence or absence of congestion is to a minimum; hence, only the presence or absence of congestion is
communicated. In contrast, ECN can provide a richer signal, e.g., to communicated. In contrast, ECN can provide a richer signal, e.g., to
indicate the current level of congestion, without the disadvantage of indicate the current level of congestion, without the disadvantage of
a larger number of packet losses. A proposed experiment in this a larger number of packet losses. A proposed experiment in this
area, Low Latency Low Loss Scalable throughput (L4S) area, Low Latency Low Loss Scalable throughput (L4S) [ECN-L4S],
[I-D.ietf-tsvwg-ecn-l4s-id] significantly increases the CE marking significantly increases the CE marking probability for traffic marked
probability for ECT(1)-marked traffic in a fashion that would as ECT(1) in a fashion that would interact badly with existing sender
interact badly with existing sender congestion response functionality congestion response functionality because that functionality assumes
because that functionality assumes that the network marks ECT packets that the network marks ECT packets as frequently as it would drop
as frequently as it would drop Not-ECT packets. If network traffic Not-ECT packets. If network traffic that uses such a conventional
that uses such a conventional sender congestion response were to sender congestion response were to encounter L4S's increased marking
encounter L4S's increased marking probability (and hence rate) at a probability (and hence rate) at a network bottleneck queue, the
network bottleneck queue, the resulting traffic throughput is likely resulting traffic throughput is likely to be much less than intended
to be much less than intended for the level of congestion at the for the level of congestion at the bottleneck queue.
bottleneck queue.
This memo updates RFC 3168 to remove that interaction for ECT(1). 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 The specific update to Section 5 of RFC 3168 is to replace the
following two paragraphs: following two paragraphs:
Senders are free to use either the ECT(0) or the ECT(1) codepoint Senders are free to use either the ECT(0) or the ECT(1) codepoint
to indicate ECT, on a packet-by-packet basis. to indicate ECT, on a packet-by-packet basis.
The use of both the two codepoints for ECT, ECT(0) and ECT(1), is 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 motivated primarily by the desire to allow mechanisms for the data
skipping to change at page 11, line 51 skipping to change at page 12, line 43
document stream. Protocols and senders MUST NOT use the ECT(1) document stream. Protocols and senders MUST NOT use the ECT(1)
codepoint to indicate ECT unless otherwise specified by an codepoint to indicate ECT unless otherwise specified by an
Experimental RFC in the IETF document stream. Guidelines for Experimental RFC in the IETF document stream. Guidelines for
senders and receivers to differentiate between the ECT(0) and senders and receivers to differentiate between the ECT(0) and
ECT(1) codepoints will be addressed in separate documents, for ECT(1) codepoints will be addressed in separate documents, for
each transport protocol. In particular, this document does not each transport protocol. In particular, this document does not
address mechanisms for TCP end-nodes to differentiate between the address mechanisms for TCP end-nodes to differentiate between the
ECT(0) and ECT(1) codepoints. ECT(0) and ECT(1) codepoints.
Congestion Marking Differences experiments SHOULD modify the network Congestion Marking Differences experiments SHOULD modify the network
behavior for ECT(1)-marked traffic rather than ECT(0)-marked traffic behavior for traffic marked as ECT(1) rather than ECT(0) if network
if network behavior for only one ECT codepoint is modified. behavior for only one ECT codepoint is modified. Congestion Marking
Congestion Marking Differences experiments MUST NOT modify the Differences experiments MUST NOT modify the network behavior for
network behavior for ECT(0)-marked traffic in a fashion that requires traffic marked as ECT(0) in a fashion that requires changes to the
changes to sender congestion response to obtain desired network sender congestion response to obtain desired network behavior. If a
behavior. If a Congestion Marking Differences experiment modifies Congestion Marking Differences experiment modifies the network
the network behavior for ECT(1)-marked traffic, e.g., CE-marking behavior for traffic marked as ECT(1), e.g., CE-marking behavior, in
behavior, in a fashion that requires changes to sender congestion a fashion that requires changes to the sender congestion response to
response to obtain desired network behavior, then the Experimental obtain desired network behavior, then the Experimental RFC in the
RFC in the IETF document stream for that experiment MUST specify: IETF document stream for that experiment MUST specify:
o The sender congestion response to CE marking in the network, and o The sender congestion response to CE marking in the network, and
o Router behavior changes, or the absence thereof, in forwarding CE- o Router behavior changes, or the absence thereof, in forwarding CE-
marked packets that are part of the experiment. marked packets that are part of the experiment.
In addition, this memo updates RFC 3168 to remove discussion of the In addition, this memo updates RFC 3168 to remove discussion of the
ECN nonce, as noted in Section 3 above. ECN nonce, as noted in Section 3 above.
4.3. TCP Control Packets and Retransmissions 4.3. TCP Control Packets and Retransmissions
With the successful use of ECN for traffic in large portions of the 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 Internet, there is interest in extending the benefits of ECN to TCP
control packets (e.g., SYNs) and retransmitted packets, e.g., as control packets (e.g., SYNs) and retransmitted packets, e.g., as
proposed by ECN++ [I-D.bagnulo-tcpm-generalized-ecn]. proposed by ECN++ [ECN-TCP].
RFC 3168 prohibits use of ECN for TCP control packets and RFC 3168 prohibits use of ECN for TCP control packets and
retransmitted packets in a number of places: retransmitted packets in a number of places:
o "To ensure the reliable delivery of the congestion indication of o Section 5.2: "To ensure the reliable delivery of the congestion
the CE codepoint, an ECT codepoint MUST NOT be set in a packet indication of the CE codepoint, an ECT codepoint MUST NOT be set
unless the loss of that packet in the network would be detected by in a packet unless the loss of that packet in the network would be
the end nodes and interpreted as an indication of congestion." detected by the end nodes and interpreted as an indication of
(Section 5.2) congestion."
o "A host MUST NOT set ECT on SYN or SYN-ACK packets." o Section 6.1.1: "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 o Section 6.1.4: "...pure acknowledgement packets (e.g., packets
any accompanying data) MUST be sent with the not-ECT codepoint." that do not contain any accompanying data) MUST be sent with the
(Section 6.1.4) not-ECT codepoint."
o "This document specifies ECN-capable TCP implementations MUST NOT o Section 6.1.5: "This document specifies ECN-capable TCP
set either ECT codepoint (ECT(0) or ECT(1)) in the IP header for implementations MUST NOT set either ECT codepoint (ECT(0) or
retransmitted data packets, and that the TCP data receiver SHOULD ECT(1)) in the IP header for retransmitted data packets, and that
ignore the ECN field on arriving data packets that are outside of the TCP data receiver SHOULD ignore the ECN field on arriving data
the receiver's current window." (Section 6.1.5) packets that are outside of the receiver's current window."
o "the TCP data sender MUST NOT set either an ECT codepoint or the o Section 6.1.6: "...the TCP data sender MUST NOT set either an ECT
CWR bit on window probe packets." (Section 6.1.6) codepoint or the CWR bit on window probe packets.
This memo updates RFC 3168 to allow the use of ECT codepoints on SYN This memo updates RFC 3168 to allow the use of ECT codepoints on SYN
and SYN-ACK packets, pure acknowledgement packets, window probe and SYN-ACK packets, pure acknowledgement packets, window probe
packets and retransmissions of packets that were originally sent with packets, and retransmissions of packets that were originally sent
an ECT codepoint, provided that the changes from RFC 3168 are with an ECT codepoint, provided that the changes from RFC 3168 are
documented in an Experimental RFC in the IETF document stream. The documented in an Experimental RFC in the IETF document stream. The
specific change to RFC 3168 is to insert the words "unless otherwise specific change to RFC 3168 is to insert the words "unless otherwise
specified by an Experimental RFC in the IETF document stream" at the specified by an Experimental RFC in the IETF document stream" at the
end of each sentence quoted above. end of each sentence quoted above.
In addition, beyond requiring TCP senders not to set ECT on TCP In addition, beyond requiring TCP senders not to set ECT on TCP
control packets and retransmitted packets, RFC 3168 is silent on control packets and retransmitted packets, RFC 3168 is silent on
whether it is appropriate for a network element, e.g. a firewall, to whether it is appropriate for a network element, e.g., a firewall, to
discard such a packet as invalid. For this area of ECN discard such a packet as invalid. For this area of ECN
experimentation to be useful, middleboxes ought not to do that, experimentation to be useful, middleboxes ought not to do that;
therefore RFC 3168 is updated by adding the following text to the end therefore, RFC 3168 is updated by adding the following text to the
of Section 6.1.1.1 on Middlebox Issues: end of Section 6.1.1.1 on Middlebox Issues:
Unless otherwise specified by an Experimental RFC in the IETF Unless otherwise specified by an Experimental RFC in the IETF
document stream, middleboxes SHOULD NOT discard TCP control document stream, middleboxes SHOULD NOT discard TCP control
packets and retransmitted TCP packets solely because the ECN field packets and retransmitted TCP packets solely because the ECN field
in the IP header does not contain Not-ECT. An exception to this in the IP header does not contain Not-ECT. An exception to this
requirement occurs in responding to an attack that uses ECN requirement occurs in responding to an attack that uses ECN
codepoints other than Not-ECT. For example, as part of the codepoints other than Not-ECT. For example, as part of the
response, it may be appropriate to drop ECT-marked TCP SYN packets response, it may be appropriate to drop ECT-marked TCP SYN packets
with higher probability than TCP SYN packets marked with not-ECT. with higher probability than TCP SYN packets marked with Not-ECT.
Any such exceptional discarding of TCP control packets and Any such exceptional discarding of TCP control packets and
retransmitted TCP packets in response to an attack MUST NOT be retransmitted TCP packets in response to an attack MUST NOT be
done routinely in the absence of an attack and SHOULD only be done 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 if it is determined that the use of ECN is contributing to the
attack. attack.
5. ECN for RTP Updates to RFC 6679 5. ECN for RTP Updates to RFC 6679
RFC 6679 [RFC6679] specifies use of ECN for RTP traffic; it allows 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 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 : following guidance on use of these codepoints in Section 7.3.1:
The sender SHOULD mark packets as ECT(0) unless the receiver The sender SHOULD mark packets as ECT(0) unless the receiver
expresses a preference for ECT(1) or for a random ECT value using 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" parameter in the "a=ecn-capable-rtp:" attribute.
The Congestion Marking Differences area of experimentation increases The Congestion Marking Differences area of experimentation increases
the potential consequences of using ECT(1) instead of ECT(0), and the potential consequences of using ECT(1) instead of ECT(0); hence,
hence the above guidance is updated by adding the following two the above guidance is updated by adding the following two sentences:
sentences:
Random ECT values MUST NOT be used, as that may expose RTP to Random ECT values MUST NOT be used, as that may expose RTP to
differences in network treatment of traffic marked with ECT(1) and differences in network treatment of traffic marked with ECT(1) and
ECT(0) and differences in associated endpoint congestion ECT(0) and differences in associated endpoint congestion
responses. In addition, ECT(0) MUST be used unless otherwise responses. In addition, ECT(0) MUST be used unless otherwise
specified in an Experimental RFC in the IETF document stream. 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 Section 7.3.3 of RFC 6679 specifies RTP's response to receipt of
marked packets as being identical to the response to dropped packets: 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 The reception of RTP packets with ECN-CE marks in the IP header is
a notification that congestion is being experienced. The default a notification that congestion is being experienced. The default
reaction on the reception of these ECN-CE-marked packets MUST be reaction on the reception of these ECN-CE-marked packets MUST be
to provide the congestion control algorithm with a congestion to provide the congestion control algorithm with a congestion
notification that triggers the algorithm to react as if packet notification that triggers the algorithm to react as if packet
loss had occurred. There should be no difference in congestion loss had occurred. There should be no difference in congestion
response if ECN-CE marks or packet drops are detected. response if ECN-CE marks or packet drops are detected.
In support of Congestion Response Differences experimentation, this In support of Congestion Response Differences experimentation, this
memo updates this text in a fashion similar to RFC 3168 to allow the 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 RTP congestion control response to a CE-marked packet to differ from
the response to a dropped packet, provided that the changes from RFC the response to a dropped packet, provided that the changes from RFC
6679 are documented in an Experimental RFC in the IETF document 6679 are documented in an Experimental RFC in the IETF document
stream. The specific change to RFC 6679 is to insert the words stream. The specific change to RFC 6679 is to insert the words
"Unless otherwise specified by an Experimental RFC in the IETF "Unless otherwise specified by an Experimental RFC in the IETF
document stream" and reformat the last two sentences to be subject to document stream" and reformat the last two sentences to be subject to
that condition, i.e.: that condition; that is:
The reception of RTP packets with ECN-CE marks in the IP header is The reception of RTP packets with ECN-CE marks in the IP header is
a notification that congestion is being experienced. Unless a notification that congestion is being experienced. Unless
otherwise specified by an Experimental RFC in the IETF document otherwise specified by an Experimental RFC in the IETF document
stream: stream:
* The default reaction on the reception of these ECN-CE-marked * The default reaction on the reception of these ECN-CE-marked
packets MUST be to provide the congestion control algorithm packets MUST be to provide the congestion control algorithm
with a congestion notification that triggers the algorithm to with a congestion notification that triggers the algorithm to
react as if packet loss had occurred. react as if packet loss had occurred.
* There should be no difference in congestion response if ECN-CE * There should be no difference in congestion response if ECN-CE
marks or packet drops are detected. marks or packet drops are detected.
The second sentence of the immediately following paragraph in RFC The second sentence of the immediately following paragraph in
6679 requires a related update: Section 7.3.3 of RFC 6679 requires a related update:
Other reactions to ECN-CE may be specified in the future, Other reactions to ECN-CE may be specified in the future,
following IETF Review. Detailed designs of such additional following IETF Review. Detailed designs of such alternative
reactions MUST be specified in a Standards Track RFC and be reactions MUST be specified in a Standards Track RFC and be
reviewed to ensure they are safe for deployment under any reviewed to ensure they are safe for deployment under any
restrictions specified. restrictions specified.
The update is to change "Standards Track RFC" to "Standards Track RFC The update is to change "Standards Track RFC" to "Standards Track RFC
or Experimental RFC in the IETF document stream" for consistency with or Experimental RFC in the IETF document stream" for consistency with
the first update. the first update.
6. ECN for DCCP Updates to RFCs 4341, 4342 and 5622 6. ECN for DCCP Updates to RFCs 4341, 4342, and 5622
The specifications of the three DCCP Congestion Control IDs (CCIDs) 2 The specifications of the three DCCP Congestion Control IDs (CCIDs),
[RFC4341], 3 [RFC4342] and 4 [RFC5622] contain broadly the same 2 [RFC4341], 3 [RFC4342], and 4 [RFC5622], contain broadly the same
wording as follows: wording as follows:
each DCCP-Data and DCCP-DataAck packet is sent as ECN Capable with each DCCP-Data and DCCP-DataAck packet is sent as ECN Capable with
either the ECT(0) or the ECT(1) codepoint set. either the ECT(0) or the ECT(1) codepoint set.
This memo updates these sentences in each of the three RFCs as This memo updates these sentences in each of the three RFCs as
follows: follows:
each DCCP-Data and DCCP-DataAck packet is sent as ECN Capable. each DCCP-Data and DCCP-DataAck packet is sent as ECN Capable.
Unless otherwise specified by an Experimental RFC in the IETF Unless otherwise specified by an Experimental RFC in the IETF
document stream, such DCCP senders MUST set the ECT(0) codepoint. document stream, such DCCP senders MUST set the ECT(0) codepoint.
In support of Congestion Marking Differences experimentation (as In support of Congestion Marking Differences experimentation (as
noted in Section 3), this memo also updates all three of these RFCs 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 to remove discussion of the ECN nonce. The specific text updates are
omitted for brevity. omitted for brevity.
7. Acknowledgements 7. IANA Considerations
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
reviews of multiple versions 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 To reflect the reclassification of RFC 3540 as Historic, IANA has
requested to update the Transmission Control Protocol (TCP) Header updated the "Transmission Control Protocol (TCP) Header Flags"
Flags registry (https://www.iana.org/assignments/tcp-header-flags/ registry <https://www.iana.org/assignments/tcp-header-flags> to
tcp-header-flags.xhtml#tcp-header-flags-1) to remove the registration remove the registration of bit 7 as the NS (Nonce Sum) bit and add an
of bit 7 as the NS (Nonce Sum) bit and add an annotation to the annotation to the registry to state that bit 7 was used by Historic
registry to state that bit 7 was used by Historic RFC 3540 as the NS RFC 3540 as the NS (Nonce Sum) bit but is now Reserved.
(Nonce Sum) bit.
9. Security Considerations 8. Security Considerations
As a process memo that only relaxes restrictions on experimentation, As a process memo that only relaxes restrictions on experimentation,
there are no protocol security considerations, as security there are no protocol security considerations, as security
considerations for any experiments that take advantage of the relaxed considerations for any experiments that take advantage of the relaxed
restrictions are discussed in the Internet-Drafts that propose the restrictions are discussed in the documents that propose the
experiments. experiments.
However, effective congestion control is crucial to the continued However, effective congestion control is crucial to the continued
operation of the Internet, and hence this memo places the operation of the Internet; hence, this memo places the responsibility
responsibility for not breaking Internet congestion control on the for not breaking Internet congestion control on the experiments and
experiments and the experimenters who propose them. This the experimenters who propose them. This responsibility includes the
responsibility includes the requirement to discuss congestion control requirement to discuss congestion control implications in an
implications in an IETF document stream Experimental RFC for each Experimental RFC in the IETF document stream for each experiment, as
experiment, as stated in Section 2.1; review of that discussion by stated in Section 2.1; review of that discussion by the IETF
the IETF community and the IESG prior to RFC publication is intended community and the IESG prior to RFC publication is intended to
to provide assurance that each experiment does not break Internet provide assurance that each experiment does not break Internet
congestion control. congestion control.
See Appendix C.1 of [I-D.ietf-tsvwg-ecn-l4s-id] for discussion of See Appendix C.1 of [ECN-L4S] for discussion of alternatives to the
alternatives to the ECN nonce. ECN nonce.
10. References 9. References
10.1. Normative References 9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41, [RFC2914] Floyd, S., "Congestion Control Principles", BCP 41,
RFC 2914, DOI 10.17487/RFC2914, September 2000, RFC 2914, DOI 10.17487/RFC2914, September 2000,
<https://www.rfc-editor.org/info/rfc2914>. <https://www.rfc-editor.org/info/rfc2914>.
skipping to change at page 17, line 37 skipping to change at page 18, line 5
Control Protocol (DCCP) Congestion ID 4: TCP-Friendly Rate Control Protocol (DCCP) Congestion ID 4: TCP-Friendly Rate
Control for Small Packets (TFRC-SP)", RFC 5622, Control for Small Packets (TFRC-SP)", RFC 5622,
DOI 10.17487/RFC5622, August 2009, DOI 10.17487/RFC5622, August 2009,
<https://www.rfc-editor.org/info/rfc5622>. <https://www.rfc-editor.org/info/rfc5622>.
[RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P., [RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
and K. Carlberg, "Explicit Congestion Notification (ECN) and K. Carlberg, "Explicit Congestion Notification (ECN)
for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August
2012, <https://www.rfc-editor.org/info/rfc6679>. 2012, <https://www.rfc-editor.org/info/rfc6679>.
10.2. Informative References [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
[I-D.bagnulo-tcpm-generalized-ecn] May 2017, <https://www.rfc-editor.org/info/rfc8174>.
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-03 (work in progress), October
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] 9.2. Informative References
Eastlake, D. and B. Briscoe, "TRILL: ECN (Explicit
Congestion Notification) Support", draft-ietf-trill-ecn-
support-03 (work in progress), May 2017.
[I-D.ietf-tsvwg-ecn-encap-guidelines] [ECN-ENCAP]
Briscoe, B., Kaippallimalil, J., and P. Thaler, Briscoe, B., Kaippallimalil, J., and P. Thaler,
"Guidelines for Adding Congestion Notification to "Guidelines for Adding Congestion Notification to
Protocols that Encapsulate IP", draft-ietf-tsvwg-ecn- Protocols that Encapsulate IP", Work in Progress,
encap-guidelines-09 (work in progress), July 2017. draft-ietf-tsvwg-ecn-encap-guidelines-09, July 2017.
[I-D.ietf-tsvwg-ecn-l4s-id] [ECN-EXPERIMENT]
Schepper, K. and B. Briscoe, "Identifying Modified Khademi, N., Welzl, M., Armitage, G., and G. Fairhurst,
"Updating the Explicit Congestion Notification (ECN)
Specification to Allow IETF Experimentation", Work in
Progress, draft-khademi-tsvwg-ecn-response-01, July 2016.
[ECN-L4S] Schepper, K. and B. Briscoe, "Identifying Modified
Explicit Congestion Notification (ECN) Semantics for Explicit Congestion Notification (ECN) Semantics for
Ultra-Low Queuing Delay", draft-ietf-tsvwg-ecn-l4s-id-01 Ultra-Low Queuing Delay", Work in Progress,
(work in progress), October 2017. draft-ietf-tsvwg-ecn-l4s-id-01, October 2017.
[I-D.ietf-tsvwg-rfc6040update-shim] [ECN-SHIM] Briscoe, B., "Propagating Explicit Congestion Notification
Briscoe, B., "Propagating Explicit Congestion Notification Across IP Tunnel Headers Separated by a Shim", Work in
Across IP Tunnel Headers Separated by a Shim", draft-ietf- Progress, draft-ietf-tsvwg-rfc6040update-shim-05, November
tsvwg-rfc6040update-shim-05 (work in progress), November
2017. 2017.
[I-D.khademi-tsvwg-ecn-response] [ECN-TCP] Bagnulo, M. and B. Briscoe, "ECN++: Adding Explicit
Khademi, N., Welzl, M., Armitage, G., and G. Fairhurst, Congestion Notification (ECN) to TCP Control Packets",
"Updating the Explicit Congestion Notification (ECN) Work in Progress, draft-ietf-tcpm-generalized-ecn-02,
Specification to Allow IETF Experimentation", draft- October 2017.
khademi-tsvwg-ecn-response-01 (work in progress), July
2016. [ECN-TRILL]
Eastlake, D. and B. Briscoe, "TRILL: ECN (Explicit
Congestion Notification) Support", Work in Progress,
draft-ietf-trill-ecn-support-04, November 2017.
[RFC4774] Floyd, S., "Specifying Alternate Semantics for the [RFC4774] Floyd, S., "Specifying Alternate Semantics for the
Explicit Congestion Notification (ECN) Field", BCP 124, Explicit Congestion Notification (ECN) Field", BCP 124,
RFC 4774, DOI 10.17487/RFC4774, November 2006, RFC 4774, DOI 10.17487/RFC4774, November 2006,
<https://www.rfc-editor.org/info/rfc4774>. <https://www.rfc-editor.org/info/rfc4774>.
[RFC4844] Daigle, L., Ed. and Internet Architecture Board, "The RFC [RFC4844] Daigle, L., Ed. and Internet Architecture Board, "The RFC
Series and RFC Editor", RFC 4844, DOI 10.17487/RFC4844, Series and RFC Editor", RFC 4844, DOI 10.17487/RFC4844,
July 2007, <https://www.rfc-editor.org/info/rfc4844>. July 2007, <https://www.rfc-editor.org/info/rfc4844>.
skipping to change at page 19, line 20 skipping to change at page 19, line 20
[RFC6040] Briscoe, B., "Tunnelling of Explicit Congestion [RFC6040] Briscoe, B., "Tunnelling of Explicit Congestion
Notification", RFC 6040, DOI 10.17487/RFC6040, November Notification", RFC 6040, DOI 10.17487/RFC6040, November
2010, <https://www.rfc-editor.org/info/rfc6040>. 2010, <https://www.rfc-editor.org/info/rfc6040>.
[RFC6660] Briscoe, B., Moncaster, T., and M. Menth, "Encoding Three [RFC6660] Briscoe, B., Moncaster, T., and M. Menth, "Encoding Three
Pre-Congestion Notification (PCN) States in the IP Header Pre-Congestion Notification (PCN) States in the IP Header
Using a Single Diffserv Codepoint (DSCP)", RFC 6660, Using a Single Diffserv Codepoint (DSCP)", RFC 6660,
DOI 10.17487/RFC6660, July 2012, DOI 10.17487/RFC6660, July 2012,
<https://www.rfc-editor.org/info/rfc6660>. <https://www.rfc-editor.org/info/rfc6660>.
[RFC8257] Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.,
and G. Judd, "Data Center TCP (DCTCP): TCP Congestion
Control for Data Centers", RFC 8257, DOI 10.17487/RFC8257,
October 2017, <https://www.rfc-editor.org/info/rfc8257>.
[TCP-ABE] Khademi, N., Welzl, M., Armitage, G., and G. Fairhurst,
"TCP Alternative Backoff with ECN (ABE)", Work in
Progress, draft-ietf-tcpm-alternativebackoff-ecn-05,
December 2017.
[Trammell15] [Trammell15]
Trammell, B., Kuehlewind, M., Boppart, D., Learmonth, I., Trammell, B., Kuehlewind, M., Boppart, D., Learmonth, I.,
Fairhurst, G., and R. Scheffenegger, "Enabling Internet- Fairhurst, G., and R. Scheffenegger, "Enabling Internet-
Wide Deployment of Explicit Congestion Notification". Wide Deployment of Explicit Congestion Notification", In
Conference Proceedings of Passive and Active Measurement
In Proc Passive & Active Measurement (PAM'15) Conference (PAM), pp. 193-205, March 2015,
(2015) <https://doi.org/10.1007/978-3-319-15509-8_15>.
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.
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
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 Acknowledgements
Changes from draft-ietf-tsvwg-ecn-experimentation-07 to -08: The content of this specification, including the specific portions of
RFC 3168 that are updated, draws heavily from [ECN-EXPERIMENT], whose
authors are gratefully acknowledged. The authors of the documents
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.
o Edits from another careful review by Bob Briscoe. The primary This specification improved as a result of comments from a number of
change is an editorial rewrite of Section 2.2 including changing reviewers, including Ben Campbell, Brian Carpenter, Benoit Claise,
its name to better reflect its content. 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 multiple draft versions of this memo resulted in numerous
improvements including addition of the updates to the DCCP RFCs.
Author's Address Author's Address
David Black David Black
Dell EMC Dell EMC
176 South Street 176 South Street
Hopkinton, MA 01748 Hopkinton, MA 01748
USA United States of America
Email: david.black@dell.com Email: david.black@dell.com
 End of changes. 94 change blocks. 
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