One document matched: draft-black-tsvwg-ecn-experimentation-00.xml
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<rfc category="std" docName="draft-black-tsvwg-ecn-experimentation-00"
ipr="trust200902" obsoletes="3540" updates="3168, 6679">
<!-- category values: std, bcp, info, exp, and historic
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<!-- ***** FRONT MATTER ***** -->
<front>
<!-- The abbreviated title is used in the page header - it is only necessary if the
full title is longer than 39 characters -->
<title abbrev="ECN Experimentation">Explicit Congestion Notification (ECN)
Experimentation</title>
<!-- add 'role="editor"' below for the editors if appropriate -->
<!-- Another author who claims to be an editor -->
<author fullname="David Black" initials="D.L." surname="Black">
<organization>Dell EMC</organization>
<address>
<postal>
<street>176 South Street</street>
<!-- Reorder these if your country does things differently -->
<city>Hopkinton</city>
<region>MA</region>
<code>01748</code>
<country>USA</country>
</postal>
<phone/>
<email>david.black@dell.com</email>
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</address>
</author>
<date year="2016"/>
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<area>Transport</area>
<workgroup>Transport Area Working Group</workgroup>
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<keyword>ECN</keyword>
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<abstract>
<t>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.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>Multiple protocol experiments have been proposed that involve changes
to Explicit Congestion Notification (ECN) as specified in <xref
target="RFC3168">RFC 3168</xref>. 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 <xref
target="RFC6679">RFC 6679</xref> 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.</t>
<t>One of these areas of experimentation involves use of the ECT(1)
codepoint that was dedicated to the ECN Nonce experiment as described in
<xref target="RFC3540">RFC 3540</xref>. This memo records the conclusion
of the ECN Nonce experiment, obsoletes RFC 3540 and reclassifies it as
Historic.</t>
<section title="Requirements Language">
<t>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 <xref
target="RFC2119">RFC 2119</xref>.</t>
</section>
</section>
<section title="Scope of ECN Experiments">
<t>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:<list style="hanging">
<t hangText="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 <xref
target="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.</t>
<t hangText="ECT Differences:">Use ECT(1) to request ECN congestion
marking behavior in the network that differs from ECT(0), e.g., as
specified in <xref target="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.</t>
<t hangText="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 <xref
target="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</t>
</list>The scope of this memo is limited to these three areas of
experimentation.</t>
</section>
<section anchor="ECNNonce" title="ECN Nonce and RFC 3540">
<t>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 <xref
target="RFC3540">RFC 3540</xref>.</t>
<t>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.</t>
<t>Therefore, in support of ECN experimentation with the ECT(1)
codepoint, this memo:<list style="symbols">
<t>Declares that the ECN Nonce experiment <xref target="RFC3540"/>
has concluded, and notes the absence of widespread deployment.</t>
<t>Obsoletes RFC 3540 in order to facilitate experimental use of the
ECT(1) codepoint.</t>
<t>Reclassifies RFC 3540 as Historic to document the ECN Nonce
experiment and discourage further implementation of the ECN
Nonce.</t>
<t>Updates <xref target="RFC3168">RFC 3168</xref> to remove
discussion of the ECN Nonce and use of ECT(1) for that Nonce. The
specific text updates are omitted for brevity.</t>
</list>The following guidance on ECT codepoint usage in Section 5 of
RFC 3168 is relevant when the ECN Nonce is not implemented:</t>
<t><list>
<t>Protocols and senders that only require a single ECT codepoint
SHOULD use ECT(0).</t>
</list></t>
</section>
<section title="Updates to RFC 3168">
<t>In support of these areas of experimentation, this memo updates <xref
target="RFC3168">RFC 3168</xref> 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.</t>
<section title="Alternative Backoff">
<t>Section 5 of RFC 3168 specifies that:</t>
<t><list style="empty">
<t>"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."</t>
</list>In support of Alternative Backoff experimentation, this memo
updates RFC 3168 to allow the congestion control response (including
the TCP Sender's congestion control response) to a CE-marked packet to
differ from the response to a dropped packet, provided that the
changes from RFC 3168 are documented in an Experimental RFC. The
specific change to RFC 3168 is to insert the words "unless otherwise
specified by an Experimental RFC" at the end of the sentence quoted
above.</t>
<t><xref target="RFC4774">RFC 4774</xref> 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.</t>
</section>
<section title="ECT Differences">
<t>Section 5 of RFC 3168 specifies that:<list style="empty">
<t>"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."</t>
</list>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:</t>
<t><list style="empty">
<t>"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."</t>
</list>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.</t>
<t>In support of ECT Differences experimentation, this memo also
updates RFC 3168 to remove discussion of the ECN Nonce, as noted in
Section <xref format="counter" target="ECNNonce"/> above.</t>
</section>
<section title="Generalized ECN">
<t>RFC 3168 prohibits use of ECN for TCP control packets and
retransmitted packets in a number of places:<list style="symbols">
<t>"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)</t>
<t>"A host MUST NOT set ECT on SYN or SYN-ACK packets." (Section
6.1.1)</t>
<t>"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)</t>
<t>"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)</t>
</list></t>
<t>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.</t>
</section>
<section anchor="ECC" title="Effective Congestion Control is Required">
<t>Congestion control remains an important aspect of the Internet
architecture <xref target="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.</t>
</section>
</section>
<section title="ECN for RTP Updates to RFC 6679">
<t><xref target="RFC6679">RFC 6679</xref> 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 :</t>
<t><list>
<t>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.</t>
</list></t>
<t>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:</t>
<t><list>
<t>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 <xref
target="I-D.briscoe-tsvwg-ecn-l4s-id"/>.</t>
</list>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:</t>
<t><list>
<t>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.</t>
</list>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.:</t>
<t><list>
<t>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: <list style="symbols">
<t>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.</t>
<t>There should be no difference in congestion response if
ECN-CE marks or packet drops are detected.</t>
</list></t>
</list>The second sentence of the immediately following paragraph in
RFC 6679 requires a related update:</t>
<t><list>
<t>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.</t>
</list>The update is to change "Standards Track RFC" to "Standards
Track RFC or Experimental RFC" for consistency with the first
update.</t>
</section>
<section anchor="Acknowledgements" title="Acknowledgements">
<t>The content of this draft, including the specific portions of RFC
3168 that are updated draws heavily from <xref
target="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.</t>
</section>
<section anchor="IANA" title="IANA Considerations">
<t>This memo includes no request to IANA.</t>
</section>
<section anchor="Security" title="Security Considerations">
<t>As a process memo that makes no changes to existing protocols, there
are no protocol security considerations.</t>
<t>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 <xref
format="counter" target="ECC"/>.</t>
</section>
</middle>
<!-- *****BACK MATTER ***** -->
<back>
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