Transport Area Working Group D. Black
Internet-Draft Dell EMC
Updates: 3168, 4341, 4342, 5622, 6679 June 21, 2017
(if approved)
Intended status: Standards Track
Expires: December 23, 2017
Explicit Congestion Notification (ECN) Experimentation
draft-ietf-tsvwg-ecn-experimentation-03
Abstract
This memo updates RFC 3168, which specifies Explicit Congestion
Notification (ECN) as a replacement for packet drops as indicators of
network congestion. It relaxes restrictions in RFC 3168 that would
otherwise hinder experimentation towards benefits beyond just removal
of loss. This memo summarizes the anticipated areas of
experimentation and updates RFC 3168 to enable experimentation in
these areas. An Experimental RFC is required to take advantage of
any of these enabling updates. In addition, this memo 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 records the
conclusion of the ECN Nonce experiment in RFC 3540, and provides the
rationale for reclassification of RFC 3540 as Historic; this
reclassification enables new experimental use of the ECT(1)
codepoint.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 23, 2017.
Black Expires December 23, 2017 [Page 1]
Internet-Draft ECN Experimentation June 2017
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. ECN Terminology . . . . . . . . . . . . . . . . . . . . . 3
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Proposed ECN Experiments: Background . . . . . . . . . . . . 4
3. ECN Nonce and RFC 3540 . . . . . . . . . . . . . . . . . . . 5
4. Updates to RFC 3168 . . . . . . . . . . . . . . . . . . . . . 6
4.1. Congestion Response Differences . . . . . . . . . . . . . 6
4.2. Congestion Marking Differences . . . . . . . . . . . . . 7
4.3. Generalized ECN . . . . . . . . . . . . . . . . . . . . . 10
4.4. Effective Congestion Control is Required . . . . . . . . 11
5. ECN for RTP Updates to RFC 6679 . . . . . . . . . . . . . . . 11
6. ECN for DCCP Updates to RFCs 4341, 4342 and 5622 . . . . . . 13
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
9. Security Considerations . . . . . . . . . . . . . . . . . . . 13
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
10.1. Normative References . . . . . . . . . . . . . . . . . . 14
10.2. Informative References . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 17
Black Expires December 23, 2017 [Page 2]
Internet-Draft ECN Experimentation June 2017
1. Introduction
This memo updates RFC 3168 [RFC3168] which specifies Explicit
Congestion Notification (ECN) as a replacement for packet drops as
indicators of network congestion. It relaxes restrictions in RFC
3168 that would otherwise hinder experimentation towards benefits
beyond just removal of loss. This memo summarizes the proposed areas
of experimentation and updates RFC 3168 to enable experimentation in
these areas. An Experimental RFC MUST be published for any protocol
or mechanism that takes advantage of any of these enabling updates.
Putting all of these updates into a single document enables
experimentation to proceed without requiring a standards process
exception for each Experimental RFC that needs changes to RFC 3168, a
Proposed Standard RFC.
There is no need to make changes for protocols and mechanisms that
are documented in Standards Track RFCs, as any Standards Track RFC
can update RFC 3168 without needing a standards process exception.
In addition, this memo makes related updates to the ECN specification
for RTP [RFC6679] and for three DCCP profiles ([RFC4341], [RFC4342]
and [RFC5622]) 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 any of these Proposed Standard RFCs when the
modification falls within the bounds established by this memo (RFC
5622 is an Experimental RFC; it is modified by this memo for
consistency with modifications to the other two DCCP RFCs).
Some of the anticipated experimentation includes use of the ECT(1)
codepoint that was dedicated to the ECN Nonce experiment in RFC 3540
[RFC3540]. This memo records the conclusion of the ECN Nonce
experiment and provides the explanation for reclassification of RFC
3540 as Historic in order to enable new experimental use of the
ECT(1) codepoint.
1.1. ECN Terminology
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
ECN-capable sender sets one of these to indicate that both transport
end-points support ECN.
Not-ECT: The ECN codepoint set by senders that indicates that the
transport is not ECN-capable.
Black Expires December 23, 2017 [Page 3]
Internet-Draft ECN Experimentation June 2017
CE: Congestion Experienced. The ECN codepoint that an intermediate
node sets to indicate congestion. A node sets an increasing
proportion of ECT packets to CE as the level of congestion increases.
1.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119].
2. Proposed ECN Experiments: Background
Three areas of ECN experimentation are covered by this memo; the
cited Internet-Drafts should be consulted for the detailed goals and
rationale of each proposed experiment:
Congestion Response Differences: As discussed further in
Section 4.1, an ECN congestion indication communicates a higher
likelihood that a shorter queue exists at the network bottleneck
node by comparison to a packet drop that indicates congestion
[I-D.ietf-tcpm-alternativebackoff-ecn]. This difference suggests
that for congestion indicated by ECN, a different sender
congestion response (e.g., reduce the response so that the sender
backs off by a smaller amount) may be appropriate by comparison to
the sender response to congestion indicated by loss, e.g., as
proposed in [I-D.ietf-tcpm-alternativebackoff-ecn] and
[I-D.ietf-tsvwg-ecn-l4s-id] - the experiment in the latter draft
couples the backoff change to Congestion Marking Differences
changes (next bullet). This is at variance with RFC 3168's
requirement that a sender's congestion control response to ECN
congestion indications be the same as to drops. IETF approval,
e.g., via an Experimental RFC, is required for any sender
congestion response used in this area of experimentation.
Congestion Marking Differences: As discussed further in Section 4.2,
when taken to its limit, congestion marking at network nodes can
be configured to maintain very shallow queues in conjunction with
a different IETF-approved congestion response to congestion
indications (CE marks) at the sender, e.g., as proposed in
[I-D.ietf-tsvwg-ecn-l4s-id]. The traffic involved needs to be
identified by the senders to the network nodes in order to avoid
damage to other network traffic whose senders do not expect the
more frequent congestion marking used to maintain nearly empty
queues. Use of different ECN codepoints, specifically ECT(0) and
ECT(1), is a promising means of traffic identification for this
purpose, but that technique is at variance with RFC 3168's
Black Expires December 23, 2017 [Page 4]
Internet-Draft ECN Experimentation June 2017
requirement that ECT(0)-marked traffic and ECT(1)-marked traffic
not receive different treatment in the network.
Generalized ECN: RFC 3168 limits the use of ECN with TCP to data
packets, excluding retransmissions. With the successful
deployment of ECN in large portions of the Internet, there is
interest in extending the benefits of ECN to TCP control packets
(e.g., SYNs) and retransmitted packets, e.g., as proposed in
[I-D.bagnulo-tcpm-generalized-ecn]. This is at variance with RFC
3168's prohibition of use of ECN for TCP control packets and
retransmitted packets.
The scope of this memo is limited to these three areas of
experimentation. This memo expresses no view on the likely outcomes
of the proposed experiments and does not specify the experiments in
detail. Additional experiments in these areas are possible, e.g., on
use of ECN to support deployment of a protocol similar to DCTCP
[I-D.ietf-tcpm-dctcp] beyond DCTCP's current applicability that is
limited to data center environments. The purpose of this memo is to
remove constraints in standards track RFCs that stand in the way of
these areas of experimentation.
3. ECN Nonce and RFC 3540
As specified in RFC 3168, ECN uses two ECN Capable Transport (ECT)
codepoints to indicate that a packet supports ECN, ECT(0) and ECT(1),
with the second codepoint used to support ECN nonce functionality to
discourage receivers from exploiting ECN to improve their throughput
at the expense of other network users, as specified in experimental
RFC 3540 [RFC3540]. This section explains why RFC 3540 is being
reclassified as Historic and makes associated updates to RFC 3168.
While the ECN Nonce works as specified, and has been deployed in
limited environments, widespread usage in the Internet has not
materialized. A study of the ECN behaviour of the Alexa top 1M web
servers using 2014 data [Trammell15] found that after ECN was
negotiated, none of the 581,711 IPv4 servers tested were using both
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
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 re-
marking of the ECN field by an erroneous middlebox or router.
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 experiment is no longer justified.
In addition, other approaches to discouraging receivers from
exploiting ECN have emerged, see Appendix B.1 of
Black Expires December 23, 2017 [Page 5]
Internet-Draft ECN Experimentation June 2017
[I-D.ietf-tsvwg-ecn-l4s-id]. Therefore, in support of ECN
experimentation with the ECT(1) codepoint, this memo:
o Declares that the ECN Nonce experiment [RFC3540] has concluded,
and notes the absence of widespread deployment.
o Updates RFC 3168 [RFC3168] to remove discussion of the ECN Nonce
and use of ECT(1) for that Nonce. The specific text updates are
omitted for brevity.
4. Updates to RFC 3168
The following subsections specify updates to RFC 3168 to enable the
three areas of experimentation summarized in Section 2.
4.1. Congestion Response Differences
RFC 3168 specifies that senders respond identically to packet drops
and ECN congestion indications. ECN congestion indications are
predominately originated by Active Queue Management (AQM) mechanisms
in intermediate buffers. AQM mechanisms are usually configured to
maintain shorter queue lengths than non-AQM based mechanisms,
particularly non-AQM drop-based mechanisms such as tail-drop, as AQM
mechanisms indicate congestion before the queue overflows. While the
occurrence of loss does not easily enable the receiver to determine
if AQM is used, the receipt of an ECN Congestion Experienced (CE)
mark conveys a strong likelihood that AQM was used to manage the
bottleneck queue. Hence an ECN congestion indication communicates a
higher likelihood that a shorter queue exists at the network
bottleneck node by comparison to a packet drop that indicates
congestion [I-D.ietf-tcpm-alternativebackoff-ecn]. This difference
suggests that for congestion indicated by ECN, a different sender
congestion response (e.g., reduce the response so that the sender
backs off by a smaller amount) may be appropriate by comparison to
the sender response to congestion indicated by loss. However,
section 5 of RFC 3168 specifies that:
Upon the receipt by an ECN-Capable transport of a single CE
packet, the congestion control algorithms followed at the end-
systems MUST be essentially the same as the congestion control
response to a *single* dropped packet.
This memo updates this RFC 3168 text 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
Black Expires December 23, 2017 [Page 6]
Internet-Draft ECN Experimentation June 2017
words "unless otherwise specified by an Experimental RFC" at the end
of the sentence quoted above.
RFC 4774 [RFC4774] quotes the above text from RFC 3168 as background,
but does not impose requirements based on that text. Therefore no
update to RFC 4774 is required to enable this area of
experimentation.
Section 6.1.2 of RFC 3168 specifies that:
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
sender knows that congestion was encountered in the network on the
path from the sender to the receiver. The indication of
congestion should be treated just as a congestion loss in non-
ECN-Capable TCP. That is, the TCP source halves the congestion
window "cwnd" and reduces the slow start threshold "ssthresh".
This memo also updates this RFC 3168 text 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 beginning of the second sentence quoted above.
4.2. Congestion Marking Differences
Taken to its limit, an AQM algorithm that uses ECN congestion
indications can be configured to maintain very shallow queues,
thereby reducing network latency by comparison to maintaining a
larger queue. Significantly more aggressive sender responses to ECN
are required to make effective use of such shallow queues; Datacenter
TCP (DCTCP) [I-D.ietf-tcpm-dctcp] provides an example. In this case,
separate network node treatments are essential, both to prevent the
aggressive low latency traffic starving conventional traffic (if
present) and to prevent any conventional traffic disruption to any
lower latency service that uses the shallow queues. Use of different
ECN codepoints is a promising means of identifying these two classes
of traffic to network nodes, and hence this area of experimentation
is based on the use of the ECT(1) codepoint to request ECN congestion
marking behavior in the network that differs from ECT(0)
counterbalanced by use of a different IETF-approved congestion
response to CE marks at the sender, e.g., as proposed in
[I-D.ietf-tsvwg-ecn-l4s-id].
Section 5 of RFC 3168 specifies that:
Black Expires December 23, 2017 [Page 7]
Internet-Draft ECN Experimentation June 2017
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
ECT(1) codepoints differently, 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 above sentence.
When an AQM is configured to use ECN congestion indications to
maintain a nearly empty queue, congestion indications are marked on
packets that would not have been dropped if ECN was not in use.
Section 5 of RFC 3168 specifies that:
For a router, the CE codepoint of an ECN-Capable packet SHOULD
only be set if the router would otherwise have dropped the packet
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
a packet to inform end nodes of incipient congestion, the router
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,
the router MAY instead set the CE codepoint in the IP header.
This memo updates RFC 3168 to allow congestion indications that are
not equivalent to drops, provided that the changes from RFC 3168 are
documented in an Experimental RFC. The specific change is to change
"For a router," to "Unless otherwise specified by an Experimental
RFC" at the beginning of the first sentence of the above paragraph.
A larger update to RFC 3168 is necessary to enable sender usage of
ECT(1) to request network congestion marking behavior that maintains
nearly empty queues at network nodes. When using loss as a
congestion signal, the number of signals provided should be reduced
to a minimum and hence only presence or absence of congestion is
communicated. In contrast, ECN can provide a richer signal, e.g., to
indicate the current level of congestion, without the disadvantage of
a larger number of packet losses. A proposed experiment in this
area, Low Latency Low Loss Scalable throughput (L4S)
[I-D.ietf-tsvwg-ecn-l4s-id] significantly increases the CE marking
probability for ECT(1)-marked traffic in a fashion that would
interact badly with existing sender congestion response functionality
because that functionality assumes that the network marks ECT packets
as frequently as it would drop Not-ECT packets. If network traffic
that uses such a conventional sender congestion response were to
encounter L4S's increased marking probability (and hence rate) at a
network bottleneck queue, the resulting traffic throughput is likely
to be much less than intended for the level of congestion at the
bottleneck queue.
Black Expires December 23, 2017 [Page 8]
Internet-Draft ECN Experimentation June 2017
To avoid that interaction, this memo reserves ECT(1) for
experimentation, initially for L4S. The specific update to Section 5
of RFC 3168 is to remove the following two paragraphs:
Senders are free to use either the ECT(0) or the ECT(1) codepoint
to indicate ECT, on a packet-by-packet basis.
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
sender to verify that network elements are not erasing the CE
codepoint, and that data receivers are properly reporting to the
sender the receipt of packets with the CE codepoint set, as
required by the transport protocol. Guidelines for the senders
and receivers to differentiate between the ECT(0) and ECT(1)
codepoints will be addressed in separate documents, for each
transport protocol. In particular, this document does not address
mechanisms for TCP end-nodes to differentiate between the ECT(0)
and ECT(1) codepoints. Protocols and senders that only require a
single ECT codepoint SHOULD use ECT(0).
and replace it with this paragraph:
Protocols and senders MUST use the ECT(0) codepoint to indicate
ECT unless otherwise specified by an Experimental RFC. Guidelines
for senders and receivers to differentiate between the ECT(0) and
ECT(1) codepoints will be addressed in separate documents, for
each transport protocol. In particular, this document does not
address mechanisms for TCP end-nodes to differentiate between the
ECT(0) and ECT(1) codepoints.
Congestion Marking Differences experiments SHOULD modify the network
behavior for ECT(1)-marked traffic rather than ECT(0)-marked traffic
if network behavior for only one ECT codepoint is modified.
Congestion Marking Differences experiments MUST NOT modify the
network behavior for ECT(0)-marked traffic in a fashion that requires
changes to sender congestion response to obtain desired network
behavior. If a Congestion Marking Differences experiment modifies
the network behavior for ECT(1)-marked traffic, e.g., CE-marking
behavior, in a fashion that requires changes to sender congestion
response to obtain desired network behavior, then the Experimental
RFC for that experiment MUST specify:
o The sender congestion response to CE marking in the network, and
o Router behavior changes, or the absence thereof, in forwarding CE-
marked packets that are part of the experiment.
Black Expires December 23, 2017 [Page 9]
Internet-Draft ECN Experimentation June 2017
In addition, until the conclusion of the L4S experiment, use of
ECT(1) in IETF RFCs is not appropriate, as the IETF may decide to
allocate ECT(1) exclusively for L4S usage if the L4S experiment is
successful.
In addition, this memo updates RFC 3168 to remove discussion of the
ECN Nonce, as noted in Section 3 above.
4.3. Generalized ECN
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
control packets (e.g., SYNs) and retransmitted packets, e.g., as
proposed in [I-D.bagnulo-tcpm-generalized-ecn].
RFC 3168 prohibits use of ECN for TCP control packets and
retransmitted packets in a number of places:
o "To ensure the reliable delivery of the congestion indication of
the CE codepoint, an ECT codepoint MUST NOT be set in a packet
unless the loss of that packet in the network would be detected by
the end nodes and interpreted as an indication of congestion."
(Section 5.2)
o "A host MUST NOT set ECT on SYN or SYN-ACK packets."
(Section 6.1.1)
o "pure acknowledgement packets (e.g., packets that do not contain
any accompanying data) MUST be sent with the not-ECT codepoint."
(Section 6.1.4)
o "This document specifies ECN-capable TCP implementations MUST NOT
set either ECT codepoint (ECT(0) or ECT(1)) in the IP header for
retransmitted data packets, and that the TCP data receiver SHOULD
ignore the ECN field on arriving data packets that are outside of
the receiver's current window." (Section 6.1.5)
o "the TCP data sender MUST NOT set either an ECT codepoint or the
CWR bit on window probe packets." (Section 6.1.6)
This memo updates RFC 3168 to allow the use of ECT codepoints on SYN
and SYN-ACK packets, pure acknowledgement packets, window probe
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.
Black Expires December 23, 2017 [Page 10]
Internet-Draft ECN Experimentation June 2017
In addition, beyond requiring TCP senders not to set ECT on TCP
control packets and retransmitted packets, RFC 3168 is silent on
whether it is appropriate for a network element, e.g. a firewall, to
discard such a packet as invalid. For Generalized ECN
experimentation to be useful, middleboxes ought not to do that,
therefore RFC 3168 is updated by adding the following text to the end
of Section 6.1.1.1 on Middlebox Issues:
Unless otherwise specified by an Experimental RFC, middleboxes
SHOULD NOT discard TCP control packets and retransmitted TCP
packets solely because the ECN field in the IP header does not
contain Not-ECT. An exception to this requirement occurs in
responding to an ongoing attack. For example, as part of the
response, it may be appropriate to drop more ECT-marked TCP SYN
packets than TCP SYN packets marked with not-ECT. Any such
exceptional discarding of TCP control packets and retransmitted
TCP packets in response to an attack MUST NOT be done routinely in
the absence of an attack and SHOULD only be done if it is
determined that the ECT capability is contributing to the attack.
4.4. Effective Congestion Control is Required
Congestion control remains an important aspect of the Internet
architecture [RFC2914]. Any Experimental RFC that takes advantage of
this memo's updates to RFC 3168 or RFC 6679 is required to discuss
the congestion control implications of the experiment(s) in order to
provide assurance that deployment of the experiment(s) does not pose
a congestion-based threat to the operation of the Internet.
5. ECN for RTP Updates to RFC 6679
RFC 6679 [RFC6679] specifies use of ECN for RTP traffic; it allows
use of both the ECT(0) and ECT(1) codepoints, and provides the
following guidance on use of these codepoints in section 7.3.1 :
The sender SHOULD mark packets as ECT(0) unless the receiver
expresses a preference for ECT(1) or for a random ECT value using
the "ect" parameter in the "a=ecn-capable-rtp:" attribute.
The Congestion Marking 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 two
sentences:
Random ECT values MUST NOT be used, as that may expose RTP to
differences in network treatment of traffic marked with ECT(1) and
ECT(0) and differences in associated endpoint congestion
responses, e.g., as proposed in [I-D.ietf-tsvwg-ecn-l4s-id]. In
Black Expires December 23, 2017 [Page 11]
Internet-Draft ECN Experimentation June 2017
addition, ECT(0) MUST be used unless otherwise specified in an
Experimental RFC.
Section 7.3.3 of RFC 6679 specifies RTP's response to receipt of CE
marked packets as being identical to the response to dropped packets:
The reception of RTP packets with ECN-CE marks in the IP header is
a notification that congestion is being experienced. The default
reaction on the reception of these ECN-CE-marked packets MUST be
to provide the congestion control algorithm with a congestion
notification that triggers the algorithm to react as if packet
loss had occurred. There should be no difference in congestion
response if ECN-CE marks or packet drops are detected.
In support of Congestion Response Differences 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 6679 is to insert the words "Unless otherwise specified by an
Experimental RFC" and reformat the last two sentences to be subject
to that condition, i.e.:
The reception of RTP packets with ECN-CE marks in the IP header is
a notification that congestion is being experienced. Unless
otherwise specified by an Experimental RFC:
* The default reaction on the reception of these ECN-CE-marked
packets MUST be to provide the congestion control algorithm
with a congestion notification that triggers the algorithm to
react as if packet loss had occurred.
* There should be no difference in congestion response if ECN-CE
marks or packet drops are detected.
The second sentence of the immediately following paragraph in RFC
6679 requires a related update:
Other reactions to ECN-CE may be specified in the future,
following IETF Review. Detailed designs of such additional
reactions MUST be specified in a Standards Track RFC and be
reviewed to ensure they are safe for deployment under any
restrictions specified.
The update is to change "Standards Track RFC" to "Standards Track RFC
or Experimental RFC" for consistency with the first update.
Black Expires December 23, 2017 [Page 12]
Internet-Draft ECN Experimentation June 2017
6. ECN for DCCP Updates to RFCs 4341, 4342 and 5622
The specifications of the three DCCP Congestion Control IDs (CCIDs) 2
[RFC4341], 3 [RFC4342] and 4 [RFC5622] contain broadly the same
wording as follows:
each DCCP-Data and DCCP-DataAck packet is sent as ECN Capable with
either the ECT(0) or the ECT(1) codepoint set.
This memo updates these sentences in each of the three RFCs as
follows:
each DCCP-Data and DCCP-DataAck packet is sent as ECN Capable.
Unless otherwise specified by an Experimental RFC, such DCCP
senders MUST set the ECT(0) codepoint.
In support of Congestion Marking Differences experimentation (as
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
omitted for brevity.
7. Acknowledgements
The content of this draft, including the specific portions of RFC
3168 that are updated draws heavily from
[I-D.khademi-tsvwg-ecn-response], whose authors are gratefully
acknowledged. The authors of the Internet Drafts describing the
experiments have motivated the production of this memo - their
interest in innovation is welcome and heartily acknowledged. Colin
Perkins suggested updating RFC 6679 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 Spencer Dawkins, Gorry Fairhurst, Ingemar
Johansson, Naeem Khademi, Mirja Kuehlewind, Karen Nielsen and Michael
Welzl. Bob Briscoe's thorough review of an early version of this
draft resulted in numerous improvements including addition of the
updates to the DCCP RFCs.
8. IANA Considerations
This memo includes no request to IANA.
9. Security Considerations
As a process memo that makes no changes to existing protocols, there
are no protocol security considerations.
Black Expires December 23, 2017 [Page 13]
Internet-Draft ECN Experimentation June 2017
However, effective congestion control is crucial to the continued
operation of the Internet, and hence this memo places the
responsibility for not breaking Internet congestion control on the
experiments and the experimenters who propose them, as specified in
Section 4.4.
Security considerations for the proposed experiments are discussed in
the Internet-Drafts that propose them.
See Appendix B.1 of [I-D.ietf-tsvwg-ecn-l4s-id] for discussion of
alternatives to the ECN Nonce.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41,
RFC 2914, DOI 10.17487/RFC2914, September 2000,
<http://www.rfc-editor.org/info/rfc2914>.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<http://www.rfc-editor.org/info/rfc3168>.
[RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
Congestion Notification (ECN) Signaling with Nonces",
RFC 3540, DOI 10.17487/RFC3540, June 2003,
<http://www.rfc-editor.org/info/rfc3540>.
[RFC4341] Floyd, S. and E. Kohler, "Profile for Datagram Congestion
Control Protocol (DCCP) Congestion Control ID 2: TCP-like
Congestion Control", RFC 4341, DOI 10.17487/RFC4341, March
2006, <http://www.rfc-editor.org/info/rfc4341>.
[RFC4342] Floyd, S., Kohler, E., and J. Padhye, "Profile for
Datagram Congestion Control Protocol (DCCP) Congestion
Control ID 3: TCP-Friendly Rate Control (TFRC)", RFC 4342,
DOI 10.17487/RFC4342, March 2006,
<http://www.rfc-editor.org/info/rfc4342>.
Black Expires December 23, 2017 [Page 14]
Internet-Draft ECN Experimentation June 2017
[RFC5622] Floyd, S. and E. Kohler, "Profile for Datagram Congestion
Control Protocol (DCCP) Congestion ID 4: TCP-Friendly Rate
Control for Small Packets (TFRC-SP)", RFC 5622,
DOI 10.17487/RFC5622, August 2009,
<http://www.rfc-editor.org/info/rfc5622>.
[RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
and K. Carlberg, "Explicit Congestion Notification (ECN)
for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August
2012, <http://www.rfc-editor.org/info/rfc6679>.
10.2. Informative References
[I-D.bagnulo-tcpm-generalized-ecn]
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-01 (work in progress), May 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-07 (work
in progress), June 2017.
[I-D.ietf-tsvwg-ecn-l4s-id]
Schepper, K. and B. Briscoe, "Identifying Modified
Explicit Congestion Notification (ECN) Semantics for
Ultra-Low Queuing Delay", draft-ietf-tsvwg-ecn-l4s-id-00
(work in progress), May 2017.
[I-D.khademi-tsvwg-ecn-response]
Khademi, N., Welzl, M., Armitage, G., and G. Fairhurst,
"Updating the Explicit Congestion Notification (ECN)
Specification to Allow IETF Experimentation", draft-
khademi-tsvwg-ecn-response-01 (work in progress), July
2016.
[RFC4774] Floyd, S., "Specifying Alternate Semantics for the
Explicit Congestion Notification (ECN) Field", BCP 124,
RFC 4774, DOI 10.17487/RFC4774, November 2006,
<http://www.rfc-editor.org/info/rfc4774>.
Black Expires December 23, 2017 [Page 15]
Internet-Draft ECN Experimentation June 2017
[Trammell15]
Trammell, B., Kuehlewind, M., Boppart, D., Learmonth, I.,
Fairhurst, G., and R. Scheffenegger, "Enabling Internet-
Wide Deployment of Explicit Congestion Notification".
In Proc Passive & Active Measurement (PAM'15) Conference
(2015)
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.
Black Expires December 23, 2017 [Page 16]
Internet-Draft ECN Experimentation June 2017
o Change the "SHOULD" for DCCP sender use of ECT(0) to a "MUST"
(overlooked in earlier editing).
o Other minor edits.
Author's Address
David Black
Dell EMC
176 South Street
Hopkinton, MA 01748
USA
Email: david.black@dell.com
Black Expires December 23, 2017 [Page 17]