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Internet Engineering Task Force (IETF) M. Kuehlewind, Ed.
Request for Comments: 7560 ETH Zurich
Category: Informational R. Scheffenegger
ISSN: 2070-1721 NetApp, Inc.
B. Briscoe
BT
August 2015
Problem Statement and Requirements for Increased Accuracy
in Explicit Congestion Notification (ECN) Feedback
Abstract
Explicit Congestion Notification (ECN) is a mechanism where network
nodes can mark IP packets, instead of dropping them, to indicate
congestion to the endpoints. An ECN-capable receiver will feed this
information back to the sender. ECN is specified for TCP in such a
way that it can only feed back one congestion signal per Round-Trip
Time (RTT). In contrast, ECN for other transport protocols, such as
RTP/UDP and SCTP, is specified with more accurate ECN feedback.
Recent new TCP mechanisms (like Congestion Exposure (ConEx) or Data
Center TCP (DCTCP)) need more accurate ECN feedback in the case where
more than one marking is received in one RTT. This document
specifies requirements for an update to the TCP protocol to provide
more accurate ECN feedback.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7560.
Kuehlewind, et al. Informational [Page 1]
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Copyright Notice
Copyright (c) 2015 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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Recap of Classic ECN and ECN Nonce in IP/TCP . . . . . . . . 5
3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Design Approaches . . . . . . . . . . . . . . . . . . . . . . 11
5.1. Redefinition of ECN/NS Header Bits . . . . . . . . . . . 11
5.2. Using Other Header Bits . . . . . . . . . . . . . . . . . 13
5.3. Using a TCP Option . . . . . . . . . . . . . . . . . . . 13
6. Security Considerations . . . . . . . . . . . . . . . . . . . 14
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.1. Normative References . . . . . . . . . . . . . . . . . . 14
7.2. Informative References . . . . . . . . . . . . . . . . . 14
Appendix A. Ambiguity of the More Accurate ECN Feedback in DCTCP 16
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
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1. Introduction
Explicit Congestion Notification (ECN) [RFC3168] is a mechanism where
network nodes can mark IP packets instead of dropping them to
indicate congestion to the endpoints. An ECN-capable receiver will
feed this information back to the sender. ECN is specified for TCP
in such a way that only one feedback signal can be transmitted per
Round-Trip Time (RTT). This is sufficient for preexisting TCP
congestion control mechanisms that perform only one reduction in
sending rate per RTT, independent of the number of ECN congestion
marks. But recently proposed or deployed mechanisms like Congestion
Exposure (ConEx) [RFC6789] or Data Center TCP (DCTCP) [DCTCP] need
more accurate ECN feedback than 'classic ECN' [RFC3168] to work
correctly in the case where more than one marking is received in any
one RTT.
For an in-depth discussion of the application benefits of using ECN
(including with sufficiently granular feedback), see [ECN-BENEFITS].
ECN is also defined for transport protocols beside TCP. ECN feedback
as defined for RTP/UDP [RFC6679] provides a very detailed level of
information, delivering individual counters for all four ECN
codepoints as well as lost and duplicate segments, but at the cost of
high signalling overhead. ECN feedback for SCTP has been proposed in
[SCTP-ECN]. This delivers a counter for the number of ECN-capable
packets that were marked due to congestion (since the last sender-
side window reduction), but it comes at the cost of increased
overhead.
Today, implementations of DCTCP already exist that alter TCP's ECN
feedback protocol in proprietary ways (DCTCP was released in
Microsoft Windows 8, and implementations exist for Linux and
FreeBSD). However, the changes DCTCP makes to TCP omit capability
negotiation, relying instead on uniform configuration across all
hosts and network devices with ECN capability. A primary motivation
for this document is to intervene before each proprietary
implementation invents its own non-interoperable handshake, which
could lead to _de facto_ consumption of the few flags or codepoints
that remain available for standardizing capability negotiation.
This document lists requirements for a robust and interoperable TCP/
ECN feedback protocol that is more accurate than classic ECN
[RFC3168] and that all implementations of new TCP extensions, like
ConEx and/or DCTCP, can use. While a new feedback scheme should
still deliver as much information as classic ECN, this document also
clarifies what has to be taken into consideration in addition. Thus,
the listed requirements should be addressed in the specification of a
more accurate ECN feedback scheme. A few solutions have already been
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proposed. Section 5 demonstrates how to use the requirements to
compare them, by briefly sketching their high-level design choices
and discussing the benefits and drawbacks of each.
The scope of these requirements is not limited to any specific
environment and is intended for general deployment over public and
private IP networks. Candidate solutions should try to adhere to all
these requirements, but, where this is not possible, they should
justify the deviation. The ordering of the requirements listed in
this document is not to be taken as an order of importance, because
each requirement might have different weight in different deployment
scenarios.
These requirements are only concerned with the type and quality of
the ECN feedback signal. The requirements do not stipulate how a TCP
sender might react to the improved ECN signal. The requirements also
do not imply that any modifications to TCP senders or receivers are
obligatory.
1.1. Terminology
We use the following terminology from [RFC3168] and [RFC3540]:
The ECN field in the IP header:
Not-ECT: the not ECN-Capable Transport codepoint,
CE: the Congestion Experienced codepoint,
ECT(0): the first ECN-Capable Transport codepoint, and
ECT(1): the second ECN-Capable Transport codepoint.
The ECN flags in the TCP header:
CWR: the Congestion Window Reduced flag,
ECE: the ECN-Echo flag, and
NS: ECN Nonce Sum.
In this document, the ECN feedback scheme as specified in [RFC3168]
is called 'classic ECN' and any new proposal is called a 'more
accurate ECN feedback' scheme. A 'congestion mark' is defined as an
IP packet where the CE codepoint is set. A 'congestion episode'
refers to one or more congestion marks that belong to the same
overload situation in the network (usually during one RTT). A TCP
segment with the acknowledgement flag set is simply called an ACK.
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2. Recap of Classic ECN and ECN Nonce in IP/TCP
ECN requires two bits in the IP header. The ECN capability of a
packet is indicated when either one of the two bits is set. A
network node can set both bits simultaneously when it experiences
congestion. This leads to the four codepoints (Not-ECT, ECT(0),
ECT(1), and CE) as listed above.
In the TCP header, the first two bits in byte 14 are defined as ECN
feedback for each half-connection. A TCP receiver signals the
reception of a congestion mark using the ECN-Echo (ECE) flag in the
TCP header. For reliability, the receiver continues to set the ECE
flag on every ACK. To enable the TCP receiver to determine when to
stop setting the ECE flag, the sender sets the CWR flag upon
reception of an ECE feedback signal. This always leads to a full RTT
of ACKs with ECE set. Thus, the receiver cannot signal back any
additional CE markings arriving within the same RTT.
The ECN Nonce [RFC3540] is an experimental addition to ECN that the
TCP sender can use to protect itself against accidental or malicious
concealment of CE-marked or dropped packets. This addition defines
the last bit of byte 13 in the TCP header as the Nonce Sum (NS) flag.
The receiver maintains a nonce sum that counts the occurrence of
ECT(1) packets and signals the least significant bit of this sum on
the NS flag. There are no known deployments of a TCP stack that
makes use of the ECN Nonce extension.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| | | N | C | E | U | A | P | R | S | F |
| Header Length | Reserved | S | W | C | R | C | S | S | Y | I |
| | | | R | E | G | K | H | T | N | N |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Figure 1: The (Post-ECN Nonce) Definition of the TCP Header Flags
An alternative for a sender to assure feedback integrity has been
proposed where the sender itself occasionally inserts a CE mark or
reorders packets, and checks that the receiver feeds these back
faithfully [TEST-RCV]. This alternative consumes no header bits or
codepoints, and it releases the ECT(1) codepoint in the IP header and
the NS flag in the TCP header for other uses.
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3. Use Cases
The following two examples serve to show where existing mechanisms
would already benefit from more accurate ECN feedback information.
However, as it is hard to predict the future, once a more accurate
ECN feedback mechanism that adheres to the requirements stated in
this document is widely deployed, it's very likely that additional
uses will be found. The examples listed below are in no particular
order.
ConEx is an experimental approach that allows a sender to relay
congestion feedback provided by the receiver into the network along
the forward data path. ConEx information can be used for traffic
management to limit traffic proportionate to the actual congestion
being caused, rather than limiting traffic based on rate or volume
[RFC6789]. A ConEx sender uses selective acknowledgements (SACK)
[RFC2018] for accurate feedback of loss signals, but until now TCP
has offered no equivalent accurate feedback for ECN.
DCTCP offers very low and predictable queuing delay. DCTCP changes
the reaction to congestion of a TCP sender and additionally requires
switches/routers to have ECN enabled and configured with a low step
threshold and no signal smoothing, so it is currently only used in
private networks, e.g., internal to data centers. DCTCP was released
in Microsoft Windows 8, and implementations exist for Linux and
FreeBSD. To retrieve sufficient congestion information, the
different DCTCP implementations use a proprietary ECN feedback
protocol, but they omit capability negotiation. Moreover, the
feedback protocol proposed in [DCTCP] only works if there are no
losses at all, and otherwise it gets very confused (see Appendix A).
Therefore, if a generic, more accurate ECN feedback scheme were
available, it would solve two problems for DCTCP: i) the need for a
consistent variant of DCTCP to be deployed network-wide and ii) the
inability to cope with ACK loss.
Classic ECN-TCP would not benefit from more accurate ECN feedback,
but it would not suffer either. The same signal that is currently
conveyed with ECN following the specification given in [RFC3168]
would be available.
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The following scenarios should briefly show where accurate ECN
feedback is needed or adds value:
A sender with standardized TCP congestion control that supports
ConEx:
In this case, the ConEx mechanism uses the extra information per
RTT to re-echo the precise congestion information, but the
congestion control algorithm still ignores multiple marks per RTT
[RFC5681].
A sender using DCTCP congestion control without ConEx:
The congestion control algorithm uses the extra info per RTT to
perform its decrease depending on the number of congestion marks.
A sender using DCTCP congestion control and supporting ConEx:
Both the congestion control algorithm and ConEx use the more
accurate ECN feedback mechanism.
As-yet-unspecified sender mechanisms:
The above are two examples of more general interest in sender
mechanisms that respond to the extent of congestion feedback, not
just its existence. It will greatly simplify incremental
deployment if the sender can unilaterally deploy new behaviours
and rely on the presence of generic receivers that have already
implemented more accurate feedback.
A TCP sender using congestion control as specified in RFC 5681
without ConEx:
No accurate feedback is necessary here. The congestion control
algorithm still reacts to only one signal per RTT. But, it is
best to feed back all the information the receiver gets, whether
or not the sender uses it -- at least as long as overhead is low
or zero.
Using CE for checking integrity:
If a more accurate ECN feedback scheme feeds all occurrences of CE
marks back, a sender could perform integrity checking by
occasionally injecting CE marks itself. Specifically, a sender
can send packets that it randomly marks with CE (at low
frequency), then check if feedback is received for these packets.
The congestion notification feedback for these self-injected
markings would not require a congestion control reaction
[TEST-RCV].
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4. Requirements
The requirements of the accurate ECN feedback protocol are to have
fairly accurate (not necessarily perfect), timely, and protected
signalling. This leads to the following requirements, which should
be discussed for any proposed more accurate ECN feedback scheme:
Resilience
The ECN feedback signal is carried within the ACK. Pure TCP ACKs
can get lost without recovery (not just due to congestion but also
due to deliberate ACK thinning). Moreover, delayed ACKs are
commonly used with TCP. Typically, an ACK is triggered after two
data segments (or more, e.g., due to receive segment coalescing,
ACK compression, ACK congestion control [RFC5690], or other
phenomena; see [RFC3449]). In a high-congestion situation where
most of the packets are marked with CE, an accurate feedback
mechanism should still be able to signal sufficient congestion
information. Thus, the accurate ECN feedback extension has to
take delayed ACKs and ACK loss into account. Also, a more
accurate feedback protocol should still provide more accurate
feedback than classic ECN when delayed ACKs cover more than two
segments, or when a thin stream disables Nagle's algorithm
[RFC896]. Finally, the feedback mechanism should not be impacted
by reordering of ACKs, even when the ACKed sequence number does
not increase.
Timeliness
A CE mark can be induced by the sending host, or more commonly a
network node on the transmission path, and is then echoed by the
receiver in the TCP ACK. Thus, when this information arrives at
the sender, it is naturally already about one RTT old. With a
sufficient ACK rate, a further delay of a small number of packets
can be tolerated. However, this information will become stale
with large delays, given the dynamic nature of networks. TCP
congestion control (which itself partly introduces these dynamics)
operates on a time scale of one RTT. Thus, to be timely,
congestion feedback information should be delivered within about
one RTT.
Integrity
The integrity of the feedback in a more accurate ECN feedback
scheme should be assured, at least as well as the ECN Nonce.
Alternatively, it should at least be possible to give strong
incentives for the receiver and network nodes to cooperate
honestly.
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Given there are known problems with ECN Nonce deployment, this
document only requires that the integrity of the more accurate ECN
feedback can be assured; it does not require that the ECN Nonce
mechanism is employed to achieve this. Indeed, if integrity could
be provided in another manner, a more accurate ECN feedback
protocol might repurpose the nonce sum (NS) flag in the TCP
header.
If the more accurate ECN feedback scheme provides sufficient
information, the integrity check could be performed by, e.g.,
deterministically setting the CE in the sender and monitoring the
respective feedback (similar to ECT(1) and the ECN Nonce sum).
Whether a sender should enforce when it detects wrong feedback
information, and what kind of enforcement it should apply, are
policy issues that need not be specified as part of the more
accurate ECN feedback signal scheme itself, but rather when
specifying an update to core TCP mechanisms like congestion
control that make use of the more accurate ECN signal.
Accuracy
Classic ECN feeds back one congestion notification per RTT; this
is sufficient for classic TCP congestion control, which reduces
the sending rate at most once per RTT. Thus, the more accurate
ECN feedback scheme should ensure that, if a congestion episode
occurs, at least one congestion notification is echoed and
received per RTT as classic ECN would do. Of course, the goal of
a more accurate ECN extension is to reconstruct the number of CE
markings more accurately. In the best case, the new scheme should
even allow reconstruction of the exact number of payload bytes
that a CE-marked packet was carrying. However, it is accepted
that it may be too complex for a sender to get the exact number of
congestion markings or marked bytes in all situations. Ideally,
the feedback scheme should preserve the order in which any (of the
four) ECN signals were received. And, ideally, it would even be
possible for the sender to determine which of the packets covered
by one delayed ACK were congestion marked, e.g., if the flow
consists of packets of different sizes, or to allow for future
protocols where the order of the markings may be important.
In the best case, a sender that sees more accurate ECN feedback
information would be able to reconstruct the occurrence of any of
the four codepoints (Not-ECT, CE, ECT(0), ECT(1)). However,
assuming the sender marks all data packets as ECN-capable and uses
a default setting of ECT(0) (as with [RFC3168]), solely feeding
back the occurrence of CE and ECT(1) might be sufficient. Because
the sender can keep account of the transmitted segments with any
of the three ECN codepoints, conveying any two of these back to
the sender is sufficient for it to reconstruct the third as
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observed by the receiver. Thus, a more accurate ECN feedback
scheme should at least provide information on two of these
signals, e.g., CE and ECT(1).
If a more accurate ECN scheme can reliably deliver feedback in
most but not all circumstances, ideally the scheme should at least
not introduce bias. In other words, undetected loss of some ACKs
should be as likely to increase as decrease the sender's estimate
of the probability of ECN marking.
Complexity
Implementation should be as simple as possible, and only a minimum
of additional state information should be needed. This will
enable more accurate ECN feedback to be used as the default
feedback mechanism, even if only one ECN feedback signal per RTT
is needed.
Overhead
A more accurate ECN feedback signal should limit the additional
network load, because ECN feedback is ultimately not critical
information (in the worst case, loss will still be available as a
congestion signal of last resort). As feedback information has to
be provided frequently and in a timely fashion, potentially all or
a large fraction of TCP acknowledgements might carry this
information. Ideally, no additional segments should be exchanged
compared to a TCP session as specified in RFC 3168, and the
overhead in each segment should be minimized.
Backward and forward compatibility
Given more accurate ECN feedback will involve a change to the TCP
protocol, it should be negotiated between the two TCP endpoints.
If either end does not support the more accurate feedback, they
should both be able to fall back to classic ECN feedback.
A more accurate ECN feedback extension should aim to traverse most
middleboxes, including firewalls and Network Address Translators
(NATs). Further, a feedback mechanism should provide a method to
fall back to classic ECN signalling if the new signal is
suppressed by certain middleboxes.
In order to avoid a fork in the TCP protocol specifications, if
experiments with the new ECN feedback protocol are successful, the
intention is to eventually update RFC 3168 for any TCP/ECN sender,
not just for ConEx or DCTCP senders. Then, future senders will be
able to unilaterally deploy new behaviours that exploit the
existence of more accurate ECN feedback in receivers (forward
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compatibility). Conversely, even if another sender only needs one
ECN feedback signal per RTT, it should be able to use more
accurate ECN feedback and simply ignore the excess information.
Furthermore, the receiver should not make assumptions about the
mechanism that was used to set the markings nor about any
interpretation or reaction to the congestion signal. The receiver
only needs to faithfully reflect congestion information back to the
sender.
5. Design Approaches
This section introduces some possible design approaches for TCP ECN
feedback. The purpose of this section is to give examples of how
trade-offs might be needed between the requirements, as input to
future IETF work to specify a protocol. The order is not
significant, and there is no intention to endorse any particular
approach.
All approaches presented below (and proposed so far) are able to
provide accurate ECN feedback information as long as no ACK loss
occurs and the congestion rate is reasonable. In the case of a high
ACK loss rate or very high congestion (CE-marking) rate, the proposed
schemes have different resilience characteristics depending on the
number of bits used for the encoding. While classic ECN provides
reliable (but inaccurate) feedback of a maximum of one congestion
signal per RTT, the proposed schemes do not implement an explicit
acknowledgement mechanism for the feedback (as, e.g., the ECE/CWR
exchange of [RFC3168]).
5.1. Redefinition of ECN/NS Header Bits
Schemes in this category can additionally use the NS bit for
capability negotiation during the TCP handshake exchange. Thus a
more accurate ECN could be negotiated without changing the classic
ECN negotiation and thus being backwards compatible.
Schemes in this category can simply redefine the ECN header flags,
ECE and CWR, to encode the occurrence of a CE marking at the
receiver. This approach provides very limited resilience against
loss of ACK, particularly pure ACKs (no payload and therefore
delivered unreliably).
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A couple of schemes have been proposed so far:
o A naive 1-bit scheme that sends one ECE for each CE received could
use CWR to increase robustness against ACK loss by introducing
redundant information on the next ACK, but this is still
vulnerable to ACK loss.
o The scheme defined for DCTCP [DCTCP], which toggles the ECE
feedback on an immediate ACK whenever the CE marking changes, and
otherwise feeds back delayed ACKs with the ECE value unchanged.
Appendix A demonstrates that this scheme is still ambiguous to the
sender if the ACKs are pure ACKs, and if some may have been lost.
Alternatively, the receiver uses the three ECN/NS header flags, ECE,
CWR, and NS, to represent a counter that signals the accumulated
number of CE markings it has received. Resilience against loss is
better than the flag-based schemes but may not suffice in the
presence of extended ACK loss that otherwise would not affect the TCP
sender's performance.
A number of coding schemes have been proposed so far in this
category:
o A 3-bit counter scheme continuously feeds back the three least
significant bits of a CE counter;
o A scheme that defines a standardized lookup table to map the eight
codepoints onto either a CE counter or an ECT(1) counter.
These proposed schemes provide accumulated information on CE marking
feedback, similar to the number of acknowledged bytes in the TCP
header. Due to the limited number of bits, the ECN feedback
information will wrap much more often than the acknowledgement field.
Thus, feedback information could be lost due to a relatively small
sequence of pure-ACK losses. Resilience could be increased by
introducing redundancy, e.g., send each counter increase two or more
times. Of course, any of these additional mechanisms will increase
the complexity. If the congestion rate is greater than the ACK rate
(multiplied by the number of congestion marks that can be signaled
per ACK), the congestion information cannot correctly be fed back.
Covering the worst case (where every packet is CE marked) can
potentially be realized by dynamically adapting the ACK rate and
redundancy. This again increases complexity and perhaps the
signalling overhead as well. Schemes that do not repurpose the ECN
NS bit could still support the ECN Nonce.
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5.2. Using Other Header Bits
As seen in Figure 1, there are currently three unused flags in the
TCP header. The proposed 3-bit counter or codepoint schemes could be
extended by one or more bits to add higher resilience against ACK
loss. The relative gain would be exponentially higher resilience
against ACK loss, while the respective drawbacks would remain
identical.
Alternatively, a new method could standardize the use of the bits in
the Urgent Pointer field (see [RFC6093]) to signal more bits of its
congestion signal counter, but only whenever the Urgent Flag is not
set. As this is often the case, resilience could be increased
without additional header overhead.
Any proposal to use such bits would need to check the likelihood that
some middleboxes might discard or 'normalize' the currently unused
flag bits or a non-zero Urgent Pointer when the Urgent Flag is
cleared. If during experimentation certain bits have been proven to
be usable, the assignment of any of these bits would then require an
IETF standards action.
5.3. Using a TCP Option
Alternatively, a new TCP option could be introduced, to help maintain
the accuracy and integrity of ECN feedback between receiver and
sender. Such an option could provide higher resilience and even more
information, e.g., as much as is provided by a proposal for SCTP that
counts the number of CE marked packet [SCTP-ECN] since the last CWR
was observed, or by ECN for RTP/UDP [RFC6679]. The latter explicitly
provides the total number of packets during a connection where the IP
ECN field is set to ECT(0), ECT(1), CE, or Not-ECT, as well as the
number of lost packets. However, deploying new TCP options has its
own challenges. Moreover, to actually achieve high resilience, this
option would need to be carried by most or all ACKs as the receiver
cannot know if and when ACKs may be dropped. Thus, this approach
would introduce considerable signalling overhead even though ECN
feedback is not extremely critical information (in the worst case,
loss will still be available to provide a strong congestion feedback
signal). Nevertheless, such a TCP option could be used in addition
to a more accurate ECN feedback scheme in the TCP header or in
addition to classic ECN, only when needed and when space is
available.
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6. Security Considerations
ECN feedback information must only be used if the other information
contained in a received TCP segment indicates that the congestion was
genuinely part of the flow and not spoofed. That is, the normal TCP
acceptance techniques have to be used to verify that the segment is
part of the flow before returning any contained ECN information, and,
similarly, ECN feedback is only accepted on valid ACKs.
Given ECN feedback is used as input for congestion control, the
respective algorithm would not react appropriately if ECN feedback
were lost and the resilience mechanism to recover it was inadequate.
This resilience requirement is articulated in Section 4. However, it
should be noted that ECN feedback is not the last resort against
congestion collapse, because if there is insufficient response to
ECN, loss will ensue, and TCP will still react appropriately to loss.
A receiver could suppress ECN feedback information leading to its
connections consuming excess sender or network resources. This
problem is similar to that seen with the classic ECN feedback scheme
and should be addressed by integrity checking as required in
Section 4.
7. References
7.1. Normative References
[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>.
7.2. Informative References
[DCTCP] Bensley, S., Eggert, L., and D. Thaler, "Microsoft's
Datacenter TCP (DCTCP): TCP Congestion Control for
Datacenters", Work in Progress,
draft-bensley-tcpm-dctcp-05, July 2015.
[ECN-BENEFITS]
Fairhurst, G. and M. Welzl, "The Benefits of using
Explicit Congestion Notification (ECN)", Work in Progress
draft-ietf-aqm-ecn-benefits-06, July 2015.
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RFC 7560 Requirements for More Accurate ECN Feedback August 2015
[RFC896] Nagle, J., "Congestion Control in IP/TCP Internetworks",
RFC 896, DOI 10.17487/RFC0896, January 1984,
<http://www.rfc-editor.org/info/rfc896>.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018,
DOI 10.17487/RFC2018, October 1996,
<http://www.rfc-editor.org/info/rfc2018>.
[RFC3449] Balakrishnan, H., Padmanabhan, V., Fairhurst, G., and M.
Sooriyabandara, "TCP Performance Implications of Network
Path Asymmetry", BCP 69, RFC 3449, DOI 10.17487/RFC3449,
December 2002, <http://www.rfc-editor.org/info/rfc3449>.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<http://www.rfc-editor.org/info/rfc5681>.
[RFC5690] Floyd, S., Arcia, A., Ros, D., and J. Iyengar, "Adding
Acknowledgement Congestion Control to TCP", RFC 5690,
DOI 10.17487/RFC5690, February 2010,
<http://www.rfc-editor.org/info/rfc5690>.
[RFC6093] Gont, F. and A. Yourtchenko, "On the Implementation of the
TCP Urgent Mechanism", RFC 6093, DOI 10.17487/RFC6093,
January 2011, <http://www.rfc-editor.org/info/rfc6093>.
[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>.
[RFC6789] Briscoe, B., Ed., Woundy, R., Ed., and A. Cooper, Ed.,
"Congestion Exposure (ConEx) Concepts and Use Cases",
RFC 6789, DOI 10.17487/RFC6789, December 2012,
<http://www.rfc-editor.org/info/rfc6789>.
[SCTP-ECN] Stewart, R., Tuexen, M., and X. Dong, "ECN for Stream
Control Transmission Protocol (SCTP)", Work in Progress,
draft-stewart-tsvwg-sctpecn-05, January 2014.
[TEST-RCV] Moncaster, T., Briscoe, B., and A. Jacquet, "A TCP Test to
Allow Senders to Identify Receiver Non-Compliance", Work
in Progress, draft-moncaster-tcpm-rcv-cheat-03, July 2014.
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Appendix A. Ambiguity of the More Accurate ECN Feedback in DCTCP
As defined in [DCTCP], a DCTCP receiver feeds back ECE=0 on delayed
ACKs as long as CE remains 0, and also immediately sends an ACK with
ECE=0 when CE transitions to 1. Similarly, it continually feeds back
ECE=1 on delayed ACKs while CE remains 1 and immediately feeds back
ECE=1 when CE transitions to 0. A sender can unambiguously decode
this scheme if there is never any ACK loss, and the sender assumes
there will never be any ACK loss.
The following two examples show that the feedback sequence becomes
highly ambiguous to the sender if either of these conditions is
broken. Below, '0' represents ECE=0, '1' represents ECE=1, and '.'
represents a gap of one segment between delayed ACKs. Now imagine
that the sender receives the following sequence of feedback on three
pure ACKs:
0.0.0
When the receiver sent this sequence, it could have been any of the
following four sequences:
a. 0.0.0 (0 x CE)
b. 010.0 (1 x CE)
c. 0.010 (1 x CE)
d. 01010 (2 x CE)
where any of the 1s represent a possible pure ACK carrying ECE
feedback that could have been lost. If the sender guesses (a), it
might be correct, or it might miss 1 or 2 congestion marks over 5
packets. Therefore, when confronted with this simple sequence (that
is not contrived), a sender can guess that congestion might have been
0%, 20%, or 40%, but it doesn't know which.
Sequences with a longer gap (e.g., 0...0.0) become far more
ambiguous. It helps a little if the sender knows the distance the
receiver uses between delayed ACKs, and it helps a lot if the
distance is 1, i.e., no delayed ACKs. However, even without delayed
ACKs there will still be ambiguity whenever there are pure ACK
losses.
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Acknowledgements
Thanks to Gorry Fairhurst for his review and for ideas on CE-based
integrity checking and to Mohammad Alizadeh for suggesting the need
to avoid bias.
Bob Briscoe was partly funded by the European Community under its
Seventh Framework Programme through the Reducing Internet Transport
Latency (RITE) project (ICT-317700) and through the Trilogy 2 project
(ICT-317756). The views expressed here are solely those of the
authors, in the context of the mentioned funding projects.
Authors' Addresses
Mirja Kuehlewind (editor)
ETH Zurich
Gloriastrasse 35
Zurich 8092
Switzerland
Email: mirja.kuehlewind@tik.ee.ethz.ch
Richard Scheffenegger
NetApp, Inc.
Am Euro Platz 2
Vienna 1120
Austria
Phone: +43 1 3676811 3146
Email: rs@netapp.com
Bob Briscoe
BT
B54/77, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
United Kingdom
Phone: +44 1473 645196
Email: ietf@bobbriscoe.net
URI: http://bobbriscoe.net/
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