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Versions: (draft-kuehlewind-tcpm-accecn-reqs)
00 01 02 03 04 05 06 07 08 RFC 7560
TCP Maintenance and Minor Extensions (tcpm) M. Kuehlewind, Ed.
Internet-Draft University of Stuttgart
Intended status: Informational R. Scheffenegger
Expires: August 16, 2014 NetApp, Inc.
B. Briscoe
BT
February 12, 2014
Problem Statement and Requirements for a More Accurate ECN Feedback
draft-ietf-tcpm-accecn-reqs-05
Abstract
Explicit Congestion Notification (ECN) is an IP/TCP mechanism where
network nodes can mark IP packets instead of dropping them to
indicate congestion to the end-points. 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 ConEx or 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 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 August 16, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Recap of Classic ECN and ECN Nonce in IP/TCP . . . . . . . . 4
3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Design Approaches . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Re-Definition of ECN/NS Header Bits . . . . . . . . . . . 10
5.2. Using Other Header Bits . . . . . . . . . . . . . . . . . 11
5.3. Using a TCP Option . . . . . . . . . . . . . . . . . . . 12
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8. Security Considerations . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. Normative References . . . . . . . . . . . . . . . . . . 13
9.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Ambiguity of the More Accurate ECN Feedback in DCTCP 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction
Explicit Congestion Notification (ECN) [RFC3168] is an IP/TCP
mechanism where network nodes can mark IP packets instead of dropping
them to indicate congestion to the end-points. 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 pre-
existing 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)
[Ali10] need more accurate ECN feedback to work correctly in the case
where more than one marking is received in any one RTT.
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
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codepoints as well as lost and duplicate segments, but at the cost of
high signaling overhead. ECN feedback for SCTP
[I-D.stewart-tsvwg-sctpecn] delivers a counter for the number of CE
marked segments between CWR chunks, but also 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). The changes DCTCP makes to TCP are not currently the
subject of any IETF standardization activity, and they omit
capability negotiation, relying instead on uniform configuration
across a 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 more
accurate TCP/ECN feedback protocol 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 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.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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
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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 acknowledgment flag set is simply called ACK.
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 ECN-Echo 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.
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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
However, as the ECN Nonce is a separate extension to ECN, even if a
sender tries to protect itself with the ECN Nonce, any receiver
wishing to conceal marked packets only has to pretend not to support
the ECN Nonce and simply does not provide any nonce sum feedback.
An alternative for a sender to assure feedback integrity has been
proposed where the sender occasionally inserts a CE mark itself (or
reordering or loss), and checks that the receiver feeds it back
faithfully [I-D.moncaster-tcpm-rcv-cheat]. This alternative requires
no standardization and consumes no header bits or codepoints, as well
as releasing the ECT(1) codepoint in the IP header and the NS flag in
the TCP header for other uses.
3. Use Cases
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 currently TCP
offers 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 [Ali10] 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) need for a
consistent variant of DCTCP to be deployed network-wide and ii)
inability to cope with ACK loss.
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The following scenarios should briefly show where accurate ECN
feedback is needed or adds value:
A sender with standardised 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 RFC5681 TCP sender 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 the sender uses it or not -- 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 which 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 [I-D.moncaster-tcpm-rcv-cheat].
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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
signaling. This leads to the following requirements, which MUST 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). 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 work if delayed ACKs
covered more than two packets.
Timeliness
A CE mark can be induced by 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
It should be possible to assure the integrity of the feedback
in a more accurate ECN feedback scheme, 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.
Given there are known problems with the ECN nonce (as
identified above), this document only requires that the
integrity of the more accurate ECN feedback can be assured as
an inherent part of the new more accurate ECN feedback
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protocol; it does not require that the ECN Nonce mechanism is
employed to achieve this. Indeed, if integrity could be
provided else-wise, a more accurate ECN feedback protocol
might re-purpose the nonce sum (NS) flag in the TCP header.
If the more accurate ECN feedback scheme provides sufficient
information, the integrity check could e.g. be performed by
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 more accurate ECN feedback scheme.
Accuracy
Classic ECN feeds back one congestion notification per RTT,
which 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 code points (non-ECT, CE,
ECT(0), ECT(1)). However, assuming the sender marks all data
packets as ECN-capable and uses the default setting of
ECT(0), solely feeding back the occurrence of CE and ECT(1)
might be sufficient. Thus a more accurate ECN feedback
scheme should at least provide information on these two
signals, CE and ECT(1).
If a more accurate ECN scheme can reliably deliver feedback
in most but not all circumstances, ideally the scheme should
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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. 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.
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
acknowledgments might carry this information. Ideally, no
additional segments should be exchanged compared to an
RFC3168 TCP session, 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 to 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 be able
to traverse most existing middleboxes. Further, a feedback
mechanism should provide a method to fall-back to classic ECN
signaling 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, it is intended to eventually update RFC3168 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 compatibility). Conversely,
even if another sender only needs one ECN feedback signal per
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RTT, it should be able to use more accurate ECN feedback, and
simply ignore the excess information.
5. Design Approaches
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 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. Re-Definition 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 re-define 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).
A couple of schemes have been proposed so far:
o A naive one-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 highly vulnerable to ACK loss.
o The scheme defined for DCTCP [Ali10], 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 highly 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 still not ideal.
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A couple 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 standardised lookup table to map the 8
codepoints onto either a CE counter or an ECT(1) counter.
These proposed schemes provide accumulated information on ECN-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
signaling overhead as well. Schemes that do not re-purpose the ECN
NS bit, could still support the ECN Nonce.
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, the receiver could use bits in the Urgent Pointer
field to signal more bits of its congestion signal counter, but only
whenever it does not set the Urgent Flag. 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.
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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. ECN for RTP/UDP [RFC6679] explicitly provides the
number of ECT(0), ECT(1), CE, non-ECT marked and lost packets, and
SCTP counts the number of ECN marks [I-D.stewart-tsvwg-sctpecn]
between CWR chunks. 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. Thus this
approach would introduce considerable signaling 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). Whatever, 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.
6. Acknowledgements
Thanks to Gorry Fairhurst for ideas on CE-based integrity checking
and to Mohammad Alizadeh for suggesting the need to avoid bias.
Moverover, thanks to Michael Welzl and Michael Scharf for their
feedback.
7. IANA Considerations
This memo includes no request to IANA.
8. Security Considerations
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.
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9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP", RFC
3168, September 2001.
[RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
Congestion Notification (ECN) Signaling with Nonces", RFC
3540, June 2003.
9.2. Informative References
[Ali10] Alizadeh, M., Greenberg, A., Maltz, D., Padhye, J., Patel,
P., Prabhakar, B., Sengupta, S., and M. Sridharan, "Data
Center TCP (DCTCP)", ACM SIGCOMM CCR 40(4)63-74, October
2010, <http://portal.acm.org/citation.cfm?id=1851192>.
[I-D.moncaster-tcpm-rcv-cheat]
Moncaster, T., Briscoe, B., and A. Jacquet, "A TCP Test to
Allow Senders to Identify Receiver Non-Compliance", draft-
moncaster-tcpm-rcv-cheat-02 (work in progress), November
2007.
[I-D.stewart-tsvwg-sctpecn]
Stewart, R., Tuexen, M., and X. Dong, "ECN for Stream
Control Transmission Protocol (SCTP)", draft-stewart-
tsvwg-sctpecn-05 (work in progress), January 2014.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009.
[RFC5690] Floyd, S., Arcia, A., Ros, D., and J. Iyengar, "Adding
Acknowledgement Congestion Control to TCP", RFC 5690,
February 2010.
[RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
and K. Carlberg, "Explicit Congestion Notification (ECN)
for RTP over UDP", RFC 6679, August 2012.
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[RFC6789] Briscoe, B., Woundy, R., and A. Cooper, "Congestion
Exposure (ConEx) Concepts and Use Cases", RFC 6789,
December 2012.
Appendix A. Ambiguity of the More Accurate ECN Feedback in DCTCP
As defined in [Ali10], 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' will represent ECE=0, '1' will represent ECE=1
and '.' will represent a gap of one segment between delayed ACKs.
Now imagine that the sender receives the following sequence of
feedback on 3 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, but even then there will still be ambiguity whenever
there are pure ACK losses.
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Authors' Addresses
Mirja Kuehlewind (editor)
University of Stuttgart
Pfaffenwaldring 47
Stuttgart 70569
Germany
Email: mirja.kuehlewind@ikr.uni-stuttgart.de
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
UK
Phone: +44 1473 645196
Email: bob.briscoe@bt.com
URI: http://bobbriscoe.net/
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