Internet DRAFT - draft-ietf-tcpm-accecn-reqs
draft-ietf-tcpm-accecn-reqs
TCP Maintenance and Minor Extensions (tcpm) M. Kuehlewind, Ed.
Internet-Draft ETH Zurich
Intended status: Informational R. Scheffenegger
Expires: September 10, 2015 NetApp, Inc.
B. Briscoe
BT
March 9, 2015
Problem Statement and Requirements for a More Accurate ECN Feedback
draft-ietf-tcpm-accecn-reqs-08
Abstract
Explicit Congestion Notification (ECN) is a 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
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 10, 2015.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
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 . . . . . . . . . . . 11
5.2. Using Other Header Bits . . . . . . . . . . . . . . . . . 12
5.3. Using a TCP Option . . . . . . . . . . . . . . . . . . . 12
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
8. Security Considerations . . . . . . . . . . . . . . . . . . . 13
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
9.1. Normative References . . . . . . . . . . . . . . . . . . 14
9.2. Informative References . . . . . . . . . . . . . . . . . 14
Appendix A. Ambiguity of the More Accurate ECN Feedback in DCTCP 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
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 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)
[I-D.bensley-tcpm-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.
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For an in-depth discussion of the application benefits of using ECN
(including with sufficiently granular feedback) see
[I-D.welzl-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 signaling overhead. ECN feedback for SCTP has been proposed in
[I-D.stewart-tsvwg-sctpecn]. This 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 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
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
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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 acknowledgment flag set is simply called an 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
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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. 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 occasionally inserts a CE mark or
reordering itself, and checks that the receiver feeds it back
faithfully [I-D.moncaster-tcpm-rcv-cheat]. This alternative 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
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 are 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
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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 [I-D.bensley-tcpm-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) need
for a consistent variant of DCTCP to be deployed network-wide and ii)
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.
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,
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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.
An 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].
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 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 [RFC0896]. Finally, the feedback mechanism
should not be impacted by reordering of ACKs, even when the
ACK'ed sequence number does not increase.
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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.
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 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 signal scheme itself, but
rather when specifying an update to core TCP mechanisms like
congestion control that makes use of the more accurate ECN
signal.
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
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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 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 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
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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 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 (NAT). 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
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 TCP ECN feedback design
approaches. 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
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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 vulnerable to ACK loss.
o The scheme defined for DCTCP [I-D.bensley-tcpm-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;
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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, a new method could standardise the use of the bits in
the Urgent Pointer field (see [RFC6093]) 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. 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, perhaps as much as ECN for RTP/UDP [RFC6679], which
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explicitly provides the number of ECT(0), ECT(1), CE, non-ECT marked
and lost packets, or as much as a proposal for SCTP that 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 as the receiver cannot know if and when
ACKs may be dropped. 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 his review and for ideas on CE-based
integrity checking and to Mohammad Alizadeh for suggesting the need
to avoid bias.
Bob Briscoe was part-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). he views expressed here are solely those of the
authors, in the context of the mentioned funding projects
7. IANA Considerations
This memo includes no request to IANA.
8. 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 - i.e. 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.
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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.
9. References
9.1. Normative References
[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
[I-D.bensley-tcpm-dctcp]
sbens@microsoft.com, s., Eggert, L., and D. Thaler,
"Microsoft's Datacenter TCP (DCTCP): TCP Congestion
Control for Datacenters", draft-bensley-tcpm-dctcp-02
(work in progress), January 2015.
[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-03 (work in progress), July 2014.
[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.
[I-D.welzl-ecn-benefits]
Welzl, M. and G. Fairhurst, "The Benefits to Applications
of using Explicit Congestion Notification (ECN)", draft-
welzl-ecn-benefits-01 (work in progress), July 2014.
[RFC0896] Nagle, J., "Congestion control in IP/TCP internetworks",
RFC 896, January 1984.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996.
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[RFC3449] Balakrishnan, H., Padmanabhan, V., Fairhurst, G., and M.
Sooriyabandara, "TCP Performance Implications of Network
Path Asymmetry", BCP 69, RFC 3449, December 2002.
[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.
[RFC6093] Gont, F. and A. Yourtchenko, "On the Implementation of the
TCP Urgent Mechanism", RFC 6093, January 2011.
[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.
[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 [I-D.bensley-tcpm-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' 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)
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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.
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
UK
Phone: +44 1473 645196
Email: bob.briscoe@bt.com
URI: http://bobbriscoe.net/
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