wdiff rfc8257v1.txt rfc8257.txt

Internet Engineering Task Force (IETF) S. Bensley
Request for Comments: 8257 D. Thaler
Category: Informational P. Balasubramanian
ISSN: 2070-1721 Microsoft
L. Eggert
NetApp
G. Judd
Morgan Stanley
October 2017

Data Center TCP (DCTCP): TCP Congestion Control for Data Centers

Abstract

This Informational RFC describes Data Center TCP (DCTCP): a TCP
congestion control scheme for data-center traffic. DCTCP extends the
Explicit Congestion Notification (ECN) processing to estimate the
fraction of bytes that encounter congestion rather than simply
detecting that some congestion has occurred. DCTCP then scales the
TCP congestion window based on this estimate. This method achieves
high-burst tolerance, low latency, and high throughput with shallow-
buffered switches. This memo also discusses deployment issues
related to the coexistence of DCTCP and conventional TCP, discusses
the lack of a negotiating mechanism between sender and receiver, and
presents some possible mitigations. This memo documents DCTCP as
currently implemented by several major operating systems. DCTCP, as
described in this specification, is applicable to deployments in
controlled environments like data centers, but it must not be
deployed over the public Internet without additional measures.

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 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8257.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://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 . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. DCTCP Algorithm . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Marking Congestion on the Level 3 L3 Switches and Routers . . . . 4
3.2. Echoing Congestion Information on the Receiver . . . . . 5
3.3. Processing Echoed Congestion Indications on the Sender . 6
3.4. Handling of Congestion Window Growth . . . . . . . . . . 8
3.5. Handling of Packet Loss . . . . . . . . . . . . . . . . . 8
3.6. Handling of SYN, SYN-ACK, and RST Packets . . . . . . . . 8
4. Implementation Issues . . . . . . . . . . . . . . . . . . . . 8
4.1. Configuration of DCTCP . . . . . . . . . . . . . . . . . 9
4.2. Computation of DCTCP.Alpha . . . . . . . . . . . . . . . 9
5. Deployment Issues . . . . . . . . . . . . . . . . . . . . . . 10
6. Known Issues . . . . . . . . . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. Normative References . . . . . . . . . . . . . . . . . . 13
9.2. Informative References . . . . . . . . . . . . . . . . . 13
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15

1. Introduction

Large data centers necessarily need many network switches to
interconnect their many servers. Therefore, a data center can
greatly reduce its capital expenditure by leveraging low-cost
switches. However, such low-cost switches tend to have limited queue
capacities; thus, they are more susceptible to packet loss due to
congestion.

Network traffic in a data center is often a mix of short and long
flows, where the short flows require low latencies and the long flows
require high throughputs. Data centers also experience incast
bursts, where many servers send traffic to a single server at the
same time. For example, this traffic pattern is a natural
consequence of the MapReduce [MAPREDUCE] workload: the worker nodes
complete at approximately the same time, and all reply to the master
node concurrently.

These factors place some conflicting demands on the queue occupancy
of a switch:

o The queue must be short enough that it does not impose excessive
latency on short flows.

o The queue must be long enough to buffer sufficient data for the
long flows to saturate the path capacity.

o The queue must be long enough to absorb incast bursts without
excessive packet loss.

Standard TCP congestion control [RFC5681] relies on packet loss to
detect congestion. This does not meet the demands described above.
First, short flows will start to experience unacceptable latencies
before packet loss occurs. Second, by the time TCP congestion
control kicks in on the senders, most of the incast burst has already
been dropped.

[RFC3168] describes a mechanism for using Explicit Congestion
Notification (ECN) from the switches for detection of congestion.
However, this method only detects the presence of congestion, not its
extent. In the presence of mild congestion, the TCP congestion
window is reduced too aggressively, and this unnecessarily reduces
the throughput of long flows.

Data Center TCP (DCTCP) changes traditional ECN processing by
estimating the fraction of bytes that encounter congestion rather
than simply detecting that some congestion has occurred. DCTCP then
scales the TCP congestion window based on this estimate. This method
achieves high-burst tolerance, low latency, and high throughput with
shallow-buffered switches. DCTCP is a modification to the processing
of ECN by a conventional TCP and requires that standard TCP
congestion control be used for handling packet loss.

DCTCP should only be deployed in an intra-data-center environment
where both endpoints and the switching fabric are under a single
administrative domain. DCTCP MUST NOT be deployed over the public
Internet without additional measures, as detailed in Section 5.

The objective of this Informational RFC is to document DCTCP as a new
approach to address TCP congestion control in data centers that (which is known to be widely implemented and deployed. deployed) to
address TCP congestion control in data centers. The IETF TCPM
Working Group reached consensus regarding the fact that a DCTCP
standard would require further work. A precise documentation of
running code enables follow-up IETF Experimental or Standards Track RFCs. RFCs
through the IETF stream.

This document describes DCTCP as implemented in Microsoft Windows
Server 2012 [WINDOWS]. The Linux [LINUX] and FreeBSD [FREEBSD]
operating systems have also implemented support for DCTCP in a way
that is believed to follow this document. Deployment experiences
with DCTCP have been documented in [MORGANSTANLEY].

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

Normative language is used to describe how necessary the various
aspects of a DCTCP implementation are for interoperability, but even
compliant implementations without the measures in Sections 4-6 would
still only be safe to deploy in controlled environments, i.e., not
over the public Internet.

3. DCTCP Algorithm

There are three components involved in the DCTCP algorithm:

o The switches (or other intermediate devices in the network) detect
congestion and set the Congestion Encountered (CE) codepoint in
the IP header.

o The receiver echoes the congestion information back to the sender,
using the ECN-Echo (ECE) flag in the TCP header.

o The sender computes a congestion estimate and reacts by reducing
the TCP congestion window (cwnd) accordingly.

3.1. Marking Congestion on the Level 3 L3 Switches and Routers

The Level Layer 3 (L3) switches and routers in a data-center fabric
indicate congestion to the end nodes by setting the CE codepoint in
the IP header as specified in Section 5 of [RFC3168]. For example,
the switches may be configured with a congestion threshold. When a
packet arrives at a switch and its queue length is greater than the
congestion threshold, the switch sets the CE codepoint in the packet.
For example, Section 3.4 of [DCTCP10] suggests threshold marking with
a threshold of K > (RTT * C)/7, where C is the link rate in packets
per second. In typical deployments, the marking threshold is set to
be a small value to maintain a short average queueing delay.
However, the actual algorithm for marking congestion is an
implementation detail of the switch and will generally not be known
to the sender and receiver. Therefore, the sender and receiver
should not assume that a particular marking algorithm is implemented
by the switching fabric.

3.2. Echoing Congestion Information on the Receiver

According to Section 6.1.3 of [RFC3168], the receiver sets the ECE
flag if any of the packets being acknowledged had the CE codepoint
set. The receiver then continues to set the ECE flag until it
receives a packet with the Congestion Window Reduced (CWR) flag set.
However, the DCTCP algorithm requires more detailed more-detailed congestion
information. In particular, the sender must be able to determine the
number of bytes sent that encountered congestion. Thus, the scheme
described in [RFC3168] does not suffice.

One possible solution is to ACK every packet and set the ECE flag in
the ACK if and only if the CE codepoint was set in the packet being
acknowledged. However, this prevents the use of delayed ACKs, which
are an important performance optimization in data centers. If the
delayed ACK frequency is m, n, then an ACK is generated every m n packets.
The typical value of m n is 2, but it could be affected by ACK
throttling or packet-coalescing techniques designed to improve
performance.

Instead, DCTCP introduces a new Boolean TCP state variable, DCTCP
Congestion Encountered (DCTCP.CE), which is initialized to false and
stored in the Transmission Control Block (TCB). When sending an ACK,
the ECE flag MUST be set if and only if DCTCP.CE is true. When
receiving packets, the CE codepoint MUST be processed as follows:

1. If the CE codepoint is set and DCTCP.CE is false, set DCTCP.CE to
true and send an immediate ACK.

2. If the CE codepoint is not set and DCTCP.CE is true, set DCTCP.CE
to false and send an immediate ACK.

3. Otherwise, ignore the CE codepoint.

Since the immediate ACK reflects the new DCTCP.CE state, it may
acknowledge any previously unacknowledged packets in the old state.

This can lead to an incorrect rate computation at the sender per
Section 3.3. To avoid this, an implementation MAY choose to send two
ACKs: one for previously unacknowledged packets and another
acknowledging the most recently received packet.

Receiver handling of the CWR bit is also per [RFC3168] (including
[Err3639]). That is, on receipt of a segment with both the CE and
CWR bits set, CWR is processed first and then CE is processed.

Figure 1: ACK Generation State Machine with DCTCP.CE abbreviated as
CE

3.3. Processing Echoed Congestion Indications on the Sender

The sender estimates the fraction of bytes sent that encountered
congestion. The current estimate is stored in a new TCP state
variable, DCTCP.Alpha, which is initialized to 1 and SHOULD be
updated as follows:

DCTCP.Alpha = DCTCP.Alpha * (1 - g) + g * M

where:

o g is the estimation gain, a real number between 0 and 1. The
selection of g is left to the implementation. See Section 4 for
further considerations.

o M is the fraction of bytes sent that encountered congestion during
the previous observation window, where the observation window is
chosen to be approximately the Round-Trip Time (RTT). In
particular, an observation window ends when all bytes in flight at
the beginning of the window have been acknowledged.

In order to update DCTCP.Alpha, the TCP state variables defined in
[RFC0793] are used, and three additional TCP state variables are
introduced:

o DCTCP.WindowEnd: the TCP sequence number threshold when one
observation window ends and another is to begin; initialized to
SND.UNA.

o DCTCP.BytesAcked: the number of sent bytes acknowledged during the
current observation window; initialized to 0.

o DCTCP.BytesMarked: the number of bytes sent during the current
observation window that encountered congestion; initialized to 0.

The congestion estimator on the sender MUST process acceptable ACKs
as follows:

1. Compute the bytes acknowledged (TCP Selective Acknowledgment
(SACK) options [RFC2018] are ignored for this computation):

BytesAcked = SEG.ACK - SND.UNA

2. Update the bytes sent:

DCTCP.BytesAcked += BytesAcked

3. If the ECE flag is set, update the bytes marked:

DCTCP.BytesMarked += BytesAcked

4. If the acknowledgment number is less than or equal to
DCTCP.WindowEnd, stop processing. Otherwise, the end of the
observation window has been reached, so proceed to update the
congestion estimate as follows:

5. Compute the congestion level for the current observation window:

M = DCTCP.BytesMarked / DCTCP.BytesAcked

6. Update the congestion estimate:

DCTCP.Alpha = DCTCP.Alpha * (1 - g) + g * M

7. Determine the end of the next observation window:

DCTCP.WindowEnd = SND.NXT

8. Reset the byte counters:

DCTCP.BytesAcked = DCTCP.BytesMarked = 0
9. Rather than always halving the congestion window as described in
[RFC3168], the sender SHOULD update cwnd as follows:

cwnd = cwnd * (1 - DCTCP.Alpha / 2)

Just as specified in [RFC3168], DCTCP does not react to congestion
indications more than once for every window of data. The setting of
the CWR bit is also as per [RFC3168]. This is required for
interoperation with classic ECN receivers due to potential
misconfigurations.

3.4. Handling of Congestion Window Growth

A DCTCP sender grows its congestion window in the same way as
conventional TCP. Slow start and congestion avoidance algorithms are
handled as specified in [RFC5681].

3.5. Handling of Packet Loss

A DCTCP sender MUST react to loss episodes in the same way as
conventional TCP, including fast retransmit and fast recovery
algorithms, as specified in [RFC5681]. For cases where the packet
loss is inferred and not explicitly signaled by ECN, the cwnd and
other state variables like ssthresh MUST be changed in the same way
that a conventional TCP would have changed them. As with ECN, a
DCTCP sender will only reduce the cwnd once per window of data across
all loss signals. Just as specified in [RFC5681], upon a timeout,
the cwnd MUST be set to no more than the loss window (1 full-sized
segment), regardless of previous cwnd reductions in a given window of
data.

3.6. Handling of SYN, SYN-ACK, and RST Packets

If SYN, SYN-ACK, and RST packets for DCTCP connections have the ECN-
Capable Transport (ECT) codepoint set in the IP header, they will
receive the same treatment as other DCTCP packets when forwarded by a
switching fabric under load. Lack of ECT in these packets can result
in a higher drop rate, depending on the switching fabric
configuration. Hence, for DCTCP connections, the sender SHOULD set
ECT for SYN, SYN-ACK, and RST packets. A DCTCP receiver ignores CE
codepoints set on any SYN, SYN-ACK, or RST packets.

4. Implementation Issues

4.1. Configuration of DCTCP

An implementation needs to know when to use DCTCP. Data-center
servers may need to communicate with endpoints outside the data
center, where DCTCP is unsuitable or unsupported. Thus, a global
configuration setting to enable DCTCP will generally not suffice.
DCTCP provides no mechanism for negotiating its use. Thus,
additional management and configuration functionality is needed to
ensure that DCTCP is not used with non-DCTCP endpoints.

Known solutions rely on either configuration or heuristics.
Heuristics need to allow endpoints to individually enable DCTCP to
ensure a DCTCP sender is always paired with a DCTCP receiver. One
approach is to enable DCTCP based on the IP address of the remote
endpoint. Another approach is to detect connections that transmit
within the bounds of a data center. For example, an implementation
could support automatic selection of DCTCP if the estimated RTT is
less than a threshold (like 10 msec) and ECN is successfully
negotiated under the assumption that if the RTT is low, then the two
endpoints are likely in the same data-center network.

[RFC3168] forbids the ECN-marking of pure ACK packets because of the
inability of TCP to mitigate ACK-path congestion. RFC 3168 also
forbids ECN-marking of retransmissions, window probes, and RSTs.
However, dropping all these control packets -- rather than ECN-
marking them -- has considerable performance disadvantages. It is
RECOMMENDED that an implementation provide a configuration knob that
will cause ECT to be set on such control packets, which can be used
in environments where such concerns do not apply. See
[ECN-EXPERIMENTATION] for details.

It is useful to implement DCTCP as an additional action on top of an
existing congestion control algorithm like Reno [RFC5681]. The DCTCP
implementation MAY also allow configuration of resetting the value of
DCTCP.Alpha as part of processing any loss episodes.

4.2. Computation of DCTCP.Alpha

As noted in Section 3.3, the implementation will need to choose a
suitable estimation gain. [DCTCP10] provides a theoretical basis for
selecting the gain. However, it may be more practical to use
experimentation to select a suitable gain for a particular network
and workload. A fixed estimation gain of 1/16 is used in some
implementations. (It should be noted that values of 0 or 1 for g
result in problematic behavior; g=0 fixes DCTCP.Alpha to its initial
value, and g=1 sets it to M without any smoothing.)
The DCTCP.Alpha computation as per the formula in Section 3.3
involves fractions. An efficient kernel implementation MAY scale the
DCTCP.Alpha value for efficient computation using shift operations.
For example, if the implementation chooses g as 1/16, multiplications
of DCTCP.Alpha by g become right-shifts by 4. A scaling
implementation SHOULD ensure that DCTCP.Alpha is able to reach 0 once
it falls below the smallest shifted value (16 in the above example).
At the other extreme, a scaled update needs to ensure DCTCP.Alpha
does not exceed the scaling factor, which would be equivalent to
greater than 100% congestion. So, DCTCP.Alpha MUST be clamped after
an update.

This results in the following computations replacing steps 5 and 6 in
Section 3.3, where SCF is the chosen scaling factor (65536 in the
example), and SHF is the shift factor (4 in the example):

1. Compute the congestion level for the current observation window:

ScaledM = SCF * DCTCP.BytesMarked / DCTCP.BytesAcked

2. Update the congestion estimate:

if (DCTCP.Alpha >> SHF) == 0, then DCTCP.Alpha = 0

DCTCP.Alpha += (ScaledM >> SHF) - (DCTCP.Alpha >> SHF)

if DCTCP.Alpha > SCF, then DCTCP.Alpha = SCF

5. Deployment Issues

DCTCP and conventional TCP congestion control do not coexist well in
the same network. In typical DCTCP deployments, the marking
threshold in the switching fabric is set to a very low value to
reduce queueing delay, and a relatively small amount of congestion
will exceed the marking threshold. During such periods of
congestion, conventional TCP will suffer packet loss and quickly and
drastically reduce cwnd. DCTCP, on the other hand, will use the
fraction of marked packets to reduce cwnd more gradually. Thus, the
rate reduction in DCTCP will be much slower than that of conventional
TCP, and DCTCP traffic will gain a larger share of the capacity
compared to conventional TCP traffic traversing the same path. If
the traffic in the data center is a mix of conventional TCP and
DCTCP, it is RECOMMENDED that DCTCP traffic be segregated from
conventional TCP traffic. [MORGANSTANLEY] describes a deployment
that uses the IP Differentiated Services Codepoint (DSCP) bits to
segregate the network such that Active Queue Management (AQM)
[RFC7567] is applied to DCTCP traffic, whereas TCP traffic is managed
via drop-tail queueing.

Deployments should take into account segregation of non-TCP traffic
as well. Today's commodity switches allow configuration of different
marking/drop profiles for non-TCP and non-IP packets. Non-TCP and
non-IP packets should be able to pass through such switches, unless
they really run out of buffer space.

Since DCTCP relies on congestion marking by the switches, DCTCP's
potential can only be realized in data centers where the entire
network infrastructure supports ECN. The switches may also support
configuration of the congestion threshold used for marking. The
proposed parameterization can be configured with switches that
implement Random Early Detection (RED) [RFC2309] (RFC 2309 was
obsoleted by RFC 7567). [RFC2309]. [DCTCP10] provides
a theoretical basis for selecting the congestion threshold, but, as
with the estimation gain, it may be more practical to rely on
experimentation or simply to use the default configuration of the
device. DCTCP will revert to loss-
based loss-based congestion control when
packet loss is experienced (e.g., when transiting a congested drop-tail drop-
tail link, or a link with an AQM drop behavior).

DCTCP requires changes on both the sender and the receiver, so both
endpoints must support DCTCP. Furthermore, DCTCP provides no
mechanism for negotiating its use, so both endpoints must be
configured through some out-of-band mechanism to use DCTCP. A
variant of DCTCP that can be deployed unilaterally and that only
requires standard ECN behavior has been described in [ODCTCP] and
[BSDCAN], but it requires additional experimental evaluation.

6. Known Issues

DCTCP relies on the sender's ability to reconstruct the stream of CE
codepoints received by the remote endpoint. To accomplish this,
DCTCP avoids using a single ACK packet to acknowledge segments
received both with and without the CE codepoint set. However, if one
or more ACK packets are dropped, it is possible that a subsequent ACK
will cumulatively acknowledge a mix of CE and non-CE segments. This
will, of course, result in a less-accurate congestion estimate.
There are some potential considerations:

o Even with an inaccurate congestion estimate, DCTCP may still
perform better than [RFC3168].

o If the estimation gain is small relative to the packet loss rate,
the estimate may not be too inaccurate.

o If ACK packet loss mostly occurs under heavy congestion, most
drops will occur during an unbroken string of CE packets, and the
estimate will be unaffected.

However, the effect of packet drops on DCTCP under real-world
conditions has not been analyzed.

DCTCP provides no mechanism for negotiating its use. The effect of
using DCTCP with a standard ECN endpoint has been analyzed in
[ODCTCP] and [BSDCAN]. Furthermore, it is possible that other
implementations may also modify behavior in the [RFC3168] style
without negotiation, causing further interoperability issues.

Much like standard TCP, DCTCP is biased against flows with longer
RTTs. A method for improving the RTT fairness of DCTCP has been
proposed in [ADCTCP], but it requires additional experimental
evaluation.

7. Security Considerations

DCTCP enhances ECN; thus, it inherits the general security
considerations discussed in [RFC3168], although additional mitigation
options exist due to the limited intra-data-center deployment of
DCTCP.

The processing changes introduced by DCTCP do not exacerbate the
considerations in [RFC3168] or introduce new ones. In particular,
with either algorithm, the network infrastructure or the remote
endpoint can falsely report congestion and, thus, cause the sender to
reduce cwnd. However, this is no worse than what can be achieved by
simply dropping packets.

[RFC3168] requires that a compliant TCP must not set ECT on SYN or
SYN-ACK packets. [RFC5562] proposes setting ECT on SYN-ACK packets
but maintains the restriction of no ECT on SYN packets. Both these
RFCs prohibit ECT in SYN packets due to security concerns regarding
malicious SYN packets with ECT set. However, these RFCs are intended
for general Internet use; they do not directly apply to a controlled
data-center environment. The security concerns addressed by both of
these RFCs might not apply in controlled environments like data
centers, and it might not be necessary to account for the presence of
non-ECN servers. Beyond the security considerations related to
virtual servers, additional security can be imposed in the physical
servers to intercept and drop traffic resembling an attack.

8. IANA Considerations

This document does not require any IANA actions.

9. References

9.1. Normative References

[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.

[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018,
DOI 10.17487/RFC2018, October 1996,
<https://www.rfc-editor.org/info/rfc2018>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[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,
<https://www.rfc-editor.org/info/rfc3168>.

[RFC5562] Kuzmanovic, A., Mondal, A., Floyd, S., and K.
Ramakrishnan, "Adding Explicit Congestion Notification
(ECN) Capability to TCP's SYN/ACK Packets", RFC 5562,
DOI 10.17487/RFC5562, June 2009,
<https://www.rfc-editor.org/info/rfc5562>.

[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<https://www.rfc-editor.org/info/rfc5681>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

9.2. Informative References

[ADCTCP] Alizadeh, M., Javanmard, A., and B. Prabhakar, "Analysis
of DCTCP: Stability, Convergence, and Fairness",
DOI 10.1145/1993744.1993753, Proceedings of the ACM
SIGMETRICS Joint International Conference on Measurement
and Modeling of Computer Systems, June 2011,
<https://dl.acm.org/citation.cfm?id=1993753>.

[BSDCAN] Kato, M., Eggert, L., Zimmermann, A., van Meter, R., and
H. Tokuda, "Extensions to FreeBSD Datacenter TCP for
Incremental Deployment Support", BSDCan 2015, June 2015,
<https://www.bsdcan.org/2015/schedule/events/559.en.html>.

[DCTCP10] Alizadeh, M., Greenberg, A., Maltz, D., Padhye, J., Patel,
P., Prabhakar, B., Sengupta, S., and M. Sridharan, "Data
Center TCP (DCTCP)", DOI 10.1145/1851182.1851192,
Proceedings of the ACM SIGCOMM 2010 Conference, August
2010,
<http://dl.acm.org/citation.cfm?doid=1851182.1851192>.

[ECN-EXPERIMENTATION]
Black, D., "Explicit Congestion Notification (ECN)
Experimentation", Work in Progress, draft-ietf-tsvwg-ecn-
experimentation-06, September 2017.

[Err3639] RFC Errata, Erratum ID 3639, RFC 3168,
<https://www.rfc-editor.org/errata/eid3639>.

[FREEBSD] Kato, M. and H. Panchasara, "DCTCP (Data Center TCP)
implementation", January 2015,
<https://github.com/freebsd/freebsd/
commit/8ad879445281027858a7fa706d13e458095b595f>.

[LINUX] Borkmann, D., Westphal, F., and Glenn. Judd, "net: tcp:
add DCTCP congestion control algorithm", LINUX DCTCP
Patch, September 2014,
<https://git.kernel.org/cgit/linux/kernel/git/davem/net-
next.git/
commit/?id=e3118e8359bb7c59555aca60c725106e6d78c5ce>.

[MAPREDUCE]
Dean, J. and S. Ghemawat, "MapReduce: Simplified Data
Processing on Large Clusters", Proceedings of the 6th
ACM/USENIX Symposium on Operating Systems Design and
Implementation, October 2004, <https://www.usenix.org/lega
cy/publications/library/proceedings/osdi04/tech/
dean.html>.

[MORGANSTANLEY]
Judd, G., "Attaining the Promise and Avoiding the Pitfalls
of TCP in the Datacenter", Proceedings of the 12th USENIX
Symposium on Networked Systems Design and Implementation,
May 2015, <https://www.usenix.org/conference/nsdi15/
technical-sessions/presentation/judd>.

[ODCTCP] Kato, M., "Improving Transmission Performance with One-
Sided Datacenter TCP", M.S. Thesis, Keio University, 2013,
<http://eggert.org/students/kato-thesis.pdf>.

[RFC2309] Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
Partridge, C., Peterson, L., Ramakrishnan, K., Shenker,
S., Wroclawski, J., and L. Zhang, "Recommendations on
Queue Management and Congestion Avoidance in the
Internet", RFC 2309, DOI 10.17487/RFC2309, April 1998,
<https://www.rfc-editor.org/info/rfc2309>.

[RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF
Recommendations Regarding Active Queue Management",
BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
<https://www.rfc-editor.org/info/rfc7567>.

[WINDOWS] Microsoft, "Data Center Transmission Control Protocol
(DCTCP)", May 2012, <https://technet.microsoft.com/en-
us/library/hh997028(v=ws.11).aspx>.

Acknowledgments

The DCTCP algorithm was originally proposed and analyzed in [DCTCP10]
by Mohammad Alizadeh, Albert Greenberg, Dave Maltz, Jitu Padhye,
Parveen Patel, Balaji Prabhakar, Sudipta Sengupta, and Murari
Sridharan.

We would like to thank Andrew Shewmaker for identifying the problem
of clamping DCTCP.Alpha and proposing a solution for it.

Lars Eggert has received funding from the European Union's Horizon
2020 research and innovation program 2014-2018 under grant agreement
No. 644866 ("SSICLOPS"). This document reflects only the authors'
views and the European Commission is not responsible for any use that
may be made of the information it contains.

Authors' Addresses

Stephen Bensley
Microsoft
One Microsoft Way
Redmond, WA 98052
United States of America

Phone: +1 425 703 5570
Email: sbens@microsoft.com
Dave Thaler
Microsoft

Phone: +1 425 703 8835
Email: dthaler@microsoft.com

Praveen Balasubramanian
Microsoft

Phone: +1 425 538 2782
Email: pravb@microsoft.com

Lars Eggert
NetApp
Sonnenallee 1
Kirchheim 85551
Germany

Phone: +49 151 120 55791
Email: lars@netapp.com
URI: http://eggert.org/

Glenn Judd
Morgan Stanley

Phone: +1 973 979 6481
Email: glenn.judd@morganstanley.com