Network Working Group S. Bensley
Internet-Draft Microsoft
Intended status: Informational L. Eggert
Expires: December 7, 2014 NetApp
D. Thaler
Microsoft
June 5, 2014
Microsoft's Datacenter TCP (DCTCP):
TCP Congestion Control for Datacenters
draft-bensley-tcpm-dctcp-01
Abstract
This memo describes Datacenter TCP (DCTCP), an improvement to TCP
congestion control for datacenter traffic, as implemented in Windows
Server 2012. DCTCP enhances 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.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. DCTCP Algorithm . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Marking Congestion on the Switch . . . . . . . . . . . . 4
3.2. Echoing Congestion Information on the Receiver . . . . . 4
3.3. Processing Congestion Indications on the Sender . . . . . 5
4. Implementation Issues . . . . . . . . . . . . . . . . . . . . 6
5. Deployment Issues . . . . . . . . . . . . . . . . . . . . . . 7
6. Known Issues . . . . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
Large datacenters necessarily need a large number of network switches
to interconnect the servers in the datacenter. Therefore, a
datacenter can greatly reduce its capital expenditure by leveraging
low cost switches. However, low cost switches tend to have limited
queue capacities and thus are more susceptible to packet loss due to
congestion.
Network traffic in the datacenter is often a mix of short and long
flows, where the short flows require low latency and the long flows
require high throughput. Datacenters also experience incast bursts,
where many endpoints send traffic to a single server at the same
time. For example, this is a natural consequence of MapReduce
algorithms. 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:
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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 bandwidth.
o The queue must be short enough to absorb incast bursts without
excessive packet loss.
Standard TCP congestion control [RFC5681] relies on segment loss to
detect congestion. This does not meet the demands described above.
First, the short flows will start to experience unacceptable
latencies before packet loss occurs. Second, by the time TCP
congestion control kicks in on the sender, most of the incast burst
has already been dropped.
[RFC3168] describes a mechanism for using Explicit Congestion
Notification (ECN) from the switch for early detection of congestion,
rather than waiting for segment loss to occur. However, this method
only detects the presence of congestion, not the extent. In the
presence of mild congestion, it reduces the TCP congestion window too
aggressively and unnecessarily affects the throughput of long flows.
Datacenter TCP (DCTCP) enhances 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 document describes DCTCP as implemented in Microsoft Windows
Server 2012.
2. 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 [RFC2119].
3. DCTCP Algorithm
There are three components involved in the DCTCP algorithm:
o The switch (or other intermediate device on the network) detects
congestion and sets the Congestion Encountered (CE) codepoint in
the IP header.
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o The receiver echoes the congestion information back to the sender
using the ECN-Echo (ECE) flag in the TCP header.
o The sender reacts to the congestion indication by reducing the TCP
congestion window (cwnd).
3.1. Marking Congestion on the Switch
The switch indicates congestion to the end nodes by setting the CE
codepoint in the IP header as specified in Section 5 of [RFC3168].
For example, the switch may be configured with a congestion
threshold. When a packet arrives at the switch and its queue length
is greater than the congestion threshold, the switch sets the CE
codepoint in the packet. 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.
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 code point
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 congestion
information. In particular, the sender must be able to determine the
number of sent bytes 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 code point was set in the packet being
acknowledged. However, this prevents the use of delayed ACKs, which
are an important performance optimization in datacenters.
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, send an ACK for
any previously unacknowledged packets and set DCTCP.CE to true.
2. If the CE codepoint is not set and DCTCP.CE is true, send an ACK
for any previously unacknowledged packets and set DCTCP.CE to
false.
3. Otherwise, ignore the CE codepoint.
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3.3. Processing Congestion Indications on the Sender
The sender estimates the fraction of sent bytes that encountered
congestion. The current estimate is stored in a new TCP state
variable, DCTCP.Alpha, which is initialized to 1 and MUST 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 sent bytes 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 the sent 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 for beginning a
new observation window; initialized to SND.UNA.
o DCTCP.BytesSent: The number of bytes sent during the current
observation window; initialized to zero.
o DCTCP.BytesMarked: The number of bytes sent during the current
observation window that encountered congestion; initialized to
zero.
The congestion estimator on the sender MUST process acceptable ACKs
as follows:
1. Compute the bytes acknowledged (TCP SACK options [RFC2018] are
ignored):
BytesAcked = SEG.ACK - SND.UNA
2. Update the bytes sent:
DCTCP.BytesSent += BytesAcked
3. If the ECE flag is set, update the bytes marked:
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DCTCP.BytesMarked += BytesAcked
4. If the sequence number is less than or equal to DCTCP.WindowEnd,
then stop processing. Otherwise, the end of the observation
window was reached, so proceed to update the congestion estimate
as follows:
5. Compute the congestion level for the current observation window:
M = DCTCP.BytesMarked / DCTCP.BytesSent
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.BytesSent = DCTCP.BytesMarked = 0
Rather than always halving the congestion window as described in
[RFC3168], when the sender receives an indication of congestion, the
sender MUST update cwnd as follows:
cwnd = cwnd * (1 - DCTCP.Alpha / 2)
Thus, when no sent byte experienced congestion, DCTCP.Alpha equals
zero, and cwnd is left unchanged. When all sent bytes experienced
congestion, DCTCP.Alpha equals one, and cwnd is reduced by half.
Lower levels of congestion will result in correspondingly smaller
reductions to cwnd.
Just as specified in [RFC3168], TCP should not react to congestion
indications more than once every window of data. The setting of the
"Congestion Window Reduced" (CWR) bit is also exactly as per
[RFC3168].
4. Implementation Issues
As noted in Section 3.3, the implementation must 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. The Microsoft implementation of DCTCP in Windows
Server 2012 uses a fixed estimation gain of 1/16.
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The implementation must also decide when to use DCTCP. Datacenter
servers may need to communicate with endpoints outside the
datacenter, where DCTCP is unsuitable or unsupported. Thus, a global
configuration setting to enable DCTCP will generally not suffice.
DCTCP may be configured based on the IP address of the remote
endpoint. Microsoft Windows Server 2012 also supports automatic
selection of DCTCP if the estimated RTT is less than 10 msec, under
the assumption that if the RTT is low, then the two endpoints are
likely on the same datacenter network.
5. Deployment Issues
Since DCTCP relies on congestion marking by the switch, DCTCP can
only be deployed in datacenters where the network infrastructure
supports ECN. The switches may also support configuration of the
congestion threshold used for marking. [DCTCP10] provides a
theoretical basis for selecting the congestion threshold, but as with
estimation gain, it may be more practical to rely on experimentation
or simply to use the default configuration of the device.
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 only requires
standard ECN behavior has been described in [ODCTCP], but 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 an
ACK packet is dropped, it's possible that a subsequent ACK will
indeed acknowledge a mix of CE and non-CE segments. This will, of
course, result in a less accurate congestion estimate. There are
some potential mitigations:
o Even with a degraded 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 degraded much.
o If packet losses mostly occur under heavy congestion, most drops
will occur during an unbroken string of CE packets, and the
estimate will be unaffected.
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However, the affect of packet drops on DCTCP under real world
conditions has not been analyzed.
DCTCP provides no mechanism for negotiating its use. Thus, there is
additional management and configuration overhead required to ensure
that DCTCP is not used with non-DCTCP endpoints. The affect of using
DCTCP with a standard ECN endpoint has been analyzed in [ODCTCP].
Furthermore, it's possible that other implementations may also modify
[RFC3168] behavior without negotiation, causing further
interoperability issues.
Much like standard TCP, DCTCP is biased against flows with longer
RTTs. A method for improving the fairness of DCTCP has been proposed
in [ADCTCP], but requires additional experimental evaluation.
7. Security Considerations
DCTCP enhances ECN and thus inherits the security considerations
discussed in [RFC3168]. The processing changes introduced by DCTCP
do not exacerbate these considerations 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.
8. IANA Considerations
This document has no actions for IANA.
9. Acknowledgements
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.
10. References
10.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP", RFC
3168, September 2001.
10.2. Informative References
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009.
[DCTCP10] Alizadeh, M., Greenberg, A., Maltz, D., Padhye, J., Patel,
P., Prabhakar, B., Sengupta, S., and M. Sridharan, "Data
Center TCP (DCTCP)", December 2010,
.
[ODCTCP] Kato, M., "Improving Transmission Performance with One-
Sided Datacenter TCP", M.S. Thesis, Keio University, 2014,
.
[ADCTCP] Alizadeh, M., Javanmard, A., and B. Prabhakar, "Analysis
of DCTCP: Stability, Convergence, and Fairness", June
2011,
.
Authors' Addresses
Stephen Bensley
Microsoft
One Microsoft Way
Redmond, WA 98052
USA
Phone: +1 425 703 5570
Email: sbens@microsoft.com
Lars Eggert
NetApp
Sonnenallee 1
Kirchheim 85551
Germany
Phone: +49 151 120 55791
Email: lars@netapp.com
URI: http://eggert.org/
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Dave Thaler
Microsoft
Phone: +1 425 703 8835
Email: dthaler@microsoft.com
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