draft-bensley-tcpm-dctcp-03.txt   draft-bensley-tcpm-dctcp-04.txt 
Network Working Group S. Bensley Network Working Group S. Bensley
Internet-Draft Microsoft Internet-Draft Microsoft
Intended status: Informational L. Eggert Intended status: Informational L. Eggert
Expires: October 15, 2015 NetApp Expires: January 4, 2016 NetApp
D. Thaler D. Thaler
P. Balasubramanian
Microsoft Microsoft
April 13, 2015 G. Judd
Morgan Stanley
July 3, 2015
Microsoft's Datacenter TCP (DCTCP): Microsoft's Datacenter TCP (DCTCP):
TCP Congestion Control for Datacenters TCP Congestion Control for Datacenters
draft-bensley-tcpm-dctcp-03 draft-bensley-tcpm-dctcp-04
Abstract Abstract
This memo describes Datacenter TCP (DCTCP), an improvement to TCP This memo describes Datacenter TCP (DCTCP), an improvement to TCP
congestion control for datacenter traffic, as implemented in Windows congestion control for datacenter traffic. DCTCP enhances Explicit
Server 2012. DCTCP enhances Explicit Congestion Notification (ECN) Congestion Notification (ECN) processing to estimate the fraction of
processing to estimate the fraction of bytes that encounter bytes that encounter congestion, rather than simply detecting that
congestion, rather than simply detecting that some congestion has some congestion has occurred. DCTCP then scales the TCP congestion
occurred. DCTCP then scales the TCP congestion window based on this window based on this estimate. This method achieves high burst
estimate. This method achieves high burst tolerance, low latency, tolerance, low latency, and high throughput with shallow-buffered
and high throughput with shallow-buffered switches. switches.
Status of This Memo Status of This Memo
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provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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Copyright Notice Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. DCTCP Algorithm . . . . . . . . . . . . . . . . . . . . . . . 3 3. DCTCP Algorithm . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Marking Congestion on the Switch . . . . . . . . . . . . 4 3.1. Marking Congestion on the Switch . . . . . . . . . . . . 4
3.2. Echoing Congestion Information on the Receiver . . . . . 4 3.2. Echoing Congestion Information on the Receiver . . . . . 4
3.3. Processing Congestion Indications on the Sender . . . . . 5 3.3. Processing Congestion Indications on the Sender . . . . . 5
3.4. Handling of SYN, SYN-ACK and RST Packets . . . . . . . . 7
4. Implementation Issues . . . . . . . . . . . . . . . . . . . . 7 4. Implementation Issues . . . . . . . . . . . . . . . . . . . . 7
5. Deployment Issues . . . . . . . . . . . . . . . . . . . . . . 7 5. Deployment Issues . . . . . . . . . . . . . . . . . . . . . . 7
6. Known Issues . . . . . . . . . . . . . . . . . . . . . . . . 7 6. Known Issues . . . . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8 7. Implementation Status . . . . . . . . . . . . . . . . . . . . 9
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
10.1. Normative References . . . . . . . . . . . . . . . . . . 9 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.2. Informative References . . . . . . . . . . . . . . . . . 9 11.1. Normative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 11.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction 1. Introduction
Large datacenters necessarily need a large number of network switches Large datacenters necessarily need a large number of network switches
to interconnect the servers in the datacenter. Therefore, a to interconnect the servers in the datacenter. Therefore, a
datacenter can greatly reduce its capital expenditure by leveraging datacenter can greatly reduce its capital expenditure by leveraging
low cost switches. However, low cost switches tend to have limited low cost switches. However, low cost switches tend to have limited
queue capacities and thus are more susceptible to packet loss due to queue capacities and thus are more susceptible to packet loss due to
congestion. congestion.
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presence of mild congestion, it reduces the TCP congestion window too presence of mild congestion, it reduces the TCP congestion window too
aggressively and unnecessarily affects the throughput of long flows. aggressively and unnecessarily affects the throughput of long flows.
Datacenter TCP (DCTCP) enhances ECN processing to estimate the Datacenter TCP (DCTCP) enhances ECN processing to estimate the
fraction of bytes that encounter congestion, rather than simply fraction of bytes that encounter congestion, rather than simply
detecting that some congestion has occurred. DCTCP then scales the detecting that some congestion has occurred. DCTCP then scales the
TCP congestion window based on this estimate. This method achieves TCP congestion window based on this estimate. This method achieves
high burst tolerance, low latency, and high throughput with shallow- high burst tolerance, low latency, and high throughput with shallow-
buffered switches. buffered switches.
This document describes DCTCP as implemented in Microsoft Windows
Server 2012. Since publication of the first versions of this
document, the Linux [LINUX] and FreeBSD [FREEBSD] operating systems
have also implemented support for DCTCP in a way that is believed to
follow this document.
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
3. DCTCP Algorithm 3. DCTCP Algorithm
There are three components involved in the DCTCP algorithm: There are three components involved in the DCTCP algorithm:
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zero, and cwnd is left unchanged. When all sent bytes experienced zero, and cwnd is left unchanged. When all sent bytes experienced
congestion, DCTCP.Alpha equals one, and cwnd is reduced by half. congestion, DCTCP.Alpha equals one, and cwnd is reduced by half.
Lower levels of congestion will result in correspondingly smaller Lower levels of congestion will result in correspondingly smaller
reductions to cwnd. reductions to cwnd.
Just as specified in [RFC3168], TCP should not react to congestion Just as specified in [RFC3168], TCP should not react to congestion
indications more than once every window of data. The setting of the indications more than once every window of data. The setting of the
"Congestion Window Reduced" (CWR) bit is also exactly as per "Congestion Window Reduced" (CWR) bit is also exactly as per
[RFC3168]. [RFC3168].
3.4. Handling of SYN, SYN-ACK and RST Packets
[RFC3168] states that "A host 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. These RFCs, however, are
intended for general Internet use, and do not directly apply to a
controlled datacenter deployment. The switching fabric can drop TCP
packets that do not have the ECT set in the IP header. If SYN and
SYN-ACK packets for DCTCP connections are non-ECT they will be
dropped with high probability. For DCTCP connections SYN, SYN-ACK
and RST packets are sent with ECT set.
4. Implementation Issues 4. Implementation Issues
As noted in Section 3.3, the implementation must choose a suitable As noted in Section 3.3, the implementation must choose a suitable
estimation gain. [DCTCP10] provides a theoretical basis for estimation gain. [DCTCP10] provides a theoretical basis for
selecting the gain. However, it may be more practical to use selecting the gain. However, it may be more practical to use
experimentation to select a suitable gain for a particular network experimentation to select a suitable gain for a particular network
and workload. The Microsoft implementation of DCTCP in Windows and workload. The Microsoft implementation of DCTCP in Windows
Server 2012 uses a fixed estimation gain of 1/16. Server 2012 uses a fixed estimation gain of 1/16.
The implementation must also decide when to use DCTCP. Datacenter The implementation must also decide when to use DCTCP. Datacenter
servers may need to communicate with endpoints outside the servers may need to communicate with endpoints outside the
datacenter, where DCTCP is unsuitable or unsupported. Thus, a global datacenter, where DCTCP is unsuitable or unsupported. Thus, a global
configuration setting to enable DCTCP will generally not suffice. configuration setting to enable DCTCP will generally not suffice.
DCTCP may be configured based on the IP address of the remote DCTCP may be configured based on the IP address of the remote
endpoint. Microsoft Windows Server 2012 also supports automatic endpoint. Microsoft Windows Server 2012 also supports automatic
selection of DCTCP if the estimated RTT is less than 10 msec, under selection of DCTCP if the estimated RTT is less than 10 ms and ECN is
the assumption that if the RTT is low, then the two endpoints are successfully negotiated, under the assumption that if the RTT is low,
likely on the same datacenter network. then the two endpoints are likely on the same datacenter network.
5. Deployment Issues 5. Deployment Issues
Since DCTCP relies on congestion marking by the switch, DCTCP can Since DCTCP relies on congestion marking by the switch, DCTCP can
only be deployed in datacenters where the network infrastructure only be deployed in datacenters where the network infrastructure
supports ECN. The switches may also support configuration of the supports ECN. The switches may also support configuration of the
congestion threshold used for marking. [DCTCP10] provides a congestion threshold used for marking. [DCTCP10] provides a
theoretical basis for selecting the congestion threshold, but as with theoretical basis for selecting the congestion threshold, but as with
estimation gain, it may be more practical to rely on experimentation estimation gain, it may be more practical to rely on experimentation
or simply to use the default configuration of the device. or simply to use the default configuration of the device.
DCTCP requires changes on both the sender and the receiver, so both DCTCP requires changes on both the sender and the receiver, so both
endpoints must support DCTCP. Furthermore, DCTCP provides no endpoints must support DCTCP. Furthermore, DCTCP provides no
mechanism for negotiating its use, so both endpoints must be mechanism for negotiating its use, so both endpoints must be
configured through some out-of-band mechanism to use DCTCP. A configured through some out-of-band mechanism to use DCTCP. A
variant of DCTCP that can be deployed unilaterally and only requires variant of DCTCP that can be deployed unilaterally and only requires
standard ECN behavior has been described in [ODCTCP], but requires standard ECN behavior has been described in [ODCTCP][BSDCAN], but
additional experimental evaluation. requires additional experimental evaluation.
6. Known Issues 6. Known Issues
DCTCP relies on the sender's ability to reconstruct the stream of CE DCTCP relies on the sender's ability to reconstruct the stream of CE
codepoints received by the remote endpoint. To accomplish this, codepoints received by the remote endpoint. To accomplish this,
DCTCP avoids using a single ACK packet to acknowledge segments DCTCP avoids using a single ACK packet to acknowledge segments
received both with and without the CE codepoint set. However, if an received both with and without the CE codepoint set. However, if an
ACK packet is dropped, it's possible that a subsequent ACK will 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 indeed acknowledge a mix of CE and non-CE segments. This will, of
course, result in a less accurate congestion estimate. There are course, result in a less accurate congestion estimate. There are
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o If packet losses mostly occur under heavy congestion, most drops o If packet losses mostly occur under heavy congestion, most drops
will occur during an unbroken string of CE packets, and the will occur during an unbroken string of CE packets, and the
estimate will be unaffected. estimate will be unaffected.
However, the affect of packet drops on DCTCP under real world However, the affect of packet drops on DCTCP under real world
conditions has not been analyzed. conditions has not been analyzed.
DCTCP provides no mechanism for negotiating its use. Thus, there is DCTCP provides no mechanism for negotiating its use. Thus, there is
additional management and configuration overhead required to ensure additional management and configuration overhead required to ensure
that DCTCP is not used with non-DCTCP endpoints. The affect of using that DCTCP is not used with non-DCTCP endpoints. The affect of using
DCTCP with a standard ECN endpoint has been analyzed in [ODCTCP]. DCTCP with a standard ECN endpoint has been analyzed in
Furthermore, it's possible that other implementations may also modify [ODCTCP][BSDCAN]. Furthermore, it's possible that other
[RFC3168] behavior without negotiation, causing further implementations may also modify [RFC3168] behavior without
interoperability issues. negotiation, causing further interoperability issues.
Much like standard TCP, DCTCP is biased against flows with longer Much like standard TCP, DCTCP is biased against flows with longer
RTTs. A method for improving the fairness of DCTCP has been proposed RTTs. A method for improving the fairness of DCTCP has been proposed
in [ADCTCP], but requires additional experimental evaluation. in [ADCTCP], but requires additional experimental evaluation.
7. Security Considerations 7. Implementation Status
This section documents the implementation status of the specification
in this document, as recommended by [RFC6982].
This document describes DCTCP as implemented in Microsoft Windows
Server 2012. Since publication of the first versions of this
document, the Linux [LINUX] and FreeBSD [FREEBSD] operating systems
have also implemented support for DCTCP in a way that is believed to
follow this document.
8. Security Considerations
DCTCP enhances ECN and thus inherits the security considerations DCTCP enhances ECN and thus inherits the security considerations
discussed in [RFC3168]. The processing changes introduced by DCTCP discussed in [RFC3168]. The processing changes introduced by DCTCP
do not exacerbate these considerations or introduce new ones. In do not exacerbate these considerations or introduce new ones. In
particular, with either algorithm, the network infrastructure or the particular, with either algorithm, the network infrastructure or the
remote endpoint can falsely report congestion and thus cause the remote endpoint can falsely report congestion and thus cause the
sender to reduce cwnd. However, this is no worse than what can be sender to reduce cwnd. However, this is no worse than what can be
achieved by simply dropping packets. achieved by simply dropping packets.
8. IANA Considerations 9. IANA Considerations
This document has no actions for IANA. This document has no actions for IANA.
9. Acknowledgements 10. Acknowledgements
The DCTCP algorithm was originally proposed and analyzed in [DCTCP10] The DCTCP algorithm was originally proposed and analyzed in [DCTCP10]
by Mohammad Alizadeh, Albert Greenberg, Dave Maltz, Jitu Padhye, by Mohammad Alizadeh, Albert Greenberg, Dave Maltz, Jitu Padhye,
Parveen Patel, Balaji Prabhakar, Sudipta Sengupta, and Murari Parveen Patel, Balaji Prabhakar, Sudipta Sengupta, and Murari
Sridharan. Sridharan.
Lars Eggert has received funding from the European Union's Horizon Lars Eggert has received funding from the European Union's Horizon
2020 research and innovation program 2014-2018 under grant agreement 2020 research and innovation program 2014-2018 under grant agreement
No. 644866. This document reflects only the authors' views and the No. 644866 ("SSICLOPS"). This document reflects only the authors'
European Commission is not responsible for any use that may be made views and the European Commission is not responsible for any use that
of the information it contains. may be made of the information it contains.
10. References 11. References
10.1. Normative References 11.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981. 793, September 1981.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP [RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996. Selective Acknowledgment Options", RFC 2018, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP", RFC of Explicit Congestion Notification (ECN) to IP", RFC
3168, September 2001. 3168, September 2001.
10.2. Informative References 11.2. Informative References
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion [RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009. Control", RFC 5681, September 2009.
[RFC5562] Kuzmanovic, A., Mondal, A., Floyd, S., and K.
Ramakrishnan, "Adding Explicit Congestion Notification
(ECN) Capability to TCP's SYN/ACK Packets", RFC 5562, June
2009.
[RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", RFC 6982, July
2013.
[DCTCP10] Alizadeh, M., Greenberg, A., Maltz, D., Padhye, J., Patel, [DCTCP10] Alizadeh, M., Greenberg, A., Maltz, D., Padhye, J., Patel,
P., Prabhakar, B., Sengupta, S., and M. Sridharan, "Data P., Prabhakar, B., Sengupta, S., and M. Sridharan, "Data
Center TCP (DCTCP)", December 2010, Center TCP (DCTCP)", Proc. ACM SIGCOMM 2010 Conference
<http://www.sigcomm.org/ccr/papers/2010/ (SIGCOMM 10), August 2010,
October/1851275.1851192/>. <http://dl.acm.org/citation.cfm?doid=1851182.1851192>.
[ODCTCP] Kato, M., "Improving Transmission Performance with One- [ODCTCP] Kato, M., "Improving Transmission Performance with One-
Sided Datacenter TCP", M.S. Thesis, Keio University, 2014, Sided Datacenter TCP", M.S. Thesis, Keio University, 2014,
<http://eggert.org/students/kato-thesis.pdf>. <http://eggert.org/students/kato-thesis.pdf>.
[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>.
[ADCTCP] Alizadeh, M., Javanmard, A., and B. Prabhakar, "Analysis [ADCTCP] Alizadeh, M., Javanmard, A., and B. Prabhakar, "Analysis
of DCTCP: Stability, Convergence, and Fairness", June of DCTCP: Stability, Convergence, and Fairness", Proc. ACM
2011, SIGMETRICS Joint International Conference on Measurement
<http://simula.stanford.edu/~alizade/Site/DCTCP_files/ and Modeling of Computer Systems (SIGMETRICS 11), June
dctcp_analysis-full.pdf>. 2011, <https://dl.acm.org/citation.cfm?id=1993753>.
[LINUX] Borkmann, D. and F. Westphal, "Linux DCTCP patch", 2014, [LINUX] Borkmann, D. and F. Westphal, "Linux DCTCP patch", 2014,
<https://git.kernel.org/cgit/linux/kernel/git/davem/net- <https://git.kernel.org/cgit/linux/kernel/git/davem/net-
next.git/ next.git/
commit/?id=e3118e8359bb7c59555aca60c725106e6d78c5ce>. commit/?id=e3118e8359bb7c59555aca60c725106e6d78c5ce>.
[FREEBSD] Kato, M. and H. Panchasara, "DCTCP (Data Center TCP) [FREEBSD] Kato, M. and H. Panchasara, "DCTCP (Data Center TCP)
implementation", 2015, implementation", 2015,
<https://github.com/freebsd/freebsd/ <https://github.com/freebsd/freebsd/
commit/8ad879445281027858a7fa706d13e458095b595f>. commit/8ad879445281027858a7fa706d13e458095b595f>.
[MORGANSTANLEY]
Judd, G., "Attaining the Promise and Avoiding the Pitfalls
of TCP in the Datacenter", Proc. 12th USENIX Symposium on
Networked Systems Design and Implementation (NSDI 15), May
2015, <https://www.usenix.org/conference/nsdi15/technical-
sessions/presentation/judd>.
Authors' Addresses Authors' Addresses
Stephen Bensley Stephen Bensley
Microsoft Microsoft
One Microsoft Way One Microsoft Way
Redmond, WA 98052 Redmond, WA 98052
USA USA
Phone: +1 425 703 5570 Phone: +1 425 703 5570
Email: sbens@microsoft.com Email: sbens@microsoft.com
skipping to change at line 461 skipping to change at page 11, line 43
Phone: +49 151 120 55791 Phone: +49 151 120 55791
Email: lars@netapp.com Email: lars@netapp.com
URI: http://eggert.org/ URI: http://eggert.org/
Dave Thaler Dave Thaler
Microsoft Microsoft
Phone: +1 425 703 8835 Phone: +1 425 703 8835
Email: dthaler@microsoft.com Email: dthaler@microsoft.com
Praveen Balasubramanian
Microsoft
Phone: +1 425 538 2782
Email: pravb@microsoft.com
Glenn Judd
Morgan Stanley
Phone: +1
Email: glenn.judd@morganstanley.com
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