Network Working Group                                         S. Bensley
Internet-Draft                                                 Microsoft
Intended status: Informational                                 L. Eggert
Expires: December 7, 2014                                         NetApp
                                                               D. Thaler
                                                            June 5, 2014

                  Microsoft's Datacenter TCP (DCTCP):
                 TCP Congestion Control for Datacenters


   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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   This Internet-Draft will expire on December 7, 2014.

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents

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   ( in effect on the date of
   publication of this document.  Please review these documents
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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

   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",
   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

   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


   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

   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

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

   1.  Compute the bytes acknowledged (TCP SACK options [RFC2018] are

          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

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

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

Authors' Addresses

   Stephen Bensley
   One Microsoft Way
   Redmond, WA  98052

   Phone: +1 425 703 5570

   Lars Eggert
   Sonnenallee 1
   Kirchheim  85551

   Phone: +49 151 120 55791

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   Dave Thaler

   Phone: +1 425 703 8835

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