Datacenter TCP (DCTCP): TCP Congestion Control for Datacenters
draft-bensley-tcpm-dctcp-00

Network Working Group                                         S. Bensley
Internet-Draft                                                 Microsoft
Intended status: Standards Track                               L. Eggert
Expires: August 18, 2014                                          NetApp
                                                               D. Thaler
                                                               Microsoft
                                                       February 14, 2014

     Datacenter TCP (DCTCP): TCP Congestion Control for Datacenters
                      draft-bensley-tcpm-dctcp-00

Abstract

   This memo describes Datacenter TCP (DCTCP), an improvement to TCP
   congestion control for datacenter traffic.  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.

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   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  DCTCP Algorithm . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Marking Congestion on the Switch  . . . . . . . . . . . .   3
     2.2.  Echoing Congestion Information on the Receiver  . . . . .   4
     2.3.  Processing Congestion Indications on the Sender . . . . .   4
   3.  Implementation Issues . . . . . . . . . . . . . . . . . . . .   6
   4.  Deployment Issues . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

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 switch's queue
   occupancy:

   o  The queue must be short enough that it doesn't impose excessive
      latency on short flows.

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

2.  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.

   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).

2.1.  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 the switch's
   queue length is greater than the congestion threshold, the switch

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   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.

2.2.  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,
   we introduce 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 is set if and only if DCTCP.CE is true.  When receiving packets,
   the CE codepoint is 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, the CE codepoint is ignored.

2.3.  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 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.

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   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).

   Whenever the TCP congestion estimate is updated, the sender also
   updates the TCP congestion window as follows:

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

   Thus, when there is no congestion at all, Alpha equals zero, and the
   congestion window is left unchanged.  When there is total congestion,
   Alpha equals one, and the congestion window is reduced by half.
   Lower levels of congestion will result in correspondingly lesser
   reductions to the congestion window.

   In order to update DCTCP.Alpha, we make use of the TCP state
   variables defined in [RFC0793], and introduce three additional TCP
   state variables:

   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
      window -- initialized to zero.

   o  DCTCP.BytesMarked - The number of bytes sent during the current
      window that encountered congestion -- initialized to zero.

   The congestion estimator on the sender processes acceptable ACKs as
   follows:

   1.  Compute the bytes acknowledged:

          BytesAcked = SEG.ACK - SND.UNA

   2.  Update the bytes sent:

          DCTCP.BytesSent += BytesAcked

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

          DCTCP.BytesMarked += BytesAcked

   4.  If the sequence number is less than or equal to DCTCP.WindowEnd,
       then stop processing.  Otherwise, we've reached the end of the
       observation window, so proceed to update the congestion estimate.

   5.  Compute the congestion for the current window:

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          M = DCTCP.BytesMarked / DCTCP.BytesSent

   6.  Update the congestion estimate:

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

   7.  Set the end of the new window:

          DCTCP.WindowEnd = SND.NXT

   8.  Reset the byte counters:

          DCTCP.BytesSent = DCTCP.BytesMarked = 0

3.  Implementation Issues

   As noted in Section 2.3, 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.

   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.

4.  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 device's default configuration.

   DCTCP requires changes on both the sender and the receiver, so in a
   heterogeneous datacenter, all the endpoints should support DCTCP and
   should be configured to use it.

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5.  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 its congestion window.  However, this is no worse
   than what can be achieved by simply dropping packets.

6.  IANA Considerations

   This document has no actions for IANA.

7.  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.

8.  References

8.1.  Normative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, September 1981.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP", RFC
              3168, September 2001.

8.2.  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,
              <http://www.sigcomm.org/ccr/papers/2010/October/
              1851275.1851192/>.

Authors' Addresses

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   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

   Dave Thaler
   Microsoft

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

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