One document matched: draft-bensley-tcpm-dctcp-00.xml
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<rfc docName="draft-bensley-tcpm-dctcp-00" ipr="trust200902" category="std">
<front>
<title abbrev='DCTCP'>Datacenter TCP (DCTCP): TCP Congestion Control for Datacenters</title>
<author initials='S.' surname='Bensley' fullname='Stephen Bensley'>
<organization>Microsoft</organization>
<address>
<postal>
<street>One Microsoft Way</street>
<city>Redmond</city>
<region>WA</region>
<code>98052</code>
<country>USA</country>
</postal>
<phone>+1 425 703 5570</phone>
<email>sbens@microsoft.com</email>
</address>
</author>
<author initials='L.' surname='Eggert' fullname='Lars Eggert'>
<organization>NetApp</organization>
<address>
<postal>
<street>Sonnenallee 1</street>
<city>Kirchheim</city>
<code>85551</code>
<country>Germany</country>
</postal>
<phone>+49 151 120 55791</phone>
<email>lars@netapp.com</email>
</address>
</author>
<author initials='D.' surname='Thaler' fullname='Dave Thaler'>
<organization>Microsoft</organization>
<address>
<phone>+1 425 703 8835</phone>
<email>dthaler@microsoft.com</email>
</address>
</author>
<date year='2014' />
<area>Transport</area>
<keyword>Some keyword</keyword>
<keyword>Another keyword</keyword>
<abstract>
<t> 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.
</t>
</abstract>
</front>
<middle>
<section title='Introduction'>
<t> 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.
</t>
<t> 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.
</t>
<t> These factors place some conflicting demands on the switch's queue occupancy:
<list style='symbols'>
<t> The queue must be short enough that it doesn't impose excessive latency on short flows.
</t>
<t> The queue must be long enough to buffer sufficient data for the long flows to saturate the bandwidth.
</t>
<t> The queue must be short enough to absorb incast bursts without excessive packet loss.
</t>
</list>
</t>
<t> Standard TCP congestion control <xref target="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.
</t>
<t> <xref target="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.
</t>
<t> 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.
</t>
</section>
<section title='DCTCP Algorithm'>
<t> There are three components involved in the DCTCP algorithm:
<list style='symbols'>
<t> The switch (or other intermediate device on the network) detects congestion and sets the Congestion Encountered (CE)
codepoint in the IP header.
</t>
<t> The receiver echoes the congestion information back to the sender using the ECN-Echo (ECE) flag in the TCP header.
</t>
<t> The sender reacts to the congestion indication by reducing the TCP congestion window (cwnd).
</t>
</list>
</t>
<section title='Marking Congestion on the Switch'>
<t> The switch indicates congestion to the end nodes by setting the CE codepoint in the IP header as specified in Section 5 of <xref target="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 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.
</t>
</section>
<section title='Echoing Congestion Information on the Receiver'>
<t> According to Section 6.1.3 of <xref target="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 <xref target="RFC3168"/> does not
suffice.
</t>
<t> 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:
<list style='numbers'>
<t> 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.
</t>
<t> 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.
</t>
<t> Otherwise, the CE codepoint is ignored.
</t>
</list>
</t>
</section>
<section title='Processing Congestion Indications on the Sender' anchor='senderprocessing'>
<t> 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:
<list>
<t> DCTCP.Alpha = DCTCP.Alpha * (1 - g) + g * M </t>
</list>
</t>
<t> where
<list style='symbols'>
<t> g is the estimation gain, a real number between 0 and 1. The selection of g is left to the implementation.
</t>
<t> 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).
</t>
</list>
</t>
<t> Whenever the TCP congestion estimate is updated, the sender also updates the TCP congestion window as follows:
<list>
<t> cwnd = cwnd * (1 - DCTCP.Alpha / 2)
</t>
</list>
</t>
<t> 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.
</t>
<t> In order to update DCTCP.Alpha, we make use of the TCP state variables defined in <xref target="RFC0793"/>, and
introduce three additional TCP state variables:
<list style='symbols'>
<t> DCTCP.WindowEnd - The TCP sequence number threshold for beginning a new observation window -- initialized to
SND.UNA.
</t>
<t> DCTCP.BytesSent - The number of bytes sent during the current window -- initialized to zero.
</t>
<t> DCTCP.BytesMarked - The number of bytes sent during the current window that encountered congestion -- initialized
to zero.
</t>
</list>
</t>
<t> The congestion estimator on the sender processes acceptable ACKs as follows:
<list style='numbers'>
<t> Compute the bytes acknowledged:
<list style='none'>
<t> BytesAcked = SEG.ACK - SND.UNA </t>
</list>
</t>
<t> Update the bytes sent:
<list style='none'>
<t> DCTCP.BytesSent += BytesAcked </t>
</list>
</t>
<t> If the ECE flag is set, update the bytes marked:
<list style='none'>
<t> DCTCP.BytesMarked += BytesAcked </t>
</list>
</t>
<t> 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.
</t>
<t> Compute the congestion for the current window:
<list style='none'>
<t> M = DCTCP.BytesMarked / DCTCP.BytesSent </t>
</list>
</t>
<t> Update the congestion estimate:
<list style='none'>
<t> DCTCP.Alpha = DCTCP.Alpha * (1 - g) + g * M </t>
</list>
</t>
<t> Set the end of the new window:
<list style='none'>
<t> DCTCP.WindowEnd = SND.NXT </t>
</list>
</t>
<t> Reset the byte counters:
<list style='none'>
<t> DCTCP.BytesSent = DCTCP.BytesMarked = 0 </t>
</list>
</t>
</list>
</t>
</section>
</section>
<section title='Implementation Issues'>
<t> As noted in <xref target='senderprocessing' />, the implementation must choose a suitable estimation gain.
<xref target='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.
</t>
<t> 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.
</t>
</section>
<section title='Deployment Issues'>
<t> 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.
<xref target='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.
</t>
<t> 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.
</t>
</section>
<section title='Security Considerations'>
<t> DCTCP enhances ECN and thus inherits the security considerations discussed in <xref target="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.
</t>
</section>
<section title='IANA Considerations'>
<t> This document has no actions for IANA.
</t>
</section>
<section title='Acknowledgements'>
<t> The DCTCP algorithm was originally proposed and analyzed in <xref target="DCTCP10"/> by Mohammad Alizadeh,
Albert Greenberg, Dave Maltz, Jitu Padhye, Parveen Patel, Balaji Prabhakar, Sudipta Sengupta, and
Murari Sridharan.
</t>
</section>
</middle>
<back>
<references title='Normative References'>
&rfc0793;
&rfc3168;
</references>
<references title='Informative References'>
&rfc5681;
<reference anchor='DCTCP10' target='http://www.sigcomm.org/ccr/papers/2010/October/1851275.1851192/'>
<front>
<title>Data Center TCP (DCTCP)</title>
<author initials="M." surname="Alizadeh" fullname="Mohammad Alizadeh">
<organization abbrev="Stanford"> Stanford University </organization>
<address>
<email> alizade@stanford.edu </email>
</address>
</author>
<author initials="A." surname="Greenberg" fullname="Albert Greenberg">
<organization abbrev="MSR"> Microsoft Research </organization>
<address>
<email> albert@microsoft.com </email>
</address>
</author>
<author initials="D.A." surname="Maltz" fullname="David A. Maltz">
<organization abbrev="MSR"> Microsoft Research </organization>
<address>
<email> dmaltz@microsoft.com </email>
</address>
</author>
<author initials="J." surname="Padhye" fullname="Jitendra Padhye">
<organization abbrev="MSR"> Microsoft Research </organization>
<address>
<email> padhye@microsoft.com </email>
</address>
</author>
<author initials="P." surname="Patel" fullname="Parveen Patel">
<organization abbrev="MSR"> Microsoft Research </organization>
<address>
<email> parveenp@microsoft.com </email>
</address>
</author>
<author initials="B." surname="Prabhakar" fullname="Balaji Prabhakar">
<organization abbrev="Stanford"> Stanford University </organization>
<address>
<email> balaji@stanford.edu </email>
</address>
</author>
<author initials="S." surname="Sengupta" fullname="Sudipta Sengupta">
<organization abbrev="MSR"> Microsoft Research </organization>
<address>
<email> sudipta@microsoft.com </email>
</address>
</author>
<author initials="M." surname="Sridharan" fullname="Murari Sridharan">
<organization abbrev="MSR"> Microsoft Research </organization>
<address>
<email> muraris@microsoft.com </email>
</address>
</author>
<date year="2010" month="December"/>
</front>
</reference>
</references>
</back>
</rfc>
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