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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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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