Network Working Group S. Bensley
Internet-Draft Microsoft
Intended status: Informational L. Eggert
Expires: March 25, 2016 NetApp
D. Thaler
P. Balasubramanian
Microsoft
G. Judd
Morgan Stanley
September 22, 2015
Datacenter TCP (DCTCP): TCP Congestion Control for Datacenters
draft-ietf-tcpm-dctcp-00
Abstract
This memo describes Datacenter TCP (DCTCP), an improvement to TCP
congestion control for datacenter traffic. DCTCP uses improved
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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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on March 25, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. DCTCP Algorithm . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Marking Congestion on the Switches . . . . . . . . . . . 4
3.2. Echoing Congestion Information on the Receiver . . . . . 4
3.3. Processing Congestion Indications on the Sender . . . . . 5
3.4. Handling of SYN, SYN-ACK, RST Packets . . . . . . . . . . 7
4. Implementation Issues . . . . . . . . . . . . . . . . . . . . 7
5. Deployment Issues . . . . . . . . . . . . . . . . . . . . . . 9
6. Known Issues . . . . . . . . . . . . . . . . . . . . . . . . 10
7. Implementation Status . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
11.1. Normative References . . . . . . . . . . . . . . . . . . 11
11.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
Large datacenters necessarily need many network switches to
interconnect its many servers. Therefore, a datacenter can greatly
reduce its capital expenditure by leveraging low-cost switches.
However, such low-cost switches tend to have limited queue capacities
and are thus more susceptible to packet loss due to congestion.
Network traffic in a datacenter is often a mix of short and long
flows, where the short flows require low latencies and the long flows
require high throughputs. Datacenters also experience incast bursts,
where many servers send traffic to a single server at the same time.
For example, this traffic pattern is a natural consequence of
MapReduce workload: The worker nodes complete at approximately the
same time, and all reply to the master node concurrently.
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These factors place some conflicting demands on the queue occupancy
of a switch:
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 capacity.
o The queue must be short enough to absorb incast bursts without
excessive packet loss.
Standard TCP congestion control [RFC5681] relies on packet loss to
detect congestion. This does not meet the demands described above.
First, short flows will start to experience unacceptable latencies
before packet loss occurs. Second, by the time TCP congestion
control kicks in on the senders, most of the incast burst has already
been dropped.
[RFC3168] describes a mechanism for using Explicit Congestion
Notification (ECN) from the switches for early detection of
congestion, rather than waiting for packet loss to occur. However,
this method only detects the presence of congestion, not its extent.
In the presence of mild congestion, the TCP congestion window is
reduced too aggressively and this unnecessarily reduces the
throughput of long flows.
Datacenter TCP (DCTCP) improves traditional ECN processing by
estimating 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.
It is recommended that DCTCP be deployed in a datacenter environment
where the endpoints and the switching fabric are under a single
administrative domain. This memo also discusses deployment issues
related to the coexistence of DCTCP and conventional TCP, the lack of
a negotiating mechanism between sender and receiver, and presents
some possible mitigations.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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3. DCTCP Algorithm
There are three components involved in the DCTCP algorithm:
o The switches (or other intermediate devices in the network) detect
congestion and set 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 computes a congestion estimate and reacts, by reducing
the TCP congestion window accordingly (cwnd).
3.1. Marking Congestion on the Switches
The switches in a datacenter fabric indicate congestion to the end
nodes by setting the CE codepoint in the IP header as specified in
Section 5 of [RFC3168]. For example, the switches may be configured
with a congestion threshold. When a packet arrives at a switch and
its queue length is greater than the congestion threshold, the switch
sets the CE codepoint in the packet. For example, Section 3.4 of
[DCTCP10] suggests threshold marking with a threshold K > (RTT *
C)/7, where C is the sending rate in packets per second. 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. Therefore, sender and receiver MUST NOT assume that a
particular marking algorithm is implemented by the switching fabric.
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,
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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
false.
3. Otherwise, ignore the CE codepoint.
The handling of the "Congestion Window Reduced" (CWR) bit is also
exactly as per [RFC3168] including [RFC3168-ERRATA3639]. That is, on
receipt of a segment with both the CE and CWR bits set, CWR is
processed first and then ECE is processed.
Send immediate
ACK with ECE=0
.----. .-------------. .---.
Send 1 ACK / v v | | \
for every | .------. .------. | Send 1 ACK
m packets | | CE=0 | | CE=1 | | for every
with ECE=0 | '------' '------' | m packets
\ | | ^ ^ / with ECE=1
'---' '------------' '----'
Send immediate
ACK with ECE=1
Figure 1: ACK generation state machine. DCTCP.CE abbreviated as CE.
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
where
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
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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
introduced:
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
zero.
The congestion estimator on the sender MUST process acceptable ACKs
as follows:
1. Compute the bytes acknowledged (TCP SACK options [RFC2018] are
ignored):
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 acknowledgment number is less than or equal to
DCTCP.WindowEnd, stop processing. Otherwise, the end of the
observation window has been 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:
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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
(ECE), the sender MUST update cwnd as follows:
cwnd = cwnd * (1 - DCTCP.Alpha / 2)
Thus, when no sent bytes 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 for every window of data. The setting of
the "Congestion Window Reduced" (CWR) bit is also exactly as per
[RFC3168].
3.4. Handling of SYN, SYN-ACK, RST Packets
[RFC3168] requires that a compliant TCP 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 environment. 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 do not have ECT set, they will
be dropped with high probability. For DCTCP connections, the sender
SHOULD set ECT for SYN, SYN-ACK and RST packets.
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.
The implementation must also decide when to use DCTCP. Datacenter
servers may need to communicate with endpoints outside the
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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 (and later) also supports
automatic selection of DCTCP if the estimated RTT is less than 10
msec and ECN is successfully negotiated, under the assumption that if
the RTT is low, then the two endpoints are likely on the same
datacenter network.
It is RECOMMENDED that the implementation deal with loss episodes in
the same way as conventional TCP. In case of a timeout or fast
retransmit or any change in delay (for delay based congestion
control), the cwnd and other state variables like ssthresh must be
changed in the same way that a conventional TCP would have changed
them. It would be useful to implement DCTCP as additional actions on
top of an existing congestion control algorithm like NewReno. The
DCTCP implementation MAY also allow configuration of resetting the
value of DCTCP.Alpha as part of processing any loss episodes.
To prevent incast throughput collapse, the minimum RTO (MinRTO) used
by TCP should be lowered significantly. The default value of MinRTO
in Windows is 300 msec, which is much greater than the maximum
latencies inside a datacenter. In Microsoft Windows Server 2012 (and
later), the MinRTO value is configurable, allowing values as low as
10 msec on a per-subnet or per-port basis (or even globally.) A
lower MinRTO value requires a correspondingly lower delayed ACK
timeout on the receiver. It is RECOMMENDED that an implementation
allow configuration of lower timeouts for DCTCP connections.
In the same vein, it is also RECOMMENDED that an implementation allow
configuration of restarting the congestion window (cwnd) of idle
DCTCP connections as described in [RFC5681], since network conditions
can change rapidly in datacenters.
[RFC3168] forbids the ECN-marking of pure ACK packets, because of the
inability of TCP to mitigate ACK-path congestion and protocol-wise
preferential treatment by routers. However, dropping pure ACKs -
rather than ECN marking them - has disadvantages for typical
datacenter traffic patterns. Because of the prevalence of bursty
traffic patterns that feature transient congestion, dropping of ACKs
causes subsequent retransmissions. It is RECOMMENDED that an
implementation provide a configuration knob that forces ECT to be set
on pure ACKs.
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5. Deployment Issues
DCTCP and conventional TCP congestion control do not coexist well in
the same network. In DCTCP, the marking threshold is set to a very
low value to reduce queueing delay, and a relatively small amount of
congestion will exceed the marking threshold. During such periods of
congestion, conventional TCP will suffer packet loss and quickly and
drastically reduce cwnd. DCTCP, on the other hand, will use the
fraction of marked packets to reduce cwnd more gradually. Thus, the
rate reduction in DCTCP will be much slower than that of conventional
TCP, and DCTCP traffic will gain a larger share of the capacity
compared to conventional TCP traffic traversing the same path. If
the traffic in the datacenter is a mix of conventional TCP and DCTCP,
it is RECOMMENDED that DCTCP traffic be segregated from conventional
TCP traffic. [MORGANSTANLEY] describes a deployment that uses the IP
DSCP bits to segregate the network such that AQM is applied to DCTCP
traffic, whereas TCP traffic is managed via drop-tail queueing.
Today's commodity switches allow configuration of different marking/
drop profiles for non-TCP and non-IP packets. Non-TCP and non-IP
packets should be able to pass through such switches, unless they
really run out of buffer space. If the datacenter traffic consists
of such traffic (including UDP), one possible mitigation would be to
mark IP packets as ECT even when there is no transport that is
reacting to the marking.
Since DCTCP relies on congestion marking by the switches, DCTCP can
only be deployed in datacenters where the entire network
infrastructure supports ECN. The switches may also support
configuration of the congestion threshold used for marking. The
proposed parameterization can be configured with switches that
implement RED. [DCTCP10] provides a theoretical basis for selecting
the congestion threshold, but as with the estimation gain, it may be
more practical to rely on experimentation or simply to use the
default configuration of the device. DCTCP will degrade to loss-
based congestion control when transiting a congested drop-tail link.
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][BSDCAN], but
requires additional experimental evaluation.
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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 one
or more ACK packets are dropped, it is possible that a subsequent ACK
will cumulatively acknowledge a mix of CE and non-CE segments. This
will, of course, result in a less accurate congestion estimate.
There are some potential considerations:
o Even with an inaccurate 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 too inaccurate.
o If packet loss mostly occurs under heavy congestion, most drops
will occur during an unbroken string of CE packets, and the
estimate will be unaffected.
However, the effect 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][BSDCAN]. Furthermore, it is 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. 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.
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8. 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.
9. IANA Considerations
This document has no actions for IANA.
10. 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.
Lars Eggert has received funding from the European Union's Horizon
2020 research and innovation program 2014-2018 under grant agreement
No. 644866 ("SSICLOPS"). This document reflects only the authors'
views and the European Commission is not responsible for any use that
may be made of the information it contains.
11. References
11.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, DOI 10.17487/RFC0793, September 1981,
<http://www.rfc-editor.org/info/rfc793>.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, DOI 10.17487/
RFC2018, October 1996,
<http://www.rfc-editor.org/info/rfc2018>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
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[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP", RFC
3168, DOI 10.17487/RFC3168, September 2001,
<http://www.rfc-editor.org/info/rfc3168>.
11.2. Informative References
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<http://www.rfc-editor.org/info/rfc5681>.
[RFC5562] Kuzmanovic, A., Mondal, A., Floyd, S., and K.
Ramakrishnan, "Adding Explicit Congestion Notification
(ECN) Capability to TCP's SYN/ACK Packets", RFC 5562, DOI
10.17487/RFC5562, June 2009,
<http://www.rfc-editor.org/info/rfc5562>.
[RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", RFC 6982, DOI
10.17487/RFC6982, July 2013,
<http://www.rfc-editor.org/info/rfc6982>.
[DCTCP10] Alizadeh, M., Greenberg, A., Maltz, D., Padhye, J., Patel,
P., Prabhakar, B., Sengupta, S., and M. Sridharan, "Data
Center TCP (DCTCP)", DOI 10.1145/1851182.1851192, Proc.
ACM SIGCOMM 2010 Conference (SIGCOMM 10), August 2010,
<http://dl.acm.org/citation.cfm?doid=1851182.1851192>.
[ODCTCP] Kato, M., "Improving Transmission Performance with One-
Sided Datacenter TCP", M.S. Thesis, Keio University, 2014,
<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
of DCTCP: Stability, Convergence, and Fairness", DOI
10.1145/1993744.1993753, Proc. ACM SIGMETRICS Joint
International Conference on Measurement and Modeling of
Computer Systems (SIGMETRICS 11), June 2011,
<https://dl.acm.org/citation.cfm?id=1993753>.
[LINUX] Borkmann, D. and F. Westphal, "Linux DCTCP patch", 2014,
<https://git.kernel.org/cgit/linux/kernel/git/davem/net-
next.git/
commit/?id=e3118e8359bb7c59555aca60c725106e6d78c5ce>.
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[FREEBSD] Kato, M. and H. Panchasara, "DCTCP (Data Center TCP)
implementation", 2015,
<https://github.com/freebsd/freebsd/
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>.
[RFC3168-ERRATA3639]
Scheffenegger, R., "RFC3168 Errata ID 3639", 2013,
<http://www.rfc-editor.org/
errata_search.php/doc/html/rfc3168&eid=3639>.
Authors' Addresses
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
URI: http://eggert.org/
Dave Thaler
Microsoft
Phone: +1 425 703 8835
Email: dthaler@microsoft.com
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Praveen Balasubramanian
Microsoft
Phone: +1 425 538 2782
Email: pravb@microsoft.com
Glenn Judd
Morgan Stanley
Phone: +1 973 979 6481
Email: glenn.judd@morganstanley.com
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