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Problem Statement and Requirements for a More Accurate ECN Feedback

The information below is for an old version of the document.
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This is an older version of an Internet-Draft that was ultimately published as RFC 7560.
Authors Mirja Kühlewind , Richard Scheffenegger
Last updated 2013-10-21
Replaces draft-kuehlewind-tcpm-accecn-reqs
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TCP Maintenance and Minor Extensions (tcpm)           M. Kuehlewind, Ed.
Internet-Draft                                   University of Stuttgart
Intended status: Informational                          R. Scheffenegger
Expires: April 24, 2014                                     NetApp, Inc.
                                                        October 21, 2013

  Problem Statement and Requirements for a More Accurate ECN Feedback


   Explicit Congestion Notification (ECN) is an IP/TCP mechanism where
   network nodes can mark IP packets instead of dropping them to
   indicate congestion to the end-points.  An ECN-capable receiver will
   feedback this information to the sender.  ECN is specified for TCP in
   such a way that only one feedback signal can be transmitted per
   Round-Trip Time (RTT).  Recently, new TCP mechanisms like ConEx or
   DCTCP need more accurate ECN feedback information in the case where
   more than one marking is received in one RTT.  This document
   specifies requirements for an update to the TCP protocol to provide
   more accurate ECN feedback than one signal per RTT.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 24, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of

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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Recap of Classic ECN and ECN Nonce in IP/TCP  . . . . . . . .   3
   3.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Design Approaches . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Re-use of ECN/NS Header Bits  . . . . . . . . . . . . . .   9
     5.2.  Using Other Header Bits . . . . . . . . . . . . . . . . .  10
     5.3.  Using a TCP Option  . . . . . . . . . . . . . . . . . . .  10
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   Explicit Congestion Notification (ECN) [RFC3168] is an IP/TCP
   mechanism where network nodes can mark IP packets instead of dropping
   them to indicate congestion to the end-points.  An ECN-capable
   receiver will feedback this information to the sender.  ECN is
   specified for TCP in such a way that only one feedback signal can be
   transmitted per Round-Trip Time (RTT).  This is sufficient for pre-
   existing congestion control mechanisms that perform only one
   reduction in sending rate per RTT, independent of the number of ECN
   congestion marks.  But recently proposed/deployed mechanisms like
   Congestion Exposure (ConEx) [RFC6789] or DCTCP [Ali10] need more
   fine-grained ECN feedback information to work correctly in the case
   where more than one marking is received in any one RTT.

   This document lists requirements for a robust and interoperable more
   accurate TCP/ECN feedback protocol that all implementations of new
   TCP extension like ConEx and/or DCTCP can use.  While a new feedback
   scheme should still deliver identical performance as classic ECN,
   this document also clarifies what has to be taken into consideration
   in addition.  Thus the listed requirements should be addressed in the
   specification of a more accurate ECN feedback scheme.  Moreover, as a

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   large set of proposals already exists, a few high level design
   choices are sketched and briefly discussed, to demonstrate some of
   the benefits and drawbacks of each of these potential schemes.  A few
   solutions have already been proposed, so Section 5 demonstrates how
   to use the requirements to compare them, by briefly sketching their
   high level design choices and discussing the benefits and drawbacks
   of each.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

   We use the following terminology from [RFC3168] and [RFC3540]:

   The ECN field in the IP header:

      CE:      the Congestion Experienced codepoint,

      ECT(0):  the first ECN-capable Transport codepoint, and

      ECT(1):  the second ECN-capable Transport codepoint.

   The ECN flags in the TCP header:

      CWR:     the Congestion Window Reduced flag,

      ECE:     the ECN-Echo flag, and

      NS:      ECN Nonce Sum.

   In this document, the ECN feedback scheme as specified in [RFC3168]
   is called the 'classic ECN' and any new proposal the 'more accurate
   ECN feedback' scheme.  A 'congestion mark' is defined as an IP packet
   where the CE codepoint is set.  A 'congestion episode' refers to one
   or more congestion marks belonging to the same overload situation in
   the network (usually during one RTT).  A TCP segment with the
   acknowledgment flag set is simply called ACK.

2.  Recap of Classic ECN and ECN Nonce in IP/TCP

   ECN requires two bits in the IP header.  The ECN capability of a
   packet is indicated when either one of the two bits is set.  An ECN

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   sender can set one or the other bit to indicate an ECN-capable
   transport (ECT) which results in two signals, ECT(0) and ECT(1).  A
   network node can set both bits simultaneously when it experiences
   congestion.  When both bits are set the packet is regarded as
   "Congestion Experienced" (CE).

   In the TCP header the first two bits in byte 14 are defined as ECN
   feedback for each half-connection.  A TCP receiver signals the
   reception of a congestion mark using the ECN-Echo (ECE) flag in the
   TCP header.  For reliability, the receiver continues to set the ECE
   flag on every ACK.  To enable the TCP receiver to determine when to
   stop setting the ECN-Echo flag, the sender sets the CWR flag upon
   reception of an ECE feedback signal.  This always leads to a full RTT
   of ACKs with ECE set.  Thus the receiver cannot signal back any
   additional CE markings arriving within the same RTT.

   The ECN Nonce [RFC3540] is an experimental addition to ECN that the
   TCP sender can use to protect itself against accidental or malicious
   concealment of marked or dropped packets.  This addition defines the
   last bit of byte 13 in the TCP header as the Nonce Sum (NS) flag.
   The receiver maintains a nonce sum that counts the occurrence of
   ECT(1) packets, and signals the least significant bit of this sum on
   the NS flag.

       0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15
     |               |           | N | C | E | U | A | P | R | S | F |
     | Header Length | Reserved  | S | W | C | R | C | S | S | Y | I |
     |               |           |   | R | E | G | K | H | T | N | N |

     Figure 1: The (post-ECN Nonce) definition of the TCP header flags

   However, as ECN is a seperate extension to ECN, even if a sender
   tries to protect itself with the nonce, any receiver wishing to
   conceal marked or dropped packets only has to pretent not supporting
   ECN Nonce and simply not provide any nonce feedback.  An alternative
   for a sender to assure feedback integrity has been proposed where the
   sender occasionally inserts a CE mark, reordering or loss itself, and
   checks that the receiver feeds it back faithfully
   [I-D.moncaster-tcpm-rcv-cheat].  This alternative requires no
   standardisation and consumes no header bits or codepoints, as well as
   releasing the ECT(1) codepoint in the IP header and the NS flag in
   the TCP header for other uses.

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3.  Use Cases

   ConEx is an experimental approach that allows the sender to re-insert
   the congestion feedback it sees into the forward data path.  This is
   primarily so that any traffic management can be proportionate to
   actual congestion caused by traffic, rather than limiting traffic
   based on rate or volume in case it might cause congestion [RFC6789].
   A ConEx sender uses selective acknowledgements (SACK [RFC2018]) for
   fine-grained feedback of loss signals, but currently TCP offers no
   equivalent fine-grained feedback for ECN.

   DCTCP offers very low and predictable queueing delay.  DCTCP requires
   switches/routers to have ECN enabled and configured with no signal
   smoothing, so it is currently only used in private networks, e.g.
   internal to data centres.  DCTCP was released in Microsoft Windows 8,
   and implementations exist for Linux and FreeBSD.

   The changes DCTCP makes to TCP are not currently the subject of any
   IETF standardisation activity.  The different DCTCP implementations
   alter TCP's ECN feedback protocol [RFC3168] in unspecified
   proprietary ways, and they either omit capability negotiation, or
   they use non-interoperable negotiation.  A primary motivation for
   this document is to prevent each proprietary implementation from
   inventing its own handshake, which could lead to _de facto_
   consumption of the few flags that remain available for standardising
   capability negotiation.  Also, those variants that use the feedback
   protocol proposed in [Ali10] only work if there are no losses at all,
   and otherwise they become confused.

   The following scenarios should briefly show where the accurate
   feedback is needed or adds value:

   An RFC5681 TCP sender that supports ConEx:
           In this case the ConEx mechanism uses the extra information
           per RTT to re-echo the precise congestion information, but
           the congestion control algorithm still ignores multiple marks
           per RTT [RFC5681].

   A sender using DCTCP congestion control without ConEx:
           The congestion control algorithm uses the extra info per RTT
           to perform its decrease depending on the number of congestion

   A sender using DCTCP congestion control and supports ConEx:
           Both the congestion control algorithm and ConEx use the fine-
           grained ECN feedback mechanism.

   A RFC5681 TCP sender without ConEx:

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           No accurate feedback is necessary here.  The congestion
           control algorithm still reacts on only one signal per RTT.
           But it is best to have one generic feedback mechanism,
           whether it is used or not.

   Using CE for checking integrity:
           If a more accurate ECN feedback scheme would feed all
           occurrences of CE marks back, a sender could perform
           interigty checking based in the injection of CE marks.
           Thereby, a sender will send packets which are
           deterministically marked with CE (at a low frequency) and
           keep track if feedback is received for these packets.  Of
           course, the congestion notification feedback for these self-
           injected markings, does not cause a congestion control react.

4.  Requirements

   The requirements of the accurate ECN feedback protocol, for the use
   of e.g. Conex or DCTCP, are to have a fairly accurate (not
   necessarily perfect), timely and protected signaling.  This leads to
   the following requirements, which should be discussed for any
   proposed more accurate ECN feedback scheme:

           The ECN feedback signal is carried within the ACK.  TCP ACKs
           can get lost.  Moreover, delayed ACKs are commonly used with
           TCP.  That means in most cases only every second data packet
           triggers an ACK.  In a high congestion situation where most
           of the packets are marked with CE, an accurate feedback
           mechanism must still be able to signal sufficient congestion
           information.  Thus the accurate ECN feedback extension has to
           take delayed ACK and ACK loss into account.  Also, a more
           accurate feedback protocol would still work if delayed ACKs
           covered more than two packets.

           a CE mark can be induced by a network node on the
           transmission path and is then echoed by the receiver in the
           TCP ACK.  Thus when this information arrives at the sender,
           it is naturally already about one RTT old.  With a sufficient
           ACK rate a further delay of a small number of ACKs can be
           tolerated.  However, this information will become stale with
           large delays, given the dynamic nature of networks.  TCP
           congestion control (which itself partly introduces these
           dynamics) operates on a time scale of one RTT.  Thus, to be
           timely, congestion feedback information should be delivered
           within about one RTT.

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           A more accurate ECN feedback scheme should assure the
           integrity of the feedback at least as well as the ECN Nonce
           or gives strong incentives for the receiver and network nodes
           to cooperate honestly.

           Given there are known problems with the ECN nonce (as
           identified above), this document only requires that the
           integrity of the more accurate ECN feedback can be assured;
           it does not require that the ECN Nonce mechanism is employed
           to achieve this.  Indeed, if integrity could be provided
           else-wise, a more accurate ECN feedback protocol might re-use
           the nonce sum (NS) flag in the TCP header.

           If the accurate ECN feedback scheme provides sufficient
           information, the integrity check could e.g. be performed by
           deterministically setting the CE in the sender and monitoring
           the respective feedback (similar to ECT(1) and the ECN Nonce
           sum).  If and what kind of enforcements a sender should do,
           when detecting wrong feedback information, is not part of
           this document.

           Classic ECN feeds back one congestion notification per RTT,
           which is sufficient for classic TCP congestion control which
           reduces the sending rate at most once per RTT.  The more
           accurate ECN feedback scheme has to ensure that if a
           congestion episode occurs, at least one congestion
           notification is echoed and received per RTT as classic ECN
           would do.  Of course, the goal of a more accurate ECN
           extension is to reconstruct the number of CE markings more
           accurately and in the best case even to reconstruct the exact
           number of payload bytes that a CE marked packet was carrying.
           However, a sender should not assume to get the exact number
           of congestion markings or marked bytes in all situations.
           Moreover, the feedback scheme should preserve the order at
           which any ECN signal was received.  And ideally, it would
           even be possible for the sender to determine which of the
           packets (covered by one delayed ACK) were congestion marked,
           e.g. if the flow consists of packets of different sizes, or
           to allow for future protocols where the order of the markings
           may be important.

           In fact, the ECN field in the IP header provides four code
           points.  In the best case, a sender that has the more
           accurate ECN feedback information, would be able to
           reconstruct the occurrence of any of the four code points.
           Assuming the sender marks all data packets will at least as

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           ECN-capable and ETC(0) will be the default setting, feeding
           the occurrence of CE and ECT(1) back might be sufficient.

           The implementation should be as simple as possible and only a
           minimum of additional state information should be needed.
           This will enable the more accurate ECN feedback to be used as
           the default feedback mechanism, even if only one ECN feedback
           signal per RTT is needed.  Furthermore, the receiver should
           not take assumptions about the mechanism that was used to set
           the markin nor about any interpretation or reaction to the
           congestion signal.  The receiver should only feed the
           information back as accurate as possible.

           A more accurate ECN feedback signal should limit the
           additional network load, because ECN feedback is ultimately
           not critical information (in the worst case, loss will still
           be available as a congestion signal of last resort).  As
           feedback information has to be provided frequently and in a
           timely fashion, potentially all or a large fraction of TCP
           acknowledgments might carry this information.  Ideally, no
           additional segments should be exchanged compared to an
           RFC3168 TCP session, and the overhead in each segment should
           be minimised.

   Backward and forward compatibility
           Given more accurate ECN feedback will involve a change to the
           TCP protocol, it will need to be negotiated between the two
           TCP endpoints.  If either end does not support the more
           accurate feedback, they should both be able to fall-back to
           classic ECN feedback.

           A more accurate ECN feedback extension should aim to be able
           to traverse most existing middleboxes.  Further, a feedback
           mechanism should provide a method to fall-back to classic
           RFC3168 signaling if the new signal is suppressed by certain

           In order to avoid a fork in the TCP protocol specifications,
           if experiments with the new ECN feedback protocol are
           successful, it is intended to eventually update RFC3168 for
           any TCP/ECN sender, not just for ConEx or DCTCP senders.
           Therefore, even if only one ECN feedback signal per RTT is
           needed, it should be possible to use the more accuarte ECN

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5.  Design Approaches

   All approaches presented below (and proposed so far) are able to
   provide accurate ECN feedback information as long as no ACK loss
   occurs and the congestion rate is reasonable.  In case of high a high
   ACK loss rate or very high congestion (CE marking) rate, the proposed
   schemes have different resilience characteristics depending on the
   number of used bits for the encoding.  While classic ECN provides a
   reliable (inaccurate) feedback of a maximum of one congestion signal
   per RTT, the proposed schemes do not implement an explicit
   acknowledgement mechanism.

5.1.  Re-use of ECN/NS Header Bits

   The three ECN/NS header, ECE, CWR and NS are re-used (not only for
   additional capability negotiation during the TCP handshake exchange
   but) to signal the current value of an CE counter at the receiver.
   This approach only provides a limited resilience against ACK lost
   depending of the number of used bits.

   Several codings have been proposed so far:

   o  A one bit scheme sends one ECE for each CE received (to increase
      the robustness against ACK loss CWR could be used to introduce
      redundant information on the next ACK);

   o  A 3-bit counter scheme continuously feeds back the three least
      significant bits of a CE counter;

   o  A 3-bit codepoint scheme encodes either a CE counter or an ECT(1)
      counter in 8 codepoints.

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   The proposed schemes provide accumulated information on ECN-CE-
   marking feedback, similar to the number of acknowledged bytes in the
   TCP header.  Due to the limited number of bits the ECN feedback
   information will wrap much more often than the acknowledgement field.
   Thus feedback information could be lost due to a relatively small
   sequence of pure-ACK losses.  Resilience could be increased by
   introducing redundancy, e.g. send each counter increase two or more
   times.  Of course any of these additional mechanisms will increasee
   the complexity.  If the congestion rate is larger that the ACK rate
   (multiplied by the number of congestion marks that can be signaled
   per ACK), the congestion information cannot correctly fed back.  Thus
   an accurate ECN feedback mechanism needs to be able to also cover the
   worst case situation where every packet is CE marked.  This can
   potentially be realized by dynamically adapting the ACK rate and
   redundancy, which again increases complexity and perhaps the
   signaling overhead as well.  Scheme that do not re-use the ECN NS
   bit, can still support ECN Nonce.

5.2.  Using Other Header Bits

   As seen in Figure 1, there are currently three unused flag bits in
   the TCP header.  The proposed 3 bit or codepoint schemes could be
   extended by one or more bits, to add higher resilience against ACK
   loss.  The relative gain would be proportionally higher resilience
   against ACK loss, while the respective drawbacks would remain

   Alternatively, the receiver could use bits in the Urgent Pointer
   field to signal more bits of its congestion signal counter, but only
   whenever it does not set the Urgent Flag.  As this is often the case,
   resilience could be increased without additional header overhead.

   Any proposal to use such bits would need to check the likelihood that
   some middleboxes might discard or 'normalise' the currently unused
   flag bits or a non-zero Urgent Pointer when the Urgent Flag is

5.3.  Using a TCP Option

   Alternatively, a new TCP option could be introduced, to help
   maintaining the accuracy and integrity of the ECN feedback between
   receiver and sender.  Such an option could provide higher resilience
   and even more information.  E.g. ECN for RTP/UDP provides explicit
   the number of ECT(0), ECT(1), CE, non-ECT marked and lost packets.
   However, deploying new TCP options has its own challenges.  Moreover,
   to actually achieve a high resilience, this option would need to be
   carried by either all or a large number ACKs.  Thus this approach
   would introduce considerable signaling overhead while ECN feedback is

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   not such a critical information (as in the worst case, loss will
   still be available to provide a strong congestion feedback signal).
   Anyway, such a TCP option could also be used in addition to a more
   accurate ECN feedback scheme in the TCP header or in addition to
   classic ECN, only when available and needed.

6.  Acknowledgements

   Thanks to Bob Briscoe for reviewing and providing valuable additons
   on DCTCP and ConEx.  Moreover, thanks to Gorry Fairhurst as well as
   Bob Briscoe for ideas on CE-based interigty checking.

7.  IANA Considerations

   This memo includes no request to IANA.

8.  Security Considerations

   Given ECN feedback is used as input for congestion control, the
   respective algorithm would not react appropriately if ECN feedback
   were lost and the resilience mechanism to recover it was inadequate.
   This resilience requirement is articulated in Section 4.  However, it
   should be noted that ECN feedback is not the last resort against
   congestion collapse, because if there is insufficient response to
   ECN, loss will ensue, and TCP will still react appropriately to loss.

   A receiver could suppress ECN feedback information leading to its
   connections consuming excess sender or network resources.  This
   problem is similar to that seen with the classic ECN feedback scheme
   and should be addressed by integrity checking as required in
   Section 4.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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

   [RFC3540]  Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
              Congestion Notification (ECN) Signaling with Nonces", RFC
              3540, June 2003.

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9.2.  Informative References

   [Ali10]    Alizadeh, M., Greenberg, A., Maltz, D., Padhye, J., Patel,
              P., Prabhakar, B., Sengupta, S., and M. Sridharan, "DCTCP:
              Efficient Packet Transport for the Commoditized Data
              Center", Jan 2010.

              Briscoe, B., Jacquet, A., Moncaster, T., and A. Smith,
              "Re-ECN: Adding Accountability for Causing Congestion to
              TCP/IP", draft-briscoe-tsvwg-re-ecn-tcp-09 (work in
              progress), October 2010.

              Kuehlewind, M. and R. Scheffenegger, "Accurate ECN
              Feedback Option in TCP", draft-kuehlewind-tcpm-accurate-
              ecn-option-01 (work in progress), July 2012.

              Moncaster, T., Briscoe, B., and A. Jacquet, "A TCP Test to
              Allow Senders to Identify Receiver Non-Compliance", draft-
              moncaster-tcpm-rcv-cheat-02 (work in progress), November

   [RFC2018]  Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
              Selective Acknowledgment Options", RFC 2018, October 1996.

   [RFC5562]  Kuzmanovic, A., Mondal, A., Floyd, S., and K.
              Ramakrishnan, "Adding Explicit Congestion Notification
              (ECN) Capability to TCP's SYN/ACK Packets", RFC 5562, June

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, September 2009.

   [RFC5690]  Floyd, S., Arcia, A., Ros, D., and J. Iyengar, "Adding
              Acknowledgement Congestion Control to TCP", RFC 5690,
              February 2010.

   [RFC6789]  Briscoe, B., Woundy, R., and A. Cooper, "Congestion
              Exposure (ConEx) Concepts and Use Cases", RFC 6789,
              December 2012.

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Authors' Addresses

   Mirja Kuehlewind (editor)
   University of Stuttgart
   Pfaffenwaldring 47
   Stuttgart  70569


   Richard Scheffenegger
   NetApp, Inc.
   Am Euro Platz 2
   Vienna  1120

   Phone: +43 1 3676811 3146

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