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Versions: (draft-kuehlewind-conex-tcp-modifications) 00 01 02 03 04 05 06 07 08 09 10 RFC 7786

Congestion Exposure (ConEx)                           M. Kuehlewind, Ed.
Internet-Draft                                   University of Stuttgart
Intended status: Experimental                           R. Scheffenegger
Expires: December 01, 2013                                  NetApp, Inc.
                                                            May 30, 2013


               TCP modifications for Congestion Exposure
                 draft-ietf-conex-tcp-modifications-03

Abstract

   Congestion Exposure (ConEx) is a mechanism by which senders inform
   the network about the congestion encountered by previous packets on
   the same flow.  This document describes the necessary modifications
   to use ConEx with the Transmission Control Protocol (TCP).

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 http://datatracker.ietf.org/drafts/current/.

   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 December 01, 2013.

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
   (http://trustee.ietf.org/license-info) in effect on the date of
   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.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Sender-side Modifications . . . . . . . . . . . . . . . . . .   3
   3.  Accounting congestion . . . . . . . . . . . . . . . . . . . .   4
     3.1.  ECN . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
       3.1.1.  Accurate ECN feedback . . . . . . . . . . . . . . . .   5
       3.1.2.  Classic ECN support . . . . . . . . . . . . . . . . .   5
     3.2.  Loss Detection with/without SACK  . . . . . . . . . . . .   7
   4.  Setting the ConEx Bits  . . . . . . . . . . . . . . . . . . .   8
     4.1.  Setting the E and the L Bit . . . . . . . . . . . . . . .   8
     4.2.  Credit Bits . . . . . . . . . . . . . . . . . . . . . . .   8
     4.3.  Loss of ConEx information . . . . . . . . . . . . . . . .   9
   5.  Timeliness of the ConEx Signals . . . . . . . . . . . . . . .  10
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Revision history . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13


1.  Introduction

   Congestion Exposure (ConEx) is a mechanism by which senders inform
   the network about the congestion encountered by previous packets on
   the same flow.  This document describes the necessary modifications
   to use ConEx with the Transmission Control Protocol (TCP).  The ConEx
   signal is based on loss or ECN marks [RFC3168] as a congestion
   indication.  This congestion information is retrieved by the sender
   based on existing feedback mechanisms from the receiver to the sender
   in TCP.

   With standard TCP without Selective Acknowledgments (SACK) [RFC2018]
   the actual number of losses is hard to detect, thus we recommend to
   enable SACK when using ConEx.  However, we discuss both cases, with
   and without SACK support, later on.

   Explicit Congestion Notification (ECN) is defined in such a way that
   only a single congestion signal is guaranteed to be delivered per
   Round-trip Time (RTT) from the receiver to the sender.  For ConEx a
   more accurate feedback signal would be beneficial.  Such an extension
   to ECN is defined in a seperate document
   [draft-kuehlewind-conex-accurate-ecn], as it can also be useful for
   other mechanisms, as e.g.  [DCTCP] or whenever the congestion control



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   reaction should be proportional to the experienced congestion.  ConEx
   also works with classic ECN but it is less accurate when multiple
   congestion markings occur within on RTT.

   ConEx is currently/will be defined as an destination option for IPv6.
   The use of four bits have been defined, namely the X (ConEx-capable),
   the L (loss experienced), the E (ECN experienced) and C (credit) bit.

1.1.  Requirements Language

   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].

2.  Sender-side Modifications

   A ConEx sender MUST negotiate for both SACK and ECN or the more
   accurate ECN feedback in the TCP handshake if these TCP extension are
   available at the sender.  Depending on the capability of the
   receiver, the following operation modes exist:

   o  Full-ConEx (SACK and accurate ECN feedback)

   o  accECN-ConEx (no SACK but accurate ECN feedback)

   o  ECN-ConEx (no SACK and no accurate ECN feedback but 'classic' ECN)

   o  SACK-ECN-ConEx (SACK and 'classic' instead of accurate ECN)

   o  SACK-ConEx (SACK but no ECN at all)

   o  Basic-ConEx (neither SACK nor ECN)

   A ConEx sender MUST expose congestion to the network according to the
   congestion information received by ECN or based on loss information
   provided by the TCP feedback loop.  A TCP sender SHOULD account
   congestion byte-wise (and not packet-wise).  A sender MUST mark
   subsequent packets (after the congestion notification) with the
   respective ConEx bit in the IP header.

   With SACK only the number of lost bytes is known, but not the number
   of packets carrying these bytes.  With classic ECN only an indication
   is given that a marking occurred which is not giving an exact number
   of bytes nor packets.  As network congestion is usually byte-
   congestion, the exact number of bytes should be taken into account if
   available to make the ConEx signal as exact as possible.





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   The congestion accounting based on different operation modes is
   described in the next section and the handling of the IPv6 bits
   itself in the subsequent section afterwards.

3.  Accounting congestion

   A TCP sender SHOULD account congestion byte-wise (and not packet-
   wise) based the congestion information received by ECN or loss
   detection provided by TCP.  For this purpose a TCP sender will
   maintain two different counters for number outstanding bytes that
   need to be ConEx marked either with the E bit or the L Bit.

   The outstanding bytes accounted based on ECN feedback information are
   maintained in the congestion exposure gauge (CEG).  The accounting of
   these bytes from the ECN feedback is explained in more detail next.

   The outstanding bytes for congestion indications based on loss are
   maintained in the loss exposure gauge (LEG) and the accounting is
   explained in subsequent to the CEG accounting.

   The subtraction of bytes which have been ConEx marked from both
   counters is explained in the next section.

   Usually all bytes of an IP packet must be accounted.  Therefore the
   sender SHOULD take the headers into account.  If equal sized packets,
   or at least equally distributed packet sizes can be assumed, the
   sender MAY only account the TCP payload bytes, as the ConEx marked
   packets as well as the original packets causing the congestion will
   both contain about the same number of headers.

   If a sender sends different sized packets with unequally distributed
   packet sizes, the sender might be able to reconstruct the exact
   number of headers based on information which packet sizes has been
   sent in the last RTT.  Otherwise if no additional information is
   available the worst case number of headers SHOULD be estimated in a
   conservative way based on a minimum packet size (of all packets sent
   in the last RTT).

3.1.  ECN

   ECN is an IP/TCP mechanism that allows network nodes to mark packets
   with the Congestion Experienced (CE) mark instead of (early dropping
   them when congestion occurs.  As soon as a CE mark is seen at the
   receiver, with classic ECN it will feed this information back to the
   sender by setting the Echo Congestion Experienced (ECE) bit in the
   TCP header until a packet with Congestion Window Reduced (CWR) bit in
   the TCP header is received to acknowledge the reception of the
   congestion notification.  The sender sets the CWR bit in the TCP



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   header once when the first ECE of a congestion notification is
   received.

   A receiver can support the accurate ECN feedback scheme, the
   'classic' ECN or neither.  In the case ECN is not supported at all,
   the transport is not ECN-capable and no ECN marks will occur, thus
   the E bit will never be set.  In the other cases a ConEx sender MUST
   maintain a gauge for the number of outstanding bytes that have to be
   ConEx marked with the E bit, the congestion exposure gauge (CEG).

   The CEG is increased when ECN information is received from an ECN-
   capable receiver supporting the 'classic' ECN scheme or the accurate
   ECN feedback scheme.  When the ConEx sender receives an ACK
   indicating one or more segments were received with a CE mark, CEG is
   increased by the appropriate number of bytes.

   In case of duplicate acknowledgements the number of acknowledged
   bytes will be zero even though (CE marked) data has been received.
   Therefore, we calculated a variable DeliveredData.  DeliveredData
   covers the total number of bytes that the current ACK indicates have
   been delivered to the receiver, relative to all past ACKs.  With
   SACK, DeliveredData is increased by the number of bytes given by
   changes in the SACK information.  Note the change in in the SACK
   information can also be negative if the number of acknowledged bytes
   increases.  Without SACK, DeliveredData is estimated to be 1 SMSS on
   duplicate acknowledgements, and on a subsequent partial or full ACK,
   DeliveredData is estimated to be the change in acknowledged bytes,
   minus one SMSS for each preceding duplicate ACK.

   DeliveredData = acked_bytes + SACK_diff + (is_dup)*1SMSS -
   (is_after_dup)*num_dup*1SMSS

   The two cases, with and without more accurate ECN depending on the
   receiver capability, are discussed in the following sections.

3.1.1.  Accurate ECN feedback

   With a more accurate ECN feedback scheme either the number of marked
   packets/received CE marks or directly the number of marked bytes is
   known.  In the later case the CEG can directly be increased by the
   number of marked bytes.  Otherwise if D is assumed to be the number
   of marks, the gauge CEG has to be increased by the amount of bytes
   sent which were marked:

   CEG += min( SMSS*D, DeliveredData )

3.1.2.  Classic ECN support




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   A ConEx sender that communicates with a classic ECN receiver
   (conforming to [RFC3168] or [RFC5562]) MAY run in one of these modes:

   o  Full compliance mode:

      The ConEx sender fully conforms to all the semantics of the ECN
      signaling as defined by [RFC5562].  In this mode, only a single
      congestion indication can be signaled by the receiver per RTT.
      Whenever the ECE flag toggles from "0" to "1", the gauge CEG is
      increased at maximum by the SMSS:

      CEG += min(SMSS, DeliveredData)

      Note that under severe congestion, a session adhering to these
      semantics may not provide enough ConEx marks.  This may cause
      appropriate sanctions by an audit device in a ConEx enabled
      network.

   o  Simple compatibility mode:

      The sender will set the CWR permanently to force the receiver to
      signal only one ECE per CE mark.  Unfortunately, the use of
      delayed ACKs [RFC5681], as it is usually done today, will prevent
      a feedback of every CE mark.  An CWR confirmation will be received
      before the ECE can be sent out with the next ACK.  With an ACK
      rate of M, about M-1/M CE indications will not be signaled back by
      the receiver (e.g.  50% with M=2 for delayed ACKs).  Thus, in this
      mode the ConEx sender MUST increase CEG as if M congestion
      notification were received for each received ECE signal:

      CEG += min(M*SMSS, DeliveredData + (M-1)*SMSS)

      In case of a congestion event with low congestion (that means when
      only a very smaller number of packets get marked), the sender
      might miss the whole congestion event.  On average the sender will
      send sufficient ConEx marks due to the scheme proposed above but
      these ConEx marks might be shifted in time.  Regarding congestion
      control it is not a general problem to miss a congestion event as,
      by chance, a marking scheme in the network node might also miss a
      certain flow.  In the case where no other flow is reacting, the
      congestion level will increase and it will get more likely that
      the congestion feedback is delivered.  To provide a fair share
      over time, a TCP sender implementing this simple ECN compatibility
      mode could react more strongly when receiving an ECN feedback
      signal.  This of course depends on the congestion control used.

   o  Advanced compatibility mode:




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      To avoid the loss of ECN feedback information in the proposed
      simple compatibility mode, a sender could set CWR only on those
      data segments, that will actually trigger a (delayed) ACK.  The
      sender would need an additional control loop to estimated which
      data segment will trigger an ACK.  Such a more sophisticated
      heuristics could extract congestion notifications more timely.  In
      addition, if this advanced compatibility mode is used, further
      heuristics SHOULD be implemented, to determine the value of each
      ECE notification.  E.g.  for each consecutive ACK received with
      the ECE flag set, CEG should be increased by min( M*SSMS,
      DeliveredData).  Else if the predecessor ACK was received with the
      ECE flag cleared, CEG need only be increase at maximum by one
      SMSS:

      if previous_marked: CEG += min( M*SSMS, DeliveredData)
      else: CEG += min(SMSS, DeliveredData)

      This heuristic is conservative during more serious congestion, and
      more relaxed at low congestion levels.

3.2.  Loss Detection with/without SACK

   For all the data segments that are determined by a ConEx sender as
   lost, (at least) the same number of TCP payload bytes MUST be be sent
   with the ConEx L bit set.  Loss detection typically happens by use of
   duplicate ACKs, or the firing of the retransmission timer.  A ConEx
   sender MUST maintain a loss exposure gauge (LEG), indicating the
   number of outstanding bytes that must be sent with the ConEx L bit.
   When a data segment is retransmitted, LEG will be increased by the
   size of the TCP payload packet containing the retransmission,
   assuming equal sized segments such that the retransmitted packet will
   have the same number of header as the original ones.  When sending
   subsequent segments, the ConEx L bit is set as long as LEG is
   positive, and LEG is decreased by the size of the sent TCP payload
   with the ConEx L bit set.

   Any retransmission may be spurious.  To accommodate that, a ConEx
   sender SHOULD make use of heuristics to detect such spurious
   retransmissions (e.g.  F-RTO [RFC5682], DSACK [RFC3708], and Eifel
   [RFC3522], [RFC4015]).  When such a heuristic has determined, that a
   certain number of packets were retransmitted erroneously, the ConEx
   sender should subtract the payload size of these TCP packets from
   LEG.

   Note that the above heuristics delays the ConEx signal by one
   segment, and also decouples them from the retransmissions themselves,
   as some control packets (e.g.  pure ACKs, window probes, or window
   updates) may be sent in between data segment retransmissions.  A



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   simpler approach would be to set the ConEx signal for each
   retransmitted data segment.  However, it is important to remember,
   that a ConEx signal and TCP segments do not natively belong together.

   If SACK is not available or SACK information has been reset for any
   reason, spurious retransmission are more likely.  In this case it
   might be valuable to slightly delay the ConEx loss feedback until a
   spurious retransmission might be detected.  But the ConEx signal MUST
   NOT be delayed more than one RTT.

4.  Setting the ConEx Bits

   ConEx is currently/will be defined as an destination option for IPv6.
   The use of four bits have been defined, namely the X (ConEx-capable),
   the L (loss experienced), the E (ECN experienced) and C (credit) bit.

   By setting the X bit a packet is marked as ConEx-capable.  All
   packets carrying payload MUST be marked with the X bit set including
   retransmissions.  No congestion feedback information are available
   about control packets as pure ACKs which are not carrying any
   payload.  Thus these packet should not be taken into account when
   determining ConEx information.  These packet MUST carry a ConEx
   Destination Option with the X bit unset.

4.1.  Setting the E and the L Bit

   As long as the CEG or LEG counter is positive, ConEx-capable packets
   SHOULD be marked with E or L respectively, and the CEG or LEG counter
   is decreased by the TCP payload bytes carried in this packet.  If the
   CEG or LEG counter is negative, the respective counter SHOULD be
   reset to zero within one RTT after it was decreased the last time or
   one RTT after recovery if no further congestion occurred.

4.2.  Credit Bits

   The ConEx abstract mechanism requires that the transport SHOULD
   signal sufficient credit in advance to cover any reasonably expected
   congestion during its feedback delay.  To be very conservative the
   number of credits would need to equal the number of packets in
   flight, as every packet could get lost or congestion marked.  With a
   more moderate view, only an increase in the sending rate should cause
   loss while the number of ECN markings within one RTT depends on
   parameterization of the used Active Queue management (AQM).

   In TCP Slow Start the sending rate will increase exponentially and
   that means double every RTT.  Thus the number of credits should equal
   at least half the number of packets in flight in every RTT.  If the
   used AQM is not overly aggressive with ECN marking, maintaining the



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   number of credit as half the number of packets in flight shuould be
   sufficient for both, congestion signaled by loss or ECN.  Under the
   assumption that all ConEx marks will not get invalid for the whole
   Slow Start phase, marks of a previous RTT have to be summed up.  Thus
   the marking of every fourth packet will allow sufficient credits in
   Slow Start as it can be seen in Figure Figure 1.

             RTT1  |------XC------>|
                   |------X------->|
                   |------X------->|   credit=1  in_flight=3
                   |               |
             RTT2  |------X------->|
                   |------XC------>|
                   |------X------->|
                   |------X------->|
                   |------X------->|
                   |------XC------>|   credit=3  in_flight=6
                   |               |
             RTT3  |------X------->|
                   |------X------->|
                   |------X------->|
                   |------XC------>|
                   |------X------->|
                   |------X------->|
                   |------X------->|
                   |------XC------>|
                   |------X------->|
                   |------X------->|
                   |------X------->|
                   |------XC------>|   credit=6  in_flight=12
                   |      .        |
                   |      :        |

       Figure 1: Credits in Slow Start (with an initial window of 3)

   Moreover, a ConEx sender should maintain a counter of the sent
   credits c.  In Congestion Avoidance phase, the sender needs to
   monitor the number of packets in flight f.  If f every gets larger
   than c, the ConEx sender should send new credits.

   The audit might loose state due to e.g.  rerouting or memory
   limitation.  Therefore, the sender needs to detect this case and
   resend credits.  Thus a ConEx sender should reset the credit count c
   if losses occur in two subsequent RTTs (assuming that the sending
   rate was correctly reduced based on the received congestion signal).

4.3.  Loss of ConEx information




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   The audit can have wrong information if e.g.  ConEx got lost on the
   channel (or a wrong number of ConEx marking has been estimated by the
   sender due to a lack of feedback information).  In this case the
   audit might penalize a sender wrongly.  The ConEx sender should
   detect this case and send further credits which should solve the
   situation (see Section 4.2).

5.  Timeliness of the ConEx Signals

   ConEx signals will anyway be evaluated with a slight time delay of
   about one RTT by a network node.  Therefore, it is not absolutely
   necessary to immediately signal ConEx bits when they become known
   (e.g.  L and E bits), but a sender SHOULD sent the ConEx signaling
   with the next available packet.  In cases where it is preferable to
   slightly delay the ConEx signal, the sender MUST NOT delay the ConEx
   signal more than one RTT.

   Multiple ConEx bits may become available for signaling at the same
   time, for example when an ACK is received by the sender, that
   indicates that at least one segment has been lost, and that one or
   more ECN marks were received at the same time.  This may happen
   during excessive congestion, where buffer queues overflow and some
   packets are marked, while others have to be dropped nevertheless.
   Another possibility when this may happen are lost ACKs, so that a
   subsequent ACK carries summary information not previously available
   to the sender.

6.  Acknowledgements

   The authors would like to thank Bob Briscoe who contributed with this
   initial ideas and valuable feedback.

7.  IANA Considerations

   This document does not have any requests to IANA.

8.  Security Considerations

   With some of the advanced ECN compatibility modes it is possible to
   miss congestion notifications.  Thus a sender will not decrease its
   sending rate.  If the congestion is persistent, the likelihood to
   receive a congestion notification increases.  In the worst case the
   sender will still react correctly to loss.  This will prevent a
   congestion collapse.

9.  References





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9.1.  Normative References

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

   [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.

   [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
              2009.

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

9.2.  Informative References

   [DCTCP]    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.

   [I-D.briscoe-tsvwg-re-ecn-tcp]
              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.

   [RFC3522]  Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm
              for TCP", RFC 3522, April 2003.

   [RFC3708]  Blanton, E. and M. Allman, "Using TCP Duplicate Selective
              Acknowledgement (DSACKs) and Stream Control Transmission
              Protocol (SCTP) Duplicate Transmission Sequence Numbers
              (TSNs) to Detect Spurious Retransmissions", RFC 3708,
              February 2004.

   [RFC4015]  Ludwig, R. and A. Gurtov, "The Eifel Response Algorithm
              for TCP", RFC 4015, February 2005.







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   [RFC5682]  Sarolahti, P., Kojo, M., Yamamoto, K., and M. Hata,
              "Forward RTO-Recovery (F-RTO): An Algorithm for Detecting
              Spurious Retransmission Timeouts with TCP", RFC 5682,
              September 2009.

   [draft-kuehlewind-conex-accurate-ecn]
              Kuehlewind, M. and R. Scheffenegger, "Accurate ECN
              Feedback in TCP", draft-kuehlewind-conex-accurate-ecn-00
              (work in progress), Jun 2011.

Appendix A.  Revision history

   RFC Editior: This section is to be removed before RFC publication.

   00 ... initial draft, early submission to meet deadline.

   01 ... refined draft, updated LEG "drain" from per-packet to RTT-
   based.

   02 ... added Section 4.3 and expanded discussion about ECN
   interaction.

   03 ... expanded the discussion around credit bits.




























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Authors' Addresses

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

   Email: mirja.kuehlewind@ikr.uni-stuttgart.de


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

   Phone: +43 1 3676811 3146
   Email: rs@netapp.com































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