TSVWG                                                   A. Ferrieux, Ed.
Internet-Draft                                         I. Hamchaoui, Ed.
Intended status: Informational                               Orange Labs
Expires: January 29, 2021                               I. Lubashev, Ed.
                                                     Akamai Technologies
                                                        D. Tikhonov, Ed.
                                                  LiteSpeed Technologies
                                                           July 28, 2020

             Packet Loss Signaling for Encrypted Protocols


   This document describes a protocol-independent method that employs
   two bits to allow endpoints to signal packet loss in a way that can
   be used by network devices to measure and locate the source of the
   loss.  The signaling method applies to all protocols with a protocol-
   specific way to identify packet loss.  The method is especially
   valuable when applied to protocols that encrypt transport header and
   do not allow an alternative method for loss detection.

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
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on January 29, 2021.

Copyright Notice

   Copyright (c) 2020 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
   (https://trustee.ietf.org/license-info) 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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Motivation for Passive On-Path Loss Observation . . . . .   3
     1.2.  On-Path Loss Observation  . . . . . . . . . . . . . . . .   3
     1.3.  On-Path Loss Signaling  . . . . . . . . . . . . . . . . .   4
     1.4.  Recommended Use of the Signals  . . . . . . . . . . . . .   4
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   4
   3.  Loss Bits . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Setting the sQuare Bit on Outgoing Packets  . . . . . . .   5
       3.1.1.  Q Run Length Selection  . . . . . . . . . . . . . . .   5
     3.2.  Setting the Loss Event Bit on Outgoing Packets  . . . . .   5
   4.  Using the Loss Bits for Passive Loss Measurement  . . . . . .   6
     4.1.  End-To-End Loss . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Upstream Loss . . . . . . . . . . . . . . . . . . . . . .   6
     4.3.  Correlating End-to-End and Upstream Loss  . . . . . . . .   7
     4.4.  Downstream Loss . . . . . . . . . . . . . . . . . . . . .   7
     4.5.  Observer Loss . . . . . . . . . . . . . . . . . . . . . .   7
   5.  ECN-Echo Event Bit  . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Setting the ECN-Echo Event Bit on Outgoing Packets  . . .   8
     5.2.  Using E Bit for Passive ECN-Reported Congestion
           Measurement . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  Protocol Ossification Considerations  . . . . . . . . . . . .   9
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
     7.1.  Optimistic ACK Attack . . . . . . . . . . . . . . . . . .  10
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  10
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   10. Change Log  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.1.  Since version 02 . . . . . . . . . . . . . . . . . . . .  10
     10.2.  Since version 01 . . . . . . . . . . . . . . . . . . . .  11
     10.3.  Since version 00 . . . . . . . . . . . . . . . . . . . .  11
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  11
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     12.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

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1.  Introduction

1.1.  Motivation for Passive On-Path Loss Observation

   Packet loss is hard and pervasive problem of day-to-day network
   operation.  Proactively detecting, measuring, and locating it is
   crucial to maintaining high QoS and timely resolution of crippling
   end-to-end throughput issues.  To this effect, in a TCP-dominated
   world, network operators have been heavily relying on information
   present in the clear in TCP headers: sequence and acknowledgment
   numbers and SACKs when enabled (see [RFC8517]).  These allow for
   quantitative estimation of packet loss by passive on-path
   observation.  Additionally, the lossy segment (upstream or downstream
   from the observation point) can be quickly identified by moving the
   passive observer around.

   With encrypted protocols, the equivalent transport headers are
   encrypted and passive packet loss observation is not possible, as
   described in [TRANSPORT-ENCRYPT].

   Measuring TCP loss between similar endpoints cannot be relied upon to
   evaluate encrypted protocol loss.  Different protools could be routed
   by the network differently and the fraction of Internet traffic
   delivered using protocols other than TCP is increasing every year.
   It is imperative to measure packet loss experienced by encrypted
   protocol users directly.

1.2.  On-Path Loss Observation

   There are three sources of loss that network operators need to
   observe to guarantee high QoS:

   -  _upstream loss_ - loss between the sender and the observation
      point (Section 4.2)

   -  _downstream loss_ - loss between the observation point and the
      destination (Section 4.4)

   -  _observer loss_ - loss by the observer itself that does not cause
      downstream loss (Section 4.5)

   The upstream and downstream loss together constitute _end-to-end
   loss_ (Section 4.1).

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1.3.  On-Path Loss Signaling

   Following the recommendation in [RFC8558] of making path signals
   explicit, this document proposes adding two explicit loss bits to the
   clear portion of the protocol headers to restore network operators'
   ability to maintain high QoS.  These bits can be added to an
   unencrypted portion of a header belonging to any protocol layer, e.g.
   IP (see [IP]) and IPv6 (see [IPv6]) headers or extensions, such as
   [IPv6AltMark], UDP surplus space (see [UDP-OPTIONS] and
   [UDP-SURPLUS]), reserved bits in a QUIC v1 header (see

1.4.  Recommended Use of the Signals

   The loss signal is not designed for use in automated control of the
   network in environments where loss bits are set by untrusted hosts,
   Instead, the signal is to be used for troubleshooting individual
   flows as well as for monitoring the network by aggregating
   information from multiple flows and raising operator alarms if
   aggregate statistics indicate a potential problem.

2.  Notational Conventions

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

3.  Loss Bits

   The draft introduces two bits that are to be present in packets
   capable of loss reporting.  These are packets that include protocol
   headers with the loss bits.  Only loss of packets capable of loss
   reporting is reported using loss bits.

   Whenever this specification refers to packets, it is referring only
   to packets capable of loss reporting.

   -  Q: The "sQuare signal" bit is toggled every N outgoing packets as
      explained below in Section 3.1.

   -  L: The "Loss event" bit is set to 0 or 1 according to the
      Unreported Loss counter, as explained below in Section 3.2.

   Each endpoint maintains appropriate counters independently and
   separately for each separately identifiable flow (each subflow for
   multipath connections).

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3.1.  Setting the sQuare Bit on Outgoing Packets

   The sQuare Value is initialized to the Initial Q Value (0) and is
   reflected in the Q bit of every outgoing packet.  The sQuare value is
   inverted after sending every N packets (a Q Run).  Hence, Q Period is
   2*N.  The Q bit represents "packet color" as defined by [RFC8321].
   The sQuare Bit can also be called an Alernate Marking bit.

   Observation points can estimate the upstream losses by counting the
   number of packets during a half period of the square signal, as
   described in Section 4.

3.1.1.  Q Run Length Selection

   The sender is expected to choose N (Q run length) based on the
   expected amount of loss and reordering on the path.  The choice of N
   strikes a compromise - the observation could become too unreliable in
   case of packet reordering and/or severe loss if N is too small, while
   short flows may not yield a useful upstream loss measurement if N is
   too large (see Section 4.2).

   The value of N MUST be at least 64 and be a power of 2.  This
   requirement allows an Observer to infer the Q run length by observing
   one period of the square signal.  It also allows the Observer to
   identify flows that set the loss bits to arbitrary values (see
   Section 6).

   If the sender does not have sufficient information to make an
   informed decision about Q run length, the sender SHOULD use N=64,
   since this value has been extensively tried in large-scale field
   tests and yielded good results.  Alternatively, the sender MAY also
   choose a random N for each flow, increasing the chances of using a Q
   run length that gives the best signal for some flows.

   The sender MUST keep the value of N constant for a given flow.

3.2.  Setting the Loss Event Bit on Outgoing Packets

   The Unreported Loss counter is initialized to 0, and L bit of every
   outgoing packet indicates whether the Unreported Loss counter is
   positive (L=1 if the counter is positive, and L=0 otherwise).  The
   value of the Unreported Loss counter is decremented every time a
   packet with L=1 is sent.

   The value of the Unreported Loss counter is incremented for every
   packet that the protocol declares lost, using whatever loss detection
   machinery the protocol employs.  If the protocol is able to rescind
   the loss determination later, a positive Unreported Loss counter MAY

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   be decremented due to the rescission, but it SHOULD NOT become
   negative due to the rescission.

   This loss signaling is similar to loss signaling in [ConEx], except
   the Loss Event bit is reporting the exact number of lost packets,
   whereas Echo Loss bit in [ConEx] is reporting an approximate number
   of lost bytes.

   For protocols, such as TCP ([TCP]), that allow network devices to
   change data segmentation, it is possible that only a part of the
   packet is lost.  In these cases, the sender MUST increment Unreported
   Loss counter by the fraction of the packet data lost (so Unreported
   Loss counter may become negative when a packet with L=1 is sent after
   a partial packet has been lost).

   Observation points can estimate the end-to-end loss, as determined by
   the upstream endpoint, by counting packets in this direction with the
   L bit equal to 1, as described in Section 4.

4.  Using the Loss Bits for Passive Loss Measurement

4.1.  End-To-End Loss

   The Loss Event bit allows an observer to calculate the end-to-end
   loss rate by counting packets with L bit value of 0 and 1 for a given
   flow.  The end-to-end loss rate is the fraction of packets with L=1.

   The assumption here is that upstream loss affects packets with L=0
   and L=1 equally.  If some loss is caused by tail-drop in a network
   device, this may be a simplification.  If the sender's congestion
   controller reduces the packet send rate after loss, there may be a
   sufficient delay before sending packets with L=1 that they have a
   greater chance of arriving at the observer.

4.2.  Upstream Loss

   Blocks of N (Q Run length) consecutive packets are sent with the same
   value of the Q bit, followed by another block of N packets with an
   inverted value of the Q bit.  Hence, knowing the value of N, an on-
   path observer can estimate the amount of upstream loss after
   observing at least N packets.  The upstream loss rate ("u") is one
   minus the average number of packets in a block of packets with the
   same Q value ("p") divided by N ("u=1-avg(p)/N").

   The observer needs to be able to tolerate packet reordering that can
   blur the edges of the square signal.

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   The observer needs to differentiate packets as belonging to different
   flows, since they use independent counters.

4.3.  Correlating End-to-End and Upstream Loss

   Upstream loss is calculated by observing packets that did not suffer
   the upstream loss.  End-to-end loss, however, is calculated by
   observing subsequent packets after the sender's protocol detected the
   loss.  Hence, end-to-end loss is generally observed with a delay of
   between 1 RTT (loss declared due to multiple duplicate
   acknowledgments) and 1 RTO (loss declared due to a timeout) relative
   to the upstream loss.

   The flow RTT can sometimes be estimated by timing protocol handshake
   messages.  This RTT estimate can be greatly improved by observing a
   dedicated protocol mechanism for conveying RTT information, such as
   the Latency Spin bit of [QUIC-TRANSPORT].

   Whenever the observer needs to perform a computation that uses both
   upstream and end-to-end loss rate measurements, it SHOULD use
   upstream loss rate leading the end-to-end loss rate by approximately
   1 RTT.  If the observer is unable to estimate RTT of the flow, it
   should accumulate loss measurements over time periods of at least 4
   times the typical RTT for the observed flows.

   If the calculated upstream loss rate exceeds the end-to-end loss rate
   calculated in Section 4.1, then either the Q Period is too short for
   the amount of packet reordering or there is observer loss, described
   in Section 4.5.  If this happens, the observer SHOULD adjust the
   calculated upstream loss rate to match end-to-end loss rate.

4.4.  Downstream Loss

   Because downstream loss affects only those packets that did not
   suffer upstream loss, the end-to-end loss rate ("e") relates to the
   upstream loss rate ("u") and downstream loss rate ("d") as
   "(1-u)(1-d)=1-e".  Hence, "d=(e-u)/(1-u)".

4.5.  Observer Loss

   A typical deployment of a passive observation system includes a
   network tap device that mirrors network packets of interest to a
   device that performs analysis and measurement on the mirrored
   packets.  The observer loss is the loss that occurs on the mirror

   Observer loss affects upstream loss rate measurement since it causes
   the observer to account for fewer packets in a block of identical Q

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   bit values (see {{upstreamloss)}).  The end-to-end loss rate
   measurement, however, is unaffected by the observer loss, since it is
   a measurement of the fraction of packets with the set L bit value,
   and the observer loss would affect all packets equally (see
   Section 4.1).

   The need to adjust the upstream loss rate down to match end-to-end
   loss rate as described in Section 4.3 is a strong indication of the
   observer loss, whose magnitude is between the amount of such
   adjustment and the entirety of the upstream loss measured in
   Section 4.2.  Alternatively, a high apparent upstream loss rate could
   be an indication of significant reordering, possibly due to packets
   belonging to a single flow being multiplexed over several upstream
   paths with different latency characteristics.

5.  ECN-Echo Event Bit

   While the primary focus of the draft is on exposing packet loss,
   modern networks can report congestion before they are forced to drop
   packets, as described in [ECN].  When transport protocols keep ECN-
   Echo feedback under encryption, this signal cannot be observed by the
   network operators.  When tasked with diagnosing network performance
   problems, knowledge of a congestion downstream of an observation
   point can be instrumental.

   If downstream congestion information is desired, this information can
   be signaled with an additional bit.

   -  E: The "ECN-Echo Event" bit is set to 0 or 1 according to the
      Unreported ECN Echo counter, as explained below in Section 5.1.

5.1.  Setting the ECN-Echo Event Bit on Outgoing Packets

   The Unreported ECN-Echo counter operates identicaly to Unreported
   Loss counter (Section 3.2), except it counts packets delivered by the
   network with CE markings, according to the ECN-Echo feedback from the

   This ECN-Echo signaling is similar to ECN signaling in [ConEx].  ECN-
   Echo mechanism in QUIC provides the number of packets received with
   CE marks.  For protocols like TCP, the method described in
   [ConEx-TCP] can be employed.  As stated in [ConEx-TCP], such feedback
   can be further improved using a method described in [ACCURATE].

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5.2.  Using E Bit for Passive ECN-Reported Congestion Measurement

   A network observer can count packets with CE codepoint and determine
   the upstream CE-marking rate directly.

   Observation points can also estimate ECN-reported end-to-end
   congestion by counting packets in this direction with a E bit equal
   to 1.

   The upstream CE-marking rate and end-to-end ECN-reported congestion
   can provide information about downstream CE-marking rate.  Presence
   of E bits along with L bits, however, can somewhat confound precise
   estimates of upstream and downstream CE-markings in case the flow
   contains packets that are not ECN-capable.

6.  Protocol Ossification Considerations

   Accurate loss information is not critical to the operation of any
   protocol, though its presence for a sufficient number of flows is
   important for the operation of networks.

   The loss bits are amenable to "greasing" described in [RFC8701], if
   the protocol designers are not ready to dedicate (and ossify) bits
   used for loss reporting to this function.  The greasing could be
   accomplished similarly to the Latency Spin bit greasing in
   [QUIC-TRANSPORT].  Namely, implementations could decide that a
   fraction of flows should not encode loss information in the loss bits
   and, instead, the bits would be set to arbitrary values.  The
   observers would need to be ready to ignore flows with loss
   information more resembling noise than the expected signal.

7.  Security Considerations

   Passive loss observation has been a part of the network operations
   for a long time, so exposing loss information to the network does not
   add new security concerns for protocols that are currently

   In the absence of upstream packet loss, the Q bit signal does not
   provide any information that cannot be observed by simply counting
   packets transiting a network path.  In the presence of upstream
   packet loss, the Q bit will disclose the loss, but this is
   information about the environment and not the endpoint state.  The L
   bit signal discloses internal state of the protocol's loss detection
   machinery, but this state can often be gleamed by timing packets and
   observing congestion controller response.  Hence, loss bits do not
   provide a viable new mechanism to attack data integrity and secrecy.

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7.1.  Optimistic ACK Attack

   A defense against an Optimistic ACK Attack, decribed in
   [QUIC-TRANSPORT], involves a sender randomly skipping packet numbers
   to detect a receiver acknowledging packet numbers that have never
   been received.  The Q bit signal may inform the attacker which packet
   numbers were skipped on purpose and which had been actually lost (and
   are, therefore, safe for the attacker to acknowledge).  To use the Q
   bit for this purpose, the attacker must first receive at least an
   entire Q Run of packets, which renders the attack ineffective against
   a delay-sensitive congestion controller.

   A protocol that is more susceptible to an Optimistic ACK Attack with
   the loss signal provided by Q bit and uses a loss-based congestion
   controller, SHOULD shorten the current Q Run by the number of skipped
   packets numbers.  For example, skipping a single packet number will
   invert the sQuare signal one outgoing packet sooner.

8.  Privacy Considerations

   To minimize unintentional exposure of information, loss bits provide
   an explicit loss signal - a preferred way to share information per

   New protocols commonly have specific privacy goals, and loss
   reporting must ensure that loss information does not compromise those
   privacy goals.  For example, [QUIC-TRANSPORT] allows changing
   Connection IDs in the middle of a connection to reduce the likelihood
   of a passive observer linking old and new subflows to the same
   device.  A QUIC implementation would need to reset all counters when
   it changes the destination (IP address or UDP port) or the Connection
   ID used for outgoing packets.  It would also need to avoid
   incrementing Unreported Loss counter for loss of packets sent to a
   different destination or with a different Connection ID.

9.  IANA Considerations

   This document makes no request of IANA.

10.  Change Log

10.1.  Since version 02

   -  Minor improvement and clarifications

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10.2.  Since version 01

   -  Clarified Q Period selection

   -  Added an optional E (ECN-Echo Event) bit

   -  Clarified L bit calculation for protocols that allow partial data
      loss due to a change in segmentation (such as TCP)

10.3.  Since version 00

   -  Addressed review comments

   -  Improved guidelines for privacy protections for QIUC

11.  Acknowledgments

   The sQuare bit was originally suggested by Kazuho Oku in early
   proposals for loss measurement and is an instance of the "alternate
   marking" as defined in [RFC8321].

12.  References

12.1.  Normative References

   [ConEx]    Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx)
              Concepts, Abstract Mechanism, and Requirements", RFC 7713,
              DOI 10.17487/RFC7713, December 2015,

              Kuehlewind, M., Ed. and R. Scheffenegger, "TCP
              Modifications for Congestion Exposure (ConEx)", RFC 7786,
              DOI 10.17487/RFC7786, May 2016,

   [ECN]      Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,

   [IP]       Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,

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   [IPv6]     Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC8321]  Fioccola, G., Ed., Capello, A., Cociglio, M., Castaldelli,
              L., Chen, M., Zheng, L., Mirsky, G., and T. Mizrahi,
              "Alternate-Marking Method for Passive and Hybrid
              Performance Monitoring", RFC 8321, DOI 10.17487/RFC8321,
              January 2018, <https://www.rfc-editor.org/info/rfc8321>.

   [RFC8558]  Hardie, T., Ed., "Transport Protocol Path Signals",
              RFC 8558, DOI 10.17487/RFC8558, April 2019,

   [TCP]      Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,

12.2.  Informative References

              Briscoe, B., Kuehlewind, M., and R. Scheffenegger, "More
              Accurate ECN Feedback in TCP", draft-ietf-tcpm-accurate-
              ecn-11 (work in progress), March 2020.

              Fioccola, G., Zhou, T., Cociglio, M., Qin, F., and R.
              Pang, "IPv6 Application of the Alternate Marking Method",
              draft-ietf-6man-ipv6-alt-mark-01 (work in progress), June

              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-29 (work
              in progress), June 2020.

   [RFC8517]  Dolson, D., Ed., Snellman, J., Boucadair, M., Ed., and C.
              Jacquenet, "An Inventory of Transport-Centric Functions
              Provided by Middleboxes: An Operator Perspective",
              RFC 8517, DOI 10.17487/RFC8517, February 2019,

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   [RFC8701]  Benjamin, D., "Applying Generate Random Extensions And
              Sustain Extensibility (GREASE) to TLS Extensibility",
              RFC 8701, DOI 10.17487/RFC8701, January 2020,

              Fairhurst, G. and C. Perkins, "Considerations around
              Transport Header Confidentiality, Network Operations, and
              the Evolution of Internet Transport Protocols", draft-
              ietf-tsvwg-transport-encrypt-16 (work in progress), July

              Touch, J., "Transport Options for UDP", draft-ietf-tsvwg-
              udp-options-08 (work in progress), September 2019.

              Herbert, T., "UDP Surplus Header", draft-herbert-udp-
              space-hdr-01 (work in progress), July 2019.

Authors' Addresses

   Alexandre Ferrieux (editor)
   Orange Labs

   EMail: alexandre.ferrieux@orange.com

   Isabelle Hamchaoui (editor)
   Orange Labs

   EMail: isabelle.hamchaoui@orange.com

   Igor Lubashev (editor)
   Akamai Technologies

   EMail: ilubashe@akamai.com

   Dmitri Tikhonov (editor)
   LiteSpeed Technologies

   EMail: dtikhonov@litespeedtech.com

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