TSVWG                                                        A. Ferrieux
Internet-Draft                                              I. Hamchaoui
Intended status: Informational                               Orange Labs
Expires: May 6, 2020                                         I. Lubashev
                                                     Akamai Technologies
                                                        November 3, 2019

             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
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   This Internet-Draft will expire on May 6, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   3
   3.  Loss Bits . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Setting the sQuare Bit on Outgoing Packets  . . . . . . .   4
       3.1.1.  Q Period Selection  . . . . . . . . . . . . . . . . .   4
     3.2.  Setting the Loss Event Bit on Outgoing Packets  . . . . .   4
   4.  Using the Loss Bits for Passive Loss Measurement  . . . . . .   5
     4.1.  End-To-End Loss . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Upstream Loss . . . . . . . . . . . . . . . . . . . . . .   6
     4.3.  Correlating End-to-End and Upstream Loss  . . . . . . . .   6
     4.4.  Downstream Loss . . . . . . . . . . . . . . . . . . . . .   7
     4.5.  Observer Loss . . . . . . . . . . . . . . . . . . . . . .   7
   5.  ECN-Echo Event Bit  . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Setting the ECN-Echo Event Bit on Outgoing Packets  . . .   8
     5.2.  Using E Bit for Passive ECN-Reported Congestion
           Measurement . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Protocol Ossification Considerations  . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   9
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   10. Change Log  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Since version 01 . . . . . . . . . . . . . . . . . . . .   9
     10.2.  Since version 00 . . . . . . . . . . . . . . . . . . . .   9
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     12.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Packet loss is a pervasive problem of day-to-day network operation,
   and 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, and the
   lossy segment (upstream or downstream from the observation point) can
   be quickly identified by moving the passive observer around.

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

   Since encrypted protocols could be routed by the network differently,
   and the fraction of Internet traffic delivered using encrypted
   protocols is increasing every year, it is imperative to measure
   packet loss experienced by encrypted protocol users directly instead
   of relying on measuring TCP loss between similar endpoints.

   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 for users of encrypted protocols.  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, UDP surplus space (see [UDP-OPTIONS] and
   [UDP-SURPLUS]), reserved bits in a QUIC v1 header (see

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 proposal introduces two bits that are to be present in every
   packet 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 connection (each subflow for multipath

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3.1.  Setting the sQuare Bit on Outgoing Packets

   The sQuare Value is initialized to the Initial Q Value (0 or 1) and
   is reflected in the Q bit of every outgoing packet.  The sQuare value
   is inverted after sending every N packets (Q Period is 2*N).  The Q
   bit represents "packet color" as defined by [RFC8321].

   The choice of the Initial Q Value and Q Period is determined by the
   protocol containing Q and L bits.  For example, the values can be
   protocol constants (e.g.  "Initial Q Value" is 0, and "Q Period" is
   128), or they can be set explicitly for each connection (e.g.
   "Initial Q Value" is whatever value the initial packet has, and "Q
   Period" is set per a dedicated TCP option on SYN and SYN/ACK), or
   they can be included with every packet (e.g.  ConEx IPv6 Destination
   Option of [ConEx-IPv6]).

   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 Period Selection

   A protocol containing Q and L bits can allow the sender to choose Q
   Period based on the expected amount of loss and reordering on the
   path (see Section 4.2).  If the explicit value of the Q Period is not
   explicitly signaled by the protocol, the Q Period value MUST be at
   least 128 and be a power of 2.  This requirement allows an Observer
   to infer the Q Period by obsering 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).

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, the Unreported Loss counter SHOULD NOT
   be decremented 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,

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   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's loss detection machinery, by counting packets
   in this direction with a L bit equal to 1, as described in Section 4.

4.  Using the Loss Bits for Passive Loss Measurement

   There are three sources of observable loss:

   -  _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).

   The Q and L bits allow detection and measurement of the types of loss
   listed above.

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
   connection.  The end-to-end loss rate is the fraction of packets with

   The simplifying assumption here is that upstream loss affects packets
   with L=0 and L=1 equally.  This may be a simplification, if some loss
   is caused by tail-drop in a network device.  If the sender 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.

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4.2.  Upstream Loss

   Blocks of N (half of Q Period) consecutive packets are sent with the
   same value of the Q bit, followed by another block of N packets with
   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.  If "p" is the average number of
   packets in a block of packets with the same Q value, then the
   upstream loss is "1-p/N".

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

   The Q Period needs to be chosen carefully, since the observation
   could become too unreliable in case of packet reordering and loss if
   Q Period is too small.  However, when Q Period is too large,
   connections that send fewer than half Q Period packets do not yield a
   useful upstream loss measurement.

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

4.3.  Correlating End-to-End and Upstream Loss

   Upstream loss is calculated by observing the actual 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 connection 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 connection,
   it should accumulate loss measurements over time periods of at least
   4 times the typical RTT for the observed connections.

   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

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   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
   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.

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 intrumental.

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

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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].

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 connections
   is important for the operation of the networks.

   The loss bits are amenable to "greasing" described in [GREASE], 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 connections 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 connections with loss
   information more resembling noise than the expected signal.

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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.

8.  Privacy Considerations

   Guarding user's privacy is an important goal for modern protocols and
   protocol extensions per [RFC7258].  While an explicit loss signal - a
   preferred way to share loss information per [RFC8558] - helps to
   minimize unintentional exposure of additional information,
   implementations of loss reporting must ensure that loss information
   does not compromise protocol's 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
   destinatoin or with a different Connection ID.

9.  IANA Considerations

   This document makes no request of IANA.

10.  Change Log

10.1.  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)

10.2.  Since version 00

   -  Addressed review comments

   -  Improved guidelines for privacy protections for QIUC

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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,

              Krishnan, S., Kuehlewind, M., Briscoe, B., and C. Ralli,
              "IPv6 Destination Option for Congestion Exposure (ConEx)",
              RFC 7837, DOI 10.17487/RFC7837, May 2016,

              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,

   [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,

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   [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-09 (work in progress), July 2019.

   [GREASE]   Benjamin, D., "Applying GREASE to TLS Extensibility",
              draft-ietf-tls-grease-04 (work in progress), August 2019.

              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-24 (work
              in progress), September 2019.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <https://www.rfc-editor.org/info/rfc7258>.

   [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,

              Fairhurst, G. and C. Perkins, "The Impact of Transport
              Header Confidentiality on Network Operation and Evolution
              of the Internet", draft-ietf-tsvwg-transport-encrypt-09
              (work in progress), August 2019.

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

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

Authors' Addresses

   Alexandre Ferrieux
   Orange Labs

   EMail: alexandre.ferrieux@orange.com

   Isabelle Hamchaoui
   Orange Labs

   EMail: isabelle.hamchaoui@orange.com

   Igor Lubashev
   Akamai Technologies
   150 Broadway
   Cambridge, MA  1122

   EMail: ilubashe@akamai.com

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