QUIC                                                    A. Ferrieux, Ed.
Internet-Draft                                         I. Hamchaoui, Ed.
Intended status: Informational                               Orange Labs
Expires: April 12, 2020                                 I. Lubashev, Ed.
                                                     Akamai Technologies
                                                        October 10, 2019

                           The QUIC Loss Bits


   This draft adapts the general technique described in draft-
   ferrieuxhamchaoui-tsvwg-lossbits for QUIC using reserved bits in QUIC
   v1 header.  It describes a 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.

Status of This Memo

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   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  . . . . . . .   3
     3.2.  Setting the Loss Event Bit on Outgoing Packets  . . . . .   3
   4.  Using the Loss Bits for Passive Loss Measurement  . . . . . .   4
     4.1.  End-To-End Loss . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  Upstream Loss . . . . . . . . . . . . . . . . . . . . . .   5
     4.3.  Correlating End-to-End and Upstream Loss  . . . . . . . .   5
     4.4.  Downstream Loss . . . . . . . . . . . . . . . . . . . . .   6
     4.5.  Observer Loss . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Ossification Considerations . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     10.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Packet loss is a hard and 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 SACK when enabled.  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 QUIC, the equivalent transport headers are encrypted and passive
   packet loss observation is not possible, as described in

   QUIC could be routed by the network differently and the fraction of
   Internet traffic delivered using QUIC is increasing every year.
   Therefore, is it imperative to measure packet loss experienced by

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   QUIC users directly instead of relying on measuring TCP loss between
   similar endpoints.

   Since explicit path signals are preferred by [RFC8558], this document
   proposes adding two explicit loss bits to the clear portion of short
   headers to restore network operators' ability to maintain high QoS
   for QUIC users.

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 packets
   with a short header.  Therefore, only loss of short header packets is
   reported using loss bits.  Whenever this specification refers to
   packets, it is referring only to packets with short headers.

   -  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 4-tuple and destination Connection ID.

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), where N
   is a parameter of the method, discussed below.

   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.

3.2.  Setting the Loss Event Bit on Outgoing Packets

   The Unreported Loss counter is initialized to 0, and the 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).

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   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 QUIC's existing loss
   detection machinery.

   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 loss after observing at
   least N packets.  The upstream loss rate is one minus the average
   number of packets in a block of packets with the same Q value divided
   by N.

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

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

   The choice of N strikes a compromise: the observation could become
   too unreliable in case of packet reordering and loss if N is too
   small; and when N is too large, short connections may not yield a
   useful upstream loss measurement.

   To leave some room for adaptation, we only constrain the sender to
   select an N that is (1) constant for a given connection and (2) equal
   to a power of two.  The latter allows on-path observers to derive N
   after a few periods.  It is thus also acceptable for a simple
   implementation to choose a global constant; N=64 has been extensively
   tried in large-scale field tests and yielded good results.

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

   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,

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   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
   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.  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
   [I-D.ietf-tls-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

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   greasing in [I-D.ietf-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.

6.  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.

7.  Privacy Considerations

   Guarding user's privacy is an important goal for modern protocols and
   protocol extensions per [RFC7285].  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, [I-D.ietf-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
   Connection ID used for outgoing packets.  It would also need to avoid
   incrementing Unreported Loss counter for loss of packets sent with a
   different Connection ID.

8.  IANA Considerations

   This document makes no request of IANA.

9.  Acknowledgments

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

10.  References

10.1.  Normative References

   [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,

10.2.  Informative References

              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-23 (work
              in progress), September 2019.

              Benjamin, D., "Applying GREASE to TLS Extensibility",
              draft-ietf-tls-grease-04 (work in progress), August 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-08
              (work in progress), August 2019.

   [RFC7285]  Alimi, R., Ed., Penno, R., Ed., Yang, Y., Ed., Kiesel, S.,
              Previdi, S., Roome, W., Shalunov, S., and R. Woundy,
              "Application-Layer Traffic Optimization (ALTO) Protocol",
              RFC 7285, DOI 10.17487/RFC7285, September 2014,

Authors' Addresses

   Alexandre Ferrieux (editor)
   Orange Labs
   2 av P.Marzin

   EMail: alexandre.ferrieux@orange.com

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   Isabelle Hamchaoui (editor)
   Orange Labs
   2 av P.Marzin

   EMail: isabelle.hamchaoui@orange.com

   Igor Lubashev (editor)
   Akamai Technologies
   150 Broadway
   Cambridge, MA  1122

   EMail: ilubashe@akamai.com

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