QUIC                                                    B. Trammell, Ed.
Internet-Draft                                               P. De Vaere
Intended status: Informational                                ETH Zurich
Expires: November 15, 2018                                       R. Even
                                                             G. Fioccola
                                                          Telecom Italia
                                                              T. Fossati
                                                                M. Ihlar
                                                               A. Morton
                                                               AT&T Labs
                                                              E. Stephan
                                                            May 14, 2018

  Adding Explicit Passive Measurability of Two-Way Latency to the QUIC
                           Transport Protocol


   This document describes the addition of a "spin bit", intended for
   explicit measurability of end-to-end RTT, to the QUIC transport
   protocol.  It proposes a detailed mechanism for the spin bit, as well
   as an additional mechanism, called the valid edge counter, to
   increase the fidelity of the latency signal in less than ideal
   network conditions.  It describes how to use the latency spin signal
   to measure end-to-end latency, discusses corner cases and their
   workarounds in the measurement, describes experimental evaluation of
   the mechanism done to date, and examines the utility and privacy
   implications of the spin bit.

Status of This Memo

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   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 November 15, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  About This Document . . . . . . . . . . . . . . . . . . .   4
   2.  The Spin Bit Mechanism  . . . . . . . . . . . . . . . . . . .   4
   3.  Using the Spin Bit for Passive RTT Measurement  . . . . . . .   5
     3.1.  Limitations and Workarounds . . . . . . . . . . . . . . .   5
     3.2.  Illustration  . . . . . . . . . . . . . . . . . . . . . .   6
   4.  The Valid Edge Counter  . . . . . . . . . . . . . . . . . . .   8
     4.1.  Proposed Short Header Format Including Spin Bit and VEC .   8
     4.2.  Setting the Valid Edge Counter (VEC)  . . . . . . . . . .   9
     4.3.  Use of the VEC by a passive observer  . . . . . . . . . .  10
   5.  Privacy and Security Considerations . . . . . . . . . . . . .  10
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  12
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Appendix A.  Experimental Evaluation  . . . . . . . . . . . . . .  15
   Appendix B.  Use Cases for Passive RTT Measurement  . . . . . . .  16
     B.1.  Inter-domain Troubleshooting  . . . . . . . . . . . . . .  17
     B.2.  Two-Point Intradomain Measurement . . . . . . . . . . . .  18
     B.3.  Bufferbloat Mitigation in Cellular Networks . . . . . . .  19
     B.4.  Locating WiFi Problems in Home Networks . . . . . . . . .  19
     B.5.  Internet Measurement Research . . . . . . . . . . . . . .  20
   Appendix C.  Alternate RTT Measurement Approaches for Diagnosing
                QUIC flows . . . . . . . . . . . . . . . . . . . . .  20
     C.1.  Handshake RTT measurement . . . . . . . . . . . . . . . .  20
     C.2.  Parallel active measurement . . . . . . . . . . . . . . .  21

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     C.3.  Frequency Analysis  . . . . . . . . . . . . . . . . . . .  21
   Appendix D.  Greasing . . . . . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

   The QUIC transport protocol [QUIC-TRANS] is a UDP-encapsulated
   protocol integrated with Transport Layer Security (TLS) [TLS] to
   encrypt most of its protocol internals, beyond those handshake
   packets needed to establish or resume a TLS session, and information
   required to reassemble QUIC streams (the packet number) and to route
   QUIC packets to the correct machine in a load-balancing situation
   (the connection ID).  In contrast to TCP, QUIC's wire image (see
   [WIRE-IMAGE]) exposes much less information about transport protocol
   state than TCP's wire image.  Specifically, the fact that sequence
   and acknowledgement numbers and timestamps (available in TCP) cannot
   be seen by on-path observers in QUIC means that passive TCP loss and
   latency measurement techniques that rely on this information (e.g.
   [CACM-TCP], [TMA-QOF]) cannot be easily ported to work with QUIC.

   This document proposes a solution to this problem by adding a
   "latency spin bit" to the QUIC short header.  This bit is designed
   solely for explicit passive measurability of the protocol.  It
   provides one RTT sample per RTT to passive observers of QUIC traffic.
   This document describes the mechanism, how it can be added to QUIC,
   and how it can be used by passive measurement facilities to generate
   RTT samples.  It explores potential corner cases and shortcomings of
   the mechanism, and proposes an extention called the Valid Edge
   Counter (VEC) to mitigate them.  It further details findings on
   privacy risk researched by the QUIC RTT Design Team, which was tasked
   by the IETF QUIC Working Group to determine the risk/utility tradeoff
   for the spin bit.

   Appendices summarize experimental results to date with an
   implementation of the spin bit built atop a recent QUIC
   implementation, describe use cases for passive RTT measurement at the
   resolution provided by the spin bit, explore alternatives to the spin
   bit for passive latency measurement of QUIC flows, and discuss the
   necessity of "greasing" the spin bit.

   The spin bit has low overhead, presents negligible privacy risk, and
   has clear utility in providing passive RTT measurability of QUIC that
   is far superior to QUIC's measurability without the spin bit, and
   equivalent to or better than TCP passive measurability.

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1.1.  About This Document

   [QUIC-SPIN-EXP] specifies the addition of the spin bit to the QUIC
   transport protocol for experimental purposes.  This document provides
   background for that specification, documents work done in the
   development of the spin bit proposal, and extends it with the VEC
   signal for loss, reordering, and delay compensation without relying
   on the QUIC packet number.

   This document is maintained in the GitHub repository
   https://github.com/britram/draft-trammell-quic-spin, and the editor's
   copy is available online at https://britram.github.io/draft-trammell-
   quic-spin.  Current open issues on the document can be seen at
   https://github.com/britram/draft-trammell-quic-spin/issues.  Comments
   and suggestions on this document can be made by filing an issue
   there, or by contacting the editor.

2.  The Spin Bit Mechanism

   The latency spin bit enables latency monitoring from observation
   points on the network path.  Each endpoint, client and server,
   maintains a spin value, 0 or 1, for each QUIC connection, and sets
   the spin bit on packets it sends for that connection to the
   appropriate value (below).  It also maintains the highest packet
   number seen from its peer on the connection.  The value is then
   determined at each endpoint as follows:

   o  The server initializes its spin value to 0.  When it receives a
      packet from the client, if that packet has a short header and if
      it increments the highest packet number seen by the server from
      the client, it sets the spin value to the spin bit in the received

   o  The client initializes its spin value to 0.  When it receives a
      packet from the server, if the packet has a short header and if it
      increments the highest packet number seen by the client from the
      server, it sets the spin value to the opposite of the spin bit in
      the received packet.

   This procedure will cause the spin bit to change value in each
   direction once per round trip.  Observation points can estimate the
   network latency by observing these changes in the latency spin bit,
   as described in Section 3.  See Section 3.2 for an illustration of
   this mechanism in action.

   The defails of the addition of the spin bit to the QUIC short header
   are given in [QUIC-SPIN-EXP].

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3.  Using the Spin Bit for Passive RTT Measurement

   When a QUIC flow is sending at full rate (i.e., neither application
   nor flow control limited), the latency spin bit in each direction
   changes value once per round-trip time (RTT).  An on-path observer
   can observe the time difference between edges in the spin bit signal
   in a single direction to measure one sample of end-to-end RTT.  Note
   that this measurement, as with passive RTT measurement for TCP,
   includes any transport protocol delay (e.g., delayed sending of
   acknowledgements) and/or application layer delay (e.g., waiting for a
   request to complete).  It therefore provides devices on path a good
   instantaneous estimate of the RTT as experienced by the application.
   A simple linear smoothing or moving minimum filter can be applied to
   the stream of RTT information to get a more stable estimate.

   An on-path observer that can see traffic in both directions (from
   client to server and from server to client) can also use the spin bit
   to measure "upstream" and "downstream" component RTT; i.e, the
   component of the end-to-end RTT attributable to the paths between the
   observer and the server and the observer and the client,
   respectively.  It does this by measuring the delay between a spin
   edge observed in the upstream direction and that observed in the
   downstream direction, and vice versa.

3.1.  Limitations and Workarounds

   Application-limited and flow-control-limited senders can have
   application and transport layer delay, respectively, that are much
   greater than network RTT.  Therefore, the spin bit provides network
   latency information only when the sender is neither application nor
   flow control limited.  When the sender is application-limited by
   periodic application traffic, where that period is longer than the
   RTT, measuring the spin bit provides information about the
   application period, not the RTT.  Simple heuristics based on the
   observed data rate per flow or changes in the RTT series can be used
   to reject bad RTT samples due to application or flow control

   Since the spin bit logic at each endpoint considers only samples on
   packets that advance the largest packet number seen, signal
   generation itself is resistant to reordering.  However, reordering
   can cause problems at an observer by causing spurious edge detection
   and therefore low RTT estimates, if reordering occurs across a spin
   bit flip in the stream.  This can be probabilistically mitigated by
   the observer also tracking the low-order bits of the packet number,
   and rejecting edges that appear out-of-order [RFC4737].

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   All of these limitations are addressed by an enhancement to the spin
   bit, the Valid Edge Counter, described in detail in Section 4.

3.2.  Illustration

   To illustrate the operation of the spin bit, we consider a simplified
   model of a single path between client and server as a queue with
   slots for five packets, and assume that both client and server sent
   packets at a constant rate.  If each packet moves one slot in the
   queue per clock tick, note that this network has a RTT of 10 ticks.

   Initially, during connection establishment, no packets with a spin
   bit are in flight, as shown in Figure 1.

   +--------+   -  -  -  -  -   +--------+
   |        |     -------->     |        |
   | Client |                   | Server |
   |        |     <--------     |        |
   +--------+   -  -  -  -  -   +--------+

      Figure 1: Initial state, no spin bit between client and server

   Either the server, the client, or both can begin sending packets with
   short headers after connection establishment, as shown in Figure 2;
   here, no spin edges are yet in transit.

   +--------+   0  0  -  -  -   +--------+
   |        |     -------->     |        |
   | Client |                   | Server |
   |        |     <--------     |        |
   +--------+   -  -  0  0  0   +--------+

       Figure 2: Client and server begin sending packets with spin 0

   Once the server's first 0-marked packet arrives at the client, the
   client sets its spin value to 1, and begins sending packets with the
   spin bit set, as shown in Figure 3.  The spin edge is now in transit
   toward the server.

   +--------+   1  0  0  0  0   +--------+
   |        |     -------->     |        |
   | Client |                   | Server |
   |        |     <--------     |        |
   +--------+   0  0  0  0  0   +--------+

                     Figure 3: The bit begins spinning

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   Five ticks later, this packet arrives at the server, which takes its
   spin value from it and reflects that value back on the next packet it
   sends, as shown in Figure 4.  The spin edge is now in transit toward
   the client.

   +--------+   1  1  1  1  1   +--------+
   |        |     -------->     |        |
   | Client |                   | Server |
   |        |     <--------     |        |
   +--------+   0  0  0  0  1   +--------+

                  Figure 4: Server reflects the spin edge

   Five ticks later, the 1-marked packet arrives at the client, which
   inverts its spin value and sends the inverted value on the next
   packet it sends, as shown in Figure 5.

         obs. points  X  Y
   +--------+   0  1  1  1  1   +--------+
   |        |     -------->     |        |
   | Client |                   | Server |
   |        |     <--------     |        |
   +--------+   1  1  1  1  1   +--------+

                  Figure 5: Client inverts the spin edge

   Here we can also see how measurement works.  An observer watching the
   signal at single observation point X in Figure 5 will see an edge
   every 10 ticks, i.e.  once per RTT.  An observer watching the signal
   at a symmetric observation point Y in Figure 5 will see a server-
   client edge 4 ticks after the client-server edge, and a client-server
   edge 6 ticks after the server-client edge, allowing it to compute
   component RTT.

   Figure 6 shows how this mechanism works in the presence of
   reordering.  Here, packet C carries the spin edge, and packet B is
   reordered on the way to the client.  In this case, the client will
   begin sending spin 1 after the arrival of C, and ignore the spin bit
   flip to 1 on packet B, since B < C; i.e. it does not increment the
   highest packet number seen.

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   +--------+   0  0  0  0  0   +--------+
   |        |     -------->     |        |
   | Client |                   | Server |
   |        |     <--------     |        |
   +--------+   1  0  1  0  0   +--------+
       PN=      A  C  B  D  E

                       Figure 6: Handling reordering

4.  The Valid Edge Counter

   This mechanism is indented to provide additional information about
   the validity of the passively observed spin edges without using
   information from a cleartext packet number.

   A one-bit spin signal is resistent to reordering during signal
   generation, since the spin value is only updated at each endpoint on
   a packet that advances the packet counter.  However, without using
   the packet number, a passive observer can neither detect reordered
   nor lost edges, and it must use heuristics to reject delayed edges.

   The Valid Edge Counter (VEC) addresses these issues with two
   additional bits added to each packet, encoding values from 0 to 3,
   indicating that an edge was considered to be valid when send out by
   the sender, and providing a possibility to detect invalid edges due
   to reordering and edge loss.

4.1.  Proposed Short Header Format Including Spin Bit and VEC

   As of the current editor's version of [QUIC-TRANS], this proposal
   specifies using bit 0x04 of the first octet in the short header for
   the spin bit, and the bits 0x03 for the valid edge counter.  Note
   that these values are subject to change as the layout of the first
   octet is finalized.

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   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |                Destination Connection ID (0..144)           ...
   |                      Packet Number (8/16/32)                ...
   |                     Protected Payload (*)                   ...

              Figure 7: Short Header Format Spin Bit and VEC

   S: The Spin bit is set 0 or 1 depending on the stored spin value that
   is updated on packet reception as explained in Section 2.

   VEC: The Valid Edge Counter is set as defined in Section 4.2.  If the
   spin bit field does not contain an edge, the VEC is set to 0.

4.2.  Setting the Valid Edge Counter (VEC)

   The VEC is set by each endpoint as follows; unlike the spin bit, note
   that there is no difference between client and server handling of the

   o  By default, the VEC is set to 0.

   o  If a packet contains an edge (transition 0->1 or 1->0) in the spin
      signal, and that edge is delayed (sent more than a configured
      delay since the edge was received, defaulting to 1ms), the VEC is
      set to 1.

   o  If a packet contains an edge in the spin signal, and that edge is
      not delayed, the VEC is set to the value of the VEC that
      accompanied the last incoming spin bit transition plus one.  This
      counter holds at 3, instead of cycling around.  In other words, an
      edge received with a VEC of 0 will be reflected as an edge with a
      VEC of 1; with a VEC of 1 as VEC of 2, and a VEC of 2 or 3 as a
      VEC of 3.

   This mechanism allows observers to recognize spurious edges due to
   reordering and delayed edges due to loss, since these packets will
   have been sent with VEC 0: they were not edges when they were sent.
   In addition, it allows senders to signal that a valid edge was
   delayed because the sender was application-limited: these edges are

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   sent with the VEC set to 1 by the sender, prompting the VEC to count
   back up over the next RTT.

4.3.  Use of the VEC by a passive observer

   The VEC can be used by observers to determine whether an edge in the
   spin bit signal is valid or not, as follows:

   o  A packet containing an apparent edge in the spin signal with a VEC
      of 0 is not a valid edge, but may be have been caused by
      reordering or loss, or was marked as delayed by the sender.  It
      should therefore be ignored.

   o  A packet containing an apparent edge in the spin signal with a VEC
      of 1 can be used as a left edge (i.e., to start measuring an RTT
      sample), but not as a right edge (i.e., to take an RTT sample
      since the last edge).

   o  A packet containing an apparent edge in the spin signal with a VEC
      of 2 can be used as a left edge, but not as a right edge.  If the
      observation point is symmetric (i.e, it can see both upstream and
      downstream packets in the flow), the packet can also be used to
      take a component RTT sample on the segment of the path between the
      observation point and the direction in which the previous VEC 1
      edge was seen.

   o  A packet containing an apparent edge in the spin signal with a VEC
      of 3 can be used as a left edge or right edge, and can be used to
      compute component RTT in either direction.

5.  Privacy and Security Considerations

   The privacy considerations for the latency spin bit are essentially
   the same as those for passive RTT measurement in general.

   A concern was raised during the discussion of this feature within the
   QUIC working group and the QUIC RTT Design Team that high-resolution
   RTT information might be usable for geolocation.  However, an
   evaluation based on RTT samples taken over 13,780 paths in the
   Internet from RIPE Atlas anchoring measurements [TRILAT] shows that
   the magnitude and uncertainty of RTT data limit the resolution of
   geolocation information that can be derived from Internet RTT to
   national- or continental-scale; i.e., less resolution than is
   generally available from free, open IP geolocation databases.

   One reason for the inaccuracy of geolocation from network RTT is that
   Internet backbone transmission facilities do not follow the great-
   circle path between major nodes.  Instead, major geographic features

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   and the efficiency of connecting adjacent major cities both influence
   the facility routing.  An evaluation of ~3500 measurements on a mesh
   of 25 backbone nodes in the continental United States shows that 85%
   had RTT to great-circle error of 3ms or more, making location within
   US State boundaries ambiguous [CONUS].

   Therefore, in the general case, when an endpoint's IP address is
   known, RTT information provides negligible additional information.

   RTT information may be used to infer the occupancy of queues along a
   path; indeed, this is part of its utility for performance measurement
   and diagnostics.  When a link on a given path has excessive buffering
   (on the order of hundreds of milliseconds or more), such that the
   difference in delay between an empty queue and a full queue dwarfs
   normal variance and RTT along the path, RTT variance during the
   lifetime of a flow can be used to infer the presence of traffic on
   the bottleneck link.  In practice, however, this is not a concern for
   passive measurement of congestion-controlled traffic, since any
   observer in a situation to observe RTT passively need not infer the
   presence of the traffic, as it can observe it directly.

   In addition, since RTT information contains application as well as
   network delay, patterns in RTT variance from minimum, and therefore
   application delay, can be used to infer or fingerprint application-
   layer behavior.  However, as with the case above, this is not a
   concern with passive measurement, since the packet size and
   interarrival time sequence, which is also directly observable,
   carries more information than RTT variance sequence.

   We therefore conclude that the high-resolution, per-flow exposure of
   RTT for passive measurement as provided by the spin bit poses
   negligible marginal risk to privacy.

   As shown in Section 2, the spin bit can be implemented separately
   from the rest of the mechanisms of the QUIC transport protocol, as it
   requires no access to any state other than that observable in the
   QUIC packet header itself.  We recommend that implementations take
   advantage of this property, to reduce the risk that errors in the
   implementation could leak private transport protocol state through
   the spin bit.

   Since the spin bit is disconnected from transport mechanics, a QUIC
   endpoint implementing the spin bit that has a model of the actual
   network RTT and a target RTT to expose can "lie" about its spin bit
   transitions, by anticipating or delaying observed transitions, even
   without coordination with and the collusion of the other endpoint.
   This is not the case with TCP, which requires coordination and
   collusion to expose false information via its sequence and

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   acknowledgment numbers and its timestamp option.  When passive
   measurement is used for purposes where one endpoint might gain a
   material advantage by representing a false RTT, e.g.  SLA
   verification or enforcement of telecommunications regulations, this
   situation raises a question about the trustworthiness of spin bit RTT

   This issue must be appreciated by users of spin bit information, but
   mitigation is simple, as QUIC implementations designed to lie about
   RTT through spin bit modification can easily be detected.  A lying
   server can be contacted by an honest client under the control of a
   verifying party, and the client's RTT estimate compared with the
   spin-bit exposed estimate.  Though in the general case, it is
   impossible to verify explicit path signals with two complicit
   endpoints (see [WIRE-IMAGE]), a lying server/client pair may be
   subject to dynamic analysis along paths with known RTTs.  We consider
   the ease of verification of lying in situations where this would be
   prohibited by regulation or contract, combined with the consequences
   of violation of said regulation or contract, to be a sufficient
   incentive in the general case not to do it.

6.  Acknowledgments

   Many thanks to Christian Huitema, who originally proposed the spin
   bit as pull request 609 on [QUIC-TRANS].  Thanks to Tobias Buehler
   for feedback on the draft, and for Alexandre Ferrieux for input on
   the Valid Edge Counter.  Special thanks to the QUIC RTT Design Team
   for discussions leading especially to the privacy and security
   considerations section.

   This work is partially supported by the European Commission under
   Horizon 2020 grant agreement no. 688421 Measurement and Architecture
   for a Middleboxed Internet (MAMI), and by the Swiss State Secretariat
   for Education, Research, and Innovation under contract no. 15.0268.
   This support does not imply endorsement.

7.  References

7.1.  Normative References

              Trammell, B. and M. Kuehlewind, "The QUIC Latency Spin
              Bit", draft-ietf-quic-spin-exp (work in progress).

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7.2.  Informative References

              Fioccola, G., Capello, A., Cociglio, M., Castaldelli, L.,
              Chen, M., Zheng, L., Mirsky, G., and T. Mizrahi,
              "Alternate Marking method for passive and hybrid
              performance monitoring", draft-ietf-ippm-alt-mark-14 (work
              in progress), December 2017.

              Strowes, S., "Passively Measuring TCP Round-Trip Times (in
              Communications of the ACM)", October 2013.

              Carra, D., Avrachenkov, K., Alouf, S., Blanc, A., Nain,
              P., and G. Post, "Passive Online RTT Estimation for Flow-
              Aware Routers Using One-Way Traffic (NETWORKING 2010, LNCS
              6091, pp. 109-121)", 2010.

   [CONUS]    Morton, A., "Comparison of Backbone Node RTT and Great
              Circle Distances (https://github.com/acmacm/CONUS-RTT)",
              September 2017.

              Luckie, M., Dhamdhere, A., Clark, D., Huffaker, B., and k.
              claffy, "Challenges in Inferring Internet Interdomain
              Congestion (in Proc. ACM IMC 2014)", November 2014.

              Sundaresan, S., Dhamdhere, A., Allman, M., and . k claffy,
              "TCP Congestion Signatures (in Proc. ACM IMC 2017)", n.d..

   [MINQ]     Rescorla, E., "MINQ, a simple Go implementation of QUIC
              (https://github.com/ekr/minq)", November 2017.

              Trammell, B., "Mokumokuren, a lightweight flow meter using
              gopacket (https://github.com/britram/mokumokuren)",
              November 2017.

   [NOSPIN]   Morton, A., "Description of a tool chain to evaluate
              Unidirectional Passive RTT measurement (and results)
              (https://github.com/acmacm/PassiveRTT)", October 2017.

              Kuehlewind, M. and B. Trammell, "Manageability of the QUIC
              Transport Protocol", draft-ietf-quic-manageability-01
              (work in progress), October 2017.

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              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-11 (work
              in progress), April 2018.

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,

   [RFC4433]  Kulkarni, M., Patel, A., and K. Leung, "Mobile IPv4
              Dynamic Home Agent (HA) Assignment", RFC 4433,
              DOI 10.17487/RFC4433, March 2006,

   [RFC4737]  Morton, A., Ciavattone, L., Ramachandran, G., Shalunov,
              S., and J. Perser, "Packet Reordering Metrics", RFC 4737,
              DOI 10.17487/RFC4737, November 2006,

   [RFC5357]  Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
              Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
              RFC 5357, DOI 10.17487/RFC5357, October 2008,

   [RFC6049]  Morton, A. and E. Stephan, "Spatial Composition of
              Metrics", RFC 6049, DOI 10.17487/RFC6049, January 2011,

   [RFC7799]  Morton, A., "Active and Passive Metrics and Methods (with
              Hybrid Types In-Between)", RFC 7799, DOI 10.17487/RFC7799,
              May 2016, <https://www.rfc-editor.org/info/rfc7799>.

   [SHBAIR]   Shbair, W., Cholez, T., Francois, J., and I. Chrisment, "A
              multi-level framework to identify HTTPS services (in Proc.
              IEEE/IFIP NOMS)", April 2016.

              De Vaere, P., "Latency Spinbit Implementation Experience
              (https://devae.re/f/eth/quic/spinbit_report/)", November

   [TLS]      Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-28 (work in progress),
              March 2018.

   [TMA-QOF]  Trammell, B., Gugelmann, D., and N. Brownlee, "Inline Data
              Integrity Signals for Passive Measurement (in Proc. TMA
              2014)", April 2014.

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              Pelsser, C., Cittadini, L., Vissicchio, S., and R. Bush,
              "From Paris to Tokyo - On the Suitability of ping to
              Measure Latency (In Proc. ACM IMC 2014)", October 2014.

   [TRILAT]   Trammell, B., "On the Suitability of RTT Measurements for
              1/paper.ipynb)", August 2017.

              Trammell, B. and M. Kuehlewind, "The Wire Image of a
              Network Protocol", draft-trammell-wire-image-04 (work in
              progress), April 2018.

              Alfredsson, S., Giudice, G., Garcia, J., Brunstrom, A.,
              Cicco, L., and S. Mascolo, "Impact of TCP Congestion
              Control on Bufferbloat in Cellular Networks (in Proc. IEEE
              WoWMoM 2013)", June 2013.

Appendix A.  Experimental Evaluation

   We have evaluated the effectiveness of the spin bit in an emulated
   network environment.  The spin bit was added to a fork of [MINQ],
   using the mechanism described in Section 2, but with the spin bit
   appearing in a measurement byte added to the header for passive
   measurability experiments.  Spin bit measurement support was added to
   [MOKUMOKUREN].  Full results of these ongoing experiments are
   available online in [SPINBIT-REPORT], but we summarize our findings

   First, we confirm that the spin bit works as advertised: it provides
   one useful RTT sample per RTT to any passive observer of the flow.
   This sample tracks each sender's local instantaneous estimate of RTT
   as well as the expected RTT (i.e., defined by the emulation) fairly
   well.  One surprising implication of this is that the spin bit
   provides _more_ information than is available by local estimation to
   an endpoint which is mostly receiving data frames and sending mainly
   ACKs, and as such can also be useful in purely endpoint-local
   observations of the RTT evolution during the flow.  The spin bit also
   works correctly under moderate to heavy packet loss and jitter.

   Second, we confirm that the spin bit can be easily implemented
   without requiring deep integration into a QUIC implementation.
   Indeed, it could be implemented completely independently, as a shim,
   aside from the requirement that the spin bit value be integrity-
   protected along with the rest of the QUIC header.

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   Third, we performed experiments focused on the intermittent-sender
   problem described in Section 3.1.  We confirm that the spin bit does
   not provide useful RTT samples after the handshake when packets are
   only sent intermittently.  Simple heuristics can be used to recognize
   this situation, however, and to reject these RTT samples.  We also
   find that a simple sender-side heuristic can be used to determine
   whether a sample will be useful.  If a sender sends a packet more
   than a specified delay (e.g. 1ms) after the last packet received by
   the client, it knows that any latency spin observation of that packet
   will be invalid.  If a second "spin valid" bit were available, the
   sender could then mark that packet "spin invalid".  Our experiments
   show that this simple heuristic and spin validity bit are successful
   in marking all packets whose RTT samples should be rejected.

   Fourth, we performed experiments focused on the reordering problem
   described in Section 3.1.  We find that while reordering can cause
   spurious samples at a naive observer, two simple approaches can be
   used to reject spurious RTT samples due to reordering.  First, a two-
   bit spin signal that always advances in a single direction (e.g. 00
   -> 01 -> 10 -> 11) successfully rejects all reordered samples,
   including under amounts of reordering that render the transport
   itself mostly useless.  However, adding a bit is not necessary:
   having the observer keep the least significant bits of the packet
   number, and rejecting samples from packets that reverse the sequence
   [RFC4737], as suggested in Section 3.1, is essentially as successful
   as a two-bit spin signal in mitigating the effects of reordering on
   RTT measurement.

   Fifth, we performed parallel active measurements using ping, as
   described in Appendix C.2.  In our emulated network, the ICMP packets
   and the QUIC packets traverse the same links with the same treatment,
   and share queues at each link, which mitigates most of the issues
   with ping.  We find that while ping works as expected in measuring
   end-to-end RTT, it does not track the sender's estimate of RTT, and
   as such does not measure the RTT experienced by the application layer
   as well as the spin bit does.

   In summary, our experiments show that the spin bit is suitable for
   purpose, can be implemented with minimal disruption, and that most of
   the identified problems can be easily mitigated.  See
   [SPINBIT-REPORT] for more.

Appendix B.  Use Cases for Passive RTT Measurement

   This section describes use cases for passive RTT measurement.  Most
   of these are currently achieved with TCP, i.e., the matching of
   packets based on sequence and acknowledgment numbers, or timestamps
   and timestamp echoes, in order to generate upstream and downstream

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   RTT samples which can be added to get end-to-end RTT.  These use
   cases could be achieved with QUIC by replacing sequence/
   acknowledgement and timestamp analysis with spin bit analysis, as
   described in Section 3.

   In any case, the measurement methodology follows one of a few basic

   o  The RTT evolution of a flow or a set of flows can be compared to
      baseline or expected RTT measurements for flows with the same
      characteristics in order to detect or localize latency issues in a
      specific network.

   o  The RTT evolution of a single flow can also be examined in detail
      to diagnose performance issues with that flow.

   o  The spin bit can be used to generate a large number of samples of
      RTT for a flow aggregate (e.g., all flows between two given
      networks) without regard to temporal evolution of the RTT, in
      order to examine the distribution of RTTs for a group of flows
      that should have similar RTT (e.g., because they should share the
      same path(s)).

B.1.  Inter-domain Troubleshooting

   Network access providers are often the first point of contact by
   their customers when network problems impact the performance of
   bandwidth-intensive and latency-sensitive applications such as video,
   regardless of whether the root cause lies within the access
   provider's network, the service provider's network, on the Internet
   paths between them, or within the customer's own network.

   The network performance is currently measured by points of presence
   on-the-path which extract spatial delay and loss metrics measurements
   [RFC6049] from fields of the transport layer (e.g.  TCP) or of
   application layer (e.g.  RTP).  The information is captured in the
   upper layer because neither the IP header nor the UDP layer includes
   fields allowing the measurement of upstream and downstream delay and

   Local network performance problems are detected with monitoring tools
   which observe the variation of upstream metrics and downstream

   Inter-domain troubleshooting relies on the same metrics but is not a
   pro-active task.  It is a recursive process which hones in on the
   domain and link responsible for the failure.  In practice, inter-
   domain troubleshooting is a communication process between the Network

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   Operations Center (NOC) teams of the networks on the path, because
   the root cause of a problem is rarely located on a single network,
   and requires cooperation and exchange of data between the NOCs.

   One example is the troubleshooting performance degradation resulting
   from a change of routing policy on one side of the path which
   increases the burden on a defective line card of a device located
   somewhere on the path.  The card's misbehavior introduces an abnormal
   reordered packets only in the traffic exchanged at line rate.

   Other examples are similar in terms of cooperation requirements and
   the need to refer to measurements.  NOCs need to share the same
   measurement metrics and to measure these metrics on the same fields
   of the packet to enable a minimal level of technical cooperation.

   Experimentation with the spinbit Appendix A has shown ability to
   replace the current RTT measurement opportunities based on clear-text
   transport or application header fields with a standard approach for
   measuring passive upstream and downstream RTT, which are a
   fundamental metric for this diagnostic process.

B.2.  Two-Point Intradomain Measurement

   The spin bit is also useful as a basic signal for instantaneous
   measurement of the treatment of QUIC traffic within a single network.
   Though the primary design goal of the spin bit signal is to enable
   single-observer on-path measurement of end-to-end RTT, the spin bit
   can also be used by two cooperating observers with access to traffic
   flowing in the same direction as an alternate marking signal, as
   described in [ALT-MARK].  The only difference from alternate marking
   with a generated signal is that the size of the alternation will
   change with the flight size each RTT.  However, these changes do not
   affect the applicability of the method that works for each marking
   batch separately applied between two measurement points on the same
   direction.  This two point measurement is an additional feature
   enabled "for free" by the spin bit signal.

   So, with more than one observer on the same direction, it can be
   useful to segment the RTT and deduce the contribution to the RTT of
   the portion of the network between two on-path observers.  This can
   be easily performed by calculating the delay between two or more
   measurement points on a single direction by applying [ALT-MARK].  In
   this way, packet loss, delay and delay variation can be measured for
   each segment of the network depending on the number and distribution
   of the available on-path observation points.  When these observation
   points are applied at network borders, the alternate-marking signal
   can be used to measure the performance of QUIC traffic within a

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   network operator's own domain of responsibility. own portion of the

B.3.  Bufferbloat Mitigation in Cellular Networks

   Cellular networks consist of multiple Radio Access Networks (RAN)
   where mobile devices are attached to base stations.  It is common
   that base stations from different vendors and different generations
   are deployed in the same cellular network.

   Due to the dynamic nature of RANs, base stations have typically been
   provisioned with large buffers to maximize throughput despite rapid
   changes in capacity.  As a side effect, bufferbloat has become a
   common issue in such networks [WWMM-BLOAT].

   An effective way of mitigating bufferbloat without sacrificing too
   much throughput is to deploy Active Queue Management (AQM) in
   bottleneck routers and base stations.  However, due to the variation
   in deployed base-stations it is not always possible to enable AQM at
   the bottlenecks, without massive infrastructure investments.

   An alternative approach is to deploy AQM as a network function in a
   more centralized location than the traditional bottleneck nodes.
   Such an AQM monitors the RTT progression of flows and drops or marks
   packets when the measured latency is indicative of congestion.  Such
   a function also has the possibility to detect misbehaving flows and
   reduce the negative impact they have on the network.

B.4.  Locating WiFi Problems in Home Networks

   Many residential networks use WiFi (802.11) on the last segment, and
   WiFi signal strength degradation manifests in high first-hop delay,
   due to the fact that the MAC layer will retransmit packets lost at
   that layer.  Measuring the RTT between endpoints on the customer
   network and parts of the service provider's own infrastructure (which
   have predictable delay characteristics) can be used to isolate this
   cause of performance problems.

   The network provider can measure the RTT and packet loss in the home
   gateway or an upstream point if there is no access to home gateway.
   A problem in the WiFi network is identified by seeing high delay and
   low packet loss.

   These measurements are particularly useful for traffic which is
   latency sensitive, such as interactive video applications.  However,
   since high latency is often correlated with other network-layer
   issues such as chronic interconnect congestion [IMC-CONGESTION], it

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   is useful for general troubleshooting of network layer issues in an
   interdomain setting.

   In this case, multiple RTT samples per flow are useful less for
   observing intraflow behavior, and more for generating sufficient
   samples for a given aggregate to make a high-quality measurement.

B.5.  Internet Measurement Research

   As a large, distributed, engineered system with no centralized
   control, the Internet has emergent properties of interest to the
   research community not just for purely scientific curiosity, but also
   to provide applicable guidance to Internet engineering, Internet
   protocol design and development, network operations, and policy
   development.  Latency measurements in particular are both an active
   area of research as well as an important tool for certain measurement
   studies (see, e.g.  [IMC-TCPSIG], from the most recent Internet
   Measurement Conference).  While much of this work is currently done
   with active measurements, the ability to generate latency samples
   passively or using a hybrid measurement approach (i.e., through
   passive observation of purpose-generated active measurement traffic;
   see [RFC7799]) can drastically increase the efficiency and
   scalability of these studies.  A latency spin bit would make these
   techniques applicable to QUIC, as well.

Appendix C.  Alternate RTT Measurement Approaches for Diagnosing QUIC

   There are three broad alternatives to explicit signaling for passive
   RTT measurement of the RTT experienced by QUIC flows.

C.1.  Handshake RTT measurement

   The first of these is handshake RTT measurement.  As described in
   [QUIC-MGT], the packets of the QUIC handshake are distinguishable on
   the wire in such a way that they can be used for one RTT measurement
   sample per flow: the delay between the client initial and the server
   cleartext packet can be used to measure "upstream" RTT (between the
   observer and the server), and the delay between the server cleartext
   packet and the next client cleartext packet can be used to measure
   "downstream" RTT (between the client and the observer).  When RTT
   measurements are used in large aggregates (all flows traversing a
   large link, for example), a methodology based on handshake RTT could
   be used to generate sufficient samples for some purposes without the
   spin bit.

   However, this methodology would rely on the assumption that the
   difference between handshake RTT and nominal in-flow RTT is

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   negligible.  Specifically, (1) any additional delay required to
   compute any cryptographic parameters must be negligible with respect
   to network RTT; (2) any additional delay required to establish state
   along the path must be negligible with respect to network RTT; and
   (3) network treatment of initial packets in a flow must be identical
   to that of later packets in the flow.  When these assumptions cannot
   be shown to hold, spin-bit based RTT measurement is preferable to
   handshake RTT measurement, even for applications for which handshake
   RTT measurement would otherwise be suitable.

C.2.  Parallel active measurement

   The second alternative is parallel active measurement: using ICMP
   Echo Request and Reply [RFC0792] [RFC4433], a dedicated measurement
   protocol like TWAMP [RFC5357], or a separate diagnostic QUIC flow to
   measure RTT.  Regardless of protocol, the active measurement must be
   initiated by a client on the same network as the client of the QUIC
   flow(s) of interest, or a network close by in the Internet topology,
   toward the server.  Note that there is no guarantee that ICMP flows
   will receive the same network treatment as the flows under study,
   both due to differential treatment of ICMP traffic and due to ECMP
   routing (see e.g.  [TOKYO-PING]).  TWAMP and QUIC diagnostic flows,
   though both use UDP, have similar issues regarding ECMP.  However, in
   situations where the entity doing the measurement can guarantee that
   the active measurement traffic will traverse the subpaths of interest
   (e.g.  residential access network measurement under a network
   architecture and business model where the network operator owns the
   CPE), active measurement can be used to generate RTT samples at the
   cost of at least two non-productive packets sent though the network
   per sample.

C.3.  Frequency Analysis

   The third alternative, proposed during the QUIC RTT design team
   process, relies on the inter-packet spacing to convey information
   about the RTT, and would therefore allow measurements confined to a
   single direction of transmission, as described in [CARRA-RTT].

   We evaluated the applicability of this work to passive RTT
   measurement in QUIC, and found it wanting.  We assembled a toolchain,
   as described in [NOSPIN], that allowed evaluation of a critical
   aspect of the [CARRA-RTT] method: extraction of inter-packet times of
   real packet streams and the analysis of frequencies present in the
   packet stream using the Lomb-Scargle Periodogram.  Several streams
   were evaluated, as summarized below:

   o  It seems that Carra et al.  [CARRA-RTT] took the noisy and low-
      confidence results of a statistical process (no RTT-related

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      frequency has been detected even after using very low alpha
      confidence) and added heuristics with sliding-window averaging to
      infer the fundamental frequency and RTT present in a
      unidirectional stream.

   o  There appear to be several limitations on the streams that are
      applicable.  Streams with long RTT (~50ms) are more likely to be
      suitable (having a better match between packet rate and relatively
      low frequencies to detect).

   o  None of the TCP streams analysed (to date) possess a sufficient
      packet rate such that the measured fundamental frequency or the
      multiples of the fundamental are actually within the detectable

   o  "Ideal" interarrival time streams were simulated with uniform
      sampling and period.  The Lomb-Scargle Periodogram is surprisingly
      unable to detect the fundamental frequency at 100 Hz from the
      constant 10 ms packet spacing.

   o  It is not clear if IETF QUIC protocol stream will possess the same
      inter-packet arrival time features as TCP streams.  Also, Carra et
      al. note that their process may not work if the TCP stream
      encounters a bottleneck, which would be an essential circumstance
      for network troubleshooting.  Mobile networks with time-slot
      service disciplines would likely cause similar issues as a
      bottleneck, by imposing their time-slot interval on the spacing of
      most packets.

   o  The Carra et al.  [CARRA-RTT] calculation of minimum and maximum
      frequencies that can be detected may not be applicable when the
      inter-arrival times are (both) the signal being detected and
      govern the non-uniform sampling frequency.

Appendix D.  Greasing

   Routes, congestion levels and therefore latency between two fixed
   QUIC endpoints, as well as the shape of individual application flows,
   fluctuate in ways that are not totally predictable by an on path
   observer.  In general, there is no a-priori pattern for the spin-bit
   distribution that will always materialise on a certain flow
   aggregate, even for a single user.

   There has been discussion in the QUIC working group that greasing
   could be a strategy to counter an evil access provider that might
   gate access to its users on a valid spin bit signal.  Let's accept
   for a moment this threat model and consider the practical case of a
   home gateway that temporarily misbehaves, for example draining its

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   queues slower than it would normally do while a firmware download is
   in progress.  It would be ill-considered for an access provider (even
   a malicious one) to block, or otherwise interfere with, QUIC flows
   originating from behind that CPE solely based on the fact that RTTs
   are now different from "usual".  In fact, providing a numerical
   assessment of what such "usual" RTT looks like would necessarily
   include many paths with different length, and considerable RTT
   variability within any fixed path, which is clearly beyond most ISPs'
   reach.  But even assuming it were, there is a simple cost-benefit
   counterargument here that the same effect (i.e., gating traffic from
   or to a given user based on observed traffic patterns) could be
   achieved with much cheaper and effective means (e.g., [SHBAIR]).

   So, the potential for ossification appears to be extremely low.
   Since it depends on so much external noise, the spin-bit result
   variability is self-greasing to an extent.  In fact, implementing
   explicit greasing around the spin-bit might even be harmful as it
   would potentially erode confidence in the veracity of the signal.

   However, if a greasing algorithm is really needed - for example, if
   we want to reuse the bit with different semantics in the future
   (i.e.: the spin-bit is not included in the header invariants), one
   very simple implementation would be as follows: each server will
   refuse to spin its bit on a per-flow basis with a given probability
   p, instead leaving it stuck to a randomly chosen value, 0 or 1.  The
   client will then end up leaving its bit stuck to the opposite value,
   or could detect this condition and also pick a randomly chosen stuck
   value.  The value chosen for p must be small enough to let the spin-
   bit mechanics work and large enough not to be seen as an error
   instead of an intentional protocol feature.

Authors' Addresses

   Brian Trammell (editor)
   ETH Zurich

   Email: ietf@trammell.ch

   Piet De Vaere
   ETH Zurich

   Email: piet@devae.re

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   Roni Even

   Email: roni.even@huawei.com

   Giuseppe Fioccola
   Telecom Italia

   Email: giuseppe.fioccola@telecomitalia.it

   Thomas Fossati

   Email: thomas.fossati@nokia.com

   Marcus Ihlar

   Email: marcus.ihlar@ericsson.com

   Al Morton
   AT&T Labs

   Email: acmorton@att.com

   Emile Stephan

   Email: emile.stephan@orange.com

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