Network Working Group                                         C. Huitema
Internet-Draft                                      Private Octopus Inc.
Intended status: Experimental                            August 10, 2020
Expires: February 11, 2021

              Quic Timestamps For Measuring One-Way Delays


   The TIME_STAMP frame can be added to Quic packets when one way delay
   measurements is useful.  The timestamp is set to the number of
   microseconds from the beginning of the connection to the time at
   which the packet is sent.  The draft defines the "enable_time_stamp"
   transport parameter for negotiating the use of this extension frame,
   and a new frame types for the time_stamped frame.

Status of This Memo

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   the Trust Legal Provisions and are provided without warranty as
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Table of Contents

   1.  Measuring One-Way Delays  . . . . . . . . . . . . . . . . . .   2
     1.1.  Terms and Definitions . . . . . . . . . . . . . . . . . .   3
   2.  Specification . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Negotiation . . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Sending TIME_STAMP frames . . . . . . . . . . . . . . . .   4
     2.3.  TIME_STAMP frame format . . . . . . . . . . . . . . . . .   4
     2.4.  RTT Measurements  . . . . . . . . . . . . . . . . . . . .   5
     2.5.  One-Way Delay Measurements  . . . . . . . . . . . . . . .   5
   3.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Management of Time  . . . . . . . . . . . . . . . . . . .   6
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Measuring One-Way Delays

   The QUIC Transport Protocol [I-D.ietf-quic-transport] provides a
   secure, multiplexed connection for transmitting reliable streams of
   application data.  The algorithms for QUIC Loss Detection and
   Congestion Control [I-D.ietf-quic-recovery] use measurement of Round
   Trip Time (RTT) to determine when packets should be retransmitted.
   RTT measurements are useful, but there are however many cases in
   which more precise One-Way Delay (1WD) measurements enable more
   efficient Loss Detection and Congestion Control.

   An example would be the Low Extra Delay Background Transport (LEDBAT)
   [RFC6817] which uses variations in transmission delay to detect
   competition for transmission resource.  Experience shows that while
   LEDBAT may be implemented using RTT measurements, it is somewhat
   inefficient because it will cause unnecessary slowdowns in case of
   queues or delayed ACKs on the return path.  Using 1WD solves these
   issues.  Similar argument can be made for most delay-based

   We propose to enable one way delay measurements in QUIC by defining a
   TIME_STAMP frame carrying the time at which a packet is sent.  The
   use of this extension frame is negotiated with a transport parameter,
   "enable_time_stamp".  When the extension is negotiated by both

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   parties, this frame can be used in conjunction with other such as ACK
   to measure one way delays.

1.1.  Terms and Definitions

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Specification

   The enable_time_stamp transport parameter used for negotiating the
   use of the extension frame is defined in Section 2.1.  The time_stamp
   frame format is defined in Section 2.3.

2.1.  Negotiation

   The use of the time_stamp frame extension is negotiated using a
   transport parameter:

   o  enable_time_stamp (TBD)

   The enable time stamp transport parameter is included if the endpoint
   wants to receive or accepts to send time_stamp frames for this
   connection.  This parameter is encoded as a variable integer as
   specified in section 16 of [I-D.ietf-quic-transport].  It can take
   one of the following three values:

   1.  I would like to receive TIME_STAMP frames

   2.  I am able to generate TIME_STAMP frames

   3.  I am able to generate TIME_STAMP frames and I would like to
       receive them

   Peers receiving another value SHOULD terminate the connection with a

   A peer that advertises its capability of sending TIME_STAMP frames
   using option values 2 or 3 MUST NOT send these frames if the other
   peer does not announce advertise its desire to receive them by
   sending the enable_time_stamp TP with option 1 or 3.  This condition
   is described as "successful sending negotiation" in Section 2.2.

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   Peers that receive TIME_STAMP frames when they have not advertised
   their desire to receive them MAY terminate the connection with a

2.2.  Sending TIME_STAMP frames

   Following successful sending negotiation, a peer SHOULD add a
   time_stamp frame to 1RTT packets carrying an ACK frame.  This
   specification does not impose a placement of TIME_STAMP frames in the
   packet.  They MAY be sent either before or after the ACK frame.

   Implementations SHOULD NOT send more than one TIME_STAMP frame per
   packet, but they MAY send more than one in rare circumstances.  When
   multiple TIME_STAMP frames are present in a packet, the receiver
   retains the frame indicating the largest timestamp.

   Implementations MUST NOT send the TIME_STAMP frame in Initial, 0-RTT
   or Handshake packets, because there is a risk that the peer will
   receive such packets before the negotiation completes.  This
   restriction may appear excessive because some Handshake packets are
   typically sent after the negotiation completes, but restricting
   TIME_STAMP frames to 1RTT packets is simpler and less error prone
   than allowing the TIME_STAMP frame in just a fraction of Handshake

2.3.  TIME_STAMP frame format

   TIME_STAMP frames are identified by the frame type:


   TIME_STAMP frames carry a single parameter, the time stamp.

    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
   |                          Time Stamp (i)                     ...

             Figure 1: TIME_STAMP Frame Format with Time Stamp

   The time stamp encodes the number of microseconds since the beginning
   of the connection, as measured by the peer at the time at which the
   packet is sent.  It is encoded using the exponent selected by the
   peer in the ack_delay_exponent.  The exponent reduced time stamp is
   encoded as a variable length integer.

   The beginning of the connection is defined as follow:

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   o  for the client, the time when the first Initial packet is sent;

   o  for the server, the time when the first Initial packet is

   TIME_STAMP frames are not ack-eliciting.  Their loss does not require

2.4.  RTT Measurements

   RTT measurements are performed as specified in Section 4 of
   [I-D.ietf-quic-recovery], without reference to the Timestamp
   parameter of the Timestamped ACK frames.

2.5.  One-Way Delay Measurements

   An endpoint generates a One Way Delay Sample on receiving a packet
   containing both a TIME_STAMP frame and an ACK frame that meets the
   following two conditions:

   o  the largest acknowledged packet number is newly acknowledged, and

   o  at least one of the newly acknowledged packets was ack-eliciting.

   The One Way Delay sample, latest_1wd, is generated as the time
   elapsed since the largest acknowledged packet was sent, corrected for
   the difference between local time at the sending peer and connection
   time at the receiving peer, phase_shift.

   latest_1wd = time_stamp - send_time_of_largest_acked - phase_shift

   By convention, the phase_shift is estimated upon reception of the
   first RTT sample, first_rtt.  It is set to:

   phase_shift = time_stamp - send_time_of_largest_acked - latest_rtt/2

   In that formula, we assume that the local time are measured in
   microseconds since the beginning of the connection.

   We understand that clocks may drift over time, and that simply
   estimating a phase shift at the beginning of a connection may be too
   simplistic for long duration connections.  Implementations MAY adopt
   different strategies to reestimate the phase shift at appropriate
   intervals.  Specifying these strategies is beyond the scope of this

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3.  Discussion

   This document replaces an earlier proposal to modify the format of
   the ACK frame by including a time stamp inside the modified frame.
   The revised proposal encodes the time stamp independently of the ACK
   frame, which requires slightly more overhead to encode the type of
   the TIME_STAMP frame.

   Defining an independent frame allows for more flexibility.  This
   draft defines the combination of TIME_STAMP with ACK frames, but they
   could be combined with other frames as well.  For example, adding a
   TIME_STAMP to packets carrying a Path Response could allow measuring
   one way delays before deciding to migrate to a new path.

3.1.  Management of Time

   There are two known issues with deducing one way delays from RTT
   measurements: clock drift and undefined phase difference.

   The phase difference problem is easy to understand.  We start from a
   list of measurements associating the send time of packet number x
   (s[x]), the receive time of the acknowledgement of packet (a[x]), and
   the time stamp indicating when packet x was received by the peer
   (p[x]).  The peer's time stamp are expressed in the peer's clock.

   Suppose that we model the peer's clock as local time plus phase
   difference f, and that we model the rtt as the sum of two one way
   delays, up (u[x]) and down (d[x]).  We get:

       u[x] = p[x] + f - s[x]

       d[x] = a[x] - p[x] - f

   Just looking at the equation shows that the value of f cannot be
   determined from the a series of measurement (s[x], a[x], p[x]).  You
   can just add constraints that all u[x] and d[x] are positive numbers,
   which gives a range of plausible values for f: max(s[x] - p[x]) < f <
   min(a[x]-p[x]).  In case you wonder, you get similar formulations in
   a multipath scenario.  The plausible range may narrow to the min rtt
   of the shortest path, but no further.

   The phase difference uncertainty is not a big issue in practice,
   because control algorithms are much more interested in the variations
   of the delays than by their absolute values.  Suppose we want to
   compare one way delays at measurement (x) and (y).  We get:

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       u[x] = p[x] + f - s[x]

       u[y] = p[y] + f - s[y]

       u[x] - u[y] = p[x] - p[y] - s[x] + s[y]

   The phase difference does not affect the measurement of variations in
   the one way delay.

   The clock drift is another matter.  All the equations above assume
   that the local clock and the remote clock have the same frequency.
   This is an approximation.  Clocks drift over time.  Instead of just
   considering a stable phase difference, one should consider the sum of
   a phase difference and a time-varying drift component.  Estimating
   drift is a complex problem.  This was studied in detail in the
   development of the Network Time Protocol (NTP) [RFC5905].  In theory,
   implementations of Quic could copy the algorithms of NTP to build a
   model of the clocks used by the local node and the peer.  That would
   be very complex.

   Fortunately, implementations of Quic no not need to implement
   something as complex as NTP.  Most time based algorithms are only
   interested in variations of delays over a short horizon.  Clock drift
   happens at a slow pace, maybe 1 millisecond per minute.  Time base
   congestion control algorithms already have to cope with the potential
   drift of the minimum RTT due to changing network conditions.  They do
   that by periodically restarting themeasurement of the minimum RTT
   after some delay, typically less than a minute.  A simple
   implementation of one way delay measurements could follow the same
   approach, for example resetting the phase difference every 30 seconds
   or so.

4.  Security Considerations

   The Timestamp value in the TIME_STAMP frame is asserted by the sender
   of the packet.  Adversarial peers could chose values of the time
   stamp designed to exercise side effects in congestion control
   algorithms or other algorithms relying on the one-way delays.  This
   can be mitigated by running plausibility checks on the received
   values.  For example, each peer can maintain statistics not just on
   the One Way Delays, but also on the differences between One Way
   Delays and RTT, and detect outlier values.  Peers can also compare
   the differences between timestamps in packets carrying
   acknowledgements and the differences between the sending times of
   corresponding packets, and detect anomalies if the delays between
   acknowledging packets appears shorter than the delays when sending

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5.  IANA Considerations

   This document registers a new value in the QUIC Transport Parameter

   Value: TBD (using value 0x7158 in early deployments)

   Parameter Name: enable_time_stamp

   Specification: Indicates that the connection should use TimeStamped
   ACK frames

   This document also registers a new value in the QUIC Frame Type

   Value: TBD (using value 757 in early deployments)

   Frame Name: TIME_STAMP

   Specification: Time stamp set at the time packet was sent

6.  Acknowledgements

   Thanks to Dmitri Tikhonov, Tal Misrahi and Watson Ladd for their
   reviews and suggestions.

7.  References

7.1.  Normative References

              Iyengar, J. and I. Swett, "QUIC Loss Detection and
              Congestion Control", draft-ietf-quic-recovery-29 (work in
              progress), June 2020.

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

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

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

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7.2.  Informative References

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,

   [RFC6817]  Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,
              "Low Extra Delay Background Transport (LEDBAT)", RFC 6817,
              DOI 10.17487/RFC6817, December 2012,

Author's Address

   Christian Huitema
   Private Octopus Inc.
   427 Golfcourse Rd
   Friday Harbor  WA 98250


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