Network Working Group C. Huitema
Internet-Draft Private Octopus Inc.
Intended status: Experimental February 29, 2020
Expires: September 1, 2020
Quic Timestamps For Measuring One-Way Delays
draft-huitema-quic-ts-02
Abstract
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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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 . . . . . . . . . . . . . . . . 3
2.3. TIME_STAMP frame format . . . . . . . . . . . . . . . . . 4
2.4. RTT Measurements . . . . . . . . . . . . . . . . . . . . 4
2.5. One-Way Delay Measurements . . . . . . . . . . . . . . . 4
3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Management of Time . . . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
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
algorithms.
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"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
accepts and sends time_stamp frames for this connection. This
parameter has a zero-length value. Negotiation is successful if both
peers support include this parameter in their transport parameter
message. Peers that receive a time_stamp frame in the absence of
successful negotiation MAY terminate the connection with a PROTOCOL
VIOLATION error.
2.2. Sending TIME_STAMP frames
If negotiation is successful the peers 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
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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
packets.
2.3. TIME_STAMP frame format
TIME_STAMP frames are identified by the frame type:
o TIME_STAMP (TBD)
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:
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
received.
TIME_STAMP frames are not ack-eliciting. Their loss does not require
retransmission.
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:
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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
document.
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
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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:
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
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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
them.
5. IANA Considerations
This document registers a new value in the QUIC Transport Parameter
Registry:
Value: TBD (using value 0x7157 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
registry:
Value: TBD (using value 757 in early deployments)
Frame Name: TIME_STAMP
Specification: Time stamp set at the time packet was sent
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6. Acknowledgements
Thanks to Dmitri Tikhonov, Tal Misrahi and Watson Ladd for their
reviews and suggestions.
7. References
7.1. Normative References
[I-D.ietf-quic-recovery]
Iyengar, J. and I. Swett, "QUIC Loss Detection and
Congestion Control", draft-ietf-quic-recovery-26 (work in
progress), February 2020.
[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-27 (work
in progress), February 2020.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
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,
<https://www.rfc-editor.org/info/rfc5905>.
[RFC6817] Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,
"Low Extra Delay Background Transport (LEDBAT)", RFC 6817,
DOI 10.17487/RFC6817, December 2012,
<https://www.rfc-editor.org/info/rfc6817>.
Author's Address
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Christian Huitema
Private Octopus Inc.
427 Golfcourse Rd
Friday Harbor WA 98250
U.S.A
Email: huitema@huitema.net
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