Internet DRAFT - draft-huitema-quic-ts
draft-huitema-quic-ts
Network Working Group C. Huitema
Internet-Draft Private Octopus Inc.
Intended status: Experimental 28 August 2022
Expires: 1 March 2023
Quic Timestamps For Measuring One-Way Delays
draft-huitema-quic-ts-08
Abstract
The TIMESTAMP frame can be added to Quic packets when one way delay
measurements are useful. The timestamp is set to the number of
microseconds from the beginning of the node's epoch to the time at
which the packet is sent. The draft defines the "enable_timestamp"
transport parameter for negotiating the use of this extension frame,
and the TIMESTAMP frame.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 1 March 2023.
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Copyright (c) 2022 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terms and Definitions . . . . . . . . . . . . . . . . . . 3
2. Specification . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Negotiation . . . . . . . . . . . . . . . . . . . . . . . 3
2.1.1. Zero RTT and Timestamp Option . . . . . . . . . . . . 4
2.2. Sending TIMESTAMP frames . . . . . . . . . . . . . . . . 4
2.3. TIMESTAMP frame format . . . . . . . . . . . . . . . . . 4
2.4. RTT Measurements . . . . . . . . . . . . . . . . . . . . 5
2.5. Choice of Epoch . . . . . . . . . . . . . . . . . . . . . 5
2.6. One-Way Delay Measurements . . . . . . . . . . . . . . . 5
3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Management of Time . . . . . . . . . . . . . . . . . . . 6
4. Use cases . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. Application to Hystart . . . . . . . . . . . . . . . . . 8
4.2. Application to QUIC Multipath . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
The QUIC Transport Protocol [QUIC-TRANSPORT] provides a secure,
multiplexed connection for transmitting reliable streams of
application data. The algorithms for QUIC Loss Detection and
Congestion Control [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
TIMESTAMP frame carrying the time at which a packet is sent. The use
of this extension frame is negotiated with a transport parameter,
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"enable_timestamp". When the extension is negotiated by both
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_timestamp transport parameter used for negotiating the use
of the extension frame is defined in Section 2.1. The timestamp
frame format is defined in Section 2.3.
2.1. Negotiation
The use of the timestamp frame extension is negotiated using a
transport parameter:
* enable_timestamp (TBD)
The enable timestamp transport parameter is included if the endpoint
wants to receive or accepts to send timestamp frames for this
connection. This parameter is encoded as a variable integer as
specified in section 16 of [QUIC-TRANSPORT]. It can take one of the
following three values:
1. I would like to receive TIMESTAMP frames
2. I am able to generate TIMESTAMP frames
3. I am able to generate TIMESTAMP frames and I would like to
receive them
Peers receiving another value SHOULD terminate the connection with a
TRANSPORT PARAMETER error.
A peer that advertises its capability of sending TIMESTAMP 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_timestamp 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 TIMESTAMP frames when they have not advertised
their desire to receive them MAY terminate the connection with a
PROTOCOL VIOLATION error.
2.1.1. Zero RTT and Timestamp Option
Implementations MUST NOT remember the value of the enable_timestamp
parameter and try to use it when attempting 0-RTT on subsequent
connections. This rules is in line with the suggestions in section
7.4.2 of [QUIC-TRANSPORT] to adopt conservative defaults and avoid
compatibility issues. It is also consistent with the specification
to only use TIMESTAMP frames in 1RTT packets, see Section 2.2.
2.2. Sending TIMESTAMP frames
Following successful sending negotiation, a peer SHOULD add a
timestamp frame to 1RTT packets carrying an ACK frame. This
specification does not impose a placement of TIMESTAMP frames in the
packet. They MAY be sent either before or after the ACK frame.
Implementations SHOULD NOT send more than one TIMESTAMP frame per
packet, but they MAY send more than one in rare circumstances. When
multiple TIMESTAMP frames are present in a packet, the receiver
retains the frame indicating the largest timestamp.
Implementations MUST NOT send the TIMESTAMP 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
TIMESTAMP frames to 1RTT packets is simpler and less error prone than
allowing the TIMESTAMP frame in just a fraction of Handshake packets.
2.3. TIMESTAMP frame format
TIMESTAMP frames are identified by the frame type:
* TIMESTAMP (TBD)
TIMESTAMP frames carry a single parameter, the timestamp.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: TIMESTAMP Frame Format with Timestamp
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The timestamp encodes the number of microseconds since the beginning
of the epoch, 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 timestamp is encoded as
a variable length integer.
TIMESTAMP frames are not ack-eliciting. Their loss does not require
retransmission.
For congestion control, TIMESTAMP frames are treated like ACK frames.
Section 7 of [QUIC-RECOVERY] specifies that "packets containing only
ACK frames do not count towards bytes in flight and are not
congestion controlled". The same applies to packets containing only
TIMESTAMP frames, or a combination of ACK frames and TIMESTAMP
frames.
2.4. RTT Measurements
RTT measurements are performed as specified in Section 4 of
[QUIC-RECOVERY], without reference to the Timestamp parameter of the
Timestamped ACK frames.
2.5. Choice of Epoch
Each peer can chose its epoch as it sees fit, but it MUST remain
constant for the duration of the connection, and the resulting
timestamps MUST be positive integers. Plausible values for the epoch
could be:
* the beginning of the connection, i.e., the time at which the first
packet for that connection was sent or received.
* the time at which the first timestamp is sent.
Chosing values close to the beginning of the connection ensures that
the timestamps value will be at most equal to the duration of the
connection, which limits the amount of bytes required to encode the
timestamps.
2.6. One-Way Delay Measurements
An endpoint generates a One Way Delay Sample on receiving a packet
containing both a TIMESTAMP frame and an ACK frame that meets the
following two conditions:
* the largest acknowledged packet number is newly acknowledged, and
* at least one of the newly acknowledged packets was ack-eliciting.
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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 = timestamp - 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 = timestamp - 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. The formula does
not depend on the choice of epoch by each peer, but simply of the
hypothesis that delays on the data path and the return path are about
equal.
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 timestamp inside the modified frame.
The revised proposal encodes the timestamp independently of the ACK
frame, which requires slightly more overhead to encode the type of
the TIMESTAMP frame.
Defining an independent frame allows for more flexibility. This
draft defines the combination of TIMESTAMP with ACK frames, but they
could be combined with other frames as well. For example, adding a
TIMESTAMP 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.
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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 timestamp indicating when packet x was received by the peer
(p[x]). The peer's timestamp 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.
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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 the measurement 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. Use cases
Time stamps have been found useful in multiple environments,
including avoid spurious exit from slow start with the Hybrid Slow
Start algorithm [HyStart], and monitoring delays for individual paths
for QUIC multipath as mentioned in Section 5 of [MULTIPATH-QUIC].
4.1. Application to Hystart
Implementations of the Cubic congestion control [RFC8312] benefit
from the Hybrid Slow Start algorithm [HyStart]. Hystart works by
monitoring the transmission delay during the initial Slow Start
phase, exiting slow start and moving to congestion avoidance when a
delay increase is noticed, before usage of a too large congestion
window causes many losses. Hystart generally improves performance of
Cubic, but can cause performance degradation if spurious delay
measurement cause an early exit.
An example of early exit was noticed when implementing Cubic
congestion control in QUIC [CH09]. Tests over a wireless connection
showed significant jitter in the RTT measurements. This was fixed by
exiting only after 5 measurements showed a delay increase. Further
investigation showed that this jitter was mostly happening on the
"upload" path. Timestamps enable measurement of the delay variations
independently on each path, and thus improved exit decisions.
4.2. Application to QUIC Multipath
Time Stamps are very useful in multipath environments, as mentioned
in [MULTIPATH-QUIC]. In the absence of time stamps, it is very hard
to estimate the contribution of each path to the end to end delay,
and the specification mandates that acknowledgements be sent on the
same path over which packets were received. If time stamps are
available, experiments show that performance are improved if the
acknowlegments are sent on the shortest available path, because the
implementation can detect packet losses or congestion events faster.
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5. Security Considerations
The Timestamp value in the TIMESTAMP frame is asserted by the sender
of the packet. Adversarial peers could chose values of the timestamp
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.
6. IANA Considerations
This document registers a new value in the QUIC Transport Parameter
Registry:
Value: TBD (using value 0x7158 in early deployments)
Parameter Name: enable_timestamp
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: TIMESTAMP
Specification: Timestamp set at the time packet was sent
7. Acknowledgements
Thanks to Dmitri Tikhonov, Tal Misrahi, Watson Ladd, Martin Thomson
and Ian Swett for their reviews and suggestions.
8. References
8.1. Normative References
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[QUIC-RECOVERY]
Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
and Congestion Control", RFC 9002,
<https://www.rfc-editor.org/rfc/rfc9002>.
[QUIC-TRANSPORT]
Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
<https://www.rfc-editor.org/rfc/rfc9000>.
[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>.
[RFC8312] Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
RFC 8312, DOI 10.17487/RFC8312, February 2018,
<https://www.rfc-editor.org/info/rfc8312>.
8.2. Informative References
[CH09] Huitema, C., "Implementing Cubic congestion control in
Quic", Blog entry, 2019,
<https://huitema.wordpress.com/2019/11/11/implementing-
cubic-congestion-control-in-quic/>.
[HyStart] Ha, S. and I. Rhee,, "Hybrid Slow Start for High-Bandwidth
and Long-Distance Networks", Conference International
Workshop on Protocols for Fast Long-Distance Networks,
2008, <https://pdfs.semanticscholar.org/25e9/
ef3f03315782c7f1cbcd31b587857adae7d1.pdf>.
[MULTIPATH-QUIC]
Liu, Y., Ed., Ma, Y., De Coninck, Q., Ed., Bonaventure,
O., Huitema, C., and M. Kuehlewind, Ed., "Multipath
Extension for QUIC", Work in Progress, Internet-Draft,
draft-ietf-quic-multipath,
<https://datatracker.ietf.org/doc/html/draft-ietf-quic-
multipath>.
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[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
Christian Huitema
Private Octopus Inc.
Email: huitema@huitema.net
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