Network Working Group C. Huitema
Internet-Draft Private Octopus Inc.
Intended status: Experimental March 17, 2021
Expires: September 18, 2021
Quic Timestamps For Measuring One-Way Delays
draft-huitema-quic-ts-05
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.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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 September 18, 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
Huitema Expires September 18, 2021 [Page 1]
Internet-Draft QUIC-TS March 2021
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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 . . . . . . . . . . . . . . . 6
3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Management of Time . . . . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
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
TIMESTAMP frame carrying the time at which a packet is sent. The use
of this extension frame is negotiated with a transport parameter,
Huitema Expires September 18, 2021 [Page 2]
Internet-Draft QUIC-TS March 2021
"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:
o 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 [I-D.ietf-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.
Huitema Expires September 18, 2021 [Page 3]
Internet-Draft QUIC-TS March 2021
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 [I-D.ietf-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:
o TIMESTAMP (TBD)
TIMESTAMP frames carry a single parameter, the timestamp.
Huitema Expires September 18, 2021 [Page 4]
Internet-Draft QUIC-TS March 2021
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
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 [I-D.ietf-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
[I-D.ietf-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:
o the beginning of the connection, i.e., the time at which the first
packet for that connection was sent or received.
o 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.
Huitema Expires September 18, 2021 [Page 5]
Internet-Draft QUIC-TS March 2021
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:
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 = 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.
Huitema Expires September 18, 2021 [Page 6]
Internet-Draft QUIC-TS March 2021
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 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,
Huitema Expires September 18, 2021 [Page 7]
Internet-Draft QUIC-TS March 2021
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 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. 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.
5. 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)
Huitema Expires September 18, 2021 [Page 8]
Internet-Draft QUIC-TS March 2021
Frame Name: TIMESTAMP
Specification: Timestamp set at the time packet was sent
6. Acknowledgements
Thanks to Dmitri Tikhonov, Tal Misrahi, Watson Ladd, Martin Thomson
and Ian Swett 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-34 (work in
progress), January 2021.
[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-34 (work
in progress), January 2021.
[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>.
Huitema Expires September 18, 2021 [Page 9]
Internet-Draft QUIC-TS March 2021
Author's Address
Christian Huitema
Private Octopus Inc.
427 Golfcourse Rd
Friday Harbor WA 98250
U.S.A
Email: huitema@huitema.net
Huitema Expires September 18, 2021 [Page 10]