Quic Timestamps For Measuring One-Way Delays
draft-huitema-quic-ts-00

Abstract

The TimeStamp 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

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 August 27, 2020.

Copyright Notice

Copyright (c) 2020 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 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

• 1. Measuring One-Way Delays
• 1.1. Terms and Definitions
• 2. Specification
• 2.1. Negotiation
• 3. Time Stamp frame format
• 3.1. RTT Measurements
• 3.2. One-Way Delay Measurements
• 4. Discussion
• 5. Security Considerations
• 6. IANA Considerations
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Author's Address

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 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 3.

2.1. Negotiation

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

• 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.

If negotiation is successful the peers SHOULD add a time_stamp frame to packets carrying an ACK frame.

3. Time Stamp frame format

Timestamped ACK are identified by the frame type:

• 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: ACK 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.

3.1. 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.

3.2. 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:

• the largest acknowledged packet number is newly acknowledged, and
• 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.

4. 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.

5. 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.

6. 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 at the time packet was sent

7. References

7.1. Normative References

7.2. Informative References

Author's Address