TCP ETS: Extensible Timestamp Options
draft-yang-tcpm-ets-00
TCP Maintenance Working Group K. Yang
Internet-Draft N. Cardwell
Intended status: Standards Track Y. Cheng
Expires: May 6, 2021 E. Dumazet
Google, Inc
November 2, 2020
TCP ETS: Extensible Timestamp Options
draft-yang-tcpm-ets-00
Abstract
This document presents ETS: an Extensible TimeStamps option for TCP.
It allows hosts to use microseconds as the unit for timestamps to
improve the precision of timestamps, and advertise the maximum ACK
delay for its own delayed ACK mechanism. Furthermore, it extends the
information provided in the [RFC7323] TCP Timestamps Option by
including the receiver delay in the TSecr echoing, so that the
receiver of the ACK is able to more accurately estimate the portion
of the RTT that resulted from time traveling through the network.
The ETS option format is extensible, so that future extensions can
add further information without the overhead of extra TCP option kind
and length fields.
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
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This Internet-Draft will expire on May 6, 2021.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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1. Terminology
The key words "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. In this document, these words will appear
with that interpretation only when in UPPER CASE. Lower case uses of
these words are not to be interpreted as carrying [RFC2119]
significance.
2. Introduction
Accurate round-trip time (RTT) estimation is necessary for TCP to
adapt to diverse and dynamic traffic conditions.
The TCP timestamp option specified in [RFC7323] is designed largely
for RTT samples intended for computing TCP's retransmission (RTO)
timer [RFC6298].
Some congestion control algorithms may wish to use a form of RTT
measurement as one of several congestion signals, since elevated RTT
measurements can reflect increases in network queueing delays. For
example, the Swift congestion control algorithm [KDJWWM20],
successfully deployed in data-center environments, requires precise
and accurate measurements of both network and host delays. However,
the existing TCP RTT sampling mechanisms that measure the delay
between data transmission and ACK receipt [RFC6298] do not separate
network and host delays, and cannot measure the RTT of retransmitted
data. Even the TCP timestamp option specified in [RFC7323] is not
well-suited to use as a congestion signal, for a number of reasons.
With the TCP Timestamps Option [RFC7323], data senders can measure an
RTT sample by computing the difference between the data sender's
current timestamp clock value and the received TSecr value. However,
there are some drawbacks in this [RFC7323] measurement method:
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1. The TCP endpoint can compute an [RFC7323] RTT measurement only if
the ACK advances the left edge of the send window, i.e. SND.UNA
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