A Transport-Independent Explicit Signal for Hybrid RTT Measurement
draft-trammell-tsvwg-spin-00

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TSVWG                                                   B. Trammell, Ed.
Internet-Draft                                                ETH Zurich
Intended status: Experimental                              July 02, 2018
Expires: January 3, 2019

   A Transport-Independent Explicit Signal for Hybrid RTT Measurement
                      draft-trammell-tsvwg-spin-00

Abstract

   This document defines an explicit per-flow transport-layer signal for
   hybrid measurement of end-to-end RTT.  This signal consists of three
   bits: a spin bit, which oscillates once per end-to-end RTT, and a
   two-bit Valid Edge Counter (VEC), which compensates for loss and
   reordering of the spin bit to increase fidelity of the signal in less
   than ideal network conditions.  It describes the algorithm for
   generating the signal, approaches for observing it to passively
   measure end-to-end latency, and proposes methods for adding it to a
   variety of IETF transport protocols.

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   Copyright (c) 2018 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
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Trammell                 Expires January 3, 2019                [Page 1]
Internet-Draft                  Spin Bits                      July 2018

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Signal Generation Mechanism . . . . . . . . . . . . . . . . .   3
     2.1.  Illustrating the Mechanism  . . . . . . . . . . . . . . .   4
   3.  Using the Signal for Hybrid RTT Measurement . . . . . . . . .   6
   4.  Binding the Hybrid RTT Measurement Signal to Transport
       Protocols . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  QUIC  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.2.  TCP . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   5.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Why the transport layer?  . . . . . . . . . . . . . . . .   9
   6.  Privacy and Security Considerations . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  12
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  12
   10. Informative References  . . . . . . . . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   Latency is a key metric to understanding network operation and
   performance, and passive measurability of round trip times (RTT) is a
   useful and important, if generally unintentional, feature of many
   transport protocols.  Passive measurement allows inspection of
   latency on productive traffic, avoiding problems with different
   treatment of productive and measurement traffic, and enables
   opportunistic measurement of latency without active measurement
   overhead.

   However, since these features are largely accidental, methods for
   passive latency measurement are transport-dependent, and different
   heuristics for deriving metrics from these accidental signals may
   lead to non-comparable values.  For example, methods applicable can
   be exclusively based on the TCP timestamp option [RFC7373] (see
   [CACM-TCP]), leverage both timestamps and matching sequence and
   acknowledgment numbers (see [TMA-QOF]), or rely on ACK-clocking in
   flows transmitting at a stable rate (see [CARRA-RTT]).  In addition,
   they rely on features that may change or have undesirable side
   effects.  For example, [CARRA-RTT] makes implicit assumptions about
   congestion control and pacing that may not hold for all senders, and
   timestamp-based methods require the TCP timestamp option to operate
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