Requirements for Time-Based Loss Detection

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Document Type Active Internet-Draft (tcpm WG)
Last updated 2020-04-30 (latest revision 2020-02-04)
Replaces draft-allman-tcpm-rto-consider
Stream IETF
Intended RFC status Best Current Practice
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Stream WG state WG Consensus: Waiting for Write-Up (wg milestone: May 2020 - Submit document on R... )
Document shepherd Yoshifumi Nishida
IESG IESG state I-D Exists (IESG: Dead)
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Internet Engineering Task Force                                M. Allman
INTERNET-DRAFT                                                      ICSI
File: draft-ietf-tcpm-rto-consider-10.txt               February 4, 2020
Intended Status: Best Current Practice
Expires: August 4, 2020

               Requirements for Time-Based Loss Detection

Status of this Memo

    This Internet-Draft is submitted in full conformance with the
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    Many protocols must detect packet loss for various reasons (e.g., to
    ensure reliability using retransmissions or to understand the level
    of congestion along a network path).  While many mechanisms have
    been designed to detect loss, protocols ultimately can only count on
    the passage of time without delivery confirmation to declare a
    packet "lost".  Each implementation of a time-based loss detection
    mechanism represents a balance between correctness and timeliness
    and therefore no implementation suits all situations.  This document

Expires: August 4, 2020                                         [Page 1]
draft-ietf-tcpm-rto-consider-10.txt                        February 2020

    provides high-level requirements for time-based loss detectors
    appropriate for general use in the Internet.  Within the
    requirements, implementations have latitude to define particulars
    that best address each situation.


    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    document are to be interpreted as described in BCP 14, RFC 2119

1   Introduction

    Loss detection is a crucial activity for many protocols and
    applications and is generally undertaken for two major reasons:

      (1) Ensuring reliable data delivery.

            This requires a data sender to develop an understanding of
            which transmitted packets have not arrived at the receiver.
            This knowledge allows the sender to retransmit missing data.
      (2) Congestion control.

            Packet loss is often taken as an indication that the sender
            is transmitting too fast and is overwhelming some portion of
            the network path.  Data senders can therefore use loss to
            trigger transmission rate reductions.

    Various mechanisms are used to detect losses in a packet stream.
    Often we use continuous or periodic acknowledgments from the
    recipient to inform the sender's notion of which pieces of data are
    missing.  However, despite our best intentions and most robust
    mechanisms we cannot place ultimate faith in receiving such
    acknowledgments, but can only truly depend on the passage of time.
    Therefore, our ultimate backstop to ensuring that we detect all loss
    is a timeout.  That is, the sender sets some expectation for how
    long to wait for confirmation of delivery for a given piece of data.
    When this time period passes without delivery confirmation the
    sender concludes the data was lost in transit.

    The specifics of time-based loss detection schemes represent a
    tradeoff between correctness and responsiveness.  In other words we
    wish to simultaneously:

      - wait long enough to ensure the detection of loss is correct, and  

      - minimize the amount of delay we impose on applications (before
        repairing loss) and the network (before we reduce the
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