Retransmission Timeout Requirements

The information below is for an old version of the document
Document Type Active Internet-Draft (tcpm WG)
Last updated 2019-12-02 (latest revision 2019-02-22)
Replaces draft-allman-tcpm-rto-consider
Stream IETF
Intended RFC status Best Current Practice
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Stream WG state WG Document (wg milestone: May 2020 - Submit document on R... )
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Responsible AD Mirja K├╝hlewind
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Internet Engineering Task Force                                M. Allman
INTERNET-DRAFT                                                      ICSI
File: draft-ietf-tcpm-rto-consider-09.txt               December 2, 2019
Intended Status: Best Current Practice
Expires: June 2, 2020

                  Retransmission Timeout Requirements

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), its areas,
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    This Internet-Draft will expire on June 2, 2020.

Copyright Notice
    Copyright (c) 2019 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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    ( in effect on the date of
    publication of this document. Please review these documents
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    Ensuring reliable communication often manifests in a timeout and
    retry mechanism.  Each implementation of a retransmission timeout
    mechanism represents a balance between correctness and timeliness
    and therefore no implementation suits all situations.  This document
    provides high-level requirements for retransmission timeout schemes
    appropriate for general use in the Internet.  Within the
    requirements, implementations have latitude to define particulars
    that best address each situation.

Expires: June 2, 2020                                           [Page 1]
draft-ietf-tcpm-rto-consider-09.txt                        December 2019


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

Editorial Note

    This version addresses Gorry's comments from the recent TCPM
    meeting.  The changes are fairly minor.

    Jana's suggestion of moving from dealing with loss recovery
    ("retransmissions") to loss detection is not reflected in this
    revision.  This change will require small changes throughout the
    document.  The goal of this version is to ensure Gorry's suggestions
    are accurately taken into account before starting on Jana's.


1   Introduction

    Reliable transmission is a key property for many network protocols
    and applications.  Our protocols use various mechanisms to achieve
    reliable data transmission.  Often we use continuous or periodic
    acknowledgments from the recipient to inform the sender's notion of
    which pieces of data are missing and need to be retransmitted to
    ensure reliability.  Alternatively, information coding---e.g.,
    FEC---can be used to achieve probabilistic reliability without
    retransmissions.  However, despite our best intentions and most
    robust mechanisms, the only thing we can truly depend on is the
    passage of time and therefore our ultimate backstop to ensuring
    reliability is a timeout and re-try mechanism.  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 assumes the data was lost
    in transit and therefore schedules a retransmission.  This process
    of ensuring reliability via time-based loss detection and resending
    lost data is commonly referred to as a "retransmission timeout
    (RTO)" mechanism.

    Various protocols have defined their own RTO mechanisms (e.g., TCP
    [RFC6298], SCTP [RFC4960], SIP [RFC3261]).  In this document, our
    use of "RTO" does not refer to any one specific scheme, but rather
    is a generic term that includes all timer-based retransmission
    mechanisms.  The specifics of retransmission timeouts often
    represent a particular tradeoff between correctness and
    responsiveness [AP99].  In other words we want to simultaneously:
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