Retransmission Timeout Requirements

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Document Type Active Internet-Draft (tcpm WG)
Last updated 2018-10-19
Replaces draft-allman-tcpm-rto-consider
Stream IETF
Intended RFC status Best Current Practice
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Internet Engineering Task Force                                M. Allman
INTERNET-DRAFT                                                      ICSI
File: draft-ietf-tcpm-rto-consider-06.txt               October 19, 2018
Intended Status: Best Current Practice
Expires: April 19, 2019

                  Retransmission Timeout Requirements

Status of this Memo

    This document may not be modified, and derivative works of it may
    not be created, except to format it for publication as an RFC or to
    translate it into languages other than English.
    This Internet-Draft is submitted in full conformance with the
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Copyright Notice
    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
    Provisions Relating to IETF Documents
    ( 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

Expires: April 19, 2019                                         [Page 1]
draft-ietf-tcpm-rto-consider-06.txt                         October 2018

    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.


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

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]).  The specifics of
    retransmission timeouts often represent a particular tradeoff
    between correctness and responsiveness [AP99].  In other words we
    want to simultaneously:

      - wait long enough to ensure the detection of loss is correct and
        therefore a retransmission is in fact needed, and

      - bound the delay we impose on applications before repairing
    Serving both of these goals is difficult as they pull in opposite
    directions.  I.e., towards either (a) withholding needed
    retransmissions too long to ensure the original transmission is
    truly lost or (b) not waiting long enough---to help application
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