Retransmission Timeout Considerations
draft-ietf-tcpm-rto-consider-01

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Last updated 2016-02-29
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Internet Engineering Task Force                                M. Allman
INTERNET-DRAFT                                                      ICSI
File: draft-ietf-tcpm-rto-consider-01.txt              February 29, 2016
Intended Status: Best Current Practice
Expires: August 29, 2016

    
                 Retransmission Timeout Considerations

Status of this Memo

    This document may not be modified, and derivative works of it may
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    Copyright (c) 2015 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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Abstract

    Each implementation of a retransmission timeout mechanism must
    balance correctness and timeliness and therefore no implementation
    suits all situations.  This document provides high-level guidance
    for retransmission timeout schemes appropriate for general use in

Expires: August 29, 2016                                        [Page 1]
draft-ietf-tcpm-rto-consider-01.txt                        February 2016

    the Internet.  Within the guidelines, implementations have latitude
    to define particulars that best address each situation.

Terminology

    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
    document are to be interpreted as described in BCP 14, RFC 2119
    [RFC2119].

1   Introduction

    Despite our best intentions and most robust mechanisms, reliability
    in networking ultimately requires a timeout and re-try mechanism.
    Often there are more timely and precise mechanisms than a timeout
    for repairing loss (e.g., TCP's fast retransmit [RFC5681], NewReno
    [RFC6582] or selective acknowledgment scheme [RFC2018,RFC6675])
    which require information exchange between components in the system.
    Such communication cannot be guaranteed.  Alternatively, information
    coding---e.g., FEC---can allow the recipient to recover from some
    amount of lost information without use of a retransmission.  This
    latter provides probabilistic reliability.  Finally, negative
    acknowledgment schemes exist that do not depend on continuous
    feedback to trigger retransmissions (e.g., [RFC3940]).  However,
    regardless of these useful alternatives, the only thing we can truly
    depend on is the passage of time and therefore our ultimate backstop
    to ensuring reliability is a timeout.  (Note: There is a case when
    we cannot count on the passage of time, but in this case we believe
    repairing loss will be a moot point and hence we do not further
    consider this case in this document.)

    Various protocols have defined their own timeout mechanisms (e.g.,
    TCP [RFC6298], SCTP [RFC4960]).  Ideally, if we know a segment will
    be lost before reaching the destination, a second copy of it would
    be sent immediately after the first transmission.  However, in
    reality 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 decision to retransmit is
        correct.  

      - Bound the delay we impose on applications before
        retransmitting. 
    
    However, serving both of these goals is difficult as they pull us in
    opposite directions.  I.e., towards either (a) withholding needed
    retransmissions too long or (b) not waiting long enough and sending
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