Retransmission Timeout Requirements

Document Type Active Internet-Draft (tcpm WG)
Last updated 2017-03-10
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 2016 - Submit document on R... )
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Internet Engineering Task Force                                M. Allman
INTERNET-DRAFT                                                      ICSI
File: draft-ietf-tcpm-rto-consider-05.txt                 March 10, 2017
Intended Status: Best Current Practice
Expires: September 10, 2017

                  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
    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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Copyright Notice
    Copyright (c) 2017 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
    carefully, as they describe your rights and restrictions with
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    warranty as described in the Simplified BSD License.


    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: October 10, 2017                                       [Page 1]
draft-ietf-tcpm-rto-consider-05.txt                           March 2017

    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
    reports 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
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