Determining SCTP's Retransmission Timer
draft-jovev-tsvwg-sctp-rto-00

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Internet Engineering Task Force                                 D. Jovev
Internet-Draft                                                  Ericsson
Intended status: Standards Track                      September 21, 2017
Expires: March 25, 2018

                Determining SCTP's Retransmission Timer
                     draft-jovev-tsvwg-sctp-rto-00

Abstract

   This document defines a modification in the RFC 4960 [RFC4960]
   defined Stream Control Transmission Protocol's (SCTP's)
   Retransmission Timer (RTO) calculation method.

   The modification is aimed to reduce the frequency of spurious T3
   timeouts, which are caused by underestimated RTO values, derived by
   the [RFC4960] defend RTO calculation method.  The proposed
   modification aligns the RTO calculation method with the
   characteristics of the statistical estimator algorithms, which are
   used for SRTT and RTTVAR calculation, the SCTP protocol data transfer
   rules and the characteristics of the data packets' arrival pattern in
   the telecom signalling networks.

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Jovev                    Expires March 25, 2018                 [Page 1]
Internet-Draft                                            September 2017

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions and Terminology . . . . . . . . . . . . . . .   3
   2.  Problem description . . . . . . . . . . . . . . . . . . . . .   3
   3.  The modified algorithm for RTO Calculation  . . . . . . . . .   6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Appendix A.  Technical background for the modifications in the
                RTO calculation algorithm  . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   Like TCP, the SCTP's reliable transfer of data is ensured by limiting
   the time in which the acknowledgement for the reception of the
   transmitted data is received, after which expiration all
   unacknowledged data is retransmitted.  The duration of this timer is
   referred to as Retransmission Timeout (RTO) and the actual timer is
   called T3-rtx or just T3.

   The expiration of the T3 timer not only invokes retransmission of the
   unacknowledged data it also drastically reduces the congestion window
   (cwnd) to 1 MTU, which are both undesirable actions: data
   retransmission increases the amount of sent data in the network, and
   1 MTU cwnd drastically reduces the SCTP association transmission
   capacity.  Because of that, determining an RTO value which reflects
   the highest RTT, or the highest feedback time, as more appropriately
   called in [ALLMAN99], is critical for reducing the probability of
   spurious T3 timeouts, which is critically important for stable SCTP
   operation.

   Namely, while in the conventional file transfer applications the
   transport layer transmission capacity reduction, due to T3 timeouts,
   only prolongs the time for completion of the file transfer, in the
   telecom signalling networks it often results in false congestion
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