RACK: a time-based fast loss detection algorithm for TCP

Document Type Active Internet-Draft (tcpm WG)
Last updated 2020-05-27 (latest revision 2020-03-09)
Replaces draft-cheng-tcpm-rack
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TCP Maintenance Working Group                                   Y. Cheng
Internet-Draft                                               N. Cardwell
Intended status: Standards Track                            N. Dukkipati
Expires: September 10, 2020                                       P. Jha
                                                             Google, Inc
                                                           March 9, 2020

        RACK: a time-based fast loss detection algorithm for TCP


   This document presents a new TCP loss detection algorithm called RACK
   ("Recent ACKnowledgment").  RACK uses the notion of time, instead of
   packet or sequence counts, to detect losses, for modern TCP
   implementations that can support per-packet timestamps and the
   selective acknowledgment (SACK) option.  It is intended to be an
   alternative to the DUPACK threshold approach [RFC6675], as well as
   other nonstandard approaches such as FACK [FACK].

Status of This Memo

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Copyright Notice

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   carefully, as they describe your rights and restrictions with respect

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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.  In this document, these words will appear
   with that interpretation only when in UPPER CASE.  Lower case uses of
   these words are not to be interpreted as carrying [RFC2119]

2.  Introduction

   This document presents a new loss detection algorithm called RACK
   ("Recent ACKnowledgment").  RACK uses the notion of time instead of
   the conventional packet or sequence counting approaches for detecting
   losses.  RACK deems a packet lost if it has not been delivered and
   some packet sent sufficiently later has been delivered.  It does this
   by recording packet transmission times and inferring losses using
   cumulative acknowledgments or selective acknowledgment (SACK) TCP

   In recent years we have been observing several increasingly common
   loss and reordering patterns in the Internet:

   1.  Slow recovery due to lost retransmissions.  Traffic policers
       [POLICER16] and burst losses often cause retransmissions to be
       lost again.  This severely increases latency because the lost
       retransmissions can only be recovered by retransmission timeouts

   2.  Tail drops.  Structured request-response traffic turns more
       losses into tail drops.  In such cases, TCP is application-
       limited, so it cannot send new data to probe losses and has to
       rely on retransmission timeouts (RTOs).

   3.  Reordering.  Link-layer protocols (e.g., 802.11 block ACK), link
       bonding, or routers' internal load-balancing can deliver TCP
       packets out of order.  The degree of such reordering is usually
       within the order of the path round trip time.

   Despite TCP stacks (e.g.  Linux) that implement many of the standard
   and proposed loss detection algorithms

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   we've found that together they do not perform well.  The main reason
   is that many of them are based on the classic rule of counting
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