RACK: a time-based fast loss detection algorithm for TCP
draft-ietf-tcpm-rack-04

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TCP Maintenance Working Group                                   Y. Cheng
Internet-Draft                                               N. Cardwell
Intended status: Experimental                               N. Dukkipati
Expires: January 3, 2019                                          P. Jha
                                                             Google, Inc
                                                            July 2, 2018

        RACK: a time-based fast loss detection algorithm for TCP
                        draft-ietf-tcpm-rack-04

Abstract

   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 replace
   the conventional DUPACK threshold approach and its variants, as well
   as other nonstandard approaches.

Status of This Memo

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   This Internet-Draft will expire on January 3, 2019.

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
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   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   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.  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 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 options.

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

   1.  Lost retransmissions.  Traffic policers [POLICER16] and burst
       losses often cause retransmissions to be lost again, severely
       increasing TCP latency.

   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) 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
   [RFC4653][RFC5827][RFC5681][RFC6675][RFC7765][FACK][THIN-STREAM],
   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
   duplicate acknowledgments [RFC5681].  They can either detect loss
   quickly or accurately, but not both, especially when the sender is
   application-limited or under reordering that is unpredictable.  And
   under these conditions none of them can detect lost retransmissions
   well.

   Also, these algorithms, including RFCs, rarely address the
   interactions with other algorithms.  For example, FACK may consider a
   packet is lost while RFC6675 may not.  Implementing N algorithms
   while dealing with N^2 interactions is a daunting task and error-
   prone.

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   The goal of RACK is to solve all the problems above by replacing many