[tcpm] clarifying applicability of PRR: draft-ietf-tcpm-prr-rfc6937bis-12.txt

[Neal Cardwell <[e-mail]>]

Based on suggestions in the TCPM session this week, we have posted updates
today to the PRR draft to try to clarify the breadth of the class of
congestion algorithms for which it is applicable.

Full diff:

https://author-tools.ietf.org/iddiff?url1=draft-ietf-tcpm-prr-rfc6937bis-10&url2=draft-ietf-tcpm-prr-rfc6937bis-12&difftype=--html

The main changes are in the following sections/paragraphs; here is the new
draft text:

2.
<https://datatracker.ietf.org/doc/html/draft-ietf-tcpm-prr-rfc6937bis-12#section-2>
Background
<https://datatracker.ietf.org/doc/html/draft-ietf-tcpm-prr-rfc6937bis-12#name-background>

Congestion control algorithms like Reno [RFC5681
<https://www.rfc-editor.org/info/rfc5681>] and CUBIC [RFC9438
<https://www.rfc-editor.org/info/rfc9438>] require that TCP (and other
protocols) reduce their congestion window (cwnd) in response to losses.
Fast recovery is the reference algorithm for making this adjustment using
feedback from acknowledgements. Its stated goal is to recover TCP's self
clock by relying on returning ACKs during recovery to clock more data into
the network. Without PRR, fast recovery typically adjusts the window by
waiting for a large fraction of a round-trip time (one half round-trip time
of ACKs for Reno [RFC5681 <https://www.rfc-editor.org/info/rfc5681>], or
30% of a round-trip time for CUBIC [RFC9438
<https://www.rfc-editor.org/info/rfc9438>]) to pass before sending any data.
¶
<https://datatracker.ietf.org/doc/html/draft-ietf-tcpm-prr-rfc6937bis-12#section-2-1>

...
4.
<https://datatracker.ietf.org/doc/html/draft-ietf-tcpm-prr-rfc6937bis-12#section-4>Relationships
to other standards
<https://datatracker.ietf.org/doc/html/draft-ietf-tcpm-prr-rfc6937bis-12#name-relationships-to-other-stan>

PRR MAY be used in conjunction with any congestion control algorithm that
intends to make a multiplicative decrease in its sending rate over
approximately the time scale of one round trip time, as long as the current
volume of in-flight data is limited by a congestion window (cwnd) and the
target volume of in-flight data during that reduction is a fixed value
given by ssthresh. In particular, PRR is applicable to both Reno [RFC5681
<https://www.rfc-editor.org/info/rfc5681>] and CUBIC [RFC9438
<https://www.rfc-editor.org/info/rfc9438>] congestion control. PRR is
described as a modification to "A Conservative Loss Recovery Algorithm
Based on Selective Acknowledgment (SACK) for TCP" [RFC6675
<https://www.rfc-editor.org/info/rfc6675>]. It is most accurate with SACK [
RFC2018 <https://www.rfc-editor.org/info/rfc2018>] but does not require
SACK.

...¶
<https://datatracker.ietf.org/doc/html/draft-ietf-tcpm-prr-rfc6937bis-12#section-4-1>

Best regards,
neal


On Sun, Jul 28, 2024 at 1:36 PM <[e-mail]> wrote:

> Internet-Draft draft-ietf-tcpm-prr-rfc6937bis-12.txt is now available. It
> is a
> work item of the TCP Maintenance and Minor Extensions (TCPM) WG of the
> IETF.
>
>    Title:   Proportional Rate Reduction for TCP
>    Authors: Matt Mathis
>             Nandita Dukkipati
>             Yuchung Cheng
>             Neal Cardwell
>    Name:    draft-ietf-tcpm-prr-rfc6937bis-12.txt
>    Pages:   20
>    Dates:   2024-07-28
>
> Abstract:
>
>    This document updates the experimental Proportional Rate Reduction
>    (PRR) algorithm, described RFC 6937, to standards track.  PRR
>    provides logic to regulate the amount of data sent by TCP or other
>    transport protocols during fast recovery.  PRR accurately regulates
>    the actual flight size through recovery such that at the end of
>    recovery it will be as close as possible to the slow start threshold
>    (ssthresh), as determined by the congestion control algorithm.
>
> The IETF datatracker status page for this Internet-Draft is:
> https://datatracker.ietf.org/doc/draft-ietf-tcpm-prr-rfc6937bis/
>
> There is also an HTML version available at:
> https://www.ietf.org/archive/id/draft-ietf-tcpm-prr-rfc6937bis-12.html
>
> A diff from the previous version is available at:
> https://author-tools.ietf.org/iddiff?url2=draft-ietf-tcpm-prr-rfc6937bis-12
>
> Internet-Drafts are also available by rsync at:
> rsync.ietf.org::internet-drafts
>
>
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