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[tsvwg] draft-kuhn-quic-careful-resume-02: Linux tcp_metrics

Piers O'Hanlon <piers.ohanlon@bbc.co.uk> Tue, 08 November 2022 08:53 UTC

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From: Piers O'Hanlon <piers.ohanlon@bbc.co.uk>
To: "tsvwg@ietf.org" <tsvwg@ietf.org>
Thread-Topic: draft-kuhn-quic-careful-resume-02: Linux tcp_metrics
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Date: Tue, 08 Nov 2022 08:52:05 +0000
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Archived-At: <https://mailarchive.ietf.org/arch/msg/tsvwg/LzfCrub0E_jmzF20DKxyvrAUSqU>
Subject: [tsvwg] draft-kuhn-quic-careful-resume-02: Linux tcp_metrics
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Hi,

As suggested by Gorry in yesterday’s WG meeting I’m posting what I brought to the mic.

I wanted to highlight that it would be useful to examine the existing related Linux kernel mechanisms which re-use cached metrics for TCP connections. I’ve briefly looked at the logic which stores a metric block for all connections that finish successfully (net/ipv4/tcp_metrics.c:tcp_update_metrics()) which are accessible via the `ip` command:
e.g.
$ ip tcp_metrics
192.168.0.1 age 528954.152sec ssthresh 14 cwnd 15 rtt 14439us rttvar 18665us source 10.10.10.3
…

When a new connection starts metrics are reused according to rules defined (net/ipv4/tcp_metrics.c:tcp_init_metrics()).

I understand some of this also happens in other operating systems but I’m not aware of the details.

Piers

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