netdev - Re: A TLP implementation question

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Date:   Tue, 13 Feb 2018 17:11:08 -0800
From:   Yuchung Cheng <ycheng@...gle.com>
To:     hiren panchasara <hiren@...ugglingcoder.info>
Cc:     netdev <netdev@...r.kernel.org>,
        Neal Cardwell <ncardwell@...gle.com>
Subject: Re: A TLP implementation question

On Tue, Feb 13, 2018 at 4:27 PM, hiren panchasara
<hiren@...ugglingcoder.info> wrote:
>
> Looking at current net-next to understand an aspect of TLP (tail loss
> probe) implementation.
>
> https://tools.ietf.org/html/draft-ietf-tcpm-rack-02 is the source of
> truth now for TLP and 6.2.1.  Phase 1: Scheduling a loss probe
> Step 1: Check conditions for scheduling a PTO. has following as one of
> the conditions:
> (d) The most recently transmitted data was not itself a TLP probe
> (i.e. a sender MUST NOT send consecutive TLP probes)
this is done by
1) calling tcp_write_xmit(push_one==2) in tcp_send_loss_probe()
2) avoid calling tcp_schedule_loss_probe() if push_one == 2 in tcp_write_xmit()
3) abort if one TLP probe is inflight by checking tlp_high_seq in
tcp-send_loss_probe()

consequently the sender will never schedule a PTO upon sending a probe
(new or rtx) to avoid consecutive probes.

hth.

>
> I would appreciate if someone can help me trace how current code is
> trying to enforce this requirement. How does it check/track that the
> last (re)transmitted packet was a tlp probe.
>
> Thanks in advance,
> Hiren