netdev - tcp_mtu_probe implementation details

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Message-ID: <op.v6s5j2i0fzo5me@stalin>
Date:	Wed, 21 Dec 2011 01:49:04 +0400
From:	"Anatoly Sivov" <mm05@...l.ru>
To:	netdev@...r.kernel.org
Subject: tcp_mtu_probe implementation details

Hi all,

Looking on TCP implementation in Linux I see that there are some things
that I don't understand in tcp_mtu_probe() implementation.
Could someone clear them for me or share the link to Linux PMTUD
documentation, please?
The first thing I don't understand is tp->snd_cwnd < 11 in
if (!icsk->icsk_mtup.enabled ||
             icsk->icsk_mtup.probe_size ||
             inet_csk(sk)->icsk_ca_state != TCP_CA_Open ||
             tp->snd_cwnd < 11 ||
             tp->rx_opt.num_sacks || tp->rx_opt.dsack)
                 return -1;
What is magic number "11" here?
The other question is about size_needed variable.
It is assigned to value probe_size + (tp->reordering + 1) * tp->mss_cache
And that is not clear for me.
What is this "(tp->reordering + 1) * tp->mss_cache" addition?

Thanks.
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