Fix broken fragmentation logic when using NCP

Fix broken fragmentation logic when using NCP

This is the 2.4 backport of master patch (commit d22ba6b).

NCP negotiation replaces worst case crypto overhead
with actual one in data channel frame. That frame
params are used by mssfix. Fragment frame still contains
worst case overhead.

Without this patch, fragmentation logic incorrectly uses
max crypto overhead when calculating packet size. It exceeds
fragment size and openvpn peforms fragmentation:

> sudo tcpdump port 1194
13:59:06.956394 IP server.fi.openvpn > nat2.panoulu.net.openvpn: UDP,
length 652
13:59:06.956489 IP server.fi.openvpn > nat2.panoulu.net.openvpn: UDP,
length 648

This patch fixes fragmentation calculation by
setting actual crypto overhead, and no unnecessary
fragmentation is performed:

> sudo tcpdump port 1194
13:58:08.685915 IP server.fi.openvpn > nat2.panoulu.net.openvpn: UDP,
length 1272
13:58:08.686007 IP server.fi.openvpn > nat2.panoulu.net.openvpn: UDP,
length 1272

Trac #1140

Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1572439499-16276-1-git-send-email-lstipakov@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18975.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>