Date: Sat, 18 Jan 2020 18:16:45 +0700 From: Eugene Grosbein <eugen@grosbein.net> To: Victor Sudakov <vas@sibptus.ru> Cc: freebsd-net@freebsd.org, "Andrey V. Elsukov" <bu7cher@yandex.ru>, Michael Tuexen <tuexen@freebsd.org> Subject: Re: IPSec transport mode, mtu, fragmentation... Message-ID: <d59805e9-3fd5-eb56-10db-26b532cb5e85@grosbein.net> In-Reply-To: <20200118105524.GA10042@admin.sibptus.ru> References: <f9b7357e-ced1-4ce5-40d5-8e3dcad42442@yandex.ru> <d263a709-63cf-7da5-1747-8a6791f6503f@grosbein.net> <20200116155305.GA465@admin.sibptus.ru> <55f7bafa-24c4-9810-0d21-f82cb332ee2d@grosbein.net> <20200116160745.GA1356@admin.sibptus.ru> <72355e03-1cf8-c58f-3aec-b0a21e631870@grosbein.net> <20200117093645.GA51899@admin.sibptus.ru> <70b0b855-189b-03c2-0712-fc1e35640702@grosbein.net> <20200117150928.GB66677@admin.sibptus.ru> <16550199-67b9-d331-0c1e-4afa0e8b361c@grosbein.net> <20200118105524.GA10042@admin.sibptus.ru>
next in thread | previous in thread | raw e-mail | index | archive | help
18.01.2020 17:55, Victor Sudakov wrote: >>>>> Back to the point. I've figured out that both encrypted (in transport >>>>> mode) and unencrypted TCP segments have the same MSS=1460. Then I'm >>>>> completely at a loss how the encrypted packets avoid being fragmented. >>>>> TCP has no way to know in advance that encryption overhead will be >>>>> added. > > Here: http://admin.sibptus.ru/~vas/ftp-pcap.tar.gz you can find two > identical FTP sessions, the only difference being ipsec=off during one > session and ipsec=on during the other one. > > As I said, in both the sessions MSS=1460 which is already odd, and I > can't explain to myself why file transfer still works without MSS > ajustment. > > Moreover, something fishy is happening in the encrypted session: there > are many TCP retransmissions (I was capturing on the FTP server's side, > so there are many segments with the same sequence number). How would you > explain this? There are almost no retransmissions in the unencrypted session. > > All this is happening in a lab environment (one bhyve VM is an FTP > server and the other downloads a file from the first), both VMs are on > the same bridge interface. There are almost 19,000 packets in the > encrypted file vs 12,000 in the plain file, I think because of those > excessive retransmissions. > > Could the retransmissions be some artifact of the enc(4) interface I was > capturing the encrypted session on? I doubt it. And I can't explain this, but maybe it's work of PMTUD Blackhole detection? Look at sysctl net.inet.tcp | fgrep blackhole_
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?d59805e9-3fd5-eb56-10db-26b532cb5e85>