IPSec transport mode, mtu, fragmentation...

[Michael Sierchio]

[apologies for  top-posting]

What is the result of

> sysctl net.enc

?  This might be a clue about the packets, which you could be seeing twice.

On Sat, Jan 18, 2020 at 3:17 AM Eugene Grosbein <eugen at grosbein.net> wrote:

> 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_
>
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