Eugene Grosbein 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 do you agree that the traffic dumps contain an anomaly? > but maybe it's work of PMTUD Blackhole detection? > Look at sysctl net.inet.tcp | fgrep blackhole_ On both 192.168.246.10 and 192.168.246.11: root@fbsd-test1:~vas # sysctl net.inet.tcp | fgrep blackhole_ net.inet.tcp.v6pmtud_blackhole_mss: 1220 net.inet.tcp.pmtud_blackhole_mss: 1200 net.inet.tcp.pmtud_blackhole_detection: 0 root@fbsd-test2:~ # sysctl net.inet.tcp | fgrep blackhole_ net.inet.tcp.v6pmtud_blackhole_mss: 1220 net.inet.tcp.pmtud_blackhole_mss: 1200 net.inet.tcp.pmtud_blackhole_detection: 0 root@fbsd-test2:~ # -- Victor Sudakov, VAS4-RIPE, VAS47-RIPN 2:5005/49@fidonet http://vas.tomsk.ru/
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