_ [comp] 続・VPN 実験再び

ちょっと大きめのデータを流すとすぐ

pptp[XXXXX]: anon log[pptp_read_some:pptp_ctrl.c:543]: read returned zero, peer has closed
pptp[XXXXX]: anon log[callmgr_main:pptp_callmgr.c:255]: Closing connection (shutdown)
pptp[XXXXX]: anon log[ctrlp_rep:pptp_ctrl.c:251]: Sent control packet type is 12 'Call-Clear-Request' 
pptp[XXXXX]: anon log[pptp_read_some:pptp_ctrl.c:543]: read returned zero, peer has closed
pptp[XXXXX]: anon log[call_callback:pptp_callmgr.c:78]: Closing connection (call state)
pppd[XXXXX]: Modem hangup

となって切れてしまう。本家の Diagnosis HOWTO に

manually restrict the MTU on the interface, by adding mtu 1404 or some smaller value as an option to the pppd program, either in the peers file for the tunnel, in the options file, or on the command line. You can also restrict the MTU on the interface after the tunnel has connected, using a command like ifconfig ppp1 mtu 1400.

The effect is that new TCP connections from your host will use an maximum segment size (MSS) that is lower. This may prevent path MTU discovery from being necessary.

ref. Connections via tunnel freeze

とあったので、MTU を落としてみる。そこに書かれている 1400 ではだめで、1000 でもだめだったので、ダイヤルアップの 576 を指定してみると、嘘のように安定した。