Technical Tip: Dial-up IPSec VPN Users Experience Slower Speeds with AES-GCM cipher in the VPN configuration

Dialup IPSec VPN users may experience slower speeds when AES-GCM ciphers are applied in both the Phase 1 and Phase 2 proposals.

This issue is caused by a bottleneck in the authentication tag calculation of the AES-GCM implementation.

Sample Configuration:

config vpn ipsec phase1-interface
    edit "dialup-GCM"
       set type dynamic
       set interface "wan1"
       set ike-version 2
       set peertype any
       set net-device disable
       set mode-cfg enable
       set proposal aes256gcm-prfsha384 aes256gcm-prfsha256 <-----
       set dpd on-idle
       set forticlient-enforcement enable
       set dhgrp 21
       set eap enable
       set eap-identity send-request
       set assign-ip-from name
       set ipv4-netmask 255.255.255.192
       set ipv4-split-include "Dialup_NET"
       set ipv4-name "FORTINET_ADDRESS_POOL"
       set psksecret ENC XXXXXXXX
       set dpd-retryinterval 60
    next
end

config vpn ipsec phase2-interface
    edit "dialup-GCM"
       set phase1name "dialup-GCM"
       set proposal aes256gcm aes128gcm  <-----
       set dhgrp 21
       set replay disable
   next
end

This issue has been resolved in FortiClient versions 7.2.9 and 7.4.3.