I followed this https://fedoraproject.org/wiki/Upgrading_Fedora_using_yum#Fedora_17_-.3E_Fedora_18 guide to upgrade my Fedora 17 to 18.
All was fine until I issued
su -c 'yum --releasever=18 --disableplugin=presto distro-sync -y'. My PuTTY disconnected. Obviously it is because
Cleanup: openssh-server-5.9p1-22.fc17.x86_64 306/574
How can I skip OpenSSH during the distro-sync so my upgrade works fine?