If you are having an issue with the Cisco VPN client after upgrading to OS X Mavericks, here is the solution...
open a terminal (iterm)
# edit the sysctl.conf file
sudo vim /etc/sysctl.conf
# comment out the following line.
kern.ipc.maxsuckbuf=512000.
Your file should look like this after you edit...
#
# Tuning network for broadband
#
# START
# kern.ipc.maxsockbuf=512000
net.inet.tcp.sendspace=131072
net.inet.tcp.recvspace=358400
# END
# Last step is to reboot.
