Blocking all ICMP traffic can disable PING, TRACERT, and PATHPING.

For a list of protocol IDs (RFC791):
	C:\WINNT\SYSTEM32\DRIVERS\ETC\SERVICES
	http://www.isi.edu/in-notes/iana/assignments/protocol-numbers/

For a list of service ports:
	C:\WINNT\SYSTEM32\DRIVERS\SERVICES

TCP/IP Ports:

	UDP:67:   DHCP: RX client messages
	UDP:68:   DHCP: RX Server replies
	UDP:500:  Internet Key Exchange (IKE) (IPSec)
	TCP:1723: PPTP Maintaince (only port needed)
	TCP:3389: RDP (Remote Desktop Protocol / Terminal Services)

IP Protocol IDs:

	47: GRE Protocol
	50: L2TP ESP Traffic
	51: L2TP AH Traffic
	89: Open Shortest Path First (OSPF) Protocol

ICMP Traffic:

	ICMP Type  5:  Redirection
	ICMP Type  9:  Router Advertisement
	ICMP Type 10:  Solicitation

To open L2TP @ firewall, open:
	IP Protocol: 50,51
	UDP:500

To open PPTP @ firewall, open:
	IP Protocol: 47
	TCP:1723