Gateways implement internet protocol to forward datagrams between networks. Gateways also implement the Gateway to Gateway Protocol (GGP) [7] to coordinate routing and other internet control information.
In a gateway the higher level protocols need not be implemented and the GGP functions are added to the IP module.
+-------------------------------+
| Internet Protocol & ICMP & GGP|
+-------------------------------+
| |
+---------------+ +---------------+
| Local Net | | Local Net |
+---------------+ +---------------+
Gateway Protocols
Figure 3.