Internet Protocol (IP) and Internet Control Message Protocol (ICMP) are the protocols present at the network layer.
IP provides machine-to-machine communication. It performs transmission routing by determining the path a transmission must take, based on the receiving machine's IP address. IP also provides transmission formatting services; it assembles data for transmission into an ``internet datagram''. The datagram defines the structure and form in which the information is sent. If the datagram is outgoing (received from the higher layer protocols), IP attaches an IP header to it. This header contains a number of parameters, including the IP addresses of the sending and receiving hosts. For more information about IP, refer to the ip(7tcp) manual page.
ICMP sends error or control messages to other hosts. For more information about ICMP, refer to the icmp(7tcp) manual page.