Internet Layer

This layer, also known as the network layer, accepts and delivers packets for the network. It includes the powerful Internet protocol (IP), the Address Resolution Protocol (ARP) protocol, and the Internet Control Message Protocol (ICMP) protocol.

IP Protocol

The IP protocol and its associated routing protocols are possibly the most significant of the entire TCP/IP suite. IP is responsible for:

• IP addressing - The IP addressing conventions are part of the IP protocol. (Chapter 5, Planning Your TCP/IP Network describes IPv4 addressing in detail and Chapter 14, Overview of IPv6 describes IPv6 addressing in detail.)

• Host-to-host communications - IP determines the path a packet must take, based on the receiving host's IP address.

• Packet formatting - IP assembles packets into units known as IP datagrams. Datagrams are fully described in "Internet Layer".

• Fragmentation - If a packet is too large for transmission over the network media, IP on the sending host breaks the packet into smaller fragments. IP on the receiving host then reconstructs the fragments into the original packet.

Previous releases of the Solaris operating environment implemented version 4 of the Internet Protocol, which is written IPv4. However, because of the rapid growth of the Internet, it was necessary to create a new Internet Protocol with improved capabilities, such as increased address space. This new version, known as version 6, is written IPv6. The Solaris operating environment supports both versions, which are described in this book. To avoid confusion when addressing the Internet Protocol, the following convention is used:

• When the term IP is used in a description, the description applies to both IPv4 and IPv6.

• When the term IPv4 is used in a description, the description applies only to IPv4.

• When the term IPv6 is used in a description, the description applies only to IPv6.

ARP Protocol

The Address Resolution Protocol (ARP) conceptually exists between the data link and Internet layers. ARP assists IP in directing datagrams to the appropriate receiving host by mapping Ethernet addresses (48 bits long) to known IP addresses (32 bits long).

ICMP Protocol

Internet Control Message Protocol (ICMP) is the protocol responsible for detecting network error conditions and reporting on them. ICMP reports on:

• Dropped packets (when packets are arriving too fast to be processed)

• Connectivity failure (when a destination host can't be reached)

• Redirection (which tells a sending host to use another router)

The "ping Command" contains more information on the operating system commands that use ICMP for error detection.