The primary function of protocols at this layer is to facilitate communication between hosts on different networks.
The predominant protocol used at this layer is the Internet Protocol (IP). Currently, two versions are in use: IPv4 and IPv6. Despite the differences, their primary features are the same:
- Routing via IP addressing
- Encapsulation of data into packets
Data Packet
The Protocol Data Unit (PDU) within the IP protocol is called a packet. It contains a Data Payload and a Header. Of course, the Data Payload of an IP packet is the PDU from the layer above it (the Transport layer). It is generally, although not always, a TCP segment or UDP datagram.
The header is split into logical fields determined by the set size of each field in bits, and the order within the packet.
The purpose of the Layer 3 header is to get the data from end to end.
There are fourteen fields in the header. The most important ones are:
- Version: IPv4 or IPv6
- ID, Flags, Fragment Offset: used to reassemble a packet in the correct order when the Transport layer PDU is too large to be sent at once.
- TTL: Time To Live value. Indicates the maximum hops a packet can take before being dropped. At each network hops the TTL value is decremented by one.
- Protocol: protocol used for the Data Payload: TCP, UDP etc.
- Checksum: error checking value. The packet is dropped if the calculated value doesn’t match the given one; this may indicate corruption somewhere on the line. IP does not manage retransmission of dropped packets.
- Source Address: the 32-bit IP address of the sender of the packet
- Destination Address: the 32-bit IP address of the intended recipient of the packet
IP Addresses
IP addresses are logical: they are not, like the MAC Addresses, tied to a specific device but assigned as required. The assigned IP address is whitin a range of available addresses, generally given automatically by a Dynamic Host Configuration Protocol (DHCP) server or router.
ARP Table
An ARP Table is a mapping of an IP address to a MAC address. All Layer 3 (this layer) devices will have an ARP table.