The Internet Protocol (IP) helps data find its way to its destination even if the data must travel through other networks, including the many and varied networks that make up the Internet. Although it’s called the Internet Protocol, local networks can use IP as well. Many communications in local networks use IP because they use its companion protocols, TCP and UDP. This section introduces IP, including how computers obtain IP addresses, the format of IP datagrams, how IP and the domain name system help in getting messages to their destinations, and how embedded systems can use IP in communicating in local networks and on the Internet.

What IP Does

Network article 4-8 shows the place of the IP layer in network communications in the networking stack introduced in Chapter 1. In transmitting, the IP layer receives a message to send from a higher-level protocol layer such as TCP or UDP. The IP layer places the message in an IP datagram that consists of an IP header, followed by the message to send. The IP layer then passes the datagram to a lower layer such as an Ethernet driver, which sends the datagram on the network. On the way to its destination, a datagram may pass through one or more routers. The router examines the destination’s IP address and uses the address in deciding where to forward the datagram. At the destination computer, the Ethernet layer or another network interface passes the IP datagram to the IP layer, which removes the IP header. Information in the header tells the computer what protocol layer, such as TCP or UDP, should receive the datagram’s message.

The Internet Protocol performs two major functions.

• It defines a way to specify source and destination addresses for use with any network interface and across networks that use different interfaces.

• It enables a datagram to pass through networks of varying capabilities by defining a protocol that allows a router to fragment, or divide, a datagram into multiple, smaller datagrams and enables the destination to reassemble the original message from the fragments.

Two things IP doesn’t provide are flow control and error checking of the data payload. When needed, a higher-level protocol such as TCP can provide these. For local communications, Ethernet frames also provide error checking. Two protocols can help in matching an IP address to a computer, or to be more precise, to a network interface (because a single computer can have multiple network interfaces). The Domain Name System (DNS) protocol described earlier in this networking tutorial enables a computer to learn the IP address that corresponds to a domain name. And in Ethernet networks, the Address Resolution Protocol (ARP) described later in this networking tutorial enables the sender of an IP datagram to match an Ethernet hardware address with an IP address in the local network. The examples in this networking tutorial use version 4 of IP (IPv4), which most networks are using at this writing. The expected replacement for IPv4 is IP version 6 (IPv6), which greatly increases the number of available IP addresses and adds other improvements for more efficient and secure transfers. It’s likely that IPv6 routers will continue to support IPv4 for some time, so computers that support only IPv4 should have no trouble communicating with any destination.

The standards for IP and related protocols are the responsibility of the Internet Engineering Task Force (IETF) and its working groups (www.ietf.org). The IETF is open to anyone who has the necessary skills and abilities and wants to contribute. The documents that define IP and many other networking protocols are available from the Request for Comments (RFC) Web site (www.rfc-editor. org). This book contains a number of references to RFC documents, so perhaps it’s appropriate to say a few words about the documents and where they come from. The RFC Editor is a group funded by the Internet Society (ISOC). ISOC in turn is an organizational home for groups who are responsible for various standards relating the Internet’s infrastructure. The RFC Web site is a repository for RFC documents, which include standards- track documents as well as technical and organizational notes relating to networking and the Internet. The standards-track documents contain specifications that have undergone a review process to become approved standards.

Request for Comments may sound like an odd designation for an approved standard, and in fact, approved standards have alternate designations that use the STD prefix. For example, the document that defines IP is RFC0791: Internet Protocol. The standards-track document that includes RFC0791 and related documents is STD0005. The IETF’s Internet Engineering Steering Group (IESG) is responsible for approving specifications as standards. A protocol doesn’t have to be an approved standard before becoming widely implemented, however.