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becarpenter-book6/1. Introduction and Foreword/How an application programmer sees IPv6.md
Brian E Carpenter 2eac5c6b15 makeBook again
2022-10-08 15:25:37 +13:00

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How an application programmer sees IPv6

In a very theoretical world, an application programmer could rely on a DNS lookup to return the best (and only) address of a remote host, and could then pass that address directly to the network socket interface without further ado. Unfortunately the real world is not that simple. Even without considering the version number, there are several types of IP address, and a DNS lookup may return a variety of addresses. In most cases, applications will use the function getaddrinfo() ("get address information") to obtain a list of valid addresses. Which is the best one to use, and should the program try more than one?

We do not go into this subject in detail, because this book is not aimed primarily at application programmers. However, operators need to be aware that the default behavior of most applications is simply to use the first address returned by getaddrinfo(). Some applications (such as web browsers) may use a smarter approach known as "happy eyeballs" (RFC8305) by means of a heuristic to detect which address gives the fastest response. However, operators need to understand the various address types in order to configure systems optimally, including the getaddrinfo() precedence table (RFC6724) in every host.

Address types are discussed further in 2. Addresses.

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