3. Terminology

In this document, the keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in RFC 2119 [RFC2119].

The following definitions are used in this document:

Name: A name is a sequence of labels. The root of the DNS tree is referred to as "the root" and is written as a zero length label (""). The fully qualified domain name (FQDN) of a node in the DNS is the concatenation of all the labels on the path from the node to the root, separated by dots ("."), and with the root itself denoted by a dot at the end of the name. By "concatenation", we mean that the labels are laid out in a sequence in the reverse order (from leaf to root) to the sequence of labels that traverse the path from the root to the node. When the FQDN is written as a sequence of characters (as opposed to a sequence of labels), all the labels are in the same uppercase or lowercase.

Label: A label is an individual component of a domain name. In the dotted string notation of a domain name, the labels are the character strings separated by dots. Note that the length of a label, in this context, is measured in characters (not bytes or octets). The maximum length of a label is 63 characters. The textual representation of a label is a string of characters which is used to represent the label. There are two kinds of labels: "data" labels, which are defined in section 11, and "compression" labels which are described in section 4.1.4 of RFC1035.

Domain Name: A domain name is a name in the domain name system. A domain name is a sequence of labels. A domain name is written as a sequence of labels separated by dots. A domain name is said to be "absolute" (or "fully qualified") if it includes the root label (i.e. it ends with a dot). A domain name is said to be "relative" if it does not include the root label.

RR (Resource Record): A Resource Record is a member of the class of data structures that are contained in the Zone Files (or in the DNS), and which have a particular structure, as defined in RFC1035. An RR is composed of a NAME, TYPE, CLASS, TTL, RDLENGTH, and RDATA fields.

RRSet (Resource Record Set): An RRSet is a set of one or more RRs with the same NAME, CLASS, and TYPE. The TTL of all RRs in an RRSet must be the same (see section 5.2).

Zone: A zone is a collection of domains, implemented in a particular data file, and which is managed by a particular name server. A zone may be, and often is, a single domain. It may also be a collection of domains. For the purposes of this document, the important aspect of a zone is that it is the unit of administrative control. That is, the zone is the smallest collection of domains over which the changes are controlled by a single entity. This is also the unit of transfer (see section 6 of RFC1034) and the unit of caching.

Authoritative: The authoritative server for a zone is the server that has the data for the zone. An answer is authoritative if it comes from the authoritative server for the zone. An answer is not authoritative if it comes from a server that is not the authoritative server for the zone, or if it comes from a cache.

Cache: A cache is a local copy of a portion of the DNS. A cache is maintained by a resolver, and is used to reduce the load on the DNS by eliminating the need to query the DNS for data that is likely to be frequently used. A cache does not contain any authoritative data.

Resolver: A resolver is a program that is used to extract information from the DNS. A resolver may be a stub resolver (which forwards all queries to a recursive resolver), or it may be a recursive resolver (which performs the queries itself).