Section: 9.1 Structure of APN
Content: The APN is composed of two parts as follows: - The APN Network Identifier; this defines to which external network the GGSN/PGW is connected and optionally a requested service by the MS. This part of the APN is mandatory. - The APN Operator Identifier; this defines in which PLMN GPRS/EPS backbone the GGSN/PGW is located. This part of the APN is optional. NOTE 1: The APN Operator Identifier is mandatory on certain interfaces, see the relevant stage 3 specifications. The APN Operator Identifier is placed after the APN Network Identifier. An APN consisting of both the Network Identifier and Operator Identifier corresponds to a DNS name of a GGSN/PGW; the APN has, after encoding as defined in the paragraph below, a maximum length of 100 octets. The encoding of the APN shall follow the Name Syntax defined in RFC 2181 [18], RFC 1035 [19] and RFC 1123 [20]. The APN consists of one or more labels. Each label is coded as a one octet length field followed by that number of octets coded as 8 bit ASCII characters. Following RFC 1035 [19] the labels shall consist only of the alphabetic characters (A-Z and a-z), digits (0-9) and the hyphen (-). Following RFC 1123 [20], the label shall begin and end with either an alphabetic character or a digit. The case of alphabetic characters is not significant. The APN is not terminated by a length byte of zero. NOTE 2: A length byte of zero is added by the SGSN/MME at the end of the APN before interrogating a DNS server. For the purpose of presentation, an APN is usually displayed as a string in which the labels are separated by dots (e.g. "Label1.Label2.Label3").