RADIUS attribute from rfc2868
This Attribute contains the address of the initiator end of the tunnel. It MAY be included in both Access-Request and Access- Accept packets to indicate the address from which a new tunnel is to be initiated. If the Tunnel-Client-Endpoint Attribute is included in an Access-Request packet, the RADIUS server should take the value as a hint; the server is not obligated to honor the hint, however. This Attribute SHOULD be included in Accounting- Request packets which contain Acct-Status-Type attributes with values of either Start or Stop, in which case it indicates the address from which the tunnel was initiated. This Attribute, along with the Tunnel-Server-Endpoint and Acct-Tunnel-Connection- ID attributes, may be used to provide a globally unique means to identify a tunnel for accounting and auditing purposes. A summary of the Tunnel-Client-Endpoint Attribute format is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Tag | String ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 66 for Tunnel-Client-Endpoint. Length >= 3Tag The Tag field is one octet in length and is intended to provide a means of grouping attributes in the same packet which refer to the same tunnel. If the value of the Tag field is greater than 0x00 and less than or equal to 0x1F, it SHOULD be interpreted as indicating which tunnel (of several alternatives) this attribute pertains. If the Tag field is greater than 0x1F, it SHOULD be interpreted as the first byte of the following String field. String The format of the address represented by the String field depends upon the value of the Tunnel-Medium-Type attribute. If Tunnel-Medium-Type is IPv4 (1), then this string is either the fully qualified domain name (FQDN) of the tunnel client machine, or it is a "dotted-decimal" IP address. Conformant implementations MUST support the dotted-decimal format and SHOULD support the FQDN format for IP addresses. If Tunnel-Medium-Type is IPv6 (2), then this string is either the FQDN of the tunnel client machine, or it is a text representation of the address in either the preferred or alternate form . Conformant implementations MUST support the preferred form and SHOULD support both the alternate text form and the FQDN format for IPv6 addresses. If Tunnel-Medium-Type is neither IPv4 nor IPv6, this string is a tag referring to configuration data local to the RADIUS client that describes the interface and medium-specific address to use.