RADIUS attribute from rfc2868

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Tunnel-Server-Endpoint

This Attribute indicates the address of the server end of the
      tunnel.  The Tunnel-Server-Endpoint Attribute MAY be included (as
      a hint to the RADIUS server) in the Access-Request packet and MUST
      be included in the Access-Accept packet if the initiation of a
      tunnel is desired.  It SHOULD be included in Accounting-Request
      packets which contain Acct-Status-Type attributes with values of
      either Start or Stop and which pertain to a tunneled session.
      This Attribute, along with the Tunnel-Client-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-Server-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
      67 for Tunnel-Server-Endpoint.

   Length
      >= 3

   Tag
      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 not IPv4 or 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.

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