3. Message Syntax

   The only element defined in this profile is the "tunnel" element.  It
   is described in the following DTD, with additional limitations as
   described afterwards.

   <!--
     DTD for the TUNNEL Profile, as of 2001-02-03

     Refer to this DTD as:

        <!ENTITY % TUNNEL PUBLIC "-//IETF//DTD TUNNEL//EN" "">
       %TUNNEL;
     -->

   <!--
     TUNNEL messages

        role           MSG                 RPY
       ======          ===                 ===
       I or L          TUNNEL              +: ok
                                           -: error
     -->

   <!ELEMENT tunnel      (tunnel?)>
   <!ATTLIST tunnel
             fqdn         CDATA    #IMPLIED
             ip4          CDATA    #IMPLIED
             ip6          CDATA    #IMPLIED
             port         CDATA    #IMPLIED
             srv          CDATA    #IMPLIED
             profile      CDATA    #IMPLIED
             endpoint     CDATA    #IMPLIED
             >

   The format of the "fqdn" attribute is a fully qualified domain name,
   such as "proxy.example.com".  The format of the "ip4" attribute is
   four sets of decimal numbers separated by periods, such as
   "10.23.34.45".  The format of the "ip6" attribute is as specified in
   RFC2373(-> 3513(-> 4291draft)) [4].  The format of the "port" attribute is a decimal number
   between one and 65535, inclusive.  The format of the "srv" attribute
   is a pair of identifiers each starting with an underline and
   separated by a period, such as "_sep._tcp".  The format of the
   "profile" attribute is a URI [5].  The format of the "endpoint"
   attribute is any string that may appear as an attribute value.

   The only allowable combinations of attributes are as follows:

   o  fqdn + port;

   o  fqdn + srv;

   o  fqdn + srv + port;

   o  ip4  + port;

   o  ip6  + port;

   o  profile, but only on the innermost element;

   o  endpoint, but only on the innermost element; or,

   o  no attributes, but only on the innermost element.