RADIUS attribute from rfc2868

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Tunnel-Medium-Type

The Tunnel-Medium-Type Attribute indicates which transport medium
      to use when creating a tunnel for those protocols (such as L2TP)
      that can operate over multiple transports.  It MAY be included in
      both Access-Request and Access-Accept packets; if it is present in
      an Access-Request packet, it SHOULD be taken as a hint to the
      RADIUS server as to the tunnel media supported by the tunnel end-
      point.  The RADIUS server MAY ignore the hint, however.

   A summary of the Tunnel-Medium-Type Attribute format is given below.
   The fields are transmitted 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      |    Value      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
      65 for Tunnel-Medium-Type

   Length
      6

   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.  Valid values for this field are 0x01 through 0x1F,
      inclusive.  If the Tag field is unused, it MUST be zero (0x00).

   Value
      The Value field is three octets and contains one of the values
      listed under "Address Family Numbers" in .  For the sake of
      convenience, a relevant excerpt of this list is reproduced below.

   1      IPv4 (IP version 4)
   2      IPv6 (IP version 6)
   3      NSAP
   4      HDLC (8-bit multidrop)
   5      BBN 1822
   6      802 (includes all 802 media plus Ethernet "canonical format")
   7      E.163 (POTS)
   8      E.164 (SMDS, Frame Relay, ATM)9      F.69 (Telex)
   10     X.121 (X.25, Frame Relay)
   11     IPX
   12     Appletalk
   13     Decnet IV
   14     Banyan Vines
   15     E.164 with NSAP format subaddress

        FreeRADIUS Vendor-Specific dictionary values:

                    1 - IP 
                    1 - IPv4 
                    2 - IPv6 
                    3 - NSAP 
                    4 - HDLC 
                    5 - BBN-1822 
                    6 - IEEE-802 
                    7 - E.163 
                    8 - E.164 
                    9 - F.69 
                    10 - X.121 
                    11 - IPX 
                    12 - Appletalk 
                    13 - DecNet-IV 
                    14 - Banyan-Vines 
                    15 - E.164-NSAP 
            

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