rfc1841.txt

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   The LAN extension interface unit responds to a LEX_RCMD_REQUEST
   packet with a LEX_RCMD_ACK packet when it correctly receives the
   request and is able to perform the request.

   LEX RCMD_NAK Packet

   The LAN extension interface unit responds to a LEX_RCMD_REQUEST
   packet with a LEX_RCMD_NAK packet when the LAN extension interface
   unit recognizes all the elements of the remote command option, but
   some elements are not acceptable. Upon receipt of a LEX_RCMD_NAK
   packet, the host router immediately stops sending the request.



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RFC 1841           LAN Extension Interface Protocol       September 1995


   LEX RCMD_REJ Packet

   The LAN extension interface unit responds to a LEX_RCMD_REQUEST
   packet with a LEX_RCMD_REJ packet when the Option-Type value in the
   request packet is invalid. Invalid Option-Type values are those less
   than 0x01 or greater than 0x0B. Currently, this is the only condition
   under which the LAN extension interface unit sends a LEX_RCMD_REJ
   packet. Upon receipt of a LEX_RCMD_REJ packet, the host router
   immediately stops sending the request.

   The following sections detail each of the 11 remote command options.
   The sections provide a general description of the option and then
   specify the option's Option-Type, Option-Flags, Option-Length, and
   Option-Data fields. In addition, the sections describe the return
   messages from the LAN extension interface unit.

3.0 Filter Protocol Type

   The host router sends a LEX_RCMD_REQUEST packet with an Option-Type
   of 0x01 to the LAN extension interface unit to configure the LAN
   extension interface unit to filter inbound packets by protocol type.
   A protocol type filter determines whether or not the LAN extension
   interface unit forwards packets of a specific protocol type to the
   host router. A protocol type filter consists of a 16-bit value, 16-
   bit mask, and a permit or deny field. (See the "Option-Data Field
   Descriptions" section for more information on these filter fields.)

   A LEX_RCMD_REQUEST packet can contain 0 to 200 (depending on MTU
   size) protocol type filters. When a LEX_RCMD_REQUEST packet contains
   multiple filters, they are concatenated.

   The LAN extension interface unit applies the protocol type filters to
   each inbound packet's protocol type field in the order in which the
   filters exist in the filter table. A packet must be permitted by one
   of the filters before the LAN extension interface unit can forward
   the packet across the serial link.

   The following example is a filtering algorithm:

   if (protocol_type_field & (~filter_mask)) == filter_value)
    if (permit/deny_field == PERMIT) <forward packet on serial LAN>
   else <DROP PACKET>

   The protocol type filter should also be applied to the DIX type code
   field of Ethernet II frames as well as to IEEE 802.2 SNAP packets.






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RFC 1841           LAN Extension Interface Protocol       September 1995


3.1 LEX RCMD_REQUEST-Filter Protocol Type

   Figure 6 shows a frame format summary of a LEX_RCMD_REQUEST packet
   for the Filter Protocol Type remote command option. The host router
   transmits the fields from left to right.

   Figure 6 LEX_RCMD_REQUEST Packet Frame Format - Filter Protocol Type

                              PPP Header
<-------------------------------------------------------------->
 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              0xFF03           |          Protocol-Type        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              (2 octets)                         (2)

                LAN Extension Interface Protocol Header
<-------------------------------------------------------------->
 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Code     |   Identifier  |             Length            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         (1)            (1)                    (2)

                   LAN Extension Interface Remote Command Options
<--------------------------------
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Option-Type  |  Option-Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         (1)            (1)

                 ---------------------------------------------->
                 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
                -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                        Option-Length          |  Option-Data  |
                -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                              (2)

   Where the Option-Data field contains the following fields:

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Value (16 bits)       |          Value (16 bits)      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Permit/Deny (16 bits)     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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RFC 1841           LAN Extension Interface Protocol       September 1995


   Summary Field Descriptions

   For a complete description of the fields in the PPP Header, the LAN
   Extension Interface Protocol Header, and the LAN Extension Interface
   Remote Command Options, refer to the "Remote Command Options" section
   earlier in this document. The following table provides a summary of
   these fields when sending a LEX_RCMD_REQUEST packet that configures
   the LAN extension interface unit to filter by protocol type.

   Table 2  Field Values for LEX_RCMD_REQUEST Packet-Filter Protocol Type
   Field               Value
   Address/Control     0xFF03 (Broadcast address/Unnumbered
                       information)
   Protocol-Type       0x8041 (Control packet)
   Code                0x40 (LEX_RCMD_REQUEST packet)
   Identifier          Valid values: 0x01-0xFF
   Length              Minimum length = 12 octets
                       Maximum length = 1212 octets
   Option-Type         0x01 (Filter Protocol Type)
   Option-Flags        None
   Option-Length       Minimum length = 4 octets
                       Maximum length = 1204 octets
   Option-Data         Zero or more filters to be applied at
                       the LAN extension interface unit. See the
                       following "Option-Data Field Descriptions"
                       for details.

   Option-Data Field Descriptions

   The following three Option-Data fields are used in conjunction to
   specify a protocol type filter:

   *  Value

   The Value field contains a 16-bit value that is any Ethernet type
   code. Refer to the "Assigned Numbers" RFC for valid Ethernet type
   codes. (References, [4]).

   *  Mask

   The Mask field contains a 16-bit "wild card" mask. That is, this
   field contains a 16-bit number whose ones bits correspond to the type
   code bits to be ignored during the comparison. Thus, the mask
   excludes bits from the comparison in the protocol type filter.







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RFC 1841           LAN Extension Interface Protocol       September 1995


   *  Permit/Deny

   The Permit/Deny field determines whether a protocol type filter
   permits or denies inbound frames to pass to the host router. A permit
   value is a non-zero value that allows frames of a specific protocol
   type to pass to the host router. A deny value is a zero value that
   does not allow frames of a specific protocol type to pass to the host
   router.

   Implementation Notes

   Each LEX_RCMD_REQUEST packet is a complete specification of all
   protocol type filters and replaces any previously established
   filters.

   Note the following special cases:

   *  A LEX_RCMD_REQUEST packet with an Option-Length field equal
      to four (without any filter entries) instructs the LAN extension
      interface unit to turn off protocol type filtering. All MAC
      protocol types are forwarded.

   *  A LEX_RCMD_REQUEST packet with a filter entry of 0x0000 in the
      Value field, 0xFFFF in the Mask field, and a positive value in the
      Permit/Deny field means that if previous filters in the filter list
      do not permit the inbound packet then this filter entry will. This
      filter entry is typically the last filter in a list of filters
      contained within the Option-Data field.

   *  A LEX_RCMD_REQUEST packet with a filter entry of 0x0000 in the
      Value field, 0xFFFF in the Mask field, and a zero in the
      Permit/Deny field, means that the LAN extension interface unit must
      deny all protocol types. This filter is typically the last filter in
      a list of filters contained within the Option-Data field.

3.2 Response Packets - Filter Protocol Type

   The following packets are valid responses to the Filter Protocol Type
   LEX_RCMD_REQUEST packet:

   *  LEX_RCMD_ACK - Filter Protocol Type

   The LAN extension interface unit sends a LEX_RCMD_ACK packet in
   response to the Filter Protocol Type LEX_RCMD_REQUEST packet when the
   LAN extension interface unit correctly receives the Filter Protocol
   Type remote command option and applies all filter entries to its
   filter table. All filter entries are returned to the host router in
   the LEX_RCMD_ACK packet.



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RFC 1841           LAN Extension Interface Protocol       September 1995


   *  LEX_RCMD_NAK - Filter Protocol Type

   The LAN extension interface unit sends a LEX_RCMD_NAK packet in
   response to the Filter Protocol Type LEX_RCMD_REQUEST packet when the
   request contains an incorrect number of bytes in the filter or when
   there are no more filter entries available. The LAN extension
   interface unit continues to use the previous filter table (that is,
   the filter table that existed prior to the receipt of the request).
   The host router should signal an error to the user/network
   administrator. All filter entries are returned to the host router in
   the LEX_RCMD_NAK packet.

   *  LEX_RCMD-REJ - Filter Protocol Type

   See the "Conditions for Sending PPP-LEX Packets" section earlier in
   this document for more information on this packet type.

   Table 3 summarizes the field values of Filter Protocol Type
   LEX_RCMD_ACK, LEX_RCMD_NAK, and LEX_RCMD_REJ packets.

     Table 3  Field Values for Response Packets - Filter Protocol Type

   Field                Value
   Address/Control      0xFF03 (Broadcast address/Unnumbered
                        information)
   Protocol-Type        0x8041 (Control packet)
   Code                 Valid values:
                        * 0x41 (LEX_RCMD_ACK packet)
                        * 0x42 (LEX_RCMD_NAK packet)
                        * 0x43 (LEX_RCMD_REJ packet)
   Identifier           The same value as that sent by the
                        LEX_RCMD_REQUEST packet
   Length               The same value as that sent by the
                        LEX_RCMD_REQUEST packet
   Option-Type          0x01 (Filter Protocol Type)
   Option-Flags         None
   Option-Length        The same value as that sent by the
                        LEX_RCMD_REQUEST packet
   Option-Data          The filter entries sent in the
                        LEX_RCMD_REQUEST packet

4.0 Filter MAC Address

   The host router sends a LEX_RCMD_REQUEST packet with an Option-Type
   of 0x02 to the LAN extension interface unit to configure the LAN
   extension interface unit to filter inbound packets by source MAC
   address. A MAC address filter determines whether or not the LAN
   extension interface unit forwards packets with a specific source MAC



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RFC 1841           LAN Extension Interface Protocol       September 1995


   address to the host router. A MAC address filter consists of MAC
   address, a MAC address mask, and a permit or deny field. (See the
   "Option-Data Field Descriptions" section later in this section for
   more information on these filter fields.)

   A LEX_RCMD_REQUEST packet can contain 0 to 100 (depending on MTU
   size) MAC address filters. When a LEX_RCMD_REQUEST packet contains
   multiple filters, they are concatenated.

   The LAN extension interface unit applies MAC address filters to each
   inbound packet's source MAC address in the order in which the filters
   exist in the filter entry list. A packet must be permitted by one of
   the filters before the LAN extension interface unit can forward the
   packet across the serial link.