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RFC 1841

Informational
Pages: 66
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PPP Network Control Protocol for LAN Extension

Part 1 of 3, p. 1 to 22
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Network Working Group                                         J. Chapman
Request For Comments: 1841                           Cisco Systems, Inc.
Category: Informational                                          D. Coli
                                                     Cisco Systems, Inc.
                                                               A. Harvey
                                                     Cisco Systems, Inc.
                                                               B. Jensen
                                                     Cisco Systems, Inc.
                                                               K. Rowett
                                                     Cisco Systems, Inc.
                                                          September 1995


            PPP Network Control Protocol for LAN Extension

Status of Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Abstract

   Telecommunications infrastructure is improving to offer higher
   bandwidth connections at lower cost. Access to the network is
   changing from modems to more intelligent devices. This informational
   RFC discusses a PPP Network Control Protocol for one such intelligent
   device. The protocol is the LAN extension interface protocol.

Table of Contents

   1.0    Introduction ...........................................    3
      1.1   LAN Extension Interface Topology .....................    4
      1.2   LAN Extension Interface Architecture .................    5
      1.3   LAN Extension Interface Protocol .....................    6

   2.0    LAN Extension Interface Protocol Control Packets........    8
      2.1   Startup Options ......................................    8
      2.2   Remote Command Options ...............................   14
      2.3   Conditions for Sending PPP-LEX Packet ................   17

   3.0    Filter Protocol Type ...................................   18
      3.1   LEX_RCMD_REQUEST - Filter Protocol Type...............   19
      3.2   Response Packets - Filter Protocol Type...............   21

   4.0    Filter MAC Address .....................................   22
      4.1   LEX_RCMD_REQUEST - Filter MAC Address ................   23
      4.2   Response Packets - Filter MAC Address.................   25

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   5.0    Set Priority ...........................................   27
      5.1   LEX_RCMD_REQUEST - Set Priority ......................   27
      5.2   Response Packets - Set Priority ......................   29

   6.0    Disable LAN Extension Ethernet Interface ...............   30
      6.1   LEX_RCMD_REQUEST - Disable LAN Extension
            Ethernet Interface ...................................   31
      6.2   Response Packets - Disable LAN Extension
            Ethernet Interface ...................................   32

   7.0    Enable LAN Extension Ethernet Interface ................   33
      7.1   LEX_RCMD_REQUEST - Enable LAN Extension
            Ethernet Interface ...................................   33
      7.2   Response Packets - Enable LAN Extension
            Ethernet Interface ...................................   34

   8.0    Reboot LAN Extension Interface Unit ....................   35
      8.1   LEX_RCMD_REQUEST - Reboot LAN Extension Interface
            Unit .................................................   35
      8.2   Response Packets - Reboot LAN Extension
            Interface Unit .......................................   36

   9.0    Request Statistics .....................................   37
      9.1   LEX_RCMD_REQUEST - Request Statistics ................   37
      9.2   LEX_RCMD_ACK - Request Statistics ....................   39
      9.3   LEX_RCMD_NAK/LEX_RCMD_REJ - Request Statistics .......   44

   10.0    Download Request ......................................   45
      10.1   LEX_RCMD_REQUEST - Download Request .................   46
      10.2   Response Packets - Download Request..................   48

   11.0    Download Data .........................................   49
      11.1   LEX_RCMD_REQUEST - Download Request .................   49
      11.2   Response Packets - Download Data ....................   51

   12.0    Download Status .......................................   52
      12.1   LEX_RCMD_REQUEST - Download Status ..................   53
      12.2   LEX_RCMD_ACK - Download Status ......................   54
      12.3   LEX_RCMD_NAK/LEX_RCMD_REJ - Download Status .........   56

   13.0    Inventory Request .....................................   56
      13.1   LEX_RCMD_REQUEST - Inventory Request ................   57
      13.2   LEX_RCMD_ACK - Inventory Request ....................   58
      13.3   LEX_RCMD_NAK/LEX_RCMD_REJ - Inventory Request .......   61

   14.0    LAN Extension Interface Protocol Data Packets .........   62
      14.1   Frame Format ........................................   62
      14.2   Summary Field Descriptions...........................   63

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   NOTES ......................................................      65
   REFERENCES .................................................      65
   SECURITY CONSIDERATIONS ....................................      66
   AUTHORS' ADDRESSES .........................................      66

1.0 Introduction

   An increasing number of corporations allow their employees to
   telecommute to work due to local government regulations on traffic
   and air pollution. Additionally, many businesses are run out of
   internetworked home offices and small branch offices. With these
   changes in the workplace, more people and businesses require Internet
   access from small LANs.

   Today, routers serve the LAN-to-LAN traffic using high-speed WAN
   links such as leased lines, ISDN, or Frame Relay. This new breed of
   Internet users from home offices and small branch offices may have a
   different, less network-literate skill set than those connecting up
   to the Internet today. These new users need an alternative to the
   complex and hard-to-configure routers currently employed for
   connectivity. One such alternative is a LAN extension interface unit.

   A LAN extension interface unit is a hardware device installed at
   remote sites (such as a home office or small branch office) that
   connects a LAN across a WAN link to a router at a central site. The
   following sections introduce a LAN extension interface topology,
   architecture, and protocol.

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1.1 LAN Extension Interface Topology

   Figure 1 shows the topology of LAN extension interfaces. The figure
   shows two LAN extension interface units connected via a WAN link to a
   central or "host router."

                 Figure 1 LAN Extension Interface Topology

                  -----------------------------------------
                                    Router
                  -----------------------------------------
                     Virtual Interface   Virtual Interface
                        123.123.78.1        123.123.89.1
                  .........................................
                           Serial 0           Serial n
                  -----------------------------------------
                             |<---- WAN Link ---->|
                ------------------                    ------------------
                | LAN Extension  |                    | LAN Extension  |
                | Interface Unit |                    | Interface Unit |
                ------------------                    ------------------
                        |                                     |
                ------------------                    ------------------
                  |                                      |
                ------------                          ------------
                | End node |                          | End node |
                ------------                          ------------
                123.123.78.2                          123.123.46.2

   Each LAN extension interface unit maps to a virtual interface at the
   host router. The virtual interface mirrors the characteristics of the
   LAN extension interface unit. To the routing protocols, the virtual
   interface looks just like a local interface, but with the bandwidth
   of a serial line. The virtual interface keeps the state (up or down)
   of the LAN extension interface unit, and identifies each LAN
   extension interface unit by its MAC address.

   A LAN extension interface protocol transfers MAC frames from the LAN
   extension interface unit across the serial line to the host router.
   At the termination point in the router, the router routes the
   packets. This topology uses only one subnet per remote LAN rather
   than two, as is the case when routers exist on both ends of a WAN
   link. Figure 1 shows this subnetting structure. The IP addresses of
   the virtual interfaces on the router are in the same subnet as the IP
   addresses of the end nodes on the LAN of the LAN extension interface
   unit. The LAN extension interface unit itself has no IP address.

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   LAN extension interface units resemble bridges, but with the
   following distinct differences:

   *  LAN extension interface units always depend on a host router.
      They cannot operate standalone or even back-to-back with other
      LAN extension interface units.

   *  LAN extension interface units need not employ any spanning tree
      algorithm.

   *  (LAN extension interface units transfer MAC frames across a
      serial line (like bridges), but a router can either route or
      bridge the LAN extension interface data packets.

1.2 LAN Extension Interface Architecture

   Figure 2 shows the basic LAN extension interface architecture.

               Figure 2 LAN Extension Interface Architecture

                 Router                        LAN Extension Interface
          -------------------                  -------------------
          |   Network Layer |                  |     MAC Layer   |
          -------------------                  -------------------
                  |                                       |
          -------------------                  -------------------
     |    |Virtual Interface|                  |       Filters   |   |
     |    -------------------                  -------------------   |
     |            |                                 |                |
     |    -------------------                  -------------         |
     |    |      PPP        |                  |   RCMD    |         |
     |    |                 |                  |   Handler |         |
     |    -------------------                  -------------         |
     |            |                                 |                |
     |    -------------------                  -------------------   |
     |    |Serial Interface |                  |        PPP      |   |
     |    -------------------                  -------------------   |
     |            |                                      |           |
     |            |                            -------------------   |
     |            |                            | Serial Interface|   |
     |            |                            -------------------   |
     |            |          WAN Link                    |           |
     |            ---------------------------------------            |
     |                                                               |
     |   Outbound                                          Inbound   |
     -------------->                                  <---------------

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   In the inbound direction (from the remote LAN, to the LAN extension
   interface unit, across the WAN link, to the host router), the LAN
   extension interface unit can filter received frames to optimize WAN
   utilization. The LAN extension interface unit can filter frames by
   protocol type or by MAC address. Frames that pass through the LAN
   extension interface filters go to the WAN protocol state machine. In
   Figure 2, this state machine is PPP. The LAN extension interface unit
   adds PPP encapsulation and forwards the packet to the router via the
   WAN serial link.

   Upon receiving the frame, the host router decapsulates the PPP header
   and passes the packet to the virtual interface. From there the
   virtual interface handles the packet like any packet received on a
   local interface -- by routing or bridging the packet to another
   interface, depending on configuration.

   In the outbound direction (from the host router, across the WAN link,
   to the LAN extension interface unit, to the LAN), the host router's
   virtual interface builds the full MAC header, before adding PPP
   encapsulation. The router then sends the packet across the WAN serial
   link to the LAN extension interface unit. The LAN extension interface
   unit strips the PPP header and forwards the packet directly onto the
   LAN. The host router has already determined that the packet needs to
   be forwarded to the LAN extension interface unit, hence there is no
   need for additional filtering or processing at that end.

   Embedded in the data stream is a control stream for configuring and
   managing the LAN extension interface unit from the host router. The
   virtual interface makes the remote LAN extension interface unit
   appear like a local router interface to the routing protocols.

   Configuration commands and statistics gathering commands are issued
   on the router to the virtual interface. The virtual interface formats
   the LAN extension interface remote commands into encoded messages and
   transfers them in-band with the data packets. The LAN extension
   interface unit decodes the remote commands and executes them.
   Responses are similarly formatted messages sent by the LAN extension
   interface unit to the host router. The remote command messages use a
   different encapsulation type than the data packets, as described
   later in this document in the "LAN Extension Interface Protocol
   Control Packets" and "LAN Extension Interface Protocol Data Packets"
   sections.

1.3 LAN Extension Interface Protocol

   To accommodate this LAN extension interface architecture, a new
   Network Control Protocol (NCP) for PPP exists. This NCP is called
   PPP-LEX. The basic functionality of PPP-LEX is to encapsulate LAN

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   extension interface control and data packets. The IETF has assigned
   two new protocol types for these functions, as shown in Table 1.

                 Table 1  IETF Protocol Types for PPP-LEX

   Protocol Type       Function

   0x8041              Encapsulates control packets
   0x0041              Encapsulates data packets (MAC frames)

   PPP is a natural choice for a LAN extension interface protocol
   because it allows for negotiating a specific control protocol and
   options at connection time. This means that network administrators do
   not have to statically configure the router interface for remote
   interfaces. Rather, remote interfaces negotiate the link at
   connection time.

   The LAN extension interface protocol employs PPP link operation as
   described in RFC 1331, which reads as follows:

      In order to establish communications of a point-to-point link,
      each end of the PPP link must first send [Link Control Protocol]
      LCP packets to configure and test the data link. After the link
      has been established,the peer may be authenticated. Then PPP must
      send NCP packets to choose and configure one or more network layer
      protocols. Once each of the chosen network-layer protocols has
      been configured, datagrams from each network layer protocol can be
      sent over the link.

      The link will remain configured for communications until explicit
      LCP or NCP packets close the link down, or until some external
      event occurs (an inactivity timer expires or network administrator
      intervention). (References, [1])

   Thus, the LAN extension interface unit and the host router exchange
   PPP-LCP packets at connection time to dynamically configure and test
   the WAN serial link. Once the link reaches an "opened" state, the LAN
   extension interface unit and host router exchange PPP-LEX NCP packets
   to configure the LAN extension interface protocol. Once it is
   configured, the NCP (PPP-LEX) reaches an "opened" state, and PPP
   carries the PPP-LEX control and data packets across the serial link.
   At this point, link traffic is a any combination of LCPs, PPP-LEX
   NCPs, PPP-LEX control packets, and PPP-LEX data packets.

   Note that the LAN extension interface protocol is not a bridging
   protocol. The only similarity to the PPP Bridging Control Protocol
   (References. [2]) is that the LAN extension interface protocol also
   encapsulates MAC frames.

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   The following sections detail PPP-LEX control packets and data
   packets.

2.0 LAN Extension Interface Protocol Control Packets

   There are two types of PPP-LEX control packets, as follows:

      *  Startup options packet
      *  Remote command options packets

   The startup options packet is the first PPP-LEX NCP packet that the
   LAN extension interface unit sends to the host router after the LCP
   has reached an "opened" state. This required startup options packet
   configures the LAN extension interface protocol and puts the PPP-LEX
   NCP in an "opened" state.

   Remote command options are the PPP-LEX NCP packets that control the
   functioning and statistics gathering of the LAN extension interface
   protocol.

2.1 Startup Options

   The LAN extension interface unit sends a startup options packet to
   the host router to negotiate the following startup options:

      *  MAC Type
      *  MAC Address
      *  LAN Extension

   The MAC Type startup option informs the host router of the type of
   media that the LAN extension interface unit is connected to. For
   example, the LAN extension interface unit may be connected to an
   Ethernet LAN or a Token Ring LAN. Currently, only Ethernet is
   supported. The MAC type tells the host router what type of traffic
   the LAN extension interface unit is prepared to receive. If the host
   router rejects the MAC type, the LAN extension interface unit sends
   the Configure-Request again.

   The MAC Address startup option sends the MAC address of the LAN
   extension interface unit to the host router to authenticate the LAN
   extension interface unit and bind it to the corresponding virtual
   interface at the host router. The host router also inserts the MAC
   address in outbound packets. The MAC address is represented in IEEE
   802.3 canonical format.

   The LAN Extension startup option establishes the network layer
   protocol (NCP) as PPP-LEX and provides the host router with the LAN
   extension interface protocol version number.

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   Each startup option is transmitted in a series of three fields:
   Option-Type, Option-Length, and Option-Data fields. The fields are
   concatenated in the startup options Configure-Request packet.

   Frame Format

   Figure 3 shows a summary of the frame format for the startup options
   packet. The LAN extension interface unit sends this startup options
   packet to the host router. The LAN extension interface unit transmits
   these fields from left to right.

        Figure 3  Startup Options Frame Format (Configure-Request)

                     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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    Address    |     Control   |          Protocol Type        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     (1 octet)          (1)                      (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 octet)         (1)                      (2)

            LAN Extension Interface Startup Options
<-------------------------------------------------------------->
 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Option-Type  | Option-Length |          Option-Data          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     (1 octet)          (1)

   Address

   This PPP-specified field is one octet and contains the binary
   sequence 11111111 (hexadecimal 0xFF), the All-Stations address. PPP
   does not assign individual station addresses. The All-Stations
   address must be recognized and received by all devices. For more
   information on this field, refer to "The Point-to-Point Protocol
   (PPP) for the Transmission of Multi-protocol Datagrams over Point-
   to-Point Links." (References, [1])

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   Control

   This PPP-specified field is one octet and contains the binary
   sequence 00000011 (hexadecimal 0x03), the Unnumbered Information (UI)
   command with the P/F bit set to zero.

   For more information on this field, refer to "The Point-to-Point
   Protocol (PPP) for the Transmission of Multi-protocol Datagrams over
   Point-to Point Links." (References, [1])

   Protocol-Type

   The Protocol-Type field is two octets and contains the IETF-assigned
   protocol type value. Valid LAN extension interface protocol type
   values are as follows:

      *  0x8041 (for control packets)
      *  0x0041 (for data packets)

   Because the startup options packet encapsulates LAN extension
   interface control data, the valid value for this field is 0x8041.

   Code

   The Code field is one octet and identifies the type of LCP packet
   that the LAN extension interface packet is sending. Valid values are
   as follows:

      *  0x01 - Configure-Request
      *  0x02 - Configure-Ack
      *  0x03 - Configure-Nak
      *  0x04 - Configure-Rej

   The LAN extension interface unit initiates the startup options
   packet; therefore, the valid value for this field is 1.

   Identifier

   The Identifier field is one octet and contains a randomly generated
   value. The value aids matching requests and replies. It is
   recommended that a non-zero value be used for the identifier. That
   is, zero could be used in the future for unsolicited messages from
   the LAN extension interface unit. Valid values are 0x01-0xFF.

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   Length

   The Length field is two octets and indicates the length of the entire
   packet in octets, including the Code, Identifier, Length, and startup
   options fields.

   Option-Type

   The Option-Type field is one octet and identifies the startup option
   being negotiated. Valid values are as follows:

      *  0x01 - MAC Type
      *  0x03 - MAC Address
      *  0x05 - LAN Extension

   Option-Length

   The Option-Length field is one octet and specifies the length of the
   startup option fields, including the Option-Type, Option-Data, and
   Option-Length fields.

   Option-Data

   The Option-Data field contains the data relating to the value
   specified in the Option-Type field. That is, if the Option-Type field
   specifies MAC type (0x01), then the Option-Data field contains the
   MAC type (Ethernet, Token Ring, and so on). If the Option-Type field
   specifies MAC address (0x03), then the Option-Data field contains the
   actual MAC address. If the Option-Type field specifies LAN Extension
   (0x05), then the Option-Data field contains LAN extension interface
   software information. The following table defines the contents of the
   Option-Data field for each possible Option-Type field value:

   Option-Type Field Value   Option-Data

   0x01 (MAC Type)           The most up-to-date value of the MAC type as
                             specified in the most recent "Assigned
                             Numbers" RFC. The current valid value from
                             that RFC follows:
                             *  0x01: IEEE 802.3/Ethernet with canonical
                                addresses
   0x03 (MAC Address)        The burned-in MAC address in IEEE 802.3
                              canonical format.
   0x05 (LAN Extension)      The LAN extension interface protocol version
                             number. 0x01 is the current protocol version
                             supported.

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   Example

   In the Configure-Request packet that it sends to the host router, the
   LAN extension interface unit concatenates the Option-Type, Option-
   Data, and Option-Length fields for each startup option, as shown in
   Figure 4. The LAN extension interface unit transmits these fields
   from left to right.

         Figure 4 Sample Startup Options Configure-Request Packet

                         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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      0xFF     |      0x03     |             0x8041            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     Address          Control             Protocol-Type

                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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     0x01      |      0x09     |             0x12              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Option-Type      Identifier               Length

              LAN Extension Interface Startup Options
<------------------------------------------------
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      0x01     |               0x03            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Option-Type             Option-Length

                                  ------------------------------>
                                  4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
                                 -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                               0x01             |
                                 -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                             Option-Data

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 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      0x03     |               0x08            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Option-Type             Option-Length        |

                                  4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
                                 -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    0A  0A  0A  0A  0B  0C      |
                                 -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                 ---------- Option-Data --------|

 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      0x05     |               0x03            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Option-Type             Option-Length

                                  4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
                                 -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                                0x01            |
                                 -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                           Option-Data

   In Figure 4, the Address field always contains 0xFF, and the Control
   field always contains 0x03. The Protocol-Type field value is 0x8041
   because the startup options packet is a LAN extension interface
   control packet. The Code field value is 0x01 because the LAN
   extension interface unit is sending an LCP Configure-Request packet
   to configure or negotiate PPP-LEX. The Identifier field contains a
   randomly generated number. The Length field gives the total length of
   the entire packet.

   The first startup option is the MAC Type startup option. The Option-
   Type value for MAC Type is 0x01. The Option-Length field value for
   the MAC type startup option is 3 octets. The Option-Data field value
   is 0x01 because, in this example, the LAN extension interface unit
   connects to an Ethernet LAN using 802.3 canonical addresses.

   The next startup option transmitted is the MAC Address. Its Option-
   Type field value is 0x03, its Option-Length field value is 8 octets,
   and its Option-Data field value is the actual MAC address.

   Lastly, the LAN Extension startup option is transmitted. Its Option-
   Type field value is 0x05, its Option-Length field value is 3 octets,
   and its Option-Data field value is the LAN extension interface
   protocol version number (0x01).

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   The host router responds to this LCP Configure-Request packet with an
   LCP Configure-Ack packet, Configure-Nak packet, or Configure-Rej
   packet. For more information on these packets, refer to "The Point-
   to-Point Protocol (PPP) for the Transmission of Multi-protocol
   Datagrams over Point-to-Point Links" RFC. (References, [1])

2.2 Remote Command Options

   Once the host router responds to the startup options Configure-
   Request packet with a Configure-Ack packet, the PPP-LEX NCP is in an
   "opened" state, and the LAN extension interface unit and the host
   router freely exchange PPP-LEX data packets and remote command
   options packets.

   The host router initiates PPP-LEX remote command options packets to
   control the configuration of the LAN extension interface unit and to
   gather statistics. There are 11 types of remote command options that
   the host router can send in a LEX_RCMD_REQUEST packet to the LAN
   extension interface unit. The LAN extension interface unit responds
   to a LEX_RCMD_REQUEST packet with a LEX_RCMD_ACK, LEX_RCMD_NAK, or
   LEX_RCMD_REJ packet.

   Frame Format

   Figure 5 shows a summary of the frame format for a remote command
   options packet. These fields are transmitted from left to right.

               Figure 5 Remote Command Options Frame Format

                        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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    Address    |     Control   |          Protocol Type        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     (1 octet)          (1)                      (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 octet)          (1)                      (2)

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               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 octet)          (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)

   Address

   This PPP-specified field is a single octet and contains the binary
   sequence 11111111 (hexadecimal 0xFF), the All-Stations address. PPP
   does not assign individual station addresses. The All-Stations
   address must be recognized and received by all devices. For more
   information on this field, refer to "The Point-to-Point Protocol
   (PPP) for the Transmission of Multi-protocol Datagrams over Point-
   to-Point Links." (References, [1])

   Control

   This PPP-specified field is a single octet and contains the binary
   sequence 00000011 (hexadecimal 0x03), the Unnumbered Information (UI)
   command with the P/F bit set to zero.

   For more information on this field, refer to "The Point-to-Point
   Protocol (PPP) for the Transmission of Multi-protocol Datagrams over
   Point-to Point Links." (References, [1])

   Note:  Hereafter the Address and Control fields will be represented
   together as a 2-octet field containing "0xFF03".

   Protocol-Type

   The Protocol-Type field is two octets and contains the IETF-assigned
   protocol type value. Valid LAN extension interface protocol type
   values follow:

      *  0x8041 (for control packets)
      *  0x0041 (for data packets)

Top      ToC       Page 16 
   Because the remote command options packet encapsulates LAN extension
   interface control data, the valid value for this field is 0x8041.

   Code

   The Code field is one octet and identifies the type of PPP-LEX
   packet. Valid values are as follows:

      *  0x40 - LEX_RCMD_REQUEST packet
      *  0x41 - LEX_RCMD_ ACK packet
      *  0x42 - LEX_RCMD_NAK packet
      *  0x43 - LEX_RCMD_REJ packet

   The host router sends the LEX_RCMD_REQUEST packet, and the LAN
   extension interface unit sends the LEX_RCMD_ACK, LEX_RCMD_NAK, and
   LEX_RCMD_REJ packets.

   Identifier

   The Identifier field is one octet and contains a randomly generated
   value. The value aids matching requests and replies. It is
   recommended that a non-zero value be used for the identifier. That
   is, zero could be used in the future for unsolicited messages from
   the LAN extension interface unit. Valid values are 0x01-0xFF.

   Length

   The Length field is two octets and indicates the length in octets of
   the entire packet, including the Code, Identifier, Length, and remote
   command options fields.

   Option-Type

   The Option-Type field is one octet and identifies the remote command
   option being transmitted. Valid values are as follows:

      *  0x01 - Filter Protocol Type
      *  0x02 - Filter MAC Address
      *  0x03 - Set Priority
      *  0x04 - Disable LAN Extension Ethernet Interface
      *  0x05 - Enable LAN Extension Ethernet Interface
      *  0x06 - Reboot LAN Extension Interface Unit
      *  0x07 - Request Statistics
      *  0x08 - Download Request
      *  0x09 - Download Data
      *  0x0A - Download Status
      *  0x0B- Inventory Request

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   Each remote command option is discussed in detail in its own section
   later in this document.

   Option-Flags

   This field is one octet and further specifies the remote command
   option, containing specific actions that must be followed.

   Option-Length

   The Option-Length field is two octets and specifies the length in
   octets of the remote command option fields, including the Option-
   Type, Option-Flags, Option-Length, and Option-Data fields.

   Option-Data

   Option-Data field contains data relating to the remote command option
   specified in the Option-Type field.

2.3 Conditions for Sending PPP-LEX Packet

   This section describes the general conditions under which PPP-LEX
   packet types are sent. For specific information by remote command,
   refer to the appropriate remote command section later in this
   document.

   LEX RCMD_REQUEST Packet

   The host router sends LEX_RCMD_REQUEST packets to the LAN extension
   interface unit to initiate a remote command request. Until the host
   router receives a LEX_RCMD_ACK, LEX_RCMD_NAK, or LEX_RCMD_REJ packet
   from the LAN extension interface unit, the host router continues to
   send the LEX_RCMD_REQUEST packet a default number of times, at which
   point the host router times out.

   LEX_RCMD_ACK Packet

   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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   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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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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   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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   *  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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   *  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



(page 22 continued on part 2)

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