rfc1841.txt

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   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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RFC 1841           LAN Extension Interface Protocol       September 1995


   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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RFC 1841           LAN Extension Interface Protocol       September 1995


   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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RFC 1841           LAN Extension Interface Protocol       September 1995


   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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RFC 1841           LAN Extension Interface Protocol       September 1995


   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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RFC 1841           LAN Extension Interface Protocol       September 1995


   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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RFC 1841           LAN Extension Interface Protocol       September 1995


   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