rfc1841.txt

<VC++网络游戏建摸与实现>源代码

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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RFC 1841           LAN Extension Interface Protocol       September 1995


   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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RFC 1841           LAN Extension Interface Protocol       September 1995


   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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RFC 1841           LAN Extension Interface Protocol       September 1995


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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RFC 1841           LAN Extension Interface Protocol       September 1995


   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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RFC 1841           LAN Extension Interface Protocol       September 1995


   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