rfc2106.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

Network Working Group                                          S. Chiang
Request for Comments: 2106                                        J. Lee
Category: Informational                              Cisco Systems, Inc.
                                                               H. Yasuda
                                               Mitsubishi Electric Corp.
                                                           February 1997


               Data Link Switching Remote Access Protocol

Status of this 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

   This memo describes the Data Link Switching Remote Access Protocol
   that is used between workstations and routers to transport SNA/
   NetBIOS traffic over TCP sessions. Any questions or comments should
   be sent to [e-mail].

1.  Introduction

   Since the Data Link Switching Protocol, RFC 1795, was published, some
   software vendors have begun implementing DLSw on workstations. The
   implementation of DLSw on a large number of workstations raises
   several important issues that must be addressed. Scalability is the
   major concern. For example, the number of TCP sessions to the DLSw
   router increases in direct proportion to the number of workstations
   added. Another concern is efficiency. Since DLSw is a switch-to-
   switch protocol, it is not efficient when implemented on
   workstations.

   DRAP addresses the above issues. It introduces a hierarchical
   structure to resolve the scalability problems. All workstations are
   clients to the router (server) rather than peers to the router. This
   creates a client/server model. It also provides a more efficient
   protocol between the workstation (client) and the router (server).











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2.  Overview

2.1.  DRAP Client/Server Model

      +-----------+              +-----------+       +---------+
      | Mainframe |              | IP Router +- ppp -+ DLSw    |
      +--+--------+              +-----+-----+       | Work    |
         |                             |             | Station |
         |                             |             +---------+
      +--+--+      +-------------+     |
      | FEP +- TR -+ DLSw Router +-- IP Backbone
      +-----+      +-------------+     |
                                       |
                                       |
                                 +-----------+       +---------+
                                 | IP Router +- ppp -+ DLSw    |
                                 +-----+-----+       | Work    |
                                                     | Station |
                                                     +---------+

                           |         DLSw Session          |
                           +-------------------------------+
  Figure 2-1. Running DLSw on a large number of workstations creates a
                         scalability problem.

   Figure 2-1 shows a typical DLSw implementation on a workstation. The
   workstations are connected to the central site DLSw router over the
   IP network.  As the network grows, scalability will become an issue
   as the number of TCP sessions increases due to the growing number of
   workstations.





















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                                  +-----------+       +-------+
       +-----------+              | DLSw/DRAP |       | DRAP  |
       | Mainframe |              |   Router  +- ppp -+ Client|
       +--+--------+              +-----+-----+       +-------+
          |                             |
          |                             |
       +--+--+      +-------------+     |
       | FEP +- TR -+ DLSw Router +-- IP Backbone
       +-----+      +-------------+     |
                                        |
                                        |
                                  +-----------+       +-------+
                                  | DLSw/DRAP |       | DRAP  |
                                  |   Router  +- ppp -+ Client|
                                  +-----+-----+       +-------+

                         | DLSw Session |  | DRAP Session |
                         +--------------+  +--------------+
Figure 2-2. DLSw Remote Access Protocol solves the scalability problem.

   In a large network, DRAP addresses the scalability problem by
   significantly reducing the number of peers that connect to the
   central site router. The workstations (DRAP client) and the router
   (DRAP server) behave in a Client/Server relationship. Workstations
   are attached to a DRAP server. A DRAP server has a single peer
   connection to the central site router.

2.2.  Dynamic Address Resolution

   In a DLSw network, each workstation needs a MAC address to
   communicate with a FEP attached to a LAN. When DLSw is implemented on
   a workstation, it does not always have a MAC address defined. For
   example, when a workstation connects to a router through a modem via
   PPP, it only consists of an IP address. In this case, the user must
   define a virtual MAC address. This is administratively intensive
   since each workstation must have an unique MAC address.

   DRAP uses the Dynamic Address Resolution protocol to solve this
   problem. The Dynamic Address Resolution protocol permits the server
   to dynamically assign a MAC address to a client without complex
   configuration.

   For a client to initiate a session to a server, the workstation sends
   a direct request to the server. The request contains the destination
   MAC address and the destination SAP. The workstation can either
   specify its own MAC address, or request the server to assign one to
   it. The server's IP address must be pre-configured on the
   workstation. If IP addresses are configured for multiple servers at a



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   workstation, the request can be sent to these servers and the first
   one to respond will be used.

   For a server to initiate a session to a client, the server sends a
   directed request to the workstation. The workstation must pre-
   register its MAC address at the server. This can be done either by
   configuration on the server or registration at the server (both MAC
   addresses and IP addresses will be registered).

2.3.  TCP Connection

   The transport used between the client and the server is TCP. Before a
   TCP session is established between the client and the server, no
   message can be sent. The default parameters associated with the TCP
   connections between the client and the server are as follows:

      Socket Family     AF_INET        (Internet protocols)
      Socket Type       SOCK_STREAM    (stream socket)
      Port Number       1973

   There is only one TCP connection between the client and the server.
   It is used for both read and write operations.

3.  DRAP Format

3.1.  General Frame Format

   The General format of the DRAP frame is as follows:

             +-------------+-----------+-----------+
             | DRAP Header | DRAP Data | User Data |
             +-------------+-----------+-----------+
                     Figure 3-1. DRAP Frame Format

   The DRAP protocol is contained in the DRAP header, which is common to
   all frames passed between the DRAP client and the server. This header
   is 4 bytes long. The next section will explain the details.

   The next part is the DRAP Data. The structure and the size are based
   on the type of messages carried in the DRAP frame. The DRAP data is
   used to process the frame, but it is optional.

   The third part of the frame is the user data, which is sent by the
   local system to the remote system. The size of this block is variable
   and is included in the frame only when there is data to be sent to
   the remote system.





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3.2.  Header Format

   The DRAP header is used to identify the message type and the length
   of the frame. This is a general purpose header used for each frame
   that is passed between the DRAP server and the client. More
   information is needed for frames like CAN_U_REACH and I_CAN_REACH,
   therefore, it is passed to the peer as DRAP data. The structure of
   the DRAP data depends on the type of frames, and will be discussed in
   detail in later sections.

   The DRAP Header is given below:

             +-------------------------------------------+
             | DRAP Packet Header (Each row is one byte) |
             +===========================================+
           0 | Protocol ID / Version Number              |
             +-------------------------------------------+
           1 | Message Type                              |
             +-------------------------------------------+
           2 | Packet Length                             |
             + - - - - - - - - - - - - - - - - - - - - - +
           3 |                                           |
             +-------------------------------------------+
                     Figure 3-2. DRAP Header Format

   o The Protocol ID uses the first 4 bits of this field and is set to
     "1000".

   o The Version Number uses the next 4 bits in this field and is set
     to "0001".

   o The message type is the DRAP message type.

   o The Total Packet length is the length of the packet including the
     DRAP header, DRAP data and User Data. The minimum size of the
     packet is 4, which is the length of the header.

3.3.  DRAP Messages

   Most of the Drap frames are based on the existing DLSw frames and
   have the same names. The information in the corresponding DRAP and
   DLSw frames may differ; but the functionalities are the same. Thus
   the DLSw State Machine is used to handle these DRAP frames. Some new
   DRAP frames were created to handle special DRAP functions. For
   example, the new DRAP frames, I_CANNOT_REACH and START_DL_FAILED
   provide negative acknowledgment. The DLSw frames not needed for DRAP,
   are dropped.




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   The following table lists and describes all available DRAP messages:

   DRAP Frame Name     Code  Function
   ---------------     ----  --------
   CAN_U_REACH         0x01  Find if the station given is reachable
   I_CAN_REACH         0x02  Positive response to CAN_U_REACH
   I_CANNOT_REACH      0x03  Negative response to CAN_U_REACH
   START_DL            0x04  Setup session for given addresses
   DL_STARTED          0x05  Session Started
   START_DL_FAILED     0x06  Session Start failed
   XID_FRAME           0x07  XID Frame
   CONTACT_STN         0x08  Contact destination to establish SABME
   STN_CONTACTED       0x09  Station contacted - SABME mode set
   DATA_FRAME          0x0A  Connectionless Data Frame for a link
   INFO_FRAME          0x0B  Connection oriented I-Frame
   HALT_DL             0x0C  Halt Data Link session
   HALT_DL_NOACK       0x0D  Halt Data Link session without ack
   DL_HALTED           0x0E  Session Halted
   FCM_FRAME           0x0F  Data Link Session Flow Control Message
   DGRM_FRAME          0x11  Connectionless Datagram Frame for a circuit
   CAP_XCHANGE         0x12  Capabilities Exchange Message
   CLOSE_PEER_REQUEST  0x13  Disconnect Peer Connection Request
   CLOSE_PEER_RESPONSE 0x14  Disconnect Peer Connection Response
   PEER_TEST_REQ       0x1D  Peer keepalive test request
   PEER_TEST_RSP       0x1E  Peer keepalive response

                         Table 3-1. DRAP Frames

3.4.  DRAP Data formats

   The DRAP data is used to carry information required for each DRAP
   frame. This information is used by the Server or the Client and it
   does not contain any user data. The DRAP data frame types are listed
   in the following sections. Please note that the sender should set the
   reserved fields to zero and the receiver should ignore these fields.

3.4.1.  CAN_U_REACH, I_CAN_REACH, and I_CANNOT_REACH Frames

   These frame types are used to locate resources in a network. A
   CAN_U_REACH frame is sent to the server to determine if the resource
   is reachable. The server responds with an I_CAN_REACH frame if it can
   reach the workstation identified in the CAN_U_REACH frame, or with an
   I_CANNOT_REACH if the station is not reachable. The server should not
   send the CAN_U_REACH frame to the clients. When a server receives an
   explorer whose destination is a known client, the server should
   respond to it directly.





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           +---------------+-----------------------+
           | Field Name    | Information           |
           +---------------+-----------------------+
           | Message Type  | 0x01, 0x02, or 0x03   |
           +---------------+-----------------------+
           | Packet Length | 0x0C                  |
           +---------------+-----------------------+
    Figure 3-3. CAN_U_REACH, I_CAN_REACH, and I_CANNOT_REACH Header

             +-----------------------------------+
             | Field Name (Each row is one byte) |
             +===================================+
           0 | Target MAC Address                |
             + - - - - - - - - - - - - - - - - - +