rfc2114.txt

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3.2.  Header Format

   The DCAP 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 DCAP 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 DCAP data. The structure of
   the DCAP data depends on the type of frames, and will be discussed in
   detail in later sections.

   The DCAP Header is given below:

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



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RFC 2114                          DCAP                     February 1997


   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 DCAP message type.

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

3.3.  DCAP Messages

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

   The following table lists and describes all available DCAP messages:

   DCAP 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








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RFC 2114                          DCAP                     February 1997


   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. DCAP Frames

3.4.  DCAP Data formats

   The DCAP data is used to carry information required for each DCAP
   frame. This information is used by the Server or the Client and it
   does not contain any user data. The DCAP 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. When a server receives a CAN_U_REACH frame, it should
   send out an LLC explorer frame to locate the destination specified in
   the CAN_U_REACH frame. If the destination is reachable, the server
   responds to the client with an I_CAN_REACH frame. If the server does
   not receive a positive acknowledgment within a recommended threshold
   value of 5 seconds, the server should send an LLC explorer to locate
   the destination again. If the server does not receive any response
   after sending out 5 explorers (recommended retry value), the
   destination is considered not reachable and an I_CANNOT_REACH frame
   is sent back to the client. The client should decide if retry
   CAN_U_REACH is necessary after the I_CANNOT_REACH frame is received
   from the server.

   When a server is in the process of searching a destination and
   receives another I_CAN_REACH with the same destination, the server
   should not send out another LLC explorer for that destination.

   The server should not send the CAN_U_REACH frame to the clients in a
   TCP session. When a server receives an LLC explorer whose destination
   is a known client, the server should respond to it directly.











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RFC 2114                          DCAP                     February 1997


           +---------------+-----------------------+
           | 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                |
             + - - - - - - - - - - - - - - - - - +
           1 |                                   |
             + - - - - - - - - - - - - - - - - - +
           2 |                                   |
             + - - - - - - - - - - - - - - - - - +
           3 |                                   |
             + - - - - - - - - - - - - - - - - - +
           4 |                                   |
             + - - - - - - - - - - - - - - - - - +
           5 |                                   |
             +-----------------------------------+
           6 | Source SAP                        |
             +-----------------------------------+
           7 | Reserved                          |
             +-----------------------------------+
     Figure 3-4. CAN_U_REACH, I_CAN_REACH, and I_CANNOT_REACH Data

   The MAC Address field carries the MAC address of the target
   workstation that is being searched. This is a six-byte MAC Address
   field. The same MAC Address is returned in the I_CAN_REACH and the
   I_CANNOT_REACH frames.

   Byte 6 is the source SAP. The destination SAP is set to zero when an
   explorer frame is sent to the network.

3.4.2.  START_DL, DL_STARTED, and START_DL_FAILED Frames

   These frame types are used by DCAP to establish a link station
   (circuit). The START_DL frame is sent directly to the server that
   responds to the CAN_U_REACH frame. When the server receives this
   frame, it establishes a link station using the source and destination
   addresses and saps provided in the START_DL frame. If the circuit
   establishment is successful, a DL_STARTED frame is sent back as a
   response. If the attempt fails within a recommended value, 5 seconds,
   the server should retry again. If the server fails to establish a



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RFC 2114                          DCAP                     February 1997


   circuit for a recommended retry value, 5 times, a START_DL_FAILED
   frame should be sent back to the client. If the client receives a
   START_DL_FAILED frame from the server, it is up to the client to
   decide if a START_DL frame needs to be sent to the server again.

   The server can also send START_DL frames to clients to establish
   circuits.

           +---------------+-----------------------+
           | Field Name    | Information           |
           +---------------+-----------------------+
           | Message Type  | 0x04, 0x05, or 0x06   |
           +---------------+-----------------------+
           | Packet Length | 0x18                  |
           +---------------+-----------------------+
      Figure 3-5. START_DL, DL_STARTED, and START_DL_FAILED Header



































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RFC 2114                          DCAP                     February 1997


             +-----------------------------------+
             | Field Name (Each row is one byte) |
             +===================================+
           0 | Host MAC Address                  |
             + - - - - - - - - - - - - - - - - - +
           1 |                                   |
             + - - - - - - - - - - - - - - - - - +
           2 |                                   |
             + - - - - - - - - - - - - - - - - - +
           3 |                                   |
             + - - - - - - - - - - - - - - - - - +
           4 |                                   |
             + - - - - - - - - - - - - - - - - - +
           5 |                                   |
             +-----------------------------------+
           6 | Host SAP                          |
             +-----------------------------------+
           7 | Client SAP                        |
             +-----------------------------------+
           8 | Origin Session ID                 |
             +-----------------------------------+
           9 |                                   |
             + - - - - - - - - - - - - - - - - - +
           10|                                   |
             + - - - - - - - - - - - - - - - - - +
           11|                                   |
             +-----------------------------------+
           12| Target Session ID                 |
             + - - - - - - - - - - - - - - - - - +
           13|                                   |
             + - - - - - - - - - - - - - - - - - +
           14|                                   |
             + - - - - - - - - - - - - - - - - - +
           15|                                   |
             +-----------------------------------+
           16| Largest Frame Size                |
             +-----------------------------------+
           17| Initial Window size               |
             +-----------------------------------+
           18| Reserved                          |
             + - - - - - - - - - - - - - - - - - +
           19|                                   |
             +-----------------------------------+
   Figure 3-6. START_DL, DL_STARTED, and START_DL_FAILED Data

   The Host MAC address is the address of the target station if the
   session is initiated from the client, or it is the address of the
   originating station if the session is initiated from the server.



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