rfc2106.txt
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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. If the sender did not receive a positive acknowledgment within a recommended threshold value of 60 seconds, the destination is considered not reachable.3.4.2. START_DL, DL_STARTED, and START_DL_FAILED Frames These frame types are used by DRAP 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 with the source and destinationChiang, et. al. Informational [Page 7] RFC 2106 DLSRAP February 1997 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. A failure will result in a START_DL_FAILED response. 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 HeaderChiang, et. al. Informational [Page 8] RFC 2106 DLSRAP 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.Chiang, et. al. Informational [Page 9] RFC 2106 DLSRAP February 1997 The next two fields are the Host and Client SAPs. Each is one byte long. The Host SAP is the SAP used by the station with the Host MAC address. The Client SAP is the SAP used by the client. The Origin Session ID, is the ID of the originating station that initiates the circuit. The originating station uses this ID to identify the newly created circuit. Before the START_DL frame is sent to the target station, the originating station sets up a control block for the circuit. This link station information is set because DRAP does not use a three-way handshake for link station establishment. In the DL_STARTED and the START_DL_FAILED messages, the Origin Session ID is returned as received in the START_DL frame. The Target Session ID is set by the target station and returned in the DL_STARTED message. The Target Session ID is not valid for the START_DL and the START_DL_FAILED frame, and should be treated as Reserved fields. In the DL_STARTED frame, it is the session ID that is used to set up this circuit by the target station. The Largest Frame Size field is used to indicate the maximum frame size that can be used by the client. It is valid only when it is set by the server. The Largest Frame Size field must be set to zero when a frame is sent by the client. Both START_DL and DL_STARTED use the Largest Frame Size field and only its rightmost 6 bits are used. The format is defined in the IEEE 802.1D Standard, Annex C, Largest Frame Bits (LF). Bit 3 to bit 5 are base bits. Bit 0 to bit 2 are extended bits. The Largest Frame Size field is not used in the START_DL_FAILED frame and must be set to zero. bit 7 6 5 4 3 2 1 0 r r b b b e e e Figure 3-7. Largest Frame Size Please note that if the client is a PU 2.1 node, the client should use the maximum I-frame size negotiated in the XID3 exchange. The Initial window size in the START_DL frame gives the receive window size on the originating side, and the target DRAP station returns its receive window size in the DL_STARTED frame. The field is reserved in the START_DL_FAILED frame. The usage of the window size is the same as the one used in DLSw. Please refer to RFC 1795 for details. The last two bits are reserved for future use. They must be set to zero by the sender and ignored by the receiver.Chiang, et. al. Informational [Page 10] RFC 2106 DLSRAP February 1997 If the sender of the START_DL frame did not receive a START_DL_FAILED frame within a recommended threshold value of 60 seconds, the connection is considered unsuccessful.3.4.3. HALT_DL, HALT_DL_NOACK, and DL_HALTED Frames These frame types are used by DRAP to disconnect a link station. A HALT_DL frame is sent directly to the remote workstation to indicate that the sender wishes to disconnect. When the receiver receives this frame, it tears down the session that is associated with the Original Session ID and the Target Session ID provided in the HALT_DL frame. The receiver should respond with the DL_HALTED frame. The DL_HALTED frame should use the same Session ID values as the received HALT_DL message without swapping them. The HALT_DL_NOACK frame is used when the response is not required. +---------------+-----------------------+ | Field Name | Information | +---------------+-----------------------+ | Message Type | 0x0C, 0x0D, or 0x0E | +---------------+-----------------------+ | Packet Length | 0x10 | +---------------+-----------------------+ Figure 3-8. HALT_DL, HALT_DL_NOACK, and DL_HALTED HeaderChiang, et. al. Informational [Page 11] RFC 2106 DLSRAP February 1997 +-----------------------------------+ | Field Name (Each row is one byte) | +===================================+ 0 | Sender Session ID | + - - - - - - - - - - - - - - - - - + 1 | | + - - - - - - - - - - - - - - - - - + 2 | | + - - - - - - - - - - - - - - - - - + 3 | | +-----------------------------------+ 4 | Receiver Session ID | + - - - - - - - - - - - - - - - - - + 5 | | + - - - - - - - - - - - - - - - - - + 6 | | + - - - - - - - - - - - - - - - - - + 7 | | +-----------------------------------+ 8 | Reserved | + - - - - - - - - - - - - - - - - - + 9 | | + - - - - - - - - - - - - - - - - - + 10| | + - - - - - - - - - - - - - - - - - + 11| | +-----------------------------------+ Figure 3-9. START_DL, DL_STARTED, and START_DL_FAILED Data3.4.4. XID_FRAME, CONTACT_STN, STN_CONTACTED, INFO_FRAME, FCM_FRAME, and DGRM_FRAME These frame types are used to carry the end-to-end data or establish a circuit. The Destination Session ID is the Session ID created in the START_DL frame or the DL_STARTED frame by the receiver. The usage of the flow control flag is the same as the one used in DLSw. Please refer to RFC 1795 for details. +---------------+----------------------------+ | Field Name | Information | +---------------+----------------------------+ | Message Type | Based on Message type | +---------------+----------------------------+ | Packet Length | 0x0C + length of user data | +---------------+----------------------------+ Figure 3-10. Generic DRAP HeaderChiang, et. al. Informational [Page 12] RFC 2106 DLSRAP February 1997 +-----------------------------------+ | Field Name (Each row is one byte) | +===================================+ 0 | Destination Session ID | + - - - - - - - - - - - - - - - - - + 1 | | + - - - - - - - - - - - - - - - - - + 2 | | + - - - - - - - - - - - - - - - - - + 3 | | +-----------------------------------+ 4 | Flow Control Flags | +-----------------------------------+ 5 | Reserved | + - - - - - - - - - - - - - - - - - + 6 | | + - - - - - - - - - - - - - - - - - + 7 | | +-----------------------------------+ Figure 3-11. Generic DRAP Data Format3.4.5. DATA_FRAME This frame type is used to send connectionless SNA and NetBIOS Datagram (UI) frames that do not have a link station associated with the source and destination MAC/SAP pair. The difference between DGRM_FRAME and DATA_FRAME is that DGRM_FRAME is used to send UI frames received for stations that have a link station opened, whereas DATA_FRAME is used for frames with no link station established. +---------------+-----------------------------+ | Field Name | Information | +---------------+-----------------------------+ | Message Type | 0x0A |⌨️ 快捷键说明
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