📄 rfc1795.txt
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|| Link | DLC Indication | | |--------------> || Control 1 |--------------->| | | |+-----------+ | Data Link | | |+-----------+ DLC Request | Switch | | || Data |<-------------- | | |Rec. SSP Frame || Link | DLC Indication | | |<------------- || Control 2 | -------------->| | | |+-----------+ +-----------+ | | Multi-Protocol Router | +-------------------------------------------+ Before Data Link Switching can occur between two routers, they must establish two TCP connections between them. Each Data Link Switch will maintain a list of DLSw capable routers and their status (active/inactive). After the TCP connection is established, SSP messages are exchanged to establish the capabilities of the two Data Link Switches. Once the exchange is complete, the DLSw will employ SSP control messages to establish end-to-end circuits over the transport connection. Within the transport connection, DLSw SSP messages are exchanged. The message formats and types for these SSP messages are documented in the following sections. The default parameters associated with the TCP connections between Data Link Switches are as follows: Socket Family AF_INET (Internet protocols) Socket Type SOCK_STREAM (stream socket) Read Port Number 2065 Write Port Number 2067Wells & Bartky [Page 6] RFC 1795 Data Link Switching April 1995 Two or more Data Link Switches may be attached to the same LAN, consisting of a number of token-ring segments interconnected by source-routing bridges. In this case, a TCP connection is not defined between bridges attached to the same LAN. This will allow using systems to select one of the possible Data Link Switches in a similar manner to the selection of a bridge path through a source- routed bridged network. The virtual ring segment in each Data Link Switch attached to a common LAN must be configured with the same ring number. This will prevent LAN frames sent by one Data Link Switch from being propagated through the other Data Link Switches.Wells & Bartky [Page 7] RFC 1795 Data Link Switching April 19953.1 SSP Frame Formats The following diagrams show the two message header formats exchanged between Data Link Switches, Control and Information. The Control message header is used for all messages except Information Frames (INFOFRAME) and Independent Flow Control Messages (IFCM), which are sent in Information header format. The INFOFRAME, KEEPALIVE and IFCM message headers are 16 bytes long, and the control message header is 72 bytes long. The fields in the first sixteen bytes of all message headers are the same. CONTROL MESSAGES (72 Bytes) (zero based offsets below shown in decimal (xx) ) +-----------------------------+-----------------------------+ | (00) Version Number | (01) Header Length (= 72) | +-----------------------------+-----------------------------+ | (02) Message Length | +-----------------------------+-----------------------------+ | (04) Remote Data Link Correlator | +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+ | | +-----------------------------+-----------------------------+ | (08) Remote DLC Port ID | +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+ | | +-----------------------------+-----------------------------+ | (12) Reserved Field | +-----------------------------+-----------------------------+ | (14) Message Type | (15) Flow Control Byte | +-----------------------------+-----------------------------+ | (16) Protocol ID | (17) Header Number | +-----------------------------+-----------------------------+ | (18) Reserved | +-----------------------------+-----------------------------+ | (20) Largest Frame Size | (21) SSP Flags | +-----------------------------+-----------------------------+ | (22) Circuit Priority | (23) Message Type (see note)| +-----------------------------+-----------------------------+ | (24) Target MAC Address (non-canonical format) | +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -| | | +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+ | | +-----------------------------+-----------------------------+ | (30) Origin MAC Address (non-canonical format) | +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -|Wells & Bartky [Page 8] RFC 1795 Data Link Switching April 1995 | | +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+ | . . | +-----------------------------+-----------------------------+ | (36) Origin Link SAP | (37) Target Link SAP | +-----------------------------+-----------------------------+ | (38) Frame Direction | (39) Reserved | +-----------------------------+-----------------------------+ | (40) Reserved | +-----------------------------+-----------------------------+ | (42) DLC Header Length | +-----------------------------+-----------------------------+ | (44) Origin DLC Port ID | +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+ | | +-----------------------------+-----------------------------+ | (48) Origin Data Link Correlator | +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+ | | +-----------------------------+-----------------------------+ | (52) Origin Transport ID | +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+ | | +-----------------------------+-----------------------------+ | (56) Target DLC Port ID | +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+ | | +-----------------------------+-----------------------------+ | (60) Target Data Link Correlator | +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+ | | +-----------------------------+-----------------------------+ | (64) Target Transport ID | +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+ | | +-----------------------------+-----------------------------+ | (68) Reserved Field | +-----------------------------+-----------------------------+ | (70) Reserved Field | +-----------------------------+-----------------------------+ (Even Byte) (Odd Byte)Wells & Bartky [Page 9] RFC 1795 Data Link Switching April 1995 INFORMATION MESSAGE (16 Bytes) +-----------------------------+-----------------------------+ | (00) Version Number | (01) Header Length (= 16) | +-----------------------------+-----------------------------+ | (02) Message Length | +-----------------------------+-----------------------------+ | (04) Remote Data Link Correlator | +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+ | | +-----------------------------+-----------------------------+ | (08) Remote DLC Port ID | +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+ | | +-----------------------------+-----------------------------+ | (12) Reserved Field | +-----------------------------+-----------------------------+ | (14) Message Type | (15) Flow Control Byte | +-----------------------------+-----------------------------+ (Even Byte) (Odd Byte) The first sixteen bytes of control and information message headers contain identical fields. A brief description of some of the fields in an SSP message are shown below (if not defined below, the fields and/or their values are described in subsequent sections). The Version Number field (offset 0) is set to 0x31 (ASCII '1'), indicating a decimal value of 49. This is used to indicate DLSw version 1. The Header Length field (offset 1) is 0x48 for control messages, indicating a decimal value of 72 bytes, and 0x10 for information and Independent Flow Control messages, indicating a decimal value of 16 bytes. The Message Length field (offset 2) defines the number of bytes within the data field following the header. The Flow Control Byte field (offset 15) is described in section 8. The Header Number field (offset 17) is 0x01, indicating a value of one. The Circuit Priority field (offset 22) is described in section 4. The Frame Direction field (offset 38) is set to 0x01 for frames sent from the origin DLSw to the target DLSw, and is set to 0x02 for frames sent from the target DLSw to the origin DLSw.Wells & Bartky [Page 10] RFC 1795 Data Link Switching April 1995 Note: The Remote Data Link Correlator and Remote DLC Port ID are set equal to the Target Data Link Correlator and Target DLC Port ID if the Frame Direction field is set to 0x01, and are set equal to the Origin Data Link Correlator and Origin DLC Port ID if the Direction Field is set to 0x02. The Protocol ID field is set to 0x42, indicating a decimal value of 66. The DLC Header Length is set to zero for SNA and is set to 0x23 for NetBIOS datagrams, indicating a length of 35 bytes. This includes the Access Control (AC) field, the Frame Control (FC) field, Destination MAC Address (DA), the Source MAC Address (SA), the Routing Information (RI) field (padded to 18 bytes), the Destination link SAP (DSAP), the Source link SAP (SSAP), and the LLC control field (UI). NOTE: The values for the Message Type field are defined in section 3.5. Note that this value is specified in two different fields (offset 14 and 23 decimal) of the control message header. Only the first field is to be used when parsing a received SSP message. The second field is to be ignored by new implementations on reception. The second field was left in for backwards compatibility with RFC 1434 implementations and this field may be used in future versions if needed. The SSP Flags field contains additional information related to the SSP message. The flags are defined as follows (bit 7 being the most significant bit and bit 0 the least significant bit of the octet): Bit(s) 76543210 Name Meaning --------- ----- ------- x....... SSPex 1 = explorer message (CANUREACH and ICANREACH) Reserved fields are set to zero upon transmission and should be ignored upon receipt.⌨️ 快捷键说明
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