1 0 Ref. B4 - NCP : INTERFACE TO RSCS (VNET) FOR RJE (Ref. B4) + ___ _________ __ ____ ____ ___ ___ ___ __ 0 Version 3 + _______ _ 0 8 March 1982 + _ _____ ____ 0 P M Girard + _ _ ______ 0 1. Introduction + ____________ 0 This paper covers both the MVT and VM versions of NCP. The original version of the paper contains slightly more detail specific to the MVT environment. In practice, RSCS calls will not be routed via MVT in future, and the relevant code may be removed from the MVT version at some stage. The purpose of this interface is to provide access to RSCS (also called VNET) from remote workstations connected via the X25 network. Card reader, line printer and card punch streams are supported. A main remote console is also supported, but no 'secondary consoles'. Additional terminals on the workstation can make network calls into the mainframe, with access at present to CMS; such calls are not routed through RSCS however, and are therefore not relevant to this paper. - 2. Addressing of Calls + __________ __ _____ 0 Workstations are expected to use Transport Service addressing, in which the required address is specified by a variable-length character string. 0 Each remote number has a default destination, built into a table in NCP, which (in the VM version) is always RSCS at present. The address must take the form HASPnnn or RSCSnnn, where nnn is the remote number. The remote number may be expressed with leading zeros optionally suppressed, in which case it must be left-adjusted in the field (i.e: no leading blanks). For example, HASP3, HASP03, or HASP003 are equivalent valid forms. - 3. Assignment of RSCS Slots + __________ __ ____ _____ 0 A pool of slots is provided for RSCS calls, and the usage of these is monitored and recorded in the log. - - 0 NCP : INTERFACE TO RSCS (VNET) FOR RJE PAGE 1 1 0 Ref. B4 0 4. Sign-on Blocks + ____ __ ______ 0 NCP remembers the remote number specified in the call address, and deposits this in any Sign-on blocks that may subsequently arrive from the station during the call. The remote number is deposited in the form expected by RSCS, namely left-adjusted with leading zeros suppressed. For instance: REMOTE4. 0 The Sign-on block is the first block sent to RSCS after call set-up: NCP does not send an initial SOH ENQ signal. - 5. Nature of the Interface + ______ __ ___ _________ 0 The interface to RSCS uses a 2-subchannel VMCF connection, as specified in the paper "A Protocol for using VMCF" (Ref. A2). One subchannel is used for passing data in each direction, and data can therefore flow simultaneously in both directions. 0 Data from RSCS is passed transparently out to the workstation, and incoming data is passed transparently to RSCS. The Binary Synchronous blocks are enclosed in X25 packets for shipping across the network, and the maximum block size is 252 bytes for output to the station and 256 bytes for input from the station. Each packet contains only one Binary Synchronous block, and blocks are never split across packet boundaries. NCP does not examine or alter FCS settings or anything else within the Binary Synchronous blocks. However, it should be noted that the Binary Synchronous protocol used across the network specifically excludes NAK signals; nor should either end implement Binary Synchronous time-outs at this level. - 6. Flow Control + ____ _______ 0 Flow control between RSCS and NCP is achieved, in both directions, simply by not taking a block which has been offered, until it can be dealt with. The Binary Synchronous WABT mechanism is not used. 0 Flow control across the network is handled by the normal X25 Level 3 mechanism. - - - NCP : INTERFACE TO RSCS (VNET) FOR RJE PAGE 2 1 0 Ref. B4 0 7. Disconnection + _____________ 0 If the station signs off, RSCS de-activates the logical link, causing NCP to receive a VMCF 'DISCONNECT' message. NCP then clears down the network call in the usual way, and the station should receive an X25 Clear packet with a code implying loss of contact with the application. 0 If the RSCS virtual machine disappears, the result is exactly the same as if it deliberately de-activates the logical link. 0 If NCP disappears, the RSCS virtual machine should receive a VMCF 'DISCONNECT' message, and the workstation should receive an X25 Clear packet generated by the GEC 4080 Exchange, with a code implying that the network connection is out of order. - 8. References + __________ 0 A2: A Protocol for Using VMCF (P M Girard) - - - - - - - - - - - 0 NCP : INTERFACE TO RSCS (VNET) FOR RJE PAGE 3