DTJ Number 3 September 1987 - Digital Technical Journals

The DECnet- VAX Product - An Integrated Approach to Ne tworkingThe command procedure can either activate animage that accepts the connection specified bySYS $NET or complete the connection directlyfrom DCL by opening a channel using the logicalname.There are both advantages and disadvantages tothe way in which DECnet-V AX uses separate processesfor diffe rent network objects . Each VMSprocess carries its own protection, defined bythe authorization parameters of the accountspecified for the process and enforced by aspectsof the VAX architecture . Therefore , using a different process for each logical link is a convenientway to maintain security and provideaccounting information . It is also a simple meansto keep separate the context of each logical link.For example, FAL must maintain the file protectionappropriate for each user accessing filesover the network. A FAL process handles only onelogical link (for one file access) at a ti me. Tohandle more would require FAL to completelyand securely replicate the security context ofeach user for each logical link concurrentlymaintained. That is a formidable task in the VMSsystem, with its complex set of authorizationprivileges.The primary disadvantage of maintaining differentprocesses for different security profiles isreduced performance. This reduction resultsfrom having to create a new process when a logicallink is established. This disadvantage can beoffset by using large timeout constants for eachaccount's NETSERVER processes and FAL logicallink caching, both of which are described later.ASSIGN and DASSGN System ServicesThe ASSIGN system service creates a channel to aspecified device. This service provides transparentaccess to the DECnet-VAX software by recognizingwhen the device specification includes anode name . ASSIGN then issues an IO$_ACCESSQIO function to NETDRIVER on behalf of thecaller to establish a logical link transparently. Inthis way, simply supplying a network task specifierin place of a device name will create a logicallink when a channel is assigned.The internal data structures associated withthe channel are defined to resemble an openfile on the channel. As a result, when called todeassign the channel, the DASSGN systemservice wi ll issue a QIO IO $_DEACCESS requestto close the file. The logical link is thendisconnected.Transparent and NontransparentModesDECnet-VAX provides two mechanisms to establishnetwork communications between applicationson different nodes. In the nontransparentmode, an image executes a network call to declareitself as a particular network task byname or object number. To use this mode a programmermust have a thorough understanding ofnetwork primitives. However, the mode providesgreater flexibility than the transparentmode . That is, the same image can support multiplelogical links concurrently and can even act asmultiple network tasks.The transparent mode provides a very simplemeans to establish a correlation between anetwork task and a process. In this mode, theimage being executed need not even be awarethat it is operating as a network task. Upon creatinga process to handle an incoming logical linkfor an undeclared ne twork object or task,NETACP creates the logical name SYS $NET.This name contains the network control block ­needed to complete the connection with theoriginator of the link. Performing a normalRMS OPEN on this logical name will start a chainof events culminating in the establishment of thelogical link:• The RMS OPEN operation issues an ASSIGNfollowed by a QIO IO$_ACCESS to confirmthe connection.• The RMS GET and PUT operations translate toQIO IOLWRITEVBLK and IOLREADVBLKcalls to the same network device .• The network device translates to networktransmit and receive operations.The standard logical names SYS SINPUT andSYS $0UTPUT can be assigned to the logicalname SYS $NET before an image is activated. Thisaction will allow programs originally writtento perform 1/0 from eithe r te rminal or diskdevices to act like distributed applications in anetwork. The programs require absolutely norewriting.At this point the various layers of the designprovide an environment in which network communicationscan proceed without any specificnetwork calls issued by the programmer. A simpleexample, using DCL command procedures,wi ll demonstrate the nontransparent mode ofnetwork communication.94Digital TecbnicalJournal'No. 3 September 1986