DTJ Number 3 September 1987 - Digital Technical Journals

New Productssystems and convert data to and from the formatthat ULTRIX users expect.Achieving this capability involved addingknowledge to the commands by · programmingseveral different record formats and attributes. Inmost cases the data is automatically convertedfor the user into the form desired. For cases inwhich that is undesirable, a means is providedfor the user to bypass that conversion. TheULTRIX FAL program must also perform dataconversions even though DAP, as a server, has nosuch responsibility. FAL is forced to convert theULTRIX format stream into the appropriate variablelength format files of the other operatingsystem so that it need not be modified to workwith the DECnet-ULTRIX code.Remote Terminal AccessIn our planning for the initial release of theDECnet-ULTRIX software , we decided not toinclude remote terminal access because itrequired too much development time . Once thebasic networking code ran in the kernel, however,we easily modified the 4.2BSD remote terminalaccess programs rlogin and rlogind to runover a DECnet system. Those programs provideULTRIX-to-ULTRIX terminal access. Later, withminor changes to the protocol , we used themodified rlogind (now called dtermd) to advertiseto other DECnet systems that it could communicatewith the TOPS-20 remote terminal protocol.Using this mechanism we provided accessto the ULTRIX system from non-ULTRIX DECnetsystems that had previously implemented supportfor the TOPS-20 software . This capabilityproved so useful that we decided to include fullremote terminal support in DECnet-ULTRIXVersion 1.0.In the DECnet system, remote terminal accessoperations are currently performed using thecommand terminal (CTERM) protocol. Remoteterminal access uses a hostjserver model inwhich the server (which controls the physicalterminal) contacts the host to request access tothe remote system. Once that connection hasbeen established, the host controls the terminalthrough the CTERM protocol.The ULTRIX system supports a "pseudoterminal"driver that allows a program to controlother programs through what appears to be anormal terminal interface. This control allowsthe daemon program ( dlogind) on the host toprovide a standard interface to users who areremotely logged in. We did, however, encountersome problems trying to use this capability.The CTERM protocol exports terminal I/0requests from the host to server, which executesthem, thus reducing! the host processingload. The pseudoterminal interface providestransparent buffering berJ.een the controllingprogram and the programs controlled. In thatway the controlling program never knows whenanother program has issued a read request; therefore,the controlling program cannot know whento ship a read request to the server. Fortunately,the protocol supports a notification function thatthe server sends to the host !if a user types a characterand there is no outstnding read request.Using this function we allowed the server toissue a "pseudoread" request when the firstcharacter is typed. Usually, the request is for afull line of input, thus allowing the server to performcharacter interrupt processing and localcharacter editing.Using the remote terminl access protocol, aterminal can connect logically to a remote systemhaving very different ontrol conventions,such as control characters and line terminators.For this reason the server program ( dlogin) disablesall special character processing by the localterminal driver. The program then processeseach character individually to perform any functionsrequested by the host ystem. A two-charactersequence (by default a filde [ -] followed bya carriage return) is reserved to allow entry to alocal command mode (the first character may bechanged by a command line switch) . This localcommand mode provides access to the shell onthe local system. This mode also provides commandsto log in the terminal session to a file andto suspend or terminate the current remote terminalsession.MailOf all the fu nctions in the bECnet-ULTRIX software,mail was the easiest to implement. Themail system included with the ULTRIX systemwas already quite sophisticated. This mail systemsupports multiple mail protocols and addressformats with a central mail program named sendmail.Sendmail is driven by a configuration filethat can be tailored on eath ULTRIX system todefine new address format and mail-forwardingrules. We decided to add upport for the mostcommon DECnet mail protocol, called mail- 11,supplied with DECnet-VAX and other DECnetjIDigital Tecbnlcal]ournalNo. 3 September 1986iIJ.105