Minor pitfall in the Telnet Protocol

　　Network Working Group John Day

　　Request for Comments: 728 Apr 1977

　　NIC #40036

　　A Minor Pitfall in the Telnet Protocol

　　Designers of Telnet options should be aware of the following possible　case in the Telnet protocol which may generate unexpected behavior on　either end of the connection. Although at present none of the existing　options are susceptible to this problem, it could arise in the future.

　　The Telnet sync sequence causes all data to be deleted from the data　stream until a data mark is encountered. Telnet control functions are　not affected by the sync sequence (see page 9 of the protocol　specification). A Telnet option subnegotiation could be defined such　that it had an affect on the data following it in the data stream. For　example, a subnegotiation might be used to indicate the terminal was to　display the following data in a particular font or should receive other　special treatment by the terminal. A Telnet sync sequence sent after　such a subnegotiation and its data and before the subnegotiation had　been processed could resuit in the subnegotiation having its affect on　data other than that intended.

　　Two possible solutions come to mind at once. First, the data to be　affected could be included as a parameter of the subnegotiation. in　other words, the data is inserted in the data stream before the IAC SE　that terminates the subnegotiation. The disadvantages of this solution　are both theoretical and practical. Theoretically, it is improper and　not really in the spirit of the Telnet protocol design to send data as　subnegotiation parameters. Practically, in a situation where this case　would arise it would be equally unexpected behavior (and perhaps　confusing if a human was affected) if all data except that affected by　the subnegotiation was flushed.

　　The second solution would be for designers of options which have such　subnegotiations define a subnegotiation or other mechanism that would　follow immediately after the Data Mark and nullify the affects of any　offending subnegotiation. The exact semantics of such a subnegotiation　would probably be very specific to the option.