RFC728 - Minor pitfall in the Telnet Protocol

Network Working Group                                                  John DayRequest for Comments: 728                                              APR 1977NIC #40036               A Minor Pitfall in the Telnet ProtocolDesigners of Telnet options should be aware of the following possiblecase in the Telnet protocol which may generate uneXPected behavior oneither end of the connection. Although at present none of the existingoptions are susceptible to this problem, it could arise in the future.The Telnet sync sequence causes all data to be deleted from the datastream until a data mark is encountered. Telnet control functions arenot affected by the sync sequence (see page 9 of the protocolspecification). A Telnet option subnegotiation could be defined sUChthat it had an affect on the data following it in the data stream. Forexample, a subnegotiation might be used to indicate the terminal was todisplay the following data in a particular font or should receive otherspecial treatment by the terminal. A Telnet sync sequence sent aftersuch a subnegotiation and its data and before the subnegotiation hadbeen processed could resuit in the subnegotiation having its affect ondata other than that intended.Two possible solutions come to mind at once. First, the data to beaffected could be included as a parameter of the subnegotiation. inother Words, the data is inserted in the data stream before the IAC SEthat terminates the subnegotiation. The disadvantages of this solutionare both theoretical and practical. Theoretically, it is improper andnot really in the spirit of the Telnet protocol design to send data assubnegotiation parameters. Practically, in a situation where this casewould arise it would be equally unexpected behavior (and perhapsconfusing if a human was affected) if all data except that affected bythe subnegotiation was flushed.The second solution would be for designers of options which have suchsubnegotiations define a subnegotiation or other mechanism that wouldfollow immediately after the Data Mark and nullify the affects of anyoffending subnegotiation. The exact semantics of such a subnegotiationwould probably be very specific to the option.