INT21CN Computer Networks

Tutorial #3

1. What is meant by the term "remote login" in the context of the telnet protocol?

2. The idea of plain ASCII text is fundamental in IT. For example, computer programs (ie, source code) are always plain text files. And, as we will see in later lectures, many Internet application protocols are based on the exchange of ASCII text messages.
   1. What do we mean by the term "printable ASCII"? How many printable ASCII characters are there?

   2. What is the conventional way to generate the ASCII control codes from a computer keyboard?

   3. What are the particular advantages of "plain ASCII text" compared to other "character sets"?

   4. What are some of the obvious disadvantages of ASCII text?

   5. All computers support ASCII text files. However, the convention for indicating the end of a text line is different in each. Unix (and Linux) systems use a single Linefeed (LF, decimal 13), Macs use a single Carriage Return (CR, decimal 10) and Microsoft systems use both CR and LF, the same as the telnet NVT. What kinds of problems could occur when text files are shared between each of these computer types?

3. What are some of the basic characteristics of the telnet Network Virtual Terminal (NVT)?

4. The telnet NVT "end-of-line" convention delimits lines of text using the two character ASCII control code sequence <CR><LF> (in English: Carriage Return followed by Line Feed). Why do you suppose the designers of the protocol adopted this two character sequence instead of just a single character <CR>, or a single <LF>, or something else entirely?

5. (Philosophical question) The NVT approach used by the telnet protocol means that servers and clients don't need to know the details of the actual terminal or host (if any) at the other end of the connection. The alternative is to perform terminal emulation. What is meant by terminal emulation?

6. (Deeply philosophical question) The notion of how the NVT is used in telnet leads on to a (moderately) serious philosophical issue about how to map the requirements of different kinds of computer systems to one another when you need to perform some kind of networked computing function. Start by comparing the telnet NVT approach to terminal emulation, and extend the discussion to the more general case.

7. When the telnet program starts up, it informs the user as follows:

   ironbark 27> telnet greybox
   Trying 149.144.20.62...
   Connected to greybox.bendigo.latrobe.edu.au.
   Escape character is '^]'.
   

   What is the "escape character" used for?

8. The^[1] text file for RFC 854 (the telnet protocol specification) contains exactly 854 lines. Do you think there is cosmic significance in this?

9. (Research and discussion question) Once upon a time, The ASCII control codes (ie, the ASCII characters less than 32_decimal) had important functions. Do some research, or just infer from the names of some of the control characters, what these functions may have been. Which of the control characters still have important meanings?

10. (Research question) How does telnet perfom option negotiation?