Here is a very nice metaphor by Andrew Tridgell (from Samba fame) on the subject: http://www.samba.org/ftp/tridge/misc/french_cafe.txt http://www.samba.org/ftp

Here is a very nice metaphor by Andrew Tridgell (from Samba fame) on the subject: http://www.samba.org/ftp/tridge/misc/french_cafe.txt http://www.samba.org/ftp/tridge/misc/french_cafe.txt "I call this method the "French Cafe technique". Imagine you wanted to learn French, and there were no books, courses etc available to teach you. You might decide to learn by flying to France and sitting in a French Cafe and just listening to the conversations around you. You take copious notes on what the customers say to the waiter and what food arrives. That way you eventually learn the words for "bread", "coffee" etc. We use the same technique to learn about protocol additions that Microsoft makes. We use a network sniffer to listen in on conversations between Microsoft clients and servers and over time we learn the "words" for "file size", "datestamp" as we observe what is sent for each query. Now one problem with the "French Cafe" technique is that you can only learn words that the customers use. What if you want to learn other words? Say for example you want to learn to swear in French? You would try ordering something at the cafe, then stepping on the waiters toe or poking him in the eye when he gives you your order. As you are being kicked out you take copious notes on the words he uses. The equivalent of "swear words" in a network protocol are "error packets". When implementing Samba we need to know how to respond to error conditions. To work this out we write a program that deliberately accesses a file that doesn't exist, or uses a buffer that is too small or accesses a file we don't own. Then we watch what error code is returned for each condition, and take notes. "