Merge branch 'master' of https://github.com/dopefishh/tt2015

[tt2015.git] / a2 / 1cases.tex

diff --git a/a2/1cases.tex b/a2/1cases.tex
index da3a34b..1bfdc39 100644 (file)
--- a/a2/1cases.tex
+++ b/a2/1cases.tex
@@ -110,7 +110,7 @@ these partitions are given.
        \item Bits flipped in \emph{payload}
                \begin{enumerate}
                        \item Correct payload
-                       \item Payload with even number of bits flipped
+                       \item Payload with bit flips 
                        \item Payload with odd number of bits flipped
                \end{enumerate}
        \item \emph{checksum}
@@ -126,6 +126,25 @@ these partitions are given.
                \end{enumerate}
 \end{enumerate}
 
+These partitions were chosen since they correspond to key parts of the TCP
+specification. 
+
+TCP segments are send over a TCP connection from a \emph{source} to a \emph{destination port}. Therefore segments which are received which have a 
+source or destination port set to an incorrect value should not be regarded
+as segments belonging to the connection. 
+
+TCP uses a \emph{checksum} to catch any error introduced in headers, when this
+checksum does not match the actual computed checksum the packet should be
+disregarded.
+
+The TCP checksum is also an inherently weak one, as it is simply the 
+bitwise negation of the addition in ones complement arithmetic 
+of all 16 bit words in the header and data of the segment (excluding the
+header). Therefore any bit error where the ones complement value of one word
+increases by one, and the value of another decreases by one, is undetected. 
+The SUT should exhibit the same behavior and accept packets where these type
+of bit error occur.
+
 \textbf{hier iets over waarom deze partities relevant zijn! Waarom odd en 
 even number of bits flipped bijv interessant?}
 \bigskip