diff --git a/Test_Cases/Test_Case3/readme b/Test_Cases/Test_Case3/readme
index 0765d80..abe2217 100644
--- a/Test_Cases/Test_Case3/readme
+++ b/Test_Cases/Test_Case3/readme
@@ -22,5 +22,7 @@ This folder contains details for Test_Case3.
 18. RSV = The reserved bits are set to zero for now.
 19. Flgas: set according to the packet type.
 20. Window Size = 0
-21. Checksum = 
+21. TCP Checksum = initially zero, then:
+(i) Concatenate TCP Pseudo header (Protocol, Source IP, Destination IP, Total Length) with TCP header and TCP data. This creates a big integer.
+(ii) Convert this integer to binary and compute bit wise 1's complement. 
 22. Urgent Pointer = 0 for now.