Needham Schroeder

I was looking for a diagram which explains Needham Schroeder Symmetric Key protocol but could not find one , so this is what I drew using the information I have on this protocol


















Step 1,2 and 3 seems to be compulsory cos all the text has described these three steps , but some text have Omitted the step 4 and 5 , and has described that lack of step 4 and 5 makes Needham Schroeder symmetric key protocol vulnerable cos "A" cannot know whether the message 3 was delivered to the intended recipient "B" ...