The title of the protocol and explanation of the steps required
This protocol is known as Kerberos, at least, as originally implemented (I believe the protocol has been extended significantly since).
Within Kerberos, the tuple $[s_2, s_A]$ is protected by $k_2$ (which both R and B know); hence no one else can modify it.
This tuple essentially tells B &quot;A wants to talk to you, and use key $k$ to do so&quot;.  A third party could forward this tuple directly to B, but it doesn't give him an advantage; after all, A really does want to talk to B.  This third party could prevent A from receiving his message from R (and so A doesn't learn $k$), however the third party doesn't learn $k$ either, so B gets a key that will never be used.
It may be that, in the original protocol, the tuple R sends to A was encrypted by $k_1$; this would obviously prevent this attack - it's been a while, and so I forget the details.  Even if it doesn't, this attack doesn't doesn't achieve anything, other than preventing A from talking to B, and an adversary that can arbitrarily modify or delete messages can trivially do that.