In 3GPP eMBMS, sometimes sessions will be disconnected unexpectedly due to the miss of session keys. Although rekeying can prevent old users from getting multicast data, it also causes authorized users to miss subsequent data if they miss the key update messages. Thus, re-authentication is needed to obtain lost keys from KMM. We point out this problem in our previous work 1. In this paper, we further propose a new KeySet algorithm, which can pre-issue a number of keys to users when they join eMBMS. The advantage is that a user can still decode multicast data even if it misses some key updates tentatively. However, the cost is that allowing some old users to freely enjoying multicast for some time. In this paper, we quantify the tradeoff and derive the optimal case.