Nuke subs can remain under water indefinitely. They only berth for loading up on food supplies. This means Indian SSBNs can be submerged under the arctic and fire a nuke missile on Rawalpindi, and then still remain untraced for a long, long time. For Pakistan, you cannot say whether the attack came from Russia or India. That is why without SSBNs you cannot have a full fledged nuclear triad. This is also the reason why only 6 countries in the world have SSBNs. They are the ultimate nuke delivery mechanism and cannot be pre-empted at all by any power. Those who have SSBNs rule the world. This is why no one gives a damn about North Korea or Pakistan though they have nukes, they have limited, unreliable delivery mechanisms. And cannot survive any second strike. At the end of the day there are only 6 countries that have the economic and military power to provide global security - P5+India. Others have to just align with one or more of these giants.