Different algorithms can be used:

Full jitter:

sleep = random(0, min(cap, base * 2 ** attempt)

Equal jitter:

temp = min(cap, base * 2 ** attempt)
sleep = temp / 2 + random_between(0, temp / 2)

Decorrelated Jitter (based on the last sleep value):

sleep = min(cap, random_between(base, sleep * 3)