How many bytes the receiver can receive
Receiver calculates advertised window and sends it in TCP Header
AdvertisedWindow = MaxRcvBuffer - ((NextByteExpected - 1) - LastByteRead)
If advertised window is 0, periodically send 1 byte packets to break deadlock
Effective window (calculated at sender side) is AdvertisedWindow - (LastByteSent - LastByteACKed)