The network explicitly tells a TCP sender how many unakcnowledged

bytes the sender can send over the network by setting the congestion window

inside the TCP header.

A TCP sender infers on its own how many unacknowledged bytes it can send

without overwhelming the receiver, based on the ACKs that the receiver sends.

A TCP receiver explicitly tells the sender how many unakcnowledged bytes the sender can send to the receiver, by setting the receiver window inside the TCP header.

A TCP sender infers on its own how many unacknowledged bytes it can send without causing network congestion, based on the ACKs it receives.

Acknowledgment number (ACK).

Sequence number (SEQ).

Checksum.

SYN.

struct some_type a = { 0, { 1.0 } };

struct some_type a;
a.s = 0;
const double init[] = { 0.0, 0.0, 0.0 };
a.tab = init;

struct some_type a;
memset(&a, 0, sizeof(a));

None of the other.

struct some_type {
  size_t s = 0;
  double tab[3] = { 0.0 };
};

struct some_type a;
a.s = 0;
a.tab = { 0.0, 0.0, 0.0 };

There is a financial crisis and everyone goes to the bank to withdraw their money.

A celebrity endorses a political candidate, and everyone votes for them.

The probability of a person winning the national lottery is 1/100000000; you are incredibly lucky and you win.

There is a damaging earthquake and everyone claims house insurance. 

Push data into the link layer.

Push data into a process.

Update the network-layer header of a packet.

Discard the link-layer header of a packet.