TCP formulas

For a segment that arrives in order, with sequence number seq and L data bytes. Because the ACK is cumulative, an out-of-order segment brings back the previous ack number again (a duplicate ACK).

rwnd is the receiver's flow-control limit; cwnd is the congestion window from the next section.

W is in bits when the rate is in bits per second. Assumes no loss, header overhead ignored and ACK transmission time negligible. When the window equals the bandwidth-delay product, the pipe is exactly full. If a question defines one cycle as RTT plus the transmission time of the window (or of a segment), use that cycle in place of RTT.

Link rate times RTT is the bandwidth-delay product. Keep it in the same unit as W (bits or bytes).

The sequence field is 32 bits and counts bytes, so $2^{32}$ bytes pass before numbers repeat, if the sender sends continuously at the full rate.

The condition is that sequence numbers (counting bytes) must not wrap around while a segment can still be alive in the network. The lifetime is the maximum segment lifetime given in the question. Convert the rate from bits to bytes per second by dividing by 8.

SRTT $\leftarrow (1-\frac{1}{8})\text{SRTT}+\frac{1}{8}R$ and RTTVAR $\leftarrow (1-\frac{1}{4})\text{RTTVAR}+\frac{1}{4}|R-\text{SRTT}|$, where R is a new RTT sample. RTTVAR is updated first, using the old SRTT. On the first sample, SRTT = R and RTTVAR = R/2, so RTO = 3R.

cwnd is the window at the moment the loss is detected. Unless the question says otherwise, take cwnd/2 in whole MSS (rounded down for an odd window).