TCP throughput has a lot to do with how much unacknowledged data can be sent by a sender before pausing transmission and waiting on ACKs from the receiver. In a TCP connection the two factors in how much un-ACK’d data can be sent is the sender Congestion Window (CWND) and the Receiver Window (RWND). The CWND is calculated from the connection agreed upon Maximum Segment Size (MSS) and the RWND is advertised from the receiving node during the initial three-way handshake. CWND size depends on the MSS multiplied by a value found in RFC 5681. The sender’s actual window size for sending data will always be the lower window of CWND and RWND.


The TCP Slow Start algorithm is used to combat substantial loss over internet when early implementations had the sender try transmitting the receiver’s entire window size after 3 way handshake. On low bandwidth networks that does not work. Slow Start works by initially sending the max SENDER WINDOW size to the receiver, and then adding one MSS value to the CWND per each ACK received from the receiver. If the original sender window was 2 MSS (2 * 1460 with common 1500 L2 MTU) and the sender receives two ACKs for each MSS/segment, then the next amount of data sent will be 4 MSS/segments. Once the four ACKs are received then the amount of transmitted MSS/segments is 8, then 16.

This exponential increase in CWND continues until the sender stops receiving ACKs for every segment sent, or the CWND hits a specific threshold. Once data starts to get lost or retransmissions begin, TCP starts to use an algorithm called ‘Congestion Avoidance Algorithm.’

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