The impact of delayed acknowledgments

Most TCP receivers implement the delayed acknowledgment algorithm [3] for reducing the number of pure acknowledgment packets sent. A TCP receiver using this algorithm will only send acknowledgments for every other received segment. If no segment is received within a specific time-frame, an acknowledgment is sent. The time-frame can be as high as 500 ms but typically is 200 ms.

A TCP sender such as uIP that only handles a single outstanding TCP segment will interact poorly with the delayed acknowledgment algorithm. Because the receiver only receives a single segment at a time, it will wait as much as 500 ms before an acknowledgment is sent. This means that the maximum possible throughput is severely limited by the 500 ms idle time.

Thus the maximum throughput equation when sending data from uIP will be $p = s / (t + t_d)$ where $s$ is the segment size and $t_d$ is the delayed acknowledgment timeout, which typically is between 200 and 500 ms. With a segment size of 1000 bytes, a round-trip time of 40 ms and a delayed acknowledgment timeout of 200 ms, the maximum throughput will be 4166 bytes per second. With the delayed acknowledgment algorithm disabled at the receiver, the maximum throughput would be 25000 bytes per second.

It should be noted, however, that since small systems running uIP are not very likely to have large amounts of data to send, the delayed acknowledgment throughput degradation of uIP need not be very severe. Small amounts of data sent by such a system will not span more than a single TCP segment, and would therefore not be affected by the throughput degradation anyway.

The maximum throughput when uIP acts as a receiver is not affected by the delayed acknowledgment throughput degradation.