The last packet in the burst has a PUSH flag. The agreed MSS is 1260 and so in real life, our 64 KB burst would appear on the wire as 52 or 53 normal sized packets. The bursts of 64 KB always appear as one very large 63,648 byte packet followed by a smaller 1888 byte packet. Unfortunately, due to capturing inside the server, we are seeing packets that appear to be very large – but would be broken up into normal sized packets after we captured them – and as they leave the server. A new 64 KB “block” cannot begin to be transmitted until the whole 64 KB (from two bursts ago) is acknowledged. So there are never more than 2 x 64 KB “blocks” in flight. There are never more that 128 KB of bytes in flight and data is sent in blocks of 64 KB.
It would appear to be a send buffer limitation (but could be FTP delivering data to TCP in the server) with a buffer size of 128 KB.
It is entirely limited by the rate that the sending server is delivering data to the network. Your throughput limitation isn’t related to Receive Window sizes (or network bandwidth limits).
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