This reports the default maximum TCP receiver window size negotiated for this specific TCP connection underlying the FCIP link. The receiver window size directly caps how much data can be in flight before an ACK is required, so on a high-latency long-haul FCIP link, a receiver window that is too small relative to the link's bandwidth-delay product will bottleneck throughput no matter how fast the underlying circuit is. A transcontinental FCIP connection reporting a receiver window of only 64KB would badly underutilize a high-bandwidth circuit with, say, 100ms of round-trip latency, since the window caps how much unacknowledged data can be outstanding at once.