This read-only Integer32 controls the size of the per-session transmit queue used to hold outbound packets when the peer has requested flow control and its receive window is currently full. A larger queue absorbs more backlog during a momentary stall on the peer side before packets start being dropped, at the cost of added latency once the queue drains. If a session to a slow peer shows frequent drops, increasing this value would give the queue more room to buffer traffic while waiting for the peer's window to reopen.