This read-write Integer32 (0-16) sets the session-level receive window size advertised to the peer when session flow control is enabled, with the special value 0 meaning the peer must include sequence numbers in payload packets without being required to flow control its transmissions, useful for forcing sequence numbers out of a non-RFC-compliant peer. An admin uses the 0 setting specifically to interoperate with a peer that needs sequence numbering enforced but doesn't support full flow control. For example, connecting to an older L2TP implementation that requires payload sequence numbers but chokes on flow-control negotiation would call for setting this to 0 rather than a nonzero window size.