code: func NewForwarder(s *stack.Stack, rcvWnd, maxInFlight int, handler func(*ForwarderRequest)) *Forwarder {
  	if rcvWnd == 0 {
  		rcvWnd = DefaultBufferSize
  	}
  	return &Forwarder{
  		maxInFlight: maxInFlight,
  		handler:     handler,
  		inFlight:    make(map[stack.TransportEndpointID]struct{}),
  		listen:      newListenContext(s, seqnum.Size(rcvWnd), true, 0),
  	}
  }
docs: // NewForwarder allocates and initializes a new forwarder with the given
  // maximum number of in-flight connection attempts. Once the maximum is reached
  // new incoming connection requests will be ignored.
  //
  // If rcvWnd is set to zero, the default buffer size is used instead.