code: func (s *Stack) SetTransportProtocolHandler(p tcpip.TransportProtocolNumber, h func(*Route, TransportEndpointID, buffer.View, buffer.VectorisedView) bool) {
  	state := s.transportProtocols[p]
  	if state != nil {
  		state.defaultHandler = h
  	}
  }
docs: // SetTransportProtocolHandler sets the per-stack default handler for the given
  // protocol.
  //
  // It must be called only during initialization of the stack. Changing it as the
  // stack is operating is not supported.