code: func (s *Stack) TransportProtocolInstance(num tcpip.TransportProtocolNumber) TransportProtocol {
  	if pState, ok := s.transportProtocols[num]; ok {
  		return pState.proto
  	}
  	return nil
  }
docs: // TransportProtocolInstance returns the protocol instance in the stack for the
  // specified transport protocol. This method is public for protocol implementers
  // and tests to use.