Web user agents range from web browsers to search engine crawlers (spiders), as well as mobile phones, screen readers and braille browsers used by people with disabilities. When a user agent operates, it typically identifies itself, its application type, operating system, software vendor, or software revision, by submitting a characteristic identification string to its operating peer. In the HTTP and SIP protocols, this is transmitted in a header field User-Agent. Bots, such as web crawlers, often also include a URL and/or e-mail address so that the webmaster can contact the operator of the bot.