URI scheme

A URI scheme is the top level of the URI (Uniform Resource Identifier) naming structure. All URIs and absolute URI references are formed with a scheme name, followed by a colon character, and the remainder of the URI called (in the outdated RFCs 1738 and 2396, but not the current STD 66/RFC 3986) the scheme-specific part. The syntax and semantics of the scheme-specific part are left largely to the specifications governing individual schemes, subject to certain constraints such as reserved characters and how to "escape" them.

URI schemes are sometimes erroneously referred to as "protocols", or specifically as URI protocols or URL protocols, since most were originally designed to be used with a particular protocol, and often have the same name. The http: scheme, for instance, is generally used for interacting with Web resources using HyperText Transfer Protocol. Today, URIs with that scheme are also used for other purposes, such as RDF resource identifiers and XML namespaces, that are not related to the protocol. Furthermore, some URI schemes are not associated with any specific protocol (e.g. "file:") and many others do not use the name of a protocol as their prefix (e.g. "news:").

URI schemes should be registered with IANA, although many non-registered schemes are used in practice. RFC 2717 describes the procedures for registering new URI schemes. RFC 2718 provides guidelines for the development of new URI schemes. Both of these RFCs are currently undergoing revision.

Generic syntax

Internet standard STD 66 (also RFC 3986) defines the generic syntax to be used in all URI schemes. Every URI is defined as consisting of four parts, as follows:

<scheme name> : <hierarchical part> [ ? <query> ] [ # <fragment> ]

The scheme name consist of a letter followed by any combination of letters, digits, and the plus ("+"), period ("."), or hyphen ("-") characters; and is terminated by a colon (":").

The hierarchical part of the URI is intended to hold identification information hierarchical in nature. Usually this part begins with a double forward slash ("//"), followed by an authority part and an optional path. The authority part holds an optional user information part terminated with "@" (e.g. username:password@), a hostname (i.e. domain name or IP address), and an optional port number preceded by a colon ":". The path part is a sequence of segments (conceptually similar to directories, though not necessarily representing them) separated by a forward slash ("/"). Each segment can contain parameters separated from it using a semicolon (";"), though this is rarely used in practice.

The query is an optional part separated with a question mark, which contains additional identification information which is not hierarchical in nature. Its syntax is not generically defined, but is commonly organized as a sequence of <key>=<value> pairs separated by an ampersand, e. g. key1=value1&key2=value2&key3=value3.

The fragment is an optional part separated from the front parts by a hash ("#"). It holds additional identifying information which allows indirect identification of a secondary resource, e.g. a section heading in an article identified by the remainder of the URI.

Examples

The following are two example URIs and their component parts (taken from STD 66):

  foo://example.com:8042/over/there?name=ferret#nose
  \ /   \______________/\_________/ \_________/ \__/
   |           |             |           |        |
scheme     authority        path       query   fragment
   |   ______________________|_
  / \ /                        \
  urn:example:animal:ferret:nose

Official IANA-registered schemes

The official URI schemes registered with the IANA follow.

Scheme	Purpose	Defined by	General format	Notes
aaa:	Diameter Protocol	RFC 3588	aaa://<host>[:<port>][;transport=<transport>][;protocol=<protocol>] example: aaa://host.example.com:1813;transport=udp;protocol=radius
aaas:	Secure equivalent of aaa:	RFC 3588	aaas://<host>[:<port>][;transport=<transport>][;protocol=<protocol>]
acap:	Application configuration access protocol	RFC 2244	acap://url-server/url-enc-entry [url-filter] [url-extension]	URL scheme used within the ACAP protocol for the "subdataset" attribute, referrals and inheritance
cap:	Calendar access protocol	RFC 4324	generic syntax	URL scheme used to designate both calendar stores and calendars accessible using the CAP protocol
cid:	Referencing individual parts of an SMTP/MIME message	RFC 2392	cid:<content-id>	e.g. referencing an attached image within a formatted e-mail. (See also mid:)
crid:	TV-Anytime Content Reference Identifier	RFC 4078	crid://<host>/<data>	Allow references to scheduled publications of broadcast media content.
data:	Inclusion of small data items inline	RFC 2397	data:<mediatype>;[base64,]<data>
dav:	HTTP Extensions for Distributed Authoring (WebDAV)	RFC 2518	dav:	Used for internal identifiers only; WebDAV itself addresses resources using the http: and https: schemes. [1]
dict:	Dictionary service protocol	RFC 2229	dict://<user>;<auth>@<host>:<port>/d:<word>:<database>:<n> dict://<user>;<auth>@<host>:<port>/m:<word>:<database>:<strat>:<n>	refer to definitions or word lists available using the DICT protocol
dns:	Domain Name System	RFC 4501	dns:[//<host>[:<port>]/]<dnsname>[?<dnsquery>] examples: dns:example?TYPE=A;CLASS=IN dns://192.168.1.1/ftp.example.org?type=A	designates a DNS resource record set, referenced by domain name, class, type, and, optionally, the authority
fax:	Used for telefacsimile numbers	RFC 2806	fax:<phonenumber>	Seems to be deprecated in RFC 3966 in favour of tel:
file:	Addressing files on local or network file systems	RFC 1738	generic syntax (often appears as file:///path, the 3rd '/' is the final delimiter when no host (authority) is specified between)	Unusual in not being bound to any network protocol, and not usable in an Internet context.
ftp:	FTP resources	RFC 1738	generic syntax
go:	Common Name Resolution Protocol	RFC 3368	go://[<host>]?[<common-name>]*[;<attribute>=[<type>,]<value>] or go:<common-name>*[;<attribute>=[<type>,]<value>]
gopher:	Used with Gopher protocol	RFC 4266	gopher://<host>:<port>/<item type>/<path>
h323:	Used with H.323 multimedia communications	RFC 3508	h323:[<user>@]<host>[:<port>][;<parameters>]
http:	HTTP resources	RFC 2616	generic syntax
https:	HTTP connections secured using SSL/TLS	RFC 2817	generic syntax
iax2:	Inter-Asterisk eXchange protocol version 2	[2]	iax2:[<username>@]<host>[:<port>][/<number>[?<context>]] examples: iax2:[2001:db8::1]:4569/alice?friends iax2:johnQ@example.com/12022561414
im:	Instant messaging protocol	RFC 3860	im:[<username>@<host>] examples: im:john@example.com
imap:	Accessing e-mail resources through IMAP	RFC 2192	imap://[<user>[;AUTH=<type>]@]<host>[:<port>]/<command>
ldap:	LDAP directory request	RFC 2255 RFC 4516	ldap://[<host>[:<port>]][/<dn> [?[<attributes>][?[<scope>][?[<filter>][?<extensions>]]]]] example: ldap://ldap1.example.net:6666/o=University%20of%20Michigan, c=US??sub?(cn=Babs%20Jensen)
mailto:	SMTP e-mail addresses and default content	RFC 2368	mailto:<address>?header1=value1&header2=value2 example: mailto:jsmith@example.com?subject=A%20Test&body=My%20idea%20is%3A%20%0A	Headers are optional, but often include subject=; body= can be used to pre-fill the body of the message.
mid:	Referencing SMTP/MIME messages, or parts of messages.	RFC 2392	mid:<message-id> or mid:<message-id>/<content-id>	(See also cid:)
news:	(Usenet) newsgroups and postings	RFC 1738	news:<newsgroupname> or news:<message-id>	References a particular resource, regardless of location.
nfs:	Network File System resources	RFC 2224	generic syntax
nntp:	Usenet NNTP	RFC 1738	nntp://<host>:<port>/<newsgroup-name>/<article-number>	Referencing a specific host is often less useful than referencing the resource generically, as NNTP servers are not always publicly accessible
pop:	Accessing mailbox through POP3	RFC 2384	pop://[<user>[;auth=<auth>]@]<host>[:<port>]
pres:	Used in Common Profile for Presence (CPP) to identify presence	RFC 3859	pres:<address>?header1=value1&header2=value2	Similar to "mailto:"
sip:	Used with Session Initiation Protocol (SIP)	RFC 3969 RFC 3261	sip:<user>[:<password>]@<host>[:<port>][;<uri-parameters>][?<headers>] examples: sip:alice@atlanta.com?subject=project%20x&priority=urgent sip:+1-212-555-1212:1234@gateway.com;user=phone
[[sips: URI scheme|sips:]]	Secure equivalent of sip:	RFC 3969 RFC 3261	sips:<user>[:<password>]@<host>[:<port>][;<uri-parameters>][?<headers>]
snmp:	Simple Network Management Protocol	RFC 4088	snmp://[user@]host[:port][/[<context>[;<contextEngineID>]][/<oid>]] examples: snmp://example.com//1.3.6.1.2.1.1.3+ snmp://tester5@example.com:8161/bridge1;800002b804616263
tel:	Used for telephone numbers	RFC 3966 RFC 2806	tel:<phonenumber>
telnet:	Used with telnet	RFC 4248	telnet://<user>:<password>@<host>[:<port>/]
urn:	Uniform Resource Names	RFC 2141	urn:<namespace>:<specificpart>
wais:	Used with Wide area information server (WAIS)	RFC 4156
xmpp:	XMPP (Jabber)	RFC 4622	xmpp:<address>/<resource>?<query>	rfc4622bis contains corrections

Unofficial but common URI schemes

Scheme	Purpose	Defined by	General format	Notes
about:	Displaying product information and internal information	Un-standardised about:blank is commonly used to display a blank page.		Widely used by web browsers, sometimes even providing interactive resources
aim:	Controlling AOL Instant Messenger.	AOL	aim:<function>?<parameters>	Functions include goim, addbuddy, and buddyicon. Authors are encouraged to use im: schema.
callto:	Launching Skype call (unofficial; see also skype:)	Skype	callto:<screenname> or callto:<phonenumber> [3]	Introduced with Microsoft NetMeeting. Works with current version of Skype with Firefox, Internet Explorer and Safari
cvs:	Provides a link to a Concurrent Versions System (CVS) Repository	Concurrent Versions System	cvs://<method:logindetails>@<repository>/<modulepath>;[date=date to retrieve | tag=tag to retrieve]
ed2k:	Resources available using the eDonkey2000 network	eDonkey2000	<ed2k_uri> ::= "ed2k://|file|" <file_name> "|" <hash_hash> "|" <file_hash> "|/" | "ed2k://|server|" <host> "|" <host_port> "|/"	Links to servers are also possible, as are additional parameters. Official documentation from
feed:	web feed subscription		<feed_uri> ::= "feed:" <absolute_uri> | "feed://" <hier_part>	See Feed:_URI_scheme for a detailed overview of common implementations, supported software, and critics.
irc:	Connecting to a server to join a channel.	IETF Draft Old IETF Draft	<irc_uri> ::= "irc://" <server> | "irc://" <server> "/" <channel> <server> ::= <host> | <host> ":" <host_port> <channel> ::= <channel_name> | <channel_name> "?" <channel_password>	Assuming the client knows a server associated with the name, "host" may optionally be an IRC network name.
ircs:	Connecting to a secure server to join a channel.	IETF Draft	<irc_uri> ::= "ircs://" <server> | "irc://" <server> "/" <channel> <server> ::= <host> | <host> ":" <host_port> <channel> ::= <channel_name> | <channel_name> "?" <channel_password>	See irc:
lastfm:	Connecting to a radio stream from Last.fm.	Last.fm	<lastfm_uri> ::= "lastfm://" <radio_stream>
ldaps:	Secure equivalent of ldap:		ldaps://[<host>[:<port>]][/<dn> [?[<attributes>][?[<scope>][?[<filter>][?<extensions>]]]]]	Not an IETF standard, but commonly used in applications.
[[magnet: URI scheme|magnet:]]	"magnet links"	Magnet-URI Project	magnet:?xt=urn:sha1:<hash of file>&dn=<display name> (other parameters are also possible)	Used by various peer-to-peer clients, usually providing the hash of a file to be located on the network.
mms:	Windows streaming media		mms://<host>:<port>/<path>	Used by Windows Media Player to stream audio and/or video.
msnim:	Adding a contact, or starting a conversation in Windows Live Messenger	Windows Live Messenger	msnim:chat?contact=<screenname>	Authors are encouraged to use im: schema.
psyc:	Used to identify or locate a person, group, place or a service and specify its ability to communicate	PSYC	psyc:[//<host>[:[<port>][<transport>]]/[<object-name>][#<channel-name>]	Official documentation from PSYC website
rsync:	Rsync		rsync://<host>[:<port>]/<path>
secondlife:	Open the Map floater in Second Life application to teleport the resident to the location.	Linden Lab	secondlife://<region name>/<x position>/<y position>/<z position>	Used by SLurl.com. Knowledge base article.
skype:	Launching Skype call (official; see also callto:)	Skype	<skype_uri> ::= "skype:" <recepient> | "skype:" <recepient> "?" <action> <recepient> ::= <skype_username> | "+" <contry_code> <phone_numer> | <skype_username> "," <skype_username> | <skype_username> ",+" <contry_code> <phone_numer> <action> ::= "add" | "call" | "chat" | "sendfile" |"userinfo"	Official documentation from Skype website. Authors are encouraged to use im: schema.
ssh:	SSH connections (like telnet:) and	IETF Draft	ssh://[<user>[;fingerprint=<host-key fingerprint>]@]<host>[:<port>]
sftp:	SFTP file transfers (not be to confused with FTPS (FTP/SSL))	IETF Draft	sftp://[<user>[;fingerprint=<host-key fingerprint>]@]<host>[:<port>]/<path>/<file>
smb:	Accessing Samba shares (like NFS), see CIFS, Windows file sharing etc.	IETF Draft	smb://[<user>@]host[:<port>][/[<path>]][?<param1>=<value1>;<param2>=<value2>] or smb://[<user>@]workgroup[:<port>][/]
sms:	Interact with SMS capable devices for composing and sending messages.	IETF draft	[tel:]sms:[<phone number>?<action>	Should be used as a subset to the tel: schema.
soldat:	Joining servers	Soldat	soldat://<host>:<port>/ example: soldat://127.0.0.1:23073/	Official note in Manual
steam:	Interact with Steam: install apps, purchase games, run games, etc.	Steam, Valve Corporation	steam:<command line arguments> or steam://<action>/<id, addon, IP, hostname, etc.>	Official documentation from Valve Developer Community website
unreal:	Joining servers	Unreal	unreal://<server>[:<port>]/	Unreal legacy "protocol"
ut2004:	Joining servers	Unreal Tournament 2004	ut2004://<server>[:<port>][/map?options]	Documentation from Unreal Developer Network
xfire:	Adding friends and servers, joining servers, changing status text.	Xfire	xfire:<function>?<parameter1>=<value1>&<parameter2>=<value2>	Official documentation from Xfire website
ymsgr:	Sending an instant message to a Yahoo! Contact.	Yahoo! Instant Messenger with Voice	<ymsgr_uri> ::= "ymsgr:sendIM?" <screenname>	Authors are encouraged to use im: schema.

External links

• Official IANA Registry of URI Schemes
• More information, including many more schemes