SIP URI scheme

The SIP URI scheme is a Uniform Resource Identifier (URI) scheme for the Session Initiation Protocol (SIP) multimedia communications protocol.wikipedia
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E.164

e164.arpae164.orgcountry code
Such a number could be a private branch exchange or an E.164 telephone number dialled through a specific gateway. These include the iNum Initiative (which issues E.164-formatted numbers, where the corresponding SIP address is the number '@sip.inum.net'), SIP Broker-style services (which associate a numeric *prefix to the SIP domain name) and the e164.org and e164.arpa domain name servers (which convert numbers to addresses one-by-one as DNS reverse-lookups).
When a number is mapped, a DNS query may be used to locate the service facilities on the Internet that accept and process telephone calls to the owner of record of the number, using, for example, the Session Initiation Protocol (SIP), a call-signaling VoIP protocol whose SIP addresses are similar in format (user@domain...) to e-mail addresses.

VoIP phone

IP PhoneIP PhonesEthernet-based phone
As the client may be a SIP phone or other device with just a numeric, telephone-like keypad, various schemes exist to associate an entirely numeric identifier to a publicly reachable SIP address.
Various schemes exist to allow one Internet telephony user to talk to another entirely via Internet and without incurring the cost of a PSTN call. Some are based on SIP addresses, some on proprietary protocol such as webcam or Internet chat applications. While it's not uncommon for two clients of the same voice over IP provider to talk to each other online for free, the various Internet telephony applications often do not talk directly to each other - requiring calls be gated to PSTN and back at full toll rates.

INum Initiative

+883-5100
These include the iNum Initiative (which issues E.164-formatted numbers, where the corresponding SIP address is the number '@sip.inum.net'), SIP Broker-style services (which associate a numeric *prefix to the SIP domain name) and the e164.org and e164.arpa domain name servers (which convert numbers to addresses one-by-one as DNS reverse-lookups).
It is possible to call an iNum as a SIP address in the format sip:+8835100xxxxxxxx@sip.inum.net or sip:+8835100xxxxxxxx@inum.net.

Caller ID spoofing

spoofspoofeda false origin is displayed
SIP is vulnerable to Caller ID spoofing as the displayed name and number, much like the return address on e-mail, is supplied by the sender and not authenticated.
In rare cases, a destination number served by voice-over-IP is reachable directly at a known SIP address (which may be published through ENUM telephone number mapping, a .tel DNS record or located using an intermediary such as SIP Broker).

Federated VoIP

Federated VoIP and telephone number mapping
SIP address

Uniform Resource Identifier

URIURIsuniform resource identifier (URI)
The SIP URI scheme is a Uniform Resource Identifier (URI) scheme for the Session Initiation Protocol (SIP) multimedia communications protocol.

Session Initiation Protocol

SIPSession Initiation Protocol (SIP)SIP Trunking
The SIP URI scheme is a Uniform Resource Identifier (URI) scheme for the Session Initiation Protocol (SIP) multimedia communications protocol.

Extension (telephone)

extensionextensionsextension number
A SIP address is a URI that addresses a specific telephone extension on a voice over IP system.

Voice over IP

VoIPvoice over Internet Protocolvoice-over-IP
A SIP address is a URI that addresses a specific telephone extension on a voice over IP system.

Business telephone system

PBXprivate branch exchangekey telephone system
Such a number could be a private branch exchange or an E.164 telephone number dialled through a specific gateway.

Email address

e-mail addressemail addressesplus addressing
A SIP address is written in user@domain.tld format in a similar fashion to an email address.

User Datagram Protocol

UDPUDP/IPUDP protocol
instructs a SIP client to make a (usually UDP) connection to voip-provider.example.net:5060 (which may be a gateway) and ask to be connected to the destination user at 1-999-123-4567.

Asterisk (PBX)

AsteriskAsterisk PBXHigh Availability for Asterisk
SIP addresses may be used directly in configuration files (for instance, in Asterisk (PBX) installations) or specified through the web interface of a voice-over-IP gateway provider (usually as a call forwarding destination or an address book entry).

Call forwarding

forwardingCall divertCall Forward on Busy
SIP addresses may be used directly in configuration files (for instance, in Asterisk (PBX) installations) or specified through the web interface of a voice-over-IP gateway provider (usually as a call forwarding destination or an address book entry).

Speed dial

Speed Callingspeed dialing
Systems which allow speed dial from a user's address book using a vertical service code may allow a short numeric code (like *75xx) to be translated to a pre-stored alphanumeric SIP address.

Vertical service code

calling featureservice featureCLASS
Systems which allow speed dial from a user's address book using a vertical service code may allow a short numeric code (like *75xx) to be translated to a pre-stored alphanumeric SIP address.

Simple Mail Transfer Protocol

SMTPemail protocolHELO command
In theory, the owner of a SIP-capable telephone handset could publish a SIP address from which they could be freely and directly reached worldwide, in much the same way that SMTP e-mail recipients may be contacted from anywhere at almost no cost to the message sender.

Softphone

soft phonePC-to-PC calling
Anyone with a broadband connection could install a softphone (such as Ekiga) and call any of these SIP addresses for free.

Ekiga

Anyone with a broadband connection could install a softphone (such as Ekiga) and call any of these SIP addresses for free.

Cybercrime countermeasures

network abuse
In practice, various forms of network abuse are discouraging creation and publication of openly reachable SIP addresses:

North American Numbering Plan

Area codeArea codesArea code(s)
an attempt to call a Palestinian mobile telephone (Israel, country code +972) by randomly trying 9- (a common code for an outside line from an office PBX), 011- (the overseas call prefix in the North American Numbering Plan) and 7- (on the off-chance a PBX is using it instead of 9- for an outside line).

Firewall (computing)

firewallfirewallspacket filter
Security tools such as firewalls or fail2ban must therefore be deployed to prevent unauthorised outside call attempts; many VoIP providers also disable overseas calls to all but countries specifically requested as enabled by the subscriber.

Fail2ban

Security tools such as firewalls or fail2ban must therefore be deployed to prevent unauthorised outside call attempts; many VoIP providers also disable overseas calls to all but countries specifically requested as enabled by the subscriber.

Spamming

spamspammersspammer
The spam (electronic) which has rendered SMTP the "spam mail transport protocol" could potentially make published sip: numbers unusable as the numbers are flooded with VoIP spam, usually automatic announcement devices delivering pre-recorded advertisements. Unlike, sip: establishes a voice call which interrupts the human recipient in real time with a ringing telephone.

VoIP spam

SPIT (SPam over Internet Telephony)SPIT
The spam (electronic) which has rendered SMTP the "spam mail transport protocol" could potentially make published sip: numbers unusable as the numbers are flooded with VoIP spam, usually automatic announcement devices delivering pre-recorded advertisements. Unlike, sip: establishes a voice call which interrupts the human recipient in real time with a ringing telephone.