SIP URI scheme

The SIP URI scheme is a

A SIP address is written in [email protected] format in a similar fashion to an email address. An address like:

sip:[email protected]

instructs a SIP client to use the

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). 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.

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

• The spam 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 mailto:, sip: establishes a voice call which interrupts the human recipient in real time with a ringing telephone.
• 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.
• Servers supporting inbound sip: connections are routinely targeted with unauthorised REGISTER attempts with random numeric usernames and passwords, a
  brute force attack intended to impersonate individual off-premises extensions
  on the local PBX
• Servers supporting inbound sip: connections are also targeted with unsolicited attempts to reach outside numbers, usually premium-rate destinations such as caller-pays-airtime mobile exchanges in foreign countries.

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). 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.

The SIPS URI scheme adheres to the syntax of the SIP URI, differing only in that the scheme is sips rather than sip. The default Internet port address for SIPS is 5061 unless explicitly specified in the URI.

SIPS allows resources to specify that they should be reached securely. It mandates that each hop over which the request is forwarded up to the target domain must be secured with TLS. The last hop from the proxy of the target domain to the user agent has to be secured according to local policies.

SIPS protects against attackers which try to listen on the signaling link. It does not provide real end-to-end security, since encryption is only hop-by-hop and every single intermediate proxy has to be trusted.

• Federated VoIP and telephone number mapping
• e164.arpa
• Security Descriptions for SDP
• Mikey key exchange method
• ZRTP end-to-end key exchange proposal