====== Locating SIP Services ======

It is very convenient to be able to use one's personal address, i.e. %%john@company.com%%, for both email and SIP communications (IP telephony, presence, video conferencing etc), just by changing the application prefix.

For example: %%email:john@company.com%% and %%sip:john@company.com%%

DNS is the mechanism for determining the IP address, port and transport protocol of the host to which a SIP request is sent.

DNS provides two different record types that controls SIP requests: SRV and NAPTR. The SRV record allows DNS administrators to use several servers for a single domain. The NAPTR record is a way for the called domain to specify which protocol it prefers a SIP request to use.\\
The usage of NAPTR and SRV for SIP is specified in [[http://www.ietf.org/rfc/rfc3263.txt|RFC3263]].

===== Setting up SRV Records =====
//(Information for DNS administrators.)//

The SRV records have a format which follows the following convention:

"_service._protocol.name TTL Class RecordType Priority Weight Port Target"

Example:

"_sip._udp.company.com 432000 IN SRV 10 10 5060 sip.company.com"

| Service: _sip	| The service is SIP. Can also be _sips for TLS encrypted communication.  |
| Protocol: _udp	| The protocol is UDP. Other values: _tcp or _sctp. |
| Name:	| |
| Cache lifetime\\ 43200	| 43,200 seconds = 12 hours. |
| Class IN	| The class is always IN. |
| Record type SRV	| The record type is SRV. |
| Priority 10	| When multiple SRV records are used the priority determines the proxy query order. Lower values are queried first. |
| Weight 10	| When multiple SRV records are used the weight determines proportionally how often a proxy is queried. Higher values are queried more often.\\ 20 = queried twice as often as 10.\\ 30 = queried three times as often as 10. |
| Port 5060	| The default SIP port is 5060. |
| Target\\ %%sip.company.com%%	| The domain name of the proxy server is %%sip.company.com%% |

The SRV records are specified in [[http://www.ietf.org/rfc/rfc2782.txt|RFC2782]].

===== Setting up NAPTR Records =====
//(Information for DNS administrators.)//

The NAPTR records have a format which follows the following convention:

“Order Preference Flags Services Regexp Replacement"

Examples:

company.com. 2508 IN NAPTR 20 50 "S" "SIP+D2T" "" _sip._tcp.company.com.

company.com. 2508 IN NAPTR 30 50 "S" "SIP+D2U" "" _sip._udp.company.com.

| Order: 20	| The order in which the NAPTR records must be processed. |
| Preference: 50	| Specifies the order in which NAPTR records with equal Order values should be processed. |
| Flags: “S"	| The flags control aspects of the rewriting and interpretation of the fields in the record. |
| Services: “SIP+D2T"	| This specifies the Service Parameters applicable to this delegation path. |
| Regexp: “"	| This contains a substitution expression that is applied to the original string held by the client in order to construct the next domain name to lookup. |
| Replacement:\\ _sip._tcp.company.com	| This is the next domain-name to query for depending on the potential values found in the flags field.|

The NAPTR records are specified in [[http://www.ietf.org/rfc/rfc3403.txt|RFC3403]].