Locating SIP servers uses DNS records like SRV and NAPTR to find the IP address, port, and transport for a SIP URI domain. ENUM uses DNS and NAPTR records to convert telephone numbers in E.164 format to SIP URIs by performing queries and applying regex rules. Both allow for features like failover and load balancing across multiple SIP servers.

1. Locating SIP Servers & ENUM
Ravi S
06 August, 2009

2. Locating SIP Server
Defined in RFC 3263
Uses DNS
Resolve the SIP URI
Next hop IP Address, Port & Transport
Failover & Load balancing

3. ENUM
Defined in RFC 3761
Uses DNS
Identify services associated with tel URI
SIP Service is of importance to us

4. A little background...
(DNS & DNS Records)

5. Very few protocols...
When Internet was invented, there were very
few protocols
E-mail – SMTP
World Wide Web – HTTP
Directory lookup – LDAP
etc.

6. Very simple DNS records...
What is the IP address of example.com ?
A Record
What is the Mail Exchange server for example.
com ?
MX Record

7. Very simple DNS procedure...
IP Address – found with DNS lookup
A record lookup & MX record lookup
Port – fixed
HTTP on 80, SMTP on 25 etc.
Transport – fixed

8. Protocols grew...
But DNS procedures remained same
IP Address is identified by A record lookup
Each protocol is bound to its port
Transport selection is handled independently
by application

9. SRV record
Query based on service & protocol
Eg. sip/udp, http/tcp etc.
Services can run on any port
SRV query returns the port
Failover & Loadbalancing support

10. SRV record – How to query?
Input to SRV query are domain-name, service
and protocol
Construct SRV query string from the input
_service ._protocol .domain-name
Perform SRV lookup on the constructed query
string

11. SRV record – How to query?
(example)
For locating a UDP SIP server of example.com
SRV query string is _sip._udp.example.com
Perform SRV lookup on _sip._udp.example.
com

12. SRV record – Result
$ORIGIN example.com.
_sip._udp SRV 0 1 5060 p1.example.com.
SRV 0 3 5061 p2.example.com.
SRV 1 0 5060 p3.example.com.
SRV 2 0 6070 p4.example.com.

13. SRV record - Failover
Least priority is "most preferred"
_sip._udp SRV 0 0 5060 p1.example.com.
SRV 1 0 5060 p2.example.com.
SRV 2 0 5060 p3.example.com.
On failure to reach "most preferred" entry, the
"next preferred" entry is attempted
_sip._udp SRV 0 0 5060 p1.example.com.
SRV 1 0 5060 p2.example.com.
SRV 2 0 5060 p3.example.com.

14. SRV record – Load Balancing
$ORIGIN example.com.
_sip._udp SRV 0 1 5060 p1.example.com.
SRV 0 3 5061 p2.example.com.
SRV 0 6 5060 p3.example.com.
p3.example.com - 60%
p2.example.com - 30%
p1.example.com - 10%

15. Limitations of SRV record
Cannot query the list of services supported
Cannot query the list of protocols supported
Multiple DNS query is required to find next-hop

16. NAPTR record
Query only on domain name
NAPTR response contains list of all services
supported as individual record
Very flexible
Application unique string -> Data

17. NAPTR record – Result
$ORIGIN example.com.
NAPTR 90 50 "s" "SIP+D2T" "" _sip._tcp
NAPTR 100 50 "s" "SIP+D2U" "" _sip._udp
NAPTR 100 50 "s" "http+I2R" "" _http._tcp
NAPTR 100 50 "s" "ftp+I2R" "" _ftp._tcp

18. Locating SIP Server

19. Locating SIP Server (Explicit IP)
sip:abc@192.168.60.150
Next hop is (192.168.60.150, 5060, UDP)
sip:abc@192.168.60.150:5070
Next hop is (192.168.60.150, 5070, UDP)
sip:abc@192.168.60.150;transport=tcp
Next hop is (192.168.60.150, 5060, TCP)
sip:abc@192.168.60.150:5070;transport=tcp
Next hop is (192.168.60.150, 5070, TCP)

20. Locating SIP Server (Domain &
Explicit Port)
sip:abc@example.com:5070
Next hop is (A record of example.com, 5070, UDP)
sip:abc@example.com:5070;transport=tcp
Next hop is (A record of example.com, 5070, TCP)

21. Locating SIP Server (Domain &
Explicit transport)
sip:abc@example.com;transport=tcp
Perform SRV query on _sip._tcp.example.com
IP Address is 'A' record of SRV response
Port is taken from SRV response
Transport is TCP

22. Locating SIP Server (Domain)
sip:abc@example.com
Perform NAPTR query on example.com
Filter responses based on service "SIP+D2U" / "SIP+D2T"
Perform SRV query on the NAPTR response
IP Address is 'A' record of SRV response
Port is taken from SRV response
Transport is taken from NAPTR service field

23. ENUM

24. ENUM
Application Unique String - Tel URI
Convert "Application Unique String" into "SIP
URI"

25. ENUM - The Process
Normalize "Application Unique String"
Create "Query String" from "Application Unique
String"
Perform NAPTR query on "Query String"
Filter the response based on service "sip+E2U"
Apply regex rule on "Application Unique String"
to get "SIP URI"

26. ENUM - An example
"Application Unique String" - + 442079460148
"Query String" - 8.4.1.0.6.4.9.7.0.2.4.4.e164.
arpa
Perform NAPTR query on
8.4.1.0.6.4.9.7.0.2.4.4.e164.arpa
Filter the response based on service "sip+E2U"
Apply regex rule on "Application Unique String"
to get "SIP URI"

27. ENUM - An example (Contd.)
$ORIGIN 8.4.1.0.6.4.9.7.0.2.4.4.e164.arpa.
NAPTR 10 100 "u" "E2U+sip" "!^.*$!sip:abc@example.com!" .