1. Location SIP Servers –RFC 3261
유비쿼터스 네트워크 연구센터
이 영

2. Contents Introduction Problem of DNS is Needed to Solve
DNS Records for SIP
Client Usage
Server Usage
Conclusion

3. Introduction(1)
Location Server in SIP

4. Introduction(2)
Location Server in SIP

5. Introduction(3)
Location and Presence

6. Introduction(4)
SIP Basic Flow

7. Introduction(5)
SIP Basic Flow - Failed

8. Introduction(6)
Load balancing

9. Problem of DNS is Needed to Solve
Determine the available address tuple for SIP server of the next hop
Client : Resolve address tuple of the next hop
Server : If the primary response has failed
Provide a scalability and high availability
Scale up through clustering
Priority and Weights

10. Contents Introduction Problem of DNS is Needed to Solve
DNS Records for SIP
Client Usage
Server Usage
Conclusion

11. DNS Records for SIP(1)
DNS lookup procedure for SIP

12. DNS Records for SIP(2) – NAPTR Record(RFC 2915)
Format
 NAPTR query for dcn.com  NAPTR records are returned
Rules
If SIPS URI, MUST select only “SIPS+D2T”
If SIP URI, select “SIPS+D2T” or “SIP+D2D”
Flag value
Replacement
Result
Select the Transport Protocol
Indexing the SRV Record
class
type
order
pref
flags
service
regexp
replacement IN
NAPTR 70 50
“s”
“ftp+I2R”
“ ”
_ftp._tcp.dcn.com 80
“SIPS+D2T”
_sips._tcp.dcn.com 90
“SIP+D2T”
_sip._tcp.dcn.com
100
“SIP+D2U”
_sip._udp.dcn.com
110
“http+I2R”
_http._tcp.dcn.com

13. DNS Records for SIP(3) – SRV Record(RFC 2782)
Format
SRV query for _sip._tcp.dcn.com  SRV records are returned
Result
Determine the port of SIP server
Provide the Load Balancing Mechanism and Fault Tolerance functions for SIP Server(Proxy Server)
class
type
priority
weight
port
Target (A record) IN
SRV 2 10
5060
server3.dcn.com 1
5063
server2.dcn.com

14. DNS Records for SIP(4) – SRV Record(RFC 2782)
SRV Record Load Balancing & Fault Tolerant Mechanism
Example
Sort Multiple SRV Record based on RFC 2782
The same Priority
Arrange in any order, except weight 0
The Running-sum
Random number(nRandom)
Next Target select (nRandom <= R-sum)
Remove the selected RR
Until there are no unordered SRV RRs.
Repeat for each Priority
Address record lookup for the Target in the order
class
type
priority
weight
port
Target (A record)
R-Sum IN
SRV 2 10
5060
sip3.dcn.com 1
5063
sip2.dcn.com 11 20
5064
sip5.dcn.com 31
100
5061
sip6.dcn.com
131
_sip._tcp.dcn.com

15. DNS Records for SIP(5) – A Record
Format
A query for server2.dcn.com  A Record are returned server2.dcn.com  IN A
Result
Determine only IP Address for SIP Server
Regardless of Service (FTP, SIP, HTTP), target resolution is one

16. Contents Introduction Problem of DNS is Needed to Solve
DNS Records for SIP
Client Usage
Server Usage
Conclusion

17. Client Usage(1) Procedure NAPTR Record SRV Record A Record
Select TARGET
Select a Transport Protocol
Determine Port and IP Address via:SIP/2.0/xxx host[:port][received=host];branch=……
Case by case, DNS Lookup
Step by step, DNS Lookup
Numeric IP Address
Port
Transport Protocol
DCN Query
Yes /
Not Using DNS No
A Record Lookup
SRV Record Lookup
NAPTR Record Lookup
NAPTR Record
SRV Record
A Record

18. Client Usage(2) If the TARGET is a Numeric IP Address Transport
If transport protocol is designated in TARGET, the transport protocol is used
If not, UDP for SIP URI and TCP for SIPS URI is Default
Port
If port is designated in TARGET, the port is used
5060 for UDP or TCP, 5061 for TLS is default port
IP Address
Use the Numeric IP Address designated in TARGET
Numeric IP Address
Port
Transport Protocol
DCN Query
Yes /
Not Using DNS No
A Record Lookup
SRV Record Lookup
NAPTR Record Lookup

19. Client Usage(3)
If a port is provided, but the TARGET is not Numeric IP Address
Transport
If Transport protocol is designated in TARGET, the transport protocol is used
If not, UDP for SIP URI and TCP for SIPS URI is Default
Port
Use the port designated in TARGET
IP Address
A record lookup or AAAA record(IPv6) with hostname designated in TARGET
Numeric IP Address
Port
Transport Protocol
DCN Query
Yes /
Not Using DNS No
A Record Lookup
SRV Record Lookup
NAPTR Record Lookup

20. Client Usage(4)
If no Transport protocol and port, not Numeric IP Address
Transport
If Transport protocol is designated in TARGET  pass 1
Attempt to NAPTR Query  pass 2
If NAPTR record is failed, SRV Record Lookup  pass 3 (_sips._tcp.dcn.com, _sip._tcp(_udp).dcn.com)
If SRV Record Lookup is failed, SIP URI(UDP) and SIPS URI(TCP) is Default  pass 4
Numeric IP Address
Port
Transport Protocol
DCN Query
Yes /
Not Using DNS No
A Record Lookup
SRV Record Lookup
NAPTR Record Lookup

21. Client Usage(4) IP Address
Case pass 1: SRV Record Lookup  A Record Lookup
Case pass 2: Selected NAPTR Record  SRV Record  A Record Lookup
Case pass 3: Selected SRV Record  A Record Lookup
Case pass 4: TARGET host  A Record Lookup
Port
If SRV Record Lookup is successful, use the port designated in SRV Record
If not, 5060 for UDP or TCP, 5061 for TLS is default port
Numeric IP Address
Port
Transport Protocol
DCN Query
Yes /
Not Using DNS No
A Record Lookup
SRV Record Lookup
NAPTR Record Lookup

22. Client Usage(5) - Consideration for stateless proxies
Stateless Proxy: Not keep up transaction state
Problem 1
Possible that retransmission go to a different server every time  Solution - Not adopt SRV Record Sorting algorithm of RFC using any algorithm to be sorted by the same order every time (example : alphabets)
Problem 2
Not contacted to the first server successfully, send to the next server, retransmit go to different server if the failed first server recovers between retransmissions  Solution - If the first server is not contacted successfully, should act as a stateful proxy.

23. Contents Introduction Problem of DNS is Needed to Solve
DNS Records for SIP
Client Usage
Server Usage
Conclusion

24. Server Usage Response Mechanism
TCP : Sent over the connection the request arrived on
UDP : the source IP Address, the port contained in the Via header
Problem
Response fails
“fails”: Any closure of the transport connection or fatal error from the transport layer  Solution - examines the value of the sent-by construction in the topmost Via header - A Record Lookup or SRV Record Lookup - It is possible that response sent to a different client - If only Client Group share transaction state, this model is worthy

25. Conclusion
Determine the available transport protocol, port, IP Address for SIP server of the next hop through DNS lookup
Provide the load balancing mechanism and fault tolerance functions for SIP server(proxy server) through DNS lookup
Requirement
UA and Proxy server should have the ability to lookup DNS and analyze DNS RRs
Administrator should have the ability
To load balancing of SIP Servers through control of DNS Server Record
To manage Back-up Server