1. SIP URI Service Discovery using DNS-SD draft-lee-sip-dns-sd-uri-02
Jae Woo Lee & Henning Schulzrinne
Columbia University
Wolfgang Kellerer & Zoran Despotovic
DoCoMo Laboratories Europe
Presented by Henning Schulzrinne
December 2007
IETF 70 - SIPPING

2. SIP UA Discovery using DNS-SD
Why?
SIP communication within subnet without servers
E.g., wireless ad-hoc network, campus LAN
Why DNS-Based Service Discovery (DNS-SD)?
Widespread implementation: Apple Bonjour, iChat
Some SIP UAs already use it: Gizmo, Ekiga
Why not SIP multicast?
UAs can track peer locations using multicast REGISTER
But no query capability
New UA won’t discover existing UAs until their registrations are refreshed (up to an hour delay)
Not reliable - may miss registrations
December 2007
IETF 70 - SIPPING

3. Overview of Operation Multicast DNS (mDNS)
Runs on each host in a local subnet
Queries & answers are sent via link-local multicast to port 5353
Special TLD reserved for local names: “.local.”
PTR records enumerate available service instances
_sipuri._udp.local. PTR
_sipuri._udp.local. PTR
SRV & A records resolve a specific service instance
SRV
bobs-machine.local.
bobs-machine.local. A
TXT records contain additional information
TXT
txtvers=1 name=Bob the Builder
December 2007
IETF 70 - SIPPING

4. SIP URI Advertisement Format
Service instance name: Instance.Service.Domain
Instance = ( SIP-URI / SIPS-URI ) [ SP description ]
Service = “_sipuri._udp” / “_sipuri._tcp” / “_sipuri._sctp”
E.g.) - PDA._sipuri._udp.local.
Contact TXT record attribute
Similar to Contact SIP header except:
It contains only a single URI
Non-SIP URIs are not allowed
UA capabilities advertised via field parameters (RFC3840)
December 2007
IETF 70 - SIPPING

5. Forming SIP Request Using Discovered URI
“To” header
SIP/SIPS URI from service instance name (normally AOR)
Request-URI
SIP/SIPS URI from TXT contact attribute if available, otherwise same as “To” header
December 2007
IETF 70 - SIPPING

6. Determining Request Destination
Service instance name:
determines transport
Contact
In TXT?
Yes No
TXT lookup
SRV lookup
Contact URI with host name:
Host name & port:
bobs-machine.local.
5060
A/AAAA lookup
Contact URI
with numeric IP:
uses default port
if contact URI doesn’t have one
IP address & port:
5060
December 2007
IETF 70 - SIPPING

7. User Interface and Security
Services are not authenticated in DNS-SD/mDNS
Anyone can advertise anything
UA should clearly distinguish two kinds of URIs:
User’s address book entries
Ad hoc URIs discovered using DNS-SD
Possibility: UA authentication using self-signed certificate
CA-signed certificate may be overkill in ad hoc network
Must warn user on first encounter of self-signed certificate
December 2007
IETF 70 - SIPPING

8. Related Mechanisms “_sip” service type used by Asterisk P2PSIP
Intended for server discovery
Gizmo & Ekiga use it for UA discovery
Now they can use “_sipuri” instead
P2PSIP
Replace centralized SIP servers with services provided by p2p endpoints
doesn’t work well in high-churn networks
significantly more complicated
Charter specifically excludes multicast-based approaches
December 2007
IETF 70 - SIPPING

9. Open Issues Believed to be complete, but...
Choice of AOR - require separate name space?
security vs. recognizability
Resolution mechanism?
deviates slightly from RFC 3263
UI recommendations ok?
December 2007
IETF 70 - SIPPING