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

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

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 sip:bob@a.com._sipuri._udp.local. _sipuri._udp.local. PTR sip:joe@a.com._sipuri._udp.local. SRV & A records resolve a specific service instance sip:bob@a.com._sipuri._udp.local. SRV 0 0 5060 bobs-machine.local. bobs-machine.local. A 192.168.0.100 TXT records contain additional information sip:bob@a.com._sipuri._udp.local. TXT txtvers=1 name=Bob the Builder contact=<sip:bob@192.168.0.100:5060>;audio;video December 2007 IETF 70 - SIPPING

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.) sip:bob@example.com - 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

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

Determining Request Destination Service instance name: sip:bob@a.com._sipuri._udp.local. determines transport Contact In TXT? Yes No TXT lookup SRV lookup Contact URI with host name: sip:bob@bobs-machine.local. Host name & port: bobs-machine.local. 5060 A/AAAA lookup Contact URI with numeric IP: sip:bob@192.168.0.100 uses default port if contact URI doesn’t have one IP address & port: 192.168.0.100 5060 December 2007 IETF 70 - SIPPING

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

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

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