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SIP URI Service Discovery using DNS-SD draft-lee-sip-dns-sd-uri-02

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. SIP UA Discovery using DNS-SD. Why?

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SIP URI Service Discovery using DNS-SD draft-lee-sip-dns-sd-uri-02

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

  6. determines transport uses default port if contact URI doesn’t have one Determining Request Destination Service instance name: sip:bob@a.com._sipuri._udp.local. Contact In TXT? No Yes 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 IP address & port: 192.168.0.100 5060 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 IETF 70 - SIPPING

  8. Related Mechanisms • “_sip” service type used by Asterisk • 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 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? IETF 70 - SIPPING

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