1. A “net head” view on SIP Henning Schulzrinne Columbia University IRT Lab Siemens Munich -- January 2003

2. Some architectural principles Independent of lower layers  minimal awareness if any, awareness of properties, rather than awareness of specific technology  support all where awareness can’t be avoided IPv4 and IPv6 all link/physical layers that support IP Network and user may not trust each other notion of “network” fuzzy neither is inherently trust-worthy Users should be able to trade privacy against services Agnostic on regulatory regimes Agnostic on architecture – can run 0 or 10 proxy layers Diagnosability across network (provider) boundaries used to paths with multiple providers that don’t know each other preference for transparency instead of network hiding

3. The disappearing network No assumption that there is an identifiable, licensed common-carrier network operator 802.11 hotspots private network extensions (e.g., IETF network) thus, no strong notion of network trust (beyond delivering packets) even with a vendor, no classical business relationship Yahoo and other freemail services  no verifiable identity Networks are service-transparent get IP packets from vendor A, SIP “dial tone’’ from vendor B may change one without changing the other no attempt to charge differently for the same color of bits DiffServ and different L2 treatment are fine

4. SIP principles Proxies are transparent dialog: Call-ID identity: To/From CSeq header fields body: no peeking, no modification media transparency: no changes in media properties topology: Via, Record-Route, Route security: S/MIME accounting functional Clear delineation of who can inspect and modify what simplifies trust analysis

5. SIP principles End-user empowerment can add SIP protocol elements headers, methods, content types can run their own proxies, recursively end user fault diagnosis traceroute, ping discussion of SIP equivalents (history, trace) REGISTER is binding registration, not a service requirement

6. SIP principles “One network, one protocol” allow single library, implementation including JAIN SIP, sip-cgi, etc. avoid repeat of national ISUP and ISDN variants multi-network clients multi-homed multi-hop: private network into 3G  thus, any 3GPP header or function should not break non- 3GPP functionality should not depend on first or last hop being a 3GPP network Avoid manual configuration at all cost users should not need to flip “3GPP” switch on device