Requirements for prioritized access to PSTN resources Henning

Requirements for prioritized access to PSTN resources Henning Schulzrinne Columbia University superset of draft-schulzrinne-ieprep-resource-req-00

Assumptions/Scope § SIP endpoint wants to access restricted (prioritized) resources on a circuit-switched network § Does not indicate request of IP resource priority – may not be available – may not be necessary § Examples: GETS, MLPP, e. MLPP, . . . § Nothing to do with 112/911 § Also, possibly call from PSTN into SIP network

Scenarios SIP RP-capable gateway PSTN w/MLPP GETS, . . . INVITE sip: 1 -212 -. . . @gets. ncs. gov INVITE tel: 1 -212 -. . . INVITE sip: command@navy. mil SIP ISUP does not know destination network (type) GSM

Assumptions § Call resource priority vs. call human priority – resource priority indicated by caller (callee can't see) – priority of call to caller: indication ("Priority", content labeling) + callee call handling policy out of scope § Resources: – IP-to-PSTN gateway channels – end-to-end PSTN circuits (PSTN network congestion, not access congestion)

Assumptions § Call destination network type may be unknown to caller § Call destination does not identify PSTN resource priority § May want to reach "any of IEPREP type 1, type 2, . . . " § May have several orthogonal indications of resource priority (e. MLPP + GETS? )

System assumptions § What do we assume about the IP side? – purpose-built: require certain capabilities (signaling, resource reservation, security, . . . ) – any network: use SIP application on standard platform or plug in own SIP phone • no network changes • firewalls may not allow protocols beyond SIP and RTP – any SIP (pay) phone • no modifications to SIP phone • not much beyond two-stage dialing possible?

General requirements § Not specific to one domain (e. g. , GETS) § Not tied to existing PSTN authentication mechanisms § Use existing namespaces different authorities that manage § Allow for default behavior § Separation of indication and policy – by reference (policy "flash"), not by value ("preempt all except class 'immediate', queue in relationship to GETS calls, but cut off after 3 minutes and only allow low-bit rate audio")

Requirement: Discovery and negotiation § Caller must be able to discover PSTN resource priority capabilities – determines authentication "hat" – gateway needs for challenge • "Resource priority FOO level 7 requires use of BAR authentication" § Network may disallow discovery administratively importance of call routing

Requirement: Testing § Must be able to test largest possible part of the system without ringing actual destination – Systems only used during emergencies are less likely to work – Exercise authentication and authorization – Exercise call routing

Requirement: Call routing § Combine with call routing: – req: specify logical destination, not physical gateway – resource priority requirement may enlarge or constrain set of destinations • e. g. , additional special GETS-only gateway • only certain gateways (carriers) are capable of particular calls – note: TRIP property? – note: cf. SIP caller preferences

Security requirements § End-to-end strong authentication and authorization of caller – not just theft of service, but system stability/performance issue § Intermediate (proxy? ) authentication – delegate responsibility – not all Vo. IP gateways may be authentication-capable (many aren't) – harden authentication DOS attacks

Security requirements § Support authentication and authorization beyond existing PIN schemes § Authentication must be DOS-resistant § Allow "early" authentication cannot wait until inside PSTN! – authentication consumes packets vs. circuits – minimize pre-authentication resource use • authenticate call signaling, not just resource signaling

Security requirements § Do not tie resource priority namespace to one authentication scheme – different hardware types • hard/soft SIP phone • SIM-equipped cell phone – from any black phone with dial pad to smartcard- and biometrics-equipped

Security requirements § Cross-domain – IP endpoint may be in different admin. domain than gateway ? Require secrets not to be pre-installed § useability from any device § Authentication of PSTN gateway – desirable; required?

Privacy requirements § Call content – very likely separate docs § Signaling (resource and/or call setup) – reveals communication relationships – cannot rely on hop-by-hop § Fact of IEPREP call – sensitivity likely same (or lower) as call signaling