Requirements for prioritized access to PSTN resources

1
Requirements for prioritized access to PSTN
resources

• Henning Schulzrinne
• Columbia University

superset of draft-schulzrinne-ieprep-resource-req-
00
2
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, eMLPP, ...
• Nothing to do with 112/911
• Also, possibly call from PSTN into SIP network

3
Scenarios
RP-capable gateway
SIP
PSTN w/MLPP GETS, ...
INVITE sip1-212-..._at_gets.ncs.gov INVITE
tel1-212-... INVITE sipcommand_at_navy.mil
SIP
ISUP
GSM
does not know destination network (type)
4
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)

5
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 (eMLPP GETS?)

6
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?

7
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")

8
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

9
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

10
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

11
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 VoIP gateways may be authentication-capabl
  e (many aren't)
• harden authentication ?? DOS attacks

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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

13
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

14
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?

15
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