Verification Involving PSTN Reachability
charter-ietf-vipr-01

There are two globally deployed address spaces for communications used
  by more than a billion people daily - phone numbers and DNS rooted
  address such as web servers and email addresses.  The inter-domain
  signaling design of SIP is primarily designed for email style addresses
  yet a large percentage of SIP deployments mostly use phone numbers for
  identifying users, thus DNS lookups are not sufficient.  The goal of 
  this working group is to enable inter-domain communications over the 
  Internet, using protocols such as SIP, while still allowing people to 
  use phone numbers to identify the person with whom they wish to 
  communicate.
  
  The VIPR WG will develop a peer to peer based approach to finding
  domains that claim to be responsible for a given phone number
  to mitigate the involvement of centralized entities to avoid some of
  the issues encountered by past attempts to support SIP inter-domain
  communications.  Validation protocols will be developed to ensure a
  reasonable likelihood that a given domain actually is responsible for
  the phone number.  In this context, "responsible" means an
  administrative domain, which is at least one of the domains, to which
  a PSTN call to this phone number would be routed.  Once the domain
  responsible for the phone number is found, existing protocols, such
  as SIP, can be used for inter-domain communications.
  
  Some validation protocols may be based on knowledge gathered around a
  PSTN call; for example, the ability to prove a call was received over
  the PSTN based on start and stop times.  Other validation schemes, such
  as examining fingerprints or watermarking of PSTN media to show that a
  domain received a particular PSTN phone call, may also be considered by
  the working group.  This validation will be accomplished using publicly
  available open interfaces to the PSTN, so the validation can be
  performed by any domain wishing to participate.  The WG will select and
  standardize at least one validation scheme.
  
  The validation mechanism requires a domain to gather and maintain
  information related to PSTN calls.  This information is used by call
  agents such as phones, SBCs and IP PBXs to route calls.  The WG will
  also develop mechanisms to detect in a timely manner that a given domain
  is no longer responsible for a phone number.  The WG will define a
  client-server protocol between these call agents and the entity within a
  domain that maintains the information.
  
  To help mitigate SPAM issues when using SIP between domains, the WG will
  define a mechanism to enable one domain to check that incoming SIP
  messages are coming from a validated phone number.  A phone number is
  considered validated if it is coming from a domain to which the calling
  domain had previously successfully placed a PSTN call.  The working
  group will define new SIP headers and option tags, as necessary, to
  enable this.
  
  The essential characteristic of VIPR is establishing authentication by
  PSTN reachability when it is not possible to use a direct reference to
  ENUM databases or other direct assertions of PSTN number
  ownership.  Elements such as public ENUM easily coexist with VIPR but no
  direct interaction with ENUM will be required.  The solution set defined
  by this WG will be incrementally deployable using only existing
  interfaces to the PSTN.  No changes will be required to existing PSTN
  capabilities, no new database access is needed nor is any new support
  from PSTN service providers required.
  
  The WG will produce the following deliverables:
  
  1) A document describing the requirements, problem statement and
  architectural approach to support SIP inter-domain communications.
  2) A document describing the use of the p2psip protocol (RELOAD) as
  applied to this problem space.
  3) A document defining a scheme for validating the phone numbers using
  publicly available open interfaces to the PSTN.
  4) A document defining client-server protocol between call agents and 
  the entity within a domain that gathers and maintains information 
  related to PSTN calls.
  5) A document describing a mechanism to mitigate SPAM issues.
  
  The working group will carefully coordinate with the security area, O&M
  area, as well as the appropriate RAI WGs such as sipcore and p2psip.