





Internet Engineering Task Force (IETF)             S. Channabasappa, Ed.
Request for Comments: 6461                                     CableLabs
Category: Informational                                     January 2012
ISSN: 2070-1721


       Data for Reachability of Inter-/Intra-NetworK SIP (DRINKS)
                  Use Cases and Protocol Requirements

Abstract

   This document captures the use cases and associated requirements for
   interfaces that provision session establishment data into Session
   Initiation Protocol (SIP) Service Provider components to assist with
   session routing.  Specifically, this document focuses on the
   provisioning of one such element termed the "registry".

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6461.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.




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Table of Contents

   1. Overview ........................................................2
   2. Terminology .....................................................5
   3. Registry Use Cases ..............................................6
      3.1. Category: Provisioning Mechanisms ..........................6
      3.2. Category: Interconnect Schemes .............................7
      3.3. Category: SED Exchange and Discovery Models ................8
      3.4. Category: SED Record Content ...............................9
      3.5. Category: Separation and Facilitation of Data Management ...9
      3.6. Category: Public Identifiers, TN Ranges, and RNs ..........10
      3.7. Category: Misc ............................................11
   4. Requirements ...................................................11
      4.1. Provisioning Mechanisms ...................................12
      4.2. Interconnect Schemes ......................................12
      4.3. SED Exchange and Discovery Requirements ...................12
      4.4. SED Record Content Requirements ...........................12
      4.5. Data Management Requirements ..............................13
      4.6. Public Identifier, TN Range, and RN Requirements ..........13
      4.7. Misc. Requirements ........................................13
   5. Security Considerations ........................................14
   6. Acknowledgments ................................................14
   7. References .....................................................15
      7.1. Normative References ......................................15
      7.2. Informative References ....................................15

1.  Overview

   [RFC5486] (Section 3.3) defines Session Establishment Data, or SED,
   as the data used to route a call to the next hop associated with the
   called domain's ingress point.  More specifically, the SED is the set
   of parameters that the outgoing signaling path border elements (SBEs)
   need to establish a session.  However, [RFC5486] does not specify the
   protocol(s) or format(s) to provision SED.  To pave the way to
   specify such a protocol, this document presents the use cases and
   associated requirements that have been proposed to provision SED.

   SED is typically created by the terminating or next-hop SIP service
   provider (SSP) and consumed by the originating SSP.  To avoid a
   multitude of bilateral exchanges, SED is often shared via
   intermediary systems -- termed "registries" within this document.
   Such registries receive data via provisioning transactions from SSPs,
   and then distribute the received data into Local Data Repositories
   (LDRs).  These LDRs are used for call routing by outgoing SBEs.  This
   is depicted in Figure 1.






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                                       *-------------*
                1. Provision SED       |             |
              -----------------------> |  Registry   |
                                       |             |
                                       *-------------*
                                            /  \
                                           /    \
                                          /      \
                                         /        \
                                        /          \
                                       /            \
                                      / 2.Distribute \
                                     /      SED       \
                                    V                  V
                              +----------+       +----------+
                              |Local Data|       |Local Data|
                              |Repository|       |Repository|
                              +----------+       +----------+

                         Figure 1: General Diagram

   In this document, we address the use cases and requirements for
   provisioning registries.  Data distribution to local data
   repositories is out of scope for this document.  The resulting
   provisioning protocol can be used to provision data into a registry
   or between multiple registries operating in parallel.  In Figure 2,
   the case of multiple registries is depicted with dotted lines.

                                  . . . . . . .
                  . . . .  . . .   registry    . . . . . . .
                .                 . . . . . . .              .
              .                        .                      .
             .                         .                       .
            .                          . provision             .
       +-----------+                   .                 +-----------+
       |           |  provision  +----------+  provision |           |
       |   SSP 1   |------------>| Registry |<-----------|   SSP 2   |
       |           |             +----------+            |           |
       |  +-----+  |                   /\                |  +-----+  |
       |  | LDR | <--------------------  ------------------>| LDR |  |
       |  +-----+  |   distribute           distribute   |  +-----+  |
       |           |                                     |           |
       +-----------+                                     +-----------+
              .                                                .
               . . . . . . . . . . . . . . . . . . . . . . . .
                              (provision / distribute)

                       Figure 2: Functional Overview



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   In addition, this document proposes two aggregation groups, as
   follows:

   o  Aggregation of public Identifiers into a destination group.

   o  Aggregation of SED records into a route group.

   The use cases in Section 3.5 provide the rationale.  The data model
   depicted in Figure 3 shows the various entities, aggregations, and
   the relationships between them.

       +---------+            +--------------+               +---------+
       |  Data   |0..n    0..n|     Route    | 1         0..n|   SED   |
       |Recipient|------------|     Group    | --------------|  Record |
       +---------+            +--------------+               +---------+
                                     |0..n                        |0..n
                                     |                            |
                                     |                            |
                                     |                            |
                                     |0..n                        |
                            1 +--------------+  0..1              |
                     ---------| Destination  |---------           |
                    |         |    Group     |         |          |
                    |         +--------------+         |          |
                    |                |                 |          |
                    |               1|                 |          |
                    |                |                 |          |
                    |                |                 |          |
               0..n |           0..n |                 | 0..n     |
               +---------+      +---------+       +----------+    |
               |   RN    |      |   TN    |       | Public   |----
               |         |      |  Range  |       |Identifier| 1
               +---------+      +---------+       +----------+


                       Figure 3: Data Model Diagram

   The relationships are as described below:

   -  A public identifier object can be directly related to zero or more
      SED Record objects, and a SED Record object can be related to
      exactly one public identifier object.

   -  A destination group object can contain zero or more TN (telephone
      number) Range objects, and a TN Range object can be contained in
      exactly one destination group object.





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   -  A destination group object can contain zero or more public
      identifier objects, and a public identifier object can be
      contained in exactly one destination group object.

   -  A destination group object can contain zero or more RN (routing
      number) objects, and an RN object can be contained in exactly one
      destination group object.

   -  A route group object can contain zero or more SED Record objects,
      and a SED Record object can be contained in exactly one route
      group object.

   -  A route group object can be associated with zero or more
      destination group objects, and a destination group object can be
      associated with zero or more route group objects.

   -  A data recipient object can be associated with zero or more route
      group objects, and a route group object can refer to zero or more
      data recipient objects.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   This document reuses terms from [RFC3261] (e.g., SIP), [RFC5486]
   (e.g., SSP, LUF, LRF, SED) and [RFC5067] (carrier-of-record and
   transit provider).  In addition, this document specifies the
   following additional terms.

   Registry:  The authoritative source for provisioned session
      establishment data (SED) and related information.  A registry can
      be part of an SSP or be an independent entity.

   Registrar:  An entity that provisions and manages data into the
      registry.  An SSP can act as its own registrar or -- additionally
      or alternatively -- delegate this function to a third party (who
      acts as its registrar).

   Local Data Repository (LDR):  The data store component of an
      addressing server that provides resolution responses.

   Public Identifier:  A public identifier refers to a telephone number
      (TN), a SIP address, or other identity as deemed appropriate, such
      as a globally routable URI of a user address (e.g.,
      sip:john.doe@example.net).




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   Telephone Number (TN) Range:  A numerically contiguous set of
      telephone numbers.

   Telephone Number (TN) Prefix:  A preceding portion of the digits
      common across a series of E.164 numbers.  A given TN prefix will
      include all the valid E.164 numbers that satisfy the expansion
      rules mandated by the country or the region with which the TNs
      comply.

   Routing Number (RN):  A Routing Number.  For more information, see
      [RFC4694].

   Destination Group:  An aggregation of a set of public identifiers, TN
      Ranges, or RNs that share common SED, which is exposed to a common
      set of peers.

   Data Recipient:  An entity with visibility into a specific set of
      public identifiers (or TN Ranges or RNs), the destination groups
      that contain these public identifiers (or TN Ranges and RNs), and
      a route group's SED records.

   Route Group:  An aggregation that contains a related set of SED
      records and is associated with a set of destination groups.  Route
      groups facilitate the management of SED records for one or more
      data recipients.

3.  Registry Use Cases

   This section documents use cases related to the provisioning of the
   registry.  Any request to provision, modify, or delete data is
   subject to several security considerations (see Section 5).  The
   protocols that implement these use cases (and associated
   requirements) will need to explicitly identify and address them.

3.1.  Category: Provisioning Mechanisms

   UC PROV #1  Real-Time Provisioning: Registrars have operational
               systems that provision public identifiers (or TN Ranges
               or RNs) in association with their SED.  These systems
               often function in a manner that expects or requires that
               these provisioning activities be completed immediately,
               as opposed to an out-of-band or batch provisioning scheme
               that can occur at a later time.  This type of
               provisioning is referred to as "real-time" or "on-demand"
               provisioning.






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   UC PROV #2  Non-Real-Time Bulk Provisioning: Operational systems that
               provision public identifiers (or TN Ranges or RNs) and
               associated SED sometimes expect that these provisioning
               activities be batched up into large sets.  These batched
               requests are then processed using a provisioning
               mechanism that is out of band and occurs at a later time.

   UC PROV #3  Multi-Request Provisioning: Regardless of whether or not
               a provisioning action is performed in real time, SSPs
               often perform several provisioning actions on several
               objects in a single request or transaction.  This is done
               for performance and scalability reasons, and for
               transactional reasons, such that the set of provisioning
               actions either fail or succeed atomically, as a complete
               set.

3.2.  Category: Interconnect Schemes

   UC INTERCONNECT #1  Inter-SSP SED: SSPs create peering relationships
                       with other SSPs in order to establish
                       interconnects.  Establishing these interconnects
                       involves, among other things, communicating and
                       enabling the points of ingress and other SED used
                       to establish sessions.

   UC INTERCONNECT #2  Direct and Indirect Peering: Some inter-SSP
                       peering relationships are created to enable the
                       establishment of sessions to the public
                       identifiers for which an SSP is the carrier-of-
                       record.  This is referred to as "direct peering".
                       Other inter-SSP peering relationships are created
                       to enable the establishment of sessions to public
                       identifiers for which an SSP is a transit
                       provider.  This is referred to as "indirect
                       peering".  Some SSPs take into consideration an
                       SSP's role as a transit or carrier-of-record
                       provider when selecting a route to a public
                       identifier.

   UC INTERCONNECT #3  Intra-SSP SED: SSPs support the establishment of
                       sessions between their own public identifiers,
                       not just to other SSPs' public identifiers.
                       Enabling this involves, among other things,
                       communicating and enabling intra-SSP signaling
                       points and other SED that can differ from inter-
                       SSP signaling points and SED.





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   UC INTERCONNECT #4  Selective Peering (a.k.a. per-peer policies):
                       SSPs create peering relationships with other SSPs
                       in order to establish interconnects.  However,
                       SSP peering relationships often result in
                       different points of ingress or other SED for the
                       same set of public identifiers.  This is referred
                       to as "selective peering" and is done on a route
                       group basis.

   UC INTERCONNECT #5  Provisioning of a delegated hierarchy: An SSP may
                       decide to maintain its own infrastructure to
                       contain the route records that constitute the
                       terminal step in the LUF.  In such cases, the SSP
                       will provision registries to direct queries for
                       the SSP's public identifiers to its own
                       infrastructure rather than provisioning the route
                       records directly.  For example, in the case of
                       DNS-based route records, such a delegated
                       hierarchy would make use of NS and CNAME records,
                       while a flat structure would make use of NAPTR
                       resource records.

3.3.  Category: SED Exchange and Discovery Models

   UC SED EXCHANGE #1  SED Exchange and Discovery using unified LUF/LRF:
                       When establishing peering relationships, some
                       SSPs may wish to communicate or receive SED
                       (e.g., points of ingress) that constitutes the
                       aggregated result of both LUF and LRF.

   UC SED EXCHANGE #2  SED Exchange and Discovery using LUF's Domain
                       Name: When establishing peering relationships,
                       some SSPs may not wish to communicate or receive
                       points of ingress and other SED using a registry.
                       They only wish to communicate or receive domain
                       names (LUF step only), and then independently
                       resolve those domain names via [RFC3263] to the
                       final points of ingress data (and other SED).

   UC SED EXCHANGE #3  SED Exchange and Discovery using LUF's
                       Administrative Domain Identifier: When
                       establishing peering relationships, some SSPs may
                       not wish to communicate or receive points of
                       ingress and other SED using a registry.  They
                       only wish to communicate or receive an
                       administrative domain identifier, which is not
                       necessarily resolvable via DNS.  The subsequent
                       process of using that administrative domain



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                       identifier to select points of ingress or other
                       SED can be SSP specific and is out of scope for
                       this document.

   UC SED EXCHANGE #4  Coexistent SED Exchange and Discovery Models:
                       When supporting multiple peering relationships,
                       some SSPs have the need to concurrently support
                       all three of the SED Exchange and Discovery
                       Models already described in this section
                       (Section 3.3) for the same set of public
                       identifiers.

3.4.  Category: SED Record Content

   UC SED RECORD #1  SED Record Content: Establishing interconnects
                     between SSPs involves, among other things,
                     communicating points of ingress, the service types
                     (SIP, SIPS, etc.) supported by each point of
                     ingress, and the relative priority of each point of
                     ingress for each service type.

   UC SED RECORD #2  Time-To-Live (TTL): For performance reasons,
                     querying SSPs sometimes cache SED that had been
                     previously looked up for a given public identifier.
                     In order to accomplish this, SSPs sometimes specify
                     the TTL associated with a given SED record.

3.5.  Category: Separation and Facilitation of Data Management

   UC DATA #1  Separation of Provisioning Responsibility: An SSP's
               operational practices often separate the responsibility
               of provisioning the points of ingress and other SED from
               the responsibility of provisioning public identifiers (or
               TN Ranges or RNs).  For example, a network engineer can
               establish a physical interconnect with a peering SSP's
               network and provision the associated domain name, host,
               and IP addressing information.  Separately, for each new
               subscriber, the SSP's provisioning systems provision the
               associated public identifiers.

   UC DATA #2  Destination Groups: SSPs often provision identical SED
               for large numbers of public identifiers (or TN Ranges or
               RNs).  For reasons of efficiency, groups of public
               identifiers that have the same SED can be aggregated.
               These aggregations are known as destination groups.  The
               SED is then indirectly associated with destination groups
               rather than with each individual public identifier (or TN
               Ranges or RNs).



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   UC DATA #3  Route Groups: SSPs often provision identical SED for
               large numbers of public identifiers (or TN Ranges or
               RNs), and then expose that relationship between a group
               of SED records and a group of public identifiers (or TN
               Ranges or RNs) to one or more SSPs.  This combined
               grouping of SED records and destination groups
               facilitates efficient management of relationships and the
               list of peers (data recipients) that can look up public
               identifiers and receive the associated SED.  This dual
               set of SED records and destination groups is termed a
               "route group".

3.6.  Category: Public Identifiers, TN Ranges, and RNs

   UC PI #1  Additions and Deletions: SSPs often allocate and de-
             allocate specific public identifiers to and from end-users.
             This involves, among other things, activating or
             deactivating specific public identifiers (TN Ranges or
             RNs), and directly or indirectly associating them with the
             appropriate points of ingress and other SED.

   UC PI #2  Carrier-of-Record versus Transit Provisioning: Some inter-
             SSP peering relationships are created to enable the
             establishment of sessions to the public identifiers (or TN
             Ranges or RNs) for which an SSP is the carrier-of-record.
             Other inter-SSP peering relationships are created to enable
             the establishment of sessions for which an SSP is a transit
             provider.  Some SSPs take into consideration an SSP's role
             as a transit or carrier-of-record provider when selecting a
             route.

   UC PI #3  Multiplicity: As described in previous use cases, SSPs
             provision public identifiers (or TN Ranges or RNs) and
             their associated SED for multiple peering SSPs, and as both
             the carrier-of-record and transit provider.  As a result, a
             given public identifier (or TN Range or RN) key can reside
             in multiple destination groups at any given time.

   UC PI #4  Destination Group Modification: SSPs often change the SED
             associated with a given public identifier (or TN Range or
             RN).  This involves, among other things, directly or
             indirectly associating them with a different point of
             ingress, different services, or different SED.

   UC PI #5  Carrier-of-Record versus Transit Modification: SSPs may
             have the need to change their carrier-of-record versus
             transit role for public identifiers (or TN Ranges or RNs)
             that they previously provisioned.



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   UC PI #6  Modification of Authority: An SSP indicates that it is the
             carrier-of-record for an existing public identifier or TN
             Range.  If the public identifier or TN Range were
             previously associated with a different carrier-of-record,
             then there are multiple possible outcomes, such as a) the
             previous carrier-of-record is disassociated, b) the
             previous carrier-of-record is relegated to transit status,
             or c) the new carrier-of-record is placed in inactive mode.
             The choice may be dependent on the deployment scenario and
             is out of scope for this document.

3.7.  Category: Misc

   UC MISC #1  Number Portability: The SSP wishes to provide, in query
               response to public identifiers, an associated routing
               number (RN).  This is the case where a set of public
               identifiers is no longer associated with the original SSP
               but has been ported to a recipient SSP, who provides
               access to these identifiers via a switch on the Signaling
               System Number 7 network identified by the RN.

   UC MISC #2  Data Recipient Offer and Accept: When a peering
               relationship is established (or invalidated), SSPs
               provision (or remove) data recipients in the registry.
               However, a peer may first need to accept its role (as a
               data recipient) before such a change is made effective.
               Alternatively, an auto-accept feature can be configured
               for a given data recipient.

   UC MISC #3  Open Numbering Plans: In several countries, an open
               numbering plan is used, where the carrier-of-record is
               only aware of a portion of the E.164 number (i.e., the TN
               prefix).  The carrier-of-record may not know the complete
               number or the number of digits in the number.  The rest
               of the digits are handled offline (e.g., by a Private
               Branch Exchange, or PBX).  For example, an SSP can be the
               carrier-of-record for "+123456789" and be the carrier-of-
               record for every possible expansion of that number, such
               as "+12345678901" and "+123456789012", even though the
               SSP does not know what those expansions could be.  This
               can be described as the carrier-of-record effectively
               being authoritative for the TN prefix.

4.  Requirements

   This section lists the requirements extracted from the use cases in
   Section 3.  The objective is to make it easier for protocol designers
   to understand the underlying requirements and to reference and list



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   the requirements that they support (or not).  The requirements listed
   here, unless explicitly indicated otherwise, are expected to be
   supported.  Protocol proposals are also expected to indicate their
   compliance with these requirements and highlight ones that they don't
   meet (if any).  Furthermore, the requirements listed here are not
   meant to be limiting, i.e., protocol implementations and deployments
   may choose to support additional requirements based on use cases that
   are not listed in this document.

4.1.  Provisioning Mechanisms

   REQ-PROV-1:  Real-time provisioning.

   REQ-PROV-2:  (Optional) Non-real-time bulk provisioning.

   REQ-PROV-3:  Multi-request provisioning.

4.2.  Interconnect Schemes

   REQ-INTERCONNECT-1:  Inter-SSP peering.

   REQ-INTERCONNECT-2:  Direct and Indirect peering.

   REQ-INTERCONNECT-3:  Intra-SSP SED.

   REQ-INTERCONNECT-4:  Selective peering.

   REQ-INTERCONNECT-5:  Provisioning of a delegated hierarchy.

4.3.  SED Exchange and Discovery Requirements

   REQ-SED-1:  SED containing unified LUF and LRF content.

   REQ-SED-2:  SED containing LUF-only data using domain names.

   REQ-SED-3:  SED containing LUF-only data using administrative
               domains.

   REQ-SED-4:  Support for all the other REQ-SED requirements (listed in
               this section), concurrently, for the same public
               identifier (or TN Range or RN).

4.4.  SED Record Content Requirements

   REQ-SED-RECORD-1:  Ability to provision SED record content.

   REQ-SED-RECORD-2:  (Optional) Communication of an associated TTL for
                      a SED Record.



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4.5.  Data Management Requirements

   REQ-DATA-MGMT-1:  Separation of responsibility for the provisioning
                     the points of ingress and other SED, from the
                     responsibility of provisioning public identifiers.

   REQ-DATA-MGMT-2:  Ability to aggregate a set of public identifiers as
                     destination groups.

   REQ-DATA-MGMT-3:  Ability to create the aggregation termed route
                     group.

4.6.  Public Identifier, TN Range, and RN Requirements

   REQ-PI-TNR-RN-1:  Provisioning of, and modifications to, the
                     following aggregations: destination group and route
                     groups.

   REQ-PI-TNR-RN-2:  Ability to distinguish an SSP as either the
                     carrier-of-record provider or the transit provider.

   REQ-PI-TNR-RN-3:  A given public identifier (or TN Range or RN) can
                     reside in multiple destination groups at the same
                     time.

   REQ-PI-TNR-RN-4:  Modification of public identifier (or TN Range or
                     RN) by allowing them to be moved to a different
                     destination group via an atomic operation.

   REQ-PI-TNR-RN-5:  SSPs can indicate a change to their role from
                     carrier-of-record provider to transit, or vice
                     versa.

   REQ-PI-TNR-RN-6:  Support for modification of authority with the
                     conditions described in UC PI #6.

4.7.  Misc. Requirements

   REQ-MISC-1:  Number portability support.

   REQ-MISC-2:  Ability for the SSP to be offered a peering relationship
                and for the SSP to accept (explicitly or implicitly) or
                reject such an offer.

   REQ-MISC-3:  Support for open numbering plans.






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5.  Security Considerations

   Session establishment data allows for the routing of SIP sessions
   within, and between, SIP Service Providers.  Access to this data can
   compromise the routing of sessions and expose a SIP Service Provider
   to attacks such as service hijacking and denial of service.  The data
   can be compromised by vulnerable functional components and interfaces
   identified within the use cases.

   A provisioning framework or protocol that implements the described
   use cases MUST, therefore, provide data confidentiality and message
   integrity.  Such frameworks and protocols MUST specify mechanisms to
   authenticate and authorize any entity that provisions data into the
   registry, i.e., that the entity is who it says it is and is allowed
   to use the provisioning interface.  The determination of whether such
   an entity is authorized to provision specific data elements (e.g., a
   certain public identifier or TN Range) -- while REQUIRED -- may be
   left to local policy.

6.  Acknowledgments

   This document is a result of various contributions from (and
   discussions within) the IETF DRINKS Working Group; specifically, in
   alphabetical order: Alexander Mayrhofer, Deborah A Guyton, Gregory
   Schumacher, Jean-Francois Mule, Kenneth Cartwright, Manjul Maharishi,
   Penn Pfautz, Ray Bellis, Richard Shockey, and Syed Ali.

   The editor also wishes to thank the following for their comments and
   suggestions: Otmar Lendl, Sohel Khan, Peter Koch, Brian Rosen, Jon
   Peterson, Gonzalo Camarillo, and Stephen Farrell.





















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RFC 6461            DRINKS Use Cases and Requirements       January 2012


7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5486]  Malas, D. and D. Meyer, "Session Peering for Multimedia
              Interconnect (SPEERMINT) Terminology", RFC 5486,
              March 2009.

7.2.  Informative References

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3263]  Rosenberg, J. and H. Schulzrinne, "Session Initiation
              Protocol (SIP): Locating SIP Servers", RFC 3263,
              June 2002.

   [RFC4694]  Yu, J., "Number Portability Parameters for the "tel" URI",
              RFC 4694, October 2006.

   [RFC5067]  Lind, S. and P. Pfautz, "Infrastructure ENUM
              Requirements", RFC 5067, November 2007.

Author's Address

   Sumanth Channabasappa (editor)
   CableLabs
   858 Coal Creek Circle
   Louisville, CO  80027
   USA

   EMail: sumanth@cablelabs.com














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