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SPEERMING Working Group                                   A. Uzelac, Ed.
Internet-Draft                                           Global Crossing
Intended status: Informational                               Y. Lee, Ed.
Expires: November 28, 2009                                 Comcast Cable
                                                            May 27, 2009


                       VoIP SIP Peering Use Cases
           draft-ietf-speermint-voip-consolidated-usecases-12

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   This document depicts many common Voice over IP (VoIP) use cases for
   Session Initiation Protocol (SIP) Peering.  These use cases are
   categorized into static and on-demand, and then further sub-
   categorized into direct and indirect.  These use cases are not an
   exhaustive set, but rather the most common use cases deployed today.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4

   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4

   3.  Reference Architecture . . . . . . . . . . . . . . . . . . . .  4

   4.  Contexts of Use Cases  . . . . . . . . . . . . . . . . . . . .  5

   5.  Use Cases  . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     5.1.  Static Peering Use Cases . . . . . . . . . . . . . . . . .  6
     5.2.  Static Direct Peering Use Case . . . . . . . . . . . . . .  6
       5.2.1.  Administrative characteristics . . . . . . . . . . . . 11
       5.2.2.  Options and Nuances  . . . . . . . . . . . . . . . . . 11
     5.3.  Static Direct Peering Use Case - Assisting LUF and LRF . . 12
       5.3.1.  Administrative Characteristics . . . . . . . . . . . . 13
       5.3.2.  Options and Nuances  . . . . . . . . . . . . . . . . . 14
     5.4.  Static Indirect Peering Use Case - Assisting LUF and
           LRF  . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
       5.4.1.  Administrative characteristics . . . . . . . . . . . . 20
       5.4.2.  Options and Nuances  . . . . . . . . . . . . . . . . . 20
     5.5.  Static Indirect Peering Use Case . . . . . . . . . . . . . 21
       5.5.1.  Administrative characteristics . . . . . . . . . . . . 21
       5.5.2.  Options and Nuances  . . . . . . . . . . . . . . . . . 22
     5.6.  On-demand Peering Use Cases  . . . . . . . . . . . . . . . 22
       5.6.1.  Administrative characteristics . . . . . . . . . . . . 22
       5.6.2.  Options and Nuances  . . . . . . . . . . . . . . . . . 22

   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 23

   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 23




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   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23

   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 23
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 24

   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24












































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1.  Introduction

   This document attempts to capture Voice over IP (VoIP) use cases for
   Session Initiation Protocol (SIP) [RFC3261] based peering.  These use
   cases will assist in identifying requirements and other issues to be
   considered for future resolution by the working group.

   Only use cases related to VoIP are considered in this document.
   Other real-time SIP communications use cases, like Instant Messaging
   (IM) and presence are out of scope for this document.  In describing
   use cases, the intent is descriptive, not prescriptive.

   The use cases contained in this document attempts to be as
   comprehensive as possible, but should not be considered the exclusive
   set of use cases.


2.  Terminology

   This document uses terms defined in [RFC5486].  Please refer to it
   for definitions.


3.  Reference Architecture

   The diagram below provides the reader with a context for the VoIP use
   cases in this document.  Terms such as Sip Service Provider (SSP),
   Look-Up Function (LUF), Location Routing Function (LRF), Signaling
   Path Border Element (SBE) and Data Path Border Element (DBE) are
   defined in [RFC5486].

   Originating SSP (O-SSP) is the SSP originating a request.
   Terminating SSP (T-SSP) is the SSP terminating the request
   originating from O-SSP.  Assisting LUF and LRF Provider offers LUF
   and LRF services to O-SSP.  Indirect SSP (I-SSP) is the SSP providing
   indirect peering service(s) to O-SSP to connect to T-SSP.















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    +--------------------+------------------------+--------------------+
    |  Originating SSP   |  Assisting LUF and LRF |  Terminating SSP   |
    |     Domain         |    Provider Domain     |      Domain        |
    |                    |                        |                    |
    |  +-----+  +-----+  |    +------+ +------+   |  +-----+  +-----+  |
    |  |O-LUF|  |O-LRF|  |    |A-LUF | | A-LRF|   |  |T-LUF|  |T-LRF|  |
    |  +-----+  +-----+  |    +------+ +------+   |  +-----+  +-----+  |
    |                    |                        |                    |
    | +-------+ +-----+  +------------------------+  +-----+ +-------+ |
    | |O-Proxy| |O-SBE|  |  Indirect SSP Domain   |  |T-SBE| |T-Proxy| |
    | +-------+ +-----+  |                        |  +-----+ +-------+ |
    |                    |    +-----+  +-----+    |                    |
    |    +---+  +-----+  |    |O-SBE|  |O-DBE|    |  +-----+  +---+    |
    |    |UAC|  |O-DBE|  |    +-----+  +-----+    |  |T-DBE|  |UAS|    |
    |    +---+  +-----+  |                        |  +-----+  +---+    |
    |                    |                        |                    |
    +--------------------+------------------------+--------------------+

                             General Overview

                                 Figure 1

   Note that in Figure 1 - some elements defined are optional in many
   use cases.


4.  Contexts of Use Cases

   Use cases are sorted into two general groups: Static and On-demand
   Peering [RFC5486].  Each group can be further sub-divided into Direct
   Peering and Indirect Peering [RFC5486].  Although there may be some
   overlap among the use cases in these categories, there are different
   requirements between the scenarios.  Each use case must specify a
   basic set of required operations to be performed by each SSP when
   peering.

   These can include:

   o  Peer Discovery - Peer discovery via a Look-Up Function (LUF) to
      determine the Session Establishment Data (SED) [RFC5486] of the
      request.  In VoIP use cases, a request normally contains a phone
      number.  The O-SSP will input the phone number to the LUF and the
      LUF will normally return a SIP AOR [RFC3261] which contains a
      domain name.

   o  Next Hop Routing Determination - Resolving the SED information is
      necessary to route the request to the T-SSP.  The LRF is used for
      this determination.  The O-SSP may also use the standard procedure



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      defined in [RFC3263] to discover the next hop.

   o  Call setup - SSPs that are interconnecting to one another may also
      define specifics on what SIP features need to be used when
      contacting the next hop in order to a) reach the next hop at all
      and b) to prove that the sender is a legitimate peering partner.

      Examples: hard-code transport (TCP/UDP/TLS), non-standard port
      number, specific source IP address (e.g. in a private Layer-3
      network), which TLS client certificate [RFC4366] to use, and other
      authentication schemes.

   o  Call reception - This step serves to ensure that the type of
      relationship (static or on-demand, indirect or direct) is
      understood and acceptable.  For example, the receiving SBE needs
      to determine whether the INVITE it received really came from a
      trusted member possibly via an access control list entry.



5.  Use Cases

   Please note there are intra-domain message flows within the use cases
   to serve as supporting background information.  Only inter-domain
   communications are germane to Speermint.

5.1.  Static Peering Use Cases

   Static Peering [RFC5486] describes the use case when two SSPs form a
   peering relationship with some form of association established prior
   to the exchange of traffic.  Pre-association is a prerequisite to
   static peering.  Static peering is used in cases when two peers want
   a consistent and tightly controlled approach to peering.  In this
   scenario, a number of variables, such as an identification method
   (remote proxy IP address) and Quality of Service (QoS) parameters,
   can be defined upfront and known by each SSP prior to peering.

5.2.  Static Direct Peering Use Case

   This is the simplest form of a peering use case.  Two SSPs negotiate
   and agree to establish a SIP peering relationship.  The peer
   connection is statically configured and is direct between the
   connected SSPs.  The peers may exchange interconnection parameters
   such as DSCP [RFC2474] policies, the maximum number of requests per
   second and proxy location prior to establishing the interconnection.
   Typically, the T-SSP only accepts traffic originating directly from
   the trusted peer.




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         +--------------------+             +---------------------+
         |        O-SSP       |             |        T-SSP        |
         |       +-----+      |             |       +-----+       |
         |       |O-LUF|      |             |       |T-LUF|       |
         |       |O-LRF|      |             |      /|T-LRF|       |
         |      /+-----+\     |             |     / +-----+       |
         |    (2)     (4,5,6) |             |    /                |
         |    /           \   |             |   /(8,9)            |
         |+-------+     +-----+             +-----+      +-------+|
         ||O-Proxy|-(3)-|O-SBE+-----(7)-----+T-SBE|-(10)-|T-Proxy||
         |+-------+     +-----+             +-----+      +-------+|
         |    |               |             |                |    |
         |   (1)              |             |               (11)  |
         |    |               |             |                |    |
         | +-----+      +-----+             +-----+       +-----+ |
         | | UAC +======|O-DBE+=====(12)====+T-DBE|=======+ UAS | |
         | +-----+      +-----+             +-----+       +-----+ |
         +--------------------+             +---------------------+
              example.com                         example.net


                      Static Direct Peering Use Case

                                 Figure 2

   The following is a high-level depiction of the use case:

   1.   UAC initiates a call via SIP INVITE to O-Proxy.  O-Proxy is the
        home proxy for UAC.

         INVITE sip:+19175550100@example.com;user=phone SIP/2.0
         Via: SIP/2.0/TCP client.example.com:5060
           ;branch=z9hG4bK74bf9
         Max-Forwards: 10
         From: Alice <sip:+14085550101@example.com;user=phone>
           ;tag=12345
         To: Bob <sip+19175550100@example.com;user=phone>
         Call-ID: abcde
         CSeq: 1 INVITE
         Contact: <sip:+19175550100@client.example.com;user=phone
           ;transport=tcp>

        Note that UAC inserted its Fully Qualified Domain Name (FQDN) in
        the VIA and CONTACT headers.  This example assumes that UAC has
        its own FQDN.  In the deployment where UAC does not have its own
        FQDN, UAC may insert an IP address into the headers.





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   2.   UAC only knows UAS's TN but not UAS's domain.  It appends its
        own domain to generate the SIP URI in Request-URI and TO header.
        O-Proxy checks the Request-URI and discovers that the Request-
        URI contains user parameter "user=phone".  This parameter
        indicates that the Request-URI is a phone number.  So O-Proxy
        will extract the TN from the Request-URI and query LUF for SED
        information from a routing database.  In this example, the LUF
        is an ENUM [RFC3761] database.  The ENUM entry looks similar to
        this:

          $ORIGIN 0.0.1.0.5.5.5.7.1.9.1.e164.arpa.
          IN NAPTR (
            10
            100
            "u"
            "E2U+SIP"
            "!^.*$!sip:+19175550100@example.net!"
            . )

        This SED data can be provisioned by O-SSP or populated by the
        T-SSP.

   3.   O-Proxy examines the SED and discovers the domain is external.
        Given the O-Proxy's internal routing policy, O-Proxy decides to
        use O-SBE to reach T-SBE.  O-Proxy routes the INVITE request to
        O-SBE and adds a Route header which contains O-SBE.

         INVITE sip:+19175550100@example.net;user=phone SIP/2.0
         Via: SIP/2.0/TCP o-proxy.example.com:5060
           ;branch=z9hG4bKye8ad
         Via: SIP/2.0/TCP client.example.com:5060
           ;branch=z9hG4bK74bf9;received=192.0.1.1
         Max-Forwards: 9
         Route: <sip:o-sbe1.example.com;lr>
         Record-Route: <sip:o-proxy.example.com;lr>
         From: Alice <sip:+14085550101@example.com;user=phone>
           ;tag=12345
         To: Bob s<ip+19175550100@example.com;user=phone>
         Call-ID: abcde
         CSeq: 1 INVITE
         Contact: <sip:+19175550100@client.example.com;user=phone
           ;transport=tcp>

   4.   O-SBE receives the requests and pops the top entry of the Route
        header which contains "o-sbe1.exapmle.com".  O-SBE examines the
        Request-URI and does a LRF for "example.net".  In this example,
        the LRF is a NAPTR DNS query [RFC3403] of the domain name.
        O-SBE receives a NAPTR response from LRF.  The response looks



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        similar to this:

          IN NAPTR (
            50
            50
            "S"
            "SIP+D2T"
            ""
            _sip._tcp.t-sbe.example.net. )

          IN NAPTR (
            90
            50
            "S"
            "SIP+D2U"
            ""
            _sip._udp.t-sbe.example.net. )

   5.   Given the lower order for TCP in the NAPTR response, O-SBE
        decides to use TCP as the transport protocol, so it sends a SRV
        DNS query for the SRV record [RFC2782] for "_sip._tcp.t-
        sbe.example.net".

        ;;     priority  weight   port  target
        IN SRV 0         2        5060  t-sbe1.example.net.
        IN SRV 0         1        5060  t-sbe2.example.net.

   6.   Given the higher weight for "t-sbe1.example.net", O-SBE sends an
        A record DNS query for "t-sbe1.example.net." to get the A
        record:

          ;; DNS ANSWER
          t-sbe1.example.net.   IN A   192.0.2.100
          t-sbe1.example.net.   IN A   192.0.2.101

   7.   O-SBE sends the INVITE to T-SBE.  O-SBE is the egress point to
        the O-SSP domain, so it should ensure subsequent mid-dialog
        requests traverse via itself.  If O-SBE chooses to act as a
        Back-to-Back User Agent (B2BUA) [RFC3261], it will terminate the
        call and generate a new INVITE request.  If O-SBC chooses to act
        as a proxy, it should record-route to stay in the call path.  In
        this example, O-SBE is a B2BUA.









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         INVITE sip:+19175550100@example.net;user=phone SIP/2.0
         Via: SIP/2.0/TCP o-sbe1.example.com:5060
           ;branch= z9hG4bK2d4zzz;
         Max-Forwards: 10
         From: Alice <sip:+14085550101@example.com;user=phone>
           ;tag=54321
         To: Bob <sip:+19175550100@example.net;user=phone>
         Call-ID: abcde-osbe1
         CSeq: 1 INVITE
         Contact: <sip:+19175550100@o-sbe1.example.com;user=phone
           ;transport=tcp>

        Note that O-SBE may re-write the Request-URI with the target
        domain in the SIP URI.  Some proxy implementations will only
        accept the request if the Request-URI contains their own
        domains.

   8.   T-SBE determines the called party home proxy and directs the
        call to the called party.  T-SBE may use ENUM or other internal
        mechanism to locate the home proxy.  If T-SSP uses ENUM, this
        internal ENUM entry is different from the external ENUM entry
        populated for O-SSP.  In this example, the internal ENUM query
        returns the UAS's home proxy.

         $ORIGIN 0.0.1.0.5.5.5.7.1.9.1.e164.arpa.
         IN NAPTR (
           10
           100
           "u"
           "E2U+SIP"
           "!^.*$!sip:+19175550100@t-proxy.example.net!"
           . )

   9.   T-SBE receives the NAPTR record and query DNS for the A record
        of domain "t-proxy.example.net.".  The DNS returns an A record:

          ;; DNS ANSWER
          t-proxy.example.net.   IN A   192.0.2.2

   10.  T-SBE is a B2BUA, so it generates a new INVITE and sends it to
        UAS's home proxy:










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         INVITE sip:bob@t-proxy.example.net;user=phone SIP/2.0
         Via: SIP/2.0/TCP t-sbe1.example.net:5060
           ;branch= z9hG4bK28uyyy;
         Max-Forwards: 10
         From: Alice <sip:+14085550101@example.com;user=phone>
           ;tag=54321
         To: Bob <sip:+19175550100@t-proxy.example.net;user=phone>
         Call-ID: abcde-tsbe1
         CSeq: 1 INVITE
         Contact: <sip:+19175550100@t-sbe1.example.net;user=phone
           ;transport=tcp>

   11.  Finally, UAS's home proxy forwards the INVITE request to the
        UAS.

         INVITE sip:+19175550100@server.example.net;user=phone SIP/2.0
         Via: SIP/2.0/TCP t-proxy.example.net:5060
           ;branch= z9hG4bK28u111;
         Via: SIP/2.0/TCP t-sbe1.example.net:5060
           ;branch= z9hG4bK28uyyy; received=192.2.0.100
         Max-Forwards: 9
         Record-Route: <sip:t-proxy.example.net:5060;lr>,
           <sip:t-sbe1.example.net:5060;lr>
         From: Alice <sip:+14085550101@example.com;user=phone>
           ;tag=54321
         To: Bob <sip:+19175550100@t-proxy.example.net;user=phone>
         Call-ID: abcde-tsbe1
         CSeq: 1 INVITE
         Contact: <sip:+19175550100@t-sbe1.example.net;user=phone
           ;transport=tcp>

   12.  RTP is established between the UAC and UAS.  Note that the media
        passes through O-DBE and T-DBE.

5.2.1.  Administrative characteristics

   The static direct peering use case is typically implemented in a
   scenario where there is a strong degree of trust between the two
   administrative domains.  Both administrative domains typically sign a
   peering agreement which state clearly the policies and terms.

5.2.2.  Options and Nuances

   In Figure 2 O-SSP and T-SSP peer via SBEs.  Normally, the operator
   will deploy the SBE at the edge of its administrative domain.  The
   signaling traffic will pass between two networks through the SBEs.
   The operator has many reasons to deploy a SBE.  For example, either
   proxy and UA may use [RFC1918] addresses that are not routable in the



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   target network.  The SBE can perform a NAT function.  Also, the SBE
   eases the operation cost for deploying or removing Layer-5 network
   elements.  Consider the deployment architecture where multiple
   proxies connect to a single SBE.  An operator can add or remove a
   proxy without coordinating with the peer operator.  The peer operator
   "sees" only the SBE.  As long as the SBE is maintained in the path,
   the peer operator does not need to be notified.

   When an operator deploys SBEs, the operator is required to advertise
   the SBE to the peer LRF so that the peer operator can locate the SBE
   and route the traffic to the SBE accordingly.

   SBE deployment is a decision within an administrative domain.  Either
   one or both administrative domains can decide to deploy SBE(s).  To
   the peer network, most important is to identify the next-hop address.
   Whether the next-hop is a proxy or SBE, the peer network will not see
   any difference.

5.3.  Static Direct Peering Use Case - Assisting LUF and LRF

   This use case shares many properties with the static direct use case.
   There must exist a pre-association between the O-SSP and T-SSP.  The
   difference is O-SSP will use the Assisting LUF/LRF Provider for LUF
   and LRF.  The LUF/LRF provider stores the SED to reach T-SSP and
   provides it to O-SSP when O-SSP requests it.


























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                            +-----------------+
                            |LUF/LRF Provider |
                            |                 |
                            |     +-------+   |
                            |   +-+ A-LUF |   |
                            |  /  | A-LRF |   |
       +--------------------+ /  ++-------+   +---------------------+
       |       O-SSP        |/  /             |         T-SSP       |
       |       +------------/(4,5,6)          |        +-----+      |
       |      /             | /               |        |T-LUF|      |
       |    (2)           +-+/                |      +-|T-LRF|      |
       |    /            /  |                 |     /  +-----+      |
       |   /            /   |                 |    /(8,9)           |
       |+-------+     +-----+                 +-----+      +-------+|
       ||O-Proxy|-(3)-|O-SBE+-------(7)-------+T-SBE|-(10)-|T-Proxy||
       |+-------+     +-----+                 +-----+      +-------+|
       |    |               |                 |                |    |
       |   (1)              |                 |              (11)   |
       |    |               |                 |                |    |
       | +-----+      +-----+                 +-----+       +-----+ |
       | | UAC +======|O-DBE+=======(12)======+T-DBE+=======+ UAS | |
       | +-----+      +-----+                 +-----+       +-----+ |
       +--------------------+                 +---------------------+
             example.com                            example.net


             Static Direct Peering with Assisting LUF and LRF

                                 Figure 3

   The call flow looks almost identical to Static Direct Peering Use
   Case except Step 2,4,5 and 6 which happen in LUF/LRF provider
   remotely instead of happening in O-SSP domain.

   Similar to Static Direct Peering Use case, O-DBE and T-DBE in the
   Figure 3 are optional.

5.3.1.  Administrative Characteristics

   The LUF/LRF provider provides the LUF and LRF services for the O-SSP.
   As such, LUF/LRF provider, O-SSP and T-SSP form a trusted
   administrative domain.  To reach T-SSP, O-SSP must still require pre-
   arranged agreements for the peer relationship with T-SSP.  The
   Layer-5 policy is maintained in the O-SSP and T-SSP domains, and the
   LUF/LRF provider may not be aware of any Layer-5 policy between the
   O-SSP and T-SSP.

   A LUF/LRF provider can serve multiple administrative domains.  The



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   LUF/LRF provider typically does not share SED from one administrative
   domain to another administrative domain without appropriate
   permission granted.

5.3.2.  Options and Nuances

   The LRF/LRF provider can use multiple methods to provide SED to
   O-SSP.  The most commonly used are an ENUM and a SIP Redirect.  The
   O-SSP should negotiate with the LUF/LRF provider which query method
   it will use prior to sending request to LUF/LRF provider.

   The T-SSP needs to populate its users' AORs and SED to the LUF/LRF
   provider.  Currently, this procedure is non-standardized and labor
   intensive.  A more detailed description of this problem has been
   documented in the work in progress [I-D.ietf-drinks-cons-rqts].

5.4.  Static Indirect Peering Use Case - Assisting LUF and LRF

   The difference between a Static Direct Use Case and a Static Indirect
   Use Case lies within the Layer-5 relationship of which O-SSP and
   T-SSP maintain.  In the Indirect use case, the O-SSP and T-SSP do not
   have direct Layer-5 connectivity.  They require one or multiple
   Indirect Domains to assist routing the SIP messages and possibly the
   associated media.

   In this use case, the O-SSP and T-SSP want to form a peer
   relationship.  For some reason, the O-SSP and T-SSP do not have
   direct Layer-5 connectivity.  The reasons may vary, for example
   business demands and/or domain policy controls.  Due to this indirect
   relationship the signaling will traverse from O-SSP to one or
   multiple I-SSP(s) to reach T-SSP.

   In addition, O-SSP decides to use a LUF/LRF provider.  This LUF/LRF
   provider stores the T-SSP's SED pre-populated by T-SSP.  One
   important motivation to use the LUF/LRF provider is that T-SSP only
   needs to populate its SED once to the provider.  Any O-SSP who wants
   to query T-SSP's SED can use this LUF/LRF provider.  Current practice
   has shown that it is rather difficult for the T-SSP to populate its
   SED to every O-SSP who likes to reach the T-SSP's subscribers.  This
   is especially true in the Enterprise environment.

   Note that the LUF/LRF provider and I-SSP can be the same provider or
   different providers.








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                            +------------------+
                            | LUF/LRF Provider |
                            |       I-SSP      |
                            |      +-------+   |
                            |   ---+ A-LUF |   |
                            |  /   | A-LRF |   |
       +--------------------+ /    +-------+   +---------------------+
       |       O-SSP        |/     /           |         T-SSP       |
       |      +-------------/     /            |        +-----+      |
       |     /              |(4,5,6)           |        |T-LUF|      |
       |    /               |   /              |   +----+T-LRF|      |
       |  (2)             + +---               |  /     +-----+      |
       |  /              /  |                  | /(9,10)             |
       |+-------+     +-----+     +-----+      +-----+      +-------+|
       ||O-Proxy|-(3)-|O-SBE+-(7)-+I-SBE+-(8)--+T-SBE+-(11)-|T-Proxy||
       |+-------+     +-----+     +-----+      +-----+      +-------+|
       |    |               |                  |                |    |
       |   (1)              |                  |               (12)  |
       |    |               |                  |                |    |
       | +-----+      +-----+     +-----+      +-----+       +-----+ |
       | | UAC +=(13)=|O-DBE+=====+I-DBE+======+T-DBE+=======+ UAS | |
       | +-----+      +-----+     +-----+      +-----+       +-----+ |
       +-------------------------------------------------------------+
            example.com          example.org         example.net


      Indirect Peering via LUF/LRF provider and I-SSP (SIP and media)

                                 Figure 4

   The following is a high-level depiction of the use case:

   1.   UAC initiates a call via SIP INVITE to O-Proxy.  O-Proxy is the
        home proxy for UAC.

         INVITE sip:+19175550100@example.com;user=phone SIP/2.0
         Via: SIP/2.0/TCP client.example.com:5060
           ;branch=z9hG4bK74bf9
         Max-Forwards: 10
         From: Alice <sip:+14085550101@example.com;user=phone>
           ;tag=12345
         To: Bob <sip+19175550100@example.com;user=phone>
         Call-ID: abcde
         CSeq: 1 INVITE
         Contact: <sip:+19175550100@client.example.com;user=phone
           ;transport=tcp>





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   2.   UAC only knows UAS's TN but not UAS's domain.  It appends its
        own domain to generate the SIP URI in Request-URI and TO header.
        O-Proxy checks the Request-URI and discovers that the Request-
        URI contains user parameter "user=phone".  This parameter
        indicates that the Request-URI is a phone number.  So O-Proxy
        will extract the TN from the Request-URI and query LUF for SED
        information from a routing database.  In this example, the LUF
        is an ENUM database.  The ENUM entry looks similar to this:

          $ORIGIN 0.0.1.0.5.5.5.7.1.9.1.e164.arpa.
          IN NAPTR (
            10
            100
            "u"
            "E2U+SIP"
            "!^.*$!sip:+19175550100@example.org!"
            . )

        Note that the response shows the next-hop is the SBE in Indirect
        SSP.

        Alternatively, O-SSP may have a pre-association with I-SSP.  As
        such, O-SSP will forward all requests which contains an external
        domain in the Request-URI or unknown TN to I-SSP.  The O-SSP
        will rely on the I-SSP to determine the T-SSP and route the
        request correctly.  In this configuration, the O-SSP can skip
        Steps 2,4,5 and 6 and forward the request directly to the I-SBE.
        This configuration is commonly used in the Enterprise
        environment.

   3.   Given the O-Proxy's internal routing policy, O-Proxy decides to
        use O-SBE to reach I-SBE.  O-Proxy routes the INVITE request to
        O-SBE and adds a Route header which contains the O-SBE.

         INVITE sip:+19175550100@example.org;user=phone SIP/2.0
         Via: SIP/2.0/TCP o-proxy.example.com:5060
           ;branch=z9hG4bKye8ad
         Via: SIP/2.0/TCP client.example.com:5060
           ;branch=z9hG4bK74bf9;received=192.0.1.1
         Max-Forwards: 9
         Route: <sip:o-sbe1.example.com;lr>
         Record-Route: <sip:o-proxy.example.com;lr>
         From: Alice <sip:+14085550101@example.com;user=phone>
           ;tag=12345
         To: Bob <sip+19175550100@example.net;user=phone>
         Call-ID: abcde
         CSeq: 1 INVITE
         Contact: <sip:+19175550100@client.example.com;user=phone



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           ;transport=tcp>

   4.   O-SBE receives the requests and pops the top entry of the Route
        header which contains "sip:o-sbe1.example.com".  O-SBE examines
        the Request-URI and does a LRF for "example.org".  In this
        example, the LRF is a NAPTR DNS query of the domain.  O-SBE
        receives a response similar to this:

          IN NAPTR (
            50
            50
            "S"
            "SIP+D2T"
            ""
            _sip._tcp.i-sbe.example.org. )

          IN NAPTR (
            90
            50
            "S"
            "SIP+D2U"
            ""
            _sip._udp.i-sbe.example.org. )

   5.   Given the lower order for TCP in the NAPTR response, O-SBE
        decides to use TCP for transport protocol, so it sends a SRV DNS
        query for the SRV record for "_sip._tcp.i-sbe.example.org.".

        ;;     priority  weight   port  target
        IN SRV 0         2        5060  i-sbe1.example.org.
        IN SRV 0         1        5060  i-sbe2.example.org.

   6.   Given the higher weight for "i-sbe1.example.org", O-SBE sends a
        DNS query for A record of "i-sbe1.example.org." to get the A
        record:

          ;; DNS ANSWER
          i-sbe1.example.org.   IN A   192.0.2.200
          i-sbe1.example.org.   IN A   192.0.2.201

   7.   O-SBE sends the INVITE to I-SBE.  O-SBE is the entry point to
        the O-SSP domain, so it should ensure subsequent mid-dialog
        requests traverse via itself.  If O-SBE chooses to act as a
        B2BUA, it will terminate the call and generate a new back-to-
        back INVITE request.  If O-SBC chooses to act as proxy, it
        should record-route to stay in the call path.  In this example,
        O-SBE is a B2BUA.




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         INVITE sip:+19175550100@example.org;user=phone SIP/2.0
         Via: SIP/2.0/TCP o-sbe1.example.com:5060
           ;branch= z9hG4bK2d4zzz;
         Max-Forwards: 10
         Route:  <sip:i-sbe1.example.org;lr>
         From: Alice <sip:+14085550101@example.com;user=phone>
           ;tag=54321
         To: Bob <sip:+19175550100@example.net;user=phone>
         Call-ID: abcde-osbe1
         CSeq: 1 INVITE
         Contact: <sip:+19175550100@o-sbe1.example.com;user=phone
           transport=tcp>

   8.   I-SBE receives the request and queries its internal routing
        database on the TN.  It determines the target belongs to T-SSP.
        Since I-SBE is a B2BUA, I-SBE generates a new INVITE request to
        T-SSP.

         INVITE sip:+19175550100@.example.net;user=phone SIP/2.0
         Via: SIP/2.0/TCP i-sbe1.example.org:5060
           ;branch= z9hG4bK2d4777;
         Max-Forwards: 10
         Route: <sip:t-sbe1.example.net;lr>
         From: Alice <sip:+14085550101@example.com;user=phone>
           ;tag=54321
         To: Bob <sip:+19175550100@example.net;user=phone>
         Call-ID: abcde-isbe1
         CSeq: 1 INVITE
         Contact: <sip:+19175550100@i-sbe1.example.org;user=phone
           transport=tcp>

        Note that if I-SSP wants the media to traverse through the
        I-DBE, I-SBE must modify the SDP in the Offer to point to its
        DBE.

   9.   T-SBE determines the called party home proxy and directs the
        call to the called party.  T-SBE may use ENUM or other internal
        mechanism to locate the home proxy.  If T-SSP uses ENUM, this
        internal ENUM entry is different from the external ENUM entry
        populated for O-SSP.  This internal ENUM entry will contain the
        information to identify the next-hop to reach the called party.
        In this example, the internal ENUM query returns the UAS's home
        proxy.








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         $ORIGIN 0.0.1.0.5.5.5.7.1.9.1.e164.arpa.
         IN NAPTR (
           10
           100
           "u"
           "E2U+SIP"
           "!^.*$!sip:+19175550100@t-proxy.example.net!"
           . )

        Note that this step is optional.  If T-SBE has other ways to
        locate the UAS home proxy, T-SBE can skip this step and send the
        request to the UAS's home proxy.  We show this step to
        illustrate one of the many possible ways to locate UAS's home
        proxy.

   10.  T-SBE receives the NAPTR record and query DNS for the A record
        of "t-proxy.example.net".  The DNS returns an A record:

          ;; DNS ANSWER
          t-proxy.example.net.   IN A   192.0.2.2

   11.  T-SBE sends the INVITE to UAS's home proxy:

         INVITE sip:+19175550100@t-proxy.example.net;user=phone SIP/2.0
         Via: SIP/2.0/TCP t-sbe1.example.net:5060
           ;branch= z9hG4bK28uyyy;
         Max-Forwards: 10
         Record-Route: <sip:t-sbe1.example.net:5060;lr>
         From: Alice <sip:+14085550101@example.com;user=phone>
           ;tag=54321
         To: Bob <sip:+19175550100@example.net;user=phone>
         Call-ID: abcde-tsbe1
         CSeq: 1 INVITE
         Contact: <sip:+19175550100@t-sbe1.example.com;user=phone
           transport=tcp>

   12.  Finally, UAS's home proxy forwards the INVITE request to UAS.














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         INVITE sip:+19175550100@server.example.net;user=phone SIP/2.0
         Via: SIP/2.0/TCP t-proxy.example.net:5060
           ;branch= z9hG4bK28u111;
         Via: SIP/2.0/TCP t-sbe1.example.net:5060
           ;branch= z9hG4bK28uyyy; received=192.2.0.100
         Max-Forwards: 9
         Record-Route: <sip:t-proxy.example.net:5060;lr>,
           <sip:t-sbe1.example.net:5060;lr>
         From: Alice <sip:+14085550101@example.com;user=phone>
           ;tag=54321
         To: Bob <sip:+19175550100@example.net;user=phone>
         Call-ID: abcde-tsbe1
         CSeq: 1 INVITE
         Contact: <sip:+19175550100@t-sbe1.example.com;user=phone
           transport=tcp>

   13.  RTP is established between UAC and UAS.

5.4.1.  Administrative characteristics

   This use case looks very similar to the Static Direct Peering with
   Assisting LUF and LRF.  The major difference is the O-SSP and T-SSP
   do not have direct Layer-5 connectivity.  Instead, O-SSP connects to
   T-SSP indirectly via I-SSP.

   Typically, a LUF/LRF provider serves multiple O-SSPs.  Two O-SSPs may
   use different I-SSP to reach the same T-SSP.  For example, O-SSP1 may
   use I-SSP1 to reach T-SSP, but O-SSP2 may use I-SSP2 to reach T-SSP.
   Given the O-SSP and T-SSP pair as input, the LUF/LRF provider will
   return the SED of I-SSP that is trusted by O-SSP to forward the
   request to T-SSP.

   In this use case. there are two levels of trust relationship.  First
   trust relationship is between the O-SSP and LUF/LRF provider.  The
   O-SSP trusts the LUF/LRF to provide the T-SSP's SED.  Second trust
   relationship is between O-SSP and I-SSP.  The O-SSP trusts the I-SSP
   to provide Layer-5 connectivity to assist the O-SSP to reach T-SSP.
   The O-SSP and I-SSP have a pre-arranged agreement for policy.  Note
   that Figure 4 shows a single provider to provide both LUF/LRF and
   I-SSP, O-SSP can choose two different providers.

5.4.2.  Options and Nuances

   Similar to the Static Direct Peering Use Case, the O-SSP and T-SSP
   may deploy SBE and DBE for NAT traversal, security, transcoding, etc.
   I-SSP can also deploy SBE and DBE for similar reasons. (as depicted
   in Figure 4)




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5.5.  Static Indirect Peering Use Case

   This use case O-SSP uses its internal LUF/LRF.  One of the reasons of
   using internal LUF/LRF is to control the routing database.  By
   controlling the database, O-SSP can apply different routing rules and
   policies to different T-SSPs.  For example, O-SSP can use I-SSP1 and
   Policy-1 to reach T-SSP1, and use I-SSP2 and Policy-2 to reach
   T-SSP2.  Note that there could be multiple I-SSPs and multiple SIP
   routes to reach the same T-SSP; this is out of scope of Speermint and
   has become a focus in the IETF DRINKS working group.

      +--------------------+-------------------+---------------------+
      |       O-SSP        |       I-SSP       |         T-SSP       |
      |      +-----+       |                   |        +-----+      |
      |     -+O-LUF|       |                   |        |T-LUF|      |
      |    / |O-LRF+\      |                   |   +----+T-LRF|      |
      |   /  +-----+ \     |                   |  /     +-----+      |
      |  /(2)         \(4,5,6)                 | /(9,10)             |
      |+-------+     +-----+      +-----+      +-----+      +-------+|
      ||O-Proxy|-(3)-|O-SBE+--(7)-+I-SBE+-(8)--+T-SBE+-(11)-|T-Proxy||
      |+-------+     +-----+      +-----+      +-----+      +-------+|
      |    |               |                   |                |    |
      |   (1)              |                   |               (12)  |
      |    |               |                   |                |    |
      | +-----+      +-----+      +-----+      +-----+       +-----+ |
      | | UAC +=(13)=+O-DBE+======+I-DBE+======+T-DBE+=======+ UAS | |
      | +-----+      +-----+      +-----+      +-----+       +-----+ |
      +--------------------------------------------------------------+
           example.com          example.org          example.net


                Indirect Peering via I-SSP (SIP and media)

                                 Figure 5

5.5.1.  Administrative characteristics

   The Static Indirect Use Case is implemented in cases where no direct
   interconnection exists between the originating and terminating
   domains due to either business or physical constraints.

   O-SSP <---> I-SSP = Relationship O-I

   In the O-I relationship, typical policies, features or functions that
   deem this relationship necessary are number portability, Ubiquity of
   termination options, security certificate management and masquerading
   of originating VoIP network gear.




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   T-SSP <---> I-SSP = Relationship T-I

   In the T-I relationship, typical policies, features or functions
   observed consist of codec "scrubbing", anonymizing, and transcoding.
   I-SSP must record-route and stay in the signaling path.  T-SSP will
   not accept message directly sent from O-SSP.

5.5.2.  Options and Nuances

   In Figure 5, we show I-DBE.  Using I-DBE is optional.  One scenario
   the I-DBE can be used is when the O-SSP and T-SSP do not have a
   common codec.  To involve I-DBE, I-SSP should know the list of codec
   supported by O-SSP and T-SSP.  When I-SBE receives the INVITE
   request, it will make a decision to invoke the I-DBE.  Another
   scenario an I-DBE can be used is if O-SSP uses SRTP [RFC3711] for
   media and T-SSP does not support SRTP.

5.6.  On-demand Peering Use Cases

   On-demand Peering [RFC5486] describes two SSPs form the peering
   relationship without a pre-arranged agreement.

   The basis of this use case is built on the fact that there is no pre-
   established relationship between the O-SSP and T-SSP.  The O-SSP and
   T-SSP does not share any information prior to the dialog initiation
   request.  When the O-Proxy invokes the LUF and LRF on the Request-
   URI, the terminating user information must be publicly available.
   Besides, when the O-Proxy routes the request to the T-Proxy, the
   T-Proxy must accept the request without any pre-arranged agreement
   with O-SSP.

5.6.1.  Administrative characteristics

   The On-demand Direct Peering Use Case is typically implemented in a
   scenario where the T-SSP allows any O-SSP to reach its serving
   subscribers.  T-SSP administrative domain does not require any pre-
   arranged agreement to accept the call.  The T-SSP makes its
   subscribers information available in public.  This model mimics the
   Internet email model.  Sender does not need an pre-arranged agreement
   to send email to the receiver.

5.6.2.  Options and Nuances

   Similar to the Static Direct Peering Use Case, the O-SSP and T-SSP
   can decide to deploy SBE.  Since T-SSP is open to the public, T-SSP
   is considered to be in higher security risk than static model because
   there is no trusted relationship between O-SSP and T-SSP.  T-SSP
   should protect itself from any attack launch by untrusted O-SSP.



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6.  Acknowledgments

   Michael Haberler, Mike Mammer, Otmar Lendl, Rohan Mahy, David
   Schwartz, Eli Katz and Jeremy Barkan are the authors of the early
   individual drafts.  Their use cases are captured in this document.
   Besides, Jason Livingood, Daryl Malas, David Meyer, Hadriel kaplan,
   John Elwell, Reinaldo Penno, Sohel Khan, James McEachern, Jon
   Peterson, Alexander Mayrhofer, and Jean-Francois Mule made many
   valuable comments to this document.


7.  Security Considerations

   This document introduces no new security consideration.  However, it
   is important to note that session interconnect, as described in this
   document, has a wide variety of security issues that should be
   considered in documents addressing both protocol and use case
   analyzes.  [I-D.niccolini-speermint-voipthreats] discuss the
   different security threats related to VoIP peering.


8.  IANA Considerations

   This document creates no new requirements on IANA namespaces
   [RFC5226].


9.  References

9.1.  Normative References

   [I-D.niccolini-speermint-voipthreats]
              Niccolini, S., Chen, E., Seedorf, J., and H. Scholz,
              "SPEERMINT Security Threats and Suggested
              Countermeasures", draft-niccolini-speermint-voipthreats-05
              (work in progress), October 2008.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

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



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              June 2002.

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

   [RFC3403]  Mealling, M., "Dynamic Delegation Discovery System (DDDS)
              Part Three: The Domain Name System (DNS) Database",
              RFC 3403, October 2002.

   [RFC3761]  Faltstrom, P. and M. Mealling, "The E.164 to Uniform
              Resource Identifiers (URI) Dynamic Delegation Discovery
              System (DDDS) Application (ENUM)", RFC 3761, April 2004.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

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

9.2.  Informative References

   [I-D.ietf-drinks-cons-rqts]
              Schwartz, D., Mahy, R., Duric, A., and E. Lewis,
              "Consolidated Provisioning Problem Statement",
              draft-ietf-drinks-cons-rqts-00 (work in progress),
              July 2008.

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              December 1998.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

   [RFC4366]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
              and T. Wright, "Transport Layer Security (TLS)
              Extensions", RFC 4366, April 2006.









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Authors' Addresses

   Adam Uzelac (editor)
   Global Crossing
   U.S.A.

   Phone:
   Email: adam.uzelac@globalcrossing.com
   URI:   http://www.globalcrossing.com


   Yiu L.Lee (editor)
   Comcast Cable
   U.S.A.

   Phone:
   Email: yiu_lee@cable.comcast.com
   URI:   http://www.comcast.com

































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