


SPEERMING Working Group                                   A. Uzelac, Ed.
Internet-Draft                                           Global Crossing
Intended status: Informational                               Y. Lee, Ed.
Expires: October 30, 2010                                  Comcast Cable
                                                          April 28, 2010


                       VoIP SIP Peering Use Cases
           draft-ietf-speermint-voip-consolidated-usecases-18

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.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on October 30, 2010.

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   Copyright (c) 2010 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
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   the Trust Legal Provisions and are provided without warranty as



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   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3

   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3

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

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

   5.  Use Cases  . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     5.1.  Static Peering Use Cases . . . . . . . . . . . . . . . . .  5
     5.2.  Static Direct Peering Use Case . . . . . . . . . . . . . .  5
       5.2.1.  Administrative characteristics . . . . . . . . . . . . 10
       5.2.2.  Options and Nuances  . . . . . . . . . . . . . . . . . 11
     5.3.  Static Direct Peering Use Case - Assisting LUF and LRF . . 11
       5.3.1.  Administrative Characteristics . . . . . . . . . . . . 12
       5.3.2.  Options and Nuances  . . . . . . . . . . . . . . . . . 13
     5.4.  Static Indirect Peering Use Case - Assisting LUF and
           LRF  . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
       5.4.1.  Administrative characteristics . . . . . . . . . . . . 19
       5.4.2.  Options and Nuances  . . . . . . . . . . . . . . . . . 20
     5.5.  Static Indirect Peering Use Case . . . . . . . . . . . . . 20
       5.5.1.  Administrative characteristics . . . . . . . . . . . . 21
       5.5.2.  Options and Nuances  . . . . . . . . . . . . . . . . . 21
     5.6.  On-demand Peering Use Case . . . . . . . . . . . . . . . . 22
       5.6.1.  Administrative characteristics . . . . . . . . . . . . 22
       5.6.2.  Options and Nuances  . . . . . . . . . . . . . . . . . 22

   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 22

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

   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 describes important Voice over IP (VoIP) use cases
   Session Initiation Protocol (SIP) [RFC3261] based peering.  These use
   cases are determined by the SPEERMINT working group and 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), Video Chat, and Presence are out of scope for this document.

   The use cases contained in this document are described 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 SIP request.
   Terminating SSP (T-SSP) is the SSP terminating the SIP 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 some elements included in Figure 1 are optional.


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.  After obtaining the SED, the O-SSP may use
      the standard procedure defined in [RFC3263] to discover the next



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      hop address.

   o  Call setup - SSPs that are interconnecting to one another may also
      define specifics on what peering policies 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 [RFC5246] 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.



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 this document.

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 the peer SSPs are directly
   connected.  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
         <allOneLine>
         Contact: <sip:+19175550100@client.example.com;user=phone;
         transport=tcp>
         </allOneLine>

        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.




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   2.   UAC knows UAS's TN, but does not know 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 signifies 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 <sip:+19175550100@example.com;user=phone>
         Call-ID: abcde
         CSeq: 1 INVITE
         <allOneLine>
         Contact: <sip:+19175550100@client.example.com;user=phone;
         transport=tcp>
         </allOneLine>

   4.   O-SBE receives the requests and pops the top entry of the Route
        header which contains "o-sbe1.example.com".  O-SBE examines the
        Request-URI and does a LRF for "example.net".  In this example,



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        the LRF is a NAPTR DNS query [RFC3403] of the domain name.
        O-SBE receives a NAPTR response from LRF.  The response looks
        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." to O-LRF.

        ;;     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 generate a
        new INVITE request in next step.  If O-SBE 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: 8
         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
         <allOneLine>
         Contact: <sip:+19175550100@o-sbe1.example.com;user=phone;
         transport=tcp>
         </allOneLine>

        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 lookup or other
        internal mechanism to locate the home proxy.  If T-SSP uses ENUM
        lookup, 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 following the requirements
        in [RFC3263] queries DNS for the SRV records indicated by the
        NAPTR result.  Not finding any, the T-SBE then queries DNS for
        the A record of domain "t-proxy.example.net.".

          ;; 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: 7
         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
         <allOneLine>
         Contact: <sip:+19175550100@t-sbe1.example.net;user=phone;
         transport=tcp>
         </allOneLine>

   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: 6
         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
         <allOneLine>
         Contact: <sip:+19175550100@t-sbe1.example.net;user=phone;
         transport=tcp>
         </allOneLine>

   12.  RTP is established between the UAC and UAS.  Note that the media
        shown in Figure 2 passes through O-DBE and T-DBE, but the use of
        DBE is optional.

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.






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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, the
   O-SSP may use [RFC1918] addresses for their UA and proxies.  These
   addresses are not routable in the 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.
   This decision does not affect the network's ability to identify the
   next-hop address.

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

   This use case shares many properties with the Static Direct Peering
   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 involve the LUF/LRF provider instead of
   happening in O-SSP domain.

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

5.3.1.  Administrative Characteristics

   The LUF/LRF provider provides the LUF and LRF services for the O-SSP.
   Taken together 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.

5.3.2.  Options and Nuances

   The LUF/LRF provider can use multiple methods to provide SED to
   O-SSP.  The most commonly used are an ENUM lookup 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 LUF/LRF providers must be populated with the T-SSP's AORs and
   SED.  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-usecases-requirements].

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 maintained by O-SSP and
   T-SSP.  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 through one or
   multiple I-SSP(s) to reach T-SSP.

   In addition, O-SSP is using 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.  Using LUF/LRF
   provider allows the T-SSP to populate its SED once, while any O-SSP
   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 must 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
         <allOneLine>
         Contact: <sip:+19175550100@client.example.com;user=phone;
         transport=tcp>
         </allOneLine>



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   2.   UAC knows UAS's TN, but does not know 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 I-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.


















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         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
         <allOneLine>
         Contact: <sip:+19175550100@client.example.com;user=phone;
         transport=tcp>
         </allOneLine>

   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." to
        O-LRF.

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



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   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 generate a new back-to-back INVITE request in
        next step.  If O-SBE chooses to act as proxy, it should record-
        route to stay in the call path.  In this example, O-SBE is a
        B2BUA.

         INVITE sip:+19175550100@example.org;user=phone SIP/2.0
         Via: SIP/2.0/TCP o-sbe1.example.com:5060
           ;branch= z9hG4bK2d4zzz
         Max-Forwards: 8
         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
         <allOneLine>
         Contact: <sip:+19175550100@o-sbe1.example.com;user=phone;
         transport=tcp>
         </allOneLine>

   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.
















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         INVITE sip:+19175550100@.example.net;user=phone SIP/2.0
         Via: SIP/2.0/TCP i-sbe1.example.org:5060
           ;branch= z9hG4bK2d4777
         Max-Forwards: 7
         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
         <allOneLine>
         Contact: <sip:+19175550100@i-sbe1.example.org;user=phone;
         transport=tcp>
         </allOneLine>

        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 lookup or other
        internal mechanism to locate the home proxy.  If T-SSP uses ENUM
        lookup, 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.

         $ORIGIN 0.0.1.0.5.5.5.7.1.9.1.e164.arpa.
         IN NAPTR (
           10
           100
           "u"
           "E2U+SIP"
           "!^.*$!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 following the requirements
        in [RFC3263] queries DNS for the SRV records indicated by the
        NAPTR result.  Not finding any, the T-SBE then queries DNS for
        the A record of domain "t-proxy.example.net.".




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          ;; 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: 6
         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
         <allOneLine>
         Contact: <sip:+19175550100@t-sbe1.example.com;user=phone;
         transport=tcp>
         </allOneLine>

   12.  Finally, UAS's home proxy forwards the INVITE request to 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: 5
         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
         <allOneLine>
         Contact: <sip:+19175550100@t-sbe1.example.com;user=phone;
         transport=tcp>
         </allOneLine>

   13.  In Figure 4, RTP is established between UAC and UAS via O-DBE,
        I-DBE and T-DBE.  The use of DBE is optional.

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



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   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, but 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)

5.5.  Static Indirect Peering Use Case

   This use case shares many properties with the Static Indirect Use
   Case with Assisting LUF and LRF.  The difference is that the O-SSP
   uses its internal LUF/LRF 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; the selection process is out of scope
   of this document.















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      +--------------------+-------------------+---------------------+
      |       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 Peering 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.

   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 sent directly from O-SSP.

5.5.2.  Options and Nuances

   In Figure 5, we show I-DBE.  Using I-DBE is optional.  For example,
   the I-DBE can be used is when the O-SSP and T-SSP do not have a



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   common codec.  To involve I-DBE, I-SSP should know the list of codecs
   supported by O-SSP and T-SSP.  When I-SBE receives the INVITE
   request, it will make a decision to invoke the I-DBE.  An I-DBE may
   also be used if O-SSP uses SRTP [RFC3711] for media and T-SSP does
   not support SRTP.

5.6.  On-demand Peering Use Case

   On-demand Peering [RFC5486] describes how 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.
   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.

   The On-demand Peering Use Case is uncommon in production.  In this
   memo, we capture only the high-level descriptions.  Further analysis
   is expected when this use case becomes more popular.

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 publicly available.  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 launched by untrusted O-SSP.


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,



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   John Elwell, Reinaldo Penno, Sohel Khan, James McEachern, Jon
   Peterson, Alexander Mayrhofer, and Jean-Francois Mule made many
   valuable comments to this document.  The editors would also like to
   extend a special thank to Spencer Dawkins for his detailed review of
   this document.


7.  Security Considerations

   Session interconnect for VoIP, as described in this document, has a
   wide variety of security issues that should be considered.  For
   example, if the O-SSP and T-SSP peer through public Internet, O-SSP
   must protect the signaling channel and accept messages only from
   authorized T-SSP.  This document does not analyze the threats in
   details.  [I-D.ietf-speermint-voipthreats] discusses the different
   security threats and countermeasures related to VoIP peering.


8.  IANA Considerations

   Note to RFC Editor: please delete this section before publication.


9.  References

9.1.  Normative References

   [I-D.ietf-speermint-voipthreats]
              Niccolini, S., Chen, E., Seedorf, J., and H. Scholz,
              "SPEERMINT Security Threats and Suggested
              Countermeasures", draft-ietf-speermint-voipthreats-01
              (work in progress), July 2009.

   [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,
              June 2002.

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



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              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.

   [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-usecases-requirements]
              Channabasappa, S., "DRINKS Use cases and Protocol
              Requirements", draft-ietf-drinks-usecases-requirements-00
              (work in progress), May 2009.

   [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.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.


Authors' Addresses

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

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









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

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













































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