Internet Draft
SIP Working Group                                           W. Marshall
Internet Draft                                          K. Ramakrishnan
Document: <draft-dcsgroup-sip-arch-00.txt>                         AT&T
Category: Informational
                                                              E. Miller
                                                             G. Russell
                                                              CableLabs

                                                               B. Beser
                                                            M. Mannette
                                                        K. Steinbrenner
                                                                   3Com

                                                                D. Oran
                                                                  Cisco

                                                             J. Pickens
                                                                  Com21

                                                            P. Lalwaney
                                                             J. Fellows
                                                     General Instrument

                                                               D. Evans
                                                           Lucent Cable

                                                               K. Kelly
                                                               NetSpeak

                                                           F. Andreasen
                                                              Telcordia

                                                          October, 1999


    Architectural Considerations for Providing Carrier Class Telephony
     Services Utilizing SIP-based Distributed Call Control Mechanisms


   Status of this Memo

   This document is an Internet-Draft and is NOT offered in accordance
   with Section 10 of RFC2026[1], and the author does not provide the
   IETF with any rights other than to publish as an Internet-Draft.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as Internet-
   Drafts. Internet-Drafts are draft documents valid for a maximum of
   six months and may be updated, replaced, or obsoleted by other
   documents at any time. It is inappropriate to use Internet- Drafts
   as reference material or to cite them other than as "work in
   progress."


DCS Group    Category: Informational - Expiration 4/30/00            1

                           DCS Architecture               October 1999


   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   h    ttp://www.ietf.org/shadow.htm                                                                  l.

   The distribution of this memo is unlimited.  It is filed as , and expires April 30, 2000. Please send
   comments to the authors.



1. Abstract

   This document provides an overview of a SIP-based Distributed Call
   Signaling (DCS) architecture to support carrier class packet-based
   voice, video, and other real time multimedia services.  Companion
   documents [3,4,5,6,7] address a specific set of SIP 2.0 protocol
   extensions and usage rules that are necessary to implement the DCS
   architecture in an interoperable fashion.

   The DCS architecture takes advantage of endpoint intelligence in
   supporting telephony services without sacrificing the network's
   ability to provide value through mechanisms such as resource
   management, lookup of directory information and translation
   databases, routing services, security protection, and privacy
   enforcement.  At the same time, the architecture provides
   flexibility to allow evolution in the services that may be provided
   by endpoints and the network.

   DCS also takes into account the need to manage access to network
   resources and account for resource usage.  The SIP usage rules
   defined in the accompanying IDs specifically address the
   coordination between Distributed Call Signaling and dynamic quality
   of service control mechanisms for managing resources over the access
   network.  In addition, the DCS architecture defines the interaction
   needed between network provided call controllers, known as a "DCS-
   proxy" (also called Gate Controller in this draft on occasion) for
   supporting these services.


2. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in
   this document are to be interpreted as described in RFC-2119 [2].


3. Introduction

   This document provides an overview of a SIP-based Distributed Call
   Signaling (DCS) architecture to support carrier class packet-based
   voice, video, and other real time multimedia services.  The DCS

DCS Group    Category: Informational - Expiration 4/30/00            2

                           DCS Architecture               October 1999


   architecture and the corresponding SIP protocol enhancements
   (described in companion documents) are being developed as part of
   the cable industry's PacketCable initiative, managed out of
   CableLabs (see www.cablelabs.com). PacketCable is defining a series
   of interface specifications that will enable vendors to develop
   interoperable products for providing internet telephony and other
   multimedia services over DOCSIS-enabled cable data networks.
   The DCS architecture described herein has its roots in the DOSA work
   performed by AT&T Laboratories ["Distributed Open Signaling
   Architecture"; Kalmanek, Marshall, Mishra, Nortz, Ramakrishnan, et
   al.; October, 1998]. AT&T contributed DOSA to PacketCable in March,
   1999 as a framework proposal for DCS.  Earlier exploratory work by
   AT&T and a select group of vendors had demonstrated that the DOSA
   concepts could be expressed in terms of SIP signaling messages.
   Since the AT&T submission, a relatively large group of vendors have
   cooperated in an intensive effort to develop the DCS architecture
   and SIP protocol extensions described here and in the accompanying
   protocol drafts.  Although DCS was originally designed with cable
   access networks in mind, the SIP signaling enhancements have general
   applicability to carrier class VOIP services running over QoS
   enabled IP networks.

   The authors are submitting this draft to the IETF in order to
   provide general information regarding the DCS architecture and to
   convey the motivation behind the SIP enhancements recommended in the
   accompanying protocol drafts.  We believe that incorporation of the
   concepts and mechanisms described in this set of drafts by the IETF
   into the SIP standard will significantly enhance SIP's ability to
   function as a carrier-class signaling protocol.  Such an enhancement
   to SIP would undoubtedly aid in its widespread acceptance and
   deployment. The PacketCable Draft Specification for DCS is available
   from the CableLabs website at:
   ftp://ftp.cablelabs.com/pub/ietfdocs/dcsdraft.pdf.


3.1 Background and Motivation

   The design of the Distributed Call Signaling (DCS) architecture
   recognizes the trend towards use of packet networks as the
   underlying framework for communications.  These networks will
   provide a broad range of services, including traditional best-effort
   data service as well as enhanced, value-added services, such as
   telephony. At the same time, improvements in silicon will reinforce
   the trend towards increased functionality in endpoints.  These
   intelligent endpoints will take advantage of the widespread
   availability of packet networks to enable a rich set of applications
   and services for users.

   However, when the network is used for real-time telephony
   applications, it is essential to have service differentiation at the
   IP layer.  The ability to control and monitor usage is needed for
   the provider to be able to provide service differentiation and to
   derive revenue from the enhanced services.  At the same time, the


DCS Group    Category: Informational - Expiration 4/30/00            3

                           DCS Architecture               October 1999


   availability of best effort communications and the migration of
   functionality to the endpoints pose a challenge to the provider to
   find incentives for users to use or pay for enhanced services.

   We see three key functions that a provider can offer, as incentives
   to use enhanced services.  First, the network service provider has
   the unique ability to manage and provide network layer quality of
   service.  When users depend on the quality of the service, as with
   telephony, there is a strong incentive to use the enhanced service,
   rather than a best effort service.  Second, the network service
   provider can play an important role as a trusted intermediary.  This
   includes ensuring the integrity of call routing, as well as ensuring
   both the accuracy and the privacy of information that is exchanged.
   The service provider can also add value by ensuring that services
   are provided consistently and reliably, even when an endpoint is
   unavailable.  Finally, there are a number of services that may be
   offered more efficiently by the network service provider rather than
   in endpoints.  For example, conference bridging may be more cost
   effective to implement in a multi-point bridge rather than in every
   endpoint attached to the network.

   A key contribution of the DCS architecture is a recognition of the
   need for coordination between call signaling, which controls access
   to telephony specific services, and resource management, which
   controls access to network-layer resources. This coordination is
   designed to meet the user expectations and human factors associated
   with telephony.   For example, the called party should not be
   alerted until the resources necessary to complete the call are
   available.  If resources were not available when the called party
   picked up, the user would experience a call defect.   In addition,
   users expect to be charged for service only after the called party
   answers the phone.  As a result, usage accounting starts only after
   the called party picks up.  Coordination between call signaling and
   resource management is also needed to prevent fraud and theft of
   service.  The coordination between DCS and Dynamic QoS protocols
   ensures that users are authenticated and authorized before receiving
   access to the enhanced QoS associated with the telephony service.

   It is important to be able to deploy a residential telephone service
   at very large scale, cost-effectively.   To achieve this, DCS
   minimizes the messaging overhead on network call servers, and does
   not require these servers to maintain call state for active calls.
   Once a call is established, call state is maintained only where it
   is needed: at the endpoints that are involved in the call, and at
   the ERs in the bearer path that are providing differentiated service
   to the media flow.  This allows the network call servers to scale to
   support more users, and imposes less stringent reliability
   requirements on those servers.

   DCS is also designed so that calling users receive consistent
   service even when a called endpoint is unavailable.  For example,
   when an endpoint is unavailable service logic in a network call
   server can forward telephone calls to a voice mailbox.

DCS Group    Category: Informational - Expiration 4/30/00            4

                           DCS Architecture               October 1999




3.2  Requirements And Design Principles

   In this section, we briefly describe the application requirements
   that led to a set of DCS signaling design principles.  In its most
   basic implementation, DCS supports a residential telephone service
   comparable to the local telephone services offered today.  In
   addition to the commonly used service features that need to be
   supported, there are important requirements in the areas of
   reliability, performance, and scalability that influence the
   signaling architecture. Supporting an IP telephony service
   comparable to the telephony service offered today requires enhanced
   bearer channel and signaling performance, including:

   @ Low delay - end-to-end packet delay must be small enough that it
     does not interfere with normal voice conversations. The ITU
     recommends no greater than 300 ms roundtrip delay for telephony
     service.

   @ Low packet loss - packet loss must be small enough to not
     perceptibly impede voice quality or performance of fax and voice
     band modems.

   @ Short post-dial delay - the delay between the user dialing the
     last digit and receiving positive confirmation from the network
     must be short enough that users do not perceive a difference with
     post-dial delay in the circuit switched network or believe that
     the network has failed.

   @ Short post pickup delay - the delay between a user picking up a
     ringing phone and the voice path being cut through must be short
     enough so that the "hello" is not clipped.

   We identify a number of key design principles that arise from the
   requirements and philosophy outlined above.

   1.      Providing differentiated network-layer quality of service is
     essential, while allowing the provider to derive revenues from the
     use of such differentiated services.

   2.      The architecture should allow, and even encourage, implementation
     of services and features in the intelligent endpoints, where
     economically feasible, while still retaining value in the network
     and network-based services

   3.      The architecture must ensure that the network is protected from
     fraud and theft of service. The service provider must be able to
     authenticate users requesting service and ensure that only those
     authorized to receive a particular service be able to obtain it.

   4.      The architecture must enable the service provider to add value by
     supporting the functions of a trusted intermediary. This includes


DCS Group    Category: Informational - Expiration 4/30/00            5

                           DCS Architecture               October 1999


     protecting the privacy of calling and called party information,
     and ensuring the accuracy of the information that is provided in
     messages from the network.

   5.      The architecture must enable the service provider to give a
     consistent view of basic services and features even when customer
     premise equipment is unavailable, and allow users to take
     advantage of functionality that is provided in the network, when
     it is cost-effective and easy to use.

   6.      The architecture must be implementable, cost-effectively, at very
     large scale.


3.3 Distributed Call Signaling Architecture

   The Distributed Call Signaling Architecture follows the principles
   outlined above to support a robust telephony service.  Figure 1
   introduces the key components in the architecture.

   The architecture assumes a broad range of DCS-compliant endpoints
   that provide telephony service to the user including Media Terminal
   Adapters (MTAs) that may be integrated with a Cable Modem or is a
   standalone device, as well as other endpoints such as personal
   computers.  The access network interfaces to an IP backbone through
   a system we refer to as the Edge Router (ER). The ER is the first
   trusted element within the provider's network and is considered to
   be the edge of the network for providing access to differentiated
   quality of service. The ER performs resource management, acts as a
   policy enforcement point and as a source of billing information.

   DCS-proxies (GCs) process call signaling messages and support number
   translation, call routing, feature support and admission control.
   In the context of SIP, a DCS-proxy is a SIP proxy that is involved
   in processing and forwarding of SIP requests.  GCs act as trusted
   decision points for controlling when resources are committed to
   particular users.  Media servers represent network-based components
   that operate on media flows to support the service.  Media servers
   perform audio bridging, play terminating announcements, provide
   interactive voice response services, etc.   Finally, PSTN gateways
   interface to the Public Switched Telephone Network.














DCS Group    Category: Informational - Expiration 4/30/00            6

                           DCS Architecture               October 1999


                 +-----+
                 | MTA |                MTA: media terminal adapter
                 +-----+
                    |                   ER: Edge Router
    Access Network  |
                    |
                 +----+
                 | ER |
                 +----+
                    |    +-----------+
                    |----| DCS Proxy |
                    |    +-----------+
                    |
                    |    +------------+
  Backbone Network  |----|Media Server|
                    |    +------------+
                    |
                    |    +------------+
                    |----|PSTN Gateway|
                    |    +------------+
                 +----+
                 | ER |
                 +----+
                    |
    Access Network  |
                    |
                 +-----+
                 | MTA |
                 +-----+
         Figure 1: A Simple view of Components of an IP Telephony
               Architecture used in a HFC Cable Environment.

   Telephony endpoints are considered to be "clients" of the telephony
   service.  Consistent with the design principles, the architecture
   allows a range of services to be implemented by intelligent
   endpoints.  They collect dialed digits, participate in signaling and
   contain the service logic required for basic call setup and feature
   support.  Endpoints also participate in end-to-end capability
   negotiation. However, endpoints are not trusted to provide accurate
   information to the network or to keep information that is received
   private, except when it is in the endpoint's best interests to do
   so.

   Access to network resources on a differentiated basis is likely to
   be controlled by the service provider. The ER receives resource
   management requests from endpoints, and is responsible for ensuring
   that packets are provided the QoS they are authorized to receive
   (either through packet marking, or through routing and queueing the
   packets as a specific QoS assured flow). The ER requires
   authorization from a network entity (on a call-by-call basis for the
   telephony service) before providing access to enhanced QoS for an
   end-to-end IP flow. The obvious point where this policy and control
   function resides is the DCS-proxy (also called gate-controller,

DCS Group    Category: Informational - Expiration 4/30/00            7

                           DCS Architecture               October 1999


   because of this responsibility for managing access to enhanced QoS).
   Thus, the ER is able to ensure that enhanced QoS is only provided
   for end-to-end flows that have been authorized and for which usage
   accounting is being done.  Since the ER knows about the resource
   usage associated with individual IP flows, it generates the usage
   events that allow a user to be charged for service.

   We introduce the concept of a "gate" in the ER, which manages access
   to enhanced quality of service. The gate is a packet classifier and
   policer that ensures that only those IP flows that have been
   authorized by the DCS-proxy are granted access to enhanced QoS in
   the access and backbone networks.  Gates are "opened" selectively
   for a flow. For the telephony service, they are opened for
   individual calls.  Opening a gate involves an admission control
   check that is performed when a resource management request is
   received from the endpoint for an individual call, and it may
   involve resource reservation in the network for the call if
   necessary. The packet filter in the gate allows a flow of packets to
   receive enhanced QoS for a call from a specific IP source address
   and port number to a specific IP destination address and port
   number.

   The DCS-proxy, in addition to implementing many of the call control
   functions, is responsible for the policy decision regarding whether
   the gate should be opened.  DCS sets up a gate in advance of a
   resource management message.  This allows the policy function, which
   is at the DCS-proxy, to be "stateless" in that it does not need to
   know the state of calls that are already in progress.

   DCS-proxies are typically organized in domains.  A DCS-proxy is
   responsible for a set of endpoints and the associated ERs.  While
   endpoints are not trusted, there is a trust relationship between the
   ER and its associated DCS-proxy, since the DCS-proxy plays a role as
   a policy server controlling when the ER can provide enhanced QoS
   service.  There is also a trust relationship among DCS-proxies.
   Details of the security model and mechanisms are work in progress.

   The DCS-proxy is designed as a simple transaction server, so that
   failure of a DCS-proxy does not affect calls in progress.  A domain
   will likely have both a primary and a secondary DCS-proxy.  If the
   primary DCS-proxy fails, only calls in a transient state are
   affected.  The endpoints involved in those calls will time out and
   retry.  All active calls are unaffected.  This is possible because
   the DCS-proxy retains no call state for stable calls. We believe
   this design makes the DCS-proxy efficient and highly scalable, and
   keeps the reliability requirements manageable.

   DCS supports inter-working with the circuit switched telephone
   network through PSTN gateways. A PSTN gateway may be realized as a
   combination of a media controller, media gateway, and a signaling
   gateway.  A media gateway acts as the IP peer of an endpoint for
   media packets, converting between the data format used over the IP
   network and the PCM format required for transmission over the PSTN.

DCS Group    Category: Informational - Expiration 4/30/00            8

                           DCS Architecture               October 1999


   The signaling gateway acts as the IP peer of an endpoint for
   signaling packets, providing signaling inter-working between DCS and
   conventional telephony signaling protocols such as ISUP/SS7. A media
   gateway control protocol is used to control the operation of the
   media gateway from the signaling gateway.

   There are additional system elements that may be involved in
   providing the telephony service.  For example, the DCS-proxy may
   interface with other servers that implement the authorization or
   translation functions.  Similarly, three way calling may be
   supported using media servers in the network.



3.4 Basic Call Flow

   Figure 2 presents a high-level overview of a basic MTA-to-MTA call
   flow in DCS.  Each MTA is associated with a DCS-proxy, which acts as
   a SIP proxy.  When a user goes off-hook and dials a telephone
   number, the originating MTA (MTA-o) collects the dialed digits and
   sends the initial call message, called an INVITE in SIP, to the
   "originating" DCS-proxy (DP-o).  DP-o verifies that MTA-o is a valid
   subscriber of the telephony service (using authentication
   information in the INVITE message) and determines whether this
   subscriber is authorized to place this call.  DP-o then translates
   the dialed number into the address of a "terminating" DCS-proxy (DP-
   t) and forwards the INVITE message to it.

   We assume that the originating and terminating DCS-proxies trust
   each other.  DP-o augments the INVITE message that it forwards with
   additional information, such as billing information containing the
   account number of the caller.  DP-t then translates the dialed
   number into the address of the terminating MTA (MTA-t) and forwards
   the INVITE message to MTA to notify it about the incoming call.

   The initial INVITE message invokes call feature handling at the
   destination MTA, such as call-forwarding.  Assuming that the call is
   not forwarded, MTA-t negotiates the coding style and bandwidth
   requirements for the media streams.  The 200 OK response to the
   initial INVITE is forwarded back through the DCS-proxies.














DCS Group    Category: Informational - Expiration 4/30/00            9

                           DCS Architecture               October 1999


   MTA-o      ER-o       DP-o       DP-t       ER-t      MTA-t
    | Invite   |          |          |          |          |
    |(Stage1) |          | Invite   |          |          |
    |----------|--------->|(Stage1) |           |          |
    |          |          |--------->|          |          |
    |          |          |          |  Invite  |          |
    |          |          |          |(Stage1) |          |
    |          |          |          |----------|--------->|
    |          |          |          |          |  200 OK  |
    |          |          |          |<---------|----------|
    |          |          |          |          |          |
    |          |  Gate    |  200 OK  |Gate Setup|          |
    |          |  Setup   |<---------|--------->|          |
    |          |<---------|          |          |          |
    |   200 OK |          |          |          |          |
    |<---------|----------|          |          |          |
    |   ACK    |          |          |          |          |
    |----------|----------|----------|----------|--------->|
    |          |          |  Reserve |          |          |
    |          |          | Resources|          |          |
    |<---------|----------|----------|----------|--------->|
    |  Invite  |          |          |          |          |
    |----------|--------- |----------|----------|--------->|
    |          |          |          |          | 180 Ring |
    |<---------|----------|----------|----------|----------|
    |          |          |          |          |          | User
    |          |          |          |          |  200 OK  | Answers
    |<---------|----------|----------|----------|----------|
    |   ACK    |          |          |          |          |
    |----------|----------|----------|----------|--------->|
    |          |          |  Active  |          |          |
    |          |          |   Call   |          |          |
    |<---------|--------- |----------|----------|--------->|
    |          |          |          |          |          | User
    |          |          |          |          |   BYE    | Hangs Up
    |<---------|----------|----------|----------|----------|
    |          |          |          |          |  200 OK  |
    |----------|----------|----------|----------|--------->|
   Figure 2: A Basic Call Flow, including Resource Management functions

   In the figure, MTA-t sends a 200 OK message to DP-t.  The 200 OK
   contains a subset of the capabilities in the INVITE message that are
   acceptable to MTA-t.  DP-t sends a GATE-SETUP message to the
   terminating ER (ER-t), conveying policy instructions allowing ER-t
   to open a gate for the IP flow associated with this phone call. The
   GATE_SETUP message contains billing information containing the
   account number of the subscriber that will pay for the call.

   DP-t forwards the 200 OK to DP-o.  DP-o sends a GATE-SETUP message
   to the originating ER (ER-o) to indicate that it can open a gate for
   the IP flow associated with the phone call.  Finally, DP-o forwards
   200 OK to MTA-o.  The initial INVITE request and 200 OK response
   contain a SIP Contact header to indicate the IP address of the

DCS Group    Category: Informational - Expiration 4/30/00           10

                           DCS Architecture               October 1999


   remote MTA to be used for subsequent end-to-end SIP signaling
   exchanges.  MTA-o acknowledges the 200 OK directly to MTA-t.

   Once the initial INVITE/200 OK exchange has completed, both MTAs
   reserve the resources that will be needed for the media streams.
   Once MTA-o has successfully made its reservation, it sends a second
   INVITE message to MTA-t containing a command to ring the destination
   telephone.  If MTA-t successfully reserved the resources needed for
   the call, it responds with a 180 Ringing to indicate that the phone
   is ringing, and that the calling party should be given a ringback
   call progress tone.  When the called party answers, by going off-
   hook, MTA-t sends a 200 OK final response, which MTA-o acknowledges.
   At this point the resources that were previously reserved are
   committed to this conversation, and the call is "cut through."

   Either party can terminate the call.  An MTA that detects an on-hook
   sends a SIP BYE message to the remote MTA, which is acknowledged.

4. Resource Management

   DCS's resource management protocols distinguish between two phases:
   Reserve and Commit.  During the first phase, resources are reserved
   but are not yet made available to the endpoint.  This ensures that
   resources are available before ringing the far-end telephone.  The
   second phase, which commits resources, is initiated after ringing
   the far end telephone and after the called party picks up.  At this
   point, resources are committed and made available to the endpoint,
   and recording is started so that the user can be billed for usage.
   The use of a two-phase protocol is essential because of the unique
   requirements associated with human communication, such as telephony.
   In addition to ensuring that resources are available before ringing
   the phone, it also preserves the semantics of billing that users are
   accustomed to, whereby usage recording is not started until the
   called party picks up the phone.  Backbone resources are reserved
   and allocated in the first phase of the two-phase resource
   reservation protocol.  This is important in order to limit the
   impact of backbone resource management on post-pickup delay (this
   minimizes the likelihood of clipping the first few syllables of the
   conversation).

5.   Distributed Call State

   In order to provide enhanced services to millions of endpoints, we
   need an architecture that can be implemented cost-effectively at
   very large scale.  Just as we enable flexibility by exploiting
   intelligence at the endpoints, services can be provided in a
   scaleable manner by storing the state associated with applications
   at the endpoints, rather than in network servers. Especially with
   telephony, endpoints are directly involved in handling calls and
   therefore need to maintain and use call state. In contrast, while
   network servers may need to be involved when setting up a call to
   gain access to enhanced QoS, there is no fundamental need for those
   servers to be involved throughout the lifetime of the call.


DCS Group    Category: Informational - Expiration 4/30/00           11

                           DCS Architecture               October 1999


   Maintaining state for every call at network servers, while
   achievable, increases the reliability requirements and load on the
   servers. The less state kept in the network, the better.

   As a result, the DCS-proxy in DCS are designed to be stateless
   transaction servers.  When a DCS-proxy processes a service request
   from an endpoint, it maintains state until the transaction is
   complete, but does not maintain any per-call state about active
   calls in the network.  There are two major advantages to this
   design.  First the reliability of the service does not depend on the
   reliability of an individual DCS-proxy. A DCS-proxy can fail without
   affecting calls that are currently in progress. Second, it removes
   many complex synchronization problems where two (or more) entities
   need to have simultaneously accurate information.  Since
   interactions with the DCS-proxies are simple stateless transactions,
   it is not necessary for consecutive calls to be processed by the
   same DCS-proxy.  DCS-proxy crashes affect only the transient calls
   (the number of calls in the process of being set up), and not stable
   conversations.  Further, it is likely that most calls in a transient
   state can be recovered and successfully established through a backup
   or spare DCS-proxy using endpoint retransmission, with no explicit
   synchronization protocol required between the DCS-proxies.  We
   believe this design principle will enable us to operate in very
   large scale, cost effectively.  Furthermore it places the function
   of managing the state of a call where it belongs - at the endpoint.
   An existing call can only be affected by failures along the path or
   by failure of the endpoints: there are no unnecessary elements
   involved in a call.

   We note that there are many services that involve the use of servers
   or proxy endpoints that communicate directly with clients.  Since
   these endpoints are directly involved in providing service, it is
   necessary and appropriate for them to maintain state.  Examples of
   proxy endpoints include application layer firewalls, caching
   servers, transcoders, network-based conference bridges, interactive
   voice response systems, and PSTN gateways.  The DCS architecture
   models these as end-points, that maintain appropriate call state.

   We now turn to the mechanisms that allow us to avoid state in the
   DCS-proxies. The DCS-proxy stores state information about an
   endpoint's calls at that endpoint in an object called a DCS-State
   string. DCS-state information are both encrypted and signed by the
   DCS-proxy to ensure the privacy and the integrity of the information
   contained in the string.  When needed, the endpoint provides the
   DCS-State to the DCS-proxy, which can use the information to provide
   additional functionality.  Because the DCS-State contains
   information encrypted by the DCS-proxy, the information it contains
   is trusted by the network even though the endpoint itself is not
   trusted.  In addition, the DCS-proxy stores service-specific opaque
   data associated with a call at the edge router.  Since charging for
   telephony services is tightly tied to the use of resources, this
   information is best stored at the edge router, where knowledge of
   resource usage exists.

DCS Group    Category: Informational - Expiration 4/30/00           12

                           DCS Architecture               October 1999



   The endpoint returns the information to the DCS-proxy when it is
   needed to implement specific features.  The endpoint cannot
   interpret the information in the string and any attempt to tamper
   with it can be detected by the DCS-proxy. There are a number of
   examples that arise in the implementation of telephony features.
   For example, when a user at an endpoint wants to return the last
   call (e.g., by dialing *69 on a traditional telephone) the "call
   return" function is invoked.  If the user had subscribed to the
   caller ID service feature, the terminating endpoint could store the
   information (phone number or IP address) associated with the last
   call.  However, it may be the case that the user does not subscribe
   to the feature, or the originator of the previous call may have
   requested that this information be blocked in order to retain
   privacy. In this case, call return can be implemented, while keeping
   the caller's identity private, by using a DCS-state string.

6. DCS Proxy - DCS Proxy Communications

   DCS-proxies implement a set of service-specific control functions
   required to support the telephony service:

   @ Authentication and authorization: Since services are only provided
     to authorized subscribers, DCS-proxies authenticate signaling
     messages and authorize requests for service on a call-by-call
     basis.

   @ Name/number translation and call routing: DCS-proxies translate
     dialed E.164 numbers, or names, to a terminating IP address based
     on call routing logic to support a wide range of call features.

   @ Service-specific admission control: DCS-proxies can implement a
     broad range of admission control policies for the telephony
     service.  For example, DCS-proxies may provide precedence for
     particular calls (e.g., 911 calls).  Admission control may also be
     used to implement overload control mechanisms, e.g. to restrict
     the number of calls to a particular location or to restrict the
     frequency of call setup to avoid signaling overload.

   @ Signaling and service feature support: While many service features
     are implemented by endpoints, the DCS-proxy also plays a role in
     feature support. DCS signaling provides a set of service
     primitives to end-points that are mediated by the DCS-proxy.  The
     DCS-proxy is involved in implementing service features that depend
     on the privacy of calling information, e.g., caller-ID blocking.
     It also plays a role in supporting service features that require
     users to receive a consistent view of feature operation even when
     an endpoint is down. For example, while an endpoint may normally
     participate in call forwarding, the DCS-proxy can control call
     forwarding on behalf of an endpoint when the endpoint is down.

   End-points MTA-o and MTA-t communicate through the DCS-Proxies DP-o
   and DP-t, as shown in Figure 2. The interface of concern in this


DCS Group    Category: Informational - Expiration 4/30/00           13

                           DCS Architecture               October 1999


   section is the one between the DCS-Proxies DP-o and DP-t. In
   contrast to a true stateless SIP proxy, the DCS-Proxy maintains
   transaction state. During the interval that a call is being setup, a
   DCS-Proxy keeps state related to a request until a response is
   received.

   For each call made to a phone number, DP-o may need to perform the
   functions needed for Local Number Portability (LNP). If a LNP
   database lookup is performed and the resulting dialed string is
   modified, DP-o must modify the Request-URI to include the result of
   the LNP lookup. When using a two-stage call setup, as described in
   [3], the information provided in the INVITE(Stage1), the originating
   proxy DP-o generates and stores the DCS-State: header. This
   information is intended to be sent to endpoint MTA-o and included
   with the final response that is returned to MTA-o. The originating
   DCS-Proxy, DP-o, may then use the call state information provided to
   it in the DCS-State: header to manipulate call-legs when requested
   by MTA-o.

   As with conventional SIP proxies, DP-o adds its address to the top
   of the Via: header list with a branch=1 field when forwarding the
   request (e.g., INVITE(Stage1)). In addition, to support billing
   functions for a carrier, DP-o appends opaque pieces of information
   called the Billing-Info: and Billing-ID:. In addition, to support
   the resource management functions (such as manipulating Gates for
   resource management in concert with call-leg manipulation), a Gate-
   Location: header is included. This allows for the subsequent
   generation of requests for access network QoS by the end-points.

   We also depend on originating DCS-Proxy, DP-o to be responsible for
   manipulating call legs. For instance, when a call is being
   forwarded, information about the new destination that the call is
   being forwarded to is provided by DP-t to DP-o. The new INVITE is
   then issued from DP-o. The information exchanged between the DCS-
   proxies enables such a function to be performed.

7. Privacy

   Many conventional telephony systems have the ability to provide
   information about the identity of the calling party to the called
   party before the latter accepts the call (such a capability is
   typically termed "Caller-ID"). Systems that support Caller-ID
   usually provide a mechanism that allows the calling party to
   instruct the network to refrain from delivering this information to
   the destination.

   In order for an IP-based network to provide a caller with a similar
   capability, a new SIP header is needed to signal the desire for
   anonymity to the network elements that would otherwise provide the
   caller's identity to the destination party. If a caller desires to
   remain anonymous, several additional changes to standard SIP are
   necessary.



DCS Group    Category: Informational - Expiration 4/30/00           14

                           DCS Architecture               October 1999


   In the cable IP network, the triplet {From:, To:, Call-ID:} is used
   to identify a call leg. Because call state information is pushed to
   the edge of the network, this information must be delivered to the
   destination endpoint.

   The SIP From: header normally contains information that identifies
   the caller. In order to hide the identity of the caller, the From:
   header information is encrypted with the originating endpoint's key.
   The destination endpoint does not possess the key to decrypt the
   From: information. No new syntax for SIP is introduced here.

   Normally, the SIP Call-ID: header also contains information about
   the caller. In the DCS architecture, to support privacy in cable-
   based IP telephony networks the value of Call-ID: header is a
   cryptographic hash string that contains no information about the
   user.

   Since all the normally-available mechanisms for passing information
   about the caller are no longer available, a new SIP header is used
   (Caller:) to pass the caller's identity to the destination if
   privacy has not been requested.  This header is introduced that
   contains the information that would normally be present in the From:
   header; the network passes it to the destination endpoint only if
   the caller has not requested anonymity.

   In addition to the usual privacy elements provided by telephone
   systems, IP-based systems must implement methods of hiding the
   source IP address from the destination if the caller requires
   privacy. The entire address must be obscured, since even a few
   address bits may provide partial location information. Likewise, IP
   addresses of the destination should not be revealed to the caller,
   in order to maintain privacy of transfer destinations.

   IP addresses typically appear in the Contact: header; they also
   appear in SDP descriptions contained in SIP messages. These must all
   be protected. The mechanism we choose is an application-level
   anonymizer that inspects the SIP call signaling messages and
   replaces any identifying information contained therein in a
   consistent manner and in such a way that when the messages are
   delivered to the destination endpoint any identifying information
   has been replaced with fields that obscure the identity of the party
   seeking privacy.

   This mechanism does not require any modification to the call
   signaling initiated by the endpoints: the application-level
   anonymizer performs these functions silently within the network.



8. Security Considerations

   Detailed security considerations related to this architecture will
   be addressed in a future companion draft.


DCS Group    Category: Informational - Expiration 4/30/00           15

                           DCS Architecture               October 1999



9. References

   1. Bradner, S., "The Internet Standards Process -- Revision 3", BCP
      9, RFC 2026, October 1996.

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

   3. "Integration of Resource Management and Call Signaling for IP
      Telephony", Internet Draft: ,
      June 1999.

   4.      "SIP Extensions for Caller Privacy", Internet Draft: , June 1999.

   5.      "SIP proxy-to-proxy extensions for supporting Distributed Call
     State", Internet Draft: <draft-dcsgroup-sip-proxy-proxy-00.txt>,
     June 1999.

   6.      "SIP extensions for supporting Distributed Call State", Internet
     Draft: , June 1999.

   7. "Integration of Call Authorization and Call Signaling for IP
      Telephony", Internet Draft: , June 1999.

10.  Acknowledgments

   The Distributed Call Signaling work in the PacketCable project is
   the work of a large number of people, representing many different
   companies.  The authors would like to recognize and thank the
   following for their assistance: John Wheeler, Motorola; David
   Boardman, Daniel Paul, Arris Interactive; Bill Blum, Jon Fellows,
   Jay Strater, Jeff Ollis, Clive Holborow, General Instruments; Doug
   Newlin, Guido Schuster, Ikhlaq Sidhu, 3Com; Jiri Matousek, Bay
   Networks; Farzi Khazai, Nortel; John Chapman, Bill Guckel, Michael
   Ramalho, Cisco; and Chuck Kalmanek, Doug Nortz, John Lawser, James
   Cheng, Tung-Hai Hsiao, and Partho Mishra, AT&T.


11. Author's Addresses

   Bill Marshall
   AT&T
   Florham Park, NJ  07932
   Email:            wtm@research.att.co                                                          m

   K. K. Ramakrishnan
   AT&T
   Florham Park, NJ  07932
   Email:            kkrama@research.att.co                                                                m


DCS Group    Category: Informational - Expiration 4/30/00           16

                           DCS Architecture               October 1999


   Ed Miller
   CableLabs
   Louisville, CO  80027
   Email:            E.Miller@Cablelabs.co                                                              m

   Glenn Russell
   CableLabs
   Louisville, CO  80027
   Email: G.Russell@Cablelabs.com

   Burcak Beser
   3Com
   Rolling Meadows, IL  60008
   Email: Burcak_Beser@3com.com

   Mike Mannette
   3Com
   Rolling Meadows, IL  60008
   Email: Michael_Mannette@3com.com

   Kurt Steinbrenner
   3Com
   Rolling Meadows, IL  60008
   Email: Kurt_Steinbrenner@3com.com

   Dave Oran
   Cisco
   Acton, MA  01720
   Email: oran@cisco.com

   John Pickens
   Com21
   San Jose, CA
   Email: jpickens@com21.com

   Poornima Lalwaney
   General Instrument
   San Diego, CA  92121
   Email: plalwaney@gi.com

   Jon Fellows
   General Instrument
   San Diego, CA  92121
   Email: jfellows@gi.com

   Doc Evans
   Lucent Cable Communications
   Westminster, CO  30120
   Email: n7dr@lucent.com

   Keith Kelly
   NetSpeak
   Boca Raton, FL  33587

DCS Group    Category: Informational - Expiration 4/30/00           17

                           DCS Architecture               October 1999


   Email: keith@netspeak.com

   Flemming Andreasen
   Telcordia
   Piscataway, NJ
   Email: fandreas@telcordia.com
















































DCS Group    Category: Informational - Expiration 4/30/00           18

                           DCS Architecture               October 1999



Full Copyright Statement

   "Copyright (C) The Internet Society (date). All Rights Reserved.
   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implmentation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph
   are included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.  The limited permissions granted above are perpetual and
   will not be revoked by the Internet Society or its successors or
   assigns.  This document and the information contained herein is
   provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE
   INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
   IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."

   Expiration Date This memo is filed as , and expires April 30, 2000.




























DCS Group    Category: Informational - Expiration 4/30/00           19