Internet Engineering Task Force                             Mike Pierce
     Internet Draft                                                    Artel
     draft-pierce-ieprep-assured-service-req-02.txt                 Don Choi
     January 2004                                                       DISA
     Expires July 2004
          Requirements for Assured Service Capabilities in Voice over IP
     Status of this memo
        This document is an Internet-Draft and is in full conformance with
        all provisions of Section 10 of RFC 2026.
        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-
        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".
        The list of current Internet-Drafts can be accessed at
        To view the list Internet-Draft Shadow Directories, see
        Copyright (C) Internet Society 2004. All rights reserved.
        Reproduction or translation of the complete document, but not of
        extracts, including this notice, is freely permitted.
        Assured Service refers to the set of capabilities used to ensure
        that critical communications are established and remain connected.
        This memo describes the requirements for such capabilities in
        support of real-time communications for voice using specific
        networks such as those used by government agencies, but they could
        also be applied in commercial environments.
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                                Table of Contents
     1.  Introduction.......................................................2
       1.1.  Changes........................................................3
     2.  Background.........................................................4
     3.  High Level Requirements............................................5
     4.  Functional Requirements............................................6
       4.1.  Precedence Level Marking.......................................6
       4.2.  Authentication/Authorization...................................7
       4.3.  Preferential Treatments........................................7
         4.3.1. Call/Session Treatment.......... ...........................7
         4.3.2. Packet Transfer Treatment........ ..........................8
         4.3.3. Procedural Requirements........... .........................8
       4.4.  Diversion if Not Answered......................................9
       4.5.  Notifications..................................................9
       4.6.  Acknowledge by Preempted Party................................10
       4.7.  Protection of Signaling/Routing Information from Disclosure...10
       4.8.  Accounting....................................................10
       4.9.  Call Control Signaling Precedence.............................11
       4.10. Interworking..................................................11
     5.  Security Considerations...........................................11
       5.1.  Authentication/authorization of User Access...................11
       5.2.  Security of Signaling Information.............................12
       5.3.  Security of Routing Data......................................12
       5.4.  Security of User Data.........................................12
     6.  IANA Considerations...............................................13
     7.  References........................................................13
     8.  Acknowledgements..................................................14
     9.  Authors' Addresses................................................14
     1.   Introduction
        Throughout many decades of evolution of the telephony network and
        its supporting protocols, there has been a need to provide
        preferential services and functionality to a limited subset of the
        users and calls within a network or domain in order to ensure
        completion of important calls that transit congested
        interconnections. Examples of this need have been in support of
        emergency traffic for natural disasters, network restoration
        traffic, and high priority traffic in a private networks. Provision
        of the required capabilities in the signaling protocols and within
        the switching systems has been defined in a number of national and
        international standards, most notably a service referred to as
        Multi-Level Precedence and Preemption (MLPP) as defined in an
        American National Standard [T1.619] in the US and in corresponding
        ITU-T Recommendations [I.255.3, Q.735.3, and Q.955.3]. In addition,
        a service called High Probability of Completion (HPC) was defined in
        the US [T1.631] and, most recently, two ITU-T Recommendations define
        the requirements for the International Emergency Preference Scheme
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        (IEPS) [E.106] and the International Preference Scheme for
        Multimedia Service in Support of Disaster Relief Operations [draft
        Other drafts submitted to the IETF have addressed aspects of IEPS.
        Among these are the Framework for ETS for Telephony over IP [ETS].
        MLPP was the solution for providing Assured Service capabilities
        within the circuit switched environment. It is essential that
        equivalent Assured Service capabilities are defined and implemented
        for the packet-based, connectionless environment of IP, and
        specifically SIP. Without these capabilities, SIP can not be used
        for those applications which require these capabilities.
        This memo builds on these references and identifies the specific
        requirements for Assured Service capabilities for Voice applications
        in support of these specific types of environments.  Although this
        memo covers only Voice, there will be similar requirements for
        "Assured Service" capabilities for all other forms of communication.
        The term "Assured Service" is used to refer to the required
        capabilities, rather than the previous term of "MLPP" or the related
        but different terms of IEPS or ETS, since the envisioned set of
        capabilities and protocols to achieve them are not expected to be
        exactly the same as those other services. For example, IEPS/ETS may
        not have a requirement for preemption at any point in the SIP
        session, whereas Assured Service may at both the session endpoint
        and in networks between endpoints.
        Although these requirements are derived from previous government
        applications, many of the same requirements and capabilities may be
        applied for non-government networks, for example, in support of
        commercial network restoration efforts. A presentation in the TEWG
        during the August 2001 meeting demonstrated real-life situations
        from the past in which total network failures required extensive
        efforts, presumably including communication via other unaffected
        networks, to bring the affected network back on line. If one
        considered a situation in which the very network which had failed
        was needed to carry the network management traffic required to get
        it back on line, it would be hard to imagine how it could ever be
        brought back up in the face of overwhelming customer attempts.
        Capabilities would be required to give priority to the network
        management traffic, even to the extent of blocking all non-emergency
        traffic for a period of time.
     1.1.   Changes
        Note: This section will be deleted before progressing as an RFC.
        This document has evolved through 2 different working groups:
        SIPPING and IEPREP. This draft was originally submitted under
        SIPPING with 2 revisions. It is now in the IEPREP WG in order to
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        ensure that the Assured Service requirements are considered along
        with those of the related IEPS discussions
        (SIPPING) -00 Original draft
        (SIPPING) -01 Indicated informative material which would not be a
        part of final. Moved some to Annex.
        (SIPPING) -02 Removed material to draft-pierce-sipping-pref-treat-
        examples-00 and draft-pierce-sipping-assured-service-arch-00.
        Added requirement to maintain records of use of service.
        (IEPREP) -00
        . Updated references.
        . Added additional requirements related to preferential treatment in
        . Added requirement in 4.8 for accounting records.
        . Added requirement in 4.9 that preferential treatment must be
            applied to call control signaling as well as to voice packets.
        . Added requirement in 4.10 for interworking between Assured Service
            and other priority schemes (e.g., IEPS)
        (IEPREP) -01
        -   Updated references
        -   Moved informative material to Annex
        -   Clarified requirement statements
        (IEPREP) -02
        -   clarified some text
        -   made individual requirements into bulleted, named items instead
            of freeform text
        -   moved additional informational material to a separate draft
     2.   Background
        In the circuit switched environment, specific circuits or channels
        are used for each call. These are typically 64 kbps channels which
        were normally part of a Time Division Multiplexed (TDM) transmission
        structure. These transmission channels are almost always
        interconnected and switched by Time Division Switching technology
        (often referred to as "TDM switching").
        More recent developments use packet/cell based transport instead of
        dedicated 64 kbps channels, often coupled with packet/cell-based
        switching, however, the effect is the same. There is still a
        dedicated transport capacity assigned for each call.
        Assured Service in the circuit switched environment may be provided
        by one or more of the following techniques.
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        .   Giving priority to return of dial tone (IEPS - note)
        .   Marking of signaling messages for better handling, for example,
            being last to be dropped in case of congestion in the signaling
            network (HPC)
        .   Extra routing possibilities for higher priority calls (IEPS -
        -   Queuing for network resources (HPC)
        .   Exemption from restrictive management controls such as hard-to-
            reach codes and code gapping (IEPS - note)
        .   Reservation of specific facilities (trunks) for higher priority
            traffic (IEPS - note)
        .   Higher priority calls may preempt existing lower priority calls,
            causing the network to release the lower priority call to free
            up resources for immediate reuse by the higher priority call
        (Note: Capabilities included within IEPS [E.106] are listed here for
        reference only but are not dealt with further in this document.)
        Identification of traffic authorized to use one or more of these
        techniques may be via the following or similar methods:
        .   Calls placed from physical lines or devices authorized for
            signaling a higher priority for a call
        .   Calls placed to specific telephone numbers or blocks of numbers
        .   Entry of a special ID code and PIN from any telephone device to
            identify that this call should receive special service.
        .   Use of a "smartcard"
     3.   High Level Requirements
        While the existing requirements and capabilities have been developed
        with the circuit switched environment in mind, many are directly
        applicable to the packet environment and specifically the Voice over
        IP application being defined using SIP. Some of the capabilities
        need to be adapted or modified for application in the packet mode
        environment. In addition, there will likely be new techniques which
        can be defined specifically for the SIP case.
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        At a high level, the Assured Service requirements can be stated as
        the need to ensure that mission critical voice-mode calls get set up
        and remain connected.
        As a result of this, calls designated as being at a lower precedence
        level are presumed to be less important and may be adversely
        affected by various techniques used to provide the preferential
        treatment to the important, mission-critical calls. For example, the
        lower precedence calls may temporarily experience reduced quality as
        their packets are discarded.
        This memo does not address issues related to incorrect assignment of
        the authority to use precedence levels or the incorrect use of
        levels, for example, if the user can not or does not specify a high
        enough precedence level for the nature of the call.
        (While this memo focuses on Voice over IP, there should be a
        consideration of the impact/solutions for other media flows which
        carry mission critical communication, for example, file transfers,
        video, and instant messaging. Most of the functional requirements
        can be equally applied to these other media.)
     4.   Functional Requirements
        While the functional requirements for Assured Service detailed here
        are specifically those needed to support the US government
        requirements for the Defense Switched Network (DSN), it is believed
        that at least a subset of those requirements are applicable to other
        government networks as well as some commercial (non-government)
        networks. This memo concentrates on those portions mentioned in
        Section 2 which are derived from the requirements for MLPP as
        defined in the American National Standard [T1.619].
        The basic requirements are defined as follows;
     4.1.   Precedence Level Marking
        Each call or session within an Assured Service network is labeled
        with a precedence level as determined by the calling party at the
        beginning of the call. If not chosen by the caller, the default is
        to the lowest precedence level. The called party does not have any
        control over the precedence level for a call or session.
        To meet this general functional requirement, the following specific
        requirements apply:
        Prec-1 It MUST be possible to assign each user the highest
               precedence level they are entitled to use.
        Prec-2 It MUST be possible for the originator of a call to select
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               and signal one of the multiple precedence levels for the
               call, with the call defaulting to the lowest level if none is
               specified. The precedence of each call is independent, that
               is, it is selected for each call.
               Note: One current network for which this is intended uses
               five levels, but other numbers of levels are possible. In no
               case is it necessary to support more than 15 levels.
        Prec-3 It MUST be possible to carry this call associated precedence
               level unchanged though the IP network as a part of the Call
               Control Signaling (for example, in SIP).
        Prec-4 It MUST be possible to deliver the originally signaled
               precedence level to the called party.
     4.2.   Authentication/Authorization
        Not all users are allowed to signal higher precedence levels.
        Therefore, a means is necessary to determine and allow only the
        authorized users the ability to signal these higher precedence
        levels. The following specific requirements apply:
        A&A-1  It MUST be possible to verify that the calling party is
               authorized to use the Assured Service and the requested
               precedence level value if higher than the lowest.
        A&A-2  It MUST be possible to take the appropriate action if the
               calling party attempts to use a  level which is higher than
               authorized. The preferred action is to reject the call, and
               send an indication of the reason to the caller.
     4.3.   Preferential Treatments
        Since it is expected that congestion may occur in various parts of a
        network, it is required that one or more preferential treatments can
        be applied to any call or session which is signaled with a higher
        precedence level relative to already existing calls or sessions if
        that call would cause congestion.  This is required to manage the
        effects of congestion, for example, delay, delay variation, and
        loss, at key points. The actual measures applied depend on the
        situation, but support for the following are required:
       4.3.1.  Call/Session Treatment
        Pref-1 It MUST be possible to block setup of a new call if that call
               would cause congestion. This is called Call Admission Control
        Pref-2 It MUST be possible to apply different limits for CAC for
               various call precedences, that is, in some cases, a higher
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               precedence call may be allowed to be established while a
               lower precedence would not.
        Pref-3 It MUST be possible for an endpoint to release an existing
               (lower precedence) session in favor of completing a new
               session signaled to it (at a higher precedence).
        Pref-4 It MUST be possible for a network node to release an existing
               network resource reservation in favor of a higher precedence
               session. This might involve releasing one or more
               reservations in the process of providing enough bandwidth for
               the new packet flow.
        Pref-5 Preferential treatment SHOULD NOT be provided through any
               scheme of dedicated or pre-reserved bandwidth or resources.
        Pref-6 In those cases in which such dedication or reservation of
               bandwidth or resources is used, when such dedicated or pre-
               reserved bandwidth or resources have been consumed by the
               high precedence traffic, further traffic of that same high
               precedence MUST NOT be provided worse treatment than any of
               the lower precedence levels.
       4.3.2. Packet Transfer Treatment
        Pref-7 It MUST be possible at any point of congestion to determine
               which packets require preferential treatment over other
               packets, including for voice media packets.
        Pref-8 It MUST be known by the device experiencing congestion what
               to do with two or more competing packets.
        Pref-9 It MUST be possible for a network node to discard packets for
               lower precedence calls in favor of those for higher
               precedence calls.
        Pref-10 Media packets MUST NOT starve all potential bandwidth of a
               node interface, thus not allowing signaling packets through
               that same interface. (Note that this requirement is not
               unique to Assured Service.)
       4.3.3. Procedural Requirements
        Pref-10 It MUST be possible to detect various congestion conditions
               which might require preferential treatments to be applied.
        Pref-11 Preferential treatment measures used to manage congestion
               MUST be automatic and MUST NOT have to be manually "turned
               on" in reaction to a congestion event of any kind.
        Pref-12 The application of preferential treatment MUST not require a
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               significant delay to activate (such that it is noticeable to
               the party originating a precedence call).
        Possible methods of providing Preferential Treatment using the
        provisions of this memo, as well as other existing IETF protocols,
        are described in [Pierce1].
     4.4.   Diversion if Not Answered
        In situations is which the called party is busy and can not be
        preempted or in which the called party does not answer, it is
        required to provide a diversion service to a predetermined address
        for any call signaled with a precedence level above the lowest. The
        following apply:
        Div-1  If a precedence call (one higher than the lowest level) is
               blocked by the called party being busy with a call of equal
               or higher precedence, the call MUST be diverted to a
               predetermined alternate party.
        Div-2  If a precedence call is not answered within a designated
               time, the call MUST be diverted to a predetermined alternate
        While the actual requirement previously was for a single "diverted-
        to" address for an entire "switch", this is not feasible in the IP
        case, so the specification of the "diverted-to" address is assumed
        to be per called user. In general, it is expected that this
        diversion capability will operate similar to a normal "Call
        Forwarding on No Answer" service.
     4.5.   Notifications
        It is required that a user who is currently on a call/session and is
        preempted either at the remote end or in between be notified of this
        event. Generally a distinct tone is provided, after which the
        call/session is released.
        Noti-1 All preempted parties MUST be provided with a distinct
               notification informing them that their call has been
        Specific notifications are required to inform the calling party of
        reasons for a precedence call not being successful. They are the
        Noti-2 When a user attempts to use a precedence level to which they
               are not authorized, the caller MUST be notified of this fact.
               The notification MUST NOT provide an indication of what level
               is authorized.
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        Noti-3 When a precedence call can not be established due to the
               called party being busy at an equal or higher precedence with
               no alternate party diversion possible or due to no
               preemptable resources in the network, the caller MUST be
               notified of this fact. The caller MUST NOT be notified what
               precedence level would be necessary to successfully complete
               the call.
     4.6.   Acknowledge by Preempted Party
        When a user is involved in a call/session and that call/session is
        preempted in favor of establishing a higher precedence call/session
        with that same user, the user is required to actively accept this
        new call before the media is connected. This is no different from
        normal calls.
        Ack-1  When an existing call has been preempted for delivery of a
               higher precedence call to the same party, the party MUST
               acknowledge the preemption before the new call is connected.
               That is, there MUST be a positive acknowledgement before any
               audio information is transferred in either direction.
     4.7.   Protection of Signaling/Routing Information from Disclosure
        Although protection is not actually an integral part of the Assured
        Service functionality, it is specifically identified here since this
        capability is generally required in those networks which are assumed
        to be the primary users of Assured Service.
        Prot-1 Sensitive information MUST NOT be made available to non-
               secure portions of the network or to any non-secure network
               through which the traffic passes.
        Prot-2 Sensitive information MUST NOT be accessible by users
               connected to the network.
        Prot-3 Precedence information regarding each call (as well as the
               other information such as calling/called party identity)
               SHOULD be protected from disclosure.
        This non-disclosure requirement especially applies to information
        which is used to control link state routing protocols based on
        knowledge of the current traffic load at each precedence level on
        each route or through each router.
     4.8.   Accounting
        Proper administration of the Assured Service capability requires
        that use of the service can be reviewed after the fact for potential
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        Acct-1 It MUST be possible for the appropriate records to be kept of
               calls made, including the calling and called parties'
               identities, time of the call, duration, and the precedence
               level used. This is similar to the requirements for Call
               Detail Recording (CDR) for billing purposes for other
               services in a commercial environment.
     4.9.   Call Control Signaling Precedence
        Since it competes for the same transport resources as the voice
        packets, it is essential that preferential treatments can be applied
        to the call control signaling. Specifically the following apply:
        CC-1   The call control signaling MUST NOT adversely affect the
               voice (e.g., by introducing excessive packet delay variation
               due to extremely long messages).
        CC-2   The voice traffic MUST NOT significantly delay important call
               control signaling (e.g., by preventing release messages from
               getting through).
     4.10.  Interworking
        Although Assured Service will likely be the only priority scheme
        within many network using it, it still needs to interwork with other
        Int-1  Assured Service calls MUST interwork with other priority
               schemes which are being provided within the same network,
               such as the one defined for [ETS]. This includes the
               following two cases:
               a. both types of traffic may exist in a single network, for
               example, an IEPS call may be originated from within a network
               which also supports "Assured Service" calls. Procedures to
               determine the relative priority and the resulting
               preferential treatment are required.
               b. a network which provides "Assured Service" needs to
               support interworking of calls to and from a network which
               provides another scheme such as IEPS as well as another
               network which provides "Assured Service". Mapping between the
               precedence levels of the two networks must be supported.
     5.   Security Considerations
     5.1.   Authentication/authorization of User Access
        There is a need within SIP  to authenticate/authorize all access to
        capabilities, since virtually any function could be misused,
        resulting in harm to the network or to other users. Because Assured
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        Service is intended to provide an authorized user with better
        service than other users, including the potential of actually
        preempting resources, it is even more important to
        authenticate/authorize the user's access to the Assured Service
        capabilities. However, the requirement already exists for all cases,
        not just Assured Service, therefore the solution is not unique to
        Assured Service.
     5.2.   Security of Signaling Information
        The need to protect signaling information from disclosure is
        independent from the provision of Assured Service. Many networks
        have long been built on the premise that such information needs to
        be protected. Bulk encryption of signaling links (as well as the
        user data channels) between secure switches provided much of this
        protection. In addition, the Signal Transfer Points of the SS#7
        network could be physically secured against unauthorized access. It
        should be noted that commercial networks have recognized the need
        for the same level of protection previously only applied to various
        government networks.
        In the IP environment, the signaling packets traverse many routers
        and could be accessed by unauthorized persons at any one of them.
        While the contents of the individual signaling messages could be
        hidden by encryption of the request and response for end-to-end
        protection of information, the IP header must be visible to
        intermediate routers. It is preferable to not require decryption/
        encryption at each router. The approach has been to encrypt the
        contents of the IP packets (the signaling message) but not the IP
        headers which are needed by the routers. However, the IP headers
        themselves may contain sensitive information such as precedence
        level and ways to identify the called party, or least the location
        of the called party.
     5.3.   Security of Routing Data
        In IP today, there is no Routing Data to secure. When enhancements
        are made to provide for route selection, especially to route around
        congestion, procedures must be developed to prevent unauthorized
        access to that data. It is presumed that procedures will also be
        required to prevent unauthorized modifications.
     5.4.   Security of User Data
        While there may typically be a greater need to protect the user data
        (voice packets) of a call which utilizes priority, since such a call
        may often be more sensitive than calls for which no priority is
        specified, this requirement is not unique to the Assured Service,
        and therefore no specific requirements are given here. The same
        requirements exist for non-Assured Service traffic.
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     6.   IANA Considerations
        There is no IANA involvement in support of Assured Service beyond
        what is described for the Resource Priority Header [Resource].
     7.   References
        [T1.523] ANSI T1.523-2001, "Telecommunications Glossary".
        [T1.619] ANSI T1.619-1992 (R1999) and ANSI T1.619a-1994 (R1999),
        "Multi-Level Precedence and Preemption (MLPP) Service, ISDN
        Supplementary Service Description".
        [T1.631] ANSI T1.631-1993 (R1999), "Telecommunications - Signalling
        System No. 7 (SS7) - High Probability of Completion (HPC) Network
        [E.106] ITU-T Recommendation E.106 (2003), "International Emergency
        Preference Scheme for Telecommunications for Disaster Relief
        [F.706] ITU-T Recommendation F.706 (draft), "International
        Preference Scheme for Multimedia Service in Support of Disaster
        Relief Operations and Mitigation".
        [I.255.3] ITU-T Recommendation I.255.3 (1990), "Multilevel
        precedence and preemption service (MLPP)".
        [Q.735.3] ITU-T Recommendation Q.735.3 (1993), "Description for
        community of interest supplementary services using SS No. 7 -
        Multilevel precedence and preemption (MLPP)".
        [Q.955.3] ITU-T Recommendation Q.955.3 (1993), "Description for
        community of interest supplementary services using DSS1 - Multilevel
        precedence and preemption (MLPP)".
        [RFC3261] "SIP: Session Initiation Protocol", J. Rosenberg, et al,
        June 2002.
        [ETS] draft-ietf-ieprep-framework-06, "Framework for Supporting ETS
        in IP Telephony", Ken Carlberg, et al, Oct 2003.
        [Pierce1] draft-pierce-ieprep-pref-treat-examples-02, "Examples for
        Provision of Preferential Treatment in Voice over IP", Mike Pierce,
        et al, January 2004.
        [Resource] draft-ietf-sip-resource-priority-01, "SIP Communications
        Resource Priority Header", Henning Schulzrinne and James Polk, July
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     8.   Acknowledgements
        The authors would like to thank James Polk and Fred Baker for the
        many suggestions made to improve this document throughout its
     9.   Authors' Addresses
        Michael Pierce
        1893 Preston White Drive
        Reston, VA 20191
        Phone: +1 410.817.4795
        Don Choi
        5600 Columbia Pike
        Falls Church, VA 22041-2717
        Phone: +1 703.681.2312
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