    Internet Engineering Task Force (IETF)                           C. Shen
    Request for Comments: 7200                                H. Schulzrinne
    Category: Standards Track                                    Columbia U.
    ISSN: 2070-1721                                                 A. Koike
                                                                         NTT
                                                                  April 2014

         A Session Initiation Protocol (SIP) Load-Control Event Package

    Abstract

       This specification defines a load-control event package for the
       Session Initiation Protocol (SIP).  It allows SIP entities to
       distribute load-filtering policies to other SIP entities in the
       network.  The load-filtering policies contain rules to throttle calls
       from a specific user or based on their source or destination domain,
       telephone number prefix.  The mechanism helps to prevent signaling
       overload and complements feedback-based SIP overload control efforts.

    Status of This Memo

       This is an Internet Standards Track document.

       This document is a product of the Internet Engineering Task Force
       (IETF).  It represents the consensus of the IETF community.  It has
       received public review and has been approved for publication by the
       Internet Engineering Steering Group (IESG).  Further information on
       Internet Standards is available in Section 2 of RFC 5741.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       http://www.rfc-editor.org/info/rfc7200.

    Copyright Notice

       Copyright (c) 2014 IETF Trust and the persons identified as the
       document authors.  All rights reserved.

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

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    Table of Contents

       1. Introduction ....................................................3
       2. Conventions .....................................................3
       3. SIP Load-Filtering Overview .....................................4
          3.1. Load-Filtering Policy Format ...............................4
          3.2. Load-Filtering Policy Computation ..........................4
          3.3. Load-Filtering Policy Distribution .........................4
          3.4. Applicable Network Domains .................................8
       4. Load-Control Event Package ......................................9
          4.1. Event Package Name .........................................9
          4.2. Event Package Parameters ...................................9
          4.3. SUBSCRIBE Bodies ...........................................9
          4.4. SUBSCRIBE Duration .........................................9
          4.5. NOTIFY Bodies .............................................10
          4.6. Notifier Processing of SUBSCRIBE Requests .................10
          4.7. Notifier Generation of NOTIFY Requests ....................10
          4.8. Subscriber Processing of NOTIFY Requests ..................10
          4.9. Handling of Forked Requests ...............................12
          4.10. Rate of Notifications ....................................12
          4.11. State Delta ..............................................12
       5. Load-Control Document ..........................................13
          5.1. Format ....................................................13
          5.2. Namespace .................................................13
          5.3. Conditions ................................................14
               5.3.1. Call Identity ......................................14
               5.3.2. Method .............................................16
               5.3.3. Target SIP Entity ..................................17
               5.3.4. Validity ...........................................18
          5.4. Actions ...................................................18
       6. XML Schema Definition for Load Control .........................20
       7. Security Considerations ........................................23
       8. IANA Considerations ............................................24
          8.1. Load-Control Event Package Registration ...................24
          8.2. application/load-control+xml Media Type Registration ......24
          8.3. URN Sub-Namespace Registration ............................25
          8.4. Load-Control Schema Registration ..........................26
       9. Acknowledgements ...............................................27
       10. References ....................................................27
          10.1. Normative References .....................................27
          10.2. Informative References ...................................28
       Appendix A. Definitions ...........................................30
       Appendix B. Design Requirements ...................................30
       Appendix C. Discussion of How This Specification Meets the
                   Requirements of RFC 5390 ..............................31
       Appendix D. Complete Examples .....................................36
          D.1. Load-Control Document Examples ............................36
          D.2. Message Flow Examples .....................................40

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       Appendix E.  Related Work .........................................41
          E.1. Relationship to Load Filtering in PSTN ....................41
          E.2. Relationship with Other IETF SIP Overload Control Efforts .42

    1.  Introduction

       SIP load-control mechanisms are needed to prevent congestion collapse
       [RFC6357] in cases of SIP server overload [RFC5390].  There are two
       types of load-control approaches.  In the first approach, feedback
       control, SIP servers provide load limits to upstream servers, to
       reduce the incoming rate of all SIP requests [SIP-OVERLOAD].  These
       upstream servers then drop or delay incoming SIP requests.  Feedback
       control is reactive and affects signaling messages that have already
       been issued by user agent clients.  This approach works well when SIP
       proxy servers in the core networks (core proxy servers) or
       destination-specific SIP proxy servers in the edge networks (edge
       proxy servers) are overloaded.  By their nature, they need to
       distribute rate, drop, or window information to all upstream SIP
       proxy servers and normally affect all calls equally, regardless of
       destination.

       This specification proposes an additional, complementary load-control
       mechanism, called "load filtering".  It is most applicable for
       situations where a traffic surge and its source/destination
       distribution can be predicted in advance.  In those cases, network
       operators create load-filtering policies that indicate calls to
       specific destinations or from specific sources should be rate-limited
       or randomly dropped.  These load-filtering policies are then
       distributed to SIP servers and possibly SIP user agents that are
       likely to generate calls to the affected destinations or from the
       affected sources.  Load filtering works best if it prevents calls as
       close to the originating user agent clients as possible.  The
       applicability of SIP load filtering can also be extended beyond
       overload control, e.g., to implement service level agreement
       commitments.

    2.  Conventions

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

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    3.  SIP Load-Filtering Overview

    3.1.  Load-Filtering Policy Format

       Load-filtering policies are specified by sets of rules.  Each rule
       contains both load-filtering conditions and actions.  The load-
       filtering conditions define identities of the targets to be filtered
       (Section 5.3.1).  For example, there are two typical resource limits
       in a possible overload situation, i.e., human destination limits
       (number of call takers) and node capacity limits.  The load-filtering
       targets in these two cases can be the specific callee numbers or the
       destination domain corresponding to the overload.  Load-filtering
       conditions also indicate the specific message type to be matched
       (Section 5.3.2), with which target SIP entity the filtering policy is
       associated (Section 5.3.3), and the period of time when the filtering
       policy should be activated and deactivated (Section 5.3.4).  Load-
       filtering actions describe the desired control functions such as
       keeping the request rate below a specified level (Section 5.4).

    3.2.  Load-Filtering Policy Computation

       When computing the load-filtering policies, one needs to take into
       consideration information such as overload time, scope and network
       topology, as well as service policies.  It is also important to make
       sure that there is no resource allocation loop and that server
       capacity is allocated in a way that both prevents overload and
       maximizes effective throughput (commonly called goodput).  In some
       cases, in order to better utilize system resources, it may be
       preferable to employ an algorithm that dynamically computes the load-
       filtering policies based on currently observed server load status,
       rather than using a purely static filtering policy assignment.  The
       computation algorithm for load-filtering policies is beyond the scope
       of this specification.

    3.3.  Load-Filtering Policy Distribution

       For distributing load-filtering policies, this specification defines
       the SIP event package for load control, which is an "instantiation"
       of the generic SIP event notification framework [RFC6665].  This
       specification also defines the XML schema of a load-control document
       (Section 5), which is used to encode load-filtering policies.

       In order for load-filtering policies to be properly distributed, each
       capable SIP entity in the network subscribes to the SIP load-control
       event package of each SIP entity to which it sends signaling
       requests.  A SIP entity that accepts subscription requests is called
       a "notifier" (Section 4.6).  Subscription is initiated and maintained
       during normal server operation.  The subscription of neighboring SIP

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       entities needs to be persistent, as described in Sections 4.1 and 4.2
       of [RFC6665].  The refresh procedure is described in Section 4.7
       below.  Subscribers may terminate the subscription if they have not
       received notifications for an extended time period, and can
       resubscribe if they determine that signaling with the notifier
       becomes active again.

       An example architecture is shown in Figure 1 to illustrate SIP load-
       filtering policy distribution.  This scenario consists of two
       networks belonging to Service Provider A and Service Provider B,
       respectively.  Each provider's network is made up of two SIP core
       proxy servers and four SIP edge proxy servers.  The core proxy
       servers and edge proxy servers of Service Provider A are denoted as
       CPa1 to CPa2 and EPa1 to EPa4; the core proxy servers and edge proxy
       servers of Service Provider B are denoted as CPb1 to CPb2 and EPb1 to
       EPb4.

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          +-----------+   +-----------+   +-----------+   +-----------+
          |           |   |           |   |           |   |           |
          |   EPa1    |   |   EPa2    |   |   EPa3    |   |   EPa4    |
          |           |   |           |   |           |   |           |
          +-----------+   +-----------+   +-----------+   +-----------+
                  \         /                    \          /
                   \       /                      \        /
                    \     /                        \      /
                  +-----------+                  +-----------+
                  |           |                  |           |
                  |   CPa1    |------------------|   CPa2    |
                  |           |                  |           |
                  +-----------+                  +-----------+
                        |                              |
          Service       |                              |
          Provider A    |                              |
                        |                              |
         =================================================================
                        |                              |
          Service       |                              |
          Provider B    |                              |
                        |                              |
                  +-----------+                  +-----------+
                  |           |                  |           |
                  |   CPb1    |------------------|   CPb2    |
                  |           |                  |           |
                  +-----------+                  +-----------+
                    /      \                        /     \
                   /        \                      /       \
                  /          \                    /         \
          +-----------+   +-----------+   +-----------+   +-----------+
          |           |   |           |   |           |   |           |
          |   EPb1    |   |   EPb2    |   |   EPb3    |   |   EPb4    |
          |           |   |           |   |           |   |           |
          +-----------+   +-----------+   +-----------+   +-----------+

          Figure 1: Example Network Scenario Using SIP Load-Control Event
                                 Package Mechanism

       During the initialization stage, the proxy servers first identify all
       their outgoing signaling neighbors and subscribe to them.  Service
       providers can provision neighbors, or the proxy servers can
       incrementally learn who their neighbors are by inspecting signaling
       messages that they send and receive.  Assuming all signaling
       relationships in Figure 1 are bidirectional, after this
       initialization stage, each proxy server will be subscribed to all its
       neighbors.

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       Case I: EPa1 serves a TV program hotline and decides to limit the
       total number of incoming calls to the hotline to prevent an overload.
       To do so, EPa1 sends a notification to CPa1 with the specific hotline
       number, time of activation, and total acceptable call rate.
       Depending on the load-filtering policy computation algorithm, CPa1
       may allocate the received total acceptable call rate among its
       neighbors, namely, EPa2, CPa2, and CPb1, and notify them about the
       resulting allocation along with the hotline number and the activation
       time.  CPa2 and CPb1 may perform further allocation among their own
       neighbors and notify the corresponding proxy servers.  This process
       continues until all edge proxy servers in the network have been
       informed about the event and have proper load-filtering policies
       configured.

       In the above case, the network entity where load-filtering policy is
       first introduced is the SIP server providing access to the resource
       that creates the overload situation.  In other cases, the network
       entry point of introducing load-filtering policy could also be an
       entity that hosts this resource.  For example, an operator may host
       an application server that performs toll-free-number ("800 number")
       translation services.  The application server itself may be a SIP
       proxy server or a SIP Back-to-Back User Agent (B2BUA).  If one of the
       toll-free numbers hosted at the application server creates the
       overload condition, the load-filtering policies can be introduced
       from the application server and then propagated to other SIP proxy
       servers in the network.

       Case II: A hurricane affects the region covered by CPb2, EPb3, and
       EPb4.  All three of these SIP proxy servers are overloaded.  The
       rescue team determines that outbound calls are more valuable than
       inbound calls in this specific situation.  Therefore, EPb3 and EPb4
       are configured with load-filtering policies to accept more outbound
       calls than inbound calls.  CPb2 may be configured the same way or
       receive dynamically computed load-filtering policies from EPb3 and
       EPb4.  Depending on the load-filtering policy computation algorithm,
       CPb2 may also send out notifications to its outside neighbors, namely
       CPb1 and CPa2, specifying a limit on the acceptable rate of inbound
       calls to CPb2's responsible domain.  CPb1 and CPa2 may subsequently
       notify their neighbors about limiting the calls to CPb2's area.  The
       same process could continue until all edge proxy servers are notified
       and have load-filtering policies configured.

       Note that this specification does not define the provisioning
       interface between the party who determines the load-filtering policy
       and the network entry point where the policy is introduced.  One of
       the options for the provisioning interface is the Extensible Markup
       Language (XML) Configuration Access Protocol (XCAP) [RFC4825].

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    3.4.  Applicable Network Domains

       This specification MUST be applied inside a "Trust Domain".  The
       concept of a Trust Domain is similar to that defined in [RFC3324] and
       [RFC3325].  A Trust Domain, for the purpose of SIP load filtering, is
       a set of SIP entities such as SIP proxy servers that are trusted to
       exchange load-filtering policies defined in this specification.  In
       the simplest case, a Trust Domain is a network of SIP entities
       belonging to a single service provider who deploys it and accurately
       knows the behavior of those SIP entities.  Such simple Trust Domains
       may be joined to form larger Trust Domains by bilateral agreements
       between the service providers of the SIP entities.

       The key requirement of a Trust Domain for the purpose of SIP load
       filtering is that the behavior of all SIP entities within a given
       Trust Domain is known to comply to the following set of
       specifications.

       o  SIP entities in the Trust Domain agree on the mechanisms used to
          secure the communication among SIP entities within the Trust
          Domain.

       o  SIP entities in the Trust Domain agree on the manner used to
          determine which SIP entities are part of the Trust Domain.

       o  SIP entities in the Trust Domain are compliant to SIP [RFC3261].

       o  SIP entities in the Trust Domain are compliant to SIP-Specific
          Event Notification[RFC6665].

       o  SIP entities in the Trust Domain are compliant to this
          specification.

       o  SIP entities in the Trust Domain agree on what types of calls can
          be affected by this SIP load-filtering mechanism.  For example,
          <call-identity> condition elements (Section 5.3.1) <one> and
          <many> might be limited to describe within certain prefixes.

       o  SIP entities in the Trust Domain agree on the destinations to
          which calls may be redirected when the "redirect" action
          (Section 5.4) is used.  For example, the URI might have to match a
          given set of domains.

       SIP load filtering is only effective if all neighbors that are
       possible signaling sources participate and enforce the designated
       load-filtering policies.  Otherwise, a single non-conforming neighbor
       could make all filtering efforts useless by pumping in excessive
       traffic to overload the server.  Therefore, the SIP server that

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       distributes load-filtering policies needs to take countermeasures
       towards any non-conforming neighbors.  A simple method is to reject
       excessive requests with 503 "Service Unavailable" response messages
       as if they were obeying the rate.  Considering the rejection costs, a
       more complicated but fairer method would be to allocate at the
       overloaded server the same amount of processing to the combination of
       both normal processing and rejection as the overloaded server would
       devote to processing requests for a conforming upstream SIP server.
       These approaches work as long as the total rejection cost does not
       overwhelm the entire server resources.  In addition, SIP servers need
       to handle message prioritization properly while performing load
       filtering, which is described in Section 4.8.

    4.  Load-Control Event Package

       The SIP load-filtering mechanism defines a load-control event package
       for SIP based on [RFC6665].

    4.1.  Event Package Name

       The name of this event package is "load-control".  This name is
       carried in the Event and Allow-Events header, as specified in
       [RFC6665].

    4.2.  Event Package Parameters

       No package-specific event header field parameters are defined for
       this event package.

    4.3.  SUBSCRIBE Bodies

       This specification does not define the content of SUBSCRIBE bodies.
       Future specifications could define bodies for SUBSCRIBE messages, for
       example, to request specific types of load-control event
       notifications.

       A SUBSCRIBE request sent without a body implies the default
       subscription behavior as specified in Section 4.7.

    4.4.  SUBSCRIBE Duration

       The default expiration time for a subscription to load-filtering
       policy is one hour.  Since the desired expiration time may vary
       significantly for subscriptions among SIP entities with different
       signaling relationships, the subscribers and notifiers are
       RECOMMENDED to explicitly negotiate appropriate subscription duration
       when knowledge about the mutual signaling relationship is available.

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    4.5.  NOTIFY Bodies

       The body of a NOTIFY request in this event package contains load-
       filtering policies.  The format of the NOTIFY request body MUST be in
       one of the formats defined in the Accept header field of the
       SUBSCRIBE request or be the default format, as specified in
       [RFC6665].  The default data format for the NOTIFY request body of
       this event package is "application/load-control+xml" (defined in
       Section 5).  This means that when a NOTIFY request body exists but no
       Accept header field is specified in a SUBSCRIBE request, the NOTIFY
       request body MUST contain content conforming to the "application/
       load-control+xml" format.

    4.6.  Notifier Processing of SUBSCRIBE Requests

       The notifier accepts a new subscription or updates an existing
       subscription upon receiving a valid SUBSCRIBE request.

       If the identity of the subscriber sending the SUBSCRIBE request is
       not allowed to receive load-filtering policies, the notifier MUST
       return a 403 "Forbidden" response.

       If none of the media types specified in the Accept header of the
       SUBSCRIBE request are supported, the notifier SHOULD return a 406
       "Not Acceptable" response.

    4.7.  Notifier Generation of NOTIFY Requests

       A notifier MUST send a NOTIFY request with its current load-filtering
       policy to the subscriber upon successfully accepting or refreshing a
       subscription.  If no load-filtering policy needs to be distributed
       when the subscription is received, the notifier SHOULD sent a NOTIFY
       request without a body to the subscriber.  The content-type header
       field of this NOTIFY request MUST indicate the correct body format as
       if the body were present (e.g., "application/load-control+xml").
       Notifiers are likely to send NOTIFY requests without a body when a
       subscription is initiated for the first time, e.g., when a SIP entity
       is just introduced, because there may be no planned events that
       require load filtering at that time.  A notifier SHOULD generate
       NOTIFY requests each time the load-filtering policy changes, with the
       maximum notification rate not exceeding values defined in
       Section 4.10.

    4.8.  Subscriber Processing of NOTIFY Requests

       The subscriber is the load-filtering server that enforces load-
       filtering policies received from the notifier.  The way subscribers
       process NOTIFY requests depends on the load-filtering policies

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       conveyed in the notifications.  Typically, load-filtering policies
       consist of rules specifying actions to be applied to requests
       matching certain conditions.  A subscriber receiving a notification
       first installs these rules and then enforces corresponding actions on
       requests matching those conditions, for example, limiting the sending
       rate of call requests destined for a specific callee.

       In the case when load-filtering policies specify a future validity,
       it is possible that when the validity time arrives, the subscription
       to the specific notifier that conveyed the rules has expired.  In
       this case, it is RECOMMENDED that the subscriber re-activate its
       subscription with the corresponding notifier.  Regardless of whether
       or not this re-activation of subscription is successful, when the
       validity time is reached, the subscriber SHOULD enforce the
       corresponding rules.

       Upon receipt of a NOTIFY request with a Subscription-State header
       field containing the value "terminated", the subscription status with
       the particular notifier will be terminated.  Meanwhile, subscribers
       MUST also terminate previously received load-filtering policies from
       that notifier.

       The subscriber MUST discard unknown bodies.  If the NOTIFY request
       contains several bodies, none of them being supported, it SHOULD
       unsubscribe unless it has knowledge that it will possibly receive
       NOTIFY requests with supported bodies from that notifier.  A NOTIFY
       request without a body indicates that no load-filtering policies need
       to be updated.

       When the subscriber enforces load-filtering policies, it needs to
       prioritize requests and select those requests that need to be
       rejected or redirected.  This selection is largely a matter of local
       policy.  It is expected that the subscriber will follow local policy
       as long as the result in reduction of traffic is consistent with the
       overload algorithm in effect at that node.  Accordingly, the
       normative behavior described in the next three paragraphs should be
       interpreted with the understanding that the subscriber will aim to
       preserve local policy to the fullest extent possible.

       o  The subscriber SHOULD honor the local policy for prioritizing SIP
          requests such as policies based on message type, e.g., INVITEs
          versus requests associated with existing sessions.

       o  The subscriber SHOULD honor the local policy for prioritizing SIP
          requests based on the content of the Resource-Priority header
          (RPH, [RFC4412]).  Specific (namespace.value) RPH contents may
          indicate high-priority requests that should be preserved as much

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          as possible during overload.  The RPH contents can also indicate a
          low-priority request that is eligible to be dropped during times
          of overload.

       o  The subscriber SHOULD honor the local policy for prioritizing SIP
          requests relating to emergency calls as identified by the sos URN
          [RFC5031] indicating an emergency request.

       A local policy can be expected to combine both the SIP request type
       and the prioritization markings and SHOULD be honored when overload
       conditions prevail.

    4.9.  Handling of Forked Requests

       Forking is not applicable when this load-control event package
       mechanism is used within a single-hop distance between neighboring
       SIP entities.  If communication scope of the load-control event
       package mechanism is among multiple hops, forking is also not
       expected to happen because the subscription request is addressed to a
       clearly defined SIP entity.  However, in the unlikely case when
       forking does happen, the load-control event package only allows the
       first potential dialog-establishing message to create a dialog, as
       specified in Section 5.4.9 of [RFC6665].

    4.10.  Rate of Notifications

       The rate of notifications is unlikely to be of concern for this local
       control event package mechanism when it is used in a non-real-time
       mode for relatively static load-filtering policies.  Nevertheless, if
       a situation does arise in which a rather frequently used load
       filtering policy update is needed, it is RECOMMENDED that the
       notifier not generate notifications at a rate higher than once per
       second in all cases, in order to avoid the NOTIFY request itself
       overloading the system.

    4.11.  State Delta

       It is likely that updates to specific load-filtering policies are
       made by changing only part of the policy parameters (e.g., acceptable
       request rate or percentage, but not matching identities).  This will
       typically be because the utilization of a resource subject to
       overload depends upon dynamic unknowns such as holding time and the
       relative distribution of offered loads over subscribing SIP entities.
       The updates could originate manually or be determined automatically
       by an algorithm that dynamically computes the load-filtering policies
       (Section 3.2).  Another factor that is usually not known precisely or

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       needs to be computed automatically is the duration of the event
       requiring load filtering.  Therefore, it would also be common for the
       validity to change frequently.

       This event package allows the use of state delta as in [RFC6665] to
       accommodate frequent updates of partial policy parameters.  For each
       NOTIFY transaction in a subscription, a version number that increases
       by exactly one MUST be included in the NOTIFY request body when the
       body is present.  When the subscriber receives a state delta, it
       associates the partial updates to the particular policy by matching
       the appropriate rule id (Appendix D).  If the subscriber receives a
       NOTIFY request with a version number that is increased by more than
       one, it knows that it has missed a state delta and needs to ask for a
       full state snapshot.  Therefore, the subscriber ignores that NOTIFY
       request containing the state delta, and resends a SUBSCRIBE request
       to force a NOTIFY request containing a complete state snapshot.

    5.  Load-Control Document

    5.1.  Format

       A load-control document is an XML document that describes the load-
       filtering policies.  It inherits and enhances the common policy
       document defined in [RFC4745].  A common policy document contains a
       set of rules.  Each rule consists of three parts: conditions,
       actions, and transformations.  The conditions part is a set of
       expressions containing attributes such as identity, domain, and
       validity time information.  Each expression evaluates to TRUE or
       FALSE.  Conditions are matched on "equality" or "greater than" style
       comparison.  There is no regular expression matching.  Conditions are
       evaluated on receipt of an initial SIP request for a dialog or
       standalone transaction.  If a request matches all conditions in a
       rule set, the action part and the transformation part are consulted
       to determine the "permission" on how to handle the request.  Each
       action or transformation specifies a positive grant to the policy
       server to perform the resulting actions.  Well-defined mechanism are
       available for combining actions and transformations obtained from
       more than one sources.

    5.2.  Namespace

       The namespace URI for elements defined by this specification is a
       Uniform Resource Namespace (URN) ([RFC2141]), using the namespace
       identifier "ietf" defined by [RFC2648] and extended by [RFC3688].
       The URN is as follows:

       urn:ietf:params:xml:ns:load-control

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    5.3.  Conditions

       [RFC4745] defines three condition elements: <identity>, <sphere>, and
       <validity>.  This specification defines new condition elements and
       reuses the <validity> element.  The <sphere> element is not used.

    5.3.1.  Call Identity

       Since the problem space of this specification is different from that
       of [RFC4745], the [RFC4745] <identity> element is not sufficient for
       use with load filtering.  First, load filtering may be applied to
       different identities contained in a request, including identities of
       both the receiving entity and the sending entity.  Second, the
       importance of authentication varies when different identities of a
       request are concerned.  This specification defines new identity
       conditions that can accommodate the granularity of specific SIP
       identity header fields.  The requirement for authentication depends
       on which field is to be matched.

       The identity condition for load filtering is specified by the
       <call-identity> element and its sub-element <sip>.  The <sip> element
       itself contains sub-elements representing SIP sending and receiving
       identity header fields: <from>, <to>, <request-uri>, and
       <p-asserted-identity>.  All those sub-elements are of an extended
       form of the [RFC4745] <identity> element.  In addition to the sub-
       elements including <one>, <except>, and <many> in the <identity>
       element from [RFC4745], the extended form adds two new sub-elements,
       namely, <many-tel> and <except-tel>, which will be explained later in
       this section.

       The [RFC4745] <one> and <except> elements may contain an "id"
       attribute, which is the URI of a single entity to be included or
       excluded in the condition.  When used in the <from>, <to>,
       <request-uri>, and <p-asserted-identity> elements, this "id" value is
       the URI contained in the corresponding SIP header field, i.e., From,
       To, Request-URI, and P-Asserted-Identity.

       When the <call-identity> element contains multiple <sip> sub-
       elements, the result is combined using logical OR.  When the <from>,
       <to>, <request-uri>, and <p-asserted-identity> elements contain
       multiple <one>, <many>, or <many-tel> sub-elements, the result is
       also combined using logical OR.  When the <many> sub-element further
       contains one or more <except> sub-elements, or when the <many-tel>
       sub-element further contains one or more <except-tel> sub-elements,
       the result of each <except> or <except-tel> sub-element is combined
       using a logical OR, similar to that of the [RFC4745] <identity>
       element.  However, when the <sip> element contains multiple <from>,
       <to>, <request-uri>, and <p-asserted-identity> sub-elements, the

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       result is combined using logical AND.  This allows the call identity
       to be specified by multiple fields of a SIP request simultaneously,
       e.g., both the From and the To header fields.

       The following shows an example of the <call-identity> element, which
       matches call requests whose To header field contains the SIP URI
       "sip:alice@hotline.example.com" or the 'tel' URI
       "tel:+1-212-555-1234".

                   <call-identity>
                       <sip>
                           <to>
                               <one id="sip:alice@hotline.example.com"/>
                               <one id="tel:+1-212-555-1234"/>
                           </to>
                       </sip>
                   </call-identity>

       Before evaluating <call-identity> conditions, the subscriber shall
       convert URIs received in SIP header fields in canonical form as per
       [RFC3261], except that the "phone-context" parameter shall not be
       removed, if present.

       The [RFC4745] <many> and <except> elements may take a "domain"
       attribute.  The "domain" attribute specifies a domain name to be
       matched by the domain part of the candidate identity.  Thus, it
       allows matching a large and possibly unknown number of entities
       within a domain.  The "domain" attribute works well for SIP URIs.

       A URI identifying a SIP user, however, can also be a 'tel' URI.
       Therefore, a similar way to match a group of 'tel' URIs is needed.
       There are two forms of 'tel' URIs: for global numbers and local
       numbers.  According to [RFC3966], "All phone numbers MUST use the
       global form unless they cannot be represented as such...Local numbers
       MUST be tagged with a 'phone-context'".  The global number 'tel' URIs
       start with a "+".  The "phone-context" parameter of local numbers may
       be labeled as a global number or any number of its leading digits or
       a domain name.  Both forms of the 'tel' URI make the resulting URI
       globally unique.

       'tel' URIs of global numbers can be grouped by prefixes consisting of
       any number of common leading digits.  For example, a prefix formed by
       a country code or both the country and area code identifies telephone
       numbers within a country or an area.  Since the length of the country
       and area code for different regions are different, the length of the
       number prefix also varies.  This allows further flexibility such as

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       grouping the numbers into sub-areas within the same area code. 'tel'
       URIs of local numbers can be grouped by the value of the
       "phone-context" parameter.

       The <many> and <except> sub-elements in the <identity> element of
       [RFC4745] do not allow additional attributes to be added directly.
       Redefining behavior of their existing "domain" attribute creates
       backward-compatibility issues.  Therefore, this specification defines
       the <many-tel> and <except-tel> sub-elements that extend the
       [RFC4745] <identity> element.  Both of them have a "prefix" attribute
       for grouping 'tel' URIs, similar to the "domain" attribute for
       grouping SIP URIs in existing <many> and <except> sub-elements.  For
       global numbers, the "prefix" attribute value holds any number of
       common leading digits, for example, "+1-212" for US phone numbers
       within area code "212" or "+1-212-854" for the organization with US
       area code "212" and local prefix "854".  For local numbers, the
       "prefix" attribute value contains the "phone-context" parameter
       value.  It should be noted that visual separators (such as the "-"
       sign) in 'tel' URIs are not used for URI comparison as per [RFC3966].

       The following example shows the use of the "prefix" attribute along
       with the "domain" attribute.  It matches those requests calling to
       the number "+1-202-999-1234" but are not calling from a "+1-212"
       prefix or a SIP From URI domain of "manhattan.example.com".

                   <call-identity>
                       <sip>
                           <from>
                               <many>
                                   <except domain="manhattan.example.com"/>
                               </many>
                               <many-tel>
                                   <except-tel prefix="+1-212"/>
                               </many-tel>
                           </from>
                           <to>
                               <one id="tel:+1-202-999-1234"/>
                           </to>
                       </sip>
                   </call-identity>

    5.3.2.  Method

       The load created on a SIP server depends on the type of initial SIP
       requests for dialogs or standalone transactions.  The <method>
       element specifies the SIP method to which the load-filtering action
       applies.  When this element is not included, the load-filtering
       actions are applicable to all applicable initial requests.  These

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       requests include INVITE, MESSAGE, REGISTER, SUBSCRIBE, OPTIONS, and
       PUBLISH.  Non-initial requests, such as ACK, BYE, and CANCEL MUST NOT
       be subjected to load filtering.  In addition, SUBSCRIBE requests are
       not filtered if the event-type header field indicates the event
       package defined in this specification.

       The following example shows the use of the <method> element in the
       case the filtering actions should be applied to INVITE requests.

               <method>INVITE</method>

    5.3.3.  Target SIP Entity

       A SIP server that performs load-filtering may have multiple paths to
       route call requests matching the same set of call identity elements.
       In those situations, the SIP load-filtering server may desire to take
       advantage of alternative paths and only apply load-filtering actions
       to matching requests for the next-hop SIP entity that originated the
       corresponding load-filtering policy.  To achieve that, the SIP load-
       filtering server needs to associate every load-filtering policy with
       its originating SIP entity.  The <target-sip-entity> element is
       defined for that purpose, and it contains the URI of the entity that
       initiated the load-filtering policy, which is generally the
       corresponding notifier.  A notifier MAY include this element as part
       of the condition of its filtering policy being sent to the
       subscriber, as below.

       <target-sip-entity>sip:biloxi.example.com</target-sip-entity>

       When a SIP load-filtering server receives a policy with a
       <target-sip-entity> element, it SHOULD record it and take it into
       consideration when making load-filtering decisions.  If the load-
       filtering server receives a load-filtering policy that does not
       contain a <target-sip-entity> element, it MAY still record the URI of
       the load-filtering policy's originator as the <target-sip-entity>
       information and consider it when making load-filtering decisions.

          The following are two examples of using the <target-sip-entity>
          element.

          Use case I: The network has user A connected to SIP Proxy 1 (SP1),
          user B connected to SIP Proxy 3 (SP3), SP1 and SP3 connected via
          SIP Proxy 2 (SP2), and SP2 connected to an Application Server
          (AS).  Under normal load conditions, a call from A to B is routed
          along the following path: A-SP1-SP2-AS-SP3-B.  The AS provides a
          nonessential service and can be bypassed in case of overload.  Now
          let's assume that AS is overloaded and sends to SP2 a load-
          filtering policy requesting that 50% of all INVITE requests be

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          dropped.  SP2 can maintain AS as the <target-sip-entity> for that
          policy so that it knows the 50% drop action is only applicable to
          call requests that must go through AS, without affecting those
          calls directly routed through SP3 to B.

          Use case II: A translation service for toll-free numbers is
          installed on two Application Servers, AS1 and AS2.  User A is
          connected to SP1 and calls 800-1234-4529, which is translated by
          AS1 and AS2 into a regular E.164 number depending on, e.g., the
          caller's location.  SP1 forwards INVITE requests with Request-URI
          = "800 number" to AS1 or AS2 based on a load-balancing strategy.
          As calls to 800-1234-4529 create a pre-overload condition in AS1,
          AS1 sends to SP1 a load-filtering policy requesting that 50% of
          calls towards 800-1234-4529 be rejected.  In this case, SP1 can
          maintain AS1 as the <target-sip-entity> for the rule, and only
          apply the load-filtering policy on incoming requests that are
          intended to be sent to AS1.  Those requests that are sent to AS2,
          although matching the <call-identity> of the filter, will not be
          affected.

    5.3.4.  Validity

       A filtering policy is usually associated with a validity period
       condition.  This specification reuses the <validity> element of
       [RFC4745], which specifies a period of validity time by pairs of
       <from> and <until> sub-elements.  When multiple time periods are
       defined, the validity condition is evaluated to TRUE if the current
       time falls into any of the specified time periods.  That is, it
       represents a logical OR operation across all validity time periods.

       The following example shows a <validity> element specifying a valid
       period from 12:00 to 15:00 US Eastern Standard Time on 2008-05-31.

                   <validity>
                       <from>2008-05-31T12:00:00-05:00</from>
                       <until>2008-05-31T15:00:00-05:00</until>
                   </validity>

    5.4.  Actions

       The actions a load-filtering server takes on loads matching the load-
       filtering conditions are defined by the <accept> element in the load-
       filtering policy, which includes any one of the three sub-elements
       <rate>, <percent>, and <win>.  The <rate> element denotes an absolute
       value of the maximum acceptable request rate in requests per second;
       the <percent> element specifies the relative percentage of incoming
       requests that should be accepted; the <win> element describes the
       acceptable window size supplied by the receiver, which is applicable

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       in window-based load-filtering.  In static load-filtering policy
       configuration scenarios, using the <rate> sub-element is RECOMMENDED
       because it is hard to enforce the percentage rate or window-based
       load filtering when incoming load from upstream or reactions from
       downstream are uncertain.  (See [SIP-OVERLOAD] and [RFC6357] for more
       details on rate-based, loss-based, and window-based load control.)

       In addition, the <accept> element takes an optional "alt-action"
       attribute that can be used to explicitly specify the desired action
       in case a request cannot be processed.  The "alt-action" can take one
       of the following three values: "reject", "redirect", or "drop".

       o  The "reject" action is the default value for "alt-action".  It
          means that the load-filtering server will reject the request with
          a 503 "Service Unavailable" response message.

       o  The "redirect" action means redirecting the request to another
          target.  When it is used, an "alt-target" attribute MUST be
          defined.  The "alt-target" specifies one URI or a list of URIs
          where the request should be redirected.  The server sends out the
          redirect URIs in a 300-class response message.

       o  The "drop" action means simply ignoring the request without doing
          anything, which can, in certain cases, help save processing
          capability during overload.  For example, when SIP is running over
          a reliable transport such as TCP, the "drop" action does not send
          out the rejection response, neither does it close the transport
          connection.  However, when running SIP over an unreliable
          transport such as UDP, using the "drop" action will create message
          retransmissions that further worsen the possible overload
          situation.  Therefore, any "drop" action applied to an unreliable
          transport MUST be treated as if it were "reject".

       The above "alt-action" processing can also be illustrated through the
       following pseudocode.

               SWITCH "alt-action"
                 "redirect": "redirect"
                 "drop":
                   IF unreliable-transport
                     THEN treat as "reject"
                   ELSE
                     "drop"
                 "reject": "reject"
                 default: "reject"
               END

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       In the following <actions> element example, the server accepts
       maximum of 100 call requests per second.  The remaining calls are
       redirected to an answering machine.

               <actions>
                   <accept alt-action="redirect" alt-target=
                           "sip:answer-machine@example.com">
                       <rate>100</rate>
                   </accept>
               </actions>

    6.  XML Schema Definition for Load Control

       This section defines the XML schema for the load-control document.
       It extends the Common Policy schema in [RFC4745] in two ways.
       Firstly, it defines two mandatory attributes for the <ruleset>
       element: "version" and "state".  The "version" attribute allows the
       recipient of the notification to properly order them.  Versions start
       at zero and increase by one for each new document sent to a
       subscriber within the same subscription.  Versions MUST be
       representable using a non-negative 32-bit integer.  The "state"
       attribute indicates whether the document contains a full load-
       filtering policy update or only state delta as partial update.
       Secondly, it defines new members of the <conditions> and <actions>
       elements.

       <?xml version="1.0" encoding="UTF-8"?>
       <xs:schema targetNamespace="urn:ietf:params:xml:ns:load-control"
           xmlns:lc="urn:ietf:params:xml:ns:load-control"
           xmlns:cp="urn:ietf:params:xml:ns:common-policy"
           xmlns:xs="http://www.w3.org/2001/XMLSchema"
           elementFormDefault="qualified"
           attributedFormDefault="unqualified">

       <xs:import namespace="urn:ietf:params:xml:ns:common-policy"/>

       <!-- RULESET -->

       <xs:element name="ruleset">
         <xs:complexType>
           <xs:complexContent>
             <xs:restriction base="xs:anyType">
               <xs:sequence>
                 <xs:element name="rule" type="cp:ruleType"
                 minOccurs="0" maxOccurs="unbounded"/>
               </xs:sequence>
             </xs:restriction>
           </xs:complexContent>

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           <xs:attribute name="version" type="xs:integer" use="required"/>
           <xs:attribute name="state" use="required">
             <xs:simpleType>
               <xs:restriction base="xs:string">
                 <xs:enumeration value="full"/>
                 <xs:enumeration value="partial"/>
               </xs:restriction>
             </xs:simpleType>
           </xs:attribute>
         </xs:complexType>
       </xs:element>

       <!-- CONDITIONS -->

       <!-- CALL IDENTITY -->
       <xs:element name="call-identity" type="lc:call-identity-type"/>

       <!-- CALL IDENTITY TYPE -->
       <xs:complexType name="call-identity-type">
         <xs:choice>
           <xs:element name="sip" type="lc:sip-id-type"/>
           <any namespace="##other" processContents="lax" minOccurs="0"
           maxOccurs="unbounded"/>
         </xs:choice>
         <anyAtrribute namespace="##other" processContents="lax"/>
       </xs:complexType>

       <!-- SIP ID TYPE -->
       <xs:complexType name="sip-id-type">
         <xs:sequence>
           <element name="from" type="lc:identityType" minOccurs="0"/>
           <element name="to" type="lc:identityType" minOccurs="0"/>
           <element name="request-uri" type="lc:identityType"
           minOccurs="0"/>
           <element name="p-asserted-identity" type="lc:identityType"
           minOccurs="0"/>
           <any namespace="##other" processContents="lax" minOccurs="0"
           maxOccurs="unbounded"/>
         </xs:sequence>
         <anyAtrribute namespace="##other" processContents="lax"/>
       </xs:complexType>

       <!-- IDENTITY TYPE -->
       <xs:complexType name="identityType">
         <xs:complexContent>
           <xs:restriction base="xs:anyType">
             <xs:choice minOccurs="1" maxOccurs="unbounded">
               <xs:element name="one" type="cp:oneType"/>

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               <xs:element name="many" type="lc:manyType"/>
               <xs:element name="many-tel" type="lc:manyTelType"/>
               <xs:any namespace="##other" processContents="lax"/>
             </xs:choice>
           </xs:restriction>
         </xs:complexContent>
       </xs:complexType>

       <!-- MANY-TEL TYPE -->
       <xs:complexType name="manyTelType">
         <xs:complexContent>
           <xs:restriction base="xs:anyType">
             <xs:choice minOccurs="0" maxOccurs="unbounded">
               <xs:element name="except-tel" type="lc:exceptTelType"/>
               <xs:any namespace="##other"
               minOccurs="0" processContents="lax"/>
             </xs:choice>
             <xs:attribute name="prefix"
             use="optional" type="xs:string"/>
           </xs:restriction>
         </xs:complexContent>
       </xs:complexType>

       <!-- EXCEPT-TEL TYPE -->
       <xs:complexType name="exceptTelType">
         <xs:attribute name="prefix" type="xs:string" use="optional"/>
         <xs:attribute name="id" type="xs:anyURI" use="optional"/>
       </xs:complexType>

       <!-- METHOD -->
       <xs:element name="method" type="lc:method-type"/>

       <!-- METHOD TYPE -->
       <xs:simpleType name="method-type">
         <xs:restriction base="xs:string">
           <xs:enumeration value="INVITE"/>
           <xs:enumeration value="MESSAGE"/>
           <xs:enumeration value="REGISTER"/>
           <xs:enumeration value="SUBSCRIBE"/>
           <xs:enumeration value="OPTIONS"/>
           <xs:enumeration value="PUBLISH"/>
         </xs:restriction>
       </xs:simpleType>

       <!-- TARGET SIP ENTITY -->
       <xs:element name="target-sip-entity" type="xs:anyURI" minOccurs="0"/>

       <!-- ACTIONS -->

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       <xs:element name="accept">
         <xs:choice>
           <element name="rate" type="xs:decimal" minOccurs="0"/>
           <element name="win" type="xs:integer" minOccurs="0"/>
           <element name="percent" type="xs:decimal" minOccurs="0"/>
           <any namespace="##other" processContents="lax" minOccurs="0"
           maxOccurs="unbounded"/>
         </xs:choice>
         <xs:attribute name="alt-action" type="xs:string" default="reject"/>
         <xs:attribute name="alt-target" type="lc:alt-target-type"
         use="optional"/>
         <anyAtrribute namespace="##other" processContents="lax"/>
       </xs:element>

       <!-- ALT TARGET TYPE -->
       <xs:simpleType name="alt-target-type">
         <xs:list itemType="xs:anyURI"/>
       </xs:simpleType>

       </xs:schema>

    7.  Security Considerations

       Two primary security considerations arise from this specification.
       One is the distribution mechanism for the load filtering policy that
       is based on the SIP event notification framework, and the other is
       the enforcement mechanism for the load-filtering policy.

       Security considerations for SIP event package mechanisms are covered
       in Section 6 of [RFC6665].  A particularly relevant security concern
       for this event package is that if the notifiers can be spoofed,
       attackers can send fake notifications asking subscribers to throttle
       all traffic, leading to denial-of-service (DoS) attacks.  Therefore,
       this SIP load-filtering mechanism MUST be used in a Trust Domain
       (Section 3.4).  But if a legitimate notifier in the Trust Domain is
       itself compromised, additional mechanisms will be needed to detect
       the attack.

       Security considerations for load-filtering policy enforcement depends
       very much on the contents of the policy.  This specification defines
       a possible match of the following SIP header fields in a load-
       filtering policy: <from>, <to>, <request-uri>, and
       <p-asserted-identity>.  The exact requirement to authenticate and
       authorize these fields is up to the service provider.  In general, if
       the identity field represents the source of the request, it SHOULD be
       authenticated and authorized; if the identity field represents the
       destination of the request, the authentication and authorization is
       optional.

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       In addition, the "redirect" action (Section 5.4) could facilitate a
       reflection denial-of-service attack.  If a number of SIP proxy
       servers in a Trust Domain are using UDP and configured to get their
       policies from a central server.  An attacker spoofs the central
       server's address to send a number of NOTIFY bodies telling the proxy
       servers to redirect all calls to victim@outside-of-trust-domain.com.
       The proxy servers then redirect all calls to the victim, who then
       becomes a victim of Denial of Service attack and becomes
       inaccessiable from the Internet.  To address this type of threat,
       this specification requires that a Trust Domain agrees on what types
       of calls can be affected as well as on the destinations to which
       calls may be redirected, as in Section 3.4.

    8.  IANA Considerations

       This specification registers a SIP event package, a new media type, a
       new XML namespace, and a new XML schema.

    8.1.  Load-Control Event Package Registration

       This section registers an event package based on the registration
       procedures defined in [RFC6665].

       Package name: load-control

       Type: package

       Published specification: This specification

       Person to contact: Charles Shen, charles@cs.columbia.edu

    8.2.  application/load-control+xml Media Type Registration

       This section registers a new media type based on the procedures
       defined in [RFC6838] and guidelines in [RFC3023].

       Type name: application

       Subtype name: load-control+xml

       Required parameters: none

       Optional parameters: Same as charset parameter of application/xml as
       specified in [RFC3023].

       Encoding considerations: Same as encoding considerations of
       application/xml as specified in [RFC3023].

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       Security considerations: See Section 10 of [RFC3023] and Section 7 of
       this specification.

       Interoperability considerations: none

       Published specification: This specification

       Applications that use this media type: Applications that perform load
       control of SIP entities.

       Fragment identifier considerations: Same as fragment identifier
       considerations of application/xml as specified in [RFC3023].

       Additional Information:

          Deprecated alias names for this type: none

          Magic Number(s): none

          File Extension(s): .xml

          Macintosh file type code(s): "TEXT"

       Person and email address for further information: Charles Shen,
       charles@cs.columbia.edu

       Intended usage: COMMON

       Restrictions on usage: none

       Author: Charles Shen, Henning Schulzrinne, Arata Koike

       Change controller: IESG

       Provisional registration? (standards tree only): no

    8.3.  URN Sub-Namespace Registration

       This section registers a new XML namespace, as per the guidelines in
       [RFC3688]

       URI: The URI for this namespace is

          urn:ietf:params:xml:ns:load-control

       Registrant Contact: IETF SOC Working Group <sip-overload@ietf.org>,
       as designated by the IESG <iesg@ietf.org>

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       XML:

       BEGIN
       <?xml version="1.0"?>
       <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN"
                    "http://www.w3.org/TR/xhtml-basic/xhtml-basic10.dtd">
       <html xmlns="http://www.w3.org/1999/xhtml">
       <head>
         <meta http-equiv="content-type"
           content="text/html;charset=iso-8859-1"/>
         <title>SIP Load-Control Namespace</title>
       </head>
       <body>
         <h1>Namespace for SIP Load Control</h1>
         <h2>urn:ietf:params:xml:ns:load-control</h2>
         <p>See <a href="http://www.rfc-editor.org/rfc/rfc7200.txt">
             RFC 7200</a>.</p>
       </body>
       </html>
       END

    8.4.  Load-Control Schema Registration

       URI: urn:ietf:params:xml:schema:load-control

       Registrant Contact: IETF SOC working group, Charles Shen
       (charles@cs.columbia.edu).

       XML: the XML schema contained in Section 6 has been registered.

       Its first line is

       <?xml version="1.0" encoding="UTF-8"?>

       and its last line is

       </xs:schema>

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    9.  Acknowledgements

       The authors would like to thank Jari Arkko, Richard Barnes, Stewart
       Bryant, Gonzalo Camarillo, Bruno Chatras, Benoit Claise, Spencer
       Dawkins, Martin Dolly, Keith Drage, Ashutosh Dutta, Donald Eastlake,
       Adrian Farrel, Stephen Farrell, Janet Gunn, Vijay Gurbani, Brian
       Haberman, Volker Hilt, Geoff Hunt, Carolyn Johnson, Hadriel Kaplan,
       Paul Kyzivat, Barry Leiba, Pearl Liang, Salvatore Loreto, Timothy
       Moran, Eric Noel, Parthasarathi R, Pete Resnick, Adam Roach, Dan
       Romascanu, Shida Schubert, Robert Sparks, Martin Stiemerling, Sean
       Turner, Phil Williams, and other members of the SOC and SIPPING
       working groups for many helpful comments.  In particular, Bruno
       Chatras proposed the <method> and <target-sip-entity> condition
       elements along with many other text improvements.  Janet Gunn
       provided detailed text suggestions including Appendix C. Eric Noel
       suggested clarification on load-filtering policy distribution
       initialization process.  Shida Schubert made many suggestions such as
       terminology usage.  Phil Williams suggested adding support for delta
       updates.  Ashutosh Dutta gave pointers to Public Switched Telephone
       Network (PSTN) references.  Adam Roach suggested improvements related
       to RFC 6665 and offered other helpful clarifications.  Richard Barnes
       made many suggestions such as referencing the Trust Domain concept of
       RFCs 3324 and 3325, the use of a separate element for 'tel' URI
       grouping, and addressing the "redirect" action security threat.

    10.  References

    10.1.  Normative References

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

       [RFC2141]  Moats, R., "URN Syntax", RFC 2141, May 1997.

       [RFC3023]  Murata, M., St. Laurent, S., and D. Kohn, "XML Media
                  Types", RFC 3023, January 2001.

       [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.

       [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
                  January 2004.

       [RFC3966]  Schulzrinne, H., "The tel URI for Telephone Numbers", RFC
                  3966, December 2004.

    Shen, et al.                 Standards Track                   [Page 27]

    RFC 7200             SIP Load-Control Event Package           April 2014

       [RFC4745]  Schulzrinne, H., Tschofenig, H., Morris, J., Cuellar, J.,
                  Polk, J., and J. Rosenberg, "Common Policy: A Document
                  Format for Expressing Privacy Preferences", RFC 4745,
                  February 2007.

       [RFC6665]  Roach, A., "SIP-Specific Event Notification", RFC 6665,
                  July 2012.

       [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
                  Specifications and Registration Procedures", BCP 13, RFC
                  6838, January 2013.

    10.2.  Informative References

       [E.300SerSup3]
                  ITU-T, "North American Precise Audible Tone Plan",
                  Recommendation E.300 Series Supplement 3, November 1988.

       [E.412]    ITU-T, "Network Management Controls", Recommendation
                  E.412-2003, January 2003.

       [Q.1248.2] ITU-T, "Interface Recommendation for Intelligent Network
                  Capability Set4:SCF-SSF interface", Recommendation
                  Q.1248.2, July 2001.

       [RFC2648]  Moats, R., "A URN Namespace for IETF Documents", RFC 2648,
                  August 1999.

       [RFC3324]  Watson, M., "Short Term Requirements for Network Asserted
                  Identity", RFC 3324, November 2002.

       [RFC3325]  Jennings, C., Peterson, J., and M. Watson, "Private
                  Extensions to the Session Initiation Protocol (SIP) for
                  Asserted Identity within Trusted Networks", RFC 3325,
                  November 2002.

       [RFC4412]  Schulzrinne, H. and J. Polk, "Communications Resource
                  Priority for the Session Initiation Protocol (SIP)", RFC
                  4412, February 2006.

       [RFC4825]  Rosenberg, J., "The Extensible Markup Language (XML)
                  Configuration Access Protocol (XCAP)", RFC 4825, May 2007.

       [RFC5031]  Schulzrinne, H., "A Uniform Resource Name (URN) for
                  Emergency and Other Well-Known Services", RFC 5031,
                  January 2008.

    Shen, et al.                 Standards Track                   [Page 28]

    RFC 7200             SIP Load-Control Event Package           April 2014

       [RFC5390]  Rosenberg, J., "Requirements for Management of Overload in
                  the Session Initiation Protocol", RFC 5390, December 2008.

       [RFC6357]  Hilt, V., Noel, E., Shen, C., and A. Abdelal, "Design
                  Considerations for Session Initiation Protocol (SIP)
                  Overload Control", RFC 6357, August 2011.

       [SIP-OVERLOAD]
                  Gurbani, V., Ed., Hilt, V., and H. Schulzrinne, "Session
                  Initiation Protocol (SIP) Overload Control", Work in
                  Progress, March 2014.

    Shen, et al.                 Standards Track                   [Page 29]

    RFC 7200             SIP Load-Control Event Package           April 2014

    Appendix A.  Definitions

       This specification reuses the definitions for "Event Package",
       "Notification", "Notifier", "Subscriber", and "Subscription" as in
       [RFC6665].  The following additional definitions are also used.

       Load Filtering:  A load-control mechanism that applies specific
          actions to selected loads (e.g., SIP requests) matching specific
          conditions.

       Load-Filtering Policy:  A set of zero or more load-filtering rules,
          also known as load-filtering rule set.

       Load-Filtering Rule:  Conditions and actions to be applied for load
          filtering.

       Load-Filtering Condition:  Elements that describe how to select loads
          to apply load-filtering actions.  This specification defines the
          <call-identity>, <method>, <target-sip-identity>, and <validity>
          condition elements (Section 5.3).

       Load-Filtering Action:  An operation to be taken by a load-filtering
          server on loads that match the load-filtering conditions.  This
          specification allows actions such as accept, reject, and redirect
          of loads (Section 5.4).

       Load-Filtering Server:  A server that performs load filtering.  In
          the context of this specification, the load-filtering server is
          the subscriber, which receives load-filtering policies from the
          notifier and enforces those policies during load filtering.

       Load-Control Document:  An XML document that describes the load-
          filtering policies (Section 5).  It inherits and enhances the
          common policy document defined in [RFC4745].

    Appendix B.  Design Requirements

       The SIP load-filtering mechanism needs to satisfy the following
       requirements:

       o  For simplicity, the solution should focus on a method for
          controlling SIP load, rather than a generic application-layer
          mechanism.

       o  The load-filtering policy needs to be distributed efficiently to
          possibly a large subset of all SIP elements.

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       o  The solution should reuse existing SIP protocol mechanisms to
          reduce implementation and deployment complexity.

       o  For predictable overload situations, such as holidays and mass
          calling events, the load-filtering policy should specify during
          what time it is to be applied, so that the information can be
          distributed ahead of time.

       o  For destination-specific overload situations, the load-filtering
          policy should be able to describe the destination domain or the
          callee.

       o  To address accidental and intentional high-volume call generators,
          the load-filtering policy should be able to specify the caller.

       o  Caller and callee need to be specified as both SIP URIs and 'tel'
          URIs [RFC3966] in load-filtering policies.

       o  It should be possible to specify particular information in the SIP
          headers (e.g., prefixes in telephone numbers) that allow load
          filtering over limited regionally focused overloads.

       o  The solution should draw upon experiences from related PSTN
          mechanisms [Q.1248.2] [E.412] [E.300SerSup3] where applicable.

       o  The solution should be extensible to meet future needs.

    Appendix C.  Discussion of How This Specification Meets the Requirements
                 of RFC 5390

       This section evaluates whether the load-control event package
       mechanism defined in this specification satisfies various SIP
       overload control requirements set forth by [RFC5390].  As mentioned
       in Section 1, this specification complements other efforts in the
       overall SIP load-control solution space.  Therefore, not all RFC 5390
       requirements are found applicable to this specification.  This
       specification categorizes the assessment results into Yes (the
       requirement is met), P/A (Partially Applicable), No (must be used in
       conjunction with another mechanism to meet the requirement), and N/A
       (Not Applicable).

          REQ 1: The overload mechanism shall strive to maintain the overall
          useful throughput (taking into consideration the quality-of-
          service needs of the using applications) of a SIP server at
          reasonable levels, even when the incoming load on the network is
          far in excess of its capacity.  The overall throughput under load
          is the ultimate measure of the value of an overload control
          mechanism.

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       P/A.  The goal of load filtering is to prevent overload or maintain
       overall goodput during the time of overload, but it is dependent on
       the predictions of the load and the computations as well as
       distribution of the filtering policies.  If the load predictions or
       filtering policy computations are incorrect, or the filtering policy
       is not properly distributed, the mechanism will be less effective.
       On the other hand, if the load can be accurately predicted and
       filtering policies be computed and distributed appropriately, this
       requirement can be met.

          REQ 2: When a single network element fails, goes into overload, or
          suffers from reduced processing capacity, the mechanism should
          strive to limit the impact of this on other elements in the
          network.  This helps to prevent a small-scale failure from
          becoming a widespread outage.

       N/A if load-filtering policies are installed in advance and do not
       change during the potential overload period, P/A if load-filtering
       policies are dynamically adjusted.  The algorithm to dynamically
       compute load-filtering policies is outside the scope of this
       specification, while the distribution of the updated filtering
       policies uses the event package mechanism of this specification.

          REQ 3: The mechanism should seek to minimize the amount of
          configuration required in order to work.  For example, it is
          better to avoid needing to configure a server with its SIP message
          throughput, as these kinds of quantities are hard to determine.

       No.  This mechanism is entirely dependent on advance configuration,
       based on advance knowledge.  In order to satisfy REQ 3, it should be
       used in conjunction with other mechanisms that are not based on
       advance configuration.

          REQ 4: The mechanism must be capable of dealing with elements that
          do not support it, so that a network can consist of a mix of
          elements that do and don't support it.  In other words, the
          mechanism should not work only in environments where all elements
          support it.  It is reasonable to assume that it works better in
          such environments, of course.  Ideally, there should be
          incremental improvements in overall network throughput as
          increasing numbers of elements in the network support the
          mechanism.

       No.  This mechanism is entirely dependent on the participation of all
       possible neighbors.  In order to satisfy REQ 4, it should be used in
       conjunction with other mechanisms, some of which are described in
       Section 3.4.

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          REQ 5: The mechanism should not assume that it will only be
          deployed in environments with completely trusted elements.  It
          should seek to operate as effectively as possible in environments
          where other elements are malicious; this includes preventing
          malicious elements from obtaining more than a fair share of
          service.

       No.  This mechanism is entirely dependent on the non-malicious
       participation of all possible neighbors.  In order to satisfy REQ 5,
       it should be used in conjunction with other mechanisms, some of which
       are described in Section 3.4.

          REQ 6: When overload is signaled by means of a specific message,
          the message must clearly indicate that it is being sent because of
          overload, as opposed to other, non overload-based failure
          conditions.  This requirement is meant to avoid some of the
          problems that have arisen from the reuse of the 503 response code
          for multiple purposes.  Of course, overload is also signaled by
          lack of response to requests.  This requirement applies only to
          explicit overload signals.

       N/A.  This mechanism signals anticipated overload, not actual
       overload.  However, the signals in this mechanism are not used for
       any other purpose.

          REQ 7: The mechanism shall provide a way for an element to
          throttle the amount of traffic it receives from an upstream
          element.  This throttling shall be graded so that it is not all-
          or-nothing as with the current 503 mechanism.  This recognizes the
          fact that "overload" is not a binary state and that there are
          degrees of overload.

       Yes.  This event package allows rate-/loss-/window-based overload
       control options as discussed in Section 5.4.

          REQ 8: The mechanism shall ensure that, when a request was not
          processed successfully due to overload (or failure) of a
          downstream element, the request will not be retried on another
          element that is also overloaded or whose status is unknown.  This
          requirement derives from REQ 1.

       N/A to the load-control event package mechanism itself.

          REQ 9: That a request has been rejected from an overloaded element
          shall not unduly restrict the ability of that request to be
          submitted to and processed by an element that is not overloaded.
          This requirement derives from REQ 1.

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       Yes.  For example, load-filtering policy (Section 3.1) can include
       alternative forwarding destinations for rejected requests.

          REQ 10: The mechanism should support servers that receive requests
          from a large number of different upstream elements, where the set
          of upstream elements is not enumerable.

       No.  Because this mechanism requires advance configuration of
       specifically identified neighbors, it does not support environments
       where the number and identity of the upstream neighbors are not known
       in advance.  In order to satisfy REQ 10, it should be used in
       conjunction with other mechanisms.

          REQ 11: The mechanism should support servers that receive requests
          from a finite set of upstream elements, where the set of upstream
          elements is enumerable.

       Yes.  See also answer to REQ 10.

          REQ 12: The mechanism should work between servers in different
          domains.

       Yes.  The load-control event package mechanism is not limited by
       domain boundaries.  However, it is likely more applicable in intra-
       domain scenarios than in inter-domain scenarios due to security and
       other concerns (see also Section 3.4).

          REQ 13: The mechanism must not dictate a specific algorithm for
          prioritizing the processing of work within a proxy during times of
          overload.  It must permit a proxy to prioritize requests based on
          any local policy, so that certain ones (such as a call for
          emergency services or a call with a specific value of the
          Resource-Priority header field [RFC4412]) are given preferential
          treatment, such as not being dropped, being given additional
          retransmission, or being processed ahead of others.

       P/A.  This mechanism does not specifically address the prioritizing
       of work during times of overload.  But it does not preclude any
       particular local policy.

          REQ 14: The mechanism should provide unambiguous directions to
          clients on when they should retry a request and when they should
          not.  This especially applies to TCP connection establishment and
          SIP registrations, in order to mitigate against avalanche restart.

       N/A to the load-control event package mechanism itself.

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          REQ 15: In cases where a network element fails, is so overloaded
          that it cannot process messages, or cannot communicate due to a
          network failure or network partition, it will not be able to
          provide explicit indications of the nature of the failure or its
          levels of congestion.  The mechanism must properly function in
          these cases.

       P/A.  Because the load-filtering policies are provisioned in advance,
       they are not affected by the overload or failure of other network
       elements.  On the other hand, they may not, in those cases, be able
       to protect the overloaded network elements (see REQ 1).

          REQ 16: The mechanism should attempt to minimize the overhead of
          the overload control messaging.

       Yes.  The standardized SIP event package mechanism [RFC6665] is used.

          REQ 17: The overload mechanism must not provide an avenue for
          malicious attack, including DoS and DDoS attacks.

       P/A.  This mechanism does provide a potential avenue for malicious
       attacks.  Therefore, the security mechanisms for SIP event packages,
       in general, [RFC6665] and Section 7 of this specification should be
       used.

          REQ 18: The overload mechanism should be unambiguous about whether
          a load indication applies to a specific IP address, host, or URI,
          so that an upstream element can determine the load of the entity
          to which a request is to be sent.

       Yes.  The identity of load indication is covered in the load-
       filtering policy format definition in Section 3.1.

          REQ 19: The specification for the overload mechanism should give
          guidance on which message types might be desirable to process over
          others during times of overload, based on SIP-specific
          considerations.  For example, it may be more beneficial to process
          a SUBSCRIBE refresh with Expires of zero than a SUBSCRIBE refresh
          with a non-zero expiration (since the former reduces the overall
          amount of load on the element), or to process re-INVITEs over new
          INVITEs.

       N/A to the load-control event package mechanism itself.

          REQ 20: In a mixed environment of elements that do and do not
          implement the overload mechanism, no disproportionate benefit
          shall accrue to the users or operators of the elements that do not
          implement the mechanism.

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       No.  This mechanism is entirely dependent on the participation of all
       possible neighbors.  In order to satisfy REQ 20, it should be used in
       conjunction with other mechanisms, some of which are described in
       Section 3.4.

          REQ 21: The overload mechanism should ensure that the system
          remains stable.  When the offered load drops from above the
          overall capacity of the network to below the overall capacity, the
          throughput should stabilize and become equal to the offered load.

       N/A to the load-control event package mechanism itself.

          REQ 22: It must be possible to disable the reporting of load
          information towards upstream targets based on the identity of
          those targets.  This allows a domain administrator who considers
          the load of their elements to be sensitive information, to
          restrict access to that information.  Of course, in such cases,
          there is no expectation that the overload mechanism itself will
          help prevent overload from that upstream target.

       N/A to the load-control event package mechanism itself.

          REQ 23: It must be possible for the overload mechanism to work in
          cases where there is a load balancer in front of a farm of
          proxies.

       Yes.  The load-control event package mechanism does not preclude its
       use in a scenario with server farms.

    Appendix D.  Complete Examples

    D.1.  Load-Control Document Examples

       This section presents two complete examples of load-control documents
       valid with respect to the XML schema defined in Section 6.

       The first example assumes that a set of hotlines are set up at
       "sip:alice@hotline.example.com" and "tel:+1-212-555-1234".  The
       hotlines are activated from 12:00 to 15:00 US Eastern Standard Time
       on 2008-05-31.  The goal is to limit the incoming calls to the
       hotlines to 100 requests per second.  Calls that exceed the rate
       limit are explicitly rejected.

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    RFC 7200             SIP Load-Control Event Package           April 2014

       <?xml version="1.0" encoding="UTF-8"?>
       <ruleset xmlns="urn:ietf:params:xml:ns:common-policy"
                   xmlns:lc="urn:ietf:params:xml:ns:load-control"
                   version="0" state="full">

           <rule id="f3g44k1">
               <conditions>
                   <lc:call-identity>
                       <lc:sip>
                           <lc:to>
                               <one id="sip:alice@hotline.example.com"/>
                               <one id="tel:+1-212-555-1234"/>
                           </lc:to>
                       </lc:sip>
                   </lc:call-identity>
                   <method>INVITE</method>
                   <validity>
                       <from>2008-05-31T12:00:00-05:00</from>
                       <until>2008-05-31T15:00:00-05:00</until>
                   </validity>
               </conditions>
               <actions>
                   <lc:accept alt-action="reject">
                       <lc:rate>100</lc:rate>
                   </lc:accept>
               </actions>

           </rule>
       </ruleset>

       The second example optimizes the usage of server resources during the
       three-day period following a hurricane.  Incoming calls to the domain
       "sandy.example.com" or to call destinations with prefix "+1-212" will
       be limited to a rate of 100 requests per second, except for those
       calls originating from a particular rescue team domain
       "rescue.example.com".  Outgoing calls from the hurricane domain or
       calls within the local domain are never limited.  All calls that are
       throttled due to the rate limit will be forwarded to an answering
       machine with updated hurricane rescue information.

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       <?xml version="1.0" encoding="UTF-8"?>
       <ruleset xmlns="urn:ietf:params:xml:ns:common-policy"
           xmlns:lc="urn:ietf:params:xml:ns:load-control"
           version="1" state="full">

           <rule id="f3g44k2">
               <conditions>
                   <lc:call-identity>
                       <lc:sip>
                           <lc:to>
                               <many domain="sandy.example.com"/>
                               <many-tel prefix="+1-212"/>
                           </lc:to>
                           <lc:from>
                               <many>
                                   <except domain="sandy.example.com"/>
                                   <except domain="rescue.example.com"/>
                               </many>
                           </lc:from>
                       </lc:sip>
                   </lc:call-identity>
                   <method>INVITE</method>
                   <validity>
                       <from>2012-10-25T09:00:00+01:00</from>
                       <until>2012-10-28T09:00:00+01:00</until>
                   </validity>
               </conditions>
               <actions>
                   <lc:accept alt-action="redirect" alt-target=
                           "sip:sandy@update.example.com">
                       <lc:rate>100</lc:rate>
                   </lc:accept>
               </actions>

           </rule>
       </ruleset>

       Sometimes it may occur that multiple rules in a ruleset define
       actions that match the same methods, call identity and validity.  In
       those cases, the "first-match-wins" principle is used.  For example,
       in the following ruleset, the first rule requires all calls from the
       "example.com" domain to be rejected.  Even though the rule following
       that one specifies that calls from "sip:alice@example.com" be
       redirected to a specific target "sip:eve@example.com", the calls from
       "sip:alice@example.com" will still be rejected because they have
       already been matched by the earlier rule.

    Shen, et al.                 Standards Track                   [Page 38]

    RFC 7200             SIP Load-Control Event Package           April 2014

       <?xml version="1.0" encoding="UTF-8"?>
       <ruleset xmlns="urn:ietf:params:xml:ns:common-policy"
           xmlns:lc="urn:ietf:params:xml:ns:load-control"
           version="1" state="full">

           <rule id="f3g44k3">
               <conditions>
                   <lc:call-identity>
                       <lc:sip>
                           <lc:from>
                               <many domain="example.com"/>
                           </lc:from>
                       </lc:sip>
                   </lc:call-identity>
                   <method>INVITE</method>
                   <validity>
                       <from>2013-7-2T09:00:00+01:00</from>
                       <until>2013-7-3T09:00:00+01:00</until>
                   </validity>
               </conditions>
               <actions>
                   <lc:accept alt-action="reject">
                       <lc:rate>0</lc:rate>
                   </lc:accept>
               </actions>
           </rule>

           <rule id="f3g44k4">
               <conditions>
                   <lc:call-identity>
                       <lc:sip>
                           <lc:from>
                               <one id="sip:alice@example.com"/>
                           </lc:from>
                       </lc:sip>
                   </lc:call-identity>
                   <method>INVITE</method>
                   <validity>
                       <from>2013-7-2T09:00:00+01:00</from>
                       <until>2013-7-3T09:00:00+01:00</until>
                   </validity>
               </conditions>
               <actions>
                   <lc:accept alt-action="redirect" alt-target=
                           "sip:eve@example.com">
                       <lc:rate>0</lc:rate>
                   </lc:accept>
               </actions>

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           </rule>
       </ruleset>

    D.2.  Message Flow Examples

       This section presents an example message flow of using the load-
       control event package mechanism defined in this specification.

          atlanta             biloxi
             | F1 SUBSCRIBE      |
             |------------------>|
             | F2 200 OK         |
             |<------------------|
             | F3 NOTIFY         |
             |<------------------|
             | F4 200 OK         |
             |------------------>|

          F1 SUBSCRIBE atlanta.example.com -> biloxi.example.com

             SUBSCRIBE sip:biloxi.example.com SIP/2.0
             Via: SIP/2.0/TCP atlanta.example.com;branch=z9hG4bKy7cjbu3
             From: sip:atlanta.example.com;tag=162ab5
             To: sip:biloxi.example.com
             Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
             CSeq: 2012 SUBSCRIBE
             Contact: sip:atlanta.example.com
             Event: load-control
             Max-Forwards: 70
             Accept: application/load-control+xml
             Expires: 3600
             Content-Length: 0

          F2 200 OK   biloxi.example.com -> atlanta.example.com

             SIP/2.0 200 OK
             Via: SIP/2.0/TCP biloxi.example.com;branch=z9hG4bKy7cjbu3
               ;received=192.0.2.1
             To: <sip:biloxi.example.com>;tag=331dc8
             From: <sip:atlanta.example.com>;tag=162ab5
             Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
             CSeq: 2012 SUBSCRIBE
             Expires: 3600
             Contact: sip:biloxi.example.com
             Content-Length: 0

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          F3 NOTIFY  biloxi.example.com -> atlanta.example.com

             NOTIFY sip:atlanta.example.com SIP/2.0
             Via: SIP/2.0/TCP biloxi.example.com;branch=z9hG4bKy71g2ks
             From: <sip:biloxi.example.com>;tag=331dc8
             To: <sip:atlanta.example.com>;tag=162ab5
             Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
             Event: load-control
             Subscription-State: active;expires=3599
             Max-Forwards: 70
             CSeq: 1775 NOTIFY
             Contact: sip:biloxi.example.com
             Content-Type: application/load-control+xml
             Content-Length: ...

             [Load-Control Document]

          F4 200 OK atlanta.example.com -> biloxi.example.com

             SIP/2.0 200 OK
             Via: SIP/2.0/TCP atlanta.example.com;branch=z9hG4bKy71g2ks
               ;received=192.0.2.2
             From: <sip:biloxi.example.com>;tag=331dc8
             To: <sip:atlanta.example.com>;tag=162ab5
             Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
             CSeq: 1775 NOTIFY
             Content-Length: 0

    Appendix E.  Related Work

    E.1.  Relationship to Load Filtering in PSTN

       It is known that an existing PSTN network also uses a load-filtering
       mechanism to prevent overload and the filtering policy configuration
       is done manually except in specific cases when the Intelligent
       Network architecture is used [Q.1248.2][E.412].  This specification
       defines a load-filtering mechanism based on the SIP event
       notification framework that allows automated filtering policy
       distribution in suitable environments.

       PSTN overload control uses messages that specify an outgoing control
       list, call gap duration, and control duration [Q.1248.2][E.412].
       These items correspond roughly to the identity, action, and time
       fields of the SIP load-filtering policy defined in this
       specification.  However, the load-filtering policy defined in this
       specification is much more generic and flexible as opposed to its
       PSTN counterpart.

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    RFC 7200             SIP Load-Control Event Package           April 2014

       Firstly, PSTN load filtering only applies to telephone numbers.  The
       identity element of SIP load-filtering policy allows both SIP URI and
       telephone numbers (through 'tel' URI) to be specified.  These
       identities can be arbitrarily grouped by SIP domains or any number of
       leading prefixes of the telephone numbers.

       Secondly, the PSTN load-filtering action is usually limited to call
       gapping.  The action field in SIP load-filtering policy allows more
       flexible possibilities such as rate throttle and others.

       Thirdly, the duration field in PSTN load filtering specifies a value
       in seconds for the load-filtering duration only, and the allowed
       values are mapped into a value set.  The time field in SIP load-
       filtering policy may specify not only a duration, but also a future
       activation time that could be especially useful for automating load
       filtering for predictable overloads.

       PSTN load filtering can be performed in both edge switches and
       transit switches; the SIP load filtering can also be applied in both
       edge proxy servers and core proxy servers, and even in capable user
       agents.

       PSTN load filtering also has special accommodation for High
       Probability of Completion (HPC) calls, which would be similar to
       calls designated by the SIP Resource Priority Headers [RFC4412].  The
       SIP load-filtering mechanism also allows prioritizing the treatment
       of these calls by specifying favorable actions for them.

       PSTN load filtering also provides an administrative option for
       routing failed call attempts to either a reorder tone [E.300SerSup3]
       indicating overload conditions or a special recorded announcement.  A
       similar capability can be provided in the SIP load-filtering
       mechanism by specifying appropriate "alt-action" attribute in the SIP
       load-filtering action field.

    E.2.  Relationship with Other IETF SIP Overload Control Efforts

       The load-filtering policies in this specification consist of
       identity, action, and time.  The identity can range from a single
       specific user to an arbitrary user aggregate, domains, or areas.  The
       user can be identified by either the source or the destination.  When
       the user is identified by the source and a favorable action is
       specified, the result is, to some extent, similar to identifying a
       priority user based on authorized Resource Priority Headers [RFC4412]
       in the requests.  Specifying a source user identity with an
       unfavorable action would cause an effect to some extent similar to an
       inverse SIP resource priority mechanism.

    Shen, et al.                 Standards Track                   [Page 42]

    RFC 7200             SIP Load-Control Event Package           April 2014

       The load-filtering policy defined in this specification is generic
       and expected to be applicable not only to the load-filtering
       mechanism but also to the feedback overload control mechanism in
       [SIP-OVERLOAD].  In particular, both mechanisms could use specific or
       wildcard identities for load control and could share well-known load-
       control actions.  The time duration field in the load-filtering
       policy could also be used in both mechanisms.  As mentioned in
       Section 1, the load-filtering policy distribution mechanism and the
       feedback overload control mechanism address complementary areas in
       the overload control problem space.  Load filtering is more proactive
       and focuses on distributing filtering policies towards the source of
       the traffic; the hop-by-hop feedback-based approach is reactive and
       reduces traffic already accepted by the network.  Therefore, they
       could also make different use of the generic load-filtering policy
       components.  For example, the load-filtering mechanism may use the
       time field in the filtering policy to specify not only a control
       duration but also a future activation time to accommodate a
       predicable overload such as the one caused by Mother's Day greetings
       or a viewer-voting program; the feedback-based control might not need
       to use the time field or might use the time field to specify an
       immediate load-control duration.

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    RFC 7200             SIP Load-Control Event Package           April 2014

    Authors' Addresses

       Charles Shen
       Columbia University
       Department of Computer Science
       1214 Amsterdam Avenue, MC 0401
       New York, NY   10027
       USA

       Phone: +1 212 854 3109
       EMail: charles@cs.columbia.edu

       Henning Schulzrinne
       Columbia University
       Department of Computer Science
       1214 Amsterdam Avenue, MC 0401
       New York, NY   10027
       USA

       Phone: +1 212 939 7004
       EMail: schulzrinne@cs.columbia.edu

       Arata Koike
       NTT Network Technology Labs
       3-9-11 Midori-cho Musashino-shi
       Tokyo  180-8585
       Japan

       Phone: +81 422 59 6099
       EMail: koike.arata@lab.ntt.co.jp

    Shen, et al.                 Standards Track                   [Page 44]