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RFC 7200 - A Session Initiation Protocol (SIP) Load-Control Even

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

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


   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

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.

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

   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

   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

2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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

   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

      +-----------+   +-----------+   +-----------+   +-----------+
      |           |   |           |   |           |   |           |
      |   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

   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

   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].

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

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

   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

   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

   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

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

   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.

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

   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

      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

   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:


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

   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

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

   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

   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".

                               <except domain="manhattan.example.com"/>
                               <except-tel prefix="+1-212"/>
                           <one id="tel:+1-202-999-1234"/>

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

   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.


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.


   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>

      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

      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

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.


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

   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"
               IF unreliable-transport
                 THEN treat as "reject"
             "reject": "reject"
             default: "reject"

   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.

               <accept alt-action="redirect" alt-target=

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>

   <?xml version="1.0" encoding="UTF-8"?>
   <xs:schema targetNamespace="urn:ietf:params:xml:ns:load-control"

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

   <!-- RULESET -->

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

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

   <!-- CONDITIONS -->

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

   <xs:complexType name="call-identity-type">
       <xs:element name="sip" type="lc:sip-id-type"/>
       <any namespace="##other" processContents="lax" minOccurs="0"
     <anyAtrribute namespace="##other" processContents="lax"/>

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

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

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

   <!-- MANY-TEL TYPE -->
   <xs:complexType name="manyTelType">
       <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:attribute name="prefix"
         use="optional" type="xs:string"/>

   <!-- 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"/>

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

   <!-- ACTIONS -->

   <xs:element name="accept">
       <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"
     <xs:attribute name="alt-action" type="xs:string" default="reject"/>
     <xs:attribute name="alt-target" type="lc:alt-target-type"
     <anyAtrribute namespace="##other" processContents="lax"/>

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


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

   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].

   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,

   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

   URI: The URI for this namespace is


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


   <?xml version="1.0"?>
   <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN"
   <html xmlns="http://www.w3.org/1999/xhtml">
     <meta http-equiv="content-type"
     <title>SIP Load-Control Namespace</title>
     <h1>Namespace for SIP Load Control</h1>
     <p>See <a href="http://www.rfc-editor.org/rfc/rfc7200.txt">
         RFC 7200</a>.</p>

8.4.  Load-Control Schema Registration

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

   Registrant Contact: IETF SOC working group, Charles Shen

   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


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.

   [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

              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.

   [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.

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

   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

   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

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

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

   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

   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

   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

   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.

      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

   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.

   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

   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.

      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

      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

   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.

   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

   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.

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

       <rule id="f3g44k1">
                           <one id="sip:alice@hotline.example.com"/>
                           <one id="tel:+1-212-555-1234"/>
               <lc:accept alt-action="reject">


   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.

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

       <rule id="f3g44k2">
                           <many domain="sandy.example.com"/>
                           <many-tel prefix="+1-212"/>
                               <except domain="sandy.example.com"/>
                               <except domain="rescue.example.com"/>
               <lc:accept alt-action="redirect" alt-target=


   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.

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

       <rule id="f3g44k3">
                           <many domain="example.com"/>
               <lc:accept alt-action="reject">

       <rule id="f3g44k4">
                           <one id="sip:alice@example.com"/>
               <lc:accept alt-action="redirect" alt-target=


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
         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

      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
         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.

   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

   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.

   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.

Authors' Addresses

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

   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

   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

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


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