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Versions: (draft-soc-load-control-event-package) 00 01 02 03 04 05 06 08 09 10 11 12 13 RFC 7200

IETF SOC Working Group                                           C. Shen
Internet-Draft                                            H. Schulzrinne
Intended status: Standards Track                             Columbia U.
Expires: June 17, 2014                                          A. Koike
                                                                     NTT
                                                       December 14, 2013


     A Session Initiation Protocol (SIP) Load Control Event Package
            draft-ietf-soc-load-control-event-package-13.txt

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
   based on their source or destination domain, telephone number prefix
   or for a specific user.  The mechanism helps to prevent signaling
   overload and complements feedback-based SIP overload control efforts.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 17, 2014.

Copyright Notice

   Copyright (c) 2013 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



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   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  . . . . . . . . . . . . . . .   7
   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  . . . . . . . . . . . . . . . . . . . . . . .  13
       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  . . . . . . . . . . . . . . . . . . . . . .  26
   10. Appendix  . . . . . . . . . . . . . . . . . . . . . . . . . .  27
     10.1.  Definitions  . . . . . . . . . . . . . . . . . . . . . .  27
     10.2.  Design Requirements  . . . . . . . . . . . . . . . . . .  28
     10.3.  Discussion of this specification meeting the
            requirements of RFC5390  . . . . . . . . . . . . . . . .  28



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     10.4.  Complete Examples  . . . . . . . . . . . . . . . . . . .  33
       10.4.1.  Load Control Document Examples . . . . . . . . . . .  33
       10.4.2.  Message Flow Examples  . . . . . . . . . . . . . . .  37
     10.5.  Related Work . . . . . . . . . . . . . . . . . . . . . .  38
       10.5.1.  Relationship with Load Filtering in PSTN . . . . . .  38
       10.5.2.  Relationship with Other IETF SIP Overload Control
                Efforts  . . . . . . . . . . . . . . . . . . . . . .  39
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  40
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  40
     11.2.  Informative References . . . . . . . . . . . . . . . . .  41
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  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
   [I-D.ietf-soc-overload-control].  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.  They work 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



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   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 (N 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 limiting the request rate below a certain level
   (Section 5.4).

3.2.  Load Filtering Policy Computation

   Computing the load filtering policies 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 which both prevents overload and
   maximizes effective throughput (aka goodput).  In some cases, in
   order to better utilize system resources, it may be preferable to
   employ an algorithm which 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 out of scope of this
   specification.

3.3.  Load Filtering Policy Distribution

   For load filtering policy distribution, 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 XML schema of a load control document
   (Section 5), which is used to encode load filtering policies.

   In order for load filtering polices 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



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   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 Section 4.1 and
   Section 4.2 of [RFC6665].  The refresh procedure is describe in
   Section 4.7.  The subscribers can terminate the subscription after an
   extended period of absence of signaling message exchange, and can
   resubscribe if it determines 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

   At initialization stage, the proxy servers first identify all their
   outgoing signaling neighbors and subscribe to them.  The neighbor
   identification process can be performed by service providers through
   direct provisioning, or by the proxy servers themselves via
   progressive learning from the signaling messages sent and received.
   Assuming all signaling relationships in Figure 1 are bi-directional,
   after this initialization stage, each proxy server will be subscribed



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   to all its neighbors.

   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 policy 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 800 number translation services.
   The application server may itself be a SIP proxy server or a SIP
   Back-to-Back User Agent (B2BUA).  If one of the 800 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 these three 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 behaviour of those SIP entities.  Such simple Trust Domains
   may be joined to form larger Trust Domains by bi-lateral 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  The mechanisms used to secure the communication among SIP entities
      within the Trust Domain.

   o  The manner used to determine which SIP entities are part of the
      Trust Domain.

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

   o  That SIP entities in the Trust Domain are compliant to SIP
      [RFC6665]

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

   o  The agreement 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 specific domains; <many-tel> and <except-tel> might be
      limited to describe within certain prefixes.

   o  The agreement 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.

   It is important to note that effectiveness of SIP load filtering
   requires that all neighbors that are possible signaling sources
   participate and enforce the designated load filtering policies.
   Otherwise, a single non-conforming neighbor could make the whole
   filtering efforts useless by pumping in excessive traffic to overload



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   the server.  Therefore, the SIP server that distributes load
   filtering policies needs to take counter-measures 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 NOTIFY request body exists but no
   Accept header field is specified in a SUBSCRIBE request, the NOTIFY
   request body MUST contain "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 policy, the notifier MUST
   return a 403 "Forbidden" response.

   If none of media types specified in the Accept header of the
   SUBSCRIBE request is supported, the notifier SHOULD return 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 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").
   Sending this NOTIFY request without body is often the case 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 which 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



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   consist of rules specifying actions to be applied to requests
   matching certain conditions.  A subscriber receiving a notification
   first installs these rules and then enforce 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 comes, 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
   this re-activation of subscription is successful or not, 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 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



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      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 not expected to
   happen either 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 5.9 of [RFC6665].

4.10.  Rate of Notifications

   Rate of notifications is likely not a 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
   situation does arise that a rather frequent load filtering policy
   update is needed, it is RECOMMENDED that the notifier do 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 only (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



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   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 (Section 10.4).  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 re-sends 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

5.3.  Conditions

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



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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 [RFC4745] <identity>
   element, 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> or <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 of the
   <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



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   "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, respectively.  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 labelled 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 [RFC4745] <identity>
   element do not allow additional attributes to be added directly.
   Redefining behavior of their existing <domain> attribute creates



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   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 U.S. phone numbers
   within area code "212" or "+1-212-854" for the organization with U.S.
   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
   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



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   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
      non-essential 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: An 800 translation service is installed on two
      Application Servers, AS1 and AS2.  User A is connected to SP1 and



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      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 creates 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. i.e., 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
   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 [I-D.ietf-soc-overload-control]
   [RFC6357] for more details on rate-based, loss-based and window-based
   load control.)




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   In addition, the <accept> element takes an optional "alt-action"
   attribute which 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" and "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

   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.








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           <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 0, 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 whether
   it contains 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>
       <xs:attribute name="version" type="xs:integer" use="required"/>
       <xs:attribute name="state" use="required">
         <xs:simpleType>
           <xs:restriction base="xs:string">



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



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       </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 -->
   <xs:element name="accept">
     <xs:choice>
       <element name="rate" type="xs:decimal" minOccurs="0"/>
       <element name="win" type="xs:integer" minOccurs="0"/>



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       <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 aspects of security considerations arise from this specification.
   One is the SIP event notification framework-based load filtering
   policy distribution mechanism, the other is the load filtering policy
   enforcement mechanism.

   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 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
   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.

   In addition, there could be one specific type of attack around the
   use the "redirect" action (Section 5.4).  Assuming a number of SIP
   proxy servers in a Trust Domain are using UDP, and configured to get



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   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 victim, who is then
   DoSed off of 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



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   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>

   XML:







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   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/rfc[?].txt">
         RFC[?]</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 to be registered is contained in Section 6.

   Its first line is

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

   and its last line is

   </xs:schema>

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 group for many helpful comments.  In particular, Bruno



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   Chatras proposed the <method> and <target-sip-entity> condition
   elements along with many other text improvements.  Janet Gunn
   provided detailed text suggestions including Section 10.3.  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 PSTN references.  Adam
   Roach suggested RFC6665-related and other helpful clarifications.
   Richard Barnes made many suggestions such as referencing the Trust
   Domain concept of RFC3324 and RFC3325, the use of a separate element
   for 'tel' URI grouping and addressing the "redirect" action security
   threat.

10.  Appendix

10.1.  Definitions

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

   Load Filtering:  A load control mechanism which 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 which 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



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      common policy document defined in [RFC4745].

10.2.  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.

   o  The solution should re-use 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) which 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.

10.3.  Discussion of this specification meeting the requirements of
       RFC5390

   This section evaluates whether the load control event package
   mechanism defined in this specification satisfies various SIP
   overload control requirements set forth by RFC5390 [RFC5390].  As
   mentioned in Section 1, this specification has its particular scope



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   that complements other efforts in the overall SIP load control
   solution space.  Therefore, not all RFC5390 requirements are found
   applicable to this specification.  This specification categorize the
   assessment results into Yes (meet the requirement), 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.

   P/A. The goal of the load filtering is to prevent overload or
   maintain overall goodput during the time of overload, but it is
   dependent on the advance 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 distribution is not properly done, the effectiveness
   of the mechanism will be affected.  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 which are not based on
   advance configuration.




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      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.

      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



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   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] allows the
   inclusion of 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



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      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.  But, 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 of 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.




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      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.

   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.

10.4.  Complete Examples

10.4.1.  Load Control Document Examples



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   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"
               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 considers optimizing server resource usage of a
   three-day period during the aftermath of 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



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   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"
       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



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   "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"
       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">



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                   <lc:rate>0</lc:rate>
               </lc:accept>
           </actions>
       </rule>

   </ruleset>


10.4.2.  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



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         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
           ;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



10.5.  Related Work

10.5.1.  Relationship with Load Filtering in PSTN

   It is known that 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.

   There are control messages associated with PSTN overload control
   which would specify an outgoing control list, call gap duration and



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   control duration [Q.1248.2][E.412].  These items could be roughly
   correlated 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 prefix 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 which could be especially useful for automating load
   filtering for predictable overloads.

   PSTN load filtering can be performed in both edge switches and
   transit switches; 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].  SIP
   load filtering mechanism also allows prioritizing the treatment of
   these calls by specifying favorable actions for them.

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

10.5.2.  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



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   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
   [I-D.ietf-soc-overload-control].  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 targets more at traffic already
   accepted in 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.

11.  References

11.1.  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.




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   [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.

11.2.  Informative References

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

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

   [I-D.ietf-soc-overload-control]
              Gurbani, V., Hilt, V., and H. Schulzrinne, "Session
              Initiation Protocol (SIP) Overload Control", draft-ietf-
              soc-overload-control-13 (work in progress), May 2013.

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




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   [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.

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 Service Integration Labs
   3-9-11 Midori-cho Musashino-shi
   Tokyo  184-0013
   Japan

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





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