IETF SOC Working Group                                           C. Shen
Internet-Draft                                            H. Schulzrinne
Intended status: Standards Track                             Columbia U.
Expires: June 18, 2011                                          A. Koike
                                                       December 15, 2010

     A Session Initiation Protocol (SIP) Load Control Event Package


   This document defines a load control event package for the Session
   Initiation Protocol (SIP).  It allows SIP servers to distribute user
   load control information to other SIP servers in the network.  The
   load control can 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
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   This Internet-Draft will expire on June 18, 2011.

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   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   ( 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 . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Requirements Notation  . . . . . . . . . . . . . . . . . . . .  5
   3.  Design Requirements  . . . . . . . . . . . . . . . . . . . . .  5
   4.  Load Filtering Control Overview  . . . . . . . . . . . . . . .  6
     4.1.  Filter Format  . . . . . . . . . . . . . . . . . . . . . .  6
     4.2.  Filter Computation . . . . . . . . . . . . . . . . . . . .  6
     4.3.  Filter Distribution  . . . . . . . . . . . . . . . . . . .  6
     4.4.  Applicability in Different Network Environments  . . . . .  9
   5.  Load Control Event Package . . . . . . . . . . . . . . . . . . 10
     5.1.  Event Package Name . . . . . . . . . . . . . . . . . . . . 10
     5.2.  Event Package Parameters . . . . . . . . . . . . . . . . . 10
     5.3.  SUBSCRIBE Bodies . . . . . . . . . . . . . . . . . . . . . 10
     5.4.  SUBSCRIBE Duration . . . . . . . . . . . . . . . . . . . . 10
     5.5.  NOTIFY Bodies  . . . . . . . . . . . . . . . . . . . . . . 11
     5.6.  Notifier Processing of SUBSCRIBE Requests  . . . . . . . . 11
     5.7.  Notifier Generation of NOTIFY Requests . . . . . . . . . . 11
     5.8.  Subscriber Processing of NOTIFY Requests . . . . . . . . . 12
     5.9.  Handling of Forked Requests  . . . . . . . . . . . . . . . 12
     5.10. Rate of Notifications  . . . . . . . . . . . . . . . . . . 12
     5.11. State Agents . . . . . . . . . . . . . . . . . . . . . . . 13
   6.  Load Control Document  . . . . . . . . . . . . . . . . . . . . 13
     6.1.  Format . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     6.2.  Namespace  . . . . . . . . . . . . . . . . . . . . . . . . 13
     6.3.  Conditions . . . . . . . . . . . . . . . . . . . . . . . . 13
       6.3.1.  Call Identity  . . . . . . . . . . . . . . . . . . . . 14
       6.3.2.  Validity . . . . . . . . . . . . . . . . . . . . . . . 16
       6.3.3.  Method . . . . . . . . . . . . . . . . . . . . . . . . 17
     6.4.  Actions  . . . . . . . . . . . . . . . . . . . . . . . . . 17
     6.5.  Complete Examples  . . . . . . . . . . . . . . . . . . . . 18
   7.  XML Schema Definition for Load Control . . . . . . . . . . . . 19
   8.  Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 21
     8.1.  Relationship with Load Filtering in PSTN . . . . . . . . . 21
     8.2.  Relationship with Other IETF SIP Load Control Efforts  . . 22
   9.  Discussion of this document meeting the requirements of
       RFC5390  . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 27
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 28
     11.1. Load Control Event Package Registration  . . . . . . . . . 28
     11.2. application/load-control+xml MIME Registration . . . . . . 28
     11.3. Load Control Schema Registration . . . . . . . . . . . . . 29

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   12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 30
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 30
     13.2. Informative References . . . . . . . . . . . . . . . . . . 31
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31

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1.  Introduction

   Proper functioning of Session Initiation Protocol (SIP) [RFC3265]
   signaling servers is critical in SIP-based communications networks.
   The performance of SIP servers can be severely degraded when the
   server is overloaded with excessive number of signaling requests.
   Both legitimate and malicious traffic can overload SIP servers,
   despite appropriate capacity planning.

   There are three common examples of legitimate short-term increases in
   call volumes.  Viewer-voting TV shows or ticket giveaways may
   generate millions of calls within a few minutes.  Call volume may
   also spike during special holidays such as New Year's Day and
   Mother's Day. Finally, callers may want to reach friends and family
   in natural disaster areas such as those affected by earthquakes.
   When possible, only calls traversing overloaded servers should be
   throttled under those conditions.

   SIP load control mechanisms are needed to prevent congestion collapse
   in these cases [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 if core or destination-
   specific SIP proxies are overloaded.  By their nature, they need to
   distribute rate, drop or window information to all upstream SIP
   proxies and generally affect all calls equally, regardless of
   destination.  However, feedback control is ineffective for edge-
   server overload.  For example, for the ticket giveaway case, almost
   all such calls will fail in the core SIP server.  If the edge server
   is also overloaded, calls to other destinations will also be rejected
   or dropped.

   Here, we propose an additional, complementary mechanism, called load
   filtering.  Network operators create filters that indicate that calls
   to specific destinations or from specific sources should be rate-
   limited or randomly dropped.  These filters are then distributed to
   SIP servers and possibly user agents likely to generate calls to the
   affected destinations or from the affected sources.  Load filters
   work best if they prevent calls as close to the user agent client as

   Performing SIP load filtering control requires three components:
   filter content format definition, filter content computation methods,
   and filter distribution mechanism.  This document addresses two of
   the three components.  The filter format is defined by the contents

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   of a SIP load control event package, while the filter distribution
   mechanism is built upon the existing SIP event framework.  The
   remaining component, filter content computation, depends heavily on
   the actual network topology and service provider policies.  Therefore
   it is out of scope of this document.

   The rest of this document is structured as follows: we begin by
   listing the design requirements for this work in Section 3.  We then
   give an overview of the load filtering control operation in
   Section 4.  The load control event package is detailed in Section 5.
   The load filter content format definition is discussed in the two
   sections that follow, with Section 6 defining the load control XML
   document and Section 7 defining the corresponding XML schema.
   Section 8 relates this work to corresponding mechanisms in PSTN and
   other IETF efforts addressing SIP load control.  Finally, Section 10
   presents security considerations and Section 11 provides IANA

2.  Requirements Notation

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

3.  Design Requirements

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

   o  To simplify the solution, we focus on SIP load control, rather
      than a generic application-layer mechanism.
   o  The load filter information 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 call-in
      events, the mechanism should specify during what time period it is
      to be applied, so that the information can be distributed ahead of
   o  For destination-specific overload situations, the load filter
      needs to be able to describe the callee.
   o  To address accidental and intentional high-volume call generators,
      the filter should allow to specify the caller.
   o  Caller and callee need to be specified as both SIP URIs and 'Tel'
   o  For telephone numbers, it should be possible to specify prefixes

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      which allow control over limited regionally-focused overloads.
   o  The solution should draw upon experiences from related PSTN
      mechanisms where applicable.
   o  The solution should be extensible to meet future needs.

4.  Load Filtering Control Overview

4.1.  Filter Format

   A load filter contains both conditions and actions.  Filter
   conditions include the identities of the targets to be controlled.
   For example, there are two typical resource limits in a possible
   overload situation, i.e., human destination limits (N call takers)
   and proxy capacity limits.  The control targets in these two cases
   can be the specific callee numbers or the destination domains
   corresponding to the overload.  Filter conditions also indicate the
   period of time during which the control should be activated, and the
   specific message type to be controlled, e.g., the INVITE message of a
   SIP session.  Filter actions describe the desired control functions
   such as limiting the request rate below a certain level.  Detailed
   formats of filter conditions and actions are defined in Section 6.

4.2.  Filter Computation

   The load filter content computation method needs to take into
   consideration information such as the overload time, scope and the
   network topology as well as service policies.  It is also important
   to make sure that there is no resource allocation loop and that loads
   are allocated in a way that both prevents overload and minimizes the
   likelihood of network under-utilization.  In some cases, in order to
   better utilize system resources, it may be preferable to employ a
   dynamic load computation algorithm which adapts to current network
   status, rather than using a purely static mechanism.  The filter
   content computation algorithm is out of scope of this document.

4.3.  Filter Distribution

   For load filter distribution, this document defines the SIP event
   package for load control, which is an "instantiation" of the generic
   SIP events framework [RFC3265].  The SIP events framework provides an
   existing method for SIP entities to subscribe to and receive
   notifications when certain events have occurred.  Such a framework
   forms a scalable event distribution architecture that suits our
   needs.  This document also defines the XML schema used to encode the
   load control document.  The choice of XML allows us to reuse existing
   SIP-specific policy related XML schemas when applicable, and also
   fits our goal of flexibility and extensibility.

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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 with SIP Load Control Event

   The load filter distribution based on the SIP load control event
   package is illustrated with an example architecture shown in
   Figure 1.  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 Proxies (CPs) and four
   SIP Edge Proxies (EPs).  The CPs and EPs of Service Provider A are
   denoted as CPa1 to CPa2 and EPa1 to EPa4; the CPs and EPs of Service

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   Provider B are denoted as CPb1 to CPb2 and EPb1 to EPb4.

   In general, each SIP proxy server in the network is required to
   subscribe to the load control event package from all its outgoing
   signaling neighbors.  Signaling neighbors are defined by sending
   signaling messages.  For instance, if A sends signaling requests to
   B, B is an outgoing signaling neighbor of A. A needs to subscribe to
   the load control event package of B in case B wants to curb requests
   from A. On the other hand, if B also sends signaling requests to A,
   then B also subscribes to A. In the example topology of Figure 1,
   assuming all signaling relationship is bi-directional, each proxy
   will need to subscribe to all its neighbors.  That is, EPa1
   subscribes to CPa1; CPa1 subscribes to EPa1, EPa2, CPa2 and CPb1, so
   on and so forth.  Notifications are always sent to all subscribing

   To begin load filter distribution on a network when the appropriate
   subscriptions among the SIP entities are ready, the initial filter
   contents are introduced to a SIP entity which acts as the network
   entry point for load filtering control.  The filter is then
   propagated to other SIP entities throughout the network.  The
   following shows two examples.

   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 filter computation algorithm, CPa1 may allocate the received
   total acceptable 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 servers.  This process continues until all edge proxies
   in the network have been informed about the event and have proper
   load filter configured.

   Case II: an earthquake affects the region covered by CPb2, EPb3 and
   EPb4.  All the three 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 filters to accept more outbound calls than inbound calls.  CPb2
   may be configured the same way or receive dynamic filters from EPb3
   and EPb4.  Depending on the filter 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

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   have filters configured.

   The network entry point for load filtering control in the above two
   cases is the SIP server to be protected.  In other cases, the
   filtering entry point could also be an entity that the protected SIP
   server is connected to.  For example, an operator may host an
   application server that performs 800 number translation services.
   The application server may itself be a SIP proxy 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
   control can be introduced from the application server and then
   propogated to other SIP proxy servers in the network.

   Note that this document does not define the provisioning interface
   between the load control policy maker and the policy entry point in
   the network.  One of the possible solutions for the provisioning
   interface is the Extensible Markup Language (XML) Configuration
   Access Protocol (XCAP) [RFC4825].

4.4.  Applicability in Different Network Environments

   Load filtering control is more effective when the filters can be
   pushed to the proximity of signaling sources.  But even if only part
   of the signaling path towards the signaling source could be covered,
   use of this mechanism can still be beneficial.  In fact, due to
   possibly sophisticated call routing and security concerns, trying to
   apply automated load filter distribution in the entire inter-domain
   network path could get extremely complicated and be unrealistic.

   The scenarios where this mechanism could be most useful are
   environments consisting of servers with secure and trust relationship
   and with relatively straightforward routing configuration known to
   the filter computation decision maker.  These scenarios may include
   intra-domain environments such as those inside a service provider or
   enterprise domain; inter-domain environments such as where enterprise
   connecting to a few service providers or between service providers
   with manageable routing configurations.

   Another important aspect that affects the applicability of the load
   filtering control is that all possible signaling source neighbors
   need to participate and enforce the designated filter.  Otherwise, a
   single non-conforming neighbor could make the whole control efforts
   useless by pumping in excessive traffic to overload the server.
   Therefore, the SIP server that initiates the filter needs to take
   counter-measures towards any non-conforming neighbors.  A simple
   policy is to reject excessive requests with 500 responses as if they
   were obeying the rate.  Considering the rejection costs, a more
   complicated but fairer policy would be to allocate at the overloaded

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   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, whatever the
   actual policy is, SIP servers SHOULD honor the Resource-Priority
   Header (RPH) [RFC4412] when processing messages.  The RPH contents
   may indicate high priority requests that should be preserved as much
   as possible, or low priority requests that could be dropped during
   overload.  The request rejection and message prioritization at an
   overloaded server are also discussed in Section 5.1 of
   [I-D.ietf-soc-overload-control] and Section 12 of

5.  Load Control Event Package

   This section defines the details of the SIP event package for load
   control according to [RFC3265].

5.1.  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

5.2.  Event Package Parameters

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

5.3.  SUBSCRIBE Bodies

   A SUBSCRIBE request for load control policy MAY contain a body to
   filter the requested load control notification.  For example, a
   subscriber may be interested in some specific types of load control
   information only.  The details of the subscription filter
   specification are not yet defined.

   A SUBSCRIBE request sent without a body implies the default
   subscription behavior as specified in Section 5.7.

5.4.  SUBSCRIBE Duration

   The default expiration time for a subscription to load control 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

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   RECOMMENDED to explicitly negotiate appropriate subscription
   durations when knowledge about the mutual signaling relationship is

5.5.  NOTIFY Bodies

   The body of a NOTIFY message in this event package contains policy
   information regarding load control.  As specified in [RFC3265], the
   format of the NOTIFY body MUST be in one of the formats defined in
   the Accept header field of the SUBSCRIBE request or be the default
   format.  The default data format for the NOTIFY body of this event
   package is "application/load-control+xml" (defined in Section 6).
   This means that if no Accept header field is specified to a SUBSCRIBE
   request, the NOTIFY will contain a body in the "application/
   load-control+xml" format.  If the Accept header field is present, it
   MUST include "application/load-control+xml" and MAY include any other

5.6.  Notifier Processing of SUBSCRIBE Requests

   The effectiveness of load filtering control relies on the scope of
   distribution and installation of the control policies in the network.
   Since wide distribution of the policy information is desirable, SIP
   entity subscribers SHOULD try to subscribe to all those SIP entity
   notifiers with which they have regular signaling exchanges, although
   not all such SIP notifiers may permit such a subscription.

   If the identity of the entity sending the SUBSCRIBE message is not
   allowed to receive overload control information, the notifier MUST
   return a 403 "Forbidden" response.

   If none of MIME types specified in the Accept header of the SUBSCRIBE
   is supported, the Notifier SHOULD return 406 "Not Acceptable"

5.7.  Notifier Generation of NOTIFY Requests

   Following the [RFC3265] specification, a notifier MUST send a NOTIFY
   with its current load control policy to the subscriber upon
   successfully accepting or refreshing a subscription.  The NOTIFY
   request MAY include a body.  If no applicable restriction is active
   when the subscription request is received, an empty document is
   attached to the NOTIFY request.  A notifier SHOULD generate NOTIFY
   requests each time the load control policy changes, with the maximum
   notification rate not exceeding values defined in Section 5.10.

   This event package does not support notifications that contain deltas
   to previous information or partial information.

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5.8.  Subscriber Processing of NOTIFY Requests

   The way subscribers process NOTIFY requests depends on the contents
   of the notifications.  Typically, a load control notification
   consists of rules that should be applied to requests matching certain
   identities.  A SIP entity subscriber receiving the notification first
   installs these rules and then filter incoming requests to enforce
   actions on appropriate requests, for example, limiting the sending
   rate of call requests destined for a specific SIP entity.

   In the case when load control rules specify a future validity time,
   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 subscriber MUST remove
   all previously received load control information and process all
   calls without applying any restriction.

   The subscriber SHALL discard unknown bodies.  If the NOTIFY request
   contains several bodies, none of them being supported, it SHOULD
   unsubscribe.  A NOTIFY request that does not contain a body MUST be

5.9.  Handling of Forked Requests

   Forking is not applicable when the load control event package is used
   within a single-hop distance between neighboring SIP entities.  If
   the communication scope of the load-control event package 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
   4.4.9 of [RFC3265].

5.10.  Rate of Notifications

   Rate of notifications is likely not a concern for this event package
   when it is used in a non-real-time mode for relatively static load
   control policies.  Nevertheless, if situation does arise that a
   rather frequent load control policy update is needed, it is
   RECOMMENDED that the notifier does not generate notifications at a

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   rate higher than once per-second in all cases, in order to avoid the
   NOTIFY message itself overloading the system.

5.11.  State Agents

   The load control policy information can be directly generated by
   concerned SIP entities distributed in the network.  Alternatively,
   qualified state agents external to the SIP entities MAY be defined to
   take charge of load control policy making.

6.  Load Control Document

6.1.  Format

   A load control document is an XML document that 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.

6.2.  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:


6.3.  Conditions

   [RFC4745] defines three condition elements: <identity>, <sphere> and
   <validity>.  In this document, we re-define an element for identity
   and reuse the <validity> element.  The <sphere> element is not used.

6.3.1.  Call Identity

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   Since the problem space of this document is different from that of
   [RFC4745], the [RFC4745] <identity> element is not sufficient for use
   with load control.  First, load control may be applied to different
   identity information 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 document defines new identity conditions
   that can accommodate the granularity of specific SIP identity header
   fields.  Requirement for authentication depends on which field is to
   be matched.

   The identity condition for load control 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>, each is of the same type as the <identity> element in
   [RFC4745].  Therefore, they also inherit the sub-elements of the
   <identity> element, including <one>, <except>, and <many>.

   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>, <except>, or <many> sub-elements, the result is also
   combined using logical OR, similar to that of the <identity> element
   in [RFC4745].  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.

                           <one id=""/>
                           <one id="tel:+1-212-555-1234"/>

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   This example matches call requests whose To header field contains the
   SIP URI "", or the 'tel' URI

   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.  We
   therefore need a similar way to match a group of 'tel' URIs.
   According to [RFC3966], there are two formats of 'tel' URIs: global
   format and local format.  All phone numbers must be expressed in the
   global format when possible.  The global format 'tel' URIs start with
   a "+".  The rest of the phone numbers are expressed in a local
   format, which must be qualified by a "phone-context" parameter.  The
   "phone-context" parameter may be labelled as a global number or any
   number of its leading digits, or a domain name.  Both formats of the
   'tel' URI make the resulting URI globally unique.

   'Tel' URIs of global format 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 is also variable.  This allows further flexibility such
   as grouping the numbers into sub-areas within the same area code.
   'Tel' URIs of local-number format can be grouped by the value of the
   "phone-context" parameter.

   To include the two formats of 'tel' URI grouping in the <many> and
   <except> elements, one approach is to add a new attribute similar to
   the "domain" attribute.  In this document, we decided on a simpler
   approach.  There are basically two forms of grouping attribute values
   for both SIP URIs and 'tel' URIs: domain name or number prefix
   starting with "+".  Both of them can be expressed as strings.
   Therefore, we re-interpret the existing "domain" attribute of the
   <many> and <except> elements to allow it to contain both forms of
   grouping attribute values.  In particular, when the "domain"
   attribute value starts with "+", it denotes a number prefix,
   otherwise, the value denotes a domain name.  Note that the tradeoff
   of this simpler approach is the overlapping in matching different
   types of URIs.  Specifically, a domain name in the "domain" attribute
   could be matched by both a SIP URI with that domain name and a local
   format 'tel' URI containing the same domain name in the "phone-
   context".  On the other hand, a number prefix in the "domain"
   attribute could be matched by both global 'tel' URIs starting with

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   those leading digits, and local 'tel' URIs having the same prefix in
   the "phone-context" parameter.  These overlapping situations would
   not be a big problem because of two reasons.  First, when the "phone-
   context" coincides with the SIP domain name or the global number
   prefix, it is usually the case that the related phone numbers indeed
   belong to the same domain or the same area, which means the
   overlapping is not inappropriate.  Second, the use of the local
   format 'tel' URI in practice is expected to be rare.  As a result,
   the chance of such overlapping happening is very small.

   The following example shows the use of the re-interpreted "domain"

                               <except domain="+1-212"/>
                               <except domain=""/>
                           <one id="tel:+1-202-999-1234"/>

   This example 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 "".

6.3.2.  Validity

   A rule is usually associated with a validity period condition.  This
   document 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.


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6.3.3.  Method

   The load created on a SIP server depends on the type of an initial
   SIP request for a dialog or standalone transaction.  The <method>
   element specifies the SIP method to which a particular action
   applies.  When this element is not included, the rule actions are
   applicable to all initial methods.

   The following example shows the use of the <method> element.


6.4.  Actions

   As [RFC4745] specified, conditions form the 'if'-part of rules, while
   actions and transformations form the 'then'-part.  Transformations
   are not used in the load control document.  The actions for load
   control are defined by the <accept> element, which takes 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 control.  In
   static load filter configuration scenarios, using the <rate> sub-
   element is RECOMMENDED because it is hard to enforce the percentage
   rate or window-based control when the incoming load from upstream or
   the reactions from downstream are uncertain.  (See
   [I-D.ietf-soc-overload-control] [I-D.ietf-soc-overload-design] for
   more details on rate-based and window-based load control)

   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 accepted.  The possible "alt-action"
   values are "drop" for simple drop, "reject" for explicit rejection
   (e.g., sending a "500 Server Internal Error" response message to an
   INVITE request), and "forward".  The default value is "reject" in
   order to avoid possible SIP retransmissions when an unreliable
   transport is used.  If the "alt-action" value is "forward", an "alt-
   target" attribute MUST be defined.  The "alt-target" specifies a URI
   where the request should be forwarded (e.g., an answering machine
   with explanation of why the request cannot be accepted).

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


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

6.5.  Complete Examples

   This section presents two complete examples of rule sets.

   The first example assumes that a set of hotlines are set up at
   "" 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"

       <rule id="f3g44k1">
                           <one id=""/>
                           <one id="tel:+1-212-555-1234"/>
               <lc:accept alt-action="reject">


   The second example considers optimizing server resource usage of a
   three-day period during the aftermath of an earthquake.  Incoming

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   calls to the earthquake domain "" will be limited
   to a rate of 100 requests per second, except for those calls
   originating from a particular rescue team domain
   "".  Outgoing calls from the earthquake domain or
   calls within the local domain are never limited.  All calls that are
   throttled due to the rate limit will be forwarded to an answering
   machine with updated earthquake rescue information.

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

       <rule id="f3g44k2">
                           <many domain=""/>
                               <except domain=""/>
                               <except domain=""/>
               <lc:accept alt-action="forward" alt-target=


7.  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] by defining new

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   members of the <conditions> and <action> elements.

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

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

   <!-- CONDITIONS -->

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

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

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

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

   <!-- 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"/>

8.  Related Work

8.1.  Relationship with Load Filtering in PSTN

   It is known that the existing PSTN network also uses a load filtering
   mechanism to prevent overload and the filter configuration is done
   manually.  This document defines the SIP event framework based
   distribution mechanism which allows automated filter distribution in
   suitable environments.

   There are control messages associated with PSTN overload control
   which would specify an outgoing control list, call gap duration and
   control duration [AINGR].  These items could be roughly correlated to
   the identity, action and the time fields in the SIP load filter
   content definition in this document.  However, the filter defined in
   this document is much more generic and flexible as opposed to its
   PSTN counterpart.

   Firstly, PSTN filtering only applies to telephone numbers, and the
   number of prefix to be matched for a group of telephone numbers is
   usually a fixed set.  The SIP filter identity allows both SIP URI and
   telephone numbers (through Tel URI) to be specified.  The identities
   can be arbitrary grouped by SIP domains or any number of leading
   prefix of the telephone number.

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   Secondly, the PSTN filtering action is usually limited to call
   gapping with a fixed set of allowed gapping intervals.  The action
   field in the SIP load filter allows more flexible rate throttle and
   other possibilities.

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

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

   PSTN overload control also has special accommodation for High
   Probability of Completion (HPC) calls, which would be similar to the
   calls designated by the SIP Resource Priority Headers [RFC4412].  SIP
   filtering mechanism can also prioritize the treatment of these calls
   by specifying favorable actions for these calls.

   PSTN filtering also provides administrative option for routing failed
   call attempts to either Reorder Tone or a special announcement.
   Similar capability can be provided in the SIP filtering mechanism by
   specifying the appropriate "alt-action" attribute in the SIP
   filtering action field.

8.2.  Relationship with Other IETF SIP Load Control Efforts

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

   The filter content defined in this document is generic and is
   expected to be applicable not only to the load filtering control
   mechanism but also to the feedback overload control mechanism in
   [I-D.ietf-soc-overload-control].  In particular, both of them could
   use specific or wildcard filter identities for load control and could
   share well-known load control actions.  The time duration field in
   the filter content could also be used in both mechanisms.  As

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   mentioned in Section 1, the load filter distribution mechanism and
   the feedback overload control mechanism address complementary areas
   in the load control problem space.  Load filtering is more proactive
   and focuses on distributing the filter 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 filter components.
   For example, the load filtering mechanism may use the time field in
   the filter to specify not only a control duration but also a future
   activation time to accommodate a predicable overload such as one
   caused by Mother's Day 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 control duration.

9.  Discussion of this document meeting the requirements of RFC5390

   This section evaluates whether the load filtering control event
   package mechanism defined in this document satisfies the various SIP
   overload control requirements set forth by RFC5390 [RFC5390].  Not
   all the RFC5390 requirements are found applicable due to the scope
   limit of this document.  Therefore, we categorize the assessment
   results into Yes (meet the requirement), P/A (partially applicable),
   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

   Yes. The goal of the load filtering control is to prevent overload or
   mantain overall goodput during the time of overload, especially when
   the overload contexts (e.g., time, scope) are known in advance.

      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 filter values are installed in advance and do not change
   during the potential overload period.  P/A if filter values are
   dynamically adjusted due to the specific filter computation
   algorithm.  The dynamic filter computation algorithm is outside the
   scope of this document, while the distribution of the updated filters

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   uses the event package mechanism of this document.

      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.

   P/A. Since a main purpose of the load filtering control approach is
   to provision the appropriate capacity with advanced knowledge,
   configuration cannot be entirely avoided, although minimizing the
   configuration efforts is still desired.

      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

   Yes. Servers need to be prepared to work in environments where not
   all the upstream neighbors support this load control event package as
   discussed in Section 4.4.

      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

   Yes. Servers need to be prepared to work in environments where not
   all the upstream neighbors conform to this load control event package
   as discussed in Section 4.4.

      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.

   Yes. The event package defined in this document is specifically for
   filter-based overload control.

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      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/windows-based overload
   control options as discussed in Section 6.4.

      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 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, the filter format [Section 4.1] allows the
   inclusion of alternative forwarding destinations for rejected

      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.

   P/A: the load filtering control is mainly for cases with advanced
   overload contexts and for scnearios with a relatively good knowledge
   of the upstream entities (e.g., core proxy and edge proxy servers)
   where the filters can be installed.  When the set of upstream
   elements is not enumerable, it is expected that the mechanism may be
   more difficult to apply.

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

   Yes. See also answer to REQ 10.

      REQ 12: The mechanism should work between servers in different

   Yes. The load control event package is not limited by domain

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

   Yes. Local policies such as honoring RPH are specified.

      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 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.

   Yes. When the filters are provisioned in advance, they do not rely on
   explicit overload feedback indication.

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

   Yes. The standardized SIP event package mechanism [RFC3265] is used.

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

   Yes. Security considerations are discussed, in particular, accepted
   filters must be authenticated and authorized.

      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 filter format
   definition in Section 4.1.

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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

   N/A to the load control event package 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.

   Yes. For example, an example approach to ensure fair server resource
   allocation in an envirnoment with both conforming and non-conforming
   entities is discussed in Section 4.4.

      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 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 itself.

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

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

10.  Security Considerations

   Two aspects of security considerations arise from this document.  One

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   is the SIP event framework based filter distribution mechanism, the
   other is the filter enforcement mechanism.

   Security considerations for SIP event framework based mechanisms are
   covered in Section 5 of [RFC3265].  A particularly relevant aspect
   for notification control is that, in order to prevent the load
   control notification being used to launch denial of service attacks,
   all load control notification MUST be authenticated and authorized
   before being accepted.  Standard authentication and authorization
   mechanisms recommended in [RFC3261] such as TLS [RFC5246] and IPSec
   [RFC4301] may serve this purpose.

   Security considerations for filter enforcements vary depending on the
   filter contents.  This document defines possible filter match of the
   following SIP header fields: <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

11.  IANA Considerations

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

11.1.  Load Control Event Package Registration

   This section registers an event package based on the registration
   procedures defined in [RFC3265].

   Package name: load-control

   Type: package

   Published specification: This document

   Person to contact: Charles Shen,

11.2.  application/load-control+xml MIME Registration

   This section registers a new MIME type based on the procedures
   defined in [RFC4288] and guidelines in [RFC3023].

      MIME media type name: application

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      MIME subtype name: load-control+xml

      Mandatory parameters: none

   Optional parameters: Same as charset parameter application/xml in

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

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

   Interpretability considerations: None

   Published Specification: This document

   Applications which use this media type: load control of SIP entities

   Additional information:

   Magic number: None

   File extension: .xml

   Macintosh file type code: 'TEXT'

   Personal and email address for further information:

   Charles Shen,

   Intended usage: COMMON

   Author/Change Controller: IETF SIPPING Working Group
   <>, as designated by the IESG <>

11.3.  Load Control Schema Registration

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

   Registrant Contact: IETF SIPPING working group, Charles Shen

   XML: the XML schema to be registered is contained in Section 7.

   Its first line is

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

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   and its last line is


12.  Acknowledgements

   The authors would like to thank Bruno Chatras, Janet Gunn, Vijay
   Gurbani, Volker Hilt, Geoff Hunt, Timothy Moran, Eric Noel,
   Parthasarathi R and other members of the SIPPING and SIP-OVERLOAD
   working group for helpful comments.  Bruno Chatras also suggested
   adding the <method> condition element.

13.  References

13.1.  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.

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

   [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.

   [RFC3265]  Roach, A., "Session Initiation Protocol (SIP)-Specific
              Event Notification", RFC 3265, June 2002.

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

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

   [RFC4288]  Freed, N. and J. Klensin, "Media Type Specifications and
              Registration Procedures", BCP 13, RFC 4288, December 2005.

   [RFC4745]  Schulzrinne, H., Tschofenig, H., Morris, J., Cuellar, J.,
              Polk, J., and J. Rosenberg, "Common Policy: A Document

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              Format for Expressing Privacy Preferences", RFC 4745,
              February 2007.

13.2.  Informative References

   [AINGR]    Bell Communications Research, "AINGR: Service Control
              Point (SCP) Network Traffic Management", GR-2938-CORE ,
              December 1996.

              Gurbani, V., Hilt, V., and H. Schulzrinne, "Session
              Initiation Protocol (SIP) Overload Control",
              draft-ietf-soc-overload-control-00 (work in progress),
              November 2010.

              Hilt, V., Noel, E., Shen, C., and A. Abdelal, "Design
              Considerations for Session Initiation Protocol (SIP)
              Overload Control", draft-ietf-soc-overload-design-03 (work
              in progress), December 2010.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

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

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

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

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

Authors' Addresses

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

   Phone: +1 212 854 3109

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

   Phone: +1 212 939 7004

   Arata Koike
   NTT Service Integration Labs &
   NTT Washington DC Representative Office
   1100 13th St., NW, Suite 900
   Washington DC,   20005

   Phone: +1 202 312 1451

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