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Versions: (draft-shen-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                                                      AT&T
Intended status: Standards Track                          H. Schulzrinne
Expires: September 3, 2012                                   Columbia U.
                                                                A. Koike
                                                                     NTT
                                                           March 2, 2012


     A Session Initiation Protocol (SIP) Load Control Event Package
            draft-ietf-soc-load-control-event-package-03.txt

Abstract

   We define a load control event package for the Session Initiation
   Protocol (SIP).  It allows SIP servers to distribute load filters to
   other SIP servers in the network.  The load filters 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 September 3, 2012.

Copyright Notice

   Copyright (c) 2012 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



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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Requirements Notation  . . . . . . . . . . . . . . . . . . . .  5
   3.  Design Requirements  . . . . . . . . . . . . . . . . . . . . .  6
   4.  SIP Load Filtering Overview  . . . . . . . . . . . . . . . . .  6
     4.1.  Filter Format  . . . . . . . . . . . . . . . . . . . . . .  6
     4.2.  Filter Computation . . . . . . . . . . . . . . . . . . . .  6
     4.3.  Filter Distribution  . . . . . . . . . . . . . . . . . . .  7
     4.4.  Applicability in Different Network Environments  . . . . . 10
   5.  Load Control Event Package . . . . . . . . . . . . . . . . . . 11
     5.1.  Event Package Name . . . . . . . . . . . . . . . . . . . . 11
     5.2.  Event Package Parameters . . . . . . . . . . . . . . . . . 11
     5.3.  SUBSCRIBE Bodies . . . . . . . . . . . . . . . . . . . . . 11
     5.4.  SUBSCRIBE Duration . . . . . . . . . . . . . . . . . . . . 11
     5.5.  NOTIFY Bodies  . . . . . . . . . . . . . . . . . . . . . . 12
     5.6.  Notifier Processing of SUBSCRIBE Requests  . . . . . . . . 12
     5.7.  Notifier Generation of NOTIFY Requests . . . . . . . . . . 12
     5.8.  Subscriber Processing of NOTIFY Requests . . . . . . . . . 12
     5.9.  Handling of Forked Requests  . . . . . . . . . . . . . . . 13
     5.10. Rate of Notifications  . . . . . . . . . . . . . . . . . . 13
     5.11. State Delta  . . . . . . . . . . . . . . . . . . . . . . . 13
     5.12. State Agents . . . . . . . . . . . . . . . . . . . . . . . 14
   6.  Load Control Document  . . . . . . . . . . . . . . . . . . . . 14
     6.1.  Format . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     6.2.  Namespace  . . . . . . . . . . . . . . . . . . . . . . . . 15
     6.3.  Conditions . . . . . . . . . . . . . . . . . . . . . . . . 15
       6.3.1.  Call Identity  . . . . . . . . . . . . . . . . . . . . 15
       6.3.2.  Validity . . . . . . . . . . . . . . . . . . . . . . . 18
       6.3.3.  Method . . . . . . . . . . . . . . . . . . . . . . . . 18
     6.4.  Actions  . . . . . . . . . . . . . . . . . . . . . . . . . 18
     6.5.  Complete Examples  . . . . . . . . . . . . . . . . . . . . 19
   7.  XML Schema Definition for Load Control . . . . . . . . . . . . 21
   8.  Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 23
     8.1.  Relationship with Load Filtering in PSTN . . . . . . . . . 23
     8.2.  Relationship with Other IETF SIP Load Control Efforts  . . 24
   9.  Discussion of this specification meeting the requirements
       of RFC5390 . . . . . . . . . . . . . . . . . . . . . . . . . . 25
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 30
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 31
     11.1. Load Control Event Package Registration  . . . . . . . . . 31



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     11.2. application/load-control+xml MIME Registration . . . . . . 31
     11.3. Load Control Schema Registration . . . . . . . . . . . . . 32
   12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 32
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 33
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 33
     13.2. Informative References . . . . . . . . . . . . . . . . . . 33
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 34












































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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 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.  However,
   feedback control is usually ineffective for overload of more general
   purpose SIP edge proxy servers.  For example, in the ticket giveaway
   case, almost all calls to the hotline will fail at the core proxy
   servers; if the edge proxy servers leading to the core proxy servers
   are 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 load filters that indicate that
   calls to specific destinations or from specific sources should be
   rate-limited or randomly dropped.  These load 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 filtering works best if it prevents calls as close to
   the user agent clients as possible.

   Performing SIP load filtering requires three components: load filter



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   format, load filter computation method, and load filter distribution
   mechanism.  This specification addresses two of these three
   components.  The load filter format is defined in a SIP load control
   event package, while the load filter distribution mechanism is built
   upon the existing SIP event framework.  The remaining component, load
   filter computation method, depends heavily on the actual network
   topology and service provider policies.  Therefore it is out of scope
   of this specification.

   It is helpful to clarify two aspects regarding some terminology used
   in this specification.  Firstly, although the SIP load filtering
   mechanism is motivated by the overload control problem, which is why
   this specification refers extensively to other parallel SIP overload
   control related efforts, the applicability of filtering extends
   beyond the overload control purpose.  For example, it can also be
   used to implement quality of service or other service level agreement
   commitments.  Therefore, we use the term SIP "load control event
   package", instead of a narrower term "overload control event
   package".  Secondly, since we are describing a specific control
   mechanism based on filtering, the term "load control" in this
   specification is used inter-changeably with the term "load filtering"
   unless when associated with other explicit context.  This
   specification, however, does not preclude the load control document
   defined here (Section 6) to be extended in the future for other forms
   of control as appropriate.

   The rest of this specification is structured as follows: we begin by
   listing the design requirements for this work in Section 3.  We then
   give an overview of load filtering operation in Section 4.  The load
   control event package for filter distribution is detailed in
   Section 5.  The load filter format is defined in the two sections
   that follow, with Section 6 introducing the XML document for load
   control and Section 7 listing the associated schema.  Section 8
   relates this work to corresponding mechanisms in PSTN and other IETF
   efforts addressing SIP load control.  Section 9 evaluates whether
   this specification meets the SIP overload control requirements set
   forth by RFC5390 [RFC5390].  Finally, Section 10 presents security
   considerations and Section 11 provides IANA considerations.


2.  Requirements Notation

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


3.  Design Requirements



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

   o  To simplify the solution, we focus on a method for controlling SIP
      load, rather than a generic application-layer mechanism.
   o  The load filter 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 load filter should specify during what time period it
      is to be applied, so that the information can be distributed ahead
      of time.
   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 load filter should allow to specify the caller.
   o  Caller and callee need to be specified as both SIP URIs and 'Tel'
      URIs[RFC3966].
   o  For telephone numbers, it should be possible to specify prefixes
      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.  SIP Load Filtering 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 number of 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

   Load filter computation needs to take into consideration information
   such as the overload time, scope and network topology, as well as
   service policies.  It is also important to make sure that there is no



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   resource allocation loop, and that loads are allocated in a way which
   both prevents overload and minimizes the likelihood of network
   resource 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 load filter
   computation algorithm is out of scope of this specification.

4.3.  Filter Distribution

   For load filter distribution, this specification 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 specification also defines the XML schema of a load
   control document (Section 6), which is used to encode load filtering
   rules.

   In order for load filters to be properly distributed, each SIP proxy
   server in the network is required to subscribe to the load control
   event package from all its outgoing signaling neighbors, known as
   notifiers (Section 5.6).  Subscription is initiated and maintained
   during normal server operation.  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. Subscription of
   neighboring SIP entities needs to be persistent so that they are in
   place independently of any specific load filtering events.  Key to
   this is the fact that notification following initial subscription
   includes an empty message body if no events are configured
   (Section 5.7), and that the subscription needs to be refreshed
   periodically to make it persistent, as described in Section 3.1.6 and
   Section 3.1.4.2 of [RFC3265].  The notifier will send a notification
   with its current control policy to its subscribers each time a new
   subscription or a subscription refreshing is accepted (Section 5.7).
   The same subscription dialog can also be used to convey policies for
   multiple different load filtering events in a set of rules
   (Section 6.1).

   We use the example architecture shown in Figure 1 to illustrate load
   filter distribution based on the SIP load control event package.
   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



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   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.


      +-----------+   +-----------+   +-----------+   +-----------+
      |           |   |           |   |           |   |           |
      |   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 the initialization stage, the proxy servers first identify all



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   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
   progressively learning from the singling messages sent and received.
   Assuming all signaling relationship in Figure 1 is bi-directional,
   after this initialization stage, each proxy server will be subscribed
   to all its neighbors.  That is, EPa1 subscribes to CPa1; CPa1
   subscribes to EPa1, EPa2, CPa2 and CPb1, so on and so forth.  The
   following cases then show two examples of how load filter
   distribution in this network works.

   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 proxy servers.  This process continues until all edge
   proxy servers 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 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 filters to accept more outbound calls than inbound calls.  CPb2
   may be configured the same way or receive dynamically computed
   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 have filters configured.

   In the above two cases, the network entity where load filtering
   policy is first introduced is the SIP server to be protected.  In
   other cases, the network entry point of load filtering policy 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 policies can be introduced from the
   application server and then propogated to other SIP proxy servers in



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   the network.

   Note that this specification does not define the provisioning
   interface between the party who determines the load control 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].

4.4.  Applicability in Different Network Environments

   SIP load filtering 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 algorithm.  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 SIP load
   filtering 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 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.  SIP
   request rejection and message prioritization at an overloaded server
   are also discussed in Section 5.1 of [I-D.ietf-soc-overload-control]



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   and Section 12 of [RFC6357].


5.  Load Control Event Package

   The SIP load filtering mechanism uses the SIP event package for load
   control.  This section defines 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
   [RFC3265].

5.2.  Event Package Parameters

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

5.3.  SUBSCRIBE Bodies

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

   A SUBSCRIBE request for the SIP load control event package MAY
   contain a body to filter the requested load control event
   notification.  For example, a subscriber may be interested in some
   specific types of load control policy 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
   RECOMMENDED to explicitly negotiate appropriate subscription
   durations when knowledge about the mutual signaling relationship is
   available.




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5.5.  NOTIFY Bodies

   The body of a NOTIFY request in this event package contains load
   control policy.  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
   request 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 types.

5.6.  Notifier Processing of SUBSCRIBE Requests

   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 control policy, 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"
   response.

5.7.  Notifier Generation of NOTIFY Requests

   Following [RFC3265] specification, a notifier MUST send a NOTIFY with
   its current load control policy to the subscriber upon successfully
   accepting or refreshing a subscription.  If no applicable restriction
   is active when the subscription request is received, an empty message
   body is attached to the NOTIFY request.  This 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
   configured at that time.  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.

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 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.



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   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 subscription status with
   the particular notifier will be terminated.  However, subscribers
   SHOULD NOT change previously received load control policies from that
   notifier because of this change in subscription status, unless it has
   other specific reasons to do so.  Modifications of existing load
   control policies at the subscriber is performed after directly
   receiving notifications containing updated load control policies.

   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
   ignored.

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

5.11.  State Delta

   It is likely that updates to specific load control events are made by



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   changing the control restriction parameter information only (e.g.
   rate, percent), but not other rule elements, such as call-identity.
   This will typically be because the utilisation 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 a dynamic filter computation algorithm
   (Section 4.2).  Another factor usually not known precisely or is
   computed automatically is the validity duration of the load control
   event.  Therefore it would also be common for the validity to change
   frequently.

   This event package allows the use of state delta to accommodate
   frequent updates of partial rule parameters.  As in [RFC3265], a
   version number that increases by exactly one is included in the
   NOTIFY body for each NOTIFY transaction in a subscription.  When the
   subscriber receives a state delta, it associates the partial updates
   to the particular rules by matching the appropriate rule id
   (Section 6.5).  If the subscriber receives a NOTIFY that has a
   version number that is increased by more than one, it knows that it
   has missed a state delta.  The subscriber then keeps the version
   number, ignores the NOTIFY request containing the state delta, and
   re-sends a SUBSCRIBE to force a NOTIFY containing a complete state
   snapshot.

5.12.  State Agents

   The load control policy 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
   determining load control policies.


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



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   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:

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

6.3.  Conditions

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

6.3.1.  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 control.  First, load control 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 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,



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   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.


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


   This example matches call requests whose To header field contains the
   SIP URI "sip:alice@hotline.example.com", or the 'tel' URI
   "tel:+1-212-555-1234".

   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 forms of 'tel' URIs for global
   numbers and local numbers, respectively.  All phone numbers must be
   expressed in global form when possible.  The global number 'tel' URIs
   start with a "+".  The rest of the numbers are expressed as local
   numbers, 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 forms of the
   'tel' URI make the resulting URI globally unique.

   'Tel' URIs of global numbers can be grouped by prefixes consisting of



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   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 numbers can be grouped by the value of the
   "phone-context" parameter.

   To include the two forms 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 specification, we decided on a
   simpler approach.  There are basically two types of grouping
   attribute values for both SIP URIs and 'tel' URIs: domain name and
   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 types
   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 overlap 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 number
   '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 number 'tel' URIs starting with those leading
   digits, and local number 'tel' URIs having the same prefix in the
   "phone-context" parameter.  These overlap 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 overlap is not
   inappropriate.  Second, use of the local number 'tel' URI in practice
   is expected to be rare.  As a result, the chance of such overlap
   happening is very small.

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

               <call-identity>
                   <sip>
                       <from>
                           <many>
                               <except domain="+1-212"/>
                               <except domain="manhattan.example.com"/>
                           </many>
                       </from>
                       <to>



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                           <one id="tel:+1-202-999-1234"/>
                       </to>
                   </sip>
               </call-identity>

   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 "manhattan.example.com".

6.3.2.  Validity

   A rule 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>

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.

               <method>INVITE</method>

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



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   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] [RFC6357] for more details on rate-
   based, loss-based and window-based load control)

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

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

6.5.  Complete Examples

   This section presents two complete examples of load control documents
   vliad with respect to the XML schema defined in Section 7.

   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"



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               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>
               <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 an earthquake.  Incoming
   calls to the earthquake domain "pompeii.example.com" 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 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"
       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="pompeii.example.com"/>



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                       </lc:to>
                       <lc:from>
                           <many>
                               <except domain="pompeii.example.com"/>
                               <except domain="rescue.example.com"/>
                           </many>
                       </lc:from>
                   </lc:sip>
               </lc:call-identity>
               <validity>
                   <from>79-08-24T09:00:00+01:00</from>
                   <until>79-08-27T09:00:00+01:00</until>
               </validity>
           </conditions>
           <actions>
               <lc:accept alt-action="forward" alt-target=
                       "sip:earthquake@update.example.com">
                   <lc:rate>100</lc:rate>
               </lc:accept>
           </actions>

       </rule>
   <ruleset>



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] 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 control 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">



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

   <!-- CALL TYPE -->
   <xs:complexType name="call-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="cp:identityType" minOccurs="0"/>
     <element name="to" type="cp:identityType" minOccurs="0"/>
     <element name="request-uri" type="cp:identityType" minOccurs="0"/>



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     <element name="p-asserted-identity" type="cp:identityType"
           minOccurs="0"/>
     <any namespace="##other" processContents="lax" minOccurs="0"
           maxOccurs="unbounded"/>
     </xs:sequence>
     <anyAtrribute namespace="##other" processContents="lax"/>
   </xs:complexType>

   <!-- 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>

   <!-- ACTIONS -->

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

   </xs:schema>



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 specification defines a SIP events framework based
   distribution mechanism which allows automated filter distribution in
   suitable environments.



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   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 time fields of the SIP load filter defined
   in this specification.  However, the filter 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, 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 arbitrarily grouped by SIP domains or any number of
   leading prefix of the telephone numbers.

   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 load 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 proxy
   servers and core proxy servers, 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 Recorder 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 load filtering rules in this specification consists 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



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   identified by either the source or the destination.  When the user is
   identified by the source and a favorable action is specified, the
   result is to some extent similar to identifying a priority user based
   on authorized Resource Priority Headers [RFC4412] in the requests.
   Specifying a source user identity with an unfavorable action would
   cause an effect to some extent similar to an inverse SIP resource
   priority mechanism.

   The load filter 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 filter identities for load control and
   could share well-known load control actions.  The time duration field
   in the load filter could also be used in both mechanisms.  As
   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 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 control duration.


9.  Discussion of this specification meeting the requirements of RFC5390

   This section evaluates whether the load control event package defined
   in this specification satisfies the various SIP overload control
   requirements set forth by RFC5390 [RFC5390].  Not all RFC5390
   requirements are found applicable due to the scope of this document.
   Therefore, we 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.



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   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.  If the predictions
   are incorrect, in either direction, the effectiveness of the
   mechanism will be affected.

      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 specification, while the distribution of the updated
   filters 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.

      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 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



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      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 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.

   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/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 load filter [Section 4.1] allows the inclusion
   of alternative forwarding destinations for rejected requests.

      REQ 10: The mechanism should support servers that receive requests



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      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 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 4.4).

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

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

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

   N/A to the load control event package 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



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      these cases.

   P/A. Because the filters are provisioned in advance, they are not
   affected by the overload or failure of other nodes.  But, on the
   other hand, they may not, in those cases, be able to protect the
   overloaded node (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 RFC3265 [RFC3265]
   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 [RFC3265] and of section 10 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 filter format
   definition in Section 4.1.

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

   N/A to the load control event package 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



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   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
      proxies.

   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 specification.
   One 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 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, 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.  On the other
   hand, if a legitimate notifier is itself compromised, additional
   mechanisms will be needed to detect the attack.

   Security considerations for filter enforcements vary depending on the
   filter itself.  This specification defines possible filter match of
   the following SIP header fields: <from>, <to>, <request-uri> and



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   <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.


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 specification

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

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

      MIME subtype name: load-control+xml

      Mandatory parameters: none

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

   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




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   Published Specification: This specification

   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, charles@cs.columbia.edu

   Intended usage: COMMON

   Author/Change Controller: IETF SOC Working Group
   <sip-overload@ietf.org>, as designated by the IESG <iesg@ietf.org>

11.3.  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 7.

   Its first line is

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

   and its last line is

   </xs:schema>


12.  Acknowledgements

   The authors would like to thank Bruno Chatras, Martin Dolly, Keith
   Drage, Ashutosh Dutta, Janet Gunn, Vijay Gurbani, Volker Hilt, Geoff
   Hunt, Hadriel Kaplan, Paul Kyzivat, Salvatore Loreto, Timothy Moran,
   Eric Noel, Parthasarathi R, Shida Schubert, Robert Sparks, Phil
   Williams and other members of the SOC and SIPPING working group for
   many helpful comments.  In addition, Bruno Chatras proposed the
   <method> condition element.  Janet Gunn provided detailed text



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   suggestions for Section 9.  Shida made many suggestions about
   terminology usage.  Phil Williams suggested adding support for delta
   updates.  Ashutosh Dutta gave pointers to PSTN references.


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.

   [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
              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.

   [I-D.ietf-soc-overload-control]
              Gurbani, V., Hilt, V., and H. Schulzrinne, "Session
              Initiation Protocol (SIP) Overload Control",



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              draft-ietf-soc-overload-control-07 (work in progress),
              January 2012.

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

   [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.

   [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
   AT&T Security Research Center
   33 Thomas Street
   New York, NY  10007
   USA

   Phone: +1 212 513 2081
   Email: shen@att.com


   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



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   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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