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rfc:rfc7415

Internet Engineering Task Force (IETF) E. Noel Request for Comments: 7415 AT&T Labs Category: Standards Track P. Williams ISSN: 2070-1721 BT Innovate & Design

                                                         February 2015
           Session Initiation Protocol (SIP) Rate Control

Abstract

 The prevalent use of the Session Initiation Protocol (SIP) in Next
 Generation Networks necessitates that SIP networks provide adequate
 control mechanisms to maintain transaction throughput by preventing
 congestion collapse during traffic overloads.  A loss-based solution
 to remedy known vulnerabilities of the SIP 503 (Service Unavailable)
 overload control mechanism has already been proposed.  Using the same
 signaling, this document proposes a rate-based control scheme to
 complement the loss-based control scheme.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7415.

Copyright Notice

 Copyright (c) 2015 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Noel & Williams Standards Track [Page 1] RFC 7415 SIP Rate Control February 2015

Table of Contents

 1. Introduction ....................................................2
 2. Terminology .....................................................3
 3. Rate-Based Algorithm Scheme .....................................3
    3.1. Overview ...................................................3
    3.2. Via Header Field Parameters for Overload Control ...........4
    3.3. Client and Server Rate-Based Control Algorithm Selection ...4
    3.4. Server Operation ...........................................5
    3.5. Client Operation ...........................................6
         3.5.1. Default Algorithm ...................................6
         3.5.2. Priority Treatment ..................................9
         3.5.3. Optional Enhancement: Avoidance of Resonance .......10
 4. Example ........................................................12
 5. Syntax .........................................................13
 6. Security Considerations ........................................13
 7. IANA Considerations ............................................13
 8. References .....................................................14
    8.1. Normative References ......................................14
    8.2. Informative References ....................................14
 Acknowledgments ...................................................14
 Contributors ......................................................14
 Authors' Addresses ................................................15

1. Introduction

 The use of SIP [RFC3261] in large-scale Next Generation Networks
 requires that SIP-based networks provide adequate control mechanisms
 for handling traffic growth.  In particular, SIP networks must be
 able to handle traffic overloads gracefully, maintaining transaction
 throughput by preventing congestion collapse.
 A promising SIP-based overload control solution has been proposed in
 [RFC7339].  That solution provides a communication scheme for
 overload control algorithms.  It also includes a default loss-based
 overload control algorithm that makes it possible for a set of
 clients to limit offered load towards an overloaded server.  However,
 such a loss control algorithm is sensitive to variations in load so
 that any increase in load would be directly reflected by the clients
 in the offered load presented to the overloaded servers.  More
 importantly, a loss-based control scheme cannot guarantee an upper
 bound on the load from the clients towards an overloaded server and
 requires frequent updates that may have implications for stability.
 In accordance with the framework defined in [RFC7339], this document
 proposes an alternate overload control scheme: the rate-based
 overload control scheme.  The rate-based control algorithm guarantees
 an upper bound on the rate, constant between server updates, of

Noel & Williams Standards Track [Page 2] RFC 7415 SIP Rate Control February 2015

 requests sent by clients towards an overloaded server.  The trade-off
 is in terms of algorithmic complexity, since the overloaded server is
 more likely to use a different target (maximum rate) for each client
 than the loss-based approach.
 The proposed rate-based overload control algorithm mitigates
 congestion in SIP networks while adhering to the overload signaling
 scheme in [RFC7339] and presenting a rate-based control scheme as an
 optional alternative to the default loss-based control scheme in
 [RFC7339].

2. Terminology

 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].
 Unless otherwise specified, all SIP entities described in this
 document are assumed to support this specification.

3. Rate-Based Algorithm Scheme

3.1. Overview

 The server is the one protected by the overload control algorithm
 defined here, and the client is the one that throttles traffic
 towards the server.
 Following the procedures defined in [RFC7339], the server and clients
 signal one another support for rate-based overload control.
 Then, periodically, the server relies on internal measurements (e.g.,
 CPU utilization or queueing delay) to evaluate its overload state and
 estimate a target maximum SIP request rate in number of requests per
 second (as opposed to target percent loss in the case of loss-based
 control).
 When in overload, the server uses the "oc" parameter in the Via
 header field [RFC7339] of SIP responses in order to inform clients of
 its overload state and of the target maximum SIP request rate for
 that client.
 Upon receiving the "oc" parameter with a target maximum SIP request
 rate, each client throttles new SIP requests towards the overloaded
 server.

Noel & Williams Standards Track [Page 3] RFC 7415 SIP Rate Control February 2015

3.2. Via Header Field Parameters for Overload Control

 Four Via header parameters are defined in [RFC7339] and are
 summarized below:
 o  oc: Used by clients in SIP requests to indicate support for
    overload control per [RFC7339] and by servers to indicate the load
    reduction amount in the loss-based algorithm and the maximum rate,
    in messages per second, for the rate-based algorithm described
    here.
 o  oc-algo: Used by clients in SIP requests to advertise supported
    overload control algorithms and by servers to notify clients of
    the algorithm in effect.  Supported values are loss (default) and
    rate (optional).
 o  oc-validity: Used by servers in SIP responses to indicate an
    interval of time (in milliseconds) that the load reduction should
    be in effect.  A value of 0 is reserved for the server to stop
    overload control.  A non-zero value is required in all other
    cases.
 o  oc-seq: A sequence number associated with the "oc" parameter.
 Consult Section 4 for an illustration of the usage of the "oc"
 parameter in the Via header field.

3.3. Client and Server Rate-Based Control Algorithm Selection

 Per [RFC7339], new clients indicate supported overload control
 algorithms to servers by inserting "oc" and "oc-algo", with the names
 of the supported algorithms, in the Via header field of SIP requests
 destined to servers.  The inclusion by the client of the token "rate"
 indicates that the client supports a rate-based algorithm.
 Conversely, servers notify clients of the selected overload control
 algorithm through the "oc-algo" parameter in the Via header field of
 SIP responses to clients.  The inclusion by the server of the token
 "rate" in the "oc-algo" parameter indicates that the rate-based
 algorithm has been selected by the server.
 Support of rate-based control MUST be indicated by clients including
 the token "rate" in the "oc-algo" list.  Selection of rate-based
 control MUST be indicated by servers by setting "oc-algo" to the
 token "rate".

Noel & Williams Standards Track [Page 4] RFC 7415 SIP Rate Control February 2015

3.4. Server Operation

 The actual algorithm used by the server to determine its overload
 state and estimate a target maximum SIP request rate is beyond the
 scope of this document.
 However, the server MUST periodically evaluate its overload state and
 estimate a target SIP request rate beyond which it would become
 overloaded.  The server must determine how it will allocate the
 target SIP request rate among its client.  The server may set the
 same rate for every client or may set different rates for different
 clients.
 The maximum rate determined by the server for a client applies to the
 entire stream of SIP requests, even though throttling may only affect
 a particular subset of the requests, since as per [RFC7339] and REQ
 13 of [RFC5390], request prioritization is a client's responsibility.
 When setting the maximum rate for a particular client, the server may
 need to take into account the workload (e.g., CPU load per request)
 of the distribution of message types from that client.  Furthermore,
 because the client may prioritize the specific types of messages it
 sends while under overload restriction, this distribution of message
 types may be different from the message distribution for that client
 under non-overload conditions (e.g., it could have either higher or
 lower CPU load).
 Note that the "oc" parameter for the rate-based algorithm is an upper
 bound (in messages per second) on the traffic sent by the client to
 the server.  The client may send traffic at a rate significantly
 lower than the upper bound for a variety of reasons.
 In other words, when multiple clients are being controlled by an
 overloaded server, at any given time, some clients may receive
 requests at a rate below their target (maximum) SIP request rate
 while others above that target rate.  But the resulting request rate
 presented to the overloaded server will converge towards the target
 SIP request rate.
 Upon detection of overload and the determination to invoke overload
 controls, the server MUST follow the specifications in [RFC7339] to
 notify its clients of the allocated target SIP request rate and to
 notify them that rate-based control is in effect.
 The server MUST use the "oc" parameter defined in [RFC7339] to send a
 target SIP request rate to each of its clients.

Noel & Williams Standards Track [Page 5] RFC 7415 SIP Rate Control February 2015

 When a client supports the default loss-based algorithm and not the
 rate-based algorithm, the client would be handled in the same way as
 in Section 5.10.2 of [RFC7339].

3.5. Client Operation

3.5.1. Default Algorithm

 In determining whether or not to transmit a specific message, the
 client may use any algorithm that limits the message rate to the "oc"
 parameter in units of messages per second.  For ease of discussion,
 we define T = 1/["oc" parameter] as the target inter-SIP request
 interval.  The algorithm may be strictly deterministic, or it may be
 probabilistic.  It may, or may not, have a tolerance factor to allow
 for short bursts, as long as the long-term rate remains below 1/T.
 The algorithm may have provisions for prioritizing traffic in
 accordance with REQ 13 of [RFC5390].
 If the algorithm requires other parameters (in addition to "T", which
 is 1/["oc" parameter]), they may be set autonomously by the client,
 or they may be negotiated between client and server independently of
 the SIP-based overload control solution.
 In either case, the coordination is out of the scope of this
 document.  The default algorithms presented here (one with and one
 without provisions for prioritizing traffic) are only examples.
 To throttle new SIP requests at the rate specified by the "oc"
 parameter sent by the server to its clients, the client MAY use the
 proposed default algorithm for rate-based control or any other
 equivalent algorithm that forward messages in conformance with the
 upper bound of 1/T messages per second.
 The default leaky bucket algorithm presented here is based on
 [ITU-T-I.371], Appendix A.2.  The algorithm makes it possible for
 clients to deliver SIP requests at a rate specified by the "oc"
 parameter with the tolerance parameter TAU (preferably configurable).
 Conceptually, the leaky bucket algorithm can be viewed as a finite
 capacity bucket whose real-valued content drains out at a continuous
 rate of 1 unit of content per time unit and whose content increases
 by the increment T for each forwarded SIP request.  T is computed as
 the inverse of the rate specified by the "oc" parameter, namely
 T = 1 / ["oc" parameter].

Noel & Williams Standards Track [Page 6] RFC 7415 SIP Rate Control February 2015

 Note that when the "oc" parameter is 0 with a non-zero "oc-validity",
 then the client should reject 100% of SIP requests destined to the
 overload server.  However, when the "oc-validity" value is 0, the
 client should immediately stop throttling.
 If, at a new SIP request arrival, the content of the bucket is less
 than or equal to the limit value TAU, then the SIP request is
 forwarded to the server; otherwise, the SIP request is rejected.
 Note that the capacity of the bucket (the upper bound of the counter)
 is (T + TAU).
 The tolerance parameter TAU determines how close the long-term
 admitted rate is to an ideal control that would admit all SIP
 requests for arrival rates less than 1/T and then admit SIP requests
 precisely at the rate of 1/T for arrival rates above 1/T.  In
 particular, at mean arrival rates close to 1/T, it determines the
 tolerance to deviation of the inter-arrival time from T (the larger
 TAU, the more tolerance to deviations from the inter-departure
 interval T).
 This deviation from the inter-departure interval influences the
 admitted rate burstiness or the number of consecutive SIP requests
 forwarded to the server (burst size proportional to TAU over the
 difference between 1/T and the arrival rate).
 In situations where clients are configured with some knowledge about
 the server (e.g., operator pre-provisioning), it can be beneficial to
 choose a value of TAU based on how many clients will be sending
 requests to the server.
 Servers with a very large number of clients, each with a relatively
 small arrival rate, will generally benefit from a smaller value for
 TAU in order to limit queuing (and hence response times) at the
 server when subjected to a sudden surge of traffic from all clients.
 Conversely, a server with a relatively small number of clients, each
 with a proportionally larger arrival rate, will benefit from a larger
 value of TAU.
 Once the control has been activated, at the arrival time of the k-th
 new SIP request, ta(k), the content of the bucket is provisionally
 updated to the value
 X' = X - (ta(k) - LCT)
 where X is the value of the leaky bucket counter after arrival of the
 last forwarded SIP request, and LCT is the time at which the last SIP
 request was forwarded.

Noel & Williams Standards Track [Page 7] RFC 7415 SIP Rate Control February 2015

 If X' is less than or equal to the limit value TAU, then the new SIP
 request is forwarded, the leaky bucket counter X is set to X' (or to
 0 if X' is negative) plus the increment T, and LCT is set to the
 current time ta(k).  If X' is greater than the limit value TAU, then
 the new SIP request is rejected, and the values of X and LCT are
 unchanged.
 When the first response from the server has been received indicating
 control activation (oc-validity>0), LCT is set to the time of
 activation, and the leaky bucket counter is initialized to the
 parameter TAU0 (preferably configurable), which is 0 or larger but
 less than or equal to TAU.
 TAU can assume any positive real number value and is not necessarily
 bounded by T.
 TAU=4*T is a reasonable compromise between burst size and throttled
 rate adaptation at low offered rates.
 Note that specification of a value for TAU and any communication or
 coordination between servers are beyond the scope of this document.
 A reference algorithm is shown below.
 No priority case:
 // T: inter-transmission interval, set to 1 / ["oc" parameter]
 // TAU: tolerance parameter
 // ta: arrival time of the most recent arrival received by the
 //     client
 // LCT: arrival time of last SIP request that was sent to the server
 //      (initialized to the first arrival time)
 // X: current value of the leaky bucket counter (initialized to
 //    TAU0)
 // After most recent arrival, calculate auxiliary variable Xp
 Xp = X - (ta - LCT);
 if (Xp <= TAU) {
   // Transmit SIP request
   // Update X and LCT
   X = max (0, Xp) + T;
   LCT = ta;
 } else {
   // Reject SIP request
   // Do not update X and LCT
 }

Noel & Williams Standards Track [Page 8] RFC 7415 SIP Rate Control February 2015

3.5.2. Priority Treatment

 As with the loss-based algorithm in [RFC7339], a client implementing
 the rate-based algorithm also prioritizes messages into two or more
 categories of requests, for example, requests that are candidates for
 reduction and requests that are not subject to reduction (except
 under extenuating circumstances when there aren't any messages in the
 first category that can be reduced).
 Accordingly, the proposed leaky bucket implementation is modified to
 support priority using two thresholds for SIP requests that are
 candidates for reduction.  With two priorities, the proposed leaky
 bucket requires two thresholds: TAU1 and TAU2 (where TAU1 < TAU2):
 o  All new requests would be admitted when the leaky bucket counter
    is at or below TAU1.
 o  Only higher-priority requests would be admitted when the leaky
    bucket counter is between TAU1 and TAU2.
 o  All requests would be rejected when the bucket counter is at or
    above TAU2.
 This can be generalized to n priorities using n thresholds for n>2 in
 the obvious way.
 With a priority scheme that relies on two tolerance parameters (TAU2
 influences the priority traffic, and TAU1 influences the non-priority
 traffic), always set TAU1 < TAU2 (TAU is replaced by TAU1 and TAU2).
 Setting both tolerance parameters to the same value is equivalent to
 having no priority.  TAU1 influences the admitted rate the same way
 as TAU does when no priority is set.  The larger the difference
 between TAU1 and TAU2, the closer the control is to strict priority
 queueing.
 TAU1 and TAU2 can assume any positive real number value and are not
 necessarily bounded by T.
 Reasonable values for TAU0, TAU1, and TAU2 are:
 o  TAU0 = 0,
 o  TAU1 = 1/2 * TAU2, and
 o  TAU2 = 10 * T.

Noel & Williams Standards Track [Page 9] RFC 7415 SIP Rate Control February 2015

 Note that specification of a value for TAU1 and TAU2 and any
 communication or coordination between servers are beyond the scope of
 this document.
 A reference algorithm is shown below.
 Priority case:
 // T: inter-transmission interval, set to 1 / ["oc" parameter]
 // TAU1: tolerance parameter of no-priority SIP requests
 // TAU2: tolerance parameter of priority SIP requests
 // ta: arrival time of the most recent arrival received by the
 //     client
 // LCT: arrival time of last SIP request that was sent to the server
 //      (initialized to the first arrival time)
 // X: current value of the leaky bucket counter (initialized to
 //    TAU0)
 // After most recent arrival, calculate auxiliary variable Xp
 Xp = X - (ta - LCT);
 if (AnyRequestReceived && Xp <= TAU1) || (PriorityRequestReceived &&
 Xp <= TAU2 && Xp > TAU1) {
   // Transmit SIP request
   // Update X and LCT
   X = max (0, Xp) + T;
   LCT = ta;
 } else {
   // Reject SIP request
   // Do not update X and LCT
 }

3.5.3. Optional Enhancement: Avoidance of Resonance

 As the number of client sources of traffic increases or the
 throughput of the server decreases, the maximum rate admitted by each
 client needs to decrease; therefore, the value of T becomes larger.
 Under some circumstances (e.g., if the traffic arises very quickly
 simultaneously at many sources), the occupancies of each bucket can
 become synchronized, resulting in the admissions from each source
 being close in time and batched or having very 'peaky' arrivals at
 the server, which gives rise not only to control instability but also
 to very poor delays and even lost messages.  An appropriate term for
 this is 'resonance' [Erramilli].

Noel & Williams Standards Track [Page 10] RFC 7415 SIP Rate Control February 2015

 If the network topology is such that resonance can occur, then a
 simple way to avoid resonance is to randomize the bucket occupancy at
 two appropriate points -- at the activation of control and whenever
 the bucket empties -- as described below.
 After updating the value of the leaky bucket to X', generate a value
 u as follows:
   if X' > 0, then u = 0
   else if X' <= 0, then let u be set to a random value uniformly
                    distributed between -1/2 and +1/2
 Then, only if the arrival is admitted, increase the bucket by an
 amount T + uT, which will therefore be just T if the bucket hadn't
 emptied or lie between T/2 and 3T/2 if it had.
 This randomization should also be done when control is activated,
 i.e., instead of simply initializing the leaky bucket counter to
 TAU0, initialize it to TAU0 + uT, where u is uniformly distributed as
 above.  Since activation would have been a result of response to a
 request sent by the client, the second term in this expression can be
 interpreted as being the bucket increment following that admission.
 This method has the following characteristics:
 o  If TAU0 is chosen to be equal to TAU and all sources activate
    control at the same time due to an extremely high request rate,
    then the time until the first request admitted by each client
    would be uniformly distributed over [0,T].
 o  The maximum occupancy is TAU + (3/2)T, rather than TAU + T without
    randomization.
 o  For the special case of 'classic gapping' where TAU=0, then the
    minimum time between admissions is uniformly distributed over
    [T/2, 3T/2], and the mean time between admissions is the same,
    i.e., T+1/R where R is the request arrival rate.
 o  As high load randomization rarely occurs, there is no loss of
    precision of the admitted rate, even though the randomized
    'phasing' of the buckets remains.

Noel & Williams Standards Track [Page 11] RFC 7415 SIP Rate Control February 2015

4. Example

 The example in this section adapts the example in Section 6 of
 [RFC7339], where client P1 sends requests to a downstream server P2:
          INVITE sips:user@example.com SIP/2.0
          Via: SIP/2.0/TLS p1.example.net;
           branch=z9hG4bK2d4790.1;received=192.0.2.111;
           oc;oc-algo="loss,rate"
          ...
          SIP/2.0 100 Trying
          Via: SIP/2.0/TLS p1.example.net;
           branch=z9hG4bK2d4790.1;received=192.0.2.111;
           oc=0;oc-algo="rate";oc-validity=0;
           oc-seq=1282321615.781
          ...
 The first message above is sent by P1 to P2.  This message is a SIP
 request; because P1 supports overload control, it inserts the "oc"
 parameter in the topmost Via header field that it created.  P1
 supports two overload control algorithms: loss and rate.
 The second message, a SIP response, shows the topmost Via header
 field amended by P2 according to this specification and sent to P1.
 Because P2 also supports overload control, it chooses the rate-based
 scheme and sends that back to P1 in the "oc-algo" parameter.  It uses
 oc-validity=0 to indicate no overload control.  In this example,
 "oc=0", but "oc" could be any value as "oc" is ignored when
 "oc-validity=0".
 At some later time, P2 starts to experience overload.  It sends the
 following SIP message indicating P1 should send SIP requests at a
 rate no greater than or equal to 150 SIP requests per second and for
 a duration of 1,000 milliseconds.

Noel & Williams Standards Track [Page 12] RFC 7415 SIP Rate Control February 2015

          SIP/2.0 180 Ringing
          Via: SIP/2.0/TLS p1.example.net;
           branch=z9hG4bK2d4790.1;received=192.0.2.111;
           oc=150;oc-algo="rate";oc-validity=1000;
           oc-seq=1282321615.782
           ...

5. Syntax

 This specification extends the existing definition of the Via header
 field parameters of [RFC7339] as follows:
 algo-list =/ "rate"

6. Security Considerations

 Aside from the resonance concerns discussed in Section 3.5.3, this
 mechanism does not introduce any security concerns beyond the general
 overload control security issues discussed in [RFC7339].  Methods to
 mitigate the risk of resonance are discussed in Section 3.5.3.

7. IANA Considerations

 IANA has registered the "oc-algo" parameter of the Via header field
 in the "Header Field Parameters and Parameter Values" subregistry of
 the "Session Initiation Protocol (SIP) Parameters" registry.  The
 entry appears as follows:
 Header     Parameter     Predefined     References
 Field      Name          Values
 ___________________________________________________________
 Via        oc-algo       Yes            [RFC7339] [RFC7415]

Noel & Williams Standards Track [Page 13] RFC 7415 SIP Rate Control February 2015

8. References

8.1. Normative References

 [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997,
               <http://www.rfc-editor.org/info/rfc2119>.
 [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,
               <http://www.rfc-editor.org/info/rfc3261>.
 [RFC5390]     Rosenberg, J., "Requirements for Management of Overload
               in the Session Initiation Protocol", RFC 5390, December
               2008, <http://www.rfc-editor.org/info/rfc5390>.
 [RFC7339]     Gurbani, V., Ed., Hilt, V., and H. Schulzrinne,
               "Session Initiation Protocol (SIP) Overload Control",
               RFC 7339, September 2014,
               <http://www.rfc-editor.org/info/rfc7339>.

8.2. Informative References

 [ITU-T-I.371] ITU-T, "Traffic control and congestion control in
               B-ISDN", ITU-T Recommendation I.371, March 2004.
 [Erramilli]   Erramilli, A., and L. Forys, "Traffic Synchronization
               Effects In Teletraffic Systems", ITC-13, 1991.

Acknowledgments

 Many thanks to the following individuals for comments and feedback on
 this document: Richard Barnes, Keith Drage, Vijay Gurbany, Volker
 Hilt, Christer Holmberg, Winston Hong, Peter Yee, and James Yu.

Contributors

 Significant contributions to this document were made by Janet Gunn.

Noel & Williams Standards Track [Page 14] RFC 7415 SIP Rate Control February 2015

Authors' Addresses

 Eric Noel
 AT&T Labs
 200 S Laurel Avenue
 Middletown, NJ 07747
 United States
 EMail: eric.noel@att.com
 Philip M. Williams
 BT Innovate & Design
 Ipswich, IP5 3RE
 United Kingdom
 EMail: phil.m.williams@bt.com

Noel & Williams Standards Track [Page 15]

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