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Internet Engineering Task Force (IETF) C. Shen Request for Comments: 7200 H. Schulzrinne Category: Standards Track Columbia U. ISSN: 2070-1721 A. Koike NTT April 2014 A Session Initiation Protocol (SIP) Load-Control Event Package Abstract This specification defines a load-control event package for the Session Initiation Protocol (SIP). It allows SIP entities to distribute load-filtering policies to other SIP entities in the network. The load-filtering policies contain rules to throttle calls from a specific user or based on their source or destination domain, telephone number prefix. The mechanism helps to prevent signaling overload and complements feedback-based SIP overload control efforts. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7200. Copyright Notice Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Shen, et al. Standards Track [Page 1]
RFC 7200 SIP Load-Control Event Package April 2014 Table of Contents 1. Introduction ....................................................3 2. Conventions .....................................................3 3. SIP Load-Filtering Overview .....................................4 3.1. Load-Filtering Policy Format ...............................4 3.2. Load-Filtering Policy Computation ..........................4 3.3. Load-Filtering Policy Distribution .........................4 3.4. Applicable Network Domains .................................8 4. Load-Control Event Package ......................................9 4.1. Event Package Name .........................................9 4.2. Event Package Parameters ...................................9 4.3. SUBSCRIBE Bodies ...........................................9 4.4. SUBSCRIBE Duration .........................................9 4.5. NOTIFY Bodies .............................................10 4.6. Notifier Processing of SUBSCRIBE Requests .................10 4.7. Notifier Generation of NOTIFY Requests ....................10 4.8. Subscriber Processing of NOTIFY Requests ..................10 4.9. Handling of Forked Requests ...............................12 4.10. Rate of Notifications ....................................12 4.11. State Delta ..............................................12 5. Load-Control Document ..........................................13 5.1. Format ....................................................13 5.2. Namespace .................................................13 5.3. Conditions ................................................14 5.3.1. Call Identity ......................................14 5.3.2. Method .............................................16 5.3.3. Target SIP Entity ..................................17 5.3.4. Validity ...........................................18 5.4. Actions ...................................................18 6. XML Schema Definition for Load Control .........................20 7. Security Considerations ........................................23 8. IANA Considerations ............................................24 8.1. Load-Control Event Package Registration ...................24 8.2. application/load-control+xml Media Type Registration ......24 8.3. URN Sub-Namespace Registration ............................25 8.4. Load-Control Schema Registration ..........................26 9. Acknowledgements ...............................................27 10. References ....................................................27 10.1. Normative References .....................................27 10.2. Informative References ...................................28 Appendix A. Definitions ...........................................30 Appendix B. Design Requirements ...................................30 Appendix C. Discussion of How This Specification Meets the Requirements of RFC 5390 ..............................31 Appendix D. Complete Examples .....................................36 D.1. Load-Control Document Examples ............................36 D.2. Message Flow Examples .....................................40 Shen, et al. Standards Track [Page 2]
RFC 7200 SIP Load-Control Event Package April 2014 Appendix E. Related Work .........................................41 E.1. Relationship to Load Filtering in PSTN ....................41 E.2. Relationship with Other IETF SIP Overload Control Efforts .42 1. Introduction SIP load-control mechanisms are needed to prevent congestion collapse [RFC6357] in cases of SIP server overload [RFC5390]. There are two types of load-control approaches. In the first approach, feedback control, SIP servers provide load limits to upstream servers, to reduce the incoming rate of all SIP requests [SIP-OVERLOAD]. These upstream servers then drop or delay incoming SIP requests. Feedback control is reactive and affects signaling messages that have already been issued by user agent clients. This approach works well when SIP proxy servers in the core networks (core proxy servers) or destination-specific SIP proxy servers in the edge networks (edge proxy servers) are overloaded. By their nature, they need to distribute rate, drop, or window information to all upstream SIP proxy servers and normally affect all calls equally, regardless of destination. This specification proposes an additional, complementary load-control mechanism, called "load filtering". It is most applicable for situations where a traffic surge and its source/destination distribution can be predicted in advance. In those cases, network operators create load-filtering policies that indicate calls to specific destinations or from specific sources should be rate-limited or randomly dropped. These load-filtering policies are then distributed to SIP servers and possibly SIP user agents that are likely to generate calls to the affected destinations or from the affected sources. Load filtering works best if it prevents calls as close to the originating user agent clients as possible. The applicability of SIP load filtering can also be extended beyond overload control, e.g., to implement service level agreement commitments. 2. Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. Shen, et al. Standards Track [Page 3]
RFC 7200 SIP Load-Control Event Package April 2014 3. SIP Load-Filtering Overview 3.1. Load-Filtering Policy Format Load-filtering policies are specified by sets of rules. Each rule contains both load-filtering conditions and actions. The load- filtering conditions define identities of the targets to be filtered (Section 5.3.1). For example, there are two typical resource limits in a possible overload situation, i.e., human destination limits (number of call takers) and node capacity limits. The load-filtering targets in these two cases can be the specific callee numbers or the destination domain corresponding to the overload. Load-filtering conditions also indicate the specific message type to be matched (Section 5.3.2), with which target SIP entity the filtering policy is associated (Section 5.3.3), and the period of time when the filtering policy should be activated and deactivated (Section 5.3.4). Load- filtering actions describe the desired control functions such as keeping the request rate below a specified level (Section 5.4). 3.2. Load-Filtering Policy Computation When computing the load-filtering policies, one needs to take into consideration information such as overload time, scope and network topology, as well as service policies. It is also important to make sure that there is no resource allocation loop and that server capacity is allocated in a way that both prevents overload and maximizes effective throughput (commonly called goodput). In some cases, in order to better utilize system resources, it may be preferable to employ an algorithm that dynamically computes the load- filtering policies based on currently observed server load status, rather than using a purely static filtering policy assignment. The computation algorithm for load-filtering policies is beyond the scope of this specification. 3.3. Load-Filtering Policy Distribution For distributing load-filtering policies, this specification defines the SIP event package for load control, which is an "instantiation" of the generic SIP event notification framework [RFC6665]. This specification also defines the XML schema of a load-control document (Section 5), which is used to encode load-filtering policies. In order for load-filtering policies to be properly distributed, each capable SIP entity in the network subscribes to the SIP load-control event package of each SIP entity to which it sends signaling requests. A SIP entity that accepts subscription requests is called a "notifier" (Section 4.6). Subscription is initiated and maintained during normal server operation. The subscription of neighboring SIP Shen, et al. Standards Track [Page 4]
RFC 7200 SIP Load-Control Event Package April 2014 entities needs to be persistent, as described in Sections 4.1 and 4.2 of [RFC6665]. The refresh procedure is described in Section 4.7 below. Subscribers may terminate the subscription if they have not received notifications for an extended time period, and can resubscribe if they determine that signaling with the notifier becomes active again. An example architecture is shown in Figure 1 to illustrate SIP load- filtering policy distribution. This scenario consists of two networks belonging to Service Provider A and Service Provider B, respectively. Each provider's network is made up of two SIP core proxy servers and four SIP edge proxy servers. The core proxy servers and edge proxy servers of Service Provider A are denoted as CPa1 to CPa2 and EPa1 to EPa4; the core proxy servers and edge proxy servers of Service Provider B are denoted as CPb1 to CPb2 and EPb1 to EPb4. Shen, et al. Standards Track [Page 5]
RFC 7200 SIP Load-Control Event Package April 2014 +-----------+ +-----------+ +-----------+ +-----------+ | | | | | | | | | EPa1 | | EPa2 | | EPa3 | | EPa4 | | | | | | | | | +-----------+ +-----------+ +-----------+ +-----------+ \ / \ / \ / \ / \ / \ / +-----------+ +-----------+ | | | | | CPa1 |------------------| CPa2 | | | | | +-----------+ +-----------+ | | Service | | Provider A | | | | ================================================================= | | Service | | Provider B | | | | +-----------+ +-----------+ | | | | | CPb1 |------------------| CPb2 | | | | | +-----------+ +-----------+ / \ / \ / \ / \ / \ / \ +-----------+ +-----------+ +-----------+ +-----------+ | | | | | | | | | EPb1 | | EPb2 | | EPb3 | | EPb4 | | | | | | | | | +-----------+ +-----------+ +-----------+ +-----------+ Figure 1: Example Network Scenario Using SIP Load-Control Event Package Mechanism During the initialization stage, the proxy servers first identify all their outgoing signaling neighbors and subscribe to them. Service providers can provision neighbors, or the proxy servers can incrementally learn who their neighbors are by inspecting signaling messages that they send and receive. Assuming all signaling relationships in Figure 1 are bidirectional, after this initialization stage, each proxy server will be subscribed to all its neighbors. Shen, et al. Standards Track [Page 6]
RFC 7200 SIP Load-Control Event Package April 2014 Case I: EPa1 serves a TV program hotline and decides to limit the total number of incoming calls to the hotline to prevent an overload. To do so, EPa1 sends a notification to CPa1 with the specific hotline number, time of activation, and total acceptable call rate. Depending on the load-filtering policy computation algorithm, CPa1 may allocate the received total acceptable call rate among its neighbors, namely, EPa2, CPa2, and CPb1, and notify them about the resulting allocation along with the hotline number and the activation time. CPa2 and CPb1 may perform further allocation among their own neighbors and notify the corresponding proxy servers. This process continues until all edge proxy servers in the network have been informed about the event and have proper load-filtering policies configured. In the above case, the network entity where load-filtering policy is first introduced is the SIP server providing access to the resource that creates the overload situation. In other cases, the network entry point of introducing load-filtering policy could also be an entity that hosts this resource. For example, an operator may host an application server that performs toll-free-number ("800 number") translation services. The application server itself may be a SIP proxy server or a SIP Back-to-Back User Agent (B2BUA). If one of the toll-free numbers hosted at the application server creates the overload condition, the load-filtering policies can be introduced from the application server and then propagated to other SIP proxy servers in the network. Case II: A hurricane affects the region covered by CPb2, EPb3, and EPb4. All three of these SIP proxy servers are overloaded. The rescue team determines that outbound calls are more valuable than inbound calls in this specific situation. Therefore, EPb3 and EPb4 are configured with load-filtering policies to accept more outbound calls than inbound calls. CPb2 may be configured the same way or receive dynamically computed load-filtering policies from EPb3 and EPb4. Depending on the load-filtering policy computation algorithm, CPb2 may also send out notifications to its outside neighbors, namely CPb1 and CPa2, specifying a limit on the acceptable rate of inbound calls to CPb2's responsible domain. CPb1 and CPa2 may subsequently notify their neighbors about limiting the calls to CPb2's area. The same process could continue until all edge proxy servers are notified and have load-filtering policies configured. Note that this specification does not define the provisioning interface between the party who determines the load-filtering policy and the network entry point where the policy is introduced. One of the options for the provisioning interface is the Extensible Markup Language (XML) Configuration Access Protocol (XCAP) [RFC4825]. Shen, et al. Standards Track [Page 7]
RFC 7200 SIP Load-Control Event Package April 2014 3.4. Applicable Network Domains This specification MUST be applied inside a "Trust Domain". The concept of a Trust Domain is similar to that defined in [RFC3324] and [RFC3325]. A Trust Domain, for the purpose of SIP load filtering, is a set of SIP entities such as SIP proxy servers that are trusted to exchange load-filtering policies defined in this specification. In the simplest case, a Trust Domain is a network of SIP entities belonging to a single service provider who deploys it and accurately knows the behavior of those SIP entities. Such simple Trust Domains may be joined to form larger Trust Domains by bilateral agreements between the service providers of the SIP entities. The key requirement of a Trust Domain for the purpose of SIP load filtering is that the behavior of all SIP entities within a given Trust Domain is known to comply to the following set of specifications. o SIP entities in the Trust Domain agree on the mechanisms used to secure the communication among SIP entities within the Trust Domain. o SIP entities in the Trust Domain agree on the manner used to determine which SIP entities are part of the Trust Domain. o SIP entities in the Trust Domain are compliant to SIP [RFC3261]. o SIP entities in the Trust Domain are compliant to SIP-Specific Event Notification[RFC6665]. o SIP entities in the Trust Domain are compliant to this specification. o SIP entities in the Trust Domain agree on what types of calls can be affected by this SIP load-filtering mechanism. For example, <call-identity> condition elements (Section 5.3.1) <one> and <many> might be limited to describe within certain prefixes. o SIP entities in the Trust Domain agree on the destinations to which calls may be redirected when the "redirect" action (Section 5.4) is used. For example, the URI might have to match a given set of domains. SIP load filtering is only effective if all neighbors that are possible signaling sources participate and enforce the designated load-filtering policies. Otherwise, a single non-conforming neighbor could make all filtering efforts useless by pumping in excessive traffic to overload the server. Therefore, the SIP server that Shen, et al. Standards Track [Page 8]
RFC 7200 SIP Load-Control Event Package April 2014 distributes load-filtering policies needs to take countermeasures towards any non-conforming neighbors. A simple method is to reject excessive requests with 503 "Service Unavailable" response messages as if they were obeying the rate. Considering the rejection costs, a more complicated but fairer method would be to allocate at the overloaded server the same amount of processing to the combination of both normal processing and rejection as the overloaded server would devote to processing requests for a conforming upstream SIP server. These approaches work as long as the total rejection cost does not overwhelm the entire server resources. In addition, SIP servers need to handle message prioritization properly while performing load filtering, which is described in Section 4.8. 4. Load-Control Event Package The SIP load-filtering mechanism defines a load-control event package for SIP based on [RFC6665]. 4.1. Event Package Name The name of this event package is "load-control". This name is carried in the Event and Allow-Events header, as specified in [RFC6665]. 4.2. Event Package Parameters No package-specific event header field parameters are defined for this event package. 4.3. SUBSCRIBE Bodies This specification does not define the content of SUBSCRIBE bodies. Future specifications could define bodies for SUBSCRIBE messages, for example, to request specific types of load-control event notifications. A SUBSCRIBE request sent without a body implies the default subscription behavior as specified in Section 4.7. 4.4. SUBSCRIBE Duration The default expiration time for a subscription to load-filtering policy is one hour. Since the desired expiration time may vary significantly for subscriptions among SIP entities with different signaling relationships, the subscribers and notifiers are RECOMMENDED to explicitly negotiate appropriate subscription duration when knowledge about the mutual signaling relationship is available. Shen, et al. Standards Track [Page 9]
RFC 7200 SIP Load-Control Event Package April 2014 4.5. NOTIFY Bodies The body of a NOTIFY request in this event package contains load- filtering policies. The format of the NOTIFY request body MUST be in one of the formats defined in the Accept header field of the SUBSCRIBE request or be the default format, as specified in [RFC6665]. The default data format for the NOTIFY request body of this event package is "application/load-control+xml" (defined in Section 5). This means that when a NOTIFY request body exists but no Accept header field is specified in a SUBSCRIBE request, the NOTIFY request body MUST contain content conforming to the "application/ load-control+xml" format. 4.6. Notifier Processing of SUBSCRIBE Requests The notifier accepts a new subscription or updates an existing subscription upon receiving a valid SUBSCRIBE request. If the identity of the subscriber sending the SUBSCRIBE request is not allowed to receive load-filtering policies, the notifier MUST return a 403 "Forbidden" response. If none of the media types specified in the Accept header of the SUBSCRIBE request are supported, the notifier SHOULD return a 406 "Not Acceptable" response. 4.7. Notifier Generation of NOTIFY Requests A notifier MUST send a NOTIFY request with its current load-filtering policy to the subscriber upon successfully accepting or refreshing a subscription. If no load-filtering policy needs to be distributed when the subscription is received, the notifier SHOULD sent a NOTIFY request without a body to the subscriber. The content-type header field of this NOTIFY request MUST indicate the correct body format as if the body were present (e.g., "application/load-control+xml"). Notifiers are likely to send NOTIFY requests without a body when a subscription is initiated for the first time, e.g., when a SIP entity is just introduced, because there may be no planned events that require load filtering at that time. A notifier SHOULD generate NOTIFY requests each time the load-filtering policy changes, with the maximum notification rate not exceeding values defined in Section 4.10. 4.8. Subscriber Processing of NOTIFY Requests The subscriber is the load-filtering server that enforces load- filtering policies received from the notifier. The way subscribers process NOTIFY requests depends on the load-filtering policies Shen, et al. Standards Track [Page 10]
RFC 7200 SIP Load-Control Event Package April 2014 conveyed in the notifications. Typically, load-filtering policies consist of rules specifying actions to be applied to requests matching certain conditions. A subscriber receiving a notification first installs these rules and then enforces corresponding actions on requests matching those conditions, for example, limiting the sending rate of call requests destined for a specific callee. In the case when load-filtering policies specify a future validity, it is possible that when the validity time arrives, the subscription to the specific notifier that conveyed the rules has expired. In this case, it is RECOMMENDED that the subscriber re-activate its subscription with the corresponding notifier. Regardless of whether or not this re-activation of subscription is successful, when the validity time is reached, the subscriber SHOULD enforce the corresponding rules. Upon receipt of a NOTIFY request with a Subscription-State header field containing the value "terminated", the subscription status with the particular notifier will be terminated. Meanwhile, subscribers MUST also terminate previously received load-filtering policies from that notifier. The subscriber MUST discard unknown bodies. If the NOTIFY request contains several bodies, none of them being supported, it SHOULD unsubscribe unless it has knowledge that it will possibly receive NOTIFY requests with supported bodies from that notifier. A NOTIFY request without a body indicates that no load-filtering policies need to be updated. When the subscriber enforces load-filtering policies, it needs to prioritize requests and select those requests that need to be rejected or redirected. This selection is largely a matter of local policy. It is expected that the subscriber will follow local policy as long as the result in reduction of traffic is consistent with the overload algorithm in effect at that node. Accordingly, the normative behavior described in the next three paragraphs should be interpreted with the understanding that the subscriber will aim to preserve local policy to the fullest extent possible. o The subscriber SHOULD honor the local policy for prioritizing SIP requests such as policies based on message type, e.g., INVITEs versus requests associated with existing sessions. o The subscriber SHOULD honor the local policy for prioritizing SIP requests based on the content of the Resource-Priority header (RPH, [RFC4412]). Specific (namespace.value) RPH contents may indicate high-priority requests that should be preserved as much Shen, et al. Standards Track [Page 11]
RFC 7200 SIP Load-Control Event Package April 2014 as possible during overload. The RPH contents can also indicate a low-priority request that is eligible to be dropped during times of overload. o The subscriber SHOULD honor the local policy for prioritizing SIP requests relating to emergency calls as identified by the sos URN [RFC5031] indicating an emergency request. A local policy can be expected to combine both the SIP request type and the prioritization markings and SHOULD be honored when overload conditions prevail. 4.9. Handling of Forked Requests Forking is not applicable when this load-control event package mechanism is used within a single-hop distance between neighboring SIP entities. If communication scope of the load-control event package mechanism is among multiple hops, forking is also not expected to happen because the subscription request is addressed to a clearly defined SIP entity. However, in the unlikely case when forking does happen, the load-control event package only allows the first potential dialog-establishing message to create a dialog, as specified in Section 5.4.9 of [RFC6665]. 4.10. Rate of Notifications The rate of notifications is unlikely to be of concern for this local control event package mechanism when it is used in a non-real-time mode for relatively static load-filtering policies. Nevertheless, if a situation does arise in which a rather frequently used load filtering policy update is needed, it is RECOMMENDED that the notifier not generate notifications at a rate higher than once per second in all cases, in order to avoid the NOTIFY request itself overloading the system. 4.11. State Delta It is likely that updates to specific load-filtering policies are made by changing only part of the policy parameters (e.g., acceptable request rate or percentage, but not matching identities). This will typically be because the utilization of a resource subject to overload depends upon dynamic unknowns such as holding time and the relative distribution of offered loads over subscribing SIP entities. The updates could originate manually or be determined automatically by an algorithm that dynamically computes the load-filtering policies (Section 3.2). Another factor that is usually not known precisely or Shen, et al. Standards Track [Page 12]
RFC 7200 SIP Load-Control Event Package April 2014 needs to be computed automatically is the duration of the event requiring load filtering. Therefore, it would also be common for the validity to change frequently. This event package allows the use of state delta as in [RFC6665] to accommodate frequent updates of partial policy parameters. For each NOTIFY transaction in a subscription, a version number that increases by exactly one MUST be included in the NOTIFY request body when the body is present. When the subscriber receives a state delta, it associates the partial updates to the particular policy by matching the appropriate rule id (Appendix D). If the subscriber receives a NOTIFY request with a version number that is increased by more than one, it knows that it has missed a state delta and needs to ask for a full state snapshot. Therefore, the subscriber ignores that NOTIFY request containing the state delta, and resends a SUBSCRIBE request to force a NOTIFY request containing a complete state snapshot. 5. Load-Control Document 5.1. Format A load-control document is an XML document that describes the load- filtering policies. It inherits and enhances the common policy document defined in [RFC4745]. A common policy document contains a set of rules. Each rule consists of three parts: conditions, actions, and transformations. The conditions part is a set of expressions containing attributes such as identity, domain, and validity time information. Each expression evaluates to TRUE or FALSE. Conditions are matched on "equality" or "greater than" style comparison. There is no regular expression matching. Conditions are evaluated on receipt of an initial SIP request for a dialog or standalone transaction. If a request matches all conditions in a rule set, the action part and the transformation part are consulted to determine the "permission" on how to handle the request. Each action or transformation specifies a positive grant to the policy server to perform the resulting actions. Well-defined mechanism are available for combining actions and transformations obtained from more than one sources. 5.2. Namespace The namespace URI for elements defined by this specification is a Uniform Resource Namespace (URN) ([RFC2141]), using the namespace identifier "ietf" defined by [RFC2648] and extended by [RFC3688]. The URN is as follows: urn:ietf:params:xml:ns:load-control Shen, et al. Standards Track [Page 13]
RFC 7200 SIP Load-Control Event Package April 2014 5.3. Conditions [RFC4745] defines three condition elements: <identity>, <sphere>, and <validity>. This specification defines new condition elements and reuses the <validity> element. The <sphere> element is not used. 5.3.1. Call Identity Since the problem space of this specification is different from that of [RFC4745], the [RFC4745] <identity> element is not sufficient for use with load filtering. First, load filtering may be applied to different identities contained in a request, including identities of both the receiving entity and the sending entity. Second, the importance of authentication varies when different identities of a request are concerned. This specification defines new identity conditions that can accommodate the granularity of specific SIP identity header fields. The requirement for authentication depends on which field is to be matched. The identity condition for load filtering is specified by the <call-identity> element and its sub-element <sip>. The <sip> element itself contains sub-elements representing SIP sending and receiving identity header fields: <from>, <to>, <request-uri>, and <p-asserted-identity>. All those sub-elements are of an extended form of the [RFC4745] <identity> element. In addition to the sub- elements including <one>, <except>, and <many> in the <identity> element from [RFC4745], the extended form adds two new sub-elements, namely, <many-tel> and <except-tel>, which will be explained later in this section. The [RFC4745] <one> and <except> elements may contain an "id" attribute, which is the URI of a single entity to be included or excluded in the condition. When used in the <from>, <to>, <request-uri>, and <p-asserted-identity> elements, this "id" value is the URI contained in the corresponding SIP header field, i.e., From, To, Request-URI, and P-Asserted-Identity. When the <call-identity> element contains multiple <sip> sub- elements, the result is combined using logical OR. When the <from>, <to>, <request-uri>, and <p-asserted-identity> elements contain multiple <one>, <many>, or <many-tel> sub-elements, the result is also combined using logical OR. When the <many> sub-element further contains one or more <except> sub-elements, or when the <many-tel> sub-element further contains one or more <except-tel> sub-elements, the result of each <except> or <except-tel> sub-element is combined using a logical OR, similar to that of the [RFC4745] <identity> element. However, when the <sip> element contains multiple <from>, <to>, <request-uri>, and <p-asserted-identity> sub-elements, the Shen, et al. Standards Track [Page 14]
RFC 7200 SIP Load-Control Event Package April 2014 result is combined using logical AND. This allows the call identity to be specified by multiple fields of a SIP request simultaneously, e.g., both the From and the To header fields. The following shows an example of the <call-identity> element, which matches call requests whose To header field contains the SIP URI "sip:email@example.com" or the 'tel' URI "tel:+1-212-555-1234". <call-identity> <sip> <to> <one id="sip:firstname.lastname@example.com"/> <one id="tel:+1-212-555-1234"/> </to> </sip> </call-identity> Before evaluating <call-identity> conditions, the subscriber shall convert URIs received in SIP header fields in canonical form as per [RFC3261], except that the "phone-context" parameter shall not be removed, if present. The [RFC4745] <many> and <except> elements may take a "domain" attribute. The "domain" attribute specifies a domain name to be matched by the domain part of the candidate identity. Thus, it allows matching a large and possibly unknown number of entities within a domain. The "domain" attribute works well for SIP URIs. A URI identifying a SIP user, however, can also be a 'tel' URI. Therefore, a similar way to match a group of 'tel' URIs is needed. There are two forms of 'tel' URIs: for global numbers and local numbers. According to [RFC3966], "All phone numbers MUST use the global form unless they cannot be represented as such...Local numbers MUST be tagged with a 'phone-context'". The global number 'tel' URIs start with a "+". The "phone-context" parameter of local numbers may be labeled as a global number or any number of its leading digits or a domain name. Both forms of the 'tel' URI make the resulting URI globally unique. 'tel' URIs of global numbers can be grouped by prefixes consisting of any number of common leading digits. For example, a prefix formed by a country code or both the country and area code identifies telephone numbers within a country or an area. Since the length of the country and area code for different regions are different, the length of the number prefix also varies. This allows further flexibility such as Shen, et al. Standards Track [Page 15]
RFC 7200 SIP Load-Control Event Package April 2014 grouping the numbers into sub-areas within the same area code. 'tel' URIs of local numbers can be grouped by the value of the "phone-context" parameter. The <many> and <except> sub-elements in the <identity> element of [RFC4745] do not allow additional attributes to be added directly. Redefining behavior of their existing "domain" attribute creates backward-compatibility issues. Therefore, this specification defines the <many-tel> and <except-tel> sub-elements that extend the [RFC4745] <identity> element. Both of them have a "prefix" attribute for grouping 'tel' URIs, similar to the "domain" attribute for grouping SIP URIs in existing <many> and <except> sub-elements. For global numbers, the "prefix" attribute value holds any number of common leading digits, for example, "+1-212" for US phone numbers within area code "212" or "+1-212-854" for the organization with US area code "212" and local prefix "854". For local numbers, the "prefix" attribute value contains the "phone-context" parameter value. It should be noted that visual separators (such as the "-" sign) in 'tel' URIs are not used for URI comparison as per [RFC3966]. The following example shows the use of the "prefix" attribute along with the "domain" attribute. It matches those requests calling to the number "+1-202-999-1234" but are not calling from a "+1-212" prefix or a SIP From URI domain of "manhattan.example.com". <call-identity> <sip> <from> <many> <except domain="manhattan.example.com"/> </many> <many-tel> <except-tel prefix="+1-212"/> </many-tel> </from> <to> <one id="tel:+1-202-999-1234"/> </to> </sip> </call-identity> 5.3.2. Method The load created on a SIP server depends on the type of initial SIP requests for dialogs or standalone transactions. The <method> element specifies the SIP method to which the load-filtering action applies. When this element is not included, the load-filtering actions are applicable to all applicable initial requests. These Shen, et al. Standards Track [Page 16]
RFC 7200 SIP Load-Control Event Package April 2014 requests include INVITE, MESSAGE, REGISTER, SUBSCRIBE, OPTIONS, and PUBLISH. Non-initial requests, such as ACK, BYE, and CANCEL MUST NOT be subjected to load filtering. In addition, SUBSCRIBE requests are not filtered if the event-type header field indicates the event package defined in this specification. The following example shows the use of the <method> element in the case the filtering actions should be applied to INVITE requests. <method>INVITE</method> 5.3.3. Target SIP Entity A SIP server that performs load-filtering may have multiple paths to route call requests matching the same set of call identity elements. In those situations, the SIP load-filtering server may desire to take advantage of alternative paths and only apply load-filtering actions to matching requests for the next-hop SIP entity that originated the corresponding load-filtering policy. To achieve that, the SIP load- filtering server needs to associate every load-filtering policy with its originating SIP entity. The <target-sip-entity> element is defined for that purpose, and it contains the URI of the entity that initiated the load-filtering policy, which is generally the corresponding notifier. A notifier MAY include this element as part of the condition of its filtering policy being sent to the subscriber, as below. <target-sip-entity>sip:biloxi.example.com</target-sip-entity> When a SIP load-filtering server receives a policy with a <target-sip-entity> element, it SHOULD record it and take it into consideration when making load-filtering decisions. If the load- filtering server receives a load-filtering policy that does not contain a <target-sip-entity> element, it MAY still record the URI of the load-filtering policy's originator as the <target-sip-entity> information and consider it when making load-filtering decisions. The following are two examples of using the <target-sip-entity> element. Use case I: The network has user A connected to SIP Proxy 1 (SP1), user B connected to SIP Proxy 3 (SP3), SP1 and SP3 connected via SIP Proxy 2 (SP2), and SP2 connected to an Application Server (AS). Under normal load conditions, a call from A to B is routed along the following path: A-SP1-SP2-AS-SP3-B. The AS provides a nonessential service and can be bypassed in case of overload. Now let's assume that AS is overloaded and sends to SP2 a load- filtering policy requesting that 50% of all INVITE requests be Shen, et al. Standards Track [Page 17]
RFC 7200 SIP Load-Control Event Package April 2014 dropped. SP2 can maintain AS as the <target-sip-entity> for that policy so that it knows the 50% drop action is only applicable to call requests that must go through AS, without affecting those calls directly routed through SP3 to B. Use case II: A translation service for toll-free numbers is installed on two Application Servers, AS1 and AS2. User A is connected to SP1 and calls 800-1234-4529, which is translated by AS1 and AS2 into a regular E.164 number depending on, e.g., the caller's location. SP1 forwards INVITE requests with Request-URI = "800 number" to AS1 or AS2 based on a load-balancing strategy. As calls to 800-1234-4529 create a pre-overload condition in AS1, AS1 sends to SP1 a load-filtering policy requesting that 50% of calls towards 800-1234-4529 be rejected. In this case, SP1 can maintain AS1 as the <target-sip-entity> for the rule, and only apply the load-filtering policy on incoming requests that are intended to be sent to AS1. Those requests that are sent to AS2, although matching the <call-identity> of the filter, will not be affected. 5.3.4. Validity A filtering policy is usually associated with a validity period condition. This specification reuses the <validity> element of [RFC4745], which specifies a period of validity time by pairs of <from> and <until> sub-elements. When multiple time periods are defined, the validity condition is evaluated to TRUE if the current time falls into any of the specified time periods. That is, it represents a logical OR operation across all validity time periods. The following example shows a <validity> element specifying a valid period from 12:00 to 15:00 US Eastern Standard Time on 2008-05-31. <validity> <from>2008-05-31T12:00:00-05:00</from> <until>2008-05-31T15:00:00-05:00</until> </validity> 5.4. Actions The actions a load-filtering server takes on loads matching the load- filtering conditions are defined by the <accept> element in the load- filtering policy, which includes any one of the three sub-elements <rate>, <percent>, and <win>. The <rate> element denotes an absolute value of the maximum acceptable request rate in requests per second; the <percent> element specifies the relative percentage of incoming requests that should be accepted; the <win> element describes the acceptable window size supplied by the receiver, which is applicable Shen, et al. Standards Track [Page 18]
RFC 7200 SIP Load-Control Event Package April 2014 in window-based load-filtering. In static load-filtering policy configuration scenarios, using the <rate> sub-element is RECOMMENDED because it is hard to enforce the percentage rate or window-based load filtering when incoming load from upstream or reactions from downstream are uncertain. (See [SIP-OVERLOAD] and [RFC6357] for more details on rate-based, loss-based, and window-based load control.) In addition, the <accept> element takes an optional "alt-action" attribute that can be used to explicitly specify the desired action in case a request cannot be processed. The "alt-action" can take one of the following three values: "reject", "redirect", or "drop". o The "reject" action is the default value for "alt-action". It means that the load-filtering server will reject the request with a 503 "Service Unavailable" response message. o The "redirect" action means redirecting the request to another target. When it is used, an "alt-target" attribute MUST be defined. The "alt-target" specifies one URI or a list of URIs where the request should be redirected. The server sends out the redirect URIs in a 300-class response message. o The "drop" action means simply ignoring the request without doing anything, which can, in certain cases, help save processing capability during overload. For example, when SIP is running over a reliable transport such as TCP, the "drop" action does not send out the rejection response, neither does it close the transport connection. However, when running SIP over an unreliable transport such as UDP, using the "drop" action will create message retransmissions that further worsen the possible overload situation. Therefore, any "drop" action applied to an unreliable transport MUST be treated as if it were "reject". The above "alt-action" processing can also be illustrated through the following pseudocode. SWITCH "alt-action" "redirect": "redirect" "drop": IF unreliable-transport THEN treat as "reject" ELSE "drop" "reject": "reject" default: "reject" END Shen, et al. Standards Track [Page 19]
RFC 7200 SIP Load-Control Event Package April 2014 In the following <actions> element example, the server accepts maximum of 100 call requests per second. The remaining calls are redirected to an answering machine. <actions> <accept alt-action="redirect" alt-target= "sip:firstname.lastname@example.com"> <rate>100</rate> </accept> </actions> 6. XML Schema Definition for Load Control This section defines the XML schema for the load-control document. It extends the Common Policy schema in [RFC4745] in two ways. Firstly, it defines two mandatory attributes for the <ruleset> element: "version" and "state". The "version" attribute allows the recipient of the notification to properly order them. Versions start at zero and increase by one for each new document sent to a subscriber within the same subscription. Versions MUST be representable using a non-negative 32-bit integer. The "state" attribute indicates whether the document contains a full load- filtering policy update or only state delta as partial update. Secondly, it defines new members of the <conditions> and <actions> elements. <?xml version="1.0" encoding="UTF-8"?> <xs:schema targetNamespace="urn:ietf:params:xml:ns:load-control" xmlns:lc="urn:ietf:params:xml:ns:load-control" xmlns:cp="urn:ietf:params:xml:ns:common-policy" xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" attributedFormDefault="unqualified"> <xs:import namespace="urn:ietf:params:xml:ns:common-policy"/> <!-- RULESET --> <xs:element name="ruleset"> <xs:complexType> <xs:complexContent> <xs:restriction base="xs:anyType"> <xs:sequence> <xs:element name="rule" type="cp:ruleType" minOccurs="0" maxOccurs="unbounded"/> </xs:sequence> </xs:restriction> </xs:complexContent> Shen, et al. Standards Track [Page 20]
RFC 7200 SIP Load-Control Event Package April 2014 <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-identity-type"/> <!-- CALL IDENTITY TYPE --> <xs:complexType name="call-identity-type"> <xs:choice> <xs:element name="sip" type="lc:sip-id-type"/> <any namespace="##other" processContents="lax" minOccurs="0" maxOccurs="unbounded"/> </xs:choice> <anyAtrribute namespace="##other" processContents="lax"/> </xs:complexType> <!-- SIP ID TYPE --> <xs:complexType name="sip-id-type"> <xs:sequence> <element name="from" type="lc:identityType" minOccurs="0"/> <element name="to" type="lc:identityType" minOccurs="0"/> <element name="request-uri" type="lc:identityType" minOccurs="0"/> <element name="p-asserted-identity" type="lc:identityType" minOccurs="0"/> <any namespace="##other" processContents="lax" minOccurs="0" maxOccurs="unbounded"/> </xs:sequence> <anyAtrribute namespace="##other" processContents="lax"/> </xs:complexType> <!-- IDENTITY TYPE --> <xs:complexType name="identityType"> <xs:complexContent> <xs:restriction base="xs:anyType"> <xs:choice minOccurs="1" maxOccurs="unbounded"> <xs:element name="one" type="cp:oneType"/> Shen, et al. Standards Track [Page 21]
RFC 7200 SIP Load-Control Event Package April 2014 <xs:element name="many" type="lc:manyType"/> <xs:element name="many-tel" type="lc:manyTelType"/> <xs:any namespace="##other" processContents="lax"/> </xs:choice> </xs:restriction> </xs:complexContent> </xs:complexType> <!-- MANY-TEL TYPE --> <xs:complexType name="manyTelType"> <xs:complexContent> <xs:restriction base="xs:anyType"> <xs:choice minOccurs="0" maxOccurs="unbounded"> <xs:element name="except-tel" type="lc:exceptTelType"/> <xs:any namespace="##other" minOccurs="0" processContents="lax"/> </xs:choice> <xs:attribute name="prefix" use="optional" type="xs:string"/> </xs:restriction> </xs:complexContent> </xs:complexType> <!-- EXCEPT-TEL TYPE --> <xs:complexType name="exceptTelType"> <xs:attribute name="prefix" type="xs:string" use="optional"/> <xs:attribute name="id" type="xs:anyURI" use="optional"/> </xs:complexType> <!-- METHOD --> <xs:element name="method" type="lc:method-type"/> <!-- METHOD TYPE --> <xs:simpleType name="method-type"> <xs:restriction base="xs:string"> <xs:enumeration value="INVITE"/> <xs:enumeration value="MESSAGE"/> <xs:enumeration value="REGISTER"/> <xs:enumeration value="SUBSCRIBE"/> <xs:enumeration value="OPTIONS"/> <xs:enumeration value="PUBLISH"/> </xs:restriction> </xs:simpleType> <!-- TARGET SIP ENTITY --> <xs:element name="target-sip-entity" type="xs:anyURI" minOccurs="0"/> <!-- ACTIONS --> Shen, et al. Standards Track [Page 22]
RFC 7200 SIP Load-Control Event Package April 2014 <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="lc:alt-target-type" use="optional"/> <anyAtrribute namespace="##other" processContents="lax"/> </xs:element> <!-- ALT TARGET TYPE --> <xs:simpleType name="alt-target-type"> <xs:list itemType="xs:anyURI"/> </xs:simpleType> </xs:schema> 7. Security Considerations Two primary security considerations arise from this specification. One is the distribution mechanism for the load filtering policy that is based on the SIP event notification framework, and the other is the enforcement mechanism for the load-filtering policy. Security considerations for SIP event package mechanisms are covered in Section 6 of [RFC6665]. A particularly relevant security concern for this event package is that if the notifiers can be spoofed, attackers can send fake notifications asking subscribers to throttle all traffic, leading to denial-of-service (DoS) attacks. Therefore, this SIP load-filtering mechanism MUST be used in a Trust Domain (Section 3.4). But if a legitimate notifier in the Trust Domain is itself compromised, additional mechanisms will be needed to detect the attack. Security considerations for load-filtering policy enforcement depends very much on the contents of the policy. This specification defines a possible match of the following SIP header fields in a load- filtering policy: <from>, <to>, <request-uri>, and <p-asserted-identity>. The exact requirement to authenticate and authorize these fields is up to the service provider. In general, if the identity field represents the source of the request, it SHOULD be authenticated and authorized; if the identity field represents the destination of the request, the authentication and authorization is optional. Shen, et al. Standards Track [Page 23]
RFC 7200 SIP Load-Control Event Package April 2014 In addition, the "redirect" action (Section 5.4) could facilitate a reflection denial-of-service attack. If a number of SIP proxy servers in a Trust Domain are using UDP and configured to get their policies from a central server. An attacker spoofs the central server's address to send a number of NOTIFY bodies telling the proxy servers to redirect all calls to email@example.com. The proxy servers then redirect all calls to the victim, who then becomes a victim of Denial of Service attack and becomes inaccessiable from the Internet. To address this type of threat, this specification requires that a Trust Domain agrees on what types of calls can be affected as well as on the destinations to which calls may be redirected, as in Section 3.4. 8. IANA Considerations This specification registers a SIP event package, a new media type, a new XML namespace, and a new XML schema. 8.1. Load-Control Event Package Registration This section registers an event package based on the registration procedures defined in [RFC6665]. Package name: load-control Type: package Published specification: This specification Person to contact: Charles Shen, email@example.com 8.2. application/load-control+xml Media Type Registration This section registers a new media type based on the procedures defined in [RFC6838] and guidelines in [RFC3023]. Type name: application Subtype name: load-control+xml Required parameters: none Optional parameters: Same as charset parameter of application/xml as specified in [RFC3023]. Encoding considerations: Same as encoding considerations of application/xml as specified in [RFC3023]. Shen, et al. Standards Track [Page 24]
RFC 7200 SIP Load-Control Event Package April 2014 Security considerations: See Section 10 of [RFC3023] and Section 7 of this specification. Interoperability considerations: none Published specification: This specification Applications that use this media type: Applications that perform load control of SIP entities. Fragment identifier considerations: Same as fragment identifier considerations of application/xml as specified in [RFC3023]. Additional Information: Deprecated alias names for this type: none Magic Number(s): none File Extension(s): .xml Macintosh file type code(s): "TEXT" Person and email address for further information: Charles Shen, email@example.com Intended usage: COMMON Restrictions on usage: none Author: Charles Shen, Henning Schulzrinne, Arata Koike Change controller: IESG Provisional registration? (standards tree only): no 8.3. URN Sub-Namespace Registration This section registers a new XML namespace, as per the guidelines in [RFC3688] URI: The URI for this namespace is urn:ietf:params:xml:ns:load-control Registrant Contact: IETF SOC Working Group <email@example.com>, as designated by the IESG <email@example.com> Shen, et al. Standards Track [Page 25]
RFC 7200 SIP Load-Control Event Package April 2014 XML: BEGIN <?xml version="1.0"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN" "http://www.w3.org/TR/xhtml-basic/xhtml-basic10.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="content-type" content="text/html;charset=iso-8859-1"/> <title>SIP Load-Control Namespace</title> </head> <body> <h1>Namespace for SIP Load Control</h1> <h2>urn:ietf:params:xml:ns:load-control</h2> <p>See <a href="http://www.rfc-editor.org/rfc/rfc7200.txt"> RFC 7200</a>.</p> </body> </html> END 8.4. Load-Control Schema Registration URI: urn:ietf:params:xml:schema:load-control Registrant Contact: IETF SOC working group, Charles Shen (email@example.com). XML: the XML schema contained in Section 6 has been registered. Its first line is <?xml version="1.0" encoding="UTF-8"?> and its last line is </xs:schema> Shen, et al. Standards Track [Page 26]
RFC 7200 SIP Load-Control Event Package April 2014 9. Acknowledgements The authors would like to thank Jari Arkko, Richard Barnes, Stewart Bryant, Gonzalo Camarillo, Bruno Chatras, Benoit Claise, Spencer Dawkins, Martin Dolly, Keith Drage, Ashutosh Dutta, Donald Eastlake, Adrian Farrel, Stephen Farrell, Janet Gunn, Vijay Gurbani, Brian Haberman, Volker Hilt, Geoff Hunt, Carolyn Johnson, Hadriel Kaplan, Paul Kyzivat, Barry Leiba, Pearl Liang, Salvatore Loreto, Timothy Moran, Eric Noel, Parthasarathi R, Pete Resnick, Adam Roach, Dan Romascanu, Shida Schubert, Robert Sparks, Martin Stiemerling, Sean Turner, Phil Williams, and other members of the SOC and SIPPING working groups for many helpful comments. In particular, Bruno Chatras proposed the <method> and <target-sip-entity> condition elements along with many other text improvements. Janet Gunn provided detailed text suggestions including Appendix C. Eric Noel suggested clarification on load-filtering policy distribution initialization process. Shida Schubert made many suggestions such as terminology usage. Phil Williams suggested adding support for delta updates. Ashutosh Dutta gave pointers to Public Switched Telephone Network (PSTN) references. Adam Roach suggested improvements related to RFC 6665 and offered other helpful clarifications. Richard Barnes made many suggestions such as referencing the Trust Domain concept of RFCs 3324 and 3325, the use of a separate element for 'tel' URI grouping, and addressing the "redirect" action security threat. 10. References 10.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997. [RFC3023] Murata, M., St. Laurent, S., and D. Kohn, "XML Media Types", RFC 3023, January 2001. [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002. [RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, January 2004. [RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC 3966, December 2004. Shen, et al. Standards Track [Page 27]
RFC 7200 SIP Load-Control Event Package April 2014 [RFC4745] Schulzrinne, H., Tschofenig, H., Morris, J., Cuellar, J., Polk, J., and J. Rosenberg, "Common Policy: A Document Format for Expressing Privacy Preferences", RFC 4745, February 2007. [RFC6665] Roach, A., "SIP-Specific Event Notification", RFC 6665, July 2012. [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, January 2013. 10.2. Informative References [E.300SerSup3] ITU-T, "North American Precise Audible Tone Plan", Recommendation E.300 Series Supplement 3, November 1988. [E.412] ITU-T, "Network Management Controls", Recommendation E.412-2003, January 2003. [Q.1248.2] ITU-T, "Interface Recommendation for Intelligent Network Capability Set4:SCF-SSF interface", Recommendation Q.1248.2, July 2001. [RFC2648] Moats, R., "A URN Namespace for IETF Documents", RFC 2648, August 1999. [RFC3324] Watson, M., "Short Term Requirements for Network Asserted Identity", RFC 3324, November 2002. [RFC3325] Jennings, C., Peterson, J., and M. Watson, "Private Extensions to the Session Initiation Protocol (SIP) for Asserted Identity within Trusted Networks", RFC 3325, November 2002. [RFC4412] Schulzrinne, H. and J. Polk, "Communications Resource Priority for the Session Initiation Protocol (SIP)", RFC 4412, February 2006. [RFC4825] Rosenberg, J., "The Extensible Markup Language (XML) Configuration Access Protocol (XCAP)", RFC 4825, May 2007. [RFC5031] Schulzrinne, H., "A Uniform Resource Name (URN) for Emergency and Other Well-Known Services", RFC 5031, January 2008. Shen, et al. Standards Track [Page 28]
RFC 7200 SIP Load-Control Event Package April 2014 [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. [SIP-OVERLOAD] Gurbani, V., Ed., Hilt, V., and H. Schulzrinne, "Session Initiation Protocol (SIP) Overload Control", Work in Progress, March 2014. Shen, et al. Standards Track [Page 29]
RFC 7200 SIP Load-Control Event Package April 2014 Appendix A. Definitions This specification reuses the definitions for "Event Package", "Notification", "Notifier", "Subscriber", and "Subscription" as in [RFC6665]. The following additional definitions are also used. Load Filtering: A load-control mechanism that applies specific actions to selected loads (e.g., SIP requests) matching specific conditions. Load-Filtering Policy: A set of zero or more load-filtering rules, also known as load-filtering rule set. Load-Filtering Rule: Conditions and actions to be applied for load filtering. Load-Filtering Condition: Elements that describe how to select loads to apply load-filtering actions. This specification defines the <call-identity>, <method>, <target-sip-identity>, and <validity> condition elements (Section 5.3). Load-Filtering Action: An operation to be taken by a load-filtering server on loads that match the load-filtering conditions. This specification allows actions such as accept, reject, and redirect of loads (Section 5.4). Load-Filtering Server: A server that performs load filtering. In the context of this specification, the load-filtering server is the subscriber, which receives load-filtering policies from the notifier and enforces those policies during load filtering. Load-Control Document: An XML document that describes the load- filtering policies (Section 5). It inherits and enhances the common policy document defined in [RFC4745]. Appendix B. Design Requirements The SIP load-filtering mechanism needs to satisfy the following requirements: o For simplicity, the solution should focus on a method for controlling SIP load, rather than a generic application-layer mechanism. o The load-filtering policy needs to be distributed efficiently to possibly a large subset of all SIP elements. Shen, et al. Standards Track [Page 30]
RFC 7200 SIP Load-Control Event Package April 2014 o The solution should reuse existing SIP protocol mechanisms to reduce implementation and deployment complexity. o For predictable overload situations, such as holidays and mass calling events, the load-filtering policy should specify during what time it is to be applied, so that the information can be distributed ahead of time. o For destination-specific overload situations, the load-filtering policy should be able to describe the destination domain or the callee. o To address accidental and intentional high-volume call generators, the load-filtering policy should be able to specify the caller. o Caller and callee need to be specified as both SIP URIs and 'tel' URIs [RFC3966] in load-filtering policies. o It should be possible to specify particular information in the SIP headers (e.g., prefixes in telephone numbers) that allow load filtering over limited regionally focused overloads. o The solution should draw upon experiences from related PSTN mechanisms [Q.1248.2] [E.412] [E.300SerSup3] where applicable. o The solution should be extensible to meet future needs. Appendix C. Discussion of How This Specification Meets the Requirements of RFC 5390 This section evaluates whether the load-control event package mechanism defined in this specification satisfies various SIP overload control requirements set forth by [RFC5390]. As mentioned in Section 1, this specification complements other efforts in the overall SIP load-control solution space. Therefore, not all RFC 5390 requirements are found applicable to this specification. This specification categorizes the assessment results into Yes (the requirement is met), P/A (Partially Applicable), No (must be used in conjunction with another mechanism to meet the requirement), and N/A (Not Applicable). REQ 1: The overload mechanism shall strive to maintain the overall useful throughput (taking into consideration the quality-of- service needs of the using applications) of a SIP server at reasonable levels, even when the incoming load on the network is far in excess of its capacity. The overall throughput under load is the ultimate measure of the value of an overload control mechanism. Shen, et al. Standards Track [Page 31]
RFC 7200 SIP Load-Control Event Package April 2014 P/A. The goal of load filtering is to prevent overload or maintain overall goodput during the time of overload, but it is dependent on the predictions of the load and the computations as well as distribution of the filtering policies. If the load predictions or filtering policy computations are incorrect, or the filtering policy is not properly distributed, the mechanism will be less effective. On the other hand, if the load can be accurately predicted and filtering policies be computed and distributed appropriately, this requirement can be met. REQ 2: When a single network element fails, goes into overload, or suffers from reduced processing capacity, the mechanism should strive to limit the impact of this on other elements in the network. This helps to prevent a small-scale failure from becoming a widespread outage. N/A if load-filtering policies are installed in advance and do not change during the potential overload period, P/A if load-filtering policies are dynamically adjusted. The algorithm to dynamically compute load-filtering policies is outside the scope of this specification, while the distribution of the updated filtering policies uses the event package mechanism of this specification. REQ 3: The mechanism should seek to minimize the amount of configuration required in order to work. For example, it is better to avoid needing to configure a server with its SIP message throughput, as these kinds of quantities are hard to determine. No. This mechanism is entirely dependent on advance configuration, based on advance knowledge. In order to satisfy REQ 3, it should be used in conjunction with other mechanisms that are not based on advance configuration. REQ 4: The mechanism must be capable of dealing with elements that do not support it, so that a network can consist of a mix of elements that do and don't support it. In other words, the mechanism should not work only in environments where all elements support it. It is reasonable to assume that it works better in such environments, of course. Ideally, there should be incremental improvements in overall network throughput as increasing numbers of elements in the network support the mechanism. No. This mechanism is entirely dependent on the participation of all possible neighbors. In order to satisfy REQ 4, it should be used in conjunction with other mechanisms, some of which are described in Section 3.4. Shen, et al. Standards Track [Page 32]
RFC 7200 SIP Load-Control Event Package April 2014 REQ 5: The mechanism should not assume that it will only be deployed in environments with completely trusted elements. It should seek to operate as effectively as possible in environments where other elements are malicious; this includes preventing malicious elements from obtaining more than a fair share of service. No. This mechanism is entirely dependent on the non-malicious participation of all possible neighbors. In order to satisfy REQ 5, it should be used in conjunction with other mechanisms, some of which are described in Section 3.4. REQ 6: When overload is signaled by means of a specific message, the message must clearly indicate that it is being sent because of overload, as opposed to other, non overload-based failure conditions. This requirement is meant to avoid some of the problems that have arisen from the reuse of the 503 response code for multiple purposes. Of course, overload is also signaled by lack of response to requests. This requirement applies only to explicit overload signals. N/A. This mechanism signals anticipated overload, not actual overload. However, the signals in this mechanism are not used for any other purpose. REQ 7: The mechanism shall provide a way for an element to throttle the amount of traffic it receives from an upstream element. This throttling shall be graded so that it is not all- or-nothing as with the current 503 mechanism. This recognizes the fact that "overload" is not a binary state and that there are degrees of overload. Yes. This event package allows rate-/loss-/window-based overload control options as discussed in Section 5.4. REQ 8: The mechanism shall ensure that, when a request was not processed successfully due to overload (or failure) of a downstream element, the request will not be retried on another element that is also overloaded or whose status is unknown. This requirement derives from REQ 1. N/A to the load-control event package mechanism itself. REQ 9: That a request has been rejected from an overloaded element shall not unduly restrict the ability of that request to be submitted to and processed by an element that is not overloaded. This requirement derives from REQ 1. Shen, et al. Standards Track [Page 33]
RFC 7200 SIP Load-Control Event Package April 2014 Yes. For example, load-filtering policy (Section 3.1) can include alternative forwarding destinations for rejected requests. REQ 10: The mechanism should support servers that receive requests from a large number of different upstream elements, where the set of upstream elements is not enumerable. No. Because this mechanism requires advance configuration of specifically identified neighbors, it does not support environments where the number and identity of the upstream neighbors are not known in advance. In order to satisfy REQ 10, it should be used in conjunction with other mechanisms. REQ 11: The mechanism should support servers that receive requests from a finite set of upstream elements, where the set of upstream elements is enumerable. Yes. See also answer to REQ 10. REQ 12: The mechanism should work between servers in different domains. Yes. The load-control event package mechanism is not limited by domain boundaries. However, it is likely more applicable in intra- domain scenarios than in inter-domain scenarios due to security and other concerns (see also Section 3.4). REQ 13: The mechanism must not dictate a specific algorithm for prioritizing the processing of work within a proxy during times of overload. It must permit a proxy to prioritize requests based on any local policy, so that certain ones (such as a call for emergency services or a call with a specific value of the Resource-Priority header field [RFC4412]) are given preferential treatment, such as not being dropped, being given additional retransmission, or being processed ahead of others. P/A. This mechanism does not specifically address the prioritizing of work during times of overload. But it does not preclude any particular local policy. REQ 14: The mechanism should provide unambiguous directions to clients on when they should retry a request and when they should not. This especially applies to TCP connection establishment and SIP registrations, in order to mitigate against avalanche restart. N/A to the load-control event package mechanism itself. Shen, et al. Standards Track [Page 34]
RFC 7200 SIP Load-Control Event Package April 2014 REQ 15: In cases where a network element fails, is so overloaded that it cannot process messages, or cannot communicate due to a network failure or network partition, it will not be able to provide explicit indications of the nature of the failure or its levels of congestion. The mechanism must properly function in these cases. P/A. Because the load-filtering policies are provisioned in advance, they are not affected by the overload or failure of other network elements. On the other hand, they may not, in those cases, be able to protect the overloaded network elements (see REQ 1). REQ 16: The mechanism should attempt to minimize the overhead of the overload control messaging. Yes. The standardized SIP event package mechanism [RFC6665] is used. REQ 17: The overload mechanism must not provide an avenue for malicious attack, including DoS and DDoS attacks. P/A. This mechanism does provide a potential avenue for malicious attacks. Therefore, the security mechanisms for SIP event packages, in general, [RFC6665] and Section 7 of this specification should be used. REQ 18: The overload mechanism should be unambiguous about whether a load indication applies to a specific IP address, host, or URI, so that an upstream element can determine the load of the entity to which a request is to be sent. Yes. The identity of load indication is covered in the load- filtering policy format definition in Section 3.1. REQ 19: The specification for the overload mechanism should give guidance on which message types might be desirable to process over others during times of overload, based on SIP-specific considerations. For example, it may be more beneficial to process a SUBSCRIBE refresh with Expires of zero than a SUBSCRIBE refresh with a non-zero expiration (since the former reduces the overall amount of load on the element), or to process re-INVITEs over new INVITEs. N/A to the load-control event package mechanism itself. REQ 20: In a mixed environment of elements that do and do not implement the overload mechanism, no disproportionate benefit shall accrue to the users or operators of the elements that do not implement the mechanism. Shen, et al. Standards Track [Page 35]
RFC 7200 SIP Load-Control Event Package April 2014 No. This mechanism is entirely dependent on the participation of all possible neighbors. In order to satisfy REQ 20, it should be used in conjunction with other mechanisms, some of which are described in Section 3.4. REQ 21: The overload mechanism should ensure that the system remains stable. When the offered load drops from above the overall capacity of the network to below the overall capacity, the throughput should stabilize and become equal to the offered load. N/A to the load-control event package mechanism itself. REQ 22: It must be possible to disable the reporting of load information towards upstream targets based on the identity of those targets. This allows a domain administrator who considers the load of their elements to be sensitive information, to restrict access to that information. Of course, in such cases, there is no expectation that the overload mechanism itself will help prevent overload from that upstream target. N/A to the load-control event package mechanism itself. REQ 23: It must be possible for the overload mechanism to work in cases where there is a load balancer in front of a farm of proxies. Yes. The load-control event package mechanism does not preclude its use in a scenario with server farms. Appendix D. Complete Examples D.1. Load-Control Document Examples This section presents two complete examples of load-control documents valid with respect to the XML schema defined in Section 6. The first example assumes that a set of hotlines are set up at "sip:email@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. Shen, et al. Standards Track [Page 36]
RFC 7200 SIP Load-Control Event Package April 2014 <?xml version="1.0" encoding="UTF-8"?> <ruleset xmlns="urn:ietf:params:xml:ns:common-policy" xmlns:lc="urn:ietf:params:xml:ns:load-control" version="0" state="full"> <rule id="f3g44k1"> <conditions> <lc:call-identity> <lc:sip> <lc:to> <one id="sip:firstname.lastname@example.com"/> <one id="tel:+1-212-555-1234"/> </lc:to> </lc:sip> </lc:call-identity> <method>INVITE</method> <validity> <from>2008-05-31T12:00:00-05:00</from> <until>2008-05-31T15:00:00-05:00</until> </validity> </conditions> <actions> <lc:accept alt-action="reject"> <lc:rate>100</lc:rate> </lc:accept> </actions> </rule> </ruleset> The second example optimizes the usage of server resources during the three-day period following a hurricane. Incoming calls to the domain "sandy.example.com" or to call destinations with prefix "+1-212" will be limited to a rate of 100 requests per second, except for those calls originating from a particular rescue team domain "rescue.example.com". Outgoing calls from the hurricane domain or calls within the local domain are never limited. All calls that are throttled due to the rate limit will be forwarded to an answering machine with updated hurricane rescue information. Shen, et al. Standards Track [Page 37]
RFC 7200 SIP Load-Control Event Package April 2014 <?xml version="1.0" encoding="UTF-8"?> <ruleset xmlns="urn:ietf:params:xml:ns:common-policy" xmlns:lc="urn:ietf:params:xml:ns:load-control" version="1" state="full"> <rule id="f3g44k2"> <conditions> <lc:call-identity> <lc:sip> <lc:to> <many domain="sandy.example.com"/> <many-tel prefix="+1-212"/> </lc:to> <lc:from> <many> <except domain="sandy.example.com"/> <except domain="rescue.example.com"/> </many> </lc:from> </lc:sip> </lc:call-identity> <method>INVITE</method> <validity> <from>2012-10-25T09:00:00+01:00</from> <until>2012-10-28T09:00:00+01:00</until> </validity> </conditions> <actions> <lc:accept alt-action="redirect" alt-target= "sip:firstname.lastname@example.com"> <lc:rate>100</lc:rate> </lc:accept> </actions> </rule> </ruleset> Sometimes it may occur that multiple rules in a ruleset define actions that match the same methods, call identity and validity. In those cases, the "first-match-wins" principle is used. For example, in the following ruleset, the first rule requires all calls from the "example.com" domain to be rejected. Even though the rule following that one specifies that calls from "sip:email@example.com" be redirected to a specific target "sip:firstname.lastname@example.com", the calls from "sip:email@example.com" will still be rejected because they have already been matched by the earlier rule. Shen, et al. Standards Track [Page 38]
RFC 7200 SIP Load-Control Event Package April 2014 <?xml version="1.0" encoding="UTF-8"?> <ruleset xmlns="urn:ietf:params:xml:ns:common-policy" xmlns:lc="urn:ietf:params:xml:ns:load-control" version="1" state="full"> <rule id="f3g44k3"> <conditions> <lc:call-identity> <lc:sip> <lc:from> <many domain="example.com"/> </lc:from> </lc:sip> </lc:call-identity> <method>INVITE</method> <validity> <from>2013-7-2T09:00:00+01:00</from> <until>2013-7-3T09:00:00+01:00</until> </validity> </conditions> <actions> <lc:accept alt-action="reject"> <lc:rate>0</lc:rate> </lc:accept> </actions> </rule> <rule id="f3g44k4"> <conditions> <lc:call-identity> <lc:sip> <lc:from> <one id="sip:email@example.com"/> </lc:from> </lc:sip> </lc:call-identity> <method>INVITE</method> <validity> <from>2013-7-2T09:00:00+01:00</from> <until>2013-7-3T09:00:00+01:00</until> </validity> </conditions> <actions> <lc:accept alt-action="redirect" alt-target= "sip:firstname.lastname@example.com"> <lc:rate>0</lc:rate> </lc:accept> </actions> Shen, et al. Standards Track [Page 39]
RFC 7200 SIP Load-Control Event Package April 2014 </rule> </ruleset> D.2. Message Flow Examples This section presents an example message flow of using the load- control event package mechanism defined in this specification. atlanta biloxi | F1 SUBSCRIBE | |------------------>| | F2 200 OK | |<------------------| | F3 NOTIFY | |<------------------| | F4 200 OK | |------------------>| F1 SUBSCRIBE atlanta.example.com -> biloxi.example.com SUBSCRIBE sip:biloxi.example.com SIP/2.0 Via: SIP/2.0/TCP atlanta.example.com;branch=z9hG4bKy7cjbu3 From: sip:atlanta.example.com;tag=162ab5 To: sip:biloxi.example.com Call-ID: firstname.lastname@example.com CSeq: 2012 SUBSCRIBE Contact: sip:atlanta.example.com Event: load-control Max-Forwards: 70 Accept: application/load-control+xml Expires: 3600 Content-Length: 0 F2 200 OK biloxi.example.com -> atlanta.example.com SIP/2.0 200 OK Via: SIP/2.0/TCP biloxi.example.com;branch=z9hG4bKy7cjbu3 ;received=192.0.2.1 To: <sip:biloxi.example.com>;tag=331dc8 From: <sip:atlanta.example.com>;tag=162ab5 Call-ID: firstname.lastname@example.com CSeq: 2012 SUBSCRIBE Expires: 3600 Contact: sip:biloxi.example.com Content-Length: 0 Shen, et al. Standards Track [Page 40]
RFC 7200 SIP Load-Control Event Package April 2014 F3 NOTIFY biloxi.example.com -> atlanta.example.com NOTIFY sip:atlanta.example.com SIP/2.0 Via: SIP/2.0/TCP biloxi.example.com;branch=z9hG4bKy71g2ks From: <sip:biloxi.example.com>;tag=331dc8 To: <sip:atlanta.example.com>;tag=162ab5 Call-ID: firstname.lastname@example.com Event: load-control Subscription-State: active;expires=3599 Max-Forwards: 70 CSeq: 1775 NOTIFY Contact: sip:biloxi.example.com Content-Type: application/load-control+xml Content-Length: ... [Load-Control Document] F4 200 OK atlanta.example.com -> biloxi.example.com SIP/2.0 200 OK Via: SIP/2.0/TCP atlanta.example.com;branch=z9hG4bKy71g2ks ;received=192.0.2.2 From: <sip:biloxi.example.com>;tag=331dc8 To: <sip:atlanta.example.com>;tag=162ab5 Call-ID: email@example.com CSeq: 1775 NOTIFY Content-Length: 0 Appendix E. Related Work E.1. Relationship to Load Filtering in PSTN It is known that an existing PSTN network also uses a load-filtering mechanism to prevent overload and the filtering policy configuration is done manually except in specific cases when the Intelligent Network architecture is used [Q.1248.2][E.412]. This specification defines a load-filtering mechanism based on the SIP event notification framework that allows automated filtering policy distribution in suitable environments. PSTN overload control uses messages that specify an outgoing control list, call gap duration, and control duration [Q.1248.2][E.412]. These items correspond roughly to the identity, action, and time fields of the SIP load-filtering policy defined in this specification. However, the load-filtering policy defined in this specification is much more generic and flexible as opposed to its PSTN counterpart. Shen, et al. Standards Track [Page 41]
RFC 7200 SIP Load-Control Event Package April 2014 Firstly, PSTN load filtering only applies to telephone numbers. The identity element of SIP load-filtering policy allows both SIP URI and telephone numbers (through 'tel' URI) to be specified. These identities can be arbitrarily grouped by SIP domains or any number of leading prefixes of the telephone numbers. Secondly, the PSTN load-filtering action is usually limited to call gapping. The action field in SIP load-filtering policy allows more flexible possibilities such as rate throttle and others. Thirdly, the duration field in PSTN load filtering specifies a value in seconds for the load-filtering duration only, and the allowed values are mapped into a value set. The time field in SIP load- filtering policy may specify not only a duration, but also a future activation time that could be especially useful for automating load filtering for predictable overloads. PSTN load filtering can be performed in both edge switches and transit switches; the SIP load filtering can also be applied in both edge proxy servers and core proxy servers, and even in capable user agents. PSTN load filtering also has special accommodation for High Probability of Completion (HPC) calls, which would be similar to calls designated by the SIP Resource Priority Headers [RFC4412]. The SIP load-filtering mechanism also allows prioritizing the treatment of these calls by specifying favorable actions for them. PSTN load filtering also provides an administrative option for routing failed call attempts to either a reorder tone [E.300SerSup3] indicating overload conditions or a special recorded announcement. A similar capability can be provided in the SIP load-filtering mechanism by specifying appropriate "alt-action" attribute in the SIP load-filtering action field. E.2. Relationship with Other IETF SIP Overload Control Efforts The load-filtering policies in this specification consist of identity, action, and time. The identity can range from a single specific user to an arbitrary user aggregate, domains, or areas. The user can be identified by either the source or the destination. When the user is identified by the source and a favorable action is specified, the result is, to some extent, similar to identifying a priority user based on authorized Resource Priority Headers [RFC4412] in the requests. Specifying a source user identity with an unfavorable action would cause an effect to some extent similar to an inverse SIP resource priority mechanism. Shen, et al. Standards Track [Page 42]
RFC 7200 SIP Load-Control Event Package April 2014 The load-filtering policy defined in this specification is generic and expected to be applicable not only to the load-filtering mechanism but also to the feedback overload control mechanism in [SIP-OVERLOAD]. In particular, both mechanisms could use specific or wildcard identities for load control and could share well-known load- control actions. The time duration field in the load-filtering policy could also be used in both mechanisms. As mentioned in Section 1, the load-filtering policy distribution mechanism and the feedback overload control mechanism address complementary areas in the overload control problem space. Load filtering is more proactive and focuses on distributing filtering policies towards the source of the traffic; the hop-by-hop feedback-based approach is reactive and reduces traffic already accepted by the network. Therefore, they could also make different use of the generic load-filtering policy components. For example, the load-filtering mechanism may use the time field in the filtering policy to specify not only a control duration but also a future activation time to accommodate a predicable overload such as the one caused by Mother's Day greetings or a viewer-voting program; the feedback-based control might not need to use the time field or might use the time field to specify an immediate load-control duration. Shen, et al. Standards Track [Page 43]
RFC 7200 SIP Load-Control Event Package April 2014
Authors' Addresses
Charles Shen
Columbia University
Department of Computer Science
1214 Amsterdam Avenue, MC 0401
New York, NY 10027
Phone: +1 212 854 3109
EMail: email@example.com
Henning Schulzrinne
Columbia University
Department of Computer Science
1214 Amsterdam Avenue, MC 0401
New York, NY 10027
Phone: +1 212 939 7004
EMail: email@example.com
Arata Koike
NTT Network Technology Labs
3-9-11 Midori-cho Musashino-shi
Tokyo 180-8585
Phone: +81 422 59 6099
EMail: firstname.lastname@example.com
Shen, et al. Standards Track [Page 44]


text:
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subset: commoncrawl


text:
Today I am very pleased to have a guest post from the Author of Death Wish, Megan Tayte! Along with the guest post I am also featuring a spotlight for Death Wish which includes an excerpt.
Make sure to check it out, and pick up your copy today!
In the course of my career I've been a mercenary, a phone sex operative, a serial dater, a war historian, a millionaire sports champion, a political refugee, a zombie fashionista and an ass-kicking superhero, among other things.
No, I wasn't voted 'Most Unlikely to Stick with a Career' at school. I'm a ghostwriter who writes for – as – other people so that their stories can be told.
Being a ghostwriter has stood me in good stead for my own writing career. It's given me experience in the many ways to write a book, and allowed me to write in different tones, styles and formats. It's taught me to pay attention to how every word choice, every nuance, affects the overall work; and how to stand back and look at the big picture with critical eyes. Above all, it's been an education in the self-discipline, organisation, determination and tenacity required to write book after book after book.
But ghosting throws up two big challenges for the would-be author:
- Writing as you: To be a ghostwriter you have to be a chameleon, and adopt the client's tone and style. It's a little like being an actress, only on paper. And as any method actress finds, the longer you play a role and the more you immerse yourself in it, the more the line between 'you' and 'other' blurs. Put simply, when you ghostwrite day after day, month after month, year after year, you can lose a sense of your authentic voice.
- Having confidence in your own writing: Ghostwriters are commissioned based on their ability to write well – and in fact writing as someone else is far more technically difficult than writing as yourself. And yet, when you've spent a long time writing 'in character', you can start to lose confidence in the writer beneath the mask. You're so used to writing to a brief, how others want you to write, that you lose touch with your early passion for writing and your belief that you can string together a sentence in your own unique style.
For me, the best way to push past these challenges is to write, and write, and write. After a long day's ghostwriting, often the last thing you want to do is write something for yourself: it's a busman's holiday. But the more you practise, the easier it becomes to find your voice and, crucially, to trust that it's all yours. Then and only then do you cease being a ghost, and become a living, breathing writer no longer haunted by the dream you've had since childhood: that someday you'll write a book of your own.
About Death Wish by Megan Tayte:
Seventeen-year-old Scarlett Blake is haunted by death. Her estranged sister has made the ultimate dramatic exit. Running away from school, joining a surfing fraternity, partying hard: that sounds like Sienna. But suicide? It makes no sense.
Following in her sister's footsteps, Scarlett comes to the isolated cove of Twycombe, Devon, with grand plans to uncover the truth. Alone. But she hasn't reckoned on meeting two boys who are determined to help her. Luke: the blue-eyed surfer who'll see the real Scarlett, who'll challenge her, who'll save her. And Jude: the elusive drifter with a knack for turning up whenever Scarlett's in need.
As Scarlett's quest for the truth unravels, so too does her grip on reality as she's always known it. Because there's something strange going on in this little cove. A dead magpie circles the skies. A dead deer watches from the undergrowth. Hands glow with light. Warmth. Power.
What transpires is a summer of discovery. Of what it means to conquer fear. To fall in love. To choose life. To choose death.
To believe the impossible.
Waves everywhere, swirling, surging, seething – a raging melange of foam and salt and inky water biting at me, pulling at me, thrusting upon me a solitary invitation:
As I fought to remain on the flimsy polystyrene surfboard that seemed more bucking bronco than wave rider, I thought: That's how easy it is – you just let go. Just release the grip on this world that in recent months had seemed so much an effort, and sink into the blue, beneath the waves, where chaos and fury turned to quiet and calm. Like she did.
Was drowning as they claim? I wondered. The easiest way to die – peaceful? How would it feel to give up all the dragging myself through the day, all the struggle to evade the aching void inside? A relief?
Another wave rose me up and slammed me down with breathtaking power. Its force stirred me. You could say a lot of things about Scarlett Blake – she's a loner, she's a wallflower, she's a menace in the kitchen – but no way was 'she's a quitter' on the list of character flaws.
'Screw you!' I shouted through the spray.
Funny, sounded like someone shouted back. But who else would be out in this tumultuous sea at six a.m. on a summer's morning? Solitude was the entire point of hauling myself out of bed in the still-dark and picking my way down the cliff path to the beach just in time to see the horizon light up with the first burnt-orange glow of the rising sun. No one to see me make a damn fool of myself on my first surfing attempt.
'Trying… yourself killed?'
Definitely a voice. Male. Angry.
Scanning the surroundings for the source proved difficult while lying stomach-to-board. On an upward surge I got a glimpse of the Devonshire cliffs that fringed the cove, all dark, jutting rocks topped by bushes of gorse, and then a flash of the beach. On a downward plummet there was nothing but eye-burning, throat-choking seawater.
'Forward… next wave!'
The voice was closer now. There was an edge to it beyond the anger. Something raw.
My eyes picked out a black form between the waves. Someone on a surfboard, paddling it expertly seaward. I took one hand off the board to push sticky tendrils of hair from my eyes. Rookie mistake. Turned out holding on one-handed was impossible. The board shot upwards, out of my feeble grip, and then it was just me and Old Man Sea.
Kicking frantically, I tried to keep my head above the surface, but the waves were burying me, one after the other, only a second or two to come up for air before the next one hit. Far away now were thoughts of letting go – I was fighting furiously for life. Never in my seventeen years had I been so desperate. But my legs were tingling with effort, and I knew it was just a matter of time.
When the final wave broke me all I could think was, Sienna. With her name on my lips I inhaled a lungful of water and I sank…
… for all of a second before something grabbed the back of my t-shirt and hauled me upward. Coughing and spluttering, I emerged from the blue and was pulled roughly onto a board, my leg shoved over so that I straddled it. I had the fleeting thought that this board was much sleeker and more substantial looking than the one I'd just lost before my rescuer settled pretty much on top of me and started paddling toward the shore.
With him in command, we crested waves and glided down the other side with apparent ease, though I seemed unable to match the rhythm of our motion and kept taking in great gulps of brine. Over the sound of the waves and the wind and the splash of powerful arms cutting into the water to propel us along, I picked out low, irate grumblings.
'… idiot tourists… total waste of… all we need… another bloody drama…'
Finally, we reached the shallow waters and he slid off the board and pulled me off to walk to the beach. But my legs didn't seem willing to respond to basic instructions like 'walk' or even 'stand' and breathing between wrenching gasps had become a challenge, so he threw an arm around me and half-carried, half-walked me, dragging his board with his spare hand.
Ten steps up the beach he let me down onto the sand.
'Head down,' he commanded. 'Between your legs. Cough it out.'
I did as I was told. Liquid spilled out of me with each retching cough, and the cool air I gulped in burned my throat. I fought the panic, I fought the pain, focusing instead on the shells and stones strewn around. Finally, breathing won out.
'You okay?'
I was reluctant to look up. For starters, I knew I must look a mess – long hair plastered to my head rat-tail style, face flushed and salt-burned, eyes teary and bloodshot. And then there was the fact that this guy, whoever he was, had just saved my life, and was evidently pretty mad about having had to do so.
'Hey, you okay?'
I lifted my head slowly. Took in broad thighs clad in black neoprene; hands reaching out, palms raised; a wide, muscular chest; a striking face – rugged, square jaw, full lips, ruddy cheeks, Grecian nose bearing a thin scar across the bridge, thick black lashes framing eyes… oh, his eyes.
I opened my mouth, tried to speak, but I was paralysed by his gaze. All at once I was home in the cottage, tucked up beneath the blue patchwork quilt of my childhood; I was watching my grandmother remove vanilla-scented fairy cakes from her powder-blue Aga; I was running through a meadow of sky-blue forget-me-nots with my sister – free, exhilarated, happy. The memories took my breath away. I felt the familiar burn in my tear ducts.
His eyebrows pulled together and he placed a hand on my trembling knee.
'Are. You. Okay?' he said with exaggerated care, as if he were speaking to an elderly lady having a turn at a bus stop.
I blinked, cleared my throat and managed a husky, 'Yes. Th-thank you.'
Concern melted into exasperation.
'What's the deal,' he demanded, 'out there on your own, clearly no idea what you're doing, children's play surfboard… you got a death wish or something?'
I cringed. I'd known the board was short, but I'd thought it was me-sized – at five foot three, what use was some enormous board?
'I'm sorry.'
'You would've been sorry if I hadn't seen you.'
'I just wanted to get a feel for it. I didn't realise it was so rough out there.'
'Rough? That's not rough. Not even optimum surfing weather. Piece of cake for someone who actually knows how to surf…'
He paused when he saw a tear escape my eye and roll traitorously down my cheek. Furrowed his brow, combed his fingers roughly through dark hair that was drying fast in the breeze.
'Listen, I didn't mean to…'
I brushed the tear away furiously. Enough with the vulnerability.
'Right, well, thank you…'
'Luke. My name's Luke.' The stress lines in his face smoothed out and his lips curved. Like this, smiling and relaxed, his scrutiny was a touch less unsettling. 'And you are…?'
'Thank you, Luke, for your, um, help, but I'm sure you've better things to do, so I'll just be…'
Before he could protest, I launched myself to my feet. He instinctively rose with me, and my water-fogged mind registered belatedly that my rescuer was a giant of a guy – my head was at the level of his chest. As I looked up to take in his stature I staggered slightly and he reached out to right me, but I stepped backwards. I didn't need his kindness.
He looked awkward, unsure of himself, as he towered over me. 'Hey, will you be okay?'
'Yes, yes, I'm fine. I'll just head home.'
'You live close?'
I pointed vaguely west. 'Yes, not far.'
'Up there?' He looked puzzled, and then interest sparked in his eyes. 'You mean the Blake place?'
Busted. Of course being vague was pointless. My grandparents' ramshackle cottage on the western cliff was the only building up there.
I made a noncommittal mnnnhnnn noise, but Luke was not to be deterred.
'But that place has been empty since…'
He was looking at me now with such scrutiny that I took a further step back. I saw the cogs turning in his mind as he took in the classic green Blake eyes and then compared her – short, spiky red hair, eternally crimson lips, tall and impossibly slender – with me – petite and curvy, hair more blond than auburn reaching to the base of my spine and a pallor worthy of a vampire. His eyes widened.
'Scarlett? Scarlett Blake!'
There was shock in his tone, and then sympathy.
About Megan Tayte:
Once upon a time a little girl told her grandmother that when she grew up she wanted to be a writer. Or a lollipop lady. Or a fairy princess fireman. 'Write, Megan,' her grandmother advised. So that's what she did.
Thirty-odd years later, Megan writes the kinds of books she loves to read: young-adult paranormal romance fiction. Young adult, because it's the time of life that most embodies freedom and discovery and first love. Paranormal, because she's always believed that there are more things in heaven and on earth than are dreamt of in our philosophy. And romance, because she's a misty-eyed dreamer who lives for those 'life is so breathtakingly beautiful' moments.
Megan grew up in the Royal County, a hop, skip and a (very long) jump from Windsor Castle, but these days she makes her home in Robin Hood's county, Nottingham. She lives with her husband, a proud Scot who occasionally kicks back in a kilt; her son, a budding artist with the soul of a paleontologist; and her baby daughter, a keen pan-and-spoon drummer who sings in her sleep. When she's not writing, you'll find her walking someplace green, reading by the fire, or creating carnage in the kitchen as she pursues her impossible dream: of baking something edible.