Internet Engineering Task Force (IETF) F. Le Faucheur, Ed. Request for Comments: 7937 Category: Standards Track G. Bertrand, Ed. ISSN: 2070-1721 I. Oprescu, Ed. R. Peterkofsky Google Inc. August 2016 Content Distribution Network Interconnection (CDNI) Logging Interface Abstract This memo specifies the Logging interface between a downstream Content Distribution Network (dCDN) and an upstream CDN (uCDN) that are interconnected as per the CDN Interconnection (CDNI) framework. First, it describes a reference model for CDNI logging. Then, it specifies the CDNI Logging File format and the actual protocol for exchange of CDNI Logging Files. 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 7841. 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/rfc7937.
Copyright Notice Copyright (c) 2016 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Requirements Language . . . . . . . . . . . . . . . . . . 5 2. CDNI Logging Reference Model . . . . . . . . . . . . . . . . 5 2.1. CDNI Logging Interactions . . . . . . . . . . . . . . . . 5 2.2. Overall Logging Chain . . . . . . . . . . . . . . . . . . 9 2.2.1. Logging Generation and During-Generation Aggregation 10 2.2.2. Logging Collection . . . . . . . . . . . . . . . . . 11 2.2.3. Logging Filtering . . . . . . . . . . . . . . . . . . 11 2.2.4. Logging Rectification and Post-Generation Aggregation 12 2.2.5. Log-Consuming Applications . . . . . . . . . . . . . 13 220.127.116.11. Maintenance and Debugging . . . . . . . . . . . . 13 18.104.22.168. Accounting . . . . . . . . . . . . . . . . . . . 14 22.214.171.124. Analytics and Reporting . . . . . . . . . . . . . 14 126.96.36.199. Content Protection . . . . . . . . . . . . . . . 14 188.8.131.52. Notions Common to Multiple Log-Consuming Applications . . . . . . . . . . . . . . . . . . 15 3. CDNI Logging File . . . . . . . . . . . . . . . . . . . . . . 17 3.1. Rules . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.2. CDNI Logging File Structure . . . . . . . . . . . . . . . 18 3.3. CDNI Logging Directives . . . . . . . . . . . . . . . . . 21 3.4. CDNI Logging Records . . . . . . . . . . . . . . . . . . 26 3.4.1. HTTP Request Logging Record . . . . . . . . . . . . . 27 3.5. CDNI Logging File Extension . . . . . . . . . . . . . . . 38 3.6. CDNI Logging File Examples . . . . . . . . . . . . . . . 38 3.7. Cascaded CDNI Logging Files Example . . . . . . . . . . . 42
4. Protocol for Exchange of CDNI Logging File after Full Collection . . . . . . . . . . . . . . . . . . . . . . . . . 44 4.1. CDNI Logging Feed . . . . . . . . . . . . . . . . . . . . 45 4.1.1. Atom Formatting . . . . . . . . . . . . . . . . . . . 45 4.1.2. Updates to Log Files and the Feed . . . . . . . . . . 46 4.1.3. Redundant Feeds . . . . . . . . . . . . . . . . . . . 47 4.1.4. Example CDNI Logging Feed . . . . . . . . . . . . . . 47 4.2. CDNI Logging File Pull . . . . . . . . . . . . . . . . . 49 5. Protocol for Exchange of CDNI Logging File During Collection 50 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 51 6.1. CDNI Logging Directive Names Registry . . . . . . . . . . 51 6.2. CDNI Logging File version Registry . . . . . . . . . . . 51 6.3. CDNI Logging record-types Registry . . . . . . . . . . . 52 6.4. CDNI Logging Field Names Registry . . . . . . . . . . . . 53 6.5. CDNI Logging Payload Type . . . . . . . . . . . . . . . . 55 7. Security Considerations . . . . . . . . . . . . . . . . . . . 55 7.1. Authentication, Authorization, Confidentiality, and Integrity Protection . . . . . . . . . . . . . . . . . . 55 7.2. Denial of Service . . . . . . . . . . . . . . . . . . . . 56 7.3. Privacy . . . . . . . . . . . . . . . . . . . . . . . . . 57 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 58 8.1. Normative References . . . . . . . . . . . . . . . . . . 58 8.2. Informative References . . . . . . . . . . . . . . . . . 61 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 63 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 63 1. Introduction This memo specifies the CDNI Logging interface between a downstream CDN (dCDN) and an upstream CDN (uCDN). First, it describes a reference model for CDNI logging. Then, it specifies the CDNI Logging File format and the actual protocol for exchange of CDNI Logging Files. The reader should be familiar with the following documents: o CDNI problem statement [RFC6707] and framework [RFC7336], which identify a Logging interface, o Section 8 of [RFC7337], which specifies a set of requirements for Logging, o [RFC6770] outlines real world use cases for interconnecting CDNs. These use cases require the exchange of Logging information between the dCDN and the uCDN.
As stated in [RFC6707], "the CDNI Logging interface enables details of content distribution and delivery activities to be exchanged between interconnected CDNs." The present document describes: o The CDNI Logging reference model (Section 2) o The CDNI Logging File format (Section 3) o The CDNI Logging File Exchange protocol (Section 4) 1.1. Terminology In this document, the first letter of each CDNI-specific term is capitalized. We adopt the terminology described in [RFC6707] and [RFC7336], and extend it with the additional terms defined below. Intra-CDN Logging information: Logging information generated and collected within a CDN. The format of the Intra-CDN Logging information may be different from the format of the CDNI Logging information. CDNI Logging information: Logging information exchanged across CDNs using the CDNI Logging interface. Logging information: Logging information generated and collected within a CDN or obtained from another CDN using the CDNI Logging interface. CDNI Logging Field: An atomic element of information that can be included in a CDNI Logging Record. The time an event/task started, the IP address of an end user to whom content was delivered, and the Uniform Resource Identifier (URI) of the content delivered, are examples of CDNI Logging fields. CDNI Logging Record: An information record providing information about a specific event. This comprises a collection of CDNI Logging fields. CDNI Logging File: A file containing CDNI Logging Records, as well as additional information facilitating the processing of the CDNI Logging Records. CDN Reporting: The process of providing the relevant information that will be used to create a formatted content delivery report provided to the Content Service Provider (CSP) in deferred time. Such information typically includes aggregated data that can cover a large
period of time (e.g., from hours to several months). Uses of reporting include the collection of charging data related to CDN services and the computation of Key Performance Indicators (KPIs). CDN Monitoring: The process of providing or displaying content delivery information in a timely fashion with respect to the corresponding deliveries. Monitoring typically includes visibility of the deliveries in progress for service operation purposes. It presents a view of the global health of the services as well as information on usage and performance, for network services supervision and operation management. In particular, monitoring data can be used to generate alarms. 1.2. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. 2. CDNI Logging Reference Model 2.1. CDNI Logging Interactions The CDNI logging reference model between a given uCDN and a given dCDN involves the following interactions: o customization by the uCDN of the CDNI Logging information to be provided by the dCDN to the uCDN (e.g., control of which CDNI Logging fields are to be communicated to the uCDN for a given task performed by the dCDN or control of which types of events are to be logged). The dCDN takes into account this CDNI Logging customization information to determine what Logging information to provide to the uCDN, but it may, or may not, take into account this CDNI Logging customization information to influence what CDN Logging information is to be generated and collected within the dCDN (e.g., even if the uCDN requests a restricted subset of the Logging information, the dCDN may elect to generate a broader set of Logging information). The mechanism to support the customization by the uCDN of CDNI Logging information is outside the scope of this document and is left for further study. Until such a mechanism is available, the uCDN and dCDN are expected to agree off-line on what exact set of CDNI Logging information is to be provided by the dCDN to the uCDN, and to rely on management- plane actions to configure the CDNI Logging functions in the dCDN to generate this information set and in the uCDN to expect this information set.
o generation and collection by the dCDN of the intra-CDN Logging information related to the completion of any task performed by the dCDN on behalf of the uCDN (e.g., delivery of the content to an end user) or related to events happening in the dCDN that are relevant to the uCDN (e.g., failures or unavailability in dCDN). This takes place within the dCDN and does not directly involve CDNI interfaces. o communication by the dCDN to the uCDN of the Logging information collected by the dCDN relevant to the uCDN. This is supported by the CDNI Logging interface and is in the scope of the present document. For example, the uCDN may use this Logging information to charge the CSP, to perform analytics and monitoring for operational reasons, to provide analytics and monitoring views on its content delivery to the CSP, or to perform troubleshooting. This document exclusively specifies non-real-time exchange of Logging information. Closer to real-time exchange of Logging information (say sub-minute or sub-second) is outside the scope of the present document and is left for further study. This document exclusively specifies exchange of Logging information related to content delivery. Exchange of Logging information related to operational events (e.g., dCDN request routing function unavailable and content acquisition failure by dCDN) for audit or operational reactive adjustments by uCDN is outside the scope of the present document and is left for further study. o customization by the dCDN of the CDNI Logging information to be provided by the uCDN on behalf of the dCDN. The mechanism to support the customization by the dCDN of CDNI Logging information is outside the scope of this document and is left for further study. o generation and collection by the uCDN of Intra-CDN Logging information related to the completion of any task performed by the uCDN on behalf of the dCDN (e.g., serving of content by uCDN to dCDN for acquisition purposes by dCDN) or related to events happening in the uCDN that are relevant to the dCDN. This takes place within the uCDN and does not directly involve CDNI interfaces. o communication by the uCDN to the dCDN of the Logging information collected by the uCDN relevant to the dCDN. For example, the dCDN might potentially benefit from this information for security auditing or content acquisition troubleshooting. This is outside the scope of this document and is left for further study.
Figure 1 provides an example of CDNI Logging interactions (focusing only on the interactions that are in the scope of this document) in a particular scenario where four CDNs are involved in the delivery of content from a given CSP: the uCDN has a CDNI interconnection with dCDN-1 and dCDN-2. In turn, dCDN-2 has a CDNI interconnection with dCDN-3, where dCDN-2 is acting as an upstream CDN relative to dCDN-3. In this example, uCDN, dCDN-1, dCDN-2, and dCDN-3 all participate in the delivery of content for the CSP. In this example, the CDNI Logging interface enables the uCDN to obtain Logging information from all the dCDNs involved in the delivery. In the example, the uCDN uses the Logging information: o to analyze the performance of the delivery performed by the dCDNs and to adjust its operations after the fact (e.g., request routing) as appropriate. o to provide (non-real-time) reporting and monitoring information to the CSP. For instance, the uCDN merges Logging information, extracts relevant KPIs, and presents a formatted report to the CSP, in addition to a bill for the content delivered by uCDN itself or by its dCDNs on the CSP's behalf. The uCDN may also provide Logging information as raw log files to the CSP, so that the CSP can use its own logging analysis tools.
+-----+ | CSP | +-----+ ^ Reporting and monitoring data * Billing ,--*--. Logging ,-' `-. Data =>( uCDN )<= Logging // `-. _,-' \\ Data || `-'-'-' || ,-----. ,-----. ,-' `-. ,-' `-. ( dCDN-1 ) ( dCDN-2 )<== Logging `-. ,-' `-. _,-' \\ Data `--'--' `--'-' || ,-----. ,' `-. ( dCDN-3 ) `. ,-' `--'--' ===> CDNI Logging interface ***> outside the scope of CDNI Figure 1: Interactions in the CDNI Logging Reference Model A downstream CDN relative to uCDN (e.g., dCDN-2) integrates the relevant Logging information obtained from its own downstream CDNs (i.e., dCDN-3) in the Logging information that it provides to the uCDN, so that the uCDN ultimately obtains all Logging information relevant to a CSP for which it acts as the authoritative CDN. Such aggregation is further discussed in Section 3.7. Note that the format of Logging information that a CDN provides over the CDNI interface might be different from the one that the CDN uses internally. In this case, the CDN needs to reformat the Logging information before it provides this information to the other CDN over the CDNI Logging interface. Similarly, a CDN might reformat the Logging information that it receives over the CDNI Logging interface before injecting it into its log-consuming applications or before providing some of this Logging information to the CSP. Such reformatting operations introduce latency in the logging distribution chain and introduce a processing burden. Therefore, there are benefits in specifying CDNI Logging formats that are suitable for use inside CDNs and also are close to the intra-CDN Logging formats commonly used in CDNs today.
2.2. Overall Logging Chain This section discusses the overall logging chain within and across CDNs to clarify how CDN Logging information is expected to fit in this overall chain. Figure 2 illustrates the overall logging chain within the dCDN, across CDNs using the CDNI Logging interface, and within the uCDN. Note that the logging chain illustrated in the figure is obviously only an example and varies depending on the specific environments. For example, there may be more or fewer instantiations of each entity (e.g., there may be 4 log-consuming applications in a given CDN). As another example, there may be one instance of a Rectification process per log-consuming application instead of a shared one.
Log-Consuming Log-Consuming App App ^ ^ | | Rectification---------- ^ | Filtering ^ | Collection ^ ^ | | | Generation | | uCDN CDNI Logging --------------------------------------------------- exchange dCDN ^ | Log-Consuming Log-Consuming | App App | ^ ^ | | | Rectification Rectification--------- ^ ^ | | Filtering ^ | Collection ^ ^ | | Generation Generation Figure 2: CDNI Logging in the Overall Logging Chain The following subsections describe each of the processes potentially involved in the logging chain of Figure 2. 2.2.1. Logging Generation and During-Generation Aggregation CDNs typically generate Logging information for all significant task completions, events, and failures. Logging information is typically generated by many devices in the CDN including the surrogates, the request routing system, and the control system.
The amount of Logging information generated can be huge. Therefore, during contract negotiations, interconnected CDNs often agree on a retention duration for Logging information, and/or potentially on a maximum volume of Logging information that the dCDN ought to keep. If this volume is exceeded, the dCDN is expected to alert the uCDN but may not keep more Logging information for the considered time period. In addition, CDNs may aggregate Logging information and transmit only summaries for some categories of operations instead of the full Logging information. Note that such aggregation leads to an information loss, which may be problematic for some usages of the Logging information (e.g., debugging). [RFC6983] discusses logging for HTTP Adaptive Streaming (HAS). In accordance with the recommendations articulated there, it is expected that a surrogate will generate separate Logging information for delivery of each chunk of HAS content. This ensures that separate Logging information can then be provided to interconnected CDNs over the CDNI Logging interface. Still in line with the recommendations of [RFC6983], the Logging information for per-chunk delivery may include some information (a Content Collection IDentifier and a Session IDentifier) intended to facilitate subsequent post-generation aggregation of per-chunk logs into per-session logs. Note that a CDN may also elect to generate aggregate per-session logs when performing HAS delivery, but this needs to be in addition to, and not instead of, the per-chunk delivery logs. We note that aggregate per-session logs for HAS delivery are for further study and are outside the scope of this document. 2.2.2. Logging Collection This is the process that continuously collects Logging information generated by the log-generating entities within a CDN. In a CDNI environment, in addition to collecting Logging information from log-generating entities within the local CDN, the Collection process also collects Logging information provided by another CDN, or other CDNs, through the CDNI Logging interface. This is illustrated in Figure 2 where we see that the Collection process of the uCDN collects Logging information from log-generating entities within the uCDN as well as Logging information coming from the dCDNs through the CDNI Logging interface. 2.2.3. Logging Filtering A CDN may be required to only present different subsets of the whole Logging information collected to various log-consuming applications. This is achieved by the Filtering process.
In particular, the Filtering process can also filter the right subset of Logging information that needs to be provided to a given interconnected CDN. For example, the filtering process in the dCDN can be used to ensure that only the Logging information related to tasks performed on behalf of a given uCDN are made available to that uCDN (thereby filtering out all the Logging information related to deliveries by the dCDN of content for its own CSPs). Similarly, the Filtering process may filter or partially mask some fields, for example, to protect end-users' privacy when communicating CDNI Logging information to another CDN. Filtering of Logging information prior to communication of this information to other CDNs via the CDNI Logging interface requires that the downstream CDN can recognize the subset of Logging information that relates to each interconnected CDN. The CDN will also filter some internal scope information such as information related to its internal alarms (security, failures, load, etc.). In some use cases described in [RFC6770], the interconnected CDNs do not want to disclose details on their internal topology. The filtering process can then also filter confidential data on the dCDNs' topology (number of servers, location, etc.). In particular, information about the requests served by each Surrogate may be confidential. Therefore, the Logging information needs to be protected so that data such as the Surrogates' hostnames are not disclosed to the uCDN. In the "Inter-Affiliates Interconnection" use case, this information may be disclosed to the uCDN because both the dCDN and the uCDN are operated by entities of the same group. 2.2.4. Logging Rectification and Post-Generation Aggregation If Logging information is generated periodically, it is important that the sessions that start in one Logging period and end in another are correctly reported. If they are reported in the starting period, then the Logging information of this period will be available only after the end of the session, which delays the Logging information generation. A simple approach is to provide the complete Logging Record for a session in the Logging Period of the session end. A Logging rectification/update mechanism could be useful to reach a good trade-off between the Logging information generation delay and the Logging information accuracy. In the presence of HAS, some log-consuming applications can benefit from aggregate per-session logs. For example, for analytics, per- session logs allow display of session-related trends, which are much more meaningful for some types of analysis than chunk-related trends.
In the case where aggregate logs have been generated directly by the log-generating entities, those can be used by the applications. In the case where aggregate logs have not been generated, the Rectification process can be extended with a Post-Generation Aggregation process that generates per-session logs from the per- chunk logs, possibly leveraging the information included in the per- chunk logs for that purpose (Content Collection IDentifier and a Session IDentifier). However, in accordance with [RFC6983], this document does not define the exchange of such aggregate logs on the CDNI Logging interface. We note that this is for further study and is outside the scope of this document. 2.2.5. Log-Consuming Applications 184.108.40.206. Maintenance and Debugging Logging information is useful to permit the detection (and limit the risk) of content delivery failures. In particular, Logging information facilitates the detection of configuration issues. To detect faults, Logging information needs to report the success and failure of CDN-delivery operations. The uCDN can summarize such information into KPIs. For instance, Logging information needs to allow the computation of the number of times, during a given time period, that content delivery related to a specific service succeeds or fails. Logging information enables the CDN providers to identify and troubleshoot performance degradations. In particular, Logging information enables tracking of traffic data (e.g., the amount of traffic that has been forwarded by a dCDN on behalf of an uCDN over a given period of time), which is particularly useful for CDN and network planning operations. Some of these maintenance and debugging applications only require aggregate Logging information highly compatible with the use of anonymization of IP addresses (as supported by the present document and specified in the definition of the c-groupid field in Section 3.4.1). However, in some situations, it may be useful, where compatible with privacy protection, to access some CDNI Logging Records containing full non-anonymized IP addresses. This is allowed in the definition of the c-groupid (in Section 3.4.1), with very significant privacy protection limitations that are discussed in the definition of the c-groupid field. For example, this may be useful for detailed fault tracking of a particular end-user content delivery issue. Where there is a hard requirement by uCDN or CSP to associate a given end user to individual CDNI Logging Records (e.g., to allow a posteriori analysis of individual delivery, for example, in
situations of performance-based penalties), instead of using aggregates containing a single client as discussed in the c-groupid field definition, an alternate approach is to ensure that a client identifier is embedded in the request fields that can be logged in a CDNI Logging Record (for example, by including the client identifier in the URI query string or in an HTTP Header). That latter approach offers two significant benefits: first, the aggregate inside the c-groupid can contain more than one client, thereby ensuring stronger privacy protection; second, it allows a reliable identification of the client while IP address does not in many situations (e.g., behind NAT, where dynamic IP addresses are used and reused, etc.). However, care SHOULD be taken so that the client identifiers exposed in other fields of the CDNI Records cannot themselves be linked back to actual users. 220.127.116.11. Accounting Logging information is essential for accounting, to permit inter-CDN billing and CSP billing by uCDNs. For instance, Logging information provided by dCDNs enables the uCDN to compute the total amount of traffic delivered by every dCDN for a particular Content Provider, as well as the associated bandwidth usage (e.g., peak, 95th percentile), and the maximum number of simultaneous sessions over a given period of time. 18.104.22.168. Analytics and Reporting The goals of analytics include gathering any relevant information in order to be able to develop statistics on content download, analyze user behavior, and monitor the performance and quality of content delivery. For instance, Logging information enables the CDN providers to report on content consumption (e.g., delivered sessions per content) in a specific geographic area. The goal of reporting is to gather any relevant information to monitor the performance and quality of content delivery, and allow detection of delivery issues. For instance, reporting could track the average delivery throughput experienced by end users in a given region for a specific CSP or content set over a period of time. 22.214.171.124. Content Protection The goal of content protection is to prevent and monitor unauthorized access, misuse, modification, and denial of access to content. A set of information is logged in a CDN for security purposes. In particular, a record of access to content is usually collected to permit the CSP to detect infringements of content delivery policies and other abnormal end-user behaviors.
126.96.36.199. Notions Common to Multiple Log-Consuming Applications 188.8.131.52.1. Logging Information Views Within a given log-consuming application, different views may be provided to different users depending on privacy, business, and scalability constraints. For example, an analytics tool run by the uCDN can provide one view to a uCDN operator that exploits all the Logging information available to the uCDN, while the tool may provide a different view to each CSP exploiting only the Logging information related to the content of the given CSP. As another example, maintenance and debugging tools may provide different views to different CDN operators, based on their operational role. 184.108.40.206.2. Key Performance Indicators (KPIs) This section presents, for explanatory purposes, a non-exhaustive list of Key Performance Indicators (KPIs) that can be extracted/ produced from logs. Multiple log-consuming applications, such as analytics, monitoring, and maintenance applications, often compute and track such KPIs. In a CDNI environment, depending on the situation, these KPIs may be computed by the uCDN or by the dCDN. But it is usually the uCDN that computes KPIs, because the uCDN and dCDN may have different definitions of the KPIs and the computation of some KPIs requires a vision of all the deliveries performed by the uCDN and all its dCDNs. Here is a list of important examples of KPIs: o Number of delivery requests received from end users in a given region for each piece of content, during a given period of time (e.g., hour/day/week/month) o Percentage of delivery successes/failures among the aforementioned requests o Number of failures listed by failure type (e.g., HTTP error code) for requests received from end users in a given region and for each piece of content, during a given period of time (e.g., hour/day/week/month)
o Number and cause of premature delivery termination for end users in a given region and for each piece of content, during a given period of time (e.g., hour/day/week/month) o Maximum and mean number of simultaneous sessions established by end users in a given region, for a given Content Provider, and during a given period of time (e.g., hour/day/week/month) o Volume of traffic delivered for sessions established by end users in a given region, for a given Content Provider, and during a given period of time (e.g., hour/day/week/month) o Maximum, mean, and minimum delivery throughput for sessions established by end users in a given region, for a given Content Provider, and during a given period of time (e.g., hour/day/week/ month) o Cache-hit and byte-hit ratios for requests received from end users in a given region for each piece of content, during a given period of time (e.g., hour/day/week/month) o Top 10 most popularly requested contents (during a given day/week/ month) o Terminal type (mobile, PC, Set-Top Box (STB), if this information can be acquired from the browser type inferred from the User Agent string, for example) Additional KPIs can be computed from other sources of information than the Logging information, for instance, data collected by a content portal or by specific client-side application programming interfaces. Such KPIs are out of scope for the present document. The KPIs used depend strongly on the considered log-consuming application -- the CDN operator may be interested in different metrics than the CSP. In particular, CDN operators are often interested in delivery and acquisition performance KPIs, information related to Surrogates' performance, caching information to evaluate the cache-hit ratio, information about the delivered file size to compute the volume of content delivered during peak hour, etc. Some of the KPIs, for instance those providing an instantaneous vision of the active sessions for a given CSP's content, are useful essentially if they are provided in a timely manner. By contrast, some other KPIs, such as those averaged over a long period of time, can be provided in non-real-time.