AAA Working Group Bernard Aboba INTERNET-DRAFT Microsoft Corporation Category: Informational Jari Arkko <draft-ietf-aaa-acct-02.txt> Ericsson 19 April 2000 David Harrington Cabletron Systems Inc. Introduction to Accounting Management 1. Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet- Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. To view the list Internet-Draft Shadow Directories, see http://www.ietf.org/shadow.html. 2. Copyright Notice Copyright (C) The Internet Society (2000). All Rights Reserved. 3. Abstract The field of Accounting Management is concerned with the collection of resource consumption data for the purposes of capacity and trend analysis, cost allocation, auditing, and billing. This document describes each of these problems, and discusses the issues involved in design of modern accounting systems. Since accounting applications do not have uniform security and reliability requirements, it is not possible to devise a single accounting protocol and set of security services that will meet all needs. Thus the goal of accounting management is to provide a set of tools that can be used to meet the requirements of each application. This document describes the currently available tools as well as the Aboba, Arkko & Harrington Informational [Page 1]
INTERNET-DRAFT Accounting Management 19 April 2000 state of the art in accounting protocol design. A companion document, draft-ietf-aaa-accounting-attributes-03.txt, reviews the state of the art in accounting attributes and record formats. 3.1. History -02 draft: added discussion of accounting proxies. Expanded discussion of accounting server faults and failover. Revised section on SNMP v3 contextName. Revised requirements and evaluation tables. Fixed spelling mistakes. 4. Table of Contents 1. Status of this Memo 1 2. Copyright notice 1 3. Abstract 1 4. Table of Contents 2 5. Introduction 3 5.1 Terminology 3 5.2 Accounting management architecture 5 5.3 Accounting management objectives 6 5.4 Intra-domain and inter-domain accounting 9 5.5 Accounting record production 10 5.6 Requirements summary 12 6. Scaling and reliability 13 6.1 Fault resilience 13 6.2 Resource consumption 21 6.3 Data collection models 24 7. Review of Accounting Protocols 30 7.1 RADIUS 30 7.2 TACACS+ 31 7.3 SNMP 31 8. Review of Accounting Data Transfer 40 8.1 SMTP 40 8.2 Other protocols 41 9. Summary 41 10. Acknowledgments 43 11. References 44 12. Authors' Addresses 48 13. Intellectual Property Statement 48 14. Full Copyright Statement 49 15. Expiration date 49 Aboba, Arkko & Harrington Informational [Page 2]
INTERNET-DRAFT Accounting Management 19 April 2000 5. Introduction The field of Accounting Management is concerned with the collection of resource consumption data for the purposes of capacity and trend analysis, cost allocation, auditing, and billing. This document describes each of these problems, and discusses the issues involved in design of modern accounting systems. Since accounting applications do not have uniform security and reliability requirements, it is not possible to devise a single accounting protocol and set of security services that will meet all needs. Thus the goal of accounting management is to provide a set of tools that can be used to meet the requirements of each application. This document describes the currently available tools as well as the state of the art in accounting protocol design. A companion document, draft-ietf-aaa-accounting-attributes-03.txt, reviews the state of the art in accounting attributes and record formats. 5.1. Terminology This document frequently uses the following terms: Accounting The collection of resource consumption data for the purposes of capacity and trend analysis, cost allocation, auditing, and billing. Accounting management requires that resource consumption be measured, rated, assigned, and communicated between appropriate parties. Archival accounting In archival accounting, the goal is to collect all accounting data, to reconstruct missing entries as best as possible in the event of data loss, and to archive data for a mandated time period. It is "usual and customary" for these systems to be engineered to be very robust against accounting data loss. Legal or financial requirements frequently mandate archival accounting practices, and may often dictate that data be kept confidential, regardless of whether it is to be used for billing purposes or not. Rating The act of determining the price to be charged for use of a resource. Billing The act of preparing an invoice. Usage sensitive billing A billing process that depends on usage information to prepare an invoice can be said to be usage-sensitive. In contrast, a Aboba, Arkko & Harrington Informational [Page 3]
INTERNET-DRAFT Accounting Management 19 April 2000 process that is independent of usage information is said to be non-usage-sensitive. Auditing The act of verifying the correctness of a procedure. In order to be able to conduct an audit it is necessary to be able to definitively determine what procedures were actually carried out so as to be able to compare this to the recommended process. Accomplishing this may require security services such as authentication and integrity protection. Cost Allocation The act of allocating costs between entities. Note that cost allocation and rating are fundamentally different processes. In cost allocation the objective is typically to allocate a known cost among several entities. In rating the objective is to determine the amount to be charged for use of a resource. In cost allocation, the cost per unit of resource may need to be determined; in rating, this is typically a given. Interim accounting An interim accounting packet provides a snapshot of usage during a user's session. This may be useful in the event of a device reboot or other network problem that prevents the reception or generation of a session summary packet or session record. Interim accounting packets can always be summarized without the loss of information. Session record A session record represents a summary of the resource consumption of a user over the entire session. Accounting gateways creating the session record may do so by processing interim accounting events or accounting events from several devices serving the same user. Accounting Protocol A protocol used to convey data for accounting purposes. Intra-domain accounting Intra-domain accounting involves the collection of information on resource usage within an administrative domain, for use within that domain. In intra-domain accounting, accounting packets and session records typically do not cross administrative boundaries. Inter-domain accounting Inter-domain accounting involves the collection of information on resource usage within an administrative domain, for use within another administrative domain. In inter-domain Aboba, Arkko & Harrington Informational [Page 4]
INTERNET-DRAFT Accounting Management 19 April 2000 accounting, accounting packets and session records will typically cross administrative boundaries. Real-time accounting Real-time accounting involves the processing of information on resource usage within a defined time window. Time constraints are typically imposed in order to limit financial risk. Accounting server The accounting server receives accounting data from devices and translates it into session records. The accounting server may also take responsibility for the routing of session records to interested parties. 5.2. Accounting management architecture The accounting management architecture involves interactions between network devices, accounting servers, and billing servers. The network device collects resource consumption data in the form of accounting metrics. This information is then transferred to an accounting server. Typically this is accomplished via an accounting protocol, although it is also possible for devices to generate their own session records. The accounting server then processes the accounting data received from the network device. This processing may include summarization of interim accounting information, elimination of duplicate data, or generation of session records. The processed accounting data is then submitted to a billing server, which typically handles rating and invoice generation, but may also carry out auditing, cost allocation, trend analysis or capacity planning functions. Session records may be batched and compressed by the accounting server prior to submission to the billing server in order to reduce the volume of accounting data and the bandwidth required to accomplish the transfer. One of the functions of the accounting server is to distinguish between inter and intra-domain accounting events and to route them appropriately. Intra-domain accounting events are typically routed to the local billing server, while inter-domain accounting events will be routed to accounting servers operating within other administrative domains. While it is not required that session record formats used in inter and intra-domain accounting be the same, this is desirable, since it eliminates translations that would otherwise be required. Aboba, Arkko & Harrington Informational [Page 5]
INTERNET-DRAFT Accounting Management 19 April 2000 The diagram below illustrates the accounting management architecture: +------------+ | | | Network | | Device | | | +------------+ | Accounting | Protocol | | V +------------+ +------------+ | | | | | Org B | Inter-domain | Org A | | Acctg. |<----------------------------->| Acctg. | | Server | session records | Server | | | | | +------------+ +------------+ | | | Intra-domain | Transfer | session records | Protocol | | | | V V +------------+ +------------+ | | | | | Org B | | Org A | | Billing | | Billing | | Server | | Server | | | | | +------------+ +------------+ 5.3. Accounting management objectives Accounting Management involves the collection of resource consumption data for the purposes of capacity and trend analysis, cost allocation, auditing, billing. Each of these tasks has different requirements. 5.3.1. Trend analysis and capacity planning In trend analysis and capacity planning, the goal is typically a forecast of future usage. Since such forecasts are inherently imperfect, high reliability is typically not required, and moderate packet loss may be tolerable. Aboba, Arkko & Harrington Informational [Page 6]
INTERNET-DRAFT Accounting Management 19 April 2000 In certain cases, it may be desirable to use statistical sampling techniques to reduce data collection requirements while still providing the forecast with the desired statistical accuracy. Such a sampling process may tolerate high packet loss as long as bias is not introduced. The security requirements for trend analysis and capacity planning depend on the circumstances of data collection and the sensitivity of the data. Additional security services may be required when data is being transferred between administrative domains. For example, when information is being collected and analyzed within the same administrative domain, integrity protection and authentication may be used in order to guard against collection of invalid data. In inter- domain applications confidentiality may be desirable to guard against snooping by third parties. 5.3.2. Billing When accounting data is used for billing purposes, the requirements depend on whether the billing process is usage-sensitive or not. 5.3.2.1. Non-usage sensitive billing Since by definition, non-usage-sensitive billing does not require usage information, in theory all accounting data can be lost without affecting the billing process. Of course this would also affect other tasks such as trend analysis or auditing, so that such wholesale data loss would still be unacceptable. 5.3.2.2. Usage-sensitive billing Since usage-sensitive billing processes depend on usage information, packet loss may translate directly to revenue loss. As a result, the billing process may need to meet requirements arising from financial reporting standards, or legal requirements, and therefore an archival accounting approach may be required. Usage-sensitive systems may also have additional requirements relating to processing delay. Today credit risk is commonly managed by computerized fraud detection systems that are designed to detect unusual activity. While efficiency concerns might otherwise dictate batched transmission of accounting data, where it is desirable to minimize financial risk, a different approach may be required. Since financial exposure increases with processing delay, it may be necessary to transmit each event individually or to minimize batch size, to require application layer acknowledgment before providing service, or even to utilize quality of service techniques to minimize queuing delays. Aboba, Arkko & Harrington Informational [Page 7]
INTERNET-DRAFT Accounting Management 19 April 2000 The degree of financial exposure is application-dependent. For dial-up Internet access from a local provider, charges are typically low and therefore the risk of loss is small. However, in the case of dial-up roaming or voice over IP, time-based charges may be substantial and therefore the risk of fraud is larger. In such situations it is highly desirable to quickly detect unusual account activity, and it may be desirable for authorization to depend on ability to pay. In situations where valuable resources can be reserved, or where charges can be high, very large bills may be rung up quickly, and processing may need to be completed within a defined time window in order to limit exposure. Since in usage-sensitive systems, accounting data translates into revenue, the security and reliability requirements are greater. Due to financial and legal requirements such systems need to be able to survive an audit. Thus security services such as authentication, integrity and replay protection are frequently required and confidentiality and data object integrity may also be desirable. 5.3.3. Auditing With enterprise networking expenditures on the rise, interest in auditing is increasing. Auditing, which is the act of verifying the correctness of a procedure, commonly relies on accounting data. Auditing tasks include verifying the correctness of an invoice submitted by a service provider, or verifying conformance to usage policy, service level agreements, or security guidelines. To permit a credible audit, the auditing data collection process must be at least as reliable as the accounting process being used by the entity that is being audited. Similarly, security policies for the audit should be at least as stringent as those used in preparation of the original invoice. Due to financial and legal requirements, archival accounting practices are frequently required in this application. Where auditing procedures are used to verify conformance to usage or security policies, security services may be desired. This typically will include authentication, integrity and replay protection as well as confidentiality and data object integrity. In order to permit response to security incidents in progress, auditing applications frequently are built to operate with low processing delay. 5.3.4. Cost allocation The application of cost allocation and billback methods by enterprise customers is not yet widespread. However, with the convergence of telephony and data communications, there is increasing interest in applying cost allocation and billback procedures to networking costs, as is now commonly practiced with telecommunications costs. Aboba, Arkko & Harrington Informational [Page 8]
INTERNET-DRAFT Accounting Management 19 April 2000 Cost allocation models, including traditional costing mechanisms described in [21]-[23] and activity-based costing techniques described in [24] are typically based on detailed analysis of usage data, and as a result they are almost always usage-sensitive. Whether these techniques are applied to allocation of costs between partners in a venture or to allocation of costs between departments in a single firm, cost allocation models often have profound behavioral and financial impacts. As a result, systems developed for this purposes are typically as concerned with reliable data collection and security as are billing applications. Due to financial and legal requirements, archival accounting practices are frequently required in this application. 5.4. Intra-domain and inter-domain accounting Much of the initial work on accounting management has focused on intra- domain accounting applications. However, with the increasing deployment of services such as dial-up roaming, Internet fax, Voice and Video over IP and QoS, applications requiring inter-domain accounting are becoming increasingly common. Inter-domain accounting differs from intra-domain accounting in several important ways. Intra-domain accounting involves the collection of information on resource consumption within an administrative domain, for use within that domain. In intra-domain accounting, accounting packets and session records typically do not cross administrative boundaries. As a result, intra-domain accounting applications typically experience low packet loss and involve transfer of data between trusted entities. In contrast, inter-domain accounting involves the collection of information on resource consumption within an administrative domain, for use within another administrative domain. In inter-domain accounting, accounting packets and session records will typically cross administrative boundaries. As a result, inter-domain accounting applications may experience substantial packet loss. In addition, the entities involved in the transfers cannot be assumed to trust each other. Since inter-domain accounting applications involve transfers of accounting data between domains, additional security measures may be desirable. In addition to authentication, replay and integrity protection, it may be desirable to deploy security services such as confidentiality and data object integrity. In inter-domain accounting each involved party also typically requires a copy of each accounting event for invoice generation and auditing. Aboba, Arkko & Harrington Informational [Page 9]
INTERNET-DRAFT Accounting Management 19 April 2000 5.5. Accounting record production Typically, a single accounting record is produced per session, or in some cases, a set of interim records which can be summarized in a single record for billing purposes. However, to support deployment of services such as wireless access or complex billing regimes, a more sophisticated approach is required. It is necessary to generate several accounting records from a single session when pricing changes during a session. For instance, the price of a service can be higher during peak hours than off-peak. For a session continuing from one tariff period to another, it becomes necessary for a device to report "packets sent" during both periods. Time is not the only factor requiring this approach. For instance, in mobile access networks the user may roam from one place to another while still being connected in the same session. If roaming causes a change in the tariffs, it is necessary to account for resource consumed in the first and second areas. Another example is where modifications are allowed to an ongoing session. For example, it is possible that a session could be re-authorized with improved QoS. This would require production of accounting records at both QoS levels. These examples could be addressed by using vectors or multi-dimensional arrays to represent resource consumption within a single session record. For example, the vector or array could describe the resource consumption for each combination of factors, e.g. one data item could be the number of packets during peak hour in the area of the home operator. However, such an approach seems complicated and inflexible and as a result, most current systems produce a set of records from one session. A session identifier needs to be present in the records to permit accounting systems to tie the records together. In most cases, the network device will determine when multiple session records are needed, as the local device is aware of factors affecting local tariffs, such as QoS changes and roaming. However, future systems are being designed that enable the home domain to control the generation of accounting records. This is of importance in inter-domain accounting or when network devices do not have tariff information. The centralized control of accounting record production can be realized, for instance, by having authorization servers require re-authorization at certain times and requiring the production of accounting records upon each re- authorization. In conclusion, in some cases it is necessary to produce multiple accounting records from a single session. It must be possible to do this without requiring the user to start a new session or to re- authenticate. The production of multiple records can be controlled Aboba, Arkko & Harrington Informational [Page 10]
INTERNET-DRAFT Accounting Management 19 April 2000 either by the network device or by the AAA server. The requirements for timeliness, security and reliability in multiple record sessions are the same as for single-record sessions. Aboba, Arkko & Harrington Informational [Page 11]
INTERNET-DRAFT Accounting Management 19 April 2000 5.6. Requirements summary +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | Usage | Intra-domain | Inter-domain | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | Robustness vs. | Robustness vs. | | | packet loss | packet loss | | Capacity | | | | Planning | Integrity, | Integrity, | | | authentication, | authentication, | | | replay protection | replay prot. | | | [confidentiality] | confidentiality | | | | [data object sec.]| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Non-usage | Integrity, | Integrity, | | Sensitive | authentication, | authentication, | | Billing | replay protection | replay protection | | | [confidentiality] | confidentiality | | | | [data object sec.]| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | Archival | Archival | | Usage | accounting | accounting | | Sensitive | Integrity, | Integrity, | | Billing, | authentication, | authentication, | | Cost | replay protection | replay prot. | | Allocation & | [confidentiality] | confidentiality | | Auditing | [Bounds on | [data object sec.]| | | processing delay] | [Bounds on | | | | processing delay] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | Archival | Archival | | Time | accounting | accounting | | Sensitive | Integrity, | Integrity, | | Billing, | authentication, | authentication, | | fraud | replay protection | replay prot. | | detection, | [confidentiality] | confidentiality | | roaming | | [Data object | | | Bounds on | security and | | | processing delay | receipt support] | | | | Bounds on | | | | processing delay | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Key [] = optional Aboba, Arkko & Harrington Informational [Page 12]
INTERNET-DRAFT Accounting Management 19 April 2000 6. Scaling and reliability With the continuing growth of the Internet, it is important that accounting management systems be scalable and reliable. This section discusses the resources consumed by accounting management systems as well as the scalability and reliability properties exhibited by various data collection and transport models. 6.1. Fault resilience As noted earlier, in applications such as usage-sensitive billing, cost allocation and auditing, an archival approach to accounting is frequently mandated, due to financial and legal requirements. Since in such situations loss of accounting data can translate to revenue loss, there is incentive to engineer a high degree of fault resilience. Faults which may be encountered include: Packet loss Accounting server failures Network failures Device reboots To date, much of the debate on accounting reliability has focused on resilience against packet loss and the differences between UDP and TCP- based transport. However, it should be understood that resilience against packet loss is only one aspect of meeting archival accounting requirements. As noted in [43], "once the cable is cut you don't need more retransmissions, you need a *lot* more voltage." Thus, the choice of UDP or TCP transport has no impact on resilience against faults such as network partition, accounting server failures or device reboots. What does provide resilience against these faults is non-volatile storage. The importance of non-volatile storage in design of reliable accounting systems cannot be over-emphasized. Without such storage, event-driven systems will lose data once the transmission timeout has been exceeded, and batching designs will experience data loss once the internal memory used for accounting data storage has been exceeded. Via use of non- volatile storage, and internally stored interim records, most of these data losses can be avoided. It may even be argued that non-volatile storage is more important to accounting reliability than network connectivity, since for many years reliable accounting systems were implemented based solely on physical storage, without any network connectivity. For example, phone usage data used to be stored on paper, film, or magnetic media and carried from the place of collection to a central location for bill processing. Aboba, Arkko & Harrington Informational [Page 13]
INTERNET-DRAFT Accounting Management 19 April 2000 6.1.1. Interim accounting Interim accounting provides protection against loss of session summary data by providing checkpoint information that can be used to reconstruct the session record in the event that the session summary information is lost. This technique may be applied to any data collection model (i.e. event-driven or polling) and is supported in both RADIUS [25] and in TACACS+ [26]. While interim accounting can provide resilience against packet loss, server failures, short-duration network failures, or device reboot, its applicability is limited. Interim accounting should not be thought of as a mainstream reliability improvement technique since it increases use of network bandwidth in normal operation, while providing benefits only in the event of a fault. Since most packet loss on the Internet is due to congestion, sending interim accounting data over the wire can make the problem worse by increasing bandwidth usage. Therefore on-the-wire interim accounting is best restricted to high-value accounting data such as information on long-lived sessions. To protect against loss of data on such sessions, the interim reporting interval is typically set several standard deviations larger than the average session duration. This ensures that most sessions will not result in generation of interim accounting events and the additional bandwidth consumed by interim accounting will be limited. However, as the interim accounting interval decreases toward the average session time, the additional bandwidth consumed by interim accounting increases markedly, and as a result, the interval must be set with caution. Where non-volatile storage is unavailable, interim accounting can also result in excessive consumption of memory that could be better allocated to storage of session data. As a result, implementors should be careful to ensure that new interim accounting data overwrites previous data rather than accumulating additional interim records in memory, thereby worsening the buffer exhaustion problem. Given the increasing popularity of non-volatile storage for use in consumer devices such as digital cameras, such devices are rapidly declining in price. This makes it increasingly feasible for network devices to include built-in support for non-volatile storage. This can be accomplished, for example, by support for compact PCMCIA cards. Where non-volatile storage is available, this can be used to store interim accounting data. Stored interim events are then replaced by updated interim events or by session data when the session completes. The session data can itself be erased once the data has been transmitted and acknowledged at the application layer. This approach avoids interim Aboba, Arkko & Harrington Informational [Page 14]
INTERNET-DRAFT Accounting Management 19 April 2000 data being transmitted over the wire except in the case of a device reboot. When a device reboots, internally stored interim records are transferred to the accounting server. 6.1.2. Multiple record sessions Generation of multiple accounting records within a session can introduce scalability problems that cannot be controlled using the techniques available in interim accounting. For example, in the case of interim records kept in non-volatile storage, it is possible to overwrite previous interim records with the most recent one or summarize them to a session record. Where interim updates are sent over the wire, it is possible to control bandwidth usage by adjusting the interim accounting interval. These measures are not applicable where multiple session records are produced from a single session, since these records cannot be summarized or overwritten without loss of information. As a result, multiple record production can result in increased consumption of bandwidth and memory. Implementors should be careful to ensure that worst-case multiple record processing requirements do not exceed the capabilities of their systems. As an example, a tariff change at a particular time of day could, if implemented carelessly, create a sudden peak in the consumption of memory and bandwidth as the records need to be stored and/or transported. Rather than attempting to send all of the records at once, it may be desirable to keep them in non-volatile storage and send all of the related records together in a batch when the session completes. It may also be desirable to shape the accounting traffic flow so as to reduce the peak bandwidth consumption. This can be accomplished by introduction of a randomized delay interval. If the home domain can also control the generation of multiple accounting records, the estimation of the worst-case processing requirements can be very difficult. 6.1.3. Packet loss As packet loss is a fact of life on the Internet, accounting protocols dealing with session data need to be resilient against packet loss. This is particularly important in inter-domain accounting, where packets often pass through Network Access Points (NAPs) where packet loss may be substantial. Resilience against packet loss can be accomplished via implementation of a retry mechanism on top of UDP, or use of TCP or SCTP [55]. On-the-wire interim accounting provides only limited benefits in mitigating the effects of packet loss. Aboba, Arkko & Harrington Informational [Page 15]
INTERNET-DRAFT Accounting Management 19 April 2000 UDP-based transport is frequently used in accounting applications. However, this is not appropriate in all cases. Where accounting data will not fit within a single UDP packet without fragmentation, use of TCP or SCTP transport may be preferred to use of multiple round-trips in UDP. As noted in [47] and [49], this may be an issue in the retrieval of large tables. In addition, in cases where congestion is likely, such as in inter- domain accounting, TCP or SCTP congestion control and round-trip time estimation will be very useful, optimizing throughput. In applications which require maintenance of session state, such as simultaneous usage control, TCP and application-layer keep alive packets or SCTP with its built-in heartbeat capabilities provide a mechanism for keeping track of session state. When implementing UDP retransmission, there are a number of issues to keep in mind: Data model Retry behavior Congestion control Timeout behavior Accounting reliability can be influenced by how the data is modeled. For example, it is almost always preferable to use cumulative variables rather than expressing accounting data in terms of a change from a previous data item. With cumulative data, the current state can be recovered by a successful retrieval, even after many packets have been lost. However, if the data is transmitted as a change then the state will not be recovered until the next cumulative update is sent. Thus, such implementations are much more vulnerable to packet loss, and should be avoided wherever possible. In designing a UDP retry mechanism, it is important that the retry timers relate to the round-trip time, so that retransmissions will not typically occur within the period in which acknowledgments may be expected to arrive. Accounting bandwidth may be significant in some circumstances, so that the added traffic due to unnecessary retransmissions may increase congestion levels. Congestion control in accounting data transfer is a somewhat controversial issue. Since accounting traffic is often considered mission-critical, it has been argued that congestion control is not a requirement; better to let other less-critical traffic back off in response to congestion. Moreover, without non-volatile storage, congestive back-off in accounting applications can result in data loss due to buffer exhaustion. Aboba, Arkko & Harrington Informational [Page 16]
INTERNET-DRAFT Accounting Management 19 April 2000 However, there are very persuasive arguments that in modern accounting implementations, it is possible to implement congestion control while improving throughput and maintaining high reliability. In circumstances where there is sustained packet loss, there simply is not sufficient capacity to maintain existing transmission rates. Thus, aggregate throughput will actually improve if congestive back-off is implemented. This is due to elimination of retransmissions and ability to utilize techniques such as RED to desynchronize flows. In addition, with QoS mechanisms such as differentiated services, it is possible to mark accounting packets for preferential handling so as to provide for lower packet loss if desired. Thus considerable leeway is available to the network administrator in controlling the treatment of accounting packets and hard coding inelastic behavior is unnecessary. Typically, systems implementing non-volatile storage allow for backlogged accounting data to be placed in non-volatile storage pending transmission, so that buffer exhaustion resulting from congestive back- off need not be a concern. Since UDP is not really a transport protocol, UDP-based accounting protocols such as [4] often do not prescribe timeout behavior. Thus implementations may exhibit widely different behavior. For example, one implementation may drop accounting data after three constant duration retries to the same server, while another may implement exponential back-off to a given server, then switch to another server, up to a total timeout interval of twelve hours, while storing the untransmitted data on non-volatile storage. The practical difference between these approaches is substantial; the former approach will not satisfy archival accounting requirements while the latter may. More predictable behavior can be achieved via use of SCTP or TCP transport. 6.1.4. Accounting server faults and failover In the event of a failure of the primary accounting server, it is desirable for the device to failover to a secondary server. Providing one or more secondary servers can remove much of the risk of accounting server failure, and as a result use of secondary servers has become commonplace. For protocols based on TCP, it is possible for the device to maintain connections to both the primary and secondary accounting servers, using the secondary connection after expiration of a timer on the primary connection. Alternatively, it is possible to open a connection to the secondary accounting server after a timeout or loss of the primary connection, or on expiration of a timer. Thus, accounting protocols based on TCP are capable of responding more rapidly to connectivity failures than TCP timeouts would otherwise allow, at the expense of an increased risk of duplicates. Aboba, Arkko & Harrington Informational [Page 17]
INTERNET-DRAFT Accounting Management 19 April 2000 With SCTP, it is possible to control transport layer timeout behavior, and therefore it is not necessary for the accounting application to maintain its own timers. However, since SCTP is not widely available, use of this transport can impose an additional implementation burden on the designer. For protocols using UDP, transmission to the secondary server can occur after a number of retries or timer expiration. For compatibility with congestion avoidance, it is advisable to incorporate techniques such as round-trip-time estimation, slow start and congestive back-off. Thus the accounting protocol designer utilizing UDP often is lead to re- inventing techniques already existing in TCP and SCTP. As a result, the use of raw UDP transport in accounting applications is not recommended. With any transport it is possible for the primary and secondary accounting servers to receive duplicate packets, so support for duplicate elimination is required. Since accounting server failures can result in data accumulation on accounting clients, use of non-volatile storage can ensure against data loss due to transmission timeouts or buffer exhaustion. On-the-wire interim accounting provides only limited benefits in mitigating the effects of accounting server failures. It is also possible for the accounting server to experience partial failures. For example, a failure in the database back end could leave the accounting retrieval process or thread operable while the process or thread responsible for storing the data is non-functional. Similarly, it is possible for the accounting application to run out of disk space, making it unable to continue storing incoming session records. In such cases it is desirable to distinguish between transport layer acknowledgment and application layer acknowledgment. Even though both acknowledgments may be sent within the same packet (such as a TCP segment carrying an application layer acknowledgment along with a piggy- backed ACK), the semantics are different. A transport-layer acknowledgment means "I have received the data" while an application layer acknowledgment means "I have accepted responsibility for the data". The former typically implies only that the data has been received and stored in a buffer, while the latter implies that the data has been written to stable storage or suitably processed so as to guard against loss. In the case of partial failures, it is possible for the accounting server to acknowledge receipt via transport layer acknowledgment, without being capable of completing the tasks necessary to take responsibility for the data, as required for an application layer acknowledgment. For example, an accounting server incapable of storing received data due to a back end database or disk space problem should not send an application layer acknowledgment, even though the data was Aboba, Arkko & Harrington Informational [Page 18]
INTERNET-DRAFT Accounting Management 19 April 2000 received, so that a transport layer acknowledgment is appropriate. In such cases, it is useful to be able to send an application layer error message such as "Backend store unavailable" so to make it clear that the accounting server is malfunctioning. 6.1.5. Network failures Network failures may result in partial or complete loss of connectivity for the accounting client. In the event of partial connectivity loss, it may not be possible to reach the primary accounting server, in which case switch over to the secondary accounting server is necessary. In the event of a network partition, it may be necessary to store accounting events in device memory or non-volatile storage until connectivity can be re-established. As with accounting server failures, on-the-wire interim accounting provides only limited benefits in mitigating the effects of network failures. 6.1.6. Device reboots In the event of a device reboot, it is desirable to minimize the loss of data on sessions in progress. Such losses may be significant even if the devices themselves are very reliable, due to long-lived sessions, which can comprise a significant fraction of total resource consumption. Sending interim accounting data over the wire is typically implemented to guard against loss of these high-value sessions. When interim accounting is combined with non-volatile storage it becomes possible to guard against data loss in much shorter sessions. This is possible since interim accounting data need only be stored in non-volatile memory until the session completes, at which time the interim data may be replaced by the session record. As a result, interim accounting data need never be sent over the wire, and it is possible to decrease the interim interval so as to provide a very high degree of protection against data loss. 6.1.7. Accounting proxies In order to maintain high reliability, it is important that accounting proxies pass through transport and application layer acknowledgments and do not store and forward accounting packets. This enables the end- systems to control re-transmission behavior and utilize techniques such as non-volatile storage and secondary servers to improve resilience. Accounting proxies sending a transport or application layer ACK to the device without receiving one from the accounting server fool the device into thinking that the accounting request had been accepted by the accounting server when this is not the case. As a result, the device can delete the accounting packet from non-volatile storage before it has Aboba, Arkko & Harrington Informational [Page 19]
INTERNET-DRAFT Accounting Management 19 April 2000 been accepted by the accounting server. The leaves the accounting proxy responsible for delivering accounting packets. If the accounting proxy involves moving parts (e.g. a disk drive) while the devices do not, overall system reliability can be reduced. Store and forward accounting proxies only add value in situations where the accounting subsystem is unreliable. For example, where devices do not implement non-volatile storage and the accounting protocol lacks transport and application layer reliability, locating the accounting proxy (with its stable storage) close to the device can reduce the risk of data loss. However, such systems are inherently unreliable so that they can only handle capacity planning or non-usage sensitive billing applications. If archival accounting reliability is desired, it is necessary to engineer a reliable accounting system from the start using the techniques described in this document, rather than attempting to repair an inherently unreliable system by adding store and forward accounting proxies. Aboba, Arkko & Harrington Informational [Page 20]
INTERNET-DRAFT Accounting Management 19 April 2000 6.1.8. Fault resilience summary +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | Fault | Counter-measures | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | Packet | Retransmission based on RTT | | loss | Congestion control | | | Well-defined timeout behavior | | | Duplicate elimination | | | Interim accounting* | | | Non-volatile storage | | | Cumulative variables | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | Accounting | Primary-secondary servers | | server & net | Duplicate elimination | | failures | Interim accounting* | | | Application layer ACK & error msgs. | | | Non-volatile storage | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | Device | Interim accounting* | | reboots | Non-volatile storage | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Key * = limited usefulness without non-volatile storage Note: Accounting proxies are not a reliability enhancement mechanism. 6.2. Resource consumption In the process of growing to meet the needs of providers and customers, accounting management systems consume a variety of resources, including: Network bandwidth Memory Non-volatile storage State on the accounting management system CPU on the management system and managed devices Aboba, Arkko & Harrington Informational [Page 21]
INTERNET-DRAFT Accounting Management 19 April 2000 In order to understand the limits to scaling of accounting management systems, we examine each of these resources in turn. 6.2.1. Network bandwidth Accounting management systems consume network bandwidth in the transferring of accounting data. The network bandwidth consumed is proportional to the amount of data transferred, as well as required network overhead. Since accounting data for a given event may be 100 octets or less, if each event is transferred individually, overhead can represent a considerable proportion of total bandwidth consumption. As a result, it is often desirable to transfer accounting data in batches, enabling network overhead to be spread over a larger payload, and enabling efficient use of compression. As noted in [48], compression can be enabled in the accounting protocol, or can be done at the IP layer as described in [50]. 6.2.2. Memory In accounting systems without non-volatile storage, accounting data must be stored in volatile memory during the period between when it is generated and when it is transferred. The resulting memory consumption will depend on retry and retransmission algorithms. Since systems designed for high reliability will typically wish to retry for long periods, or may store interim accounting data, the resulting memory consumption can be considerable. As a result, if non-volatile storage is unavailable, it may be desirable to compress accounting data awaiting transmission. As noted earlier, implementors of interim accounting should take care to ensure against excessive memory usage by overwriting older interim accounting data with newer data for the same session rather than accumulating interim data in the buffer. 6.2.3. Non-volatile storage Since accounting data stored in memory will typically be lost in the event of a device reboot or a timeout, it may be desirable to provide non-volatile storage for undelivered accounting data. With the costs of non-volatile storage declining rapidly, network devices will be increasingly capable of incorporating non-volatile storage support over the next few years. As described in [11], non-volatile storage may be used to store interim or session records in a standard ASCII format. As with memory utilization, interim accounting overwrite is desirable so as to prevent excessive storage consumption. Note that the use of ASCII data representation enables use of highly efficient text compression Aboba, Arkko & Harrington Informational [Page 22]
INTERNET-DRAFT Accounting Management 19 April 2000 algorithms that can minimize storage requirements. Such compression algorithms are only typically applied to session records so as to enable implementation of interim data overwrite. 6.2.4. State on the accounting management system In order to keep track of received accounting data, accounting management systems may need to keep state on managed devices or concurrent sessions. Since the number of devices is typically much smaller than the number of concurrent sessions, it is desirable to keep only per-device state if possible. 6.2.5. CPU requirements CPU consumption of the managed and managing nodes will be proportional to the complexity of the required accounting processing. Operations such as ASN.1 encoding and decoding, compression/decompression, and encryption/decryption can consume considerable resources, both on accounting clients and servers. The effect of these operations on accounting system reliability should not be under-estimated, particularly in the case of devices with moderate CPU resources. In the event that devices are over-taxed by accounting tasks, it is likely that overall device reliability will suffer. Aboba, Arkko & Harrington Informational [Page 23]
INTERNET-DRAFT Accounting Management 19 April 2000 6.2.6. Efficiency measures +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | Resource | Efficiency measures | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | Network | Batching | | Bandwidth | Compression | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | Memory | Compression | | | Interim accounting overwrite | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | Non-volatile | Compression | | Storage | Interim accounting overwrite | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | System | Per-device state | | state | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | CPU | Hardware assisted | | requirements | compression/encryption | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 6.3. Data collection models Several data collection models are currently in use today for the purposes of accounting data collection. These include: Polling model Event-driven model without batching Event-driven model with batching Event-driven polling model Aboba, Arkko & Harrington Informational [Page 24]
INTERNET-DRAFT Accounting Management 19 April 2000 6.3.1. Polling model In the polling model, an accounting manager will poll devices for accounting information at regular intervals. In order to ensure against loss of data, the polling interval will need to be shorter than the maximum time that accounting data can be stored on the polled device. For devices without non-volatile stage, this is typically determined by available memory; for devices with non-volatile storage the maximum polling interval is determined by the size of non-volatile storage. The polling model results in an accumulation of data within individual devices, and as a result, data is typically transferred to the accounting manager in a batch, resulting in an efficient transfer process. In terms of Accounting Manager state, polling systems scale with the number of managed devices, and system bandwidth usage scales with the amount of data transferred. Without non-volatile storage, the polling model results in loss of accounting data due to device reboots, but not due to packet loss or network failures of sufficiently short duration to be handled within available memory. This is because the Accounting Manager will continue to poll until the data is received. In situations where operational difficulties are encountered, the volume of accounting data will frequently increase so as to make data loss more likely. However, in this case the polling model will detect the problem since attempts to reach the managed devices will fail. The polling model scales poorly for implementation of shared use or roaming services, including wireless data, Internet telephony, QoS provisioning or Internet access. This is because in order to retrieve accounting data for users within a given domain, the Accounting Management station would need to periodically poll all devices in all domains, most of which would not contain any relevant data. There are also issues with processing delay, since use of a polling interval also implies an average processing delay of half the polling interval, which may be too high for accounting data that requires low processing delay. Thus the event-driven polling or the pure event-driven approach is more appropriate for shared use or roaming implementations. Per-device state is typical of polling-based network management systems, which often also carry out accounting management functions, since network management systems need to keep track of the state of network devices for operational purposes. These systems offer average processing delays equal to half the polling interval. Aboba, Arkko & Harrington Informational [Page 25]
INTERNET-DRAFT Accounting Management 19 April 2000 6.3.2. Event-driven model without batching In the event-driven model, a device will contact the accounting server or manager when it is ready to transfer accounting data. Most event- driven accounting systems, such as those based on RADIUS accounting, described in [4], transfer only one accounting event per packet, which is inefficient. Without non-volatile storage, a pure event-driven model typically stores accounting events that have not yet been delivered only until the timeout interval expires. As a result this model has the smallest memory requirements. Once the timeout interval has expired, the accounting event is lost, even if the device has sufficient buffer space to continue to store it. As a result, the event-driven model is the least reliable, since accounting data loss will occur due to device reboots, sustained packet loss, or network failures of duration greater than the timeout interval. In event-driven protocols without a "keep alive" message, accounting servers cannot assume a device failure should no messages arrive for an extended period. Thus, event-driven accounting systems are typically not useful in monitoring of device health. The event-driven model is frequently used in shared use networks and roaming, since this model sends data to the recipient domains without requiring them to poll a large number of devices, most of which have no relevant data. Since the event-driven model typically does not support batching, it permits accounting records to be sent with low processing delay, enabling application of fraud prevention techniques. However, because roaming accounting events are frequently of high value, the poor reliability of this model is an issue. As a result, the event-driven polling model may be more appropriate. Per-session state is typical of event-driven systems without batching. As a result, the event-driven approach scales poorly. However, event- driven systems offer the lowest processing delay since events are processed immediately and there is no possibility of an event requiring low processing delay being caught behind a batch transfer. 6.3.3. Event-driven model with batching In the event-driven model with batching, a device will contact the accounting server or manager when it is ready to transfer accounting data. The device can contact the server when a batch of a given size has been gathered, when data of a certain type is available or after a minimum time period has elapsed. Such systems can transfer more than one accounting event per packet and are thus more efficient. An event-driven system with batching will store accounting events that have not yet been delivered up to the limits of memory. As a result, Aboba, Arkko & Harrington Informational [Page 26]
INTERNET-DRAFT Accounting Management 19 April 2000 accounting data loss will occur due to device reboots, but not due to packet loss or network failures of sufficiently short duration to be handled within available memory. Note that while transfer efficiency will increase with batch size, without non-volatile storage, the potential data loss from a device reboot will also increase. Where event-driven systems with batching have a keep-alive interval and run over reliable transport, the accounting server can assume that a failure has occurred if no messages are received within the keep-alive interval. Thus, such implementations can be useful in monitoring of device health. When used for this purpose the average time delay prior to failure detection is one half the keep-alive interval. Through implementation of a scheduling algorithm, event-driven systems with batching can deliver appropriate service to accounting events that require low processing delay. For example, high-value inter-domain accounting events could be sent immediately, thus enabling use of fraud- prevention techniques, while all other events would be batched. However, there is a possibility that an event requiring low processing delay will be caught behind a batch transfer in progress. Thus the maximum processing delay is proportional to the maximum batch size divided by the link speed. Event-driven systems with batching scale with the number of active devices. As a result this approach scales better than the pure event- driven approach, or even the polling approach, and is equivalent in terms of scaling to the event-driven polling approach. However, the event-driven batching approach has lower processing delay than the event-driven polling approach, since delivery of accounting data requires fewer round-trips and events requiring low processing delay can be accommodated if a scheduling algorithm is employed. 6.3.4. Event-driven polling model In the event-driven polling model an accounting manager will poll the device for accounting data only when it receives an event. The accounting client can generate an event when a batch of a given size has been gathered, when data of a certain type is available or after a minimum time period has elapsed. Note that while transfer efficiency will increase with batch size, without non-volatile storage, the potential data loss from a device reboot will also increase. Without non-volatile storage, an event-driven polling model will lose data due to device reboots, but not due to packet loss, or network partitions of short-duration. Unless a minimum delivery interval is set, event-driven polling systems are not useful in monitoring of device health. Aboba, Arkko & Harrington Informational [Page 27]
INTERNET-DRAFT Accounting Management 19 April 2000 The event-driven polling model can be suitable for use in roaming since it permits accounting data to be sent to the roaming partners with low processing delay. At the same time non-roaming accounting can be handled via more efficient polling techniques, thereby providing the best of both worlds. Where batching can be implemented, the state required in event-driven polling can be reduced to scale with the number of active devices. If portions of the network vary widely in usage, then this state may actually be less than that of the polling approach. Note that processing delay in this approach is higher than in event-driven accounting with batching since at least two round-trips are required to deliver data: one for the event notification, and one for the resulting poll. Aboba, Arkko & Harrington Informational [Page 28]
INTERNET-DRAFT Accounting Management 19 April 2000 6.3.5. Data collection summary +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | Model | Pros | Cons | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Polling | Per-device state | Not robust | | | Robust against | against device | | | packet loss | reboot, server | | | Batch transfers | or network | | | | failures* | | | | Polling interval | | | | determined by | | | | storage limit | | | | High processing | | | | delay | | | | Unsuitable for | | | | use in roaming | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Event-driven, | Lowest processing | Not robust | | no batching | delay | against packet | | | Suitable for | loss, device | | | use in roaming | reboot, or | | | | network | | | | failures* | | | | Low efficiency | | | | Per-session state | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Event-driven, | Single round-trip | Not robust | | with batching | latency | against device | | and | Batch transfers | reboot, network | | scheduling | Suitable for | failures* | | | use in roaming | | | | Per active device | | | | state | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Event-driven | Batch transfers | Not robust | | polling | Suitable for | against device | | | use in roaming | reboot, network | | | Per active device | failures* | | | state | Two round-trip | | | | latency | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Key * = addressed by non-volatile storage Aboba, Arkko & Harrington Informational [Page 29]
INTERNET-DRAFT Accounting Management 19 April 2000 7. Review of Accounting Protocols Accounting systems have been successfully implemented using protocols such as RADIUS, TACACS+, and SNMP. This section describes the characteristics of each of these protocols. 7.1. RADIUS RADIUS accounting, described in [4], was developed as an add-on to the RADIUS authentication protocol, described in [3]. As a result, RADIUS accounting shares the event-driven approach of RADIUS authentication, without support for batching or polling. As a result, RADIUS accounting scales with the number of accounting events instead of the number of devices, and accounting transfers are inefficient. Since RADIUS accounting is based on UDP and timeout and retry parameters are not specified, implementations vary widely in their approach to reliability, with some implementations retrying until delivery or buffer exhaustion, and others losing accounting data after a few retries. Since RADIUS accounting does not provide for Application-layer acknowledgments or error messages, a RADIUS Accounting-Response is equivalent to a transport-layer acknowledgment and provides no protection against application layer malfunctions. Due to the lack of reliability, it is not possible to do simultaneous usage control based on RADIUS accounting alone. Typically another device data source is required, such as polling of a session MIB or a command-line session over telnet. RADIUS accounting implementations are vulnerable to packet loss as well as application layer failures, network failures and device reboots. These deficiencies are magnified in inter-domain accounting as is required in roaming ([1],[2]). On the other hand, the event-driven approach of RADIUS accounting is useful where low processing delay is required, such as credit risk management or fraud detection. While RADIUS accounting does provide hop-by-hop authentication and integrity protection, and IPSEC can be employed to provide hop-by-hop confidentiality, data object security is not supported, and thus systems based on RADIUS accounting are not capable of being deployed with untrusted proxies, or in situations requiring auditability, as noted in [2]. While RADIUS does not support compression, IP compression, described in [50], can be employed to provide this. While in principle extensible with the definition of new attributes, RADIUS suffers from the very small standard attribute space (256 attributes). Aboba, Arkko & Harrington Informational [Page 30]
INTERNET-DRAFT Accounting Management 19 April 2000 7.2. TACACS+ TACACS+ as defined in [26] offers an accounting model with start, stop, and interim update messages. Since TACACS+ is based on TCP, implementations are typically resilient against packet loss and short- lived network partitions, and TACACS+ scales with the number of devices. Since TACACS+ runs over TCP, it offers support for both transport layer and application layer acknowledgments, and is suitable for simultaneous usage control and handling of accounting events that require moderate though not the lowest processing delay. TACACS+ provides for hop-by-hop authentication and integrity protection as well as hop-by-hop confidentiality. Data object security is not supported, and therefore systems based on TACACS+ accounting are not deployable in the presence of untrusted proxies. While TACACS+ does not support compression, IP compression, described in [50], can be employed to provide this. 7.3. SNMP SNMP, described in [27]-[41], has been widely deployed in a wide variety of intra-domain accounting applications, typically using the polling data collection model. Since polling allows data to be collected on multiple accounting events simultaneously, this model results in per- device state. Since the management agent is able to retry requests when a response is not received, such systems are resilient against packet loss or even short-lived network partitions. While implementations without non-volatile storage can only store accounting events up to the limits of their memory, and thus are not robust against device reboots or network failures, when combined with non-volatile storage, they can be made highly reliable. With SMIv2 it is possible to support confirmed notifications, so as to implement an event-driven polling model or even an event-driven batching model. However, we are not aware of any SNMP- based accounting implementations built on these models. 7.3.1. NMRG extensions As discussed in [49], there are a number of efficiency and latency issues that arise when using SNMP for accounting. In such applications it is often necessary for management stations to retrieve large tables. In such situations, the latency can be quite high, even with the getbulk operation. This is because the response must fit into the largest supported packet size, requiring multiple round-trips. Unless multiple threads are employed, the transfers will be serialized and the resulting latency will be a combination of multiple round-trip times, timeout and re-transmission delays and processing overhead, which may result in unacceptable performance. Since data may change during the course of multiple retrievals, it can be difficult to get a consistent snapshot. Aboba, Arkko & Harrington Informational [Page 31]
INTERNET-DRAFT Accounting Management 19 April 2000 In addition, [49] notes that SNMP is inefficient for transfer of accounting data, due to lack of compression, use of BER encoding, transmission of redundant OID prefixes, and the "get bulk overshoot" problem. As described in [54] the overshoot problem occurs when using the getbulk PDU because the manager typically does not know the number of rows in the table. As a result, it must either request too many rows, retrieving unneeded data, or too few, resulting in the need for multiple getbulk requests. To address these issues, a number of changes to SNMP are now under discussion within the IRTF Network Management Research Group (NMRG), described in [46]. These include an SNMP-over-TCP transport mapping, described in [47]; SNMP payload compression, described in [48]; and the addition of a "get subtree" PDU or the subtree retrieval MIB [54]. Taken together, we will refer to these changes as the "NMRG extensions." The SNMP-over-TCP transport mapping, particularly when combined with a subtree retrieval solution, results in substantial latency reductions in table retrieval. While it is possible for SNMP to operate in polling, event-driven, event-driven batching and event-driven polling modes, the latency reduction from the SNMP-over-TCP transport mapping manifests itself primarily in the polling, event-driven polling and event-driven batching modes. While use of TCP transport provides for transport layer acknowledgment, this still leaves SNMP without the equivalent of an application layer acknowledgment for confirmed notifications. Since the Response to an Inform-Request is typically sent by the master agent, not the subagent, there is no requirement that the Response wait until the accounting application has accepted responsibility for the data and thus the Response does not correspond to an application layer acknowledgment. Reference [49] also discusses file-based storage of SNMP data, as described in [43], and the FTP MIB, described in [44]. Together these MIBs enable storage of SNMP data in non-volatile storage, and subsequent transfer via SNMP. It is noted that this approach requires implementation of additional MIBs as well as FTP, and requires separate security mechanisms such as IPSEC to provide authentication, replay and integrity protection and confidentiality for the data in transit. However, the the file-based transfer approach also has an important benefit, which is compatibility with non-volatile storage. While an SNMP over TCP transport mapping is easily implemented, it does require SNMP agents to listen on TCP ports 161 and 162. Addition of a "get subtree" PDU implies changes to every agent that the management station will interact with. However, the subtree retrieval MIB described in [54] requires no changes to the SNMP protocol or SNMP protocol engine and thus can be implemented and deployed more easily. Aboba, Arkko & Harrington Informational [Page 32]
INTERNET-DRAFT Accounting Management 19 April 2000 7.3.2. SNMP v3 SNMPv3 includes support for security services as well as access controls. This section describes how this functionality may be applied in intra and inter-domain accounting. In inter-domain accounting, it is necessary to control access on a per- domain basis. In order to apply domain-based access controls, it is first necessary to identify the data subset that is to have its access controlled. Several conceptual abstractions are used for identifying subsets of data in SNMP v3. These include engines, contexts, and views. 7.3.2.1. Engines SNMP v3 supports the use of the contextEngineID field in order to identify the engine which provides access to the data. In traditional terms, this is the agent. contextEngineID support was added in order to improve handling of mobility as well as well as to improve SNMP proxy support. Use of contextEngineID enables improved mobility, allowing the agent on a laptop to be identified independently of the IP address where it is attached to the network. In SNMPv1 and v2, different endpoint addresses imply different agents. This is not the case with SNMPv3. contextEngineID also enables SNMP proxies to identify the data origin. While in SNMPv1 and v2, the data origin is automatically assumed to be the communications endpoint (SNMP agent), with SNMPv3 it is possible to distinguish the data origin from the communications endpoint. For example, let us assume that agent A sends a trap to manager M through agent B, who forwards it. When SNMPv3 is used, while the trap received by manager M will have a source address corresponding to agent B, contextEngineID will identify agent A as the data origin of the trap. Thus, by using contextEngineID, M can identify the data origin, no matter how many intermediate SNMP agents have forwarded it. With SNMPv1 and v2, M would need to assume that the data in the trap (from A) refers to the instrumentation of the agent at the last hop (B). Note that in SNMPv3 there is only a single contextEngineID per SNMP implementation. 7.3.2.2. Contexts Contexts are used to identify subsets of objects that are tied to instrumentation. These subsets are defined by the agent rather than the manager since if the manager defined them, the agent would not know how to tie the contexts to the underlying instrumentation. Aboba, Arkko & Harrington Informational [Page 33]
INTERNET-DRAFT Accounting Management 19 April 2000 In SNMPv3, contextName represents a slice of the data contained within a particular engine. Contexts are defined in a dynamic table, with the names defined as read-only. The agent uses the dynamic table to tell the manager what contexts it recognizes. Contexts are commonly tied to hardware components, to logical entities related to the hardware components, or to logical services. For example, contextNames might include board5, board7, repeater1, repeater2, etc. However, it is also possible for contextNames to correspond to domains. For example, for a shared use device, contextNames might include bigco.com, foo.com, etc. While each context may support multiple MIB modules, each contextName is limited to one instance of a particular MIB module. Thus, if multiple instances of a MIB module are required per engine, then unique contextNames must be defined. If it only makes sense to have one instance of a MIB module in an engine, such as the USM userTable, such a MIB will typically fall into the default context "". Note that while a MIB module may allow more than one instance per engine, a given SNMPv3 implementation may not support this. 7.3.2.3. Views A view is a mask for a particular contextName (subset of data). The view identifies which objects are visible, by specifying OIDs of the subtrees involved. There is also a mechanism to allow wildcards in the OID specification. For example, it is possible to define a view that includes the RMON tables, and another view that includes only the SNMPv3 security related tables. Using these views, it is possible to allow access to the RMON view for users Joe and Josephine (the RMON administrators), and access to the SNMPv3 security tables for user Adam (the security policy administrator). Views can be set up with wildcards. For a table that is indexed using IP addresses, Joe can be allowed access to all RMON rows that are in the subnet 134.141.x.x, with Josephine given access to all rows for subnet 134.200.x.x. However, for this to work the table must be indexed by the differentiating variable, since views filter at the OID level, not at the data level. It is therefore not possible to define a view that filters on the value of non-index data. In this example, were the IP address to have been used merely as a data item rather than an index, it would not be possible to utilize view-based access control to achieve the desired objective (delegation of administrative responsibility according to subnet). Aboba, Arkko & Harrington Informational [Page 34]
INTERNET-DRAFT Accounting Management 19 April 2000 7.3.2.4. SNMPv3 security services SNMPv1 and SNMPv2 do not incorporate security services. With SNMPv3, it is possible to incorporate view-based access controls, described in [40], as well as user-based security, described in [38]. As a result, SNMPv3-based accounting implementations can provide hop-by-hop authentication, integrity and replay protection, confidentiality and access-control. SNMPv3 does not support data-object security. Merely providing security for individual MIB variables is not sufficient. In order to prevent a cut and paste attack by an untrusted proxy, it is necessary to provide integrity protection covering enough of the packet (including other MIB variables) to protect against replay. A Kerberos Security Model (KSM) for SNMPv3 is described in [51]. This approach requires storage of a key on the KDC for each device and domain, while dynamically generating a session key for conversations between domains and devices. Thus, in terms of stored keys the KSM approach scales with the sum of devices and domains, whereas in terms of dynamic session keys, it scales as the product of domains and devices. As Kerberos is extended to allow initial authentication via public key, as described in [52], and cross-realm authentication, as described in [53], the KSM inherits these capabilities. As a result, this approach may have potential to reduce or even eliminate the shared secret management problem in the long-term. However, it should also be noted that certificate-based authentication can strain the limits of UDP packet sizes supported in SNMP implementations, so that the SNMP-over- TCP transport mapping may be required to support this. An IPSEC-based security model for SNMPv3 has also been discussed. Implementation of such a security model would require the SNMPv3 engine to be able to retrieve the properties of the IPSEC security association used to protect the SNMPv3 traffic. This would include the security services invoked, as well as information relating to the other endpoint such as the authentication method and presented identity and certificate. To date such APIs have not been widely implemented, and in addition, most IPSEC implementations only support machine certificates, which may not provide the required granularity of identification. Thus, an IPSEC-based security model for SNMPv3 will probably take several years to come to fruition. 7.3.3. Domain-based access control in SNMPv3 Domain-based access controls are required where multiple administrative domains are involved, such as in the shared use networks and roaming associations described in [1]. Since the same device may be accessed by multiple organizations, it is often necessary to control access to accounting data according to the user's organization. This ensures that Aboba, Arkko & Harrington Informational [Page 35]
INTERNET-DRAFT Accounting Management 19 April 2000 organizations may be given access to accounting data relating to their users, but not to data relating to users of other organizations. In order to be able to control access to accounting data on a per-domain basis, there are several alternatives. These include use of the domain as an index, engines, contexts and proxies. 7.3.3.1. Domain as index Through use of view-based access control [40], it is possible to define multiple fine-grained views of an SNMP MIB, and to assign views to specific groups of users, such that access rights to the included data elements will depend on the identity of the user making the request. For example, all users of bigco.com which are allowed access to the device would be defined in the User-based security MIB (or other security model MIB). For simplicity in administering access control, the users can be grouped using a vacmGroupName, e.g. bigco. A view of a subset of the data objects in the MIB can be defined in the vacmViewFamilyTreeTable. A vacmAccessTable pairs groups and views. For messages received from users in the bigco group, access would only be provided to the data permitted to be viewed by bigco users, as defined in the view family tree. This requires that each domain accessing the data be given one or more separate vacmGroupNames, an appropriate ViewTable be defined, and the vacmAccessTable be configured for each group. As the number of network devices within the shared use or roaming network grows, the polling model of data collection becomes increasingly impractical since most devices will not carry data relating to the polling organization. As a result, shared-use networks or roaming associations relying on SNMP-based accounting have generally collected data for all organizations and then sorted the resulting session records for delivery to each organization. While functional, this approach will typically result in increased processing delay as the number of organizations and data records grows. This issue can be addressed in SNMPv3 through use of view-based access control and the SNMP notification tables, using the event-driven, event- driven polling or event-driven batching approaches. This permits SNMPv3-enabled devices to notify domains that have accounting data awaiting collection. However, since views filter at the OID level, not at the data level, when using views to filter by domain it is necessary to use the domain as an index. For example, a table of session data could be indexed by record number and domain, allowing a view to be defined that could restrict access to domain data to the administrators of that domain. For example, user bigco could be allowed to view data relating to users Aboba, Arkko & Harrington Informational [Page 36]
INTERNET-DRAFT Accounting Management 19 April 2000 within the bigco.com domain, but user smallco would not be allowed access to this view. An advantage of using domains as an index is that this technique can be used with SNMPv1 and v2 agents as well as with v3 agents. A disadvantage is that the MIBs must be specifically designed for this purpose. Since existing MIBs rarely use the domain as an index, domain separation cannot be enabled within legacy MIBs using this technique. 7.3.3.2. Engines Another approach is to use contextEngineID to differentiate between data within individual domains. This approach would only be feasible for use with SNMPv3, where contextEngineID is supported. Since this technique can work with existing MIBs it enables domain separation to be applied to MIBs not specifically designed with domain separation in mind. One way this can be implemented is to provide multiple SNMP agents on the same system, one for each domain, differentiated by contextEngineID. However, this approach is not very scalable, particularly if there are a large number of domains involved, since it would require multiple agent implementations each with their own separate data space. An alternative is to allow one engine to be known by multiple contextEngineIDs. This would require that an engine be built where the engineID MIB module is multiply-instanced with different engineID values, in different contexts. However, this was not the intent of the original authors of SNMP v3, who assumed only a single contextEngineID per agent. As a result, doing this may introduce complications. For example, the MIB module that contains the contextEngineID may explicitly require only one instance per engine. 7.3.3.3. Contexts Still another approach is to use contextName to differentiate between data within individual domains. This approach would only be feasible for use with SNMPv3, where contextName is supported. Since this technique can work with existing MIBs it enables domain separation to be applied to MIBs not specifically designed with domain separation in mind. Since views filter at the OID level, not the data level, it is not possible to use VACM to handle the domain separation. Instead, the domain separation needs to be handled at the sub-agent level. For example, if the contextName is made available to the sub-agent, then it can only expose those elements of a table that correspond to the given context. For example, when a getbulk request is made with a contextName of bigco.com, then only table entries for bigco.com users would be visible. For this to work, it is necessary that the MIB not require that Aboba, Arkko & Harrington Informational [Page 37]
INTERNET-DRAFT Accounting Management 19 April 2000 index entries be consecutive, so that table rows may be ommitted as needed. In order to implement the required access control within the sub-agent, this approach requires that the sub-agent APIs pass the contextName to the sub-agent. Since SNMPv2 does not support contextName, sub-agents originally created for SNMPv2 cannot take advantage of this and therefore would need to be rewritten even though the MIB might not require modification. 7.3.3.4. Proxies Another approach is to support domain separation via use of a proxy. However, the proxy application is forbidden to provide access control at the varbind level, and is designed so that the proxy does not need to look inside the PDU of the message except to determine the contextEngineID to verify it is not destined to itself. If contextEngine == securityEngine, with other qualifications, then the message is being sent to the current engine, so it is processed locally rather than being sent to the proxy forwarder. Restrictions on use of proxies to provide access control at the varbind level also affect the ability to provide support for legacy devices. If legacy devices do not support view-based access control, then the proxy will not be able to provide this capability. Issues of legacy support also exist with the NMRG extensions. A proxy receiving a "get subtree" PDU going to a non-NMRG capable device would need to translate the "get subtree" PDU into multiple getnext or getbulk requests. This issue would also exist with the subtree retrieval MIB described in [54], since unless the legacy devices also support the subtree retrieval MIB, the proxy would encounter the "getbulk overshoot" problem. Similarly, unless a device supports the SNMP-over-TCP transport mapping, deployment of an NMRG-capable proxy will not provide much benefit, since the proxy will need to fall back to UDP-based getnext or getbulk operations. This will result in multiple round-trips and high latency and in addition the risk of inconsistent tables would remain. Existing proxies are built to support only the current standard operations so that new proxy code would be needed to support these NMRG extensions. Where the product of the number of domains and devices is large, such as in inter-domain accounting applications, the number of shared secrets can get out of hand. The localized key capability in the SNMPv3 USM allows a manager to have one central key, sharing it with many agents in a localized way while preventing the agents from getting at each other's data. This can assist in cross-domain security if deployed properly. Aboba, Arkko & Harrington Informational [Page 38]
INTERNET-DRAFT Accounting Management 19 April 2000 Another solution is to implement a proxy for the purposes of shared secret reduction. In such a scheme, the domains will share a secret with the proxy, and the proxy will share a secret with each of the devices. Thus the number of shared secrets will scale with the sum of the number of devices and domains rather than the product. 7.3.4. SNMP summary Given the wealth of existing accounting-related MIBs, it is likely that SNMP will remain a popular accounting protocol for the foreseeable future. Given the SNMPv3 enhancements, it is desirable for SNMP-based intra-domain accounting implementations to upgrade to SNMPv3. Such an upgrade is virtually mandatory for inter-domain applications. With SNMPv3, it is now possible to provide hop-by-hop security services. Through use of view-based access control, it is possible for SNMPv3 to provide per-domain access controls that are backward compatible with existing MIBs. Through use of the SNMPv3 notify tables, and confirmed notifications, it is possible to implement the event-driven, event- driven polling and event-driven batching models, making it possible to notify domains of available data rather than requiring them to poll for it. This is critical in shared use or roaming implementations. In inter-domain accounting, management of SNMPv3 shared secrets can be assisted by the localized key capability or via implementation of a proxy. In the long term, alternative security models such as the Kerberos Security Model may further reduce the effort required to manage security. As noted in [49], SNMP-based accounting has limitations in terms of efficiency and latency that may make it inappropriate for use in situations requiring low processing delay or low overhead. These issues can be addressed via addition of extensions currently under discussion in the IRTF Network Management Research Group (NMRG). Compatibility with non-volatile storage can be achieved via implementation of the MIBs described in [43]-[44]. Since few current MIBs support the domain as an index, it is typically not possible to support domain-based access control for existing MIBs without requiring SNMPv3. Thus for SNMPv1 and v2 it is typically necessary to collect data from devices and sort it by domain, resulting in high processing delay. Elimination of this problem would require upgrading all devices to SNMPv3, as well as rewriting existing subagents to provide access control based on contextName. This approach provides backward compatibility with existing MIBs. Alternatively if compatibility with SNMPv1 and v2 is required and a new MIB can be written, then it may be Aboba, Arkko & Harrington Informational [Page 39]
INTERNET-DRAFT Accounting Management 19 April 2000 desirable to use the domain as an index. In order to reduce latency and make the polling, or event-driven batching modes viable, it is necessary to support the NMRG extensions. Since SNMPv3 is not widely deployed today and the NMRG extensions are still under development, this approach is probably not feasible in the short to medium term. 8. Review of Accounting Data Transfer In order for session records to be transmitted between accounting servers, a transfer protocol is required. Transfer protocols in use today include SMTP, FTP, and HTTP. For a review of accounting attributes and record formats, see [45]. 8.1. SMTP To date, few accounting management systems have been built on SMTP since the implementation of a store-and-forward message system has traditionally required access to non-volatile storage which has not been widely available on network devices. However, SMTP-based implementations have many desirable characteristics, particularly with regards to security. Accounting management systems using SMTP for accounting transfer will typically support batching so that message processing overhead will be spread over multiple accounting records. As a result, these systems result in per-active device state. Since accounting systems using SMTP as a transfer mechanism have access to substantial non-volatile storage, they can generate, compress if necessary, and store accounting records until they are transferred to the collection site. As a result, accounting systems implemented using SMTP can be highly efficient and scalable. Using IPSEC, TLS or Kerberos, hop-by-hop security services such as authentication, integrity protection and confidentiality can be provided. As described in [13] and [15], data object security is available for SMTP, and in addition, the facilities described in [12] make it possible to request and receive signed receipts, which enables non-repudiation as described in [12]-[18]. As a result, accounting systems utilizing SMTP for accounting data transfer are capable of satisfying the most demanding security requirements. However, such systems are not typically capable of providing low processing delay, although this may be addressed by the enhancements described in [20]. Aboba, Arkko & Harrington Informational [Page 40]
INTERNET-DRAFT Accounting Management 19 April 2000 8.2. Other protocols File transfer protocols such as FTP and HTTP have been used for transfer of accounting data. For example, Reference [9] describes a means for representing ASN.1-based accounting data for storage on archival media. Through the use of the Bulk File MIB, described in [43], accounting data from an SNMP MIB can be stored in ASN.1, bulk binary or Bulk ASCII format, and then subsequently retrieved as required using the FTP Client MIB described in [44]. Given access to sufficient non-volatile storage, accounting systems based on record formats and transfer protocols can avoid loss of data due to long-duration network partitions, server failures or device reboots. Since it is possible for the transfer to be driven from the collection site, the collector can retry transfers until successful, or with HTTP may even be able to restart partially completed transfers. As a result, file transfer-based systems can be made highly reliable, and the batching of accounting records makes possible efficient transfers and application of required security services with lessened overhead. 9. Summary As noted previously in this document, accounting applications vary in their security and reliability requirements. Some uses such as capacity planning may only require authentication, integrity and replay protection, and modest reliability. Other applications such as inter- domain usage-sensitive billing may require the highest degree of security and reliability, since in these cases the transfer of accounting data will lead directly to the transfer of funds. Since accounting applications do not have uniform security and reliability requirements, it is not possible to devise a single accounting protocol and set of security services that will meet all needs. Rather, the goal of accounting management should be to provide a set of tools that can be used to construct accounting systems meeting the requirements of an individual application. As a result, it is important to analyze a given accounting application to ensure that the methods chosen meet the security and reliability requirements of the application. Based on the analysis given previously, it appears that existing protocols are capable of meeting the security and reliability requirements for intra-domain capacity planning and non- usage sensitive billing. In these applications efficient transfer of bulk data is important. In order to improve SNMP bulk data transport and reduce latency, the NMRG has proposed a TCP transport mapping as well as support for sub-tree retrieval. Both of these extensions would be welcome; in addition support for file-based storage should be investigated. For inter-domain use, existing protocols lack data object Aboba, Arkko & Harrington Informational [Page 41]
INTERNET-DRAFT Accounting Management 19 April 2000 security support and extensions for inter-domain authentication are required, such as the Kerberos Security Model (KSM) for SNMPv3. For usage sensitive billing, as well as cost allocation and auditing applications, the reliability requirement are greater. Here existing protocols can only provide transport layer reliability, but do not support application layer acknowledgments required for robustness against accounting server failures. As with capacity planning and non- usage sensitive billing, efficient transfer of bulk data is important. In these applications it may also be necessary to put bounds on processing delay. This implies the need to reduce table retrieval latency, as addressed by the NMRG extensions to SNMP. Inter-domain operation requires data object security (which no existing protocol provides) as well as security model extensions (such as the KSM for SNMPv3). Where high-value sessions are involved, such as in roaming, Mobile IP, or telephony, fraud prevention may require low processing delay. To meet the latency requirements, SNMP requires support for a TCP transport mapping as well as subtree retrieval. High reliability is also required, implying the need for application layer as well as transport layer acknowledgments. SNMP does not provide this. In inter-domain use, additional security precautions such as data object security and receipt support may be required. No existing protocol can meet these requirements. A summary is given in the table on the next page. Aboba, Arkko & Harrington Informational [Page 42]
INTERNET-DRAFT Accounting Management 19 April 2000 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | Usage | Intra-domain | Inter-domain | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | Capacity | SNMP v3 w/NMRG | SNMP v3 & KSM * | | Planning | RADIUS #%@ | w/NMRG | | | TACACS+ @ | | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | Non-usage | SNMP v3 w/NMRG | SNMP v3 & KSM * | | Sensitive | RADIUS #%@ | w/NMRG | | Billing | TACACS+ @ | | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | Usage | | | | Sensitive | | | | Billing, | SNMP v3 w/NMRG &$ |SNMP v3 & KSM *&$ | | Cost | TACACS+ &$@ | w/ NMRG | | Allocation & | | | | Auditing | | | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | Time | | | | Sensitive | SNMP v3 w/NMRG &$ | No existing | | Billing, | | protocol | | fraud | | | | detection, | | | | roaming | | | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Key # = lacks confidentiality support * = lacks data object security % = limited robustness against packet loss & = lacks application layer acknowledgment $ = requires non-volatile storage @ = lacks batching support 10. Acknowledgments The authors would like to thank Bert Wijnen (Lucent), Jan Melen (Ericsson) and Jarmo Savolainen (Ericsson) for many useful discussions Aboba, Arkko & Harrington Informational [Page 43]
INTERNET-DRAFT Accounting Management 19 April 2000 of this problem space. 11. References [1] Aboba, B., Lu J., Alsop J., Ding J., and W. Wang, "Review of Roaming Implementations", RFC 2194, September 1997. [2] Aboba, B., and G. Zorn, "Criteria for Evaluating Roaming Protocols", RFC 2477, January 1999. [3] Rigney, C., Rubens, A., Simpson, W., Willens, S., "Remote Authentication Dial In User Service (RADIUS)", RFC 2138, April, 1997. [4] Rigney, C., "RADIUS Accounting", RFC 2139, April 1997. [5] Gray, J., Reuter, A., Transaction Processing: Concepts and Techniques, Morgan Kaufmann Publishers, San Francisco, California, 1993. [6] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [7] Crocker, D., Overrell, P., "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, November 1997. [8] Aboba, B., and M. Beadles, "The Network Access Identifier", RFC 2486, January 1999. [9] McCloghrie, K., Heinanen, J., Greene, W., Prasad, A., "Accounting Information for ATM Networks", RFC 2512, February 1999. [10] McCloghrie, K., Heinanen, J., Greene, W., Prasad, A., "Managed Objects for Controlling the Collection and Storage of Accounting Information for Connection-Oriented Networks", RFC 2513, February 1999. [11] Aboba, B., Lidyard, D., "The Accounting Data Interchange Format (ADIF)", Internet draft (work in progress), draft-ietf-roamops- actng-05.txt, November 1998. [12] Fajman, R., "An Extensible Message Format for Message Disposition Notifications", RFC 2298, March 1998. [13] Elkins, M., "MIME Security with Pretty Good Privacy (PGP)", RFC 2015, October 1996. Aboba, Arkko & Harrington Informational [Page 44]
INTERNET-DRAFT Accounting Management 19 April 2000 [14] Vaudreuil, G., "The Multipart/Report Content Type for the Reporting of Mail System Administrative Messages", RFC 1892, January 1996. [15] Galvin, J., et al. "Security Multiparts for MIME: Multi- part/Signed and Multipart/Encrypted", RFC 1847, October 1995. [16] Crocker, D., "MIME Encapsulation of EDI Objects", RFC 1767, March 1995. [17] Harding, T., Drummond, R., Shih, C., "MIME-based Secure EDI", Internet draft (work in progress), draft-ietf-ediint- as1-11.txt, September 1999. [18] Harding, T., Drummond, R., Shih, C., "Requirements for Inter- operable Internet EDI", Internet draft (work in progress), draft-ietf-ediint-req-08.txt, September 1999. [19] Borenstein, N., Freed, N, "MIME (Multipurpose Internet Mail Extensions) Part One: Mechanisms for Specifying and Describing the Format of Internet Message Bodies", RFC 1521, December 1993. [20] Klyne, G., "Timely Delivery for Facsimile Using Internet Mail", Internet draft (work in progress), draft-ietf-fax-timely- delivery-00.txt, October 1999. [21] Johnson, H. T., Kaplan, R. S., Relevance Lost: The Rise and Fall of Management Accounting, Harvard Business School Press, Boston, Massachusetts, 1987. [22] Horngren, C. T., Foster, G., Cost Accounting: A Managerial Emphasis. Prentice Hall, Englewood Cliffs, New Jersey, 1991. [23] Kaplan, R. S., Atkinson, Anthony A., Advanced Management Accounting, Prentice Hall, Englewood Cliffs, New Jersey, 1989. [24] Cooper, R., Kaplan, R. S., The Design of Cost Management Systems. Prentice Hall, Englewood Cliffs, New Jersey, 1991. [25] Rigney, C., Willens, S., Calhoun, P., "RADIUS Extensions", draft- ietf-radius-ext-07.txt, Internet Draft (work in progress), February 2000. [26] Carrel, D., Grant, L., "The TACACS+ Protocol Version 1.78", Internet draft (work in progress), draft-grant-tacacs-02.txt, January 1997. [27] Harrington, D., Presuhn, R., and B. Wijnen, "An Architecture for Describing SNMP Management Frameworks", RFC 2571, April 1999. Aboba, Arkko & Harrington Informational [Page 45]
INTERNET-DRAFT Accounting Management 19 April 2000 [28] Rose, M., and K. McCloghrie, "Structure and Identification of Management Information for TCP/IP-based Internets", RFC 1155, May 1990. [29] Rose, M., and K. McCloghrie, "Concise MIB Definitions", RFC 1212, March 1991. [30] M. Rose, "A Convention for Defining Traps for use with the SNMP", RFC 1215, March 1991. [31] Case, J., McCloghrie, K., Rose, M., and S. Waldbusser, "Structure of Management Information for Version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1902, January 1996. [32] Case, J., McCloghrie, K., Rose, M., and S. Waldbusser, "Textual Conventions for Version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1903, January 1996. [33] Case, J., McCloghrie, K., Rose, M., and S. Waldbusser, "Conformance Statements for Version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1904, January 1996. [34] Case, J., Fedor, M., Schoffstall, M., and J. Davin, "Simple Network Management Protocol", RFC 1157, May 1990. [35] Case, J., McCloghrie, K., Rose, M., and S. Waldbusser, "Introduction to Community-based SNMPv2", RFC 1901, January 1996. [36] Case, J., McCloghrie, K., Rose, M., and S. Waldbusser, "Transport Mappings for Version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1906, January 1996. [37] Case, J., Harrington D., Presuhn R., and B. Wijnen, "Message Processing and Dispatching for the Simple Network Management Protocol (SNMP)", RFC 2572, April 1999. [38] Blumenthal, U., and B. Wijnen, "User-based Security Model (USM) for version 3 of the Simple Network Management Protocol (SNMPv3)", RFC 2574, April 1999. [39] Levi, D., Meyer, P., and B. Stewart, "SNMPv3 Applications", RFC 2573, April 1999. [40] Wijnen, B., Presuhn, R., and K. McCloghrie, "View-based Access Control Model (VACM) for the Simple Network Management Protocol (SNMP)", RFC 2575, April 1999. Aboba, Arkko & Harrington Informational [Page 46]
INTERNET-DRAFT Accounting Management 19 April 2000 [41] Case, J., McCloghrie, K., Rose, M., and S. Waldbusser, "Protocol Operations for Version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1905, January 1996. [42] Rose, M.T., The Simple Book, Second Edition, Prentice Hall, Upper Saddle River, NJ, 1996. [43] Stewart, B., "Bulk File MIB", Internet draft (work in progress), draft-stewart-bulk-file-mib-00.txt, November 1998. [44] Stewart, B., "FTP Client MIB", Internet draft (work in progress), draft-stewart-ftp-client-mib-00.txt, November 1998. [45] Brownlee, N., Blount, A., "Accounting Attributes and Record Formats", Internet draft (work in progress), draft-ietf-aaa- accounting-attributes-02.txt, March 2000. [46] Network Management Research Group Web page, http://www.ibr.cs.tu- bs.de/projects/nmrg/ [47] Schoenwaelder, J.,"SNMP-over-TCP Transport Mapping", Internet draft (work in progress), draft-irtf-nmrg-snmp-tcp-01.txt, June 1999. [48] Schoenwaelder, J.,"SNMP Payload Compression", Internet draft (work in progress), draft-irtf-nmrg-snmp-compression-00.txt, June 1999. [49] Sprenkels, R., Martin-Flatin, J.,"Bulk Transfers of MIB Data", Simple Times, http://www.simple-times.org/pub/simple- times/issues/7-1.html, March 1999. [50] Shacham, A., Monsour, R., Pereira, R., Thomas, M., "IP Payload Compression Protocol (IPComp)", RFC 2393, December 1998. [51] Hornstein, K., Hardaker, W., "A Kerberos Security Model for SNMP v3", Internet draft (work in progress), draft-hornstein- snmpv3-ksm-00.txt, June 1999. [52] Tung, B., Neuman, C., Hur, M., Medvinsky, A., Medvinsky, S., Wray, J., Trostle, J., "Public Key Cryptography for Initial Authentication in Kerberos", Internet draft (work in progress), draft-ietf-cat-kerberos-pk-init-09.txt, June 1999. [53] Tung, B., Ryutov, T., Neuman, C., Tsudik, G., Sommerfeld, B., Medvinsky, A., Hur, M., "Public Key Cryptography for Cross-Realm Authentication in Kerberos", Internet draft (work in progress), draft-ietf-cat-kerberos-pk-cross-04.txt, June 1999. Aboba, Arkko & Harrington Informational [Page 47]
INTERNET-DRAFT Accounting Management 19 April 2000 [54] Thaler, D., "Get Subtree Retrieval MIB", Internet draft (work in progress), draft-irtf-nmrg-get-subtree-mib-00.txt, October 1999. [55] Stewart, R. R., et al., "Simple Control Transmission Protocol", Internet draft (work in progress), draft-ietf-sigtran-sctp-05.txt, January 2000. 12. Author's Addresses Bernard Aboba Microsoft Corporation One Microsoft Way Redmond, WA 98052 USA Phone: +1 425 936 6605 EMail: bernarda@microsoft.com Jari Arkko Oy LM Ericsson Ab 02420 Jorvas Finland Phone: +358 40 5079256 EMail: Jari.Arkko@ericsson.com David Harrington Cabletron Systems Inc. P.O.Box 5005 Rochester NH 03867-5005 USA Phone: +1 603 337 7357 EMail: dbh@cabletron.com 13. Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards- related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general Aboba, Arkko & Harrington Informational [Page 48]
INTERNET-DRAFT Accounting Management 19 April 2000 license or permission for the use of such proprietary rights by implementors or users of this specification can be obtained from the IETF Secretariat. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director. 14. Full Copyright Statement Copyright (C) The Internet Society (2000). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE." 15. Expiration Date This memo is filed as <draft-ietf-aaa-acct-02.txt>, and expires November 1, 2000. Aboba, Arkko & Harrington Informational [Page 49]
