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rfc:rfc2975

Network Working Group B. Aboba Request for Comments: 2975 Microsoft Corporation Category: Informational J. Arkko

                                                             Ericsson
                                                        D. Harrington
                                               Cabletron Systems Inc.
                                                         October 2000
               Introduction to Accounting Management

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2000).  All Rights Reserved.

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
 state of the art in accounting protocol design.  A companion
 document, RFC 2924, reviews the state of the art in accounting
 attributes and record formats.

Aboba, et al. Informational [Page 1] RFC 2975 Introduction to Accounting Management October 2000

Table of Contents

 1.  Introduction                                             2
     1.1   Requirements language                              3
     1.2   Terminology                                        3
     1.3   Accounting management architecture                 5
     1.4   Accounting management objectives                   7
     1.5   Intra-domain and inter-domain accounting          10
     1.6   Accounting record production                      11
     1.7   Requirements summary                              13
 2.  Scaling and reliability                                 14
     2.1   Fault resilience                                  14
     2.2   Resource consumption                              23
     2.3   Data collection models                            26
 3.  Review of Accounting Protocols                          32
     3.1 RADIUS                                              32
     3.2 TACACS+                                             33
     3.3 SNMP                                                33
 4.  Review of Accounting Data Transfer                      43
     4.1 SMTP                                                44
     4.2 Other protocols                                     44
 5.  Summary                                                 45
 6. Security Considerations                                  48
 7. Acknowledgments                                          48
 8. References                                               48
 9. Authors' Addresses                                       52
 10. Intellectual Property Statement                         53
 11. Full Copyright Statement                                54

1. 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, RFC 2924, reviews the state of the art in accounting
 attributes and record formats.

Aboba, et al. Informational [Page 2] RFC 2975 Introduction to Accounting Management October 2000

1.1. Requirements language

 In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
 "recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
 described in [6].

1.2. 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.  This may include provisions for
           transport layer as well as application layer
           acknowledgments, use of non-volatile storage, interim
           accounting capabilities (stored or transmitted over the
           wire), etc.  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 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

Aboba, et al. Informational [Page 3] RFC 2975 Introduction to Accounting Management October 2000

           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
           Interim accounting 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 records can always be
           summarized without the loss of information.  Note that
           interim accounting records may be stored internally on the
           device (such as in non-volatile storage) so as to survive a
           reboot and thus may not always be transmitted over the
           wire.
 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

Aboba, et al. Informational [Page 4] RFC 2975 Introduction to Accounting Management October 2000

           domain, for use within another administrative domain.  In
           inter-domain 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.

1.3. 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.  For session records containing a Network Access
 Identifier (NAI), described in [8], the distinction can be made by
 examining the domain portion of the NAI.  If the domain portion is
 absent or corresponds to the local domain, then the session record is
 treated as an intra-domain accounting event.  Otherwise, it is
 treated as an inter-domain accounting event.

Aboba, et al. Informational [Page 5] RFC 2975 Introduction to Accounting Management October 2000

 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.
 Where a proxy forwarder is employed, domain-based access controls may
 be employed by the proxy forwarder, rather than by the devices
 themselves.  The network device will typically speak an accounting
 protocol to the proxy forwarder, which may then either convert the
 accounting packets to session records, or forward the accounting
 packets to another domain.  In either case, domain separation is
 typically achieved by having the proxy forwarder sort the session
 records or accounting messages by destination.
 Where the accounting proxy is not trusted, it may be difficult to
 verify that the proxy is issuing correct session records based on the
 accounting messages it receives, since the original accounting
 messages typically are not forwarded along with the session records.
 Therefore where trust is an issue, the proxy typically forwards the
 accounting packets themselves.  Assuming that the accounting protocol
 supports data object security, this allows the end-points to verify
 that the proxy has not modified the data in transit or snooped on the
 packet contents.

Aboba, et al. Informational [Page 6] RFC 2975 Introduction to Accounting Management October 2000

 The diagram below illustrates the accounting management architecture:
      +------------+
      |            |
      |   Network  |
      |   Device   |
      |            |
      +------------+
            |
 Accounting |
 Protocol   |
            |
            V
      +------------+                               +------------+
      |            |                               |            |
      |   Org B    |  Inter-domain session records |  Org A     |
      |   Acctg.   |<----------------------------->|  Acctg.    |
      |Proxy/Server|   or accounting protocol      |  Server    |
      |            |                               |            |
      +------------+                               +------------+
            |                                            |
            |                                            |
 Transfer   | Intra-domain                               |
 Protocol   | Session records                            |
            |                                            |
            V                                            V
      +------------+                               +------------+
      |            |                               |            |
      |  Org B     |                               |  Org A     |
      |  Billing   |                               |  Billing   |
      |  Server    |                               |  Server    |
      |            |                               |            |
      +------------+                               +------------+

1.4. 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.

1.4.1. 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 can be tolerated.  Where it is possible to use
 statistical sampling techniques to reduce data collection

Aboba, et al. Informational [Page 7] RFC 2975 Introduction to Accounting Management October 2000

 requirements while still providing the forecast with the desired
 statistical accuracy, it may be possible to 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.

1.4.2. Billing

 When accounting data is used for billing purposes, the requirements
 depend on whether the billing process is usage-sensitive or not.

1.4.2.1. 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.

1.4.2.2. 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 conform to financial reporting and legal
 requirements, and therefore an archival accounting approach may be
 needed.
 Usage-sensitive systems may also require low 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 there is a risk of fraud, financial exposure
 increases with processing delay.  Thus it may be advisable to
 transmit each event individually to minimize batch size, or even to
 utilize quality of service techniques to minimize queuing delays.  In
 addition, it may be necessary for authorization to be dependent on
 ability to pay.

Aboba, et al. Informational [Page 8] RFC 2975 Introduction to Accounting Management October 2000

 Whether these techniques will be useful varies by application since
 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.
 Application-layer acknowledgments are also often required so as to
 guard against accounting server failures.

1.4.3. 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.

Aboba, et al. Informational [Page 9] RFC 2975 Introduction to Accounting Management October 2000

1.4.4. 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.
 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.

1.5. 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.

Aboba, et al. Informational [Page 10] RFC 2975 Introduction to Accounting Management October 2000

 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.

1.6. 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

Aboba, et al. Informational [Page 11] RFC 2975 Introduction to Accounting Management October 2000

 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
 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, et al. Informational [Page 12] RFC 2975 Introduction to Accounting Management October 2000

1.7. 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, et al. Informational [Page 13] RFC 2975 Introduction to Accounting Management October 2000

2. 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.

2.1. 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, SCTP
 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 [18], "once the cable is cut you don't need more
 retransmissions, you need a *lot* more voltage."  Thus, the choice of
 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 non-volatile
 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,

Aboba, et al. Informational [Page 14] RFC 2975 Introduction to Accounting Management October 2000

 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.

2.1.1. 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+.
 While interim accounting can provide resilience against packet loss,
 server failures, short-duration network failures, or device reboot,
 its applicability is limited.  Transmission of interim accounting
 data over the wire 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.

Aboba, et al. Informational [Page 15] RFC 2975 Introduction to Accounting Management October 2000

 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 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.

2.1.2. 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.

2.1.3. 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

Aboba, et al. Informational [Page 16] RFC 2975 Introduction to Accounting Management October 2000

 (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 [7] or SCTP [26].  On-the-wire
 interim accounting provides only limited benefits in mitigating the
 effects of packet loss.
 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.

Aboba, et al. Informational [Page 17] RFC 2975 Introduction to Accounting Management October 2000

 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.
 However, it can also be argued 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 the 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.

2.1.4. Accounting server 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.

Aboba, et al. Informational [Page 18] RFC 2975 Introduction to Accounting Management October 2000

 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.
 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.  SCTP also enables
 multiplexing of multiple connections within a single transport
 connection, all maintaining the same congestion control state,
 avoiding the "head of line blocking" issues that can occur with TCP.
 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.

2.1.5. Application layer acknowledgments

 It is 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.

Aboba, et al. Informational [Page 19] RFC 2975 Introduction to Accounting Management October 2000

 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 "the transport layer has taken responsibility
 for delivering the data to the application", while an application-
 layer acknowledgment means "the application has taken responsibility
 for the data".
 A common misconception is that use of TCP transport guarantees that
 data is delivered to the application.  However, as noted in RFC 793
 [7]:
  An acknowledgment by TCP does not guarantee that the data has been
  delivered to the end user, but only that the receiving TCP has taken
  the responsibility to do so.
 Therefore, if receiving TCP fails after sending the ACK, the
 application may not receive the data.  Similarly, if the application
 fails prior to committing the data to stable storage, the data may be
 lost.  In order for a sending application to be sure that the data it
 sent was received by the receiving application, either a graceful
 close of the TCP connection or an application-layer acknowledgment is
 required. In order to protect against data loss, it is necessary that
 the application-layer acknowledgment imply 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 transport
 layer to acknowledge receipt via transport layer acknowledgment,
 without having delivered the data to the application.  Similarly, the
 application may not complete the tasks necessary to take
 responsibility for the data.
 For example, an accounting server may receive data from the transport
 layer but be incapable of storing it data due to a back end database
 problem or disk fault.  In this case it should not send an
 application layer acknowledgment, even though a a transport layer
 acknowledgment is appropriate.  Rather, an application layer error
 message should be sent indicating the source of the problem, such as
 "Backend store unavailable".
 Thus application-layer acknowledgment capability requires not only
 the ability to acknowledge when the application has taken
 responsibility for the data, but also the ability to indicate when
 the application has not taken responsibility for the data, and why.

Aboba, et al. Informational [Page 20] RFC 2975 Introduction to Accounting Management October 2000

2.1.6. 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.

2.1.7. 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.  To guard against loss of these high-value sessions,
 interim accounting data is typically transmitted over the wire.  When
 interim accounting in-place 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.

2.1.8. 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 been accepted by the accounting server.  The leaves the

Aboba, et al. Informational [Page 21] RFC 2975 Introduction to Accounting Management October 2000

 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 are only
 appropriate for use in 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 patch an inherently unreliable system by adding store
 and forward accounting proxies.

Aboba, et al. Informational [Page 22] RFC 2975 Introduction to Accounting Management October 2000

2.1.9. 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.

2.2. 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, et al. Informational [Page 23] RFC 2975 Introduction to Accounting Management October 2000

 In order to understand the limits to scaling, we examine each of
 these resources in turn.

2.2.1. Network bandwidth

 Accounting management systems consume network bandwidth in
 transferring 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 [5].

2.2.2. 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.

2.2.3. 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.
 Non-volatile storage may be used to store interim or session records.
 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 algorithms that can minimize storage requirements.  Such

Aboba, et al. Informational [Page 24] RFC 2975 Introduction to Accounting Management October 2000

 compression algorithms are only typically applied to session records
 so as to enable implementation of interim data overwrite.

2.2.4. 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.

2.2.5. 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, et al. Informational [Page 25] RFC 2975 Introduction to Accounting Management October 2000

2.2.6. 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            |
 |                 |                                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

2.3. 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, et al. Informational [Page 26] RFC 2975 Introduction to Accounting Management October 2000

2.3.1. 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.  This 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 usage sensitive
 billing applications such as 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, et al. Informational [Page 27] RFC 2975 Introduction to Accounting Management October 2000

2.3.2. 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.

2.3.3. 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.

Aboba, et al. Informational [Page 28] RFC 2975 Introduction to Accounting Management October 2000

 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, 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.

2.3.4. 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.

Aboba, et al. Informational [Page 29] RFC 2975 Introduction to Accounting Management October 2000

 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.
 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, et al. Informational [Page 30] RFC 2975 Introduction to Accounting Management October 2000

2.3.5. 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, et al. Informational [Page 31] RFC 2975 Introduction to Accounting Management October 2000

3. 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.

3.1. 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 [5], 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, et al. Informational [Page 32] RFC 2975 Introduction to Accounting Management October 2000

3.2. TACACS+

 TACACS+ 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 [5], can be employed to provide this.

3.3. SNMP

 SNMP, described in [19],[27]-[41], has been widely deployed in a wide
 variety of intra-domain accounting applications, typically using the
 polling data collection model.  Polling allows data to be collected
 on multiple accounting events simultaneously, resulting in per-device
 state.  Management applications are able to retry requests when a
 response is not received, providing resiliency against packet loss or
 even short-lived network partitions.  Implementations without non-
 volatile storage are not robust against device reboots or network
 failures, but when combined with non-volatile storage they can be
 made highly reliable.
 SMIv1, the data modeling language of SNMPv1, has traps to permit
 trap-directed polling, but the traps are not acknowledged, and lost
 traps can lead to a loss of data.  SMIv2, used by SNMPv2c and SNMPv3,
 has Inform Requests which are acknowledged notifications.  This makes
 it possible to implement a more reliable event-driven polling model
 or event-driven batching model.  However, we are not aware of any
 SNMP-based accounting implementations currently built on the use of
 Informs.

3.3.1. Security services

 SNMPv1 and SNMPv2c support per-packet authentication and read-only
 and read-write access profiles, via the community string.  This
 clear-text password approach provides only trivial authentication,
 and no per-packet integrity checks, replay protection or
 confidentiality.  View-based access control [40] can be supported
 using the snmpCommunityMIB, defined in [11], and SNMPv1 or SNMPv2c

Aboba, et al. Informational [Page 33] RFC 2975 Introduction to Accounting Management October 2000

 messages.  The updated SNMP architecture [rfc2571] supports per-
 packet hop-by-hop authentication, integrity and replay protection,
 confidentiality and access control.
 The SNMP User Security Model (USM) [38] uses shared secrets, and when
 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 USM allows a
 manager to have one central key, sharing it with many SNMP entities
 in a localized way while preventing the other entities from getting
 at each other's data.  This can assist in cross-domain security if
 deployed properly.
 SNMPv3 does not support end-to-end data object integrity and
 confidentiality; SNMP proxy entities decrypt and re-encrypt the data
 they forward.  In the presence of an untrusted proxy entity, this
 would be inadequate.

3.3.2. Application layer acknowledgments

 SNMP uses application-layer acknowledgment to indicate that data has
 been processed.  SNMP Responses to get, get-next, or get-bulk
 requests return the requested data, or an error code indicating the
 nature of the error encountered.
 A noError SNMP Response to a SET command indicates that the requested
 assignments were made by the application.  SNMP SETs are atomic; the
 command either succeeds or fails.  An error-response indicates that
 the entity received the request, but did not succeed in executing it.
 Notifications do not use acknowledgements to indicate that data has
 been processed.  The Inform notification returns an acknowledgement
 of receipt, but not of processing, by design.  Since the updated SNMP
 architecture treats entities as peers with varying levels of
 functionality, it is possible to use SETs in either direction between
 cooperating entities to achieve processing acknowledgements.
 There are eighteen SNMP error codes.  The design of SNMP makes
 service-specific error codes unnecessary and undesirable.

3.3.3. Proxy forwarders

 In the accounting management architecture, proxy forwarders play an
 important role, forwarding intra and inter-domain accounting events
 to the correct destinations.  The proxy forwarder may also play a
 role in a polling or event-driven polling architecture.

Aboba, et al. Informational [Page 34] RFC 2975 Introduction to Accounting Management October 2000

 The functionality of an SNMP Proxy Forwarder is defined in [39].  For
 example, the network devices may be configured to send notifications
 for all domains to the Proxy Forwarder, and the devices may be
 configured to allow the Proxy Forwarder to access all MIB data.
 The use of proxy forwarders may reduce the number of shared secrets
 required for inter-domain accounting.  With Proxy Forwarders, the
 domains could share a secret with the Proxy Forwarder, and in turn,
 the Proxy Forwarder could 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.
 The engine of an SNMP Proxy Forwarder does not look inside the PDU of
 the message except to determine to which SNMP engine the PDU should
 be forwarded or which local SNMP application should process the PDU.
 The SNMP Proxy Forwarder does not modify the varbind values; it does
 not modify the varbind list except to translate between SNMP
 versions; and it does not provide any varbind level access control.

3.3.4. Domain-based access controls in SNMP

 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 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 apply domain-based access controls, in inter-domain
 accounting, 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.  These include engines,
 contexts, and views.  This section describes how this functionality
 may be applied in intra and inter-domain accounting.

3.3.4.1. Engines

 The new SNMP architecture, described in [27], added the concept of an
 SNMP engine to improve mobility support and to identify which data
 source is being referenced.  The engine is the portion of an SNMP
 entity that constructs messages, provides security functions, and
 maps to the transport layer.  Traditional agents and traditional
 managers each contain an SNMP engine.  engineID allows an SNMP engine
 to be uniquely identified, independent of the address where it is
 attached to the network.

Aboba, et al. Informational [Page 35] RFC 2975 Introduction to Accounting Management October 2000

 A securityEngineID field in a message identifies the engine which
 provides access to the security credentials contained in the message
 header.  A contextEngineID field in a message identifies the engine
 which provides access to the data contained in the PDU.
 The SNMPv3 message format explicitly passes both.  In SNMPv1 and
 SNMPv2c, the data origin is typically assumed to be the
 communications endpoint (SNMP agent).  SNMPv1 and SNMPv2c messages
 contain a community name; the community name and the source address
 can be mapped to an engineID via the snmpCommunityTable, described in
 [11].

3.3.4.2. Contexts

 Contexts are used to identify subsets of objects, within the scope of
 an engine, that are tied to instrumentation.  A contextName refers to
 a particular subset within an engine.
 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.
 An SNMP agent populates a read-only dynamic table to tell the manager
 what contexts it recognizes.  Typically contexts 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.  It is possible that MIB modules could be defined to
 allow a manager to assign contextNames to a logical subset of
 instrumentation.
 While each context may support instances of multiple MIB modules,
 each contextName is limited to one instance of a particular MIB
 module.  If multiple instances of a MIB module are required per
 engine, then unique contextNames must be defined (e.g. repeater1,
 repeater2).  The default context "" is used for engines which only
 support single instances of MIB modules, and it is used for MIB
 modules where it only makes sense to have one instance of that MIB
 module in an engine and that instance must be easy to locate, such as
 the system MIB or the security MIBs.
 SNMPv3 messages contain contextNames which are limited to the scope
 of the contextEngineID in the message.  SNMPv1 and SNMPv2c messages
 contain communities which can be mapped to contextNames within the
 local engine, or can be mapped to contextNames within other engines
 via the snmpCommunityTable, described in [11].

Aboba, et al. Informational [Page 36] RFC 2975 Introduction to Accounting Management October 2000

3.3.4.3. Views

 Views are defined in the View-based Access Control Model.  A view is
 a mask which is used to determine access to the managed objects in a
 particular context.  The view identifies which objects are visible,
 by specifying OIDs of the subtrees included and excluded.  There is
 also a mechanism to allow wildcards in the OID specification.
 For example, it is possible to define a view that includes 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 SNMP
 security Administrator).
 Views can be set up with wildcards.  For a table that is indexed
 using IP addresses, Joe can be allowed access to all rows in given
 RMON tables (e.g. the RMON hostTable) that are in the subnet
 10.2.x.x, while Josephine is given access to all rows for subnet
 10.200.x.x.
 Views filter at the name level (OIDs), not at the value level, so
 defining views based on the values of non-index data is not
 supported.  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).
 View-based access control is independent of message version.  It can
 be utilized by entities using SNMPv1, SNMPv2c, or SNMPv3 message
 formats.

3.3.5. Inter-domain access-control alternatives

 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 SNMP using the event-driven, event-
 driven polling or event-driven batching approaches.  Traps and
 Informs permit SNMP-enabled devices to notify domains that have

Aboba, et al. Informational [Page 37] RFC 2975 Introduction to Accounting Management October 2000

 accounting data awaiting collection.  SNMP Applications [39] defines
 a standard module for managing notifications.
 To use the event-driven approaches, the device must be able to
 determine when information is available for a domain.  Domain-
 specific data can be differentiated at the SNMP agent level through
 the use of the domain as an index, and the separation of data into
 domain-specific contexts.

3.3.5.1. Domain as index

 View-based access control [40] allows multiple fine-grained views of
 an SNMP MIB to be assigned to specific groups of users, such that
 access rights to the included data elements 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 module (or
 other security model MIB module).  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.
 Views filter at the name (OID) level, not at the data (value) level.
 When using views to filter by domain it is necessary to use the
 domain as an index.  Standard view-based access control is not
 designed to filter based on the values on non-indexed fields.
 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 bigco data to the administrators of the bigco domain.
 An advantage of using domains as an index is that this technique can
 be used with SNMPv1 and SNMPv2c agents as well as with SNMPv3 agents.
 A disadvantage is that the MIB modules must be specifically designed
 for this purpose.  Since existing MIB modules rarely use the domain
 as an index, domain separation cannot be enabled within legacy MIB
 modules using this technique.
 SNMP does support a RowPointer convention that could be used to
 define a new table, indexed by domain, which holds tuples between the
 domain and existing rows of data.  This would introduce issues of

Aboba, et al. Informational [Page 38] RFC 2975 Introduction to Accounting Management October 2000

 synchronization between tables.

3.3.5.2. Contexts

 ContextNames can be used to differentiate multiple instances of a MIB
 module within an engine.
 Individual domains, such as bigco.com, could be mapped to logical
 contexts, such as a bigco context.  The agent would need to create
 and recognize new contexts and to know which instrumentation is
 associated with the logical context.  The agent needs to collect
 accounting data by domain and make the data accessible via distinct
 contexts, so that access control can be applied to the context to
 prevent disclosure of sensitive information to the wrong domain.  The
 VACM access control views are applied relative to the context, so an
 operation can be permitted or denied a user based on the context
 which contains the data.
 Domain separation is handled by using contextName to differentiate
 multiple virtual tables.  For example, if accounting data has been
 collected on users with the bigco.com and smallco.com domains, then a
 separate virtual instance of the accounting session record table
 would exist for each domain, and each domain would have a
 corresponding contextName.  When a get-bulk request is made with a
 contextName of bigco, then data from the virtual table in the bigco
 context, i.e.  corresponding to the bigco.com domain, would be
 returned.
 There are a number of design approaches to creating new contexts and
 associating the contexts with appropriate instrumentation, most
 notably a sub-agent approach and a manager-configured MIB approach.
 AgentX [51], which standardizes a registration protocol between sub-
 agents and master agents to simplify SNMP agent implementation,
 allows for the creation and recognition of new contextNames when a
 subagent registers to provide support for a particular MIB subtree
 range.  The sub-agent knows how to support a particular
 functionality, e.g.  instrumentation exposed via a range of MIB
 objects.  Based on values detected in the data, such as
 source=bigco.com, the sub-agent could determine that a new domain
 needed to be tracked and create the appropriate context for the
 collection of the data, plus the appropriate access control entries.
 The determination could be table-driven, using MIB configuration.
 A manager-driven approach could use a MIB module to predefine
 contextNames corresponding to the domains of interest, and to
 indicate which objects should be collected, how to differentiate to
 which domain the data should be applied based on a specified

Aboba, et al. Informational [Page 39] RFC 2975 Introduction to Accounting Management October 2000

 condition, and what access control rules apply to the context.
 Either technique could associate existing MIB modules to domain-
 specific contexts, so domain separation can be applied to MIB modules
 not specifically designed with domain separation in mind.  Legacy
 agents would not be designed to do this, so they would need to be
 updated to support inter-domain separation and VACM access control.
 The use of contextNames for inter-domain separation represents new
 territory, so careful consideration would be needed in designing the
 MIB modules and applications to provide domain to context and context
 to instrumentation mappings, and to ensure that security is not
 weakened.

3.3.6. Outstanding issues

 There are issues that arise when using SNMP for transfer of bulk
 data, including issues of latency, network overhead, and table
 retrieval, as discussed in [49].
 In accounting applications, management stations often must retrieve
 large tables.  Latency can be high, even with the get-bulk operation,
 because the response must fit into the largest supported packet size,
 requiring multiple round-trips.  Transfers may be serialized and the
 resulting latency will be a combination of multiple round-trip times,
 possible 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.
 For bulk transfers, SNMP network overhead can be high due to the lack
 of compression, inefficiency of BER encoding, the  transmission of
 redundant OID prefixes, and the "get-bulk overshoot problem".  In
 bulk transfer of a table, the OIDs transferred are redundant: all OID
 prefixes up to the column number are identical, as are the instance
 identifier postfixes of all entries of a single table row.  Thus it
 may be possible to reduce this redundancy by compressing the OIDs, or
 by not transferring an OID with each variable.
 The "get-bulk overshoot problem", described in reference [50], occurs
 when using the get-bulk PDU.  The problem is that 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 get-bulk
 requests.  Note that the "get-bulk overshoot" problem may be
 preventable on the agent side.  Reference [41] states that an agent
 can terminate the get-bulk because of "local constraints" (see items
 1 and 3 on pages 15/16 of [41]).  This could be interpreted to mean

Aboba, et al. Informational [Page 40] RFC 2975 Introduction to Accounting Management October 2000

 that it is possible to stop at the end of a table.

3.3.6.1. Ongoing research

 To address issues of latency and efficiency, the Network Management
 Research Group (NMRG) was formed within the Internet Research Task
 Force (IRTF).  Since the NMRG work is research and is not on the
 standards track, it should be understood that the NMRG proposals may
 never be standardized, or may change substantially during the
 standardization process.  As a result, these proposals represent
 works in progress and are not readily available for use.
 The proposals under discussion in the IRTF Network Management
 Research Group (NMRG) are 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 [50].
 The SNMP-over-TCP transport mapping may result in substantial latency
 reductions in table retrieval.  The latency reduction of an SNMP-
 over-TCP transport mapping will likely manifest itself primarily in
 the polling, event-driven polling and event-driven batching modes.
 Payload compression methods include compression of the IP packet, as
 described in [5] or compression of the SNMP payload, described in
 [48].
 Proposed improvements to table retrieval include a subtree retrieval
 MIB and the addition of a get-subtree PDU.  The subtree retrieval MIB
 [50] requires no changes to the SNMP protocol or SNMP protocol
 engine, so it can be implemented and deployed more easily than a
 change to the protocol.  The addition of a get-subtree PDU implies
 changes to the protocol and to the engines of all SNMP entities which
 would support it.  Since it may be possible to address the "get-bulk
 overshoot problem" without changes to the SNMP protocol, the
 necessity of this modification is controversial.
 Reference [49] also discusses file-based storage of SNMP data, and
 use of an FTP MIB, to enable storage of SNMP data in non-volatile
 storage, and subsequent bulk transfer via FTP.  This approach would
 require implementation of additional MIB modules as well as FTP, and
 requires separate security mechanisms such as IPSEC to provide
 authentication, replay, integrity protection and confidentiality for
 the data in transit.  The file-based transfer approach has an
 important benefit - compatibility with non-volatile storage.

Aboba, et al. Informational [Page 41] RFC 2975 Introduction to Accounting Management October 2000

 Issues of legacy support exist with the NMRG proposals.  Devices
 which do not implement the new functionality would need to be
 accommodated.  This is especially problematic for proxy forwarders,
 which may need to act as translators between new and legacy entities.
 In these situations, the overhead of translation may offset the
 benefits of the new technologies.

3.3.6.2. On-going security extension research

 In order to simplify key management and enable use of certificate-
 based security in SNMPv3, a Kerberos Security Model (KSM) for SNMPv3
 has been proposed in [44].  This memo is not on the standards track,
 and therefore is not yet readily available for use.
 Use of Kerberos with SNMPv3 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.  In terms of stored
 keys, the KSM approach scales with the sum of devices and domains; 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 [42], and cross-realm authentication, as
 described in [43], the KSM inherits these capabilities.  As a result,
 this approach may have potential to reduce or even eliminate the
 shared secret management problem.  However, it should also be noted
 that certificate-based authentication can strain the limits of UDP
 packet sizes supported in SNMP implementations, so that alternate
 transport mappings may be required to support this.
 An IPSEC-based security model for SNMPv3 has 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
 would probably take several years to come to fruition.

3.3.7. SNMP summary

 Given the wealth of existing accounting-related MIB modules, it is
 likely that SNMP will remain a popular accounting protocol for the
 foreseeable future.

Aboba, et al. Informational [Page 42] RFC 2975 Introduction to Accounting Management October 2000

 Support for notifications makes it possible to implement the event-
 driven, event-driven polling and event-driven batching models.  This
 makes it possible to notify domains of available data rather than
 requiring them to poll for it, which is critical in shared use
 networks and roaming.
 Given the SNMPv3 security 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.
 In inter-domain accounting, the burden of managing SNMPv3 shared
 secrets can be reduced via the localized key capability or via
 implementation of a Proxy Forwarder.  In the long term, alternative
 security models such as the Kerberos Security Model may further
 reduce the effort required to manage security and enable streamlined
 inter-domain operation.
 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.  This includes usage
 sensitive billing applications where fraud detection may be required.
 These issues can be addressed via proposals under discussion in the
 IRTF Network Management Research Group (NMRG).  The experimental SNMP
 over TCP transport mapping may prove helpful at reducing latency.
 Depending on the volume of data, some form of compression may also be
 worth considering.  However, since these proposals are still in the
 research stage, and are not on the standards track, these
 capabilities are not readily available, and the specifications could
 change considerably before they reach their final form.
 SNMP supports separation of accounting data by domain, using either
 of two general approaches with the VACM access control model.  The
 domain as index approach can be used if the desired MIB module
 supports domain indexing, or it can implemented using an additional
 table.  The domain-context approach can be used in agents which
 support dynamic logical contexts and a domain-to-context and
 context-to-instrumentation mapping mechanism.  Either approach can be
 supported using SNMPv1, SNMPv2c, or SNMPv3 messages, by utilizing the
 snmpCommunitytable [11] to provide a community-to-context mapping.

4. 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].

Aboba, et al. Informational [Page 43] RFC 2975 Introduction to Accounting Management October 2000

 Reference [49] contains a discussion of alternative encodings for SMI
 data types, as well as alternative protocols for transmission of
 accounting data.  For example, [49] describes how MIME tags and XML
 DTDs may be used for encoding of SNMP messages or SMI data types.
 This enables data from SNMP MIBs to be transported using any protocol
 that can encapsulate MIME or XML, including SMTP and HTTP.

4.1. 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]-[17].  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].

4.2. 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, 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.

Aboba, et al. Informational [Page 44] RFC 2975 Introduction to Accounting Management October 2000

 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.

5. 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 an analysis of the requirements, it appears that existing
 deployed protocols are capable of meeting the requirements for
 intra-domain capacity planning and non-usage sensitive billing.  In
 these applications efficient transfer of bulk data is useful although
 not critical.  Thus, it is possible to use SNMPv3 to satisfy these
 requirements, without the NMRG extensions.  These include TCP
 transport mapping, sub-tree retrieval, and OID compression.
 In inter-domain capacity planning and non-usage sensitive billing,
 the security and reliability requirements are greater.  As a result,
 no existing deployed protocol satisfies the requirements.  For
 example, existing protocols lack data object security support and
 extensions to improve scalability of inter-domain authentication are
 needed, such as the Kerberos Security Model (KSM) for SNMPv3.

Aboba, et al. Informational [Page 45] RFC 2975 Introduction to Accounting Management October 2000

 For usage sensitive billing, as well as cost allocation and auditing
 applications, the reliability requirement are greater.  Here
 transport layer reliability is required to provide robustness against
 packet loss, as well as application layer acknowledgments to provide
 robustness against accounting server failures.  SNMP operations with
 the exception of InforRequest provide application layer
 acknowledgments, and the TCP transport mapping proposed by NMRG
 provides robustness against packet loss.  Inter-domain operation can
 benefit from data object security (which no existing protocol
 provides) as well as inter-domain security model enhancements (such
 as the KSM).
 Where high-value sessions are involved, such as in roaming, Mobile
 IP, or telephony, it may be necessary to put bounds on processing
 delay.  This implies the need to reduce latency.  As a result, the
 NMRG extensions are required in time sensitive billing applications,
 including TCP transport mapping, get-subtree capabilities and OID
 compression.  High reliability is also required in this application,
 implying the need for application layer as well as transport layer
 acknowledgments.  SNMPv3 with the NMRG extensions and security
 scalability improvements such as the KSM can satisfy the requirements
 in intra-domain use.
 However, in inter-domain use, additional security precautions such as
 data object security and receipt support are required.  No existing
 protocol can meet these requirements.  A summary is given in the
 table on the next page.

Aboba, et al. Informational [Page 46] RFC 2975 Introduction to Accounting Management October 2000

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 |                     |                   |
 |  Usage          |   Intra-domain      | Inter-domain      |
 |                 |                     |                   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 |                     |                   |
 |  Capacity       | SNMPv3 &            | SNMPv3 &<*        |
 |  Planning       | RADIUS #%@          |                   |
 |                 | TACACS+ @           |                   |
 |                 |                     |                   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 |                     |                   |
 |  Non-usage      | SNMPv3 &            | SNMPv3 &<*        |
 |  Sensitive      | RADIUS #%@          |                   |
 |  Billing        | TACACS+ @           |                   |
 |                 |                     |                   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 |                     |                   |
 |  Usage          |                     |                   |
 |  Sensitive      |                     |                   |
 |  Billing,       | SNMPv3 &>$          | SNMPv3 &<>*$      |
 |  Cost           | TACACS+ &$@         |                   |
 |  Allocation &   |                     |                   |
 |  Auditing       |                     |                   |
 |                 |                     |                   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 |                     |                   |
 |  Time           |                     |                   |
 |  Sensitive      | SNMPv3 &>$          |  No existing      |
 |  Billing,       |                     |  protocol         |
 |  fraud          |                     |                   |
 |  detection,     |                     |                   |
 |  roaming        |                     |                   |
 |                 |                     |                   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Key
 # = lacks confidentiality support
 * = lacks data object security
 % = limited robustness against packet loss
 & = lacks application layer acknowledgment (e.g. SNMP InformRequest)
 $ = requires non-volatile storage
 @ = lacks batching support
 < = lacks certificate support (KSM, work in progress)
 > = lacks support for large packet sizes (TCP transport mapping,
     experimental)

Aboba, et al. Informational [Page 47] RFC 2975 Introduction to Accounting Management October 2000

6. Security Considerations

 Security issues are discussed throughout this memo.

7. Acknowledgments

 The authors would like to thank Bert Wijnen (Lucent), Keith
 McCloghrie (Cisco Systems), Jan Melen (Ericsson) and Jarmo Savolainen
 (Ericsson) for useful discussions of this problem space.

8. 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. and S. Willens, "Remote
      Authentication Dial In User Service (RADIUS)", RFC  2138, April,
      1997.
 [4]  Rigney, C., "RADIUS  Accounting", RFC 2139, April 1997.
 [5]  Shacham, A., Monsour, R., Pereira, R. and M. Thomas, "IP Payload
      Compression Protocol (IPComp)", RFC 2393, December 1998.
 [6]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.
 [7]  Information Sciences Institute, "Transmission Control Protocol",
      RFC 793, September 1981.
 [8]  Aboba,  B. and  M.  Beadles, "The Network Access Identifier",
      RFC 2486, January 1999.
 [9]  McCloghrie, K., Heinanen, J., Greene, W. and A. Prasad,
      "Accounting Information for ATM Networks", RFC 2512, February
      1999.
 [10] McCloghrie, K., Heinanen, J., Greene, W., and A. Prasad,
      "Managed Objects for Controlling the Collection and Storage of
      Accounting Information for Connection-Oriented Networks", RFC
      2513, February 1999.
 [11] Frye, R., Levi, D., Routhier, S. and B. Wijnen, "Coexistence
      between Version 1, Version 2, and Version 3 of the Internet-
      standard Management Framework", RFC 2576, March 2000.

Aboba, et al. Informational [Page 48] RFC 2975 Introduction to Accounting Management October 2000

 [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.
 [14] Vaudreuil, G., "The Multipart/Report Content Type for the
      Reporting of  Mail System Administrative Messages", RFC 1892,
      January 1996.
 [15] Galvin, J., Murphy, S., Crocker, S. and N. Freed, "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] Borenstein, N. and N. Freed, "MIME (Multipurpose Internet Mail
      Extensions) Part One: Mechanisms for Specifying and Describing
      the Format of Internet Message Bodies", RFC 1521, December 1993.
 [18] Rose, M.T., The Simple Book, Second Edition, Prentice Hall,
      Upper Saddle River, NJ, 1996.
 [19] Case, J., Mundy, R., Partain, D. and B. Stewart, "Introduction
      to Version 3 of the Internet-standard Network Management
      Framework", RFC 2570, April 1999.
 [20] Klyne, G., "Timely Delivery for Facsimile Using Internet Mail",
      Work in Progress.
 [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., Willats, S. and P. Calhoun, "RADIUS Extensions", RFC
      2869, June 2000.

Aboba, et al. Informational [Page 49] RFC 2975 Introduction to Accounting Management October 2000

 [26] Stewart, R., et al., "Simple Control Transmission Protocol", RFC
      2960, October 2000.
 [27] Harrington, D., Presuhn, R., and B. Wijnen, "An Architecture for
      Describing SNMP Management Frameworks", RFC 2571, April 1999.
 [28] Rose, M., and K. McCloghrie, "Structure and Identification of
      Management Information for TCP/IP-based Internets", STD 16, RFC
      1155, May 1990.
 [29] Rose, M. and K. McCloghrie, "Concise MIB Definitions", STD 16,
      RFC 1212, March 1991.
 [30] Rose, M., "A Convention for Defining Traps for use with the
      SNMP", RFC 1215, March 1991.
 [31] McCloghrie, K., Perkins, D. and J. Schoenwaelder, "Structure of
      Management Information Version 2 (SMIv2)", STD 58, RFC 2578,
      April 1999.
 [32] McCloghrie, K., Perkins, D. and J. Schoenwaelder, "Textual
      Conventions for SMIv2", STD 58, RFC 2579, April 1999.
 [33] McCloghrie, K., Perkins, D. and J. Schoenwaelder, "Conformance
      Statements for SMIv2", STD 58, RFC 2580, April 1999.
 [34] Case, J., Fedor, M., Schoffstall, M. and J. Davin, "Simple
      Network Management Protocol", STD 15, 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.

Aboba, et al. Informational [Page 50] RFC 2975 Introduction to Accounting Management October 2000

 [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.
 [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] Tung, B., Neuman, C., Hur, M., Medvinsky, A., Medvinsky, S.,
      Wray, J. and J. Trostle, "Public Key Cryptography for Initial
      Authentication in Kerberos", Work in Progress.
 [43] Tung, B., Ryutov, T., Neuman, C., Tsudik, G., Sommerfeld, B.,
      Medvinsky, A. and M. Hur, "Public Key Cryptography for Cross-
      Realm Authentication in Kerberos", Work in Progress.
 [44] Hornstein, K. and W. Hardaker, "A Kerberos Security Model for
      SNMPv3", Work in Progress.
 [45] Brownlee, N. and A. Blount, "Accounting Attributes and Record
      Formats", RFC 2924, September 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", Work in
      Progress.
 [48] Schoenwaelder, J., "SNMP Payload Compression", Work in Progress.
 [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] Thaler, D., "Get Subtree Retrieval MIB", Work in Progress.
 [51] Daniele, M., Wijnen, B., Ellison, M. and D. Francisco, "Agent
      Extensibility (AgentX) Protocol Version 1", RFC 2741, January
      2000.

Aboba, et al. Informational [Page 51] RFC 2975 Introduction to Accounting Management October 2000

9. Authors' 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

Aboba, et al. Informational [Page 52] RFC 2975 Introduction to Accounting Management October 2000

10. 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 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.

Aboba, et al. Informational [Page 53] RFC 2975 Introduction to Accounting Management October 2000

11. 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
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 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
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 followed, or as required to translate it into languages other than
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 The limited permissions granted above are perpetual and will not be
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 This document and the information contained herein is provided on an
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 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Aboba, et al. Informational [Page 54]

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