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Versions: 00 01 02 03 04 05 06 rfc2975                                  
AAA Working Group                                          Bernard Aboba
INTERNET-DRAFT                                     Microsoft Corporation
Category: Informational                                       Jari Arkko
<draft-ietf-aaa-acct-03.txt>                                    Ericsson
2 May 2000                                              David Harrington
                                                  Cabletron Systems Inc.


                 Introduction to Accounting Management


1.  Status of this Memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC 2026.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups.  Note that other groups
may also distribute working documents as Internet-Drafts.  Internet-
Drafts are draft documents valid for a maximum of six months and may be
updated, replaced, or obsoleted by other documents at any time.  It is
inappropriate to use Internet-Drafts as reference material or to cite
them other than as "work in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.

To view the list Internet-Draft Shadow Directories, see
http://www.ietf.org/shadow.html.

2.  Copyright Notice

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

3.  Abstract

The field of Accounting Management is concerned with the collection of
resource consumption data for the purposes of capacity and trend
analysis, cost allocation, auditing, and billing. This document
describes each of these problems, and discusses the issues involved in
design of modern accounting systems.

Since accounting applications do not have uniform security and
reliability requirements, it is not possible to devise a single
accounting protocol and set of security services that will meet all
needs. Thus the goal of accounting management is to provide a set of
tools that can be used to meet the requirements of each application.
This document describes the currently available tools as well as the



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state of the art in accounting protocol design. A companion document,
draft-ietf-aaa-accounting-attributes-03.txt, reviews the state of the
art in accounting attributes and record formats.

3.1.  History

-03 draft: rewrote SNMPv3 contextName section.

-02 draft: added discussion of accounting proxies. Expanded discussion
of accounting server faults and failover. Revised section on SNMPv3
contextName. Revised requirements and evaluation tables. Fixed spelling
mistakes.

4.  Table of Contents

     1.  Status of this Memo                                      1
     2.  Copyright notice                                         1
     3.  Abstract                                                 1
     4.  Table of Contents                                        2
     5.  Introduction                                             3
         5.1   Terminology                                        3
         5.2   Accounting management architecture                 5
         5.3   Accounting management objectives                   6
         5.4   Intra-domain and inter-domain accounting           9
         5.5   Accounting record production                      10
         5.6   Requirements summary                              12
     6.  Scaling and reliability                                 13
         6.1   Fault resilience                                  13
         6.2   Resource consumption                              21
         6.3   Data collection models                            24
     7.  Review of Accounting Protocols                          30
         7.1 RADIUS                                              30
         7.2 TACACS+                                             31
         7.3 SNMP                                                31
     8.  Review of Accounting Data Transfer                      40
         8.1 SMTP                                                40
         8.2 Other protocols                                     41
     9.  Summary                                                 41
     10. Acknowledgments                                         43
     11. References                                              44
     12. Authors' Addresses                                      48
     13. Intellectual Property Statement                         48
     14. Full Copyright Statement                                49
     15. Expiration date                                         49







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5.  Introduction

The field of Accounting Management is concerned with the collection of
resource consumption data for the purposes of capacity and trend
analysis, cost allocation, auditing, and billing. This document
describes each of these problems, and discusses the issues involved in
design of modern accounting systems.

Since accounting applications do not have uniform security and
reliability requirements, it is not possible to devise a single
accounting protocol and set of security services that will meet all
needs. Thus the goal of accounting management is to provide a set of
tools that can be used to meet the requirements of each application.
This document describes the currently available tools as well as the
state of the art in accounting protocol design. A companion document,
draft-ietf-aaa-accounting-attributes-03.txt, reviews the state of the
art in accounting attributes and record formats.

5.1.  Terminology

This document frequently uses the following terms:

Accounting
          The collection of resource consumption data for the purposes
          of capacity and trend analysis, cost allocation, auditing, and
          billing.  Accounting management requires that resource
          consumption be  measured, rated, assigned, and communicated
          between appropriate parties.

Archival accounting
          In archival accounting, the goal is to collect all accounting
          data, to reconstruct missing entries as best as possible in
          the event of data loss, and to archive data for a mandated
          time period. It is "usual and customary" for these systems to
          be engineered to be very robust against accounting data loss.
          Legal or financial requirements frequently mandate archival
          accounting practices, and may often dictate that data be kept
          confidential, regardless of whether it is to be used for
          billing purposes or not.

Rating    The act of determining the price to be charged for use of a
          resource.

Billing   The act of preparing an invoice.

Usage sensitive billing
          A billing process that depends on usage information to prepare
          an invoice can be said to be usage-sensitive. In contrast, a



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          process that is independent of usage information is said to be
          non-usage-sensitive.

Auditing  The act of verifying the correctness of a procedure. In order
          to be able to conduct an audit it is necessary to be able to
          definitively determine what procedures were actually carried
          out so as to be able to compare this to the recommended
          process. Accomplishing this may require security services such
          as authentication and integrity protection.

Cost Allocation
          The act of allocating costs between entities. Note that cost
          allocation and rating are fundamentally different processes.
          In cost allocation the objective is typically to allocate a
          known cost among several entities.  In rating the objective is
          to determine the amount to be charged for use of a resource.
          In cost allocation, the cost per unit of resource may need to
          be determined; in rating, this is typically a given.

Interim accounting
          An interim accounting packet provides a snapshot of usage
          during a user's session. This may be useful in the event of a
          device reboot or other network problem that prevents the
          reception or generation of a session summary packet or session
          record. Interim accounting packets can always be summarized
          without the loss of information.

Session record
          A session record represents a summary of the resource
          consumption of a user over the entire session. Accounting
          gateways creating the session record may do so by processing
          interim accounting events or accounting events from several
          devices serving the same user.

Accounting Protocol
          A protocol used to convey data for accounting purposes.

Intra-domain accounting
          Intra-domain accounting involves the collection of information
          on resource usage within an administrative domain, for use
          within that domain. In intra-domain accounting, accounting
          packets and session records typically do not cross
          administrative boundaries.

Inter-domain accounting
          Inter-domain accounting involves the collection of information
          on resource usage within an administrative domain, for use
          within another administrative domain. In inter-domain



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          accounting, accounting packets and session records will
          typically cross administrative boundaries.

Real-time accounting
          Real-time accounting involves the processing of information on
          resource usage within a defined time window. Time constraints
          are typically imposed in order to limit financial risk.

Accounting server
          The accounting server receives accounting data from devices
          and translates it into session records. The accounting server
          may also take responsibility for the routing of session
          records to interested parties.

5.2.  Accounting management architecture

The accounting management architecture involves interactions between
network devices, accounting servers, and billing servers.  The network
device collects resource consumption data in the form of accounting
metrics.  This information is then transferred to an accounting server.
Typically this is accomplished via an accounting protocol, although it
is also possible for devices to generate their own session records.

The accounting server then processes the accounting data received from
the network device. This processing may include summarization of interim
accounting information, elimination of duplicate data, or generation of
session records.

The processed accounting data is then submitted to a billing server,
which typically handles rating and invoice generation, but may also
carry out auditing, cost allocation, trend analysis or capacity planning
functions.  Session records may be batched and compressed by the
accounting server prior to submission to the billing server in order to
reduce the volume of accounting data and the bandwidth required to
accomplish the transfer.

One of the functions of the accounting server is to distinguish between
inter and intra-domain accounting events and to route them
appropriately.  Intra-domain accounting events are typically routed to
the local billing server, while inter-domain accounting events will be
routed to accounting servers operating within other administrative
domains.  While it is not required that session record formats used in
inter and intra-domain accounting be the same, this is desirable, since
it eliminates translations that would otherwise be required.







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The diagram below illustrates the accounting management architecture:

     +------------+
     |            |
     |   Network  |
     |   Device   |
     |            |
     +------------+
           |
Accounting |
Protocol   |
           |
           V
     +------------+                               +------------+
     |            |                               |            |
     |   Org B    |      Inter-domain             |  Org A     |
     |   Acctg.   |<----------------------------->|  Acctg.    |
     |   Server   |      session records          |  Server    |
     |            |                               |            |
     +------------+                               +------------+
           |                                            |
           |  Intra-domain                              |
Transfer   |  session records                           |
Protocol   |                                            |
           |                                            |
           V                                            V
     +------------+                               +------------+
     |            |                               |            |
     |  Org B     |                               |  Org A     |
     |  Billing   |                               |  Billing   |
     |  Server    |                               |  Server    |
     |            |                               |            |
     +------------+                               +------------+

5.3.  Accounting management objectives

Accounting Management involves the collection of resource consumption
data for the purposes of capacity and trend analysis, cost allocation,
auditing, billing. Each of these tasks has different requirements.

5.3.1.  Trend analysis and capacity planning

In trend analysis and capacity planning, the goal is typically a
forecast of future usage.  Since such forecasts are inherently
imperfect, high reliability is typically not required, and moderate
packet loss may be tolerable.





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In certain cases, it may be desirable to use statistical sampling
techniques to reduce data collection requirements while still providing
the forecast with the desired statistical accuracy.  Such a sampling
process may tolerate high packet loss as long as bias is not introduced.

The security requirements for trend analysis and capacity planning
depend on the circumstances of data collection and the sensitivity of
the data.  Additional security services may be required when data is
being transferred between administrative domains.  For example, when
information is being collected and analyzed within the same
administrative domain, integrity protection and authentication may be
used in order to guard against collection of invalid data.  In inter-
domain applications confidentiality may be desirable to guard against
snooping by third parties.

5.3.2.  Billing

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

5.3.2.1.  Non-usage sensitive billing

Since by definition, non-usage-sensitive billing does not require usage
information, in theory all accounting data can be lost without affecting
the billing process. Of course this would also affect other tasks such
as trend analysis or auditing, so that such wholesale data loss would
still be unacceptable.

5.3.2.2.  Usage-sensitive billing

Since usage-sensitive billing processes depend on usage information,
packet loss may translate directly to revenue loss. As a result, the
billing process may need to meet requirements arising from financial
reporting standards, or legal requirements, and therefore an archival
accounting approach may be required.

Usage-sensitive systems may also have additional requirements relating
to processing delay. Today credit risk is commonly managed by
computerized fraud detection systems that are designed to detect unusual
activity. While efficiency concerns might otherwise dictate batched
transmission of accounting data, where it is desirable to minimize
financial risk, a different approach may be required.

Since financial exposure increases with processing delay, it may be
necessary to transmit each event individually or to minimize batch size,
to require application layer acknowledgment before providing service, or
even to utilize quality of service techniques to minimize queuing
delays.



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The degree of financial exposure is application-dependent.  For dial-up
Internet access from a local provider, charges are typically low and
therefore the risk of loss is small.  However, in the case of dial-up
roaming or voice over IP, time-based charges may be substantial and
therefore the risk of fraud is larger. In such situations it is highly
desirable to quickly detect unusual account activity, and it may be
desirable for authorization to depend on ability to pay. In situations
where valuable resources can be reserved, or where charges can be high,
very large bills may be rung up quickly, and processing may need to be
completed within a defined time window in order to limit exposure.

Since in usage-sensitive systems, accounting data translates into
revenue, the security and reliability requirements are greater. Due to
financial and legal requirements such systems need to be able to survive
an audit.  Thus security services such as authentication, integrity and
replay protection are frequently required and confidentiality and data
object integrity may also be desirable.

5.3.3.  Auditing

With enterprise networking expenditures on the rise, interest in
auditing is increasing.  Auditing, which is the act of verifying the
correctness of a procedure, commonly relies on accounting data. Auditing
tasks include verifying the correctness of an invoice submitted by a
service provider, or verifying conformance to usage policy, service
level agreements, or security guidelines.

To permit a credible audit, the auditing data collection process must be
at least as reliable as the accounting process being used by the entity
that is being audited. Similarly, security policies for the audit should
be at least as stringent as those used in preparation of the original
invoice. Due to financial and legal requirements, archival accounting
practices are frequently required in this application.

Where auditing procedures are used to verify conformance to usage or
security policies, security services may be desired. This typically will
include authentication, integrity and replay protection as well as
confidentiality and data object integrity. In order to permit response
to security incidents in progress, auditing applications frequently are
built to operate with low processing delay.

5.3.4.  Cost allocation

The application of cost allocation and billback methods by enterprise
customers is not yet widespread. However, with the convergence of
telephony and data communications, there is increasing interest in
applying cost allocation and billback procedures to networking costs, as
is now commonly practiced with telecommunications costs.



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Cost allocation models, including traditional costing mechanisms
described in [21]-[23] and activity-based costing techniques described
in [24] are typically based on detailed analysis of usage data, and as a
result they are almost always usage-sensitive. Whether these techniques
are applied to allocation of costs between partners in a venture or to
allocation of costs between departments in a single firm, cost
allocation models often have profound behavioral and financial impacts.
As a result, systems developed for this purposes are typically as
concerned with reliable data collection and security as are billing
applications. Due to financial and legal requirements, archival
accounting practices are frequently required in this application.

5.4.  Intra-domain and inter-domain accounting

Much of the initial work on accounting management has focused on intra-
domain accounting applications. However, with the increasing deployment
of services such as dial-up roaming, Internet fax, Voice and Video over
IP and QoS, applications requiring inter-domain accounting are becoming
increasingly common.

Inter-domain accounting differs from intra-domain accounting in several
important ways. Intra-domain accounting involves the collection of
information on resource consumption within an administrative domain, for
use within that domain. In intra-domain accounting, accounting packets
and session records typically do not cross administrative boundaries. As
a result, intra-domain accounting applications typically experience low
packet loss and involve transfer of data between trusted entities.

In contrast, inter-domain accounting involves the collection of
information on resource consumption within an administrative domain, for
use within another administrative domain. In inter-domain accounting,
accounting packets and session records will typically cross
administrative boundaries. As a result, inter-domain accounting
applications may experience substantial packet loss. In addition, the
entities involved in the transfers cannot be assumed to trust each
other.

Since inter-domain accounting applications involve transfers of
accounting data between domains, additional security measures may be
desirable. In addition to authentication, replay and integrity
protection, it may be desirable to deploy security services such as
confidentiality and data object integrity.  In inter-domain accounting
each involved party also typically requires a copy of each accounting
event for invoice generation and auditing.







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5.5.  Accounting record production

Typically, a single accounting record is produced per session, or in
some cases, a set of interim records which can be summarized in a single
record for billing purposes. However, to support deployment of services
such as wireless access or complex billing regimes, a more sophisticated
approach is required.

It is necessary to generate several accounting records from a single
session when pricing changes during a session.  For instance, the price
of a service can be higher during peak hours than off-peak. For a
session continuing from one tariff period to another, it becomes
necessary for a device to report "packets sent" during both periods.

Time is not the only factor requiring this approach.  For instance, in
mobile access networks the user may roam from one place to another while
still being connected in the same session. If roaming causes a change in
the tariffs, it is necessary to account for resource consumed in the
first and second areas. Another example is where modifications are
allowed to an ongoing session. For example, it is possible that a
session could be re-authorized with improved QoS. This would require
production of accounting records at both QoS levels.

These examples could be addressed by using vectors or multi-dimensional
arrays to represent resource consumption within a single session record.
For example, the vector or array could describe the resource consumption
for each combination of factors, e.g. one data item could be the number
of packets during peak hour in the area of the home operator.  However,
such an approach seems complicated and inflexible and as a result, most
current systems produce a set of records from one session. A session
identifier needs to be present in the records to permit accounting
systems to tie the records together.

In most cases, the network device will determine when multiple session
records are needed, as the local device is aware of factors affecting
local tariffs, such as QoS changes and roaming.  However, future systems
are being designed that enable the home domain to control the generation
of accounting records. This is of importance in inter-domain accounting
or when network devices do not have tariff information. The centralized
control of accounting record production can be realized, for instance,
by having authorization servers require re-authorization at certain
times and requiring the production of accounting records upon each re-
authorization.

In conclusion, in some cases it is necessary to produce multiple
accounting records from a single session.  It must be possible to do
this without requiring the user to start a new session or to re-
authenticate. The production of multiple records can be controlled



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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.
















































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5.6.  Requirements summary

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 |                     |                   |
   |  Usage          |   Intra-domain      | Inter-domain      |
   |                 |                     |                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 | Robustness vs.      | Robustness vs.    |
   |                 | packet loss         | packet loss       |
   |  Capacity       |                     |                   |
   |  Planning       | Integrity,          | Integrity,        |
   |                 | authentication,     | authentication,   |
   |                 | replay protection   | replay prot.      |
   |                 | [confidentiality]   | confidentiality   |
   |                 |                     | [data object sec.]|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Non-usage      | Integrity,          | Integrity,        |
   |  Sensitive      | authentication,     | authentication,   |
   |  Billing        | replay protection   | replay protection |
   |                 | [confidentiality]   | confidentiality   |
   |                 |                     | [data object sec.]|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 | Archival            | Archival          |
   |  Usage          | accounting          | accounting        |
   |  Sensitive      | Integrity,          | Integrity,        |
   |  Billing,       | authentication,     | authentication,   |
   |  Cost           | replay protection   | replay prot.      |
   |  Allocation &   | [confidentiality]   | confidentiality   |
   |  Auditing       | [Bounds on          | [data object sec.]|
   |                 |  processing delay]  | [Bounds on        |
   |                 |                     | processing delay] |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 | Archival            | Archival          |
   |  Time           | accounting          | accounting        |
   |  Sensitive      | Integrity,          | Integrity,        |
   |  Billing,       | authentication,     | authentication,   |
   |  fraud          | replay protection   | replay prot.      |
   |  detection,     | [confidentiality]   | confidentiality   |
   |  roaming        |                     | [Data object      |
   |                 | Bounds on           |  security and     |
   |                 |  processing delay   |  receipt support] |
   |                 |                     | Bounds on         |
   |                 |                     |  processing delay |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Key
   [] = optional




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6.  Scaling and reliability

With the continuing growth of the Internet, it is important that
accounting management systems be scalable and reliable. This section
discusses the resources consumed by accounting management systems as
well as the scalability and reliability properties exhibited by various
data collection and transport models.

6.1.  Fault resilience

As noted earlier, in applications such as usage-sensitive billing, cost
allocation and auditing, an archival approach to accounting is
frequently mandated, due to financial and legal requirements. Since in
such situations loss of accounting data can translate to revenue loss,
there is incentive to engineer a high degree of fault resilience. Faults
which may be encountered include:

   Packet loss
   Accounting server failures
   Network failures
   Device reboots

To date, much of the debate on accounting reliability has focused on
resilience against packet loss and the differences between UDP and TCP-
based transport. However, it should be understood that resilience
against packet loss is only  one aspect of meeting archival accounting
requirements.

As noted in [43], "once the cable is cut you don't need more
retransmissions, you need a *lot* more voltage."  Thus, the choice of
UDP or TCP transport has no impact on resilience against faults such as
network partition, accounting server failures or device reboots. What
does provide resilience against these faults is non-volatile storage.

The importance of non-volatile storage in design of reliable accounting
systems cannot be over-emphasized. Without such storage, event-driven
systems will lose data once the transmission timeout has been exceeded,
and batching designs will experience data loss once the internal memory
used for accounting data storage has been exceeded.  Via use of non-
volatile storage, and internally stored interim records, most of these
data losses can be avoided.

It may even be argued that non-volatile storage is more important to
accounting reliability than network connectivity, since for many years
reliable accounting systems were implemented based solely on physical
storage, without any network connectivity. For example, phone usage data
used to be stored on paper, film, or magnetic media and carried from the
place of collection to a central location for bill processing.



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6.1.1.  Interim accounting

Interim accounting provides protection against loss of session summary
data by providing checkpoint information that can be used to reconstruct
the session record in the event that the session summary information is
lost. This technique may be applied to any data collection model (i.e.
event-driven or polling) and is supported in both RADIUS [25] and in
TACACS+ [26].

While interim accounting can provide resilience against packet loss,
server failures, short-duration network failures, or device reboot, its
applicability is limited.  Interim accounting should not be thought of
as a mainstream reliability improvement technique since it increases use
of network bandwidth in normal operation, while providing benefits only
in the event of a fault.

Since most packet loss on the Internet is due to congestion, sending
interim accounting data over the wire can make the problem worse by
increasing bandwidth usage.  Therefore on-the-wire interim accounting is
best restricted to high-value accounting data such as information on
long-lived sessions. To protect against loss of data on such sessions,
the interim reporting interval is typically set several standard
deviations larger than the average session duration. This ensures that
most sessions will not result in generation of interim accounting events
and the additional bandwidth consumed by interim accounting will be
limited.  However, as the interim accounting interval decreases toward
the average session time, the additional bandwidth consumed by interim
accounting increases markedly, and as a result, the interval must be set
with caution.

Where non-volatile storage is unavailable, interim accounting can also
result in excessive consumption of memory that could be better allocated
to storage of session data. As a result, implementors should be careful
to ensure that new interim accounting data overwrites previous data
rather than accumulating additional interim records in memory, thereby
worsening the buffer exhaustion problem.

Given the increasing popularity of non-volatile storage for use in
consumer devices such as digital cameras, such devices are rapidly
declining in price. This makes it increasingly feasible for network
devices to include built-in support for non-volatile storage. This can
be accomplished, for example, by support for compact PCMCIA cards.

Where non-volatile storage is available, this can be used to store
interim accounting data.  Stored interim events are then replaced by
updated interim events or by session data when the session completes.
The session data can itself be erased once the data has been transmitted
and acknowledged at the application layer.  This approach avoids interim



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data being transmitted over the wire except in the case of a device
reboot.  When a device reboots, internally stored interim records are
transferred to the accounting server.

6.1.2.  Multiple record sessions

Generation of multiple accounting records within a session can introduce
scalability problems that cannot be controlled using the techniques
available in interim accounting.

For example, in the case of interim records kept in non-volatile
storage, it is possible to  overwrite previous interim records with the
most recent one or summarize them to a session record.  Where interim
updates are sent over the wire, it is possible to control bandwidth
usage by adjusting the interim accounting interval.

These measures are not applicable where multiple session records are
produced from a single session, since these records cannot be summarized
or overwritten without loss of information.  As a result, multiple
record production can result in increased consumption of bandwidth and
memory.  Implementors should be careful to ensure that worst-case
multiple record processing requirements do not exceed the capabilities
of their systems.

As an example, a tariff change at a particular time of day could, if
implemented carelessly, create a sudden peak in the consumption of
memory and bandwidth as the records need to be stored and/or
transported. Rather than attempting to send all of the records at once,
it may be desirable to keep them in non-volatile storage and send all of
the related records together in a batch when the session completes.  It
may also be desirable to shape the accounting traffic flow so as to
reduce the peak bandwidth consumption.  This can be accomplished by
introduction of a randomized delay interval.  If the home domain can
also control the generation of multiple accounting records, the
estimation of the worst-case processing requirements can be very
difficult.

6.1.3.  Packet loss

As packet loss is a fact of life on the Internet, accounting protocols
dealing with session data need to be resilient against packet loss. This
is particularly important in inter-domain accounting, where packets
often pass through Network Access Points (NAPs) where packet loss may be
substantial. Resilience against packet loss can be accomplished via
implementation of a retry mechanism on top of UDP, or use of TCP or SCTP
[55]. On-the-wire interim accounting provides only limited benefits in
mitigating the effects of packet loss.




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UDP-based transport is frequently used in accounting applications.
However, this is not appropriate in all cases. Where accounting data
will not fit within a single UDP packet without fragmentation, use of
TCP or SCTP transport may be preferred to use of multiple round-trips in
UDP. As noted in [47] and [49], this may be an issue in the retrieval of
large tables.

In addition, in cases where congestion is likely, such as in inter-
domain accounting, TCP or SCTP congestion control and round-trip time
estimation will be very useful, optimizing throughput.  In applications
which require maintenance of session state, such as simultaneous usage
control, TCP and application-layer keep alive packets or SCTP with its
built-in heartbeat capabilities provide a mechanism for keeping track of
session state.

When implementing UDP retransmission, there are a number of issues to
keep in mind:

   Data model
   Retry behavior
   Congestion control
   Timeout behavior

Accounting reliability can be influenced by how the data is modeled.
For example, it is almost always preferable to use cumulative variables
rather than expressing accounting data in terms of a change from a
previous data item. With cumulative data, the current state can be
recovered by a successful retrieval, even after many packets have been
lost. However, if the data is transmitted as a change then the state
will not be recovered until the next cumulative update is sent. Thus,
such implementations are much more vulnerable to packet loss, and should
be avoided wherever possible.

In designing a UDP retry mechanism, it is important that the retry
timers relate to the round-trip time, so that retransmissions will not
typically occur within the period in which acknowledgments may be
expected to arrive.  Accounting bandwidth may be significant in some
circumstances, so that the added traffic due to unnecessary
retransmissions may increase congestion levels.

Congestion control in accounting data transfer is a somewhat
controversial issue. Since accounting traffic is often considered
mission-critical, it has been argued that congestion control is not a
requirement; better to let other less-critical traffic back off in
response to congestion. Moreover, without non-volatile storage,
congestive back-off in accounting applications can result in data loss
due to buffer exhaustion.




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However, there are very persuasive arguments that in modern accounting
implementations, it is possible to implement congestion control while
improving throughput and maintaining high reliability.

In circumstances where there is sustained packet loss, there simply is
not sufficient capacity to maintain existing transmission rates. Thus,
aggregate throughput will actually improve if congestive back-off is
implemented. This is due to elimination of retransmissions and ability
to utilize techniques such as RED to desynchronize flows. In addition,
with QoS mechanisms such as differentiated services, it is possible to
mark accounting packets for preferential handling so as to provide for
lower packet loss if desired. Thus considerable leeway is available to
the network administrator in controlling the treatment of accounting
packets and hard coding inelastic behavior is unnecessary. Typically,
systems implementing non-volatile storage allow for backlogged
accounting data to be placed in non-volatile storage pending
transmission, so that buffer exhaustion resulting from congestive back-
off need not be a concern.

Since UDP is not really a transport protocol, UDP-based accounting
protocols such as [4] often do not prescribe timeout behavior. Thus
implementations may exhibit widely different behavior. For example, one
implementation may drop accounting data after three constant duration
retries to the same server, while another may implement exponential
back-off to a given server, then switch to another server, up to a total
timeout interval of twelve hours, while storing the untransmitted data
on non-volatile storage. The practical difference between these
approaches is substantial; the former approach will not satisfy archival
accounting requirements while the latter may. More predictable behavior
can be achieved via use of SCTP or TCP transport.

6.1.4.  Accounting server faults and failover

In the event of a failure of the primary accounting server, it is
desirable for the device to failover to a secondary server.  Providing
one or more secondary servers can remove much of the risk of accounting
server failure, and as a result use of secondary servers has become
commonplace.

For protocols based on TCP, it is possible for the device to maintain
connections to both the primary and secondary accounting servers, using
the secondary connection after expiration of a timer on the primary
connection. Alternatively,  it is possible to open a connection to the
secondary accounting server after a timeout or loss of the primary
connection, or on  expiration of a timer. Thus, accounting protocols
based on TCP are capable of responding more rapidly to connectivity
failures than TCP timeouts would otherwise allow, at the expense of an
increased risk of duplicates.



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With SCTP, it is possible to control transport layer timeout behavior,
and therefore it is not necessary for the accounting application to
maintain its own timers. However, since SCTP is not widely available,
use of this transport can impose an additional implementation burden on
the designer.

For protocols using UDP, transmission to the secondary  server can occur
after a number of retries or timer expiration. For compatibility with
congestion avoidance, it is advisable to incorporate techniques such as
round-trip-time estimation, slow start and congestive back-off.  Thus
the accounting protocol designer utilizing UDP often is lead to re-
inventing techniques already existing in TCP and SCTP. As a result, the
use of raw UDP transport in accounting applications is not recommended.

With any transport it is possible for the primary and secondary
accounting servers to receive duplicate packets, so support for
duplicate elimination is required. Since accounting server failures can
result in data accumulation on accounting clients, use of non-volatile
storage can ensure against data loss due to transmission timeouts or
buffer exhaustion. On-the-wire interim accounting provides only limited
benefits in mitigating the effects of accounting server failures.

It is also possible for the accounting server to experience partial
failures.  For example, a failure in the database back end could leave
the accounting retrieval process or thread operable while the process or
thread responsible for storing the data is non-functional. Similarly, it
is possible for the accounting application to run out of disk space,
making it unable to continue storing incoming session records.

In such cases it is desirable to distinguish between transport layer
acknowledgment and application layer acknowledgment.  Even though both
acknowledgments may be sent within the same packet (such as a TCP
segment carrying an application layer acknowledgment along with a piggy-
backed ACK), the semantics are different. A transport-layer
acknowledgment means "I have received the data" while an application
layer acknowledgment means "I have accepted responsibility for the
data".  The former typically implies only that the data has been
received and stored in a buffer, while the latter implies that the data
has been written to stable storage or suitably processed so as to guard
against loss.

In the case of partial failures, it is possible for the accounting
server to acknowledge receipt via transport layer acknowledgment,
without being capable of completing the tasks necessary to take
responsibility for the data, as required for an application layer
acknowledgment. For example, an accounting server incapable of storing
received data due to a back end database or disk space problem should
not send an application layer acknowledgment, even though the data was



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received, so that a transport layer acknowledgment is appropriate. In
such cases, it is useful to be able to send an application layer error
message such as "Backend store unavailable" so to make it clear that the
accounting server is malfunctioning.

6.1.5.  Network failures

Network failures may result in partial or complete loss of connectivity
for the accounting client. In the event of partial connectivity loss, it
may not be possible to reach the primary accounting server, in which
case switch over to the secondary accounting server is necessary.  In
the event of a network partition, it may be necessary to store
accounting events in device memory or non-volatile storage until
connectivity can be re-established.

As with accounting server failures, on-the-wire interim accounting
provides only limited benefits in mitigating the effects of network
failures.

6.1.6.  Device reboots

In the event of a device reboot, it is desirable to minimize the loss of
data on sessions in progress. Such losses may be significant even if the
devices themselves are very reliable, due to long-lived sessions, which
can comprise a significant fraction of total resource consumption.
Sending interim accounting data over the wire is typically implemented
to guard against loss of these high-value sessions. When interim
accounting is combined with non-volatile storage it becomes possible to
guard against data loss in much shorter sessions. This is possible since
interim accounting data need only be stored in non-volatile memory until
the session completes, at which time the interim data may be replaced by
the session record. As a result, interim accounting data need never be
sent over the wire, and it is possible to decrease the interim interval
so as to provide a very high degree of protection against data loss.

6.1.7.  Accounting proxies

In order to maintain high reliability, it is important that accounting
proxies pass through transport and application layer acknowledgments and
do not store and forward accounting packets. This enables the end-
systems to control re-transmission behavior and utilize techniques such
as non-volatile storage and secondary servers to improve resilience.

Accounting proxies sending a transport or application layer ACK to the
device without receiving one from the accounting server fool the device
into thinking that the accounting request had been accepted by the
accounting server when this is not the case. As a result, the device can
delete the accounting packet from non-volatile storage before it has



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been accepted by the accounting server. The leaves the accounting proxy
responsible for delivering accounting packets. If the accounting proxy
involves moving parts (e.g. a disk drive) while the devices do not,
overall system reliability can be reduced.

Store and forward accounting proxies only add value in situations where
the accounting subsystem is unreliable.  For example, where devices do
not implement non-volatile storage and the accounting protocol lacks
transport and application layer reliability, locating the accounting
proxy (with its stable storage) close to the device can reduce the risk
of data loss.

However, such systems are inherently unreliable so that they can only
handle capacity planning or non-usage sensitive billing applications.
If archival accounting reliability is desired, it is necessary to
engineer a reliable accounting system from the start using the
techniques described in this document, rather than attempting to repair
an inherently unreliable system by adding store and forward accounting
proxies.
































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6.1.8.  Fault resilience summary

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 |                                       |
   |  Fault          |   Counter-measures                    |
   |                 |                                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 |                                       |
   |  Packet         |   Retransmission based on RTT         |
   |  loss           |   Congestion control                  |
   |                 |   Well-defined timeout behavior       |
   |                 |   Duplicate elimination               |
   |                 |   Interim accounting*                 |
   |                 |   Non-volatile storage                |
   |                 |   Cumulative variables                |
   |                 |                                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 |                                       |
   |  Accounting     |   Primary-secondary servers           |
   |  server & net   |   Duplicate elimination               |
   |  failures       |   Interim accounting*                 |
   |                 |   Application layer ACK & error msgs. |
   |                 |   Non-volatile storage                |
   |                 |                                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 |                                       |
   |  Device         |   Interim accounting*                 |
   |  reboots        |   Non-volatile storage                |
   |                 |                                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Key
   * = limited usefulness without non-volatile storage

   Note: Accounting proxies are not a reliability
   enhancement mechanism.

6.2.  Resource consumption

In the process of growing to meet the needs of providers and customers,
accounting management systems consume a variety of resources, including:

   Network bandwidth
   Memory
   Non-volatile storage
   State on the accounting management system
   CPU on the management system and managed devices




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

6.2.1.  Network bandwidth

Accounting management systems consume network bandwidth in the
transferring of accounting data. The network bandwidth consumed is
proportional to the amount of data transferred, as well as required
network overhead.  Since accounting data for a given event may be 100
octets or less, if each event is transferred individually, overhead can
represent a considerable proportion of total bandwidth consumption.  As
a result, it is often desirable to transfer accounting data in batches,
enabling network overhead to be spread over a larger payload, and
enabling efficient use of compression.  As noted in [48], compression
can be enabled in the accounting protocol, or can be done at the IP
layer as described in [50].

6.2.2.  Memory

In accounting systems without non-volatile storage, accounting data must
be stored in volatile memory during the period between when it is
generated and when it is transferred. The resulting memory consumption
will depend on retry and retransmission algorithms. Since systems
designed for high reliability will typically wish to retry for long
periods, or may store interim accounting data, the resulting memory
consumption can be considerable. As a result, if non-volatile storage is
unavailable, it may be desirable to compress accounting data awaiting
transmission.

As noted earlier, implementors of interim accounting should take care to
ensure against excessive memory usage by overwriting older interim
accounting data with newer data for the same session rather than
accumulating interim data in the buffer.

6.2.3.  Non-volatile storage

Since accounting data stored in memory will typically be lost in the
event of a device reboot or a timeout, it may be desirable to provide
non-volatile storage for undelivered accounting data. With the costs of
non-volatile storage declining rapidly, network devices will be
increasingly capable of incorporating non-volatile storage support over
the next few years.

As described in [11], non-volatile storage may be used to store interim
or session records in a standard ASCII format. As with memory
utilization, interim accounting overwrite is desirable so as to prevent
excessive storage consumption. Note that the use of ASCII data
representation enables use of highly efficient text compression



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algorithms that can minimize storage requirements. Such compression
algorithms are only typically applied to session records so as to enable
implementation of interim data overwrite.

6.2.4.  State on the accounting management system

In order to keep track of received accounting data, accounting
management systems may need to keep state on managed devices or
concurrent sessions.  Since the number of devices is typically much
smaller than the number of concurrent sessions, it is desirable to keep
only per-device state if possible.

6.2.5.  CPU requirements

CPU consumption of the managed and managing nodes will be proportional
to the complexity of the required accounting processing. Operations such
as ASN.1 encoding and decoding, compression/decompression, and
encryption/decryption can consume considerable resources, both on
accounting clients and servers.

The effect of these operations on accounting system reliability should
not be under-estimated, particularly in the case of devices with
moderate CPU resources. In the event that devices are over-taxed by
accounting tasks, it is likely that overall device reliability will
suffer.


























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6.2.6.  Efficiency measures

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 |                                       |
   |  Resource       |   Efficiency measures                 |
   |                 |                                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 |                                       |
   |  Network        |   Batching                            |
   |  Bandwidth      |   Compression                         |
   |                 |                                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 |                                       |
   |  Memory         |   Compression                         |
   |                 |   Interim accounting overwrite        |
   |                 |                                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 |                                       |
   |  Non-volatile   |   Compression                         |
   |  Storage        |   Interim accounting overwrite        |
   |                 |                                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 |                                       |
   |  System         |   Per-device state                    |
   |  state          |                                       |
   |                 |                                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 |                                       |
   |  CPU            |   Hardware assisted                   |
   |  requirements   |     compression/encryption            |
   |                 |                                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

6.3.

Data collection models

Several data collection models are currently in use today for the
purposes of accounting data collection. These include:

   Polling model
   Event-driven model without batching
   Event-driven model with batching
   Event-driven polling model







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6.3.1.  Polling model

In the polling model, an accounting manager will poll devices for
accounting information at regular intervals. In order to ensure against
loss of data, the polling interval will need to be shorter than the
maximum time that accounting data can be stored on the polled device.
For devices without non-volatile stage, this is typically determined by
available memory; for devices with non-volatile storage the maximum
polling interval is determined by the size of non-volatile storage.

The polling model results in an accumulation of data within individual
devices, and as a result, data is typically transferred to the
accounting manager in a batch, resulting in an efficient transfer
process. In terms of Accounting Manager state, polling systems scale
with the number of managed devices, and system bandwidth usage scales
with the amount of data transferred.

Without non-volatile storage, the polling model results in loss of
accounting data due to device reboots, but not due to packet loss or
network failures of sufficiently short duration to be handled within
available memory. This is because the Accounting Manager will continue
to poll until the data is received. In situations where operational
difficulties are encountered, the volume of accounting data will
frequently increase so as to make data loss more likely. However, in
this case the polling model will detect the problem since attempts to
reach the managed devices will fail.

The polling model scales poorly for implementation of shared use or
roaming services, including wireless data, Internet telephony, QoS
provisioning or Internet access. This is because in order to retrieve
accounting data for users within a given domain, the Accounting
Management station would need to periodically poll all devices in all
domains, most of which would not contain any relevant data.  There are
also issues with processing delay, since use of a polling interval also
implies an average processing delay of half the polling interval, which
may be too high for accounting data that requires low processing delay.
Thus the event-driven polling or the pure event-driven approach is more
appropriate for shared use or roaming implementations.

Per-device state is typical of polling-based network management systems,
which often also carry out accounting management functions, since
network management systems need to  keep track of the state of network
devices for operational purposes. These systems offer average processing
delays equal to half the polling interval.







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6.3.2.  Event-driven model without batching

In the event-driven model, a device will contact the accounting server
or manager when it is ready to transfer accounting data. Most event-
driven accounting systems, such as those based on RADIUS accounting,
described in [4], transfer only one accounting event per packet, which
is inefficient.

Without non-volatile storage, a pure event-driven model typically stores
accounting events that have not yet been delivered only until the
timeout interval expires. As a result this model has the smallest memory
requirements. Once the timeout interval has expired, the accounting
event is lost, even if the device has sufficient buffer space to
continue to store it. As a result, the event-driven model is the least
reliable, since accounting data loss will occur due to device reboots,
sustained packet loss, or network failures of duration greater than the
timeout interval. In event-driven protocols without a "keep alive"
message, accounting servers cannot assume a device failure should no
messages arrive for an extended period. Thus, event-driven accounting
systems are typically not useful in monitoring of device health.

The event-driven model is frequently used in shared use networks and
roaming, since this model sends data to the recipient domains without
requiring them to poll a large number of devices, most of which have no
relevant data. Since the event-driven model typically does not support
batching, it permits accounting records to be sent with low processing
delay, enabling application of fraud prevention techniques. However,
because roaming accounting events are frequently of high value, the poor
reliability of this model is an issue. As a result, the event-driven
polling model may be more appropriate.

Per-session state is typical of event-driven systems without batching.
As a result, the event-driven approach scales poorly. However, event-
driven systems offer the lowest processing delay since events are
processed immediately and there is no possibility of an event requiring
low processing delay being caught behind a batch transfer.

6.3.3.  Event-driven model with batching

In the event-driven model with batching, a device will contact the
accounting server or manager when it is ready to transfer accounting
data. The device can contact the server when a batch of a given size has
been gathered, when data of a certain type is available or after a
minimum time period has elapsed. Such systems can transfer more than one
accounting event per packet and are thus more efficient.

An event-driven system with batching will store accounting events that
have not yet been delivered up to the limits of memory.  As a result,



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accounting data loss will occur due to device reboots, but not due to
packet loss or network failures of sufficiently short duration to be
handled within available memory. Note that while transfer efficiency
will increase with batch size, without non-volatile storage, the
potential data loss from a device reboot will also increase.

Where event-driven systems with batching have a keep-alive interval and
run over reliable transport, the accounting server can assume that a
failure has occurred if no messages are received within the keep-alive
interval. Thus, such implementations can be useful in monitoring of
device health. When used for this purpose the average time delay prior
to failure detection is one half the keep-alive interval.

Through implementation of a scheduling algorithm, event-driven systems
with batching can deliver appropriate service to accounting events that
require low processing delay. For example, high-value inter-domain
accounting events could be sent immediately, thus enabling use of fraud-
prevention techniques, while all other events would be batched. However,
there is a possibility that an event requiring low processing delay will
be caught behind a batch transfer in progress. Thus the maximum
processing delay is proportional to the maximum batch size divided by
the link speed.

Event-driven systems with batching scale with the number of active
devices. As a result this approach scales better than the pure event-
driven approach, or even the polling approach, and is equivalent in
terms of scaling to the event-driven polling approach.  However, the
event-driven batching approach has lower processing delay than the
event-driven polling approach, since delivery of accounting data
requires fewer round-trips and events requiring low processing delay can
be accommodated if a scheduling algorithm is employed.

6.3.4.  Event-driven polling model

In the event-driven polling model an accounting manager will poll the
device for accounting data only when it receives an event. The
accounting client can generate an event when a batch of a given size has
been gathered, when data of a certain type is available or after a
minimum time period has elapsed. Note that while transfer efficiency
will increase with batch size, without non-volatile storage, the
potential data loss from a device reboot will also increase.

Without non-volatile storage, an event-driven polling model will lose
data due to device reboots, but not due to packet loss, or network
partitions of short-duration. Unless a minimum delivery interval is set,
event-driven polling systems are not useful in monitoring of device
health.




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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.






































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6.3.5.  Data collection summary

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 |                   |                   |
   |     Model       |       Pros        |      Cons         |
   |                 |                   |                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Polling        | Per-device state  | Not robust        |
   |                 | Robust against    |  against device   |
   |                 |   packet loss     |  reboot, server   |
   |                 | Batch transfers   |  or network       |
   |                 |                   |  failures*        |
   |                 |                   | Polling interval  |
   |                 |                   |  determined by    |
   |                 |                   |  storage limit    |
   |                 |                   | High processing   |
   |                 |                   |  delay            |
   |                 |                   | Unsuitable for    |
   |                 |                   |  use in roaming   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Event-driven,  | Lowest processing | Not robust        |
   |   no batching   |  delay            |  against packet   |
   |                 | Suitable for      |  loss, device     |
   |                 |  use in roaming   |  reboot, or       |
   |                 |                   |  network          |
   |                 |                   |  failures*        |
   |                 |                   | Low efficiency    |
   |                 |                   | Per-session state |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Event-driven,  | Single round-trip | Not robust        |
   |   with batching |  latency          |  against device   |
   |      and        | Batch transfers   |  reboot, network  |
   |   scheduling    | Suitable for      |  failures*        |
   |                 |  use in roaming   |                   |
   |                 | Per active device |                   |
   |                 |  state            |                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Event-driven   | Batch transfers   | Not robust        |
   |   polling       | Suitable for      |  against device   |
   |                 |  use in roaming   |  reboot, network  |
   |                 | Per active device |  failures*        |
   |                 |  state            | Two round-trip    |
   |                 |                   |  latency          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Key
   * = addressed by non-volatile storage




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7.  Review of Accounting Protocols

Accounting systems have been successfully implemented using protocols
such as RADIUS, TACACS+, and SNMP. This section  describes the
characteristics of each of these protocols.

7.1.  RADIUS

RADIUS accounting, described in [4], was developed as an add-on to the
RADIUS authentication protocol, described in [3]. As a result, RADIUS
accounting shares the event-driven approach of RADIUS authentication,
without support for batching or polling. As a result, RADIUS accounting
scales with the number of accounting events instead of the number of
devices, and accounting transfers are inefficient.

Since RADIUS accounting is based on UDP and timeout and retry parameters
are not specified, implementations vary widely in their approach to
reliability, with some implementations retrying until delivery or buffer
exhaustion, and others losing accounting data after a few retries. Since
RADIUS accounting does not provide for Application-layer acknowledgments
or error messages, a RADIUS Accounting-Response is equivalent to a
transport-layer acknowledgment and provides no protection against
application layer malfunctions.  Due to the lack of reliability, it is
not possible to do simultaneous usage control based on RADIUS accounting
alone. Typically another device data source is required, such as polling
of a session MIB or a command-line session over telnet.

RADIUS accounting implementations are vulnerable to packet loss as well
as application layer failures, network failures and device reboots.
These deficiencies are magnified in inter-domain accounting as is
required in roaming ([1],[2]). On the other hand, the event-driven
approach of RADIUS accounting is useful where low processing delay is
required, such as credit risk management or fraud detection.

While RADIUS accounting does provide hop-by-hop authentication and
integrity protection, and IPSEC can be employed to provide hop-by-hop
confidentiality, data object security is not supported, and thus systems
based on RADIUS accounting are not capable of being deployed with
untrusted proxies, or in situations requiring auditability, as noted in
[2].

While RADIUS does not support compression, IP compression, described in
[50], can be employed to provide this.  While in principle extensible
with the definition of new attributes, RADIUS suffers from the very
small standard attribute space (256 attributes).






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7.2.  TACACS+

TACACS+ as defined in [26] offers an accounting model with start, stop,
and interim update messages. Since TACACS+ is based on TCP,
implementations are typically resilient against packet loss and short-
lived network partitions, and TACACS+ scales with the number of devices.
Since TACACS+ runs over TCP, it offers support for both transport layer
and application layer acknowledgments, and is suitable for simultaneous
usage control and handling of accounting events that require moderate
though not the lowest processing delay.

TACACS+ provides for hop-by-hop authentication and integrity protection
as well as hop-by-hop confidentiality. Data object security is not
supported, and therefore systems based on TACACS+ accounting are not
deployable in the presence of untrusted proxies.  While TACACS+ does not
support compression, IP compression, described in [50], can be employed
to provide this.

7.3.  SNMP

SNMP, described in [27]-[41], has been widely deployed in a wide variety
of intra-domain accounting applications, typically using the polling
data collection model. Since polling allows data to be collected on
multiple accounting events simultaneously, this model results in per-
device state.  Since the management agent is able to retry requests when
a response is not received, such systems are resilient against packet
loss or even short-lived network partitions.  While implementations
without non-volatile storage can only store accounting events up to the
limits of their memory, and thus are not robust against device reboots
or network failures, when combined with non-volatile storage, they can
be made highly reliable. With SMIv2 it is possible to support confirmed
notifications, so as to implement an event-driven polling model or even
an event-driven batching model. However, we are not aware of any SNMP-
based accounting implementations built on these models.

7.3.1.  NMRG extensions

As discussed in [49], there are a number of efficiency and latency
issues that arise when using SNMP for accounting. In such applications
it is often necessary for management stations to retrieve large tables.
In such situations, the latency can be quite high, even with the getbulk
operation. This is because the response must fit into the largest
supported packet size, requiring multiple round-trips. Unless multiple
threads are employed, the transfers will be serialized and the resulting
latency will be a combination of multiple round-trip times, timeout and
re-transmission delays and processing overhead, which may result in
unacceptable performance. Since data may change during the course of
multiple retrievals, it can be difficult to get a consistent snapshot.



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In addition, [49] notes that SNMP is inefficient for transfer of
accounting data, due to lack of compression, use of BER encoding,
transmission of redundant OID prefixes, and the "get bulk overshoot"
problem. As described in [54] the overshoot problem occurs when using
the getbulk PDU because the manager typically does not know the number
of rows in the table. As a result, it must either request too many rows,
retrieving unneeded data, or too few, resulting in the need for multiple
getbulk requests.

To address these issues, a number of changes to SNMP are now under
discussion within the IRTF Network Management Research Group (NMRG),
described in [46]. These include an SNMP-over-TCP transport mapping,
described in [47]; SNMP payload compression, described in [48]; and the
addition of a "get subtree" PDU or the subtree retrieval MIB [54].
Taken together, we will refer to these changes as the "NMRG extensions."

The SNMP-over-TCP transport mapping, particularly when combined with a
subtree retrieval solution, results in substantial latency reductions in
table retrieval. While it is possible for SNMP to operate in polling,
event-driven, event-driven batching and event-driven polling modes, the
latency reduction from the SNMP-over-TCP transport mapping manifests
itself primarily in the polling, event-driven polling and event-driven
batching modes.

While use of TCP transport provides for transport layer acknowledgment,
this still leaves SNMP without the equivalent of an application layer
acknowledgment for confirmed notifications. Since the Response to an
Inform-Request is typically sent by the master agent, not the subagent,
there is no requirement that the Response wait until the accounting
application has accepted responsibility for the data and thus the
Response does not correspond to an application layer acknowledgment.

Reference [49] also discusses file-based storage of SNMP data, as
described in [43], and the FTP MIB, described in [44]. Together these
MIBs enable storage of SNMP data in non-volatile storage, and subsequent
transfer via SNMP. It is noted that this approach requires
implementation of additional MIBs as well as FTP, and requires separate
security mechanisms such as IPSEC to provide authentication, replay and
integrity protection and confidentiality for the data in transit.
However, the the file-based transfer approach also has an important
benefit, which is compatibility with non-volatile storage.

While an SNMP over TCP transport mapping is easily implemented, it does
require SNMP agents to listen on TCP ports 161 and 162.  Addition of a
"get subtree" PDU implies changes to every agent that the management
station will interact with. However, the subtree retrieval MIB described
in [54] requires no changes to the SNMP protocol or SNMP protocol engine
and thus can be implemented and deployed more easily.



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7.3.2.  SNMP v3

SNMPv3 includes support for security services as well as access
controls.  This section describes how this functionality may be applied
in intra and inter-domain accounting.

In inter-domain accounting, it is necessary to control access on a per-
domain basis. In order to apply domain-based access controls, it is
first necessary to  identify the data subset that is to have its access
controlled. Several conceptual abstractions are used for identifying
subsets of data in SNMP v3.  These include engines, contexts, and views.

7.3.2.1.  Engines

SNMP v3 supports the use of the contextEngineID field in order to
identify the engine which provides access to the data. In traditional
terms, this is the agent. contextEngineID support was added in order to
improve handling of mobility as well as well as to improve SNMP proxy
support. Use of contextEngineID enables improved mobility, allowing the
agent on a laptop to be identified independently of the IP address where
it is attached to the network. In SNMPv1 and v2, different endpoint
addresses imply different agents. This is not the case with SNMPv3.

contextEngineID also enables SNMP proxies to identify the data origin.
While in SNMPv1 and v2, the data origin is automatically assumed to be
the communications endpoint (SNMP agent), with SNMPv3 it is possible to
distinguish the data origin from the communications endpoint.

For example, let us assume that agent A sends a trap to manager M
through agent B, who forwards it. When SNMPv3 is used, while the trap
received by manager M will have a source address corresponding to agent
B, contextEngineID will identify agent A as the data origin of the trap.
Thus, by using contextEngineID, M can identify the data origin, no
matter how many intermediate SNMP agents have forwarded it.  With SNMPv1
and v2, M would need to assume that the data in the trap (from A) refers
to the instrumentation  of the agent at the last hop (B).

Note that in SNMPv3 there is only a single contextEngineID per SNMP
implementation.

7.3.2.2.  Contexts

Contexts are used to identify subsets of objects that are tied to
instrumentation. These subsets are defined by the agent rather than the
manager since if the manager defined them, the agent would not know how
to tie the contexts to the underlying instrumentation.





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In SNMPv3, contextName represents a slice of the data contained within a
particular engine. Contexts are defined in a dynamic table, with the
names defined as read-only. The agent uses the dynamic table to tell the
manager what contexts it recognizes.

Contexts are commonly tied to hardware components, to logical entities
related to the hardware components, or to logical services. For example,
contextNames might include board5, board7, repeater1, repeater2, etc.

While each context may support multiple MIB modules, each contextName is
limited to one instance of a particular MIB module. Thus, if multiple
instances of a MIB module are required per engine, then unique
contextNames must be defined. If it only makes sense to have one
instance of a MIB module in an engine, such as the USM userTable, such a
MIB will typically fall into the default context "". Note that while a
MIB module may allow more than one instance per engine, a given SNMPv3
implementation may not support this.

7.3.2.3.  Views

A view is a mask for a particular contextName (subset of data). The view
identifies which objects are visible, by specifying OIDs of the subtrees
involved. There is also a mechanism to allow wildcards in the OID
specification.

For example, it is possible to define a view that includes the RMON
tables, and another view that includes only the SNMPv3 security related
tables. Using these views, it is possible to allow access to the RMON
view for users Joe and Josephine (the RMON administrators), and access
to the SNMPv3 security tables for user Adam (the security policy
administrator).

Views can be set up with wildcards. For a table that is indexed using IP
addresses, Joe can be allowed access to all RMON rows that are in the
subnet 134.141.x.x, with Josephine given access to all rows for subnet
134.200.x.x. However, for this to work the table must be indexed by the
differentiating variable, since views filter at the OID level, not at
the data level. It is therefore not possible to define a view that
filters on the value of non-index data. In this example, were the IP
address to have been used merely as a data item rather than an index, it
would not be possible to utilize view-based access control to achieve
the desired objective (delegation of administrative responsibility
according to subnet).

7.3.2.4.  SNMPv3 security services

SNMPv1 and SNMPv2 do not incorporate security services. With SNMPv3, it
is possible to incorporate view-based access controls, described in



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[40], as well as user-based security, described in [38]. As a result,
SNMPv3-based accounting implementations can provide hop-by-hop
authentication, integrity and replay protection, confidentiality and
access-control. SNMPv3 does not support data-object security. Merely
providing security for individual MIB variables is not sufficient. In
order to prevent a cut and paste attack by an untrusted proxy, it is
necessary to provide integrity protection covering enough of the packet
(including other MIB variables) to protect against replay.

A Kerberos Security Model (KSM) for SNMPv3 is described in [51]. This
approach requires storage of a key on the KDC for each device and
domain, while dynamically generating a session key for conversations
between domains and devices. Thus, in terms of stored keys the KSM
approach scales with the sum of devices and domains, whereas in terms of
dynamic session keys, it scales as the product of domains and devices.

As Kerberos is extended to allow initial authentication via public key,
as described in [52], and cross-realm authentication, as described in
[53], the KSM inherits these capabilities. As a result, this approach
may have potential to reduce or even eliminate the shared secret
management problem in the long-term.  However, it should also be noted
that certificate-based authentication can strain the limits of UDP
packet sizes supported in SNMP implementations, so that the SNMP-over-
TCP transport mapping may be required to support this.

An IPSEC-based security model for SNMPv3 has also been discussed.
Implementation of such a security model would require the SNMPv3 engine
to be able to retrieve the properties of the IPSEC security association
used to protect the SNMPv3 traffic.  This would include the security
services invoked, as well as information relating to the other endpoint
such as the authentication method and presented identity and
certificate. To date such APIs have not been widely implemented, and in
addition, most IPSEC implementations only support machine certificates,
which may not provide the required granularity of identification. Thus,
an IPSEC-based security model for SNMPv3 will probably take several
years to come to fruition.

7.3.3.  Domain-based access control in SNMPv3

Domain-based access controls are required where multiple administrative
domains are involved, such as in the shared use networks and roaming
associations described in [1]. Since the same device may be accessed by
multiple organizations, it is often necessary to control access to
accounting data according to the user's organization. This ensures that
organizations may be given access to accounting data  relating to their
users, but not to data relating to users of other organizations.





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In order to be able to control access to accounting data on a per-domain
basis, there are several alternatives.  These include use of the domain
as an index, engines, contexts and proxies.

7.3.3.1.  Domain as index

Through use of view-based access control [40], it is possible to define
multiple fine-grained views of an SNMP MIB, and to assign views to
specific groups of users, such that access rights to the included data
elements will depend on the identity of the user making the request.
For example, all users of bigco.com which are allowed access to the
device would be defined in the User-based security MIB (or other
security model MIB).  For simplicity in administering access control,
the users can be grouped using a vacmGroupName, e.g. bigco. A view of a
subset of the data objects in the MIB can be defined in the
vacmViewFamilyTreeTable. A vacmAccessTable pairs groups and views. For
messages received from users in the bigco group, access would only be
provided to the data permitted to be viewed by bigco users, as defined
in the view family tree. This requires that each domain accessing the
data be given one or more separate vacmGroupNames, an appropriate
ViewTable be defined, and the vacmAccessTable be configured for each
group.

As the number of network devices within the shared use or roaming
network grows, the polling model of data collection becomes increasingly
impractical since most devices will not carry data relating to the
polling organization. As a result, shared-use networks or roaming
associations relying on SNMP-based accounting have generally collected
data for all organizations and then sorted the resulting session records
for delivery to each organization. While functional, this approach will
typically result in increased processing delay as the number of
organizations and data records grows.

This issue can be addressed in SNMPv3 through use of view-based access
control and the SNMP notification tables, using the event-driven, event-
driven polling or event-driven batching approaches.  This permits
SNMPv3-enabled  devices to notify domains that have accounting data
awaiting collection.

However, since views filter at the OID level, not at the data level,
when using views to filter by domain it is necessary to use the domain
as an index. For example, a table of session data could be indexed by
record number and domain, allowing a view to be defined that could
restrict access to domain data to the administrators of that domain. For
example, user bigco could be allowed to view data relating to users
within the bigco.com domain, but user smallco would not be allowed
access to this view.




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An advantage of using domains as an index is that this technique can be
used with SNMPv1 and v2 agents as well as with v3 agents. A disadvantage
is that the MIBs must be specifically designed for this purpose. Since
existing MIBs rarely use the domain as an index, domain separation
cannot be enabled within legacy MIBs using this technique.

7.3.3.2.  Engines

Another approach is to use contextEngineID to differentiate between data
within individual domains. This approach would only be feasible for use
with SNMPv3, where contextEngineID is supported. Since this technique
can work with existing MIBs it enables domain separation to be applied
to MIBs not specifically designed with domain separation in mind.

One way this can be implemented is to provide multiple SNMP agents on
the same system, one for each domain, differentiated by contextEngineID.
However, this approach is not very scalable, particularly if there are a
large number of domains involved, since it would require multiple agent
implementations each with their own separate data space.

An alternative is to allow one engine to be known by multiple
contextEngineIDs. This would require that an engine be built where the
engineID MIB module is multiply-instanced with different engineID
values, in different contexts. However, this was not the intent of the
original authors of SNMP v3, who assumed only a single contextEngineID
per agent. As a result, doing this may introduce complications. For
example, the MIB module that contains the contextEngineID may explicitly
require only one instance per engine.

7.3.3.3.  Contexts

Still another approach is to use contextName to differentiate between
data within individual domains.  contextName offers a mechanism for
demultiplexing MIB modules, just as community names did in SNMPv1. The
distinction is that community was overloaded to serve multiple purposes,
while contextName is not.  This approach would only be feasible for use
with SNMPv3, where contextName is supported. Since this technique can
work with existing MIBs it enables domain separation to be applied to
MIBs not specifically designed with domain separation in mind.

However, since contextName was not originally designed for the purpose
of domain separation, use for this purpose may be problematic. In the
design of SNMP v3, contexts were intended to be used by an agent to
inform a manager about the contexts known to the agent.  As a result,
vacmContextName is read-only and so cannot be configured directly using
SNMP.  Since contextName is not manager configurable, this implies that
agents must dynamically create contexts. Manager-defined contexts are
problematic because the agent doesn't know what objects are encompassed



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by such a manager-defined context.

Since views filter at the OID level, not the data level, it is not
possible to use VACM to handle the domain separation. Instead, the
domain separation needs to be handled at the sub-agent level.  For
example, if the contextName is made available to the sub-agent, then it
can only expose those elements of a table that correspond to the given
context. For example, when a getbulk request is made with a contextName
of bigco.com, then only table entries for bigco.com users would be
visible. For this to work, it is necessary that the MIB not require that
index entries be consecutive, so that table rows may be ommitted as
needed.

In order to implement the required access control within the sub-agent,
this approach requires that the sub-agent APIs pass the contextName to
the sub-agent. Since SNMPv2 does not support contextName, therefore
would need to be rewritten even though the MIB might not require
modification. As a result, agent implementations may not be

7.3.3.4.  Proxies

Another approach is to support domain separation via use of a proxy.
However, the proxy application is forbidden to provide access control at
the varbind level, and is designed so that the proxy does not need to
look inside the PDU of the message except to determine the
contextEngineID to verify it is not destined to itself.  If
contextEngine == securityEngine, with other qualifications, then the
message is being sent to the current engine, so it is processed locally
rather than being sent to the proxy forwarder.

Restrictions on use of proxies to provide access control at the varbind
level also affect the ability to provide support for legacy devices.  If
legacy devices do not support view-based access control, then the proxy
will not be able to provide this capability.

Issues of legacy support also exist with the NMRG extensions.  A proxy
receiving a "get subtree" PDU going to a non-NMRG capable device would
need to translate the "get subtree" PDU into multiple getnext or getbulk
requests. This issue would also exist with the subtree retrieval MIB
described in [54], since unless the legacy devices also support the
subtree retrieval MIB, the proxy would encounter the "getbulk overshoot"
problem.

Similarly, unless a device supports the SNMP-over-TCP transport mapping,
deployment of an NMRG-capable proxy will not provide much benefit, since
the proxy will need to fall back to UDP-based getnext or getbulk
operations. This will result in multiple round-trips and high latency
and in addition the risk of inconsistent tables would remain.  Existing



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proxies are built to support only the current standard operations so
that new proxy code would be needed to support these NMRG extensions.

Where the product of the number of domains and devices is large, such as
in inter-domain accounting applications, the number of shared secrets
can get out of hand.  The localized key capability in the SNMPv3 USM
allows a manager to have one central key, sharing it with many agents in
a localized way while preventing the agents from getting at each other's
data. This can assist in cross-domain security if deployed properly.

Another solution is to implement a proxy for the purposes of shared
secret reduction. In such a scheme, the domains will share a secret with
the proxy, and the proxy will share a secret with each of the devices.
Thus the number of shared secrets will scale with the sum of the number
of devices and domains rather than the product.

7.3.4.  SNMP summary

Given the wealth of existing accounting-related MIBs, it is likely that
SNMP will remain a popular accounting protocol for the foreseeable
future.  Given the SNMPv3 enhancements, it is desirable for SNMP-based
intra-domain accounting implementations to upgrade to SNMPv3. Such an
upgrade is virtually mandatory for inter-domain applications.

With SNMPv3, it is now possible to provide hop-by-hop security services.
Through use of the SNMPv3 notify tables, and confirmed notifications, it
is possible to implement the event-driven, event-driven polling and
event-driven batching models, making it possible to notify domains of
available data rather than requiring them to poll for it. This is
critical in shared use or roaming implementations.

In inter-domain accounting, management of SNMPv3 shared secrets can be
assisted by the localized key capability or via implementation of a
proxy. In the long term, alternative security models such as the
Kerberos Security Model may further reduce the effort required to manage
security.

As noted in [49], SNMP-based accounting has limitations in terms of
efficiency and latency that may make it inappropriate for use in
situations requiring low processing delay or low overhead.  These issues
can be addressed via addition of extensions currently under discussion
in the IRTF Network Management Research Group (NMRG).  Compatibility
with non-volatile storage can be achieved via implementation of the MIBs
described in [43]-[44].

Since SNMPv3 was not designed for use in inter-domain accounting,
facilities such as contextEngineID and contextName are not helpful for
domain separation.  As a result, the domain as index approach is



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recommended.  While few current MIBs support the domain as an index, it
may be possible to retrofit existing MIBs to support domain separation.

For example, if all data is table-based, it may be possible to use
AUGMENTS to add a domain-specific index to an existing table.  If this
is not possible, it may be necessary to collect data from devices and
sort it by domain, resulting in high processing delay.

In order to reduce latency and make the polling, or event-driven
batching modes viable, it is necessary to support the NMRG extensions.
Since SNMPv3 is not widely deployed today and the NMRG extensions are
still under development, this approach is probably not feasible in the
short to medium term.

8.  Review of Accounting Data Transfer

In order for session records to be transmitted between accounting
servers, a transfer protocol is required. Transfer protocols in use
today include SMTP, FTP, and HTTP.  For a review of accounting
attributes and record formats, see [45].

8.1.  SMTP

To date, few accounting management systems have been built on SMTP since
the implementation of a store-and-forward message system has
traditionally required access to non-volatile storage which has not been
widely available on network devices.  However, SMTP-based
implementations have many desirable characteristics, particularly with
regards to security.

Accounting management systems using SMTP for accounting transfer will
typically support batching so that message processing overhead will be
spread over multiple accounting records. As a result, these systems
result in per-active device state. Since accounting systems using SMTP
as a transfer mechanism have access to substantial non-volatile storage,
they can generate, compress if necessary, and store accounting records
until they are transferred to the collection site. As a result,
accounting systems implemented using SMTP can be highly efficient and
scalable.  Using IPSEC, TLS or Kerberos, hop-by-hop security services
such as authentication, integrity protection and confidentiality can be
provided.

As described in [13] and [15], data object security is available for
SMTP, and in addition, the facilities described in [12] make it possible
to request and receive signed receipts, which enables non-repudiation as
described in [12]-[18]. As a result, accounting systems utilizing SMTP
for accounting data transfer are capable of satisfying the most
demanding security requirements. However, such systems are not typically



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capable of providing low processing delay, although this may be
addressed by the enhancements described in [20].

8.2.  Other protocols

File transfer protocols such as FTP and HTTP have been used for transfer
of accounting data. For example, Reference [9] describes a means for
representing ASN.1-based accounting data for storage on archival media.
Through the use of the Bulk File MIB, described in [43], accounting data
from an SNMP MIB can be stored in ASN.1, bulk binary or Bulk ASCII
format, and then subsequently retrieved as required using the FTP Client
MIB described in [44].

Given access to sufficient non-volatile storage, accounting systems
based on record formats and transfer protocols can avoid loss of data
due to long-duration network partitions, server failures or device
reboots.  Since it is possible for the transfer to be driven from the
collection site, the collector can retry transfers until successful, or
with HTTP may even be able to restart partially completed transfers. As
a result, file transfer-based systems can be made highly reliable, and
the batching of accounting records makes possible efficient transfers
and application of required security services with lessened overhead.

9.  Summary

As noted previously in this document, accounting applications vary in
their security and reliability requirements. Some uses such as capacity
planning may only require authentication, integrity and replay
protection, and modest reliability.  Other applications such as inter-
domain usage-sensitive billing may require the highest degree of
security and reliability, since in these cases the transfer of
accounting data will lead directly to the transfer of funds.

Since accounting applications do not have uniform security and
reliability requirements, it is not possible to devise a single
accounting protocol and set of security services that will meet all
needs. Rather, the goal of accounting management should be to provide a
set of tools that can be used to construct accounting systems meeting
the requirements of an individual application.

As a result, it is important to analyze a given accounting application
to ensure that the methods chosen meet the security and reliability
requirements of the application. Based on the analysis given previously,
it appears that existing protocols are capable of meeting the security
and reliability requirements for intra-domain capacity planning and non-
usage sensitive billing. In these applications efficient transfer of
bulk data is important.  In order to improve SNMP bulk data transport
and reduce latency, the NMRG has proposed a TCP transport mapping as



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well as support for sub-tree retrieval. Both of these extensions would
be welcome; in addition support for file-based storage should be
investigated.  For inter-domain use, existing protocols lack data object
security support and extensions for inter-domain authentication are
required, such as the Kerberos Security Model (KSM) for SNMPv3.

For usage sensitive billing, as well as cost allocation and auditing
applications, the reliability requirement are greater.  Here existing
protocols can only provide transport layer reliability, but do not
support application layer acknowledgments required for robustness
against accounting server failures. As with capacity planning and non-
usage sensitive billing, efficient transfer of bulk data is important.
In these applications it may also be necessary to put bounds on
processing delay. This implies the need to reduce table retrieval
latency, as addressed by the NMRG extensions to SNMP.  Inter-domain
operation requires data object security (which no existing protocol
provides) as well as security model extensions (such as the KSM for
SNMPv3).

Where high-value sessions are involved, such as in roaming, Mobile IP,
or telephony, fraud prevention may require low processing delay.  To
meet the latency requirements, SNMP requires support for a TCP transport
mapping as well as subtree retrieval.  High reliability is also
required, implying the need for application layer as well as transport
layer acknowledgments. SNMP does not provide this.  In inter-domain use,
additional security precautions such as data object security and receipt
support may be required. No existing protocol can meet these
requirements.  A summary is given in the table on the next page.























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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 |                     |                   |
   |  Usage          |   Intra-domain      | Inter-domain      |
   |                 |                     |                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 |                     |                   |
   |  Capacity       | SNMP v3 w/NMRG      | SNMP v3 & KSM *   |
   |  Planning       | RADIUS #%@          |  w/NMRG           |
   |                 | TACACS+ @           |                   |
   |                 |                     |                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 |                     |                   |
   |  Non-usage      | SNMP v3 w/NMRG      | SNMP v3 & KSM *   |
   |  Sensitive      | RADIUS #%@          |  w/NMRG           |
   |  Billing        | TACACS+ @           |                   |
   |                 |                     |                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 |                     |                   |
   |  Usage          |                     |                   |
   |  Sensitive      |                     |                   |
   |  Billing,       | SNMP v3 w/NMRG &$   |SNMP v3 & KSM  *&$ |
   |  Cost           | TACACS+ &$@         | w/ NMRG           |
   |  Allocation &   |                     |                   |
   |  Auditing       |                     |                   |
   |                 |                     |                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 |                     |                   |
   |  Time           |                     |                   |
   |  Sensitive      | SNMP v3 w/NMRG  &$  | No existing       |
   |  Billing,       |                     |  protocol         |
   |  fraud          |                     |                   |
   |  detection,     |                     |                   |
   |  roaming        |                     |                   |
   |                 |                     |                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Key
   # = lacks confidentiality support
   * = lacks data object security
   % = limited robustness against packet loss
   & = lacks application layer acknowledgment
   $ = requires non-volatile storage
   @ = lacks batching support

10.  Acknowledgments

The authors would like to thank Bert Wijnen (Lucent), Jan Melen
(Ericsson) and Jarmo Savolainen (Ericsson) for many useful discussions



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of this problem space.

11.  References


[1]  Aboba, B., Lu J., Alsop J., Ding J., and W. Wang, "Review of
     Roaming Implementations", RFC 2194, September 1997.

[2]  Aboba, B., and G. Zorn, "Criteria for Evaluating Roaming
     Protocols", RFC 2477, January 1999.

[3]  Rigney, C., Rubens, A., Simpson, W., Willens, S., "Remote
     Authentication Dial In User Service (RADIUS)", RFC  2138, April,
     1997.

[4]  Rigney, C., "RADIUS  Accounting", RFC 2139, April 1997.

[5]  Gray, J., Reuter, A., Transaction Processing: Concepts and
     Techniques, Morgan Kaufmann Publishers, San Francisco, California,
     1993.

[6]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
     Levels", BCP 14, RFC 2119, March 1997.

[7]  Crocker, D., Overrell, P., "Augmented BNF for Syntax
     Specifications: ABNF", RFC 2234, November 1997.

[8]  Aboba,  B.,  and  M.  Beadles,  "The Network Access Identifier",
     RFC 2486, January 1999.

[9]  McCloghrie, K., Heinanen, J., Greene, W., Prasad, A., "Accounting
     Information for ATM Networks",  RFC 2512, February 1999.

[10] McCloghrie, K., Heinanen, J., Greene, W., Prasad, A., "Managed
     Objects for Controlling the Collection and Storage of Accounting
     Information for Connection-Oriented Networks", RFC 2513, February
     1999.

[11] Aboba, B., Lidyard, D., "The Accounting Data Interchange Format
     (ADIF)", Internet draft (work in progress), draft-ietf-roamops-
     actng-05.txt, November 1998.

[12] Fajman, R., "An Extensible Message Format for Message Disposition
     Notifications", RFC 2298, March 1998.

[13] Elkins, M., "MIME  Security with Pretty Good Privacy (PGP)", RFC
     2015, October 1996.




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[14] Vaudreuil, G., "The Multipart/Report Content Type for the Reporting
     of  Mail System Administrative Messages", RFC 1892, January 1996.

[15] Galvin, J.,  et  al.  "Security  Multiparts  for  MIME:  Multi-
     part/Signed and  Multipart/Encrypted",  RFC 1847, October 1995.

[16] Crocker, D., "MIME Encapsulation of EDI Objects", RFC 1767, March
     1995.

[17] Harding, T., Drummond, R., Shih, C., "MIME-based Secure EDI",
     Internet  draft   (work   in   progress),   draft-ietf-ediint-
     as1-11.txt, September 1999.

[18] Harding, T., Drummond, R., Shih, C., "Requirements for Inter-
     operable Internet EDI",   Internet  draft  (work  in  progress),
     draft-ietf-ediint-req-08.txt, September 1999.

[19] Borenstein, N.,  Freed,  N, "MIME  (Multipurpose  Internet  Mail
     Extensions)  Part  One:  Mechanisms  for Specifying and Describing
     the Format of Internet Message  Bodies",  RFC  1521, December 1993.

[20] Klyne, G., "Timely Delivery for Facsimile Using Internet Mail",
     Internet draft (work in progress), draft-ietf-fax-timely-
     delivery-00.txt, October 1999.

[21] Johnson, H. T., Kaplan, R. S., Relevance Lost: The Rise and Fall of
     Management Accounting, Harvard Business School Press, Boston,
     Massachusetts, 1987.

[22] Horngren, C. T., Foster, G., Cost Accounting: A Managerial
     Emphasis.  Prentice Hall, Englewood Cliffs, New Jersey, 1991.

[23] Kaplan, R. S., Atkinson, Anthony A., Advanced Management
     Accounting, Prentice Hall, Englewood Cliffs, New Jersey, 1989.

[24] Cooper, R., Kaplan, R. S., The Design of Cost Management Systems.
     Prentice Hall, Englewood Cliffs, New Jersey, 1991.

[25] Rigney, C., Willens, S., Calhoun, P., "RADIUS Extensions", draft-
     ietf-radius-ext-07.txt, Internet Draft (work in progress), February
     2000.

[26] Carrel, D., Grant, L., "The TACACS+ Protocol Version 1.78",
     Internet draft (work in progress), draft-grant-tacacs-02.txt,
     January 1997.

[27] Harrington, D., Presuhn, R., and B. Wijnen, "An Architecture for
     Describing SNMP Management Frameworks", RFC 2571, April 1999.



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[28] Rose, M., and K. McCloghrie, "Structure and Identification of
     Management Information for TCP/IP-based Internets", RFC 1155, May
     1990.

[29] Rose, M., and K. McCloghrie, "Concise MIB Definitions", RFC 1212,
     March 1991.

[30] M. Rose, "A Convention for Defining Traps for use with the SNMP",
     RFC 1215, March 1991.

[31] Case, J., McCloghrie, K., Rose, M., and S. Waldbusser, "Structure
     of Management Information for Version 2 of the Simple Network
     Management Protocol (SNMPv2)", RFC 1902, January 1996.

[32] Case, J., McCloghrie, K., Rose, M., and S. Waldbusser, "Textual
     Conventions for Version 2 of the Simple Network Management Protocol
     (SNMPv2)", RFC 1903, January 1996.

[33] Case, J., McCloghrie, K., Rose, M., and S. Waldbusser, "Conformance
     Statements for Version 2 of the Simple Network Management Protocol
     (SNMPv2)", RFC 1904, January 1996.

[34] Case, J., Fedor, M., Schoffstall, M., and J. Davin, "Simple Network
     Management Protocol", RFC 1157, May 1990.

[35] Case, J., McCloghrie, K., Rose, M., and S. Waldbusser,
     "Introduction to Community-based SNMPv2", RFC 1901, January 1996.

[36] Case, J., McCloghrie, K., Rose, M., and S. Waldbusser, "Transport
     Mappings for Version 2 of the Simple Network Management Protocol
     (SNMPv2)", RFC 1906, January 1996.

[37] Case, J., Harrington D., Presuhn R., and B. Wijnen, "Message
     Processing and Dispatching for the Simple Network Management
     Protocol (SNMP)", RFC 2572, April 1999.

[38] Blumenthal, U., and B. Wijnen, "User-based Security Model (USM) for
     version 3 of the Simple Network Management Protocol (SNMPv3)", RFC
     2574, April 1999.

[39] Levi, D., Meyer, P., and B. Stewart, "SNMPv3 Applications", RFC
     2573, April 1999.

[40] Wijnen, B., Presuhn, R., and K. McCloghrie, "View-based Access
     Control Model (VACM) for the Simple Network Management Protocol
     (SNMP)", RFC 2575, April 1999.





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[41] Case, J., McCloghrie, K., Rose, M., and S. Waldbusser, "Protocol
     Operations for Version 2 of the Simple Network Management Protocol
     (SNMPv2)", RFC 1905, January 1996.

[42] Rose, M.T., The Simple Book, Second Edition, Prentice Hall, Upper
     Saddle River, NJ, 1996.

[43] Stewart, B., "Bulk File MIB", Internet draft (work in progress),
     draft-stewart-bulk-file-mib-00.txt, November 1998.

[44] Stewart, B., "FTP Client MIB", Internet draft (work in progress),
     draft-stewart-ftp-client-mib-00.txt, November 1998.

[45] Brownlee, N., Blount, A., "Accounting Attributes and Record
     Formats", Internet draft (work in progress), draft-ietf-aaa-
     accounting-attributes-02.txt, March 2000.

[46] Network Management Research Group Web page, http://www.ibr.cs.tu-
     bs.de/projects/nmrg/

[47] Schoenwaelder, J.,"SNMP-over-TCP Transport Mapping", Internet draft
     (work in progress), draft-irtf-nmrg-snmp-tcp-01.txt, June 1999.

[48] Schoenwaelder, J.,"SNMP Payload Compression", Internet draft (work
     in progress),  draft-irtf-nmrg-snmp-compression-00.txt, June 1999.

[49] Sprenkels, R., Martin-Flatin, J.,"Bulk Transfers of MIB Data",
     Simple Times, http://www.simple-times.org/pub/simple-
     times/issues/7-1.html, March 1999.

[50] Shacham, A., Monsour, R., Pereira, R., Thomas, M., "IP Payload
     Compression Protocol (IPComp)", RFC 2393, December 1998.

[51] Hornstein, K., Hardaker, W., "A Kerberos Security Model for SNMP
     v3", Internet draft (work in progress), draft-hornstein-
     snmpv3-ksm-00.txt, June 1999.

[52] Tung, B., Neuman, C., Hur, M., Medvinsky, A., Medvinsky, S., Wray,
     J., Trostle, J., "Public Key Cryptography for Initial
     Authentication in Kerberos", Internet draft (work in progress),
     draft-ietf-cat-kerberos-pk-init-09.txt, June 1999.

[53] Tung, B., Ryutov, T., Neuman, C., Tsudik, G., Sommerfeld, B.,
     Medvinsky, A., Hur, M.,  "Public Key Cryptography for Cross-Realm
     Authentication in Kerberos", Internet draft (work in progress),
     draft-ietf-cat-kerberos-pk-cross-04.txt, June 1999.





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[54] Thaler, D., "Get Subtree Retrieval MIB", Internet draft (work in
     progress), draft-irtf-nmrg-get-subtree-mib-00.txt, October 1999.

[55] Stewart, R. R., et al., "Simple Control Transmission Protocol",
     Internet draft (work in progress), draft-ietf-sigtran-sctp-05.txt,
     January 2000.

12.  Author's Addresses

Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
USA

Phone: +1 425 936 6605
EMail: bernarda@microsoft.com

Jari Arkko
Oy LM Ericsson Ab
02420 Jorvas
Finland

Phone: +358 40 5079256
EMail: Jari.Arkko@ericsson.com

David Harrington
Cabletron Systems Inc.
P.O.Box 5005
Rochester NH 03867-5005
USA

Phone: +1 603 337 7357
EMail: dbh@cabletron.com


13.  Intellectual Property Statement

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be made available, or the result of an attempt made to obtain a general



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license or permission for the use of such proprietary rights by
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The IETF invites any interested party to bring to its attention any
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Copyright (C) The Internet Society (2000).  All Rights Reserved.
This document and translations of it may be copied and furnished to
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15.  Expiration Date

This memo is filed as <draft-ietf-aaa-acct-03.txt>, and  expires
December 1, 2000.














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