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Versions: 00 01 02 03 04 05 06 rfc2975                     Informational
AAA Working Group                                          Bernard Aboba
INTERNET-DRAFT                                     Microsoft Corporation
Category: Informational                                       Jari Arkko
<draft-ietf-aaa-acct-06.txt>                                    Ericsson
21 June 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

To view the list Internet-Draft Shadow Directories, see

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

-06 draft: rewrote SNMP section yet again.
-05 draft: rewrote SNMP section.
-04 draft: rewrote SNMP section, cleaned up references
-03 draft: rewrote SNMPv3 section.
-02 draft: added discussion of accounting proxies. Expanded
discussion of accounting server faults and failover. Revised
section on SNMPv3. 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   Requirements language                              3
         5.2   Terminology                                        3
         5.3   Accounting management architecture                 5
         5.4   Accounting management objectives                   7
         5.5   Intra-domain and inter-domain accounting          10
         5.6   Accounting record production                      11
         5.7   Requirements summary                              13
     6.  Scaling and reliability                                 14
         6.1   Fault resilience                                  14
         6.2   Resource consumption                              22
         6.3   Data collection models                            25
     7.  Review of Accounting Protocols                          31
         7.1 RADIUS                                              31
         7.2 TACACS+                                             32
         7.3 SNMP                                                32
     8.  Review of Accounting Data Transfer                      42
         8.1 SMTP                                                42
         8.2 Other protocols                                     43
     9.  Summary                                                 43
     10. Acknowledgments                                         46
     11. References                                              46
     12. Authors' Addresses                                      49
     13. Intellectual Property Statement                         50
     14. Full Copyright Statement                                51
     15. Expiration date                                         51

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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.  Requirements language

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

5.2.  Terminology

This document frequently uses the following terms:

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

Archival accounting
          In archival accounting, the goal is to collect all accounting
          data, to reconstruct missing entries as best as possible in
          the event of data loss, and to archive data for a mandated
          time period. It is "usual and customary" for these systems to
          be engineered to be very robust against accounting data loss.
          This may include provisions for transport layer as well as
          application layer acknowledgments, use of non-volatile
          storage, interim accounting capabilities (stored or
          transmitted over the wire), etc.  Legal or financial
          requirements frequently mandate archival accounting practices,
          and may often dictate that data be kept confidential,
          regardless of whether it is to be used for billing purposes or

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

Billing   The act of preparing an invoice.

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

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
          Interim accounting provides a snapshot of usage during a
          user's session. This may be useful in the event of a device
          reboot or other network problem that prevents the reception or
          generation of a session summary packet or session record.
          Interim accounting records can always be summarized without
          the loss of information. Note that interim accounting records
          may be stored internally on the device (such as in non-
          volatile storage) so as to survive a reboot and thus may not
          always be transmitted over the wire.

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

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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
          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.

5.3.  Accounting management architecture

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

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

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

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accomplish the transfer.

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

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

Where a proxy forwarder is employed, domain-based access controls may be
employed by the proxy forwarder, rather than by the devices themselves.
The network device will typically speak an accounting protocol to the
proxy forwarder, which may then either convert the accounting packets to
session records, or forward the accounting packets to another domain.
In either case, domain separation is typically achieved by having the
proxy forwarder sort the session records or accounting messages by

Where the accounting proxy is not trusted, it may be difficult to verify
that the proxy is issuing correct session records based on the
accounting messages it receives, since the original accounting messages
typically are not forwarded along with the session records. Therefore
where trust is an issue, the proxy typically forwards the accounting
packets themselves.  Assuming that the accounting protocol supports data
object security, this allows the end-points to verify that the proxy has
not modified the data in transit or snooped on the packet contents.

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The diagram below illustrates the accounting management architecture:

     |            |
     |   Network  |
     |   Device   |
     |            |
Accounting |
Protocol   |
     +------------+                               +------------+
     |            |                               |            |
     |   Org B    |  Inter-domain session records |  Org A     |
     |   Acctg.   |<----------------------------->|  Acctg.    |
     |Proxy/Server|   or accounting protocol      |  Server    |
     |            |                               |            |
     +------------+                               +------------+
           |                                            |
           |                                            |
Transfer   | Intra-domain                               |
Protocol   | Session records                            |
           |                                            |
           V                                            V
     +------------+                               +------------+
     |            |                               |            |
     |  Org B     |                               |  Org A     |
     |  Billing   |                               |  Billing   |
     |  Server    |                               |  Server    |
     |            |                               |            |
     +------------+                               +------------+

5.4.  Accounting management objectives

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

5.4.1.  Trend analysis and capacity planning

In trend analysis and capacity planning, the goal is typically a
forecast of future usage.  Since such forecasts are inherently
imperfect, high reliability is typically not required, and moderate
packet loss can be tolerated.  Where it is possible to use statistical
sampling techniques to reduce data collection requirements while still
providing the forecast with the desired statistical accuracy, it may be

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possible to tolerate high packet loss as long as bias is not introduced.

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

5.4.2.  Billing

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

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

Usage-sensitive systems may also require low processing delay. Today
credit risk is commonly managed by computerized fraud detection systems
that are designed to detect unusual activity. While efficiency concerns
might otherwise dictate batched transmission of accounting data, where
there is a risk of fraud, financial exposure increases with processing
delay. Thus it may be advisable to transmit each event individually to
minimize batch size, or even to utilize quality of service techniques to
minimize queuing delays. In addition, it may be necessary for
authorization to be dependent on ability to pay.

Whether these techniques will be useful varies by application since the
degree of financial exposure is application-dependent.  For dial-up
Internet access from a local provider, charges are typically low and
therefore the risk of loss is small.  However, in the case of dial-up
roaming or voice over IP, time-based charges may be substantial and

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therefore the risk of fraud is larger. In such situations it is highly
desirable to quickly detect unusual account activity, and it may be
desirable for authorization to depend on ability to pay. In situations
where valuable resources can be reserved, or where charges can be high,
very large bills may be rung up quickly, and processing may need to be
completed within a defined time window in order to limit exposure.

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

5.4.3.  Auditing

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

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

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

5.4.4.  Cost allocation

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

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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.

5.5.  Intra-domain and inter-domain accounting

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

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

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

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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5.6.  Accounting record production

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

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

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

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

In most cases, the network device will determine when multiple session
records are needed, as the local device is aware of factors affecting
local tariffs, such as QoS changes and roaming.  However, future systems
are being designed that enable the home domain to 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-

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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5.7.  Requirements summary

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

   [] = 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, SCTP and
TCP-based transport. However, it should be understood that resilience
against packet loss is only  one aspect of meeting archival accounting

As noted in [18], "once the cable is cut you don't need more
retransmissions, you need a *lot* more voltage."  Thus, the choice of
transport has no impact on resilience against faults such as network
partition, accounting server failures or device reboots. What does
provide resilience against these faults is non-volatile storage.

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

It may even be argued that non-volatile storage is more important to
accounting reliability than network connectivity, since for many years
reliable accounting systems were implemented based solely on physical
storage, without any network connectivity. For example, 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

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

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

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

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

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

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 [7] or
SCTP [26]. 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, it can also be argued that in modern accounting
implementations, it is possible to implement congestion control while
improving throughput and maintaining high reliability.  In circumstances
where there is sustained packet loss, there simply is not sufficient
capacity to maintain existing transmission rates. Thus, aggregate
throughput will actually improve if congestive back-off is implemented.
This is due to elimination of retransmissions and the ability to utilize
techniques such as RED to desynchronize flows. In addition, with QoS
mechanisms such as differentiated services, it is possible to mark
accounting packets for preferential handling so as to provide for lower
packet loss if desired. Thus considerable leeway is available to the
network administrator in controlling the treatment of accounting packets
and hard coding inelastic behavior is unnecessary. Typically, systems
implementing non-volatile storage allow for backlogged accounting data
to be placed in non-volatile storage pending transmission, so that
buffer exhaustion resulting from congestive back-off need not be a

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 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

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. SCTP also enables multiplexing of multiple
connections within a single transport connection, all maintaining the
same congestion control state, avoiding the "head of line blocking"
issues that can occur with TCP.  However, since SCTP is not widely
available, use of this transport can impose an additional implementation
burden on the designer.

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

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

6.1.5.  Application layer acknowledgments

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

In such cases it is desirable to distinguish between transport layer
acknowledgment and application layer acknowledgment.  Even though both
acknowledgments may be sent within the same packet (such as a TCP
segment carrying an application layer acknowledgment along with a piggy-
backed ACK), the semantics are different. A transport-layer
acknowledgment means "the transport layer has taken responsibility for
delivering the data to the application", while an application-layer
acknowledgment means "the application has taken responsibility for the

A common misconception is that use of TCP transport guarantees that data
is delivered to the application.  However, as noted in RFC 793 [7]:

  An acknowledgment by TCP does not guarantee that the data has been

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  delivered to the end user, but only that the receiving TCP has taken
  the responsibility to do so.

Therefore, if receiving TCP fails after sending the ACK, the application
may not receive the data. Similarly, if the application fails prior to
committing the data to stable storage, the data may be lost.  In order
for a sending application to be sure that the data it sent was received
by the receiving application, either a graceful close of the TCP
connection or an application-layer acknowledgment is required. In order
to protect against data loss, it is necessary that the application-layer
acknowledgment imply that the data has been written to stable storage or
suitably processed so as to guard against loss.

In the case of partial failures, it is possible for the transport layer
to acknowledge receipt via transport layer acknowledgment, without
having delivered the data to the application. Similarly, the application
may not complete the tasks necessary to take responsibility for the

For example, an accounting server may receive data from the transport
layer but be incapable of storing it data due to a back end database
problem or disk fault. In this case it should not send an application
layer acknowledgment, even though a a transport layer acknowledgment is
appropriate. Rather, an application layer error message should be sent
indicating the source of the problem, such as "Backend store

Thus application-layer acknowledgment capability requires not only the
ability to acknowledge when the application has taken responsibility for
the data, but also the ability to indicate when the application has not
taken responsibility for the data, and why.

6.1.6.  Network failures

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

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

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6.1.7.  Device reboots

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

6.1.8.  Accounting proxies

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

Accounting proxies sending a transport or application layer ACK to the
device without receiving one from the accounting server fool the device
into thinking that the accounting request had been accepted by the
accounting server when this is not the case. As a result, the device can
delete the accounting packet from non-volatile storage before it has
been accepted by the accounting server. The leaves the accounting proxy
responsible for delivering accounting packets. If the accounting proxy
involves moving parts (e.g. a disk drive) while the devices do not,
overall system reliability can be reduced.

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

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

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6.1.9.  Fault resilience summary

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

   * = 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
   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, we examine each of these
resources in turn.

6.2.1.  Network bandwidth

Accounting management systems consume network bandwidth in transferring
accounting data. The network bandwidth consumed is proportional to the
amount of data transferred, as well as required network overhead.  Since
accounting data for a given event may be 100 octets or less, if each
event is transferred individually, overhead can represent a considerable
proportion of total bandwidth consumption.  As a result, it is often
desirable to transfer accounting data in batches, enabling network
overhead to be spread over a larger payload, and enabling efficient use
of compression.  As noted in [48], compression can be enabled in the
accounting protocol, or can be done at the IP layer as described in [5].

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

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.

Non-volatile storage may be used to store interim or session records. As
with memory utilization, interim accounting overwrite is desirable so as
to prevent excessive storage consumption. Note that the use of ASCII
data representation enables use of highly efficient text compression
algorithms that can minimize storage requirements. Such compression
algorithms are only typically applied to session records so as to enable

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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

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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            |
   |                 |                                       |


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. This
may be too high for accounting data that requires low processing delay.
Thus the event-driven polling or the pure event-driven approach is more
appropriate for usage sensitive billing applications such as shared use
or roaming implementations.

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

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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

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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          |

   * = 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

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

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7.2.  TACACS+

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

TACACS+ provides for hop-by-hop authentication and integrity protection
as well as hop-by-hop confidentiality. Data object security is not
supported, and therefore systems based on TACACS+ accounting are not
deployable in the presence of untrusted proxies.  While TACACS+ does not
support compression, IP compression, described in [5], can be employed
to provide this.

7.3.  SNMP

SNMP, described in [19],[27]-[41], has been widely deployed in a wide
variety of intra-domain accounting applications, typically using the
polling data collection model. Polling allows data to be collected on
multiple accounting events simultaneously, resulting in per- device
state. Management applications are able to retry requests when a
response is not received, providing resiliency against packet loss or
even short-lived network partitions.  Implementations without non-
volatile storage are not robust against device reboots or network
failures, but when combined with non-volatile storage they can be made
highly reliable.

SMIv1, the data modeling language of SNMPv1, has traps to permit trap-
directed polling, but the traps are not acknowledged, and lost traps can
lead to a loss of data. SMIv2, used by SNMPv2c and SNMPv3, has Inform
Requests which are acknowledged notifications. This makes it possible to
implement a more reliable event-driven polling model or event-driven
batching model. However, we are not aware of any SNMP-based accounting
implementations currently built on the use of Informs.

7.3.1.  Security services

SNMPv1 and SNMPv2c support per-packet authentication and read-only and
read-write access profiles, via the community string. This clear-text
password approach provides only trivial authentication, and no per-
packet integrity checks, replay protection or confidentiality. View-
based access control [40] can be supported using the snmpCommunityMIB,
defined in [11], and SNMPv1 or SNMPv2c messages.  The updated SNMP
architecture [rfc2571] supports per-packet hop-by-hop authentication,

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integrity and replay protection, confidentiality and access control.

The SNMP User Security Model (USM) [38] uses shared secrets, and when
the product of the number of domains and devices is large, such as in
inter-domain accounting applications, the number of shared secrets can
get out of hand.  The localized key capability in USM allows a manager
to have one central key, sharing it with many SNMP entities in a
localized way while preventing the other entities from getting at each
other's data. This can assist in cross-domain security if deployed

SNMPv3 does not support end-to-end data object integrity and
confidentiality; SNMP proxy entities decrypt and re-encrypt the data
they forward. In the presence of an untrusted proxy entity, this would
be inadequate.

7.3.2.  Application layer acknowledgments

SNMP uses application-layer acknowledgment to indicate that data has
been processed. SNMP Responses to get, get-next, or get-bulk requests
return the requested data, or an error code indicating the nature of the
error encountered.

A noError SNMP Response to a SET command indicates that the requested
assignments were made by the application. SNMP SETs are atomic; the
command either succeeds or fails. An error-response indicates that the
entity received the request, but did not succeed in executing it.

Notifications do not use acknowledgements to indicate that data has been
processed. The Inform notification returns an acknowledgement of
receipt, but not of processing, by design. Since the updated SNMP
architecture treats entities as peers with varying levels of
functionality, it is possible to use SETs in either direction between
cooperating entities to achieve processing acknowledgements.

There are eighteen SNMP error codes. The design of SNMP makes service-
specific error codes unnecessary and undesirable.

7.3.3.  Proxy forwarders

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

The functionality of an SNMP Proxy Forwarder is defined in [39].  For
example, the network devices may be configured to send notifications for
all domains to the Proxy Forwarder, and the devices may be configured to

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allow the Proxy Forwarder to access all MIB data.

The use of proxy forwarders may reduce the number of shared secrets
required for inter-domain accounting. With Proxy Forwarders, the domains
could share a secret with the Proxy Forwarder, and in turn, the Proxy
Forwarder could share a secret with each of the devices.  Thus the
number of shared secrets will scale with the sum of the number of
devices and domains rather than the product.

The engine of an SNMP Proxy Forwarder does not look inside the PDU of
the message except to determine to which SNMP engine the PDU should be
forwarded or which local SNMP application should process the PDU. The
SNMP Proxy Forwarder does not modify the varbind values; it does not
modify the varbind list except to translate between SNMP versions; and
it does not provide any varbind level access control.

7.3.4.  Domain-based access controls in SNMP

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

In order to apply domain-based access controls, in inter-domain
accounting, it is first necessary to identify the data subset that is to
have its access controlled. Several conceptual abstractions are used for
identifying subsets of data in SNMP.  These include engines, contexts,
and views.  This section describes how this functionality may be applied
in intra and inter-domain accounting.  Engines

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

A securityEngineID field in a message identifies the engine which
provides access to the security credentials contained in the message
header. A contextEngineID field in a message identifies the engine which
provides access to the data contained in the PDU.

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The SNMPv3 message format explicitly passes both. In SNMPv1 and SNMPv2c,
the data origin is typically assumed to be the communications endpoint
(SNMP agent). SNMPv1 and SNMPv2c messages contain a community name; the
community name and the source address can be mapped to an engineID via
the snmpCommunityTable, described in [11].  Contexts

Contexts are used to identify subsets of objects, within the scope of an
engine, that are tied to instrumentation. A contextName refers to a
particular subset within an engine.

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

An SNMP agent populates a read-only dynamic table to tell the manager
what contexts it recognizes. Typically contexts are defined by the agent
rather than the manager since if the manager defined them, the agent
would not know how to tie the contexts to the underlying
instrumentation. It is possible that MIB modules could be defined to
allow a manager to assign contextNames to a logical subset of

While each context may support instances of multiple MIB modules, each
contextName is limited to one instance of a particular MIB module. If
multiple instances of a MIB module are required per engine, then unique
contextNames must be defined (e.g. repeater1, repeater2). The default
context "" is used for engines which only support single instances of
MIB modules, and it is used for MIB modules where it only makes sense to
have one instance of that MIB module in an engine and that instance must
be easy to locate, such as the system MIB or the security MIBs.

SNMPv3 messages contain contextNames which are limited to the scope of
the contextEngineID in the message. SNMPv1 and SNMPv2c messages contain
communities which can be mapped to contextNames within the local engine,
or can be mapped to contextNames within other engines via the
snmpCommunityTable, described in [11].  Views

Views are defined in the View-based Access Control Model. A view is a
mask which is used to determine access to the managed objects in a
particular context. The view identifies which objects are visible, by
specifying OIDs of the subtrees included and excluded. There is also a
mechanism to allow wildcards in the OID specification.

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

Views can be set up with wildcards. For a table that is indexed using IP
addresses, Joe can be allowed access to all rows in given RMON tables
(e.g. the RMON hostTable) that are in the subnet 10.2.x.x, while
Josephine is given access to all rows for subnet 10.200.x.x.

Views filter at the name level (OIDs), not at the value level, so
defining views based on the values of non-index data is not supported.
In this example, were the IP address to have been used merely as a data
item rather than an index, it would not be possible to utilize view-
based access control to achieve the desired objective (delegation of
administrative responsibility according to subnet).

View-based access control is independent of message version. It can be
utilized by entities using SNMPv1, SNMPv2c, or SNMPv3 message formats.

7.3.5.  Inter-domain access-control alternatives

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

This issue can be addressed in SNMP using the event-driven, event-
driven polling or event-driven batching approaches. Traps and Informs
permit SNMP-enabled devices to notify domains that have accounting data
awaiting collection. SNMP Applications [39] defines a standard module
for managing notifications.

To use the event-driven approaches, the device must be able to determine
when information is available for a domain. Domain-specific data can be
differentiated at the SNMP agent level through the use of the domain as
an index, and the separation of data into domain-specific contexts.  Domain as index

View-based access control [40] allows multiple fine-grained views of an
SNMP MIB to be assigned to specific groups of users, such that access

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rights to the included data elements depend on the identity of the user
making the request.

For example, all users of bigco.com which are allowed access to the
device would be defined in the User-based security MIB module (or other
security model MIB module). For simplicity in administering access
control, the users can be grouped using a vacmGroupName, e.g. bigco. A
view of a subset of the data objects in the MIB can be defined in the
vacmViewFamilyTreeTable. A vacmAccessTable pairs groups and views. For
messages received from users in the bigco group, access would only be
provided to the data permitted to be viewed by bigco users, as defined
in the view family tree. This requires that each domain accessing the
data be given one or more separate vacmGroupNames, an appropriate
ViewTable be defined, and the vacmAccessTable be configured for each

Views filter at the name (OID) level, not at the data (value) level.
When using views to filter by domain it is necessary to use the domain
as an index. Standard view-based access control is not designed to
filter based on the values on non-indexed fields.

For example, a table of session data could be indexed by record number
and domain, allowing a view to be defined that could restrict access to
bigco data to the administrators of the bigco domain.

An advantage of using domains as an index is that this technique can be
used with SNMPv1 and SNMPv2c agents as well as with SNMPv3 agents. A
disadvantage is that the MIB modules must be specifically designed for
this purpose. Since existing MIB modules rarely use the domain as an
index, domain separation cannot be enabled within legacy MIB modules
using this technique.

SNMP does support a RowPointer convention that could be used to define a
new table, indexed by domain, which holds tuples between the domain and
existing rows of data. This would introduce issues of synchronization
between tables.  Contexts

ContextNames can be used to differentiate multiple instances of a MIB
module within an engine.

Individual domains, such as bigco.com, could be mapped to logical
contexts, such as a bigco context. The agent would need to create and
recognize new contexts and to know which instrumentation is associated
with the logical context. The agent needs to collect accounting data by
domain and make the data accessible via distinct contexts, so that
access control can be applied to the context to prevent disclosure of

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sensitive information to the wrong domain. The VACM access control views
are applied relative to the context, so an operation can be permitted or
denied a user based on the context which contains the data.

Domain separation is handled by using contextName to differentiate
multiple virtual tables. For example, if accounting data has been
collected on users with the bigco.com and smallco.com domains, then a
separate virtual instance of the accounting session record table would
exist for each domain, and each domain would have a corresponding
contextName. When a get-bulk request is made with a contextName of
bigco, then data from the virtual table in the bigco context, i.e.
corresponding to the bigco.com domain, would be returned.

There are a number of design approaches to creating new contexts and
associating the contexts with appropriate instrumentation, most notably
a sub-agent approach and a manager-configured MIB approach.

AgentX [51], which standardizes a registration protocol between sub-
agents and master agents to simplify SNMP agent implementation, allows
for the creation and recognition of new contextNames when a subagent
registers to provide support for a particular MIB subtree range. The
sub-agent knows how to support a particular functionality, e.g.
instrumentation exposed via a range of MIB objects. Based on values
detected in the data, such as source=bigco.com, the sub-agent could
determine that a new domain needed to be tracked and create the
appropriate context for the collection of the data, plus the appropriate
access control entries. The determination could be table-driven, using
MIB configuration.

A manager-driven approach could use a MIB module to predefine
contextNames corresponding to the domains of interest, and to indicate
which objects should be collected, how to differentiate to which domain
the data should be applied based on a specified condition, and what
access control rules apply to the context.

Either technique could associate existing MIB modules to domain-specific
contexts, so domain separation can be applied to MIB modules not
specifically designed with domain separation in mind. Legacy agents
would not be designed to do this, so they would need to be updated to
support inter-domain separation and VACM access control.

The use of contextNames for inter-domain separation represents new
territory, so careful consideration would be needed in designing the MIB
modules and applications to provide domain to context and context to
instrumentation mappings, and to ensure that security is not weakened.

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7.3.6.  Outstanding issues

There are issues that arise when using SNMP for transfer of bulk data,
including issues of latency, network overhead, and table retrieval, as
discussed in [49].

In accounting applications, management stations often must retrieve
large tables. Latency can be high, even with the get-bulk operation,
because the response must fit into the largest supported packet size,
requiring multiple round-trips. Transfers may be serialized and the
resulting latency will be a combination of multiple round-trip times,
possible timeout and re-transmission delays and processing overhead,
which may result in unacceptable performance.  Since data may change
during the course of multiple retrievals, it can be difficult to get a
consistent snapshot.

For bulk transfers, SNMP network overhead can be high due to the lack of
compression, inefficiency of BER encoding, the  transmission of
redundant OID prefixes, and the "get-bulk overshoot problem". In bulk
transfer of a table, the OIDs transferred are redundant: all OID
prefixes up to the column number are identical, as are the instance
identifier postfixes of all entries of a single table row. Thus it may
be possible to reduce this redundancy by compressing the OIDs, or by not
transferring an OID with each variable.

The "get-bulk overshoot problem", described in reference [50], occurs
when using the get-bulk PDU. The problem is that the manager typically
does not know the number of rows in the table. As a result, it must
either request too many rows, retrieving unneeded data, or too few,
resulting in the need for multiple get-bulk requests.  Note that the
"get-bulk overshoot" problem may be preventable on the agent side.
Reference [41] states that an agent can terminate the get-bulk because
of "local constraints" (see items 1 and 3 on pages 15/16 of [41]). This
could be interpreted to mean that it is possible to stop at the end of a
table.  Ongoing research

To address issues of latency and efficiency, the Network Management
Research Group (NMRG) was formed within the Internet Research Task Force
(IRTF). Since the NMRG work is research and is not on the standards
track, it should be understood that the NMRG proposals may never be
standardized, or may change substantially during the standardization
process. As a result, these proposals represent works in progress and
are not readily available for use.

The proposals under discussion in the IRTF Network Management Research
Group (NMRG) are described in [46]. These include an SNMP-over-TCP

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transport mapping, described in [47]; SNMP payload compression,
described in [48]; and the addition of a "get subtree" PDU or the
subtree retrieval MIB [50].

The SNMP-over-TCP transport mapping may result in substantial latency
reductions in table retrieval. The latency reduction of an SNMP-over-TCP
transport mapping will likely manifest itself primarily in the polling,
event-driven polling and event-driven batching modes.

Payload compression methods include compression of the IP packet, as
described in [5] or compression of the SNMP payload, described in [48].

Proposed improvements to table retrieval include a subtree retrieval MIB
and the addition of a get-subtree PDU. The subtree retrieval MIB [50]
requires no changes to the SNMP protocol or SNMP protocol engine, so it
can be implemented and deployed more easily than a change to the
protocol.  The addition of a get-subtree PDU implies changes to the
protocol and to the engines of all SNMP entities which would support it.
Since it may be possible to address the "get-bulk overshoot problem"
without changes to the SNMP protocol, the necessity of this modification
is controversial.

Reference [49] also discusses file-based storage of SNMP data, and use
of an FTP MIB, to enable storage of SNMP data in non-volatile storage,
and subsequent bulk transfer via FTP. This approach would require
implementation of additional MIB modules as well as FTP, and requires
separate security mechanisms such as IPSEC to provide authentication,
replay, integrity protection and confidentiality for the data in
transit.  The file-based transfer approach has an important benefit -
compatibility with non-volatile storage.

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

In order to simplify key management and enable use of certificate-based
security in SNMPv3, a Kerberos Security Model (KSM) for SNMPv3 has been
proposed in [44]. This draft is not on the standards track, and
therefore is not yet readily available for use.

Use of Kerberos with SNMPv3 requires storage of a key on the KDC for
each device and domain, while dynamically generating a session key for
conversations between domains and devices. In terms of stored keys, the
KSM approach scales with the sum of devices and domains; in terms of

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dynamic session keys, it scales as the product of domains and devices.

As Kerberos is extended to allow initial authentication via public key,
as described in [42], and cross-realm authentication, as described in
[43], the KSM inherits these capabilities. As a result, this approach
may have potential to reduce or even eliminate the shared secret
management problem.  However, it should also be noted that certificate-
based authentication can strain the limits of UDP packet sizes supported
in SNMP implementations, so that alternate transport mappings may be
required to support this.

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

7.3.7.  SNMP summary

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

Support for notifications makes it possible to implement the event-
driven, event-driven polling and event-driven batching models.  This
makes it possible to notify domains of available data rather than
requiring them to poll for it, which is critical in shared use networks
and roaming.

Given the SNMPv3 security enhancements, it is desirable for SNMP-based
intra-domain accounting implementations to upgrade to SNMPv3. Such an
upgrade is virtually mandatory for inter-domain applications.

In inter-domain accounting, the burden of managing SNMPv3 shared secrets
can be reduced via the localized key capability or via implementation of
a Proxy Forwarder. In the long term, alternative security models such as
the Kerberos Security Model may further reduce the effort required to
manage security and enable streamlined inter-domain operation.

SNMP-based accounting has limitations in terms of efficiency and latency
that may make it inappropriate for use in situations requiring low
processing delay or low overhead. This includes usage sensitive billing

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applications where fraud detection may be required.  These issues can be
addressed via proposals under discussion in the IRTF Network Management
Research Group (NMRG). The experimental SNMP over TCP transport mapping
may prove helpful at reducing latency.  Depending on the volume of data,
some form of compression may also be worth considering. However, since
these proposals are still in the research stage, and are not on the
standards track, these capabilities are not readily available, and the
specifications could change considerably before they reach their final

SNMP supports separation of accounting data by domain, using either of
two general approaches with the VACM access control model. The domain as
index approach can be used if the desired MIB module supports domain
indexing, or it can implemented using an additional table. The domain-
context approach can be used in agents which support dynamic logical
contexts and a domain-to-context and context-to-instrumentation mapping
mechanism. Either approach can be supported using SNMPv1, SNMPv2c, or
SNMPv3 messages, by utilizing the snmpCommunitytable [11] to provide a
community-to-context mapping.

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].

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

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,

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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

As described in [13] and [15], data object security is available for
SMTP, and in addition, the facilities described in [12] make it possible
to request and receive signed receipts, which enables non-repudiation as
described in [12]-[17]. As a result, accounting systems utilizing SMTP
for accounting data transfer are capable of satisfying the most
demanding security requirements. However, such systems are not typically
capable of providing low processing delay, although this may be
addressed by the enhancements described in [20].

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, 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.

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

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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

Based on an analysis of the requirements, it appears that existing
deployed protocols are capable of meeting the requirements for intra-
domain capacity planning and non-usage sensitive billing.  In these
applications efficient transfer of bulk data is useful although not
critical. Thus, it is possible to use SNMPv3 to satisfy these
requirements, without the NMRG extensions.  These include TCP transport
mapping, sub-tree retrieval, and OID compression.

In inter-domain capacity planning and non-usage sensitive billing, the
security and reliability requirements are greater. As a result, no
existing deployed protocol satisfies the requirements. For example,
existing protocols lack data object security support and extensions to
improve scalability of inter-domain authentication are needed, such as
the Kerberos Security Model (KSM) for SNMPv3.

For usage sensitive billing, as well as cost allocation and auditing
applications, the reliability requirement are greater.  Here transport
layer reliability is required to provide robustness against packet loss,
as well as application layer acknowledgments to provide robustness
against accounting server failures. SNMP operations with the exception
of InforRequest provide application layer acknowledgments, and the TCP
transport mapping proposed by NMRG provides robustness against packet
loss.  Inter-domain operation can benefit from data object security
(which no existing protocol provides) as well as inter-domain security
model enhancements (such as the KSM).

Where high-value sessions are involved, such as in roaming, Mobile IP,
or telephony, it may be necessary to put bounds on processing delay.
This implies the need to reduce latency. As a result, the NMRG
extensions are required in time sensitive billing applications,
including TCP transport mapping, get-subtree capabilities and OID
compression.  High reliability is also required in this application,
implying the need for application layer as well as transport layer
acknowledgments. SNMPv3 with the NMRG extensions and security
scalability improvements such as the KSM can satisfy the requirements in
intra-domain use.

However, in inter-domain use, additional security precautions such as
data object security and receipt support are required. No existing
protocol can meet these requirements.  A summary is given in the table
on the next page.

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   |                 |                     |                   |
   |  Usage          |   Intra-domain      | Inter-domain      |
   |                 |                     |                   |
   |                 |                     |                   |
   |  Capacity       | SNMPv3 &            | SNMPv3 &<*        |
   |  Planning       | RADIUS #%@          |                   |
   |                 | TACACS+ @           |                   |
   |                 |                     |                   |
   |                 |                     |                   |
   |  Non-usage      | SNMPv3 &            | SNMPv3 &<*        |
   |  Sensitive      | RADIUS #%@          |                   |
   |  Billing        | TACACS+ @           |                   |
   |                 |                     |                   |
   |                 |                     |                   |
   |  Usage          |                     |                   |
   |  Sensitive      |                     |                   |
   |  Billing,       | SNMPv3 &>$          | SNMPv3 &<>*$      |
   |  Cost           | TACACS+ &$@         |                   |
   |  Allocation &   |                     |                   |
   |  Auditing       |                     |                   |
   |                 |                     |                   |
   |                 |                     |                   |
   |  Time           |                     |                   |
   |  Sensitive      | SNMPv3 &>$          |  No existing      |
   |  Billing,       |                     |  protocol         |
   |  fraud          |                     |                   |
   |  detection,     |                     |                   |
   |  roaming        |                     |                   |
   |                 |                     |                   |

   # = lacks confidentiality support
   * = lacks data object security
   % = limited robustness against packet loss
   & = lacks application layer acknowledgment (e.g. SNMP InformRequest)
   $ = requires non-volatile storage
   @ = lacks batching support
   < = lacks certificate support (KSM, work in progress)
   > = lacks support for large packet sizes (TCP transport mapping, experimental)

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10.  Acknowledgments

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

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,

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

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

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

[7]  Information Sciences Institute, "Transmission Control Protocol",
     RFC 793, September 1981.

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

[9]  McCloghrie, K., Heinanen, J., Greene, W., 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

[11] Frye, R., Levi, D., Routhier, S., Wijnen, B., "Coexistence between
     Version 1, Version 2, and Version 3 of the Internet-standard
     Management Framework", RFC 2576, March 2000.

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

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

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

[15] Galvin, J.,  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

[17] 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.

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

[19] Case, J., Mundy, R., Partain, D., Stewart, B., "Introduction to
     Version 3 of the Internet-standard Network Management Framework",
     RFC 2570, April 1999.

[20] Klyne, G., "Timely Delivery for Facsimile Using Internet Mail",
     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

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

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

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

[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

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     (SNMP)", RFC 2575, April 1999.

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

[42] Tung, B., Neuman, C., Hur, M., Medvinsky, A., Medvinsky, S., Wray,
     J., 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.

[43] 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.

[44] Hornstein, K., Hardaker, W., "A Kerberos Security Model for
     SNMPv3", Internet draft (work in progress), draft-hornstein-
     snmpv3-ksm-00.txt, June 1999.

[45] Brownlee, N., Blount, A., "Accounting Attributes and Record
     Formats", Internet draft (work in progress), draft-ietf-aaa-
     accounting-attributes-04.txt, June 2000.

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

[47] Schoenwaelder, J.,"SNMP-over-TCP Transport Mapping", Internet draft
     (work in progress), draft-irtf-nmrg-snmp-tcp-03.txt, April 2000.

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

[51] Daniele, M., Wijnen, B., Ellison, M., Francisco, D., "Agent
     Extensibility (AgentX) Protocol Version 1", RFC 2741, January 2000.

12.  Author's Addresses

Bernard Aboba
Microsoft Corporation
One Microsoft Way

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Redmond, WA 98052

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

Jari Arkko
Oy LM Ericsson Ab
02420 Jorvas

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

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

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

13.  Intellectual Property Statement

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The IETF invites any interested party to bring to its attention any
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standard.  Please address the information to the IETF Executive

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14.  Full Copyright Statement

Copyright (C) The Internet Society (2000).  All Rights Reserved.
This document and translations of it may be copied and furnished to
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15.  Expiration Date

This memo is filed as <draft-ietf-aaa-acct-06.txt>, and  expires January
1, 2001.

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