RFC2975 - Introduction to Accounting Management

Network Working Group B. Aboba
Request for Comments: 2975 Microsoft Corporation
Category: Informational J. Arkko
EriCsson
D. Harrington
Cabletron Systems Inc.
October 2000
IntrodUCtion to Accounting Management
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
The field of Accounting Management is concerned with the collection
of resource consumption data for the purposes of capacity and trend
analysis, cost allocation, auditing, and billing. This document
describes each of these problems, and discusses the issues involved
in design of modern accounting systems.
Since accounting applications do not have uniform security and
reliability requirements, it is not possible to devise a single
accounting protocol and set of security services that will meet all
needs. Thus the goal of accounting management is to provide a set of
tools that can be used to meet the requirements of each application.
This document describes the currently available tools as well as the
state of the art in accounting protocol design. A companion
document, RFC2924, reviews the state of the art in accounting
attributes and record formats.
Table of Contents
1. Introduction 2
1.1 Requirements language 3
1.2 Terminology 3
1.3 Accounting management architecture 5
1.4 Accounting management objectives 7
1.5 Intra-domain and inter-domain accounting 10
1.6 Accounting record production 11
1.7 Requirements summary 13
2. Scaling and reliability 14
2.1 Fault resilience 14
2.2 Resource consumption 23
2.3 Data collection models 26
3. Review of Accounting Protocols 32
3.1 RADIUS 32
3.2 TACACS+ 33
3.3 SNMP 33
4. Review of Accounting Data Transfer 43
4.1 SMTP 44
4.2 Other protocols 44
5. Summary 45
6. Security Considerations 48
7. Acknowledgments 48
8. References 48
9. Authors" Addresses 52
10. Intellectual Property Statement 53
11. Full Copyright Statement 54
1. Introduction
The field of Accounting Management is concerned with the collection
of resource consumption data for the purposes of capacity and trend
analysis, cost allocation, auditing, and billing. This document
describes each of these problems, and discusses the issues involved
in design of modern accounting systems.
Since accounting applications do not have uniform security and
reliability requirements, it is not possible to devise a single
accounting protocol and set of security services that will meet all
needs. Thus the goal of accounting management is to provide a set of
tools that can be used to meet the requirements of each application.
This document describes the currently available tools as well as the
state of the art in accounting protocol design. A companion
document, RFC2924, reviews the state of the art in accounting
attributes and record formats.
1.1. Requirements language
In this document, the key Words "MAY", "MUST, "MUST NOT", "optional",
"recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
described in [6].
1.2. Terminology
This document frequently uses the following terms:
Accounting
The collection of resource consumption data for the
purposes of capacity and trend analysis, cost allocation,
auditing, and billing. Accounting management requires that
resource consumption be measured, rated, assigned, and
communicated between appropriate parties.
Archival accounting
In archival accounting, the goal is to collect all
accounting data, to reconstruct missing entries as best as
possible in the event of data loss, and to archive data for
a mandated time period. It is "usual and customary" for
these systems to be engineered to be very robust against
accounting data loss. This may include provisions for
transport layer as well as application layer
acknowledgments, use of non-volatile storage, interim
accounting capabilities (stored or transmitted over the
wire), etc. Legal or financial requirements frequently
mandate archival accounting practices, and may often
dictate that data be kept confidential, regardless of
whether it is to be used for billing purposes or not.
Rating The act of determining the price to be charged for use of a
resource.
Billing The act of preparing an invoice.
Usage sensitive billing
A billing process that depends on usage information to
prepare an invoice can be said to be usage-sensitive. In
contrast, a process that is independent of usage
information is said to be non-usage-sensitive.
Auditing The act of verifying the correctness of a procedure. In
order to be able to conduct an audit it is necessary to be
able to definitively determine what procedures were
actually carried out so as to be able to compare this to
the recommended process. Accomplishing this may require
security services such as authentication and integrity
protection.
Cost Allocation
The act of allocating costs between entities. Note that
cost allocation and rating are fundamentally different
processes. In cost allocation the objective is typically
to allocate a known cost among several entities. In rating
the objective is to determine the amount to be charged for
use of a resource. In cost allocation, the cost per unit
of resource may need to be determined; in rating, this is
typically a given.
Interim accounting
Interim accounting provides a snapshot of usage during a
user"s session. This may be useful in the event of a
device reboot or other network problem that prevents the
reception or generation of a session summary packet or
session record. Interim accounting records can always be
summarized without the loss of information. Note that
interim accounting records may be stored internally on the
device (such as in non-volatile storage) so as to survive a
reboot and thus may not always be transmitted over the
wire.
Session record
A session record represents a summary of the resource
consumption of a user over the entire session. Accounting
gateways creating the session record may do so by
processing interim accounting events or accounting events
from several devices serving the same user.
Accounting Protocol
A protocol used to convey data for accounting purposes.
Intra-domain accounting
Intra-domain accounting involves the collection of
information on resource usage within an administrative
domain, for use within that domain. In intra-domain
accounting, accounting packets and session records
typically do not cross administrative boundaries.
Inter-domain accounting
Inter-domain accounting involves the collection of
information on resource usage within an administrative
domain, for use within another administrative domain. In
inter-domain accounting, accounting packets and session
records will typically cross administrative boundaries.
Real-time accounting
Real-time accounting involves the processing of information
on resource usage within a defined time window. Time
constraints are typically imposed in order to limit
financial risk.
Accounting server
The accounting server receives accounting data from devices
and translates it into session records. The accounting
server may also take responsibility for the routing of
session records to interested parties.
1.3. Accounting management architecture
The accounting management architecture involves interactions between
network devices, accounting servers, and billing servers. The
network device collects resource consumption data in the form of
accounting metrics. This information is then transferred to an
accounting server. Typically this is accomplished via an accounting
protocol, although it is also possible for devices to generate their
own session records.
The accounting server then processes the accounting data received
from the network device. This processing may include summarization
of interim accounting information, elimination of duplicate data, or
generation of session records.
The processed accounting data is then submitted to a billing server,
which typically handles rating and invoice generation, but may also
carry out auditing, cost allocation, trend analysis or capacity
planning functions. Session records may be batched and compressed by
the accounting server prior to submission to the billing server in
order to reduce the volume of accounting data and the bandwidth
required to accomplish the transfer.
One of the functions of the accounting server is to distinguish
between inter and intra-domain accounting events and to route them
appropriately. For session records containing a Network Access
Identifier (NAI), described in [8], the distinction can be made by
examining the domain portion of the NAI. If the domain portion is
absent or corresponds to the local domain, then the session record is
treated as an intra-domain accounting event. Otherwise, it is
treated as an inter-domain accounting event.
Intra-domain accounting events are typically routed to the local
billing server, while inter-domain accounting events will be routed
to accounting servers operating within other administrative domains.
While it is not required that session record formats used in inter
and intra-domain accounting be the same, this is desirable, since it
eliminates translations that would otherwise be required.
Where a proxy forwarder is employed, domain-based access controls may
be employed by the proxy forwarder, rather than by the devices
themselves. The network device will typically speak an accounting
protocol to the proxy forwarder, which may then either convert the
accounting packets to session records, or forward the accounting
packets to another domain. In either case, domain separation is
typically achieved by having the proxy forwarder sort the session
records or accounting messages by destination.
Where the accounting proxy is not trusted, it may be difficult to
verify that the proxy is issuing correct session records based on the
accounting messages it receives, since the original accounting
messages typically are not forwarded along with the session records.
Therefore where trust is an issue, the proxy typically forwards the
accounting packets themselves. Assuming that the accounting protocol
supports data object security, this allows the end-points to verify
that the proxy has not modified the data in transit or snooped on the
packet contents.
The diagram below illustrates the accounting management architecture:
+------------+

Network
Device

+------------+

Accounting
Protocol

V
+------------+ +------------+

Org B Inter-domain session records Org A
Acctg. <-----------------------------> Acctg.
Proxy/Server or accounting protocol Server

+------------+ +------------+


Transfer Intra-domain
Protocol Session records

V V
+------------+ +------------+

Org B Org A
Billing Billing
Server Server

+------------+ +------------+
1.4. Accounting management objectives
Accounting Management involves the collection of resource consumption
data for the purposes of capacity and trend analysis, cost
allocation, auditing, billing. Each of these tasks has different
requirements.
1.4.1. Trend analysis and capacity planning
In trend analysis and capacity planning, the goal is typically a
forecast of future usage. Since such forecasts are inherently
imperfect, high reliability is typically not required, and moderate
packet loss can be tolerated. Where it is possible to use
statistical sampling techniques to reduce data collection
requirements while still providing the forecast with the desired
statistical accuracy, it may be possible to tolerate high packet loss
as long as bias is not introduced.
The security requirements for trend analysis and capacity planning
depend on the circumstances of data collection and the sensitivity of
the data. Additional security services may be required when data is
being transferred between administrative domains. For example, when
information is being collected and analyzed within the same
administrative domain, integrity protection and authentication may be
used in order to guard against collection of invalid data. In
inter-domain applications confidentiality may be desirable to guard
against snooping by third parties.
1.4.2. Billing
When accounting data is used for billing purposes, the requirements
depend on whether the billing process is usage-sensitive or not.
1.4.2.1. Non-usage sensitive billing
Since by definition, non-usage-sensitive billing does not require
usage information, in theory all accounting data can be lost without
affecting the billing process. Of course this would also affect
other tasks such as trend analysis or auditing, so that such
wholesale data loss would still be unacceptable.
1.4.2.2. Usage-sensitive billing
Since usage-sensitive billing processes depend on usage information,
packet loss may translate directly to revenue loss. As a result, the
billing process may need to conform to financial reporting and legal
requirements, and therefore an archival accounting approach may be
needed.
Usage-sensitive systems may also require low processing delay. Today
credit risk is commonly managed by computerized fraud detection
systems that are designed to detect unusual activity. While
efficiency concerns might otherwise dictate batched transmission of
accounting data, where there is a risk of fraud, financial eXPosure
increases with processing delay. Thus it may be advisable to
transmit each event individually to minimize batch size, or even to
utilize quality of service techniques to minimize queuing delays. In
addition, it may be necessary for authorization to be dependent on
ability to pay.
Whether these techniques will be useful varies by application since
the degree of financial exposure is application-dependent. For
dial-up Internet access from a local provider, charges are typically
low and therefore the risk of loss is small. However, in the case of
dial-up roaming or voice over IP, time-based charges may be
substantial and therefore the risk of fraud is larger. In such
situations it is highly desirable to quickly detect unusual account
activity, and it may be desirable for authorization to depend on
ability to pay. In situations where valuable resources can be
reserved, or where charges can be high, very large bills may be rung
up quickly, and processing may need to be completed within a defined
time window in order to limit exposure.
Since in usage-sensitive systems, accounting data translates into
revenue, the security and reliability requirements are greater. Due
to financial and legal requirements such systems need to be able to
survive an audit. Thus security services such as authentication,
integrity and replay protection are frequently required and
confidentiality and data object integrity may also be desirable.
Application-layer acknowledgments are also often required so as to
guard against accounting server failures.
1.4.3. Auditing
With enterprise networking expenditures on the rise, interest in
auditing is increasing. Auditing, which is the act of verifying the
correctness of a procedure, commonly relies on accounting data.
Auditing tasks include verifying the correctness of an invoice
submitted by a service provider, or verifying conformance to usage
policy, service level agreements, or security guidelines.
To permit a credible audit, the auditing data collection process must
be at least as reliable as the accounting process being used by the
entity that is being audited. Similarly, security policies for the
audit should be at least as stringent as those used in preparation of
the original invoice. Due to financial and legal requirements,
archival accounting practices are frequently required in this
application.
Where auditing procedures are used to verify conformance to usage or
security policies, security services may be desired. This typically
will include authentication, integrity and replay protection as well
as confidentiality and data object integrity. In order to permit
response to security incidents in progress, auditing applications
frequently are built to operate with low processing delay.
1.4.4. Cost allocation
The application of cost allocation and billback methods by enterprise
customers is not yet widespread. However, with the convergence of
telephony and data communications, there is increasing interest in
applying cost allocation and billback procedures to networking costs,
as is now commonly practiced with telecommunications costs.
Cost allocation models, including traditional costing mechanisms
described in [21]-[23] and activity-based costing techniques
described in [24] are typically based on detailed analysis of usage
data, and as a result they are almost always usage-sensitive.
Whether these techniques are applied to allocation of costs between
partners in a venture or to allocation of costs between departments
in a single firm, cost allocation models often have profound
behavioral and financial impacts. As a result, systems developed for
this purposes are typically as concerned with reliable data
collection and security as are billing applications. Due to
financial and legal requirements, archival accounting practices are
frequently required in this application.
1.5. Intra-domain and inter-domain accounting
Much of the initial work on accounting management has focused on
intra-domain accounting applications. However, with the increasing
deployment of services such as dial-up roaming, Internet fax, Voice
and Video over IP and QoS, applications requiring inter-domain
accounting are becoming increasingly common.
Inter-domain accounting differs from intra-domain accounting in
several important ways. Intra-domain accounting involves the
collection of information on resource consumption within an
administrative domain, for use within that domain. In intra-domain
accounting, accounting packets and session records typically do not
cross administrative boundaries. As a result, intra-domain
accounting applications typically experience low packet loss and
involve transfer of data between trusted entities.
In contrast, inter-domain accounting involves the collection of
information on resource consumption within an administrative domain,
for use within another administrative domain. In inter-domain
accounting, accounting packets and session records will typically
cross administrative boundaries. As a result, inter-domain
accounting applications may experience substantial packet loss. In
addition, the entities involved in the transfers cannot be assumed to
trust each other.
Since inter-domain accounting applications involve transfers of
accounting data between domains, additional security measures may be
desirable. In addition to authentication, replay and integrity
protection, it may be desirable to deploy security services such as
confidentiality and data object integrity. In inter-domain
accounting each involved party also typically requires a copy of each
accounting event for invoice generation and auditing.
1.6. Accounting record production
Typically, a single accounting record is produced per session, or in
some cases, a set of interim records which can be summarized in a
single record for billing purposes. However, to support deployment
of services such as wireless access or complex billing regimes, a
more sophisticated approach is required.
It is necessary to generate several accounting records from a single
session when pricing changes during a session. For instance, the
price of a service can be higher during peak hours than off-peak.
For a session continuing from one tariff period to another, it
becomes necessary for a device to report "packets sent" during both
periods.
Time is not the only factor requiring this approach. For instance,
in mobile access networks the user may roam from one place to another
while still being connected in the same session. If roaming causes a
change in the tariffs, it is necessary to account for resource
consumed in the first and second areas. Another example is where
modifications are allowed to an ongoing session. For example, it is
possible that a session could be re-authorized with improved QoS.
This would require production of accounting records at both QoS
levels.
These examples could be addressed by using vectors or multi-
dimensional arrays to represent resource consumption within a single
session record. For example, the vector or array could describe the
resource consumption for each combination of factors, e.g. one data
item could be the number of packets during peak hour in the area of
the home operator. However, such an approach seems complicated and
inflexible and as a result, most current systems produce a set of
records from one session. A session identifier needs to be present
in the records to permit accounting systems to tie the records
together.
In most cases, the network device will determine when multiple
session records are needed, as the local device is aware of factors
affecting local tariffs, such as QoS changes and roaming. However,
future systems are being designed that enable the home domain to
control the generation of accounting records. This is of importance
in inter-domain accounting or when network devices do not have tariff
information. The centralized control of accounting record production
can be realized, for instance, by having authorization servers
require re-authorization at certain times and requiring the
production of accounting records upon each re-authorization.
In conclusion, in some cases it is necessary to produce multiple
accounting records from a single session. It must be possible to do
this without requiring the user to start a new session or to re-
authenticate. The production of multiple records can be controlled
either by the network device or by the AAA server. The requirements
for timeliness, security and reliability in multiple record sessions
are the same as for single-record sessions.
1.7. Requirements summary
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Usage Intra-domain Inter-domain

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Robustness vs. Robustness vs.
packet loss packet loss
Capacity
Planning Integrity, Integrity,
authentication, authentication,
replay protection replay prot.
[confidentiality] confidentiality
[data object sec.]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Non-usage Integrity, Integrity,
Sensitive authentication, authentication,
Billing replay protection replay protection
[confidentiality] confidentiality
[data object sec.]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Archival Archival
Usage accounting accounting
Sensitive Integrity, Integrity,
Billing, authentication, authentication,
Cost replay protection replay prot.
Allocation & [confidentiality] confidentiality
Auditing [Bounds on [data object sec.]
processing delay] [Bounds on
processing delay]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Archival Archival
Time accounting accounting
Sensitive Integrity, Integrity,
Billing, authentication, authentication,
fraud replay protection replay prot.
detection, [confidentiality] confidentiality
roaming [Data object
Bounds on security and
processing delay receipt support]
Bounds on
processing delay
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Key
[] = optional
2. Scaling and reliability
With the continuing growth of the Internet, it is important that
accounting management systems be scalable and reliable. This section
discusses the resources consumed by accounting management systems as
well as the scalability and reliability properties exhibited by
various data collection and transport models.
2.1. Fault resilience
As noted earlier, in applications such as usage-sensitive billing,
cost allocation and auditing, an archival approach to accounting is
frequently mandated, due to financial and legal requirements. Since
in such situations loss of accounting data can translate to revenue
loss, there is incentive to engineer a high degree of fault
resilience. Faults which may be encountered include:
Packet loss
Accounting server failures
Network failures
Device reboots
To date, much of the debate on accounting reliability has focused on
resilience against packet loss and the differences between UDP, SCTP
and TCP-based transport. However, it should be understood that
resilience against packet loss is only one ASPect of meeting
archival accounting requirements.
As noted in [18], "once the cable is cut you don"t need more
retransmissions, you need a *lot* more voltage." Thus, the choice of
transport has no impact on resilience against faults such as network
partition, accounting server failures or device reboots. What does
provide resilience against these faults is non-volatile storage.
The importance of non-volatile storage in design of reliable
accounting systems cannot be over-emphasized. Without non-volatile
storage, event-driven systems will lose data once the transmission
timeout has been exceeded, and batching designs will experience data
loss once the internal memory used for accounting data storage has
been exceeded. Via use of non-volatile storage, and internally
stored interim records, most of these data losses can be avoided.
It may even be argued that non-volatile storage is more important to
accounting reliability than network connectivity, since for many
years reliable accounting systems were implemented based solely on
physical storage, without any network connectivity. For example,
phone usage data used to be stored on paper, film, or magnetic media
and carried from the place of collection to a central location for
bill processing.
2.1.1. Interim accounting
Interim accounting provides protection against loss of session
summary data by providing checkpoint information that can be used to
reconstruct the session record in the event that the session summary
information is lost. This technique may be applied to any data
collection model (i.e. event-driven or polling) and is supported in
both RADIUS [25] and in TACACS+.
While interim accounting can provide resilience against packet loss,
server failures, short-duration network failures, or device reboot,
its applicability is limited. Transmission of interim accounting
data over the wire should not be thought of as a mainstream
reliability improvement technique since it increases use of network
bandwidth in normal operation, while providing benefits only in the
event of a fault.
Since most packet loss on the Internet is due to congestion, sending
interim accounting data over the wire can make the problem worse by
increasing bandwidth usage. Therefore on-the-wire interim accounting
is best restricted to high-value accounting data such as information
on long-lived sessions. To protect against loss of data on such
sessions, the interim reporting interval is typically set several
standard deviations larger than the average session duration. This
ensures that most sessions will not result in generation of interim
accounting events and the additional bandwidth consumed by interim
accounting will be limited. However, as the interim accounting
interval decreases toward the average session time, the additional
bandwidth consumed by interim accounting increases markedly, and as a
result, the interval must be set with caution.
Where non-volatile storage is unavailable, interim accounting can
also result in excessive consumption of memory that could be better
allocated to storage of session data. As a result, implementors
should be careful to ensure that new interim accounting data
overwrites previous data rather than accumulating additional interim
records in memory, thereby worsening the buffer exhaustion problem.
Given the increasing popularity of non-volatile storage for use in
consumer devices such as digital cameras, such devices are rapidly
declining in price. This makes it increasingly feasible for network
devices to include built-in support for non-volatile storage. This
can be accomplished, for example, by support for compact PCMCIA
cards.
Where non-volatile storage is available, this can be used to store
interim accounting data. Stored interim events are then replaced by
updated interim events or by session data when the session completes.
The session data can itself be erased once the data has been
transmitted and acknowledged at the application layer. This approach
avoids interim data being transmitted over the wire except in the
case of a device reboot. When a device reboots, internally stored
interim records are transferred to the accounting server.
2.1.2. Multiple record sessions
Generation of multiple accounting records within a session can
introduce scalability problems that cannot be controlled using the
techniques available in interim accounting.
For example, in the case of interim records kept in non-volatile
storage, it is possible to overwrite previous interim records with
the most recent one or summarize them to a session record. Where
interim updates are sent over the wire, it is possible to control
bandwidth usage by adjusting the interim accounting interval.
These measures are not applicable where multiple session records are
produced from a single session, since these records cannot be
summarized or overwritten without loss of information. As a result,
multiple record production can result in increased consumption of
bandwidth and memory. Implementors should be careful to ensure that
worst-case multiple record processing requirements do not exceed the
capabilities of their systems.
As an example, a tariff change at a particular time of day could, if
implemented carelessly, create a sudden peak in the consumption of
memory and bandwidth as the records need to be stored and/or
transported. Rather than attempting to send all of the records at
once, it may be desirable to keep them in non-volatile storage and
send all of the related records together in a batch when the session
completes. It may also be desirable to shape the accounting traffic
flow so as to reduce the peak bandwidth consumption. This can be
accomplished by introduction of a randomized delay interval. If the
home domain can also control the generation of multiple accounting
records, the estimation of the worst-case processing requirements can
be very difficult.
2.1.3. Packet loss
As packet loss is a fact of life on the Internet, accounting
protocols dealing with session data need to be resilient against
packet loss. This is particularly important in inter-domain
accounting, where packets often pass through Network Access Points
(NAPs) where packet loss may be substantial. Resilience against
packet loss can be accomplished via implementation of a retry
mechanism on top of UDP, or use of TCP [7] or SCTP [26]. On-the-wire
interim accounting provides only limited benefits in mitigating the
effects of packet loss.
UDP-based transport is frequently used in accounting applications.
However, this is not appropriate in all cases. Where accounting data
will not fit within a single UDP packet without fragmentation, use of
TCP or SCTP transport may be preferred to use of multiple round-trips
in UDP. As noted in [47] and [49], this may be an issue in the
retrieval of large tables.
In addition, in cases where congestion is likely, such as in inter-
domain accounting, TCP or SCTP congestion control and round-trip time
estimation will be very useful, optimizing throughput. In
applications which require maintenance of session state, such as
simultaneous usage control, TCP and application-layer keep alive
packets or SCTP with its built-in heartbeat capabilities provide a
mechanism for keeping track of session state.
When implementing UDP retransmission, there are a number of issues to
keep in mind:
Data model
Retry behavior
Congestion control
Timeout behavior
Accounting reliability can be influenced by how the data is modeled.
For example, it is almost always preferable to use cumulative
variables rather than expressing accounting data in terms of a change
from a previous data item. With cumulative data, the current state
can be recovered by a successful retrieval, even after many packets
have been lost. However, if the data is transmitted as a change then
the state will not be recovered until the next cumulative update is
sent. Thus, such implementations are much more vulnerable to packet
loss, and should be avoided wherever possible.
In designing a UDP retry mechanism, it is important that the retry
timers relate to the round-trip time, so that retransmissions will
not typically occur within the period in which acknowledgments may be
expected to arrive. Accounting bandwidth may be significant in some
circumstances, so that the added traffic due to unnecessary
retransmissions may increase congestion levels.
Congestion control in accounting data transfer is a somewhat
controversial issue. Since accounting traffic is often considered
mission-critical, it has been argued that congestion control is not a
requirement; better to let other less-critical traffic back off in
response to congestion. Moreover, without non-volatile storage,
congestive back-off in accounting applications can result in data
loss due to buffer exhaustion.
However, it can also be argued that in modern accounting
implementations, it is possible to implement congestion control while
improving throughput and maintaining high reliability. In
circumstances where there is sustained packet loss, there simply is
not sufficient capacity to maintain existing transmission rates.
Thus, aggregate throughput will actually improve if congestive back-
off is implemented. This is due to elimination of retransmissions
and the ability to utilize techniques such as RED to desynchronize
flows. In addition, with QoS mechanisms such as differentiated
services, it is possible to mark accounting packets for preferential
handling so as to provide for lower packet loss if desired. Thus
considerable leeway is available to the network administrator in
controlling the treatment of accounting packets and hard coding
inelastic behavior is unnecessary. Typically, systems implementing
non-volatile storage allow for backlogged accounting data to be
placed in non-volatile storage pending transmission, so that buffer
exhaustion resulting from congestive back-off need not be a concern.
Since UDP is not really a transport protocol, UDP-based accounting
protocols such as [4] often do not prescribe timeout behavior. Thus
implementations may exhibit widely different behavior. For example,
one implementation may drop accounting data after three constant
duration retries to the same server, while another may implement
exponential back-off to a given server, then switch to another
server, up to a total timeout interval of twelve hours, while storing
the untransmitted data on non-volatile storage. The practical
difference between these approaches is substantial; the former
approach will not satisfy archival accounting requirements while the
latter may. More predictable behavior can be achieved via use of
SCTP or TCP transport.
2.1.4. Accounting server failover
In the event of a failure of the primary accounting server, it is
desirable for the device to failover to a secondary server.
Providing one or more secondary servers can remove much of the risk
of accounting server failure, and as a result use of secondary
servers has become commonplace.
For protocols based on TCP, it is possible for the device to maintain
connections to both the primary and secondary accounting servers,
using the secondary connection after expiration of a timer on the
primary connection. Alternatively, it is possible to open a
connection to the secondary accounting server after a timeout or loss
of the primary connection, or on expiration of a timer. Thus,
accounting protocols based on TCP are capable of responding more
rapidly to connectivity failures than TCP timeouts would otherwise
allow, at the expense of an increased risk of duplicates.
With SCTP, it is possible to control transport layer timeout
behavior, and therefore it is not necessary for the accounting
application to maintain its own timers. SCTP also enables
multiplexing of multiple connections within a single transport
connection, all maintaining the same congestion control state,
avoiding the "head of line blocking" issues that can occur with TCP.
However, since SCTP is not widely available, use of this transport
can impose an additional implementation burden on the designer.
For protocols using UDP, transmission to the secondary server can
occur after a number of retries or timer expiration. For
compatibility with congestion avoidance, it is advisable to
incorporate techniques such as round-trip-time estimation, slow start
and congestive back-off. Thus the accounting protocol designer
utilizing UDP often is lead to re-inventing techniques already
existing in TCP and SCTP. As a result, the use of raw UDP transport
in accounting applications is not recommended.
With any transport it is possible for the primary and secondary
accounting servers to receive duplicate packets, so support for
duplicate elimination is required. Since accounting server failures
can result in data accumulation on accounting clients, use of non-
volatile storage can ensure against data loss due to transmission
timeouts or buffer exhaustion. On-the-wire interim accounting
provides only limited benefits in mitigating the effects of
accounting server failures.
2.1.5. Application layer acknowledgments
It is possible for the accounting server to experience partial
failures. For example, a failure in the database back end could
leave the accounting retrieval process or thread operable while the
process or thread responsible for storing the data is non-functional.
Similarly, it is possible for the accounting application to run out
of disk space, making it unable to continue storing incoming session
records.
In such cases it is desirable to distinguish between transport layer
acknowledgment and application layer acknowledgment. Even though
both acknowledgments may be sent within the same packet (such as a
TCP segment carrying an application layer acknowledgment along with a
piggy-backed ACK), the semantics are different. A transport-layer
acknowledgment means "the transport layer has taken responsibility
for delivering the data to the application", while an application-
layer acknowledgment means "the application has taken responsibility
for the data".
A common misconception is that use of TCP transport guarantees that
data is delivered to the application. However, as noted in RFC793
[7]:
An acknowledgment by TCP does not guarantee that the data has been
delivered to the end user, but only that the receiving TCP has taken
the responsibility to do so.
Therefore, if receiving TCP fails after sending the ACK, the
application may not receive the data. Similarly, if the application
fails prior to committing the data to stable storage, the data may be
lost. In order for a sending application to be sure that the data it
sent was received by the receiving application, either a graceful
close of the TCP connection or an application-layer acknowledgment is
required. In order to protect against data loss, it is necessary that
the application-layer acknowledgment imply that the data has been
written to stable storage or suitably processed so as to guard
against loss.
In the case of partial failures, it is possible for the transport
layer to acknowledge receipt via transport layer acknowledgment,
without having delivered the data to the application. Similarly, the
application may not complete the tasks necessary to take
responsibility for the data.
For example, an accounting server may receive data from the transport
layer but be incapable of storing it data due to a back end database
problem or disk fault. In this case it should not send an
application layer acknowledgment, even though a a transport layer
acknowledgment is appropriate. Rather, an application layer error
message should be sent indicating the source of the problem, such as
"Backend store unavailable".
Thus application-layer acknowledgment capability requires not only
the ability to acknowledge when the application has taken
responsibility for the data, but also the ability to indicate when
the application has not taken responsibility for the data, and why.
2.1.6. Network failures
Network failures may result in partial or complete loss of
connectivity for the accounting client. In the event of partial
connectivity loss, it may not be possible to reach the primary
accounting server, in which case switch over to the secondary
accounting server is necessary. In the event of a network partition,
it may be necessary to store accounting events in device memory or
non-volatile storage until connectivity can be re-established.
As with accounting server failures, on-the-wire interim accounting
provides only limited benefits in mitigating the effects of network
failures.
2.1.7. Device reboots
In the event of a device reboot, it is desirable to minimize the loss
of data on sessions in progress. Such losses may be significant even
if the devices themselves are very reliable, due to long-lived
sessions, which can comprise a significant fraction of total resource
consumption. To guard against loss of these high-value sessions,
interim accounting data is typically transmitted over the wire. When
interim accounting in-place is combined with non-volatile storage it
becomes possible to guard against data loss in much shorter sessions.
This is possible since interim accounting data need only be stored in
non-volatile memory until the session completes, at which time the
interim data may be replaced by the session record. As a result,
interim accounting data need never be sent over the wire, and it is
possible to decrease the interim interval so as to provide a very
high degree of protection against data loss.
2.1.8. Accounting proxies
In order to maintain high reliability, it is important that
accounting proxies pass through transport and application layer
acknowledgments and do not store and forward accounting packets.
This enables the end-systems to control re-transmission behavior and
utilize techniques such as non-volatile storage and secondary servers
to improve resilience.
Accounting proxies sending a transport or application layer ACK to
the device without receiving one from the accounting server fool the
device into thinking that the accounting request had been accepted by
the accounting server when this is not the case. As a result, the
device can delete the accounting packet from non-volatile storage
before it has been accepted by the accounting server. The leaves the
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.
2.1.9. Fault resilience summary
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Fault Counter-measures

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Packet Retransmission based on RTT
loss Congestion control
Well-defined timeout behavior
Duplicate elimination
Interim accounting*
Non-volatile storage
Cumulative variables

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Accounting Primary-secondary servers
server & net Duplicate elimination
failures Interim accounting*
Application layer ACK & error msgs.
Non-volatile storage

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Device Interim accounting*
reboots Non-volatile storage

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Key
* = limited usefulness without non-volatile storage
Note: Accounting proxies are not a reliability
enhancement mechanism.
2.2. Resource consumption
In the process of growing to meet the needs of providers and
customers, accounting management systems consume a variety of
resources, including:
Network bandwidth
Memory
Non-volatile storage
State on the accounting management system
CPU on the management system and managed devices
In order to understand the limits to scaling, we examine each of
these resources in turn.
2.2.1. Network bandwidth
Accounting management systems consume network bandwidth in
transferring accounting data. The network bandwidth consumed is
proportional to the amount of data transferred, as well as required
network overhead. Since accounting data for a given event may be 100
octets or less, if each event is transferred individually, overhead
can represent a considerable proportion of total bandwidth
consumption. As a result, it is often desirable to transfer
accounting data in batches, enabling network overhead to be spread
over a larger payload, and enabling efficient use of compression. As
noted in [48], compression can be enabled in the accounting protocol,
or can be done at the IP layer as described in [5].
2.2.2. Memory
In accounting systems without non-volatile storage, accounting data
must be stored in volatile memory during the period between when it
is generated and when it is transferred. The resulting memory
consumption will depend on retry and retransmission algorithms.
Since systems designed for high reliability will typically wish to
retry for long periods, or may store interim accounting data, the
resulting memory consumption can be considerable. As a result, if
non-volatile storage is unavailable, it may be desirable to compress
accounting data awaiting transmission.
As noted earlier, implementors of interim accounting should take care
to ensure against excessive memory usage by overwriting older interim
accounting data with newer data for the same session rather than
accumulating interim data in the buffer.
2.2.3. Non-volatile storage
Since accounting data stored in memory will typically be lost in the
event of a device reboot or a timeout, it may be desirable to provide
non-volatile storage for undelivered accounting data. With the costs
of non-volatile storage declining rapidly, network devices will be
increasingly capable of incorporating non-volatile storage support
over the next few years.
Non-volatile storage may be used to store interim or session records.
As with memory utilization, interim accounting overwrite is desirable
so as to prevent excessive storage consumption. Note that the use of
ASCII data representation enables use of highly efficient text
compression algorithms that can minimize storage requirements. Such
compression algorithms are only typically applied to session records
so as to enable implementation of interim data overwrite.
2.2.4. State on the accounting management system
In order to keep track of received accounting data, accounting
management systems may need to keep state on managed devices or
concurrent sessions. Since the number of devices is typically much
smaller than the number of concurrent sessions, it is desirable to
keep only per-device state if possible.
2.2.5. CPU requirements
CPU consumption of the managed and managing nodes will be
proportional to the complexity of the required accounting processing.
Operations such as ASN.1 encoding and decoding,
compression/decompression, and encryption/decryption can consume
considerable resources, both on accounting clients and servers.
The effect of these operations on accounting system reliability
should not be under-estimated, particularly in the case of devices
with moderate CPU resources. In the event that devices are over-
taxed by accounting tasks, it is likely that overall device
reliability will suffer.
2.2.6. Efficiency measures
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Resource Efficiency measures

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Network Batching
Bandwidth Compression

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Memory Compression
Interim accounting overwrite

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Non-volatile Compression
Storage Interim accounting overwrite

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System Per-device state
state

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CPU Hardware assisted
requirements compression/encryption

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2.3. Data collection models
Several data collection models are currently in use today for the
purposes of accounting data collection. These include:
Polling model
Event-driven model without batching
Event-driven model with batching
Event-driven polling model
2.3.1. Polling model
In the polling model, an accounting manager will poll devices for
accounting information at regular intervals. In order to ensure
against loss of data, the polling interval will need to be shorter
than the maximum time that accounting data can be stored on the
polled device. For devices without non-volatile stage, this is
typically determined by available memory; for devices with non-
volatile storage the maximum polling interval is determined by the
size of non-volatile storage.
The polling model results in an accumulation of data within
individual devices, and as a result, data is typically transferred to
the accounting manager in a batch, resulting in an efficient transfer
process. In terms of Accounting Manager state, polling systems scale
with the number of managed devices, and system bandwidth usage scales
with the amount of data transferred.
Without non-volatile storage, the polling model results in loss of
accounting data due to device reboots, but not due to packet loss or
network failures of sufficiently short duration to be handled within
available memory. This is because the Accounting Manager will
continue to poll until the data is received. In situations where
operational difficulties are encountered, the volume of accounting
data will frequently increase so as to make data loss more likely.
However, in this case the polling model will detect the problem since
attempts to reach the managed devices will fail.
The polling model scales poorly for implementation of shared use or
roaming services, including wireless data, Internet telephony, QoS
provisioning or Internet access. This is because in order to
retrieve accounting data for users within a given domain, the
Accounting Management station would need to periodically poll all
devices in all domains, most of which would not contain any relevant
data. There are also issues with processing delay, since use of a
polling interval also implies an average processing delay of half the
polling interval. This may be too high for accounting data that
requires low processing delay. Thus the event-driven polling or the
pure event-driven approach is more appropriate for usage sensitive
billing applications such as shared use or roaming implementations.
Per-device state is typical of polling-based network management
systems, which often also carry out accounting management functions,
since network management systems need to keep track of the state of
network devices for operational purposes. These systems offer
average processing delays equal to half the polling interval.
2.3.2. Event-driven model without batching
In the event-driven model, a device will contact the accounting
server or manager when it is ready to transfer accounting data. Most
event-driven accounting systems, such as those based on RADIUS
accounting, described in [4], transfer only one accounting event per
packet, which is inefficient.
Without non-volatile storage, a pure event-driven model typically
stores accounting events that have not yet been delivered only until
the timeout interval expires. As a result this model has the
smallest memory requirements. Once the timeout interval has expired,
the accounting event is lost, even if the device has sufficient
buffer space to continue to store it. As a result, the event-driven
model is the least reliable, since accounting data loss will occur
due to device reboots, sustained packet loss, or network failures of
duration greater than the timeout interval. In event-driven
protocols without a "keep alive" message, accounting servers cannot
assume a device failure should no messages arrive for an extended
period. Thus, event-driven accounting systems are typically not
useful in monitoring of device health.
The event-driven model is frequently used in shared use networks and
roaming, since this model sends data to the recipient domains without
requiring them to poll a large number of devices, most of which have
no relevant data. Since the event-driven model typically does not
support batching, it permits accounting records to be sent with low
processing delay, enabling application of fraud prevention
techniques. However, because roaming accounting events are
frequently of high value, the poor reliability of this model is an
issue. As a result, the event-driven polling model may be more
appropriate.
Per-session state is typical of event-driven systems without
batching. As a result, the event-driven approach scales poorly.
However, event-driven systems offer the lowest processing delay since
events are processed immediately and there is no possibility of an
event requiring low processing delay being caught behind a batch
transfer.
2.3.3. Event-driven model with batching
In the event-driven model with batching, a device will contact the
accounting server or manager when it is ready to transfer accounting
data. The device can contact the server when a batch of a given size
has been gathered, when data of a certain type is available or after
a minimum time period has elapsed. Such systems can transfer more
than one accounting event per packet and are thus more efficient.
An event-driven system with batching will store accounting events
that have not yet been delivered up to the limits of memory. As a
result, accounting data loss will occur due to device reboots, but
not due to packet loss or network failures of sufficiently short
duration to be handled within available memory. Note that while
transfer efficiency will increase with batch size, without non-
volatile storage, the potential data loss from a device reboot will
also increase.
Where event-driven systems with batching have a keep-alive interval
and run over reliable transport, the accounting server can assume
that a failure has occurred if no messages are received within the
keep-alive interval. Thus, such implementations can be useful in
monitoring of device health. When used for this purpose the average
time delay prior to failure detection is one half the keep-alive
interval.
Through implementation of a scheduling algorithm, event-driven
systems with batching can deliver appropriate service to accounting
events that require low processing delay. For example, high-value
inter-domain accounting events could be sent immediately, thus
enabling use of fraud-prevention techniques, while all other events
would be batched. However, there is a possibility that an event
requiring low processing delay will be caught behind a batch transfer
in progress. Thus the maximum processing delay is proportional to
the maximum batch size divided by the link speed.
Event-driven systems with batching scale with the number of active
devices. As a result this approach scales better than the pure
event-driven approach, or even the polling approach, and is
equivalent in terms of scaling to the event-driven polling approach.
However, the event-driven batching approach has lower processing
delay than the event-driven polling approach, since delivery of
accounting data requires fewer round-trips and events requiring low
processing delay can be accommodated if a scheduling algorithm is
employed.
2.3.4. Event-driven polling model
In the event-driven polling model an accounting manager will poll the
device for accounting data only when it receives an event. The
accounting client can generate an event when a batch of a given size
has been gathered, when data of a certain type is available or after
a minimum time period has elapsed. Note that while transfer
efficiency will increase with batch size, without non-volatile
storage, the potential data loss from a device reboot will also
increase.
Without non-volatile storage, an event-driven polling model will lose
data due to device reboots, but not due to packet loss, or network
partitions of short-duration. Unless a minimum delivery interval is
set, event-driven polling systems are not useful in monitoring of
device health.
The event-driven polling model can be suitable for use in roaming
since it permits accounting data to be sent to the roaming partners
with low processing delay. At the same time non-roaming accounting
can be handled via more efficient polling techniques, thereby
providing the best of both worlds.
Where batching can be implemented, the state required in event-driven
polling can be reduced to scale with the number of active devices.
If portions of the network vary widely in usage, then this state may
actually be less than that of the polling approach. Note that
processing delay in this approach is higher than in event-driven
accounting with batching since at least two round-trips are required
to deliver data: one for the event notification, and one for the
resulting poll.
2.3.5. Data collection summary
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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
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Event-driven, Lowest processing Not robust
no batching del