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RFC 2975


Introduction to Accounting Management

Part 2 of 2, p. 32 to 54
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3.  Review of Accounting Protocols

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

3.1.  RADIUS

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

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

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

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

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

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3.2.  TACACS+

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

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

3.3.  SNMP

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

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

3.3.1.  Security services

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

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   messages.  The updated SNMP architecture [rfc2571] supports per-
   packet hop-by-hop authentication, integrity and replay protection,
   confidentiality and access control.

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

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

3.3.2.  Application layer acknowledgments

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

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

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

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

3.3.3.  Proxy forwarders

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

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

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

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

3.3.4.  Domain-based access controls in SNMP

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

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

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

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

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

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

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

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

   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

3.3.5.  Inter-domain access-control alternatives

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

   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

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

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

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

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

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

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

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   synchronization between tables.  Contexts

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

   Individual domains, such as, could be mapped to logical
   contexts, such as a bigco context.  The agent would need to create
   and recognize new contexts and to know which instrumentation is
   associated with the logical context.  The agent needs to collect
   accounting data by domain and make the data accessible via distinct
   contexts, so that access control can be applied to the context to
   prevent disclosure of sensitive information to the wrong domain.  The
   VACM access control views are applied relative to the context, so an
   operation can be permitted or denied a user based on the context
   which contains the data.

   Domain separation is handled by using contextName to differentiate
   multiple virtual tables.  For example, if accounting data has been
   collected on users with the and 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 domain, would be

   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, 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

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   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

3.3.6.  Outstanding issues

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

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

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

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

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   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 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

   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.

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   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 memo is not on the standards track,
   and therefore is not yet readily available for use.

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

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

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

3.3.7.  SNMP summary

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

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

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

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

   SNMP-based accounting has limitations in terms of efficiency and
   latency that may make it inappropriate for use in situations
   requiring low processing delay or low overhead.  This includes usage
   sensitive billing applications where fraud detection may be required.
   These issues can be addressed via proposals under discussion in the
   IRTF Network Management Research Group (NMRG).  The experimental SNMP
   over TCP transport mapping may prove helpful at reducing latency.
   Depending on the volume of data, some form of compression may also be
   worth considering.  However, since these proposals are still in the
   research stage, and are not on the standards track, these
   capabilities are not readily available, and the specifications could
   change considerably before they reach their final form.

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

4.  Review of Accounting Data Transfer

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

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

4.1.  SMTP

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

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

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

4.2.  Other protocols

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

Top      Up      ToC       Page 45 
   Given access to sufficient non-volatile storage, accounting systems
   based on record formats and transfer protocols can avoid loss of data
   due to long-duration network partitions, server failures or device
   reboots.  Since it is possible for the transfer to be driven from the
   collection site, the collector can retry transfers until successful,
   or with HTTP may even be able to restart partially completed
   transfers.  As a result, file transfer-based systems can be made
   highly reliable, and the batching of accounting records makes
   possible efficient transfers and application of required security
   services with lessened overhead.

5.  Summary

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

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

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

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

Top      Up      ToC       Page 46 
   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.

Top      Up      ToC       Page 47 
   |                 |                     |                   |
   |  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,

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6.  Security Considerations

   Security issues are discussed throughout this memo.

7.  Acknowledgments

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

8.  References

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

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

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

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

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

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

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

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

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

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

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

Top      Up      ToC       Page 49 
   [12] Fajman, R., "An Extensible Message Format for Message
        Disposition Notifications", RFC 2298, March 1998.

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

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

   [15] Galvin, J., Murphy, S., Crocker, S. and N. Freed, "Security
        Multiparts for MIME:  Multi-part/Signed and
        Multipart/Encrypted", RFC 1847, October 1995.

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

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

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

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

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

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

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

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

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

   [25] Rigney, C., Willats, S. and P. Calhoun, "RADIUS Extensions", RFC
        2869, June 2000.

Top      Up      ToC       Page 50 
   [26] Stewart, R., et al., "Simple Control Transmission Protocol", RFC
        2960, October 2000.

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

   [28] Rose, M., and K. McCloghrie, "Structure and Identification of
        Management Information for TCP/IP-based Internets", STD 16, RFC
        1155, May 1990.

   [29] Rose, M. and K. McCloghrie, "Concise MIB Definitions", STD 16,
        RFC 1212, March 1991.

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

   [31] McCloghrie, K., Perkins, D. and J. Schoenwaelder, "Structure of
        Management Information Version 2 (SMIv2)", STD 58, RFC 2578,
        April 1999.

   [32] McCloghrie, K., Perkins, D. and J. Schoenwaelder, "Textual
        Conventions for SMIv2", STD 58, RFC 2579, April 1999.

   [33] McCloghrie, K., Perkins, D. and J. Schoenwaelder, "Conformance
        Statements for SMIv2", STD 58, RFC 2580, April 1999.

   [34] Case, J., Fedor, M., Schoffstall, M. and J. Davin, "Simple
        Network Management Protocol", STD 15, RFC 1157, May 1990.

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

   [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.

Top      Up      ToC       Page 51 
   [40] Wijnen, B., Presuhn, R. and K. McCloghrie, "View-based Access
        Control Model (VACM) for the Simple Network Management Protocol
        (SNMP)", RFC 2575, April 1999.

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

   [42] Tung, B., Neuman, C., Hur, M., Medvinsky, A., Medvinsky, S.,
        Wray, J. and J. Trostle, "Public Key Cryptography for Initial
        Authentication in Kerberos", Work in Progress.

   [43] Tung, B., Ryutov, T., Neuman, C., Tsudik, G., Sommerfeld, B.,
        Medvinsky, A. and M. Hur, "Public Key Cryptography for Cross-
        Realm Authentication in Kerberos", Work in Progress.

   [44] Hornstein, K. and W. Hardaker, "A Kerberos Security Model for
        SNMPv3", Work in Progress.

   [45] Brownlee, N. and A. Blount, "Accounting Attributes and Record
        Formats", RFC 2924, September 2000.

   [46] Network Management Research Group Web page,

   [47] Schoenwaelder, J.,"SNMP-over-TCP Transport Mapping", Work in

   [48] Schoenwaelder, J., "SNMP Payload Compression", Work in Progress.

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

   [50] Thaler, D., "Get Subtree Retrieval MIB", Work in Progress.

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

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9.  Authors' Addresses

   Bernard Aboba
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

   Phone: +1 425 936 6605

   Jari Arkko
   Oy LM Ericsson Ab
   02420 Jorvas

   Phone: +358 40 5079256

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

   Phone: +1 603 337 7357

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