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 , was developed as an add-on to the RADIUS authentication protocol, described in . As a result, RADIUS accounting shares the event-driven approach of RADIUS authentication, without support for batching or polling. As a result, RADIUS accounting scales with the number of accounting events instead of the number of devices, and accounting transfers are inefficient. Since RADIUS accounting is based on UDP and timeout and retry parameters are not specified, implementations vary widely in their approach to reliability, with some implementations retrying until delivery or buffer exhaustion, and others losing accounting data after a few retries. Since RADIUS accounting does not provide for application-layer acknowledgments or error messages, a RADIUS Accounting-Response is equivalent to a transport-layer acknowledgment and provides no protection against application layer malfunctions. Due to the lack of reliability, it is not possible to do simultaneous usage control based on RADIUS accounting alone. Typically another device data source is required, such as polling of a session MIB or a command-line session over telnet. RADIUS accounting implementations are vulnerable to packet loss as well as application layer failures, network failures and device reboots. These deficiencies are magnified in inter-domain accounting as is required in roaming (,). On the other hand, the event- driven approach of RADIUS accounting is useful where low processing delay is required, such as credit risk management or fraud detection. While RADIUS accounting does provide hop-by-hop authentication and integrity protection, and IPSEC can be employed to provide hop-by-hop confidentiality, data object security is not supported, and thus systems based on RADIUS accounting are not capable of being deployed with untrusted proxies, or in situations requiring auditability, as noted in . While RADIUS does not support compression, IP compression, described in , 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).
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 , can be employed to provide this. 3.3. SNMP SNMP, described in ,-, has been widely deployed in a wide variety of intra-domain accounting applications, typically using the polling data collection model. Polling allows data to be collected on multiple accounting events simultaneously, resulting in per-device state. Management applications are able to retry requests when a response is not received, providing resiliency against packet loss or even short-lived network partitions. Implementations without non- volatile storage are not robust against device reboots or network failures, but when combined with non-volatile storage they can be made highly reliable. SMIv1, the data modeling language of SNMPv1, has traps to permit trap-directed polling, but the traps are not acknowledged, and lost traps can lead to a loss of data. SMIv2, used by SNMPv2c and SNMPv3, has Inform Requests which are acknowledged notifications. This makes it possible to implement a more reliable event-driven polling model or event-driven batching model. However, we are not aware of any SNMP-based accounting implementations currently built on the use of Informs. 3.3.1. Security services SNMPv1 and SNMPv2c support per-packet authentication and read-only and read-write access profiles, via the community string. This clear-text password approach provides only trivial authentication, and no per-packet integrity checks, replay protection or confidentiality. View-based access control  can be supported using the snmpCommunityMIB, defined in , and SNMPv1 or SNMPv2c
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)  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.
The functionality of an SNMP Proxy Forwarder is defined in . 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 . 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. 126.96.36.199. Engines The new SNMP architecture, described in , 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.
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 . 188.8.131.52. Contexts Contexts are used to identify subsets of objects, within the scope of an engine, that are tied to instrumentation. A contextName refers to a particular subset within an engine. Contexts are commonly tied to hardware components, to logical entities related to the hardware components, or to logical services. For example, contextNames might include board5, board7, repeater1, repeater2, etc. An SNMP agent populates a read-only dynamic table to tell the manager what contexts it recognizes. Typically contexts are defined by the agent rather than the manager since if the manager defined them, the agent would not know how to tie the contexts to the underlying instrumentation. It is possible that MIB modules could be defined to allow a manager to assign contextNames to a logical subset of instrumentation. While each context may support instances of multiple MIB modules, each contextName is limited to one instance of a particular MIB module. If multiple instances of a MIB module are required per engine, then unique contextNames must be defined (e.g. repeater1, repeater2). The default context "" is used for engines which only support single instances of MIB modules, and it is used for MIB modules where it only makes sense to have one instance of that MIB module in an engine and that instance must be easy to locate, such as the system MIB or the security MIBs. SNMPv3 messages contain contextNames which are limited to the scope of the contextEngineID in the message. SNMPv1 and SNMPv2c messages contain communities which can be mapped to contextNames within the local engine, or can be mapped to contextNames within other engines via the snmpCommunityTable, described in .
184.108.40.206. Views Views are defined in the View-based Access Control Model. A view is a mask which is used to determine access to the managed objects in a particular context. The view identifies which objects are visible, by specifying OIDs of the subtrees included and excluded. There is also a mechanism to allow wildcards in the OID specification. For example, it is possible to define a view that includes RMON tables, and another view that includes only the SNMPv3 security related tables. Using these views, it is possible to allow access to the RMON view for users Joe and Josephine (the RMON administrators), and access to the SNMPv3 security tables for user Adam (the SNMP security Administrator). Views can be set up with wildcards. For a table that is indexed using IP addresses, Joe can be allowed access to all rows in given RMON tables (e.g. the RMON hostTable) that are in the subnet 10.2.x.x, while Josephine is given access to all rows for subnet 10.200.x.x. Views filter at the name level (OIDs), not at the value level, so defining views based on the values of non-index data is not supported. In this example, were the IP address to have been used merely as a data item rather than an index, it would not be possible to utilize view-based access control to achieve the desired objective (delegation of administrative responsibility according to subnet). View-based access control is independent of message version. It can be utilized by entities using SNMPv1, SNMPv2c, or SNMPv3 message formats. 3.3.5. Inter-domain access-control alternatives As the number of network devices within the shared use or roaming network grows, the polling model of data collection becomes increasingly impractical since most devices will not carry data relating to the polling organization. As a result, shared-use networks or roaming associations relying on SNMP-based accounting have generally collected data for all organizations and then sorted the resulting session records for delivery to each organization. While functional, this approach will typically result in increased processing delay as the number of organizations and data records grows. This issue can be addressed in SNMP using the event-driven, event- driven polling or event-driven batching approaches. Traps and Informs permit SNMP-enabled devices to notify domains that have
accounting data awaiting collection. SNMP Applications  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. 220.127.116.11. Domain as index View-based access control  allows multiple fine-grained views of an SNMP MIB to be assigned to specific groups of users, such that access rights to the included data elements depend on the identity of the user making the request. For example, all users of bigco.com which are allowed access to the device would be defined in the User-based security MIB module (or other security model MIB module). For simplicity in administering access control, the users can be grouped using a vacmGroupName, e.g. bigco. A view of a subset of the data objects in the MIB can be defined in the vacmViewFamilyTreeTable. A vacmAccessTable pairs groups and views. For messages received from users in the bigco group, access would only be provided to the data permitted to be viewed by bigco users, as defined in the view family tree. This requires that each domain accessing the data be given one or more separate vacmGroupNames, an appropriate ViewTable be defined, and the vacmAccessTable be configured for each group. Views filter at the name (OID) level, not at the data (value) level. When using views to filter by domain it is necessary to use the domain as an index. Standard view-based access control is not designed to filter based on the values on non-indexed fields. For example, a table of session data could be indexed by record number and domain, allowing a view to be defined that could restrict access to bigco data to the administrators of the bigco domain. An advantage of using domains as an index is that this technique can be used with SNMPv1 and SNMPv2c agents as well as with SNMPv3 agents. A disadvantage is that the MIB modules must be specifically designed for this purpose. Since existing MIB modules rarely use the domain as an index, domain separation cannot be enabled within legacy MIB modules using this technique. SNMP does support a RowPointer convention that could be used to define a new table, indexed by domain, which holds tuples between the domain and existing rows of data. This would introduce issues of
synchronization between tables. 18.104.22.168. Contexts ContextNames can be used to differentiate multiple instances of a MIB module within an engine. Individual domains, such as bigco.com, could be mapped to logical contexts, such as a bigco context. The agent would need to create and recognize new contexts and to know which instrumentation is associated with the logical context. The agent needs to collect accounting data by domain and make the data accessible via distinct contexts, so that access control can be applied to the context to prevent disclosure of sensitive information to the wrong domain. The VACM access control views are applied relative to the context, so an operation can be permitted or denied a user based on the context which contains the data. Domain separation is handled by using contextName to differentiate multiple virtual tables. For example, if accounting data has been collected on users with the bigco.com and smallco.com domains, then a separate virtual instance of the accounting session record table would exist for each domain, and each domain would have a corresponding contextName. When a get-bulk request is made with a contextName of bigco, then data from the virtual table in the bigco context, i.e. corresponding to the bigco.com domain, would be returned. There are a number of design approaches to creating new contexts and associating the contexts with appropriate instrumentation, most notably a sub-agent approach and a manager-configured MIB approach. AgentX , which standardizes a registration protocol between sub- agents and master agents to simplify SNMP agent implementation, allows for the creation and recognition of new contextNames when a subagent registers to provide support for a particular MIB subtree range. The sub-agent knows how to support a particular functionality, e.g. instrumentation exposed via a range of MIB objects. Based on values detected in the data, such as source=bigco.com, the sub-agent could determine that a new domain needed to be tracked and create the appropriate context for the collection of the data, plus the appropriate access control entries. The determination could be table-driven, using MIB configuration. A manager-driven approach could use a MIB module to predefine contextNames corresponding to the domains of interest, and to indicate which objects should be collected, how to differentiate to which domain the data should be applied based on a specified
condition, and what access control rules apply to the context. Either technique could associate existing MIB modules to domain- specific contexts, so domain separation can be applied to MIB modules not specifically designed with domain separation in mind. Legacy agents would not be designed to do this, so they would need to be updated to support inter-domain separation and VACM access control. The use of contextNames for inter-domain separation represents new territory, so careful consideration would be needed in designing the MIB modules and applications to provide domain to context and context to instrumentation mappings, and to ensure that security is not weakened. 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 . 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 , 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  states that an agent can terminate the get-bulk because of "local constraints" (see items 1 and 3 on pages 15/16 of ). This could be interpreted to mean
that it is possible to stop at the end of a table. 22.214.171.124. 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 . These include an SNMP- over-TCP transport mapping, described in ; SNMP payload compression, described in ; and the addition of a "get subtree" PDU or the subtree retrieval MIB . 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  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  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  also discusses file-based storage of SNMP data, and use of an FTP MIB, to enable storage of SNMP data in non-volatile storage, and subsequent bulk transfer via FTP. This approach would require implementation of additional MIB modules as well as FTP, and requires separate security mechanisms such as IPSEC to provide authentication, replay, integrity protection and confidentiality for the data in transit. The file-based transfer approach has an important benefit - compatibility with non-volatile storage.
Issues of legacy support exist with the NMRG proposals. Devices which do not implement the new functionality would need to be accommodated. This is especially problematic for proxy forwarders, which may need to act as translators between new and legacy entities. In these situations, the overhead of translation may offset the benefits of the new technologies. 126.96.36.199. 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 . 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 , and cross-realm authentication, as described in , 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.
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  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 .
Reference  contains a discussion of alternative encodings for SMI data types, as well as alternative protocols for transmission of accounting data. For example,  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  and , data object security is available for SMTP, and in addition, the facilities described in  make it possible to request and receive signed receipts, which enables non- repudiation as described in -. 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 . 4.2. Other protocols File transfer protocols such as FTP and HTTP have been used for transfer of accounting data. For example, Reference  describes a means for representing ASN.1-based accounting data for storage on archival media. Through the use of the Bulk File MIB, accounting data from an SNMP MIB can be stored in ASN.1, bulk binary or Bulk ASCII format, and then subsequently retrieved as required using the FTP Client MIB.
Given access to sufficient non-volatile storage, accounting systems based on record formats and transfer protocols can avoid loss of data due to long-duration network partitions, server failures or device reboots. Since it is possible for the transfer to be driven from the collection site, the collector can retry transfers until successful, or with HTTP may even be able to restart partially completed transfers. As a result, file transfer-based systems can be made highly reliable, and the batching of accounting records makes possible efficient transfers and application of required security services with lessened overhead. 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.
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.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | Usage | Intra-domain | Inter-domain | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | Capacity | SNMPv3 & | SNMPv3 &<* | | Planning | RADIUS #%@ | | | | TACACS+ @ | | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | Non-usage | SNMPv3 & | SNMPv3 &<* | | Sensitive | RADIUS #%@ | | | Billing | TACACS+ @ | | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | Usage | | | | Sensitive | | | | Billing, | SNMPv3 &>$ | SNMPv3 &<>*$ | | Cost | TACACS+ &$@ | | | Allocation & | | | | Auditing | | | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | Time | | | | Sensitive | SNMPv3 &>$ | No existing | | Billing, | | protocol | | fraud | | | | detection, | | | | roaming | | | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Key # = lacks confidentiality support * = lacks data object security % = limited robustness against packet loss & = lacks application layer acknowledgment (e.g. SNMP InformRequest) $ = requires non-volatile storage @ = lacks batching support < = lacks certificate support (KSM, work in progress) > = lacks support for large packet sizes (TCP transport mapping, experimental)
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  Aboba, B., Lu J., Alsop J., Ding J. and W. Wang, "Review of Roaming Implementations", RFC 2194, September 1997.  Aboba, B. and G. Zorn, "Criteria for Evaluating Roaming Protocols", RFC 2477, January 1999.  Rigney, C., Rubens, A., Simpson, W. and S. Willens, "Remote Authentication Dial In User Service (RADIUS)", RFC 2138, April, 1997.  Rigney, C., "RADIUS Accounting", RFC 2139, April 1997.  Shacham, A., Monsour, R., Pereira, R. and M. Thomas, "IP Payload Compression Protocol (IPComp)", RFC 2393, December 1998.  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.  Information Sciences Institute, "Transmission Control Protocol", RFC 793, September 1981.  Aboba, B. and M. Beadles, "The Network Access Identifier", RFC 2486, January 1999.  McCloghrie, K., Heinanen, J., Greene, W. and A. Prasad, "Accounting Information for ATM Networks", RFC 2512, February 1999.  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.  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.
 Fajman, R., "An Extensible Message Format for Message Disposition Notifications", RFC 2298, March 1998.  Elkins, M., "MIME Security with Pretty Good Privacy (PGP)", RFC 2015, October 1996.  Vaudreuil, G., "The Multipart/Report Content Type for the Reporting of Mail System Administrative Messages", RFC 1892, January 1996.  Galvin, J., Murphy, S., Crocker, S. and N. Freed, "Security Multiparts for MIME: Multi-part/Signed and Multipart/Encrypted", RFC 1847, October 1995.  Crocker, D., "MIME Encapsulation of EDI Objects", RFC 1767, March 1995.  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.  Rose, M.T., The Simple Book, Second Edition, Prentice Hall, Upper Saddle River, NJ, 1996.  Case, J., Mundy, R., Partain, D. and B. Stewart, "Introduction to Version 3 of the Internet-standard Network Management Framework", RFC 2570, April 1999.  Klyne, G., "Timely Delivery for Facsimile Using Internet Mail", Work in Progress.  Johnson, H. T., Kaplan, R. S., Relevance Lost: The Rise and Fall of Management Accounting, Harvard Business School Press, Boston, Massachusetts, 1987.  Horngren, C. T., Foster, G., Cost Accounting: A Managerial Emphasis. Prentice Hall, Englewood Cliffs, New Jersey, 1991.  Kaplan, R. S., Atkinson, Anthony A., Advanced Management Accounting, Prentice Hall, Englewood Cliffs, New Jersey, 1989.  Cooper, R., Kaplan, R. S., The Design of Cost Management Systems. Prentice Hall, Englewood Cliffs, New Jersey, 1991.  Rigney, C., Willats, S. and P. Calhoun, "RADIUS Extensions", RFC 2869, June 2000.
 Stewart, R., et al., "Simple Control Transmission Protocol", RFC 2960, October 2000.  Harrington, D., Presuhn, R., and B. Wijnen, "An Architecture for Describing SNMP Management Frameworks", RFC 2571, April 1999.  Rose, M., and K. McCloghrie, "Structure and Identification of Management Information for TCP/IP-based Internets", STD 16, RFC 1155, May 1990.  Rose, M. and K. McCloghrie, "Concise MIB Definitions", STD 16, RFC 1212, March 1991.  Rose, M., "A Convention for Defining Traps for use with the SNMP", RFC 1215, March 1991.  McCloghrie, K., Perkins, D. and J. Schoenwaelder, "Structure of Management Information Version 2 (SMIv2)", STD 58, RFC 2578, April 1999.  McCloghrie, K., Perkins, D. and J. Schoenwaelder, "Textual Conventions for SMIv2", STD 58, RFC 2579, April 1999.  McCloghrie, K., Perkins, D. and J. Schoenwaelder, "Conformance Statements for SMIv2", STD 58, RFC 2580, April 1999.  Case, J., Fedor, M., Schoffstall, M. and J. Davin, "Simple Network Management Protocol", STD 15, RFC 1157, May 1990.  Case, J., McCloghrie, K., Rose, M. and S. Waldbusser, "Introduction to Community-based SNMPv2", RFC 1901, January 1996.  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.  Case, J., Harrington D., Presuhn R. and B. Wijnen, "Message Processing and Dispatching for the Simple Network Management Protocol (SNMP)", RFC 2572, April 1999.  Blumenthal, U. and B. Wijnen, "User-based Security Model (USM) for version 3 of the Simple Network Management Protocol (SNMPv3)", RFC 2574, April 1999.  Levi, D., Meyer, P. and B. Stewart, "SNMPv3 Applications", RFC 2573, April 1999.
 Wijnen, B., Presuhn, R. and K. McCloghrie, "View-based Access Control Model (VACM) for the Simple Network Management Protocol (SNMP)", RFC 2575, April 1999.  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.  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.  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.  Hornstein, K. and W. Hardaker, "A Kerberos Security Model for SNMPv3", Work in Progress.  Brownlee, N. and A. Blount, "Accounting Attributes and Record Formats", RFC 2924, September 2000.  Network Management Research Group Web page, http://www.ibr.cs.tu-bs.de/projects/nmrg/  Schoenwaelder, J.,"SNMP-over-TCP Transport Mapping", Work in Progress.  Schoenwaelder, J., "SNMP Payload Compression", Work in Progress.  Sprenkels, R., Martin-Flatin, J.,"Bulk Transfers of MIB Data", Simple Times, http://www.simple-times.org/pub/simple- times/issues/7-1.html, March 1999.  Thaler, D., "Get Subtree Retrieval MIB", Work in Progress.  Daniele, M., Wijnen, B., Ellison, M. and D. Francisco, "Agent Extensibility (AgentX) Protocol Version 1", RFC 2741, January 2000.
9. Authors' Addresses Bernard Aboba Microsoft Corporation One Microsoft Way Redmond, WA 98052 USA Phone: +1 425 936 6605 EMail: email@example.com Jari Arkko Oy LM Ericsson Ab 02420 Jorvas Finland Phone: +358 40 5079256 EMail: Jari.Arkko@ericsson.com David Harrington Cabletron Systems Inc. P.O.Box 5005 Rochester NH 03867-5005 USA Phone: +1 603 337 7357 EMail: firstname.lastname@example.org
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