The Simple Times
The Quarterly Newsletter of SNMP Technology, Comment, and Events
Volume 8, Number 1
The Simple Times (tm) is an openly-available publication devoted
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In This Issue:
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of this issue.
This issue of The Simple Times focuses on configuration management,
which is still considered an important and insufficiently solved problem
in many networked environments. Of course, configuration management is
not a new problem. So why is there again so much interest in effective
The movement from a simple best-effort Internet towards an Internet
which supports multiple service levels is currently the main driving force.
The technologies which have been developed to implement QoS in the Internet
require much more configuration data to be effective. It is therefore of
key importance to solve the configuration management problem in order to
deploy an Internet which provides multiple service levels.
The articles in this issue of The Simple Times discuss various
aspects of the configuration management problem. The authors address questions
such as whether SNMP can be used to effectively address configuration management
problems or whether policy-based configuration management solutions can
address the problem appropriately.
An important aspect is the role standards can play to reduce the amount
of time and money spent on configuration management. Some people believe
that commonly accepted and implemented configuration management standards
are the only way to solve the problems. Others believe that device vendors
have little interest to support common configuration management standards
in order to differentiate their products and because it takes too long
to define configuration management standards for new emerging technologies.
You may want to keep these different views in mind while reading the articles
in this issue.
In the last issue of The Simple Times, we asked our readers to
fill out an online survey since we considered to publish the first twenty
issues of The Simple Times as a reference book. A total of 144 readers
completed the form. A majority of 77 percent expressed their interest in
such a book and the amount of money they were willing to pay was more or
less evenly distributed in the range US $10-$50.
Comparing the survey results with the page hit rates for the last issue
of The Simple Times (18,500 HTML, 7,750 PDF, 700 PostScript), we
concluded that only a minority actually participated in our online survey.
Of course, the online HTML version is frequently reloaded by the same group
of people. Hence, we believe that the PDF and PostScript page hit counts
provide a better base for an estimation of the size of The Simple Times
readership. We also know that some people continue to distribute paper
copies of The Simple Times. So a reasonable estimate of the size
of The Simple Times readership is perhaps 10,000. This, however,
implies that our survey only reached about one percent of the total readership
and thus we concluded that the interest is not big enough to bundle the
first twenty issues of
The Simple Times in a reference book.
Configuration Management Services for the Large
The need for more robust and effective configuration management tools has
always been a pressing issue within the enterprise networking space. Network
administrators continue to be under tremendous pressure to make their network
infrastructure provide more robust and timely services to more users.
John Roese, Enterasys
Compounding this pressure is the fact that enterprise IT organizations
continue to face growing shortages of qualified staff as well as budgets
that are not growing at the same pace as the scale of their systems. Examining
these trends, it becomes clear that in order to deliver the services required,
with the limited resources available, more intelligent management services
First Some History...
While network management has been part of the overall network solution
for over a decade, beyond basic fault isolation the real value of network
management tools has been very limited.
Vendors offer many ``configuration management'' tools and services,
but most have failed to provide real cost versus contribution value. What
is clear by looking at which tools are used and which tools are put idle
after purchase (whether due to inadequate design, limited device support
or excess complexity), is that the vast majority of configuration management
tools and services have failed to provide real value. This failure is sometimes
due to a disconnect between the management interfaces supported by the
tools, e.g. MIB modules, and the management interfaces supported by the
devices under their management.
Many configuration tools operate on the flawed principle that the configuration
action can be manually configured on a device-by-device basis. Most web-based
configuration tools fall into this category. They might be interesting
when configuring a single switch in a lab environment, but they do not
provide value when used to configure an enterprise consisting of thousands
The tangible result of the inadequacy of configuration management tools
can be seen in the development of the preferred configuration interface,
the Command Line Interface (CLI). The fact that most seasoned and technical
networking professionals prefer to use the cryptic and non-intuitive CLI
versus external software based configuration tools defies logic until you
look at the historical
record of the effectiveness of configuration management
tools. This is a simple but valid observation of the current state of configuration
We need to look at how we might correct this situation as we move into
the next generation of configuration management architectures. A deeper
analysis shows that a few specific issues come forward as an explanation
of what has gone wrong and how we might correct them as we move into the
next generation of configuration management architectures:
Too much detail. Building a huge monolithic configuration management
service that offers a direct interface into every attribute of a device
will overwhelm the network administrator. Too many tools present raw data
without converting it into useful management information.
Single Manager Assumption. Because the CLI is commonly used, and
split horizon management concepts are becoming the rule of the day, any
assumption that one management interface will have absolute control over
a device configuration is incorrect. Configuration Management services
that assume exclusive control of the systems under management are destined
Lack of support for standard management interfaces. Many vendors
require customers to learn and to use that vendorís proprietary tools,
such as the CLI, for configuring their devices. Even many of the core standards
developed by the IETF and IEEE considered network management a secondary
concern. Lack of standardized management increases the burden of staff
training, and increases the likelihood of unintended inconsistencies in
the configuration of multiple devices. There are two, not necessarily consequential,
effects here. Even a well-trained operator has difficulty ensuring consistency
of configuration in a large network of heterogeneous devices. This is particularly
problematic in the configuration of network security.
Lack of support for remote management in network devices. Many vendors
have traditionally failed to provide the ability to configure a device
remotely. Many router vendors have chosen not to instrument their routers
with open services that can facilitate remote configuration. This is a
significant deficiency in configuration management services because routers
are such an important factor in core internetworking.
Lack of security. One excuse to avoid developing and deploying configuration
management tools has been the lack of secure network transactions to facilitate
the service. The standards bodies have been responding to this need for
security with new standards, such as 802.1X, IPSec and SNMPv3.
Lack of scalability. Many tools work well in a demonstration or
lab setting but often fail when subjected to the scale of an enterprise
network. The design flaws occur either as a result of a failure to develop
a distributed model for the software or as a result of heavyweight protocols
that cannot operate when thousands of devices must be under system control.
What is Required to Succeed -- or -- What We Have Learned
There is hope for the next generation of configuration management tools.
Many of the needed solutions to the above issues have already been developed
and are simply awaiting market adoption and integration into better solutions.
As the CTO for a vendor of both network infrastructure and management
tools, I have seen the company and its competitors deliver solutions that
provided very effective configuration management services. I have also
seen them deliver tools that ended up being under utilized by customers.
After a decade of experience in this area, several key factors required
for successful configuration management services have become apparent:
The best solutions are the ones with the simplest interface and most
focused mission. Tools that hide the underlying complexity and raw
data from the network administrator -- and use elegant and focused interfaces
to present information that allows desired tasks to be accomplished easily
-- are the tools that will get used.
Multiple management interfaces must be supported. Configuration
management tools must be designed to expect configuration modifications
by other tools that are using other management interfaces. Even where tools
are designed to enforce policy-based configuration throughout the network,
the need to support exceptions to the rule exists for troubleshooting and
other atypical conditions.
Standardized management is critical for coordination of network-wide
configuration. For tools to apply configuration consistently throughout
a network, the ``language'' for expressing the configuration must be consistent
throughout the network, even across different vendorsí devices. The strong
future emphasis on policy based configuration will require standardized
The standards organizations recognize the importance of consistent
management interfaces. The IETF has made discussion of management a requirement
of all protocol proposals; IEEE standards typically include a discussion
of how a protocol should be managed. Likewise, many consortia submit proposals
to the IETF to provide a standard management interface.
Interfaces for remote management are necessary to allow for scalable
enterprise configuration. As networks continue to grow, it is no longer
acceptable to expect each device to be configured manually. The use of
automation reduces the staff burden and the consistency of configuration
throughout a large enterprise network.
The standards organizations recognize the importance of remote configuration,
especially using the Internet Standard Management Protocol, SNMP. Most
IETF working groups developing new protocols submit SNMP MIBs for standardized
management; IEEE has been submitting SNMP MIB proposals to the IETF to
accompany IEEE protocols and many consortia submit proposed SNMP MIBs to
the IETF to manage the protocol that is their focus.
Security must be approached in a holistic manner. As configuration
management tasks become easier to use, they also become more risky if left
exposed. There are many simple techniques to control the access to configuration
management interfaces that are rarely utilized today. Some of these will
become part of an overall holistic security solution in the future.
Logical side-band management channels via 802.1QVLANs, IPSEC tunnels
or other means are an easy way to secure the management interfaces of the
devices. More fine-grained user-authentication and data-access control
schemes will be possible using SNMPv3. To prevent security holes, it will
be important that authentication and data access control be consistent
between different management interfaces and between devices of different
vendors. All aspects of security must be integrated to create a cohesive
Scalable configuration management requires both bulk transfers and minimal
sized packets. An efficient bulk transfer mechanism is needed to distribute
network- and device-wide configurations during an initialization phase.
Minimal sized packets are needed to distribute incremental updates quickly,
using little bandwidth. In both the initialization and the update scenarios,
connectionless datagram-based protocols are much better suited for large-scale
configuration management tasks.
Although TCP based transport can offer some efficiency in bulk transfers
of data, when system-wide configuration is applied in a large enterprise
network, the resulting connection setup and maintenance overhead is unacceptable.
The connection setup and maintenance overhead is especially unsuitable
for incremental configuration management. For example, if a manager wants
to change the default QoS services in the 802.1p switches, it is important
that the thousands of ports affected be touched quickly, and that only
the minimum necessary data be transferred.
Policy-based networking and configuration management are not mutually
exclusive. In the confusion over the emergence of policy-based networking
as a marketing architecture, the focus of network management has been shifted
towards a future ``nirvana'' of user-based, profile-derived IT services.
While that end goal is worth pursuing, the typical swing of focus has failed
to recognize that this nirvana will be reachable via a combination of both
current and future management technologies.
Policy services can be viewed as falling into two classifications:
edge policy and core policy.
Edge policy deals with dynamic and individual entities in the
IT system. These include mapping policy to authenticated users at their
ingress port or possibly delivering policy-based on the dynamic content
of a connection (and the users involved) through a firewall. Edge policy
is somewhat served by protocols such as RADIUS and more recently via COPS-RSVP
and even LDAPv3.
Core policy, however, deals with static and aggregate entities
(subnets, traffic classes, system-wide application, etc.) in the IT system.
Core policy is almost exactly the same problem space as traditional configuration
management. Leveraging existing configuration management services and technologies
is an excellent way to effectively deploy a core policy model. When you
evaluate ways to deliver core policy services the existing configuration
management protocols of SNMPv1 -- and especially SNMPv3 -- are more than
adequate to deploy system wide or multiple device configuration changes
over large systems.
Leverage what you have before inventing and imposing new protocols.
Any vendor of network infrastructure solutions should be very cautious
about adding any new protocol when an existing protocol could be extended
and leveraged to solve the same problem. In development, including standards
work, it is often too easy to lose sight of the fact that existing protocols
and methods already in devices could be leveraged to solve the problem
Adding a new protocol to a device typically has a huge impact in
the complexity of the device because the change affects much more than
just the service it is tied to. For example, new protocols affect the deviceís
operating system because it must allocate resources to the new service
at the expense of other on-board services.
With the move to upgrade existing SNMPv1 management support to SNMPv3
and tri-lingual SNMP stacks, maximizing the use of that protocol is the
most logical direction for the development of management interfaces for
configuration and policy-based configuration.
With the increase in the complexity of networking services (DiffServ, 802.1X,
L4 Classification, Bandwidth control/rate limiting, policy-based routing,
802.1s Multiple Spanning Trees, ...) the need for effective and usable
configuration management services is critical. These evolutions have the
opportunity to solve many administration and configuration problems in
the next enterprise network.
Attention to these lessons will make the possibility of success for
next-generation configuration management services much higher:
As this article has discussed, we must make configuration management tools
that better address customersí needs, and obviate the need to rely on proprietary
We need to understand the historical failings of the predecessors of our
current configuration tools.
We need to solicit and study customerís present and future demands for
We must strive to evolve configuration management to address the failings
that plagued early implementations.
The standards bodies (IEEE and IETF) must continue to pay careful attention
to the delivery of management interfaces with their emerging technologies.
SNMPv3 must be deployed to provide a scalable, secure management architecture.
Core policy services should be delivered over existing protocols, such
Policy-Based Configuration Management: A Perspective
from a Network Management Vendor
One of the principle costs associated with maintaining a network is the
time spent on reconfiguration. This is not necessarily the replacement
of switches, concentrators, bridges, etc., but also adding, moving and
changing of users connected to the network. Simply moving a person from
one desk on one floor to another desk on another floor may involve changing
router ports, routing tables, IP addresses, making desktop changes and
even doing some physical rewiring.
Lundy Lewis, Aprisma
The average cost of adds, moves, and changes on today's router-centric
networks has been conservatively estimated at $300-500 per user. With the
average company moving each user 1.1 times per year, it is clear where
many of the support dollars are going. The administrators overseeing these
operations would appreciate a reduction in the time it takes to implement
As the cost of maintaining networks has risen, the network operators
able to oversee such operations are becoming harder to find. Many networks
are understaffed to meet the increasing demands placed on them. A management
system is needed which allows someone who is not a networking expert to
perform the mundane operations such as moving users around, adding users,
or changing the access constraints of specific users.
For example, the ability to connect to a network will often depend on
the location from which a user is accessing the network and the destination
a user is trying to reach. It is a complicated job to control access between
what could be thousands of users, and it is made more complicated by the
fact that the same user might access the system from different locations
and might need different levels of access as a function of the location.
The possible combinations of access increase quickly because of these ``nomadic''
Thus, it is desirable to have a management system for controlling, simplifying,
and/or automating various aspects of configuration management tasks so
that the cost of maintaining the network, and/or using the network, can
be better controlled.
Requirements for Policy-Based Configuration Management
What would a framework for implementing policy in configuration management
look like? The framework should include (i) a method for defining network
domains, (ii) a method for defining policies, (iii) a method for attaching
policies to domains, and (iv) a policy driver to monitor objects, execute
policies that apply to the objects, and adjudicate among conflicting policies.
Given this framework, one developing an application in a particular
network management area may ask the following questions:
With answers to these questions, one has made a good start towards implementing
policy in a particular management domain. For example, in the area of configuration
management, we desire a configuration application provided with policies
What are the objects in my application?
What are the attributes of the objects?
What (if any) are the ways in which I should group the objects?
Which attributes do I want to monitor and control?
What policies apply to attributes, objects, or groups of objects?
Which events will trigger the policy driver?
What are the actions I want when policies are violated or close to being
Let us describe such a policy framework in a little more detail .
The addition of users and resources on the network;
The deletion of users and resources from the network;
Changes in resource operating parameters;
The access rights of users and end stations to databases, applications,
and other users and end stations;
Authentication of users (for security); and
Tracking the usage of network resources.
A Generic Policy Framework
Figure 1 shows a generic policy framework. A domain space and a rule space
make up policy space, and together provide input to a policy driver. The
output of the policy driver is an action space which generally brings about
an enforcement of a policy in the network. The network communicates attribute
values to the domain space via SNMP polling or traps.
Figure 1: A Generic Policy Framework.
The domain space, at the lowest level of abstraction, consists of objects
of interest in the application. Objects are the smallest units in the domain
space, and they are defined in terms of their attributes. In access management
for example, the objects might be transmissions, where the attributes of
transmissions are source Internet Protocol (IP) address, destination IP
address, and service type. In fault management, objects might be alarms,
where the attributes of alarms are alarm severity, device type, and device
At higher levels of abstraction, objects are grouped into domains. A
particular grouping principle depends on the objects of interest in the
application and the attributes of the objects. Possible domains in access
management include all transmissions of a certain service type, or all
transmissions whose destination IP address falls into the address block
192.168/16. Possible domains in fault management include all red alarms,
or all alarms in Building 2. The domains include both objects and other
domains, as one domain may be a member of another domain.
The rule space consists of if-then rules, where the left-hand side of
the rule is written in terms of the attributes of objects in the domain
space, and the right-hand side is an action. For example, a rule in fault
management might be: ``If an alarm is red, then forward the alarm parameters
to the trouble ticket application.'' In a security application, an example
of a rule is: ``If the transmission source is X and the transmission destination
is Y, then block the transmission.''
The elements of the action space are just the right-hand sides of the
rules in the rule space. Actions are dependent on the application. They
may include permission or forbiddance of an operation on the network, the
modification of attributes in other objects, the display of a console message,
or an entry in a log file. For example, there might be just two kinds of
actions in fault management: forward an alarm to an external application,
or discard the alarm.
It is important to note that a policy in this framework is the attachment
of a rule or rule set to an element of the domain space. Thus, a policy
is inherently a two-place relation ``attaches to.'' For example, the statement
``All kids have to be in bed by 8 p.m.'' is a rule, but ``All kids have
to be in bed by 8 p.m. and this applies to you'' is a policy. In general,
we may say that ``rules that do not apply to objects are empty; and objects
without rules applying to the them are blind.''
The functions of the policy driver are to:
In general, the policy driver is triggered by an event, and takes an element
in the domain space as a parameter. In fault management, the policy driver
can be triggered by an alarm, where the parameter is just the alarm. In
configuration management, the policy driver can be triggered by a device
being switched on, and the parameter is the name of the device.
monitor the attributes of objects in the domain;
compare the values of attributes with the left-hand sides of rules;
resolve conflicts when two or more rules are applicable to the same object;
execute the right-hand side of a selected rule.
The general operation of the policy driver is as follows:
for domain element E do:
1. Collect all domains D of which E is a
member (either directly or indirectly).
2. Collect the rules that apply to each domain
D (if any), plus the rules for E (if any).
3. Resolve any conflicting rules,
producing an enforceable rule set.
4. Execute the action of each rule in the
enforceable rule set.
Note that a single iteration of the policy driver over the policy space
may result in actions that change the attributes of elements in the domain.
On subsequent iterations of the driver, other policies may be applicable
and thus change other attributes.
Conflicts occur when two rules issue two inconsistent actions. Consider
Figure 2, which illustrates a simple example of a domain space, a rule
space, and a policy space. If the policy driver is triggered for Object
1, and Object 1 inherits policies from parent Domain 1 and grandparent
Domain 2, it is possible that Rule 1 and Rule 2 are triggered and that
they have inconsistent actions (i.e. y and not-y).
Figure 2: How Conflicts Happen.
Now, the purpose of the conflict resolution strategy is to adjudicate when
conflicts occur. Note that conflict resolution strategies are a form of
``metapolicy'' about policies. There are several ways to specify such strategies
in a generic way in order to resolve conflicts. Possible strategies include
Disallow overlapping domains, thereby precluding the possibility of conflicts.
Uncover possible conflicts and resolve them via verification/validation
For the situation in Figure 2, for example, the strategy ``Select the rule
that issues from the most specific domain element'' would select Rule 1.
Select the rule that issues from the most specific domain element.
Select the rule that issues from the least specific domain element.
Select the rule that satisfies the largest number of conditions.
Report conflicting rules to a user and allow the user to adjudicate.
Select the rule according to a predefined priority ranking of rules.
Implementing Policy for Device Configuration Management (CM)
In this section we will describe a policy-based configuration manager (PCM)
based on the ideas above. The PCM monitors and controls the configuration
of network devices with respect to a prescribed policy. The PCM modifies
configurations (if needed) under alternative network scenarios, including
for example, when a device is added to the network and switched on, when
network traffic becomes overstressed, and when an administrator wishes
to perform a spot check on the network configuration.
The embodiment of the PCM uses the Spectrum Management System built
by Aprisma Management Technologies (formerly Cabletron Spectrum). Spectrum
provides the necessary underpinnings for a PCM application, including device
modeling, management information base (MIB) compilation, and interfaces
for monitoring and controlling devices via SNMP. The system is illustrated
in Figure 3.
Figure 3: A Policy-Based Configuration Management System.
A live network is monitored and controlled by the network management system,
which in turn communicates with a PCM system. The functions of the PCM
system are listed in Figure 3. We will describe them in a little more detail
Device configuration management in communications networks generally
includes the tasks of keeping an inventory of network devices, knowing/verifying
the configuration of each device, resetting or updating configurations
as the need arises, and scheduling configuration changes.
A configuration is a set of particular values of MIB attributes that
govern the operational characteristics of a device (e.g., port thresholds,
on/off switches, access, security, etc.). Devices that are reconfigured
routinely in communications networks are routers, switches, bridges, and
A configuration record is a copy of a configuration for a particular
device, e.g. a Cisco router. The configuration record includes a list of
attributes and their corresponding values. A configuration record may be
obtained by interrogating a selected device through a template, or by manual
construction and editing. The apparatus for doing so exists in the PCM.
Note that a configuration record may consist of a list of records that
are desired to be in effect for particular devices in a domain. For example,
a compound configuration record might consist of a record for SGI workstations
and another record for Cisco routers.
A configuration policy expresses a relation between a configuration
record and a device; the expression ``attaches to'' represents this relation.
For example, a policy could be that a network administrator wishes a particular
configuration record (i.e., rule) to be in force for a particular device
(i.e., object), regardless of whether the current configuration of the
device is equivalent to the desired configuration record.
As in the generic policy framework in Figure 1, the PCM includes the
The elements in the domain space are network devices such as hubs, bridges,
routers, and workstations. Domains are constructed in accordance with an
organizational principle by which devices are grouped in the network. In
general, network devices may be grouped in any way that serves as an aid
in understanding and managing the network. Common grouping principles include
grouping with respect to topology, device type, location, managerial domains,
and/or the organizational structure of a enterprise network.
an apparatus for defining a domain space;
an apparatus for defining configuration records (a rule space);
an apparatus for attaching configuration records to elements in the domain
space to create configuration policies; and
a policy driver for monitoring and enforcing configuration policies.
The data structure that records domain membership is of the form ``X
is a member of Y,'' where X identifies a device or a domain, and Y identifies
a domain. For example, Figure 4 shows five individual devices which are
grouped in a two-level grouping structure: (i) domains with respect to
device type (WS and RTR) and (ii) domains with respect to topology (LAN-1,
LAN-2). The arrows in the figure represent ``is a member of'' relationships.
Figure 4: A Sample Structure of a Domain Space.
Configuration policies are attachments of configuration records to elements
of the domain space. In Figure 4, a policy is represented by the expression
"CR-X " resting on top of an element in the domain space (i.e., CR-1, CR-2,
The general form of a configuration policy is ``X is attached to Y with
Ordering Index I if Conditions condition1, condition2, ...'' where X is
a configuration record, and Y is an element in the domain space. The Ordering
Index and Conditions parameters are optional. The former controls the order
in which configurations are loaded into a device, and the latter constrains
the enforceability of the attachments. For example:
CR-1.1 is attached to Y with Ordering Index 2 if
segment load (Z) > 40% and
CR-1.1 is not equal to the current
configuration of Y.
CR-1.2 is attached to Y with Ordering Index 3 if
segment load (Z) > 40% and
CR-1.2 is not equal to the current
configuration of Y.
CR-1.3 is attached to Y if
segment load (Z) =< 40%.
Here, if the condition segment load (Z) > 40% is true and neither
CR-1.1 nor CR-1.2 match the existing configuration of Y, then configuration
record CR-1.1 is downloaded on Y first, then CR-1.2. Other examples of
CM policies include the following:
CR-1.4 is attached to LAN-1 if
"the time is between 8 a.m. and 5 p.m."
CR-1.5 is attached to LAN-1 if
"the time is between 5 p.m. and 1 a.m."
As shown in Figure 5, the function of the policy driver is to monitor objects
in the domain space and to enforce configuration policies. The inputs to
the driver are a domain space, a trigger issuing from the network management
system, and a set of configuration records attached to elements in the
Figure 5: Policy-Based Configuration Management Framework.
The output of the policy driver is an instruction to the network management
A note of caution is in order. Looking back at Figure 3, we see that the
PCM interacts with network devices via the network management system, requesting
certain configuration actions. However, that introduces a big assumption,
namely that the network management system can configure devices according
to instruction. Unfortunately, not all configuration methods are SNMP-based.
Some vendors introduce additional methods such as proprietary command line
interfaces that by-pass MIB-based control. Such exceptions, then, may call
for additional (ugly) patches to the pure SNMP-based system.
download a configuration;
issue a notice of conflicting configurations;
issue a notice of ``no action required''; or
issue a report of the state of overall network configuration.
The policy driver may be triggered by one or more of the following events
issuing from the network management system:
The operation of the driver is a modification of the general operation
of the driver described in the Generic Policy Framework section:
a device goes up or down;
a new device is added to the network;
the network goes up or down;
a scheduler triggers the driver; and
a user manually triggers the driver.
for domain element E do:
1. Collect all domains D of which E is a
2. Collect the CRs that attach to each domain
D (if any), plus the CRs for E (if any).
3. From each CR, pick out those that
attach to the individual devices that
are members of E.
4. Resolve conflicting attachments, producing
one enforceable configuration record (ECR).
5. Do one of the following (user-selectable):
(a) Appeal to the administrator with
(b) Load the ECR into the devices in E and
report the transaction (unsupervised
Steps 1 to 3 are performed by cycling through a network grouping structure
following ``is a member of'' links and collecting attachments for E recursively
with prevention of infinite loops. A method for preventing infinite loops
is to keep a record of where you have been and stop if you revisit the
same spot. Step 4 is performed by the conflict resolution strategy incorporated
into the policy driver. Step 5 is performed by the network management platform.
Configuration conflicts occur when two configuration records issue enforcements
for two nonidentical values of a single device attribute. The purpose of
the conflict resolution strategy is to adjudicate when this happens. For
example, CR-2 issues
ifAdminStatus.5 = up(1) while CR-4 issues
There are several strategies one may employ to resolve such conflicts.
The PCM provides the following strategies, which are user-selectable:
These strategies reflect the common-sense notion that the exception overrides
the rule. If this strategy is not acceptable, the burden of conflict resolution
rests with the user of the system.
Select the value that issues from the CR which is attached to the most
specific network domain.
Select the value that issues from the CR which satisfies the greatest number
If both #1 and #2 issue conflicts, favor #1.
Report conflicting values to a user and allow the user to adjudicate among
A good sample of early work in policy-based management is the author's
paper ``Implementing Policy in Enterprise Networks'' . One may find
additional details of constructing the above policy-based configuration
management system in the three patents [2-4], which are available on-line
at the United States Patent and Trademark
Office. The IETF
Policy Framework Working Group is defining a policy framework and information
model to apply to quality-of-service (QoS). The DMTF
Information Service Level Agreement Working Group is extending the
Common Information Model (CIM) to include policies, rules, and expressions.
Good web sites on policy management in general are at the
College and at the University
of Southern California. Finally, there is a steady body of research
in policy-based management reported in the IEEE/IFIP Network Operations
and Management Symposium and IEEE/IFIP Integrated Management Symposium.
One can go to the
Society web site to find out more, including a brand new ``Workshop
on Policies for Distributed Systems and Networks''. See the ``Calendar
and Announcements'' section at the end of this issue for more details.
 Lewis, L., Implementing Policy in Enterprise Networks, IEEE
Communications Magazine, 34(1), January 1996.
 Malik, R., Sycamore, S., Tracy, B., Method and Apparatus for Configuration
Management in Communications Networks, Patent # 5,832,503, United
States Patent and Trademark Office, November 1998.
 Lewis, L., Malik, R., Sycamore, S., Thebaut, S., Scott, W., Rustici,
E., Kaikini, P., Method and Apparatus for Defining and Enforcing Policies
for Configuration Management in Communications Networks, Patent # 5,872,928,
United States Patent and Trademark Office,
 Thebaut, S., Scott, W., Rustici. E., Kaikini, P., Lewis, L., Malik,
R., Sycamore, S., Dev, R., Ibe, O., Aggarwal, A., Wohlers, T.,
Management and Conflict Resolution in Computer Networks, Patent # 5,889,953,
United States Patent and Trademark Office,
What are MODULE-COMPLIANCE and AGENT-CAPABILITIES constructs?
A MODULE-COMPLIANCE construct is used to formally write a requirements
specification for implementation of objects and/or notifications from one
or more MIB modules. An
AGENT-CAPABILITIES construct is used to
formally write an implementation capabilities specification for objects
and/or notifications from one or more MIB modules. The definitions of these
constructs are in RFC
2580, which is a companion to the other SMIv2 documents - RFC
2578 and RFC 2579.
How is a MODULE-COMPLIANCE construct used?
In IETF standards-track RFC documents, one or more
specifications are defined in each MIB module to specify implementation
requirements for that MIB module. They provide developers with a precise
definition of what support is needed to claim compliance with the RFC.
Note that the
MODULE-COMPLIANCE specifications could be put in
another MIB module. However, the current style is to include them inside
the MIB modules of standards-track RFCs. MIB modules developed by equipment
vendors do not typically include
Another use of MODULE-COMPLIANCE specifications is by equipment
users, such as Internet service providers (ISP). ISPs have a set of management
applications that are used to manage their networks. These applications
use a set of objects and notifications. An ISP can use a MODULE-COMPLIANCE
construct to specify exactly the objects and notification that must be
supported by SNMP agents in the network equipment for the management applications
to function. The buyer of equipment for the ISP can use
specifications to determine if new equipment can be managed with existing
How is an AGENT-CAPABILITIES construct used?
A provider of SNMP-managed devices can use one or more
specifications to precisely specify the objects and notifications that
are supported by an SNMP agent. The
sysORTable defined in RFC
1907 provides a mechanism for an SNMP agent to list a set of AGENT-CAPABILITIES
specifications that define what is presently supported. An
specification is identified by an OID value, which uniquely defines it
for all space and time. (And, thus, once written cannot be modified.) The
sysORTable allows runtime modification, so that extensible SNMP
agents, such as those that use the AgentX protocol, can also be supported.
Are agent capabilities a ``good thing''?
What AGENT-CAPABILITIES specifications are trying to accomplish
is very important and useful. A formal list of supported objects and notifications
helps in the understanding between agent developers and all the users of
SNMP agents, which include testing groups within the equipment manufacturer
(and independent test organizations), product managers writing product
description literature, support groups within the manufacturer, evaluators
of the equipment, developers of management applications, and integrators
that customize generic management applications (such as those that generate
trend charts). However, agent capabilities specifications have seen limited
usage, and there appears to be a few places for improvement. Even though,
they are still quite useful.
The most successful usage of agent capabilities specifications so far
is probably in automated agent testing. Given agent capabilities definitions
and a few control files, an automated test system can determine if an agent
supports everything specified in an agent capabilities specification.
Should I create one or several agent capabilities specifications to describe
an SNMP agent?
Typically, when a company starts developing network equipment, there is
a single product with a single developer for the SNMP agent. In this case,
a single agent capabilities specification seems appropriate. However, over
time (if the company is successful), there will be several releases of
the agent (with different and probably expanding capabilities), and several
products with similar capabilities in the agents. Generally, there will
be common chunks of capabilities in each release and across several products.
Back many years ago I worked at a company that had after about five
years of development, approximately 15 products with SNMP agents, and over
80 total versions! There was much similarity between versions and between
products. There were over 12 engineers developing SNMP agents. Since there
was a lot of shared code, the approach that was taken was to have an equivalent
of an agent capabilities specification per version of a ``library.'' Note
that sometimes the library would be updated, for example to fix a bug or
to apply some optimization, without changing SNMP support. So, even without
dynamic configuration of SNMP agents, it is still advantageous to have
multiple agent capabilities specifications. On the other hand, some developers
create a single agent capabilities specification per release of an agent,
even if nothing has changed in the support for objects and notifications.
What about agents which can dynamically adapt their capabilities?
Some SNMP agents are able to dynamically adapt their capabilities to the
execution environment. This adaptation can either happen at installation
time or during runtime. As an example, consider an SNMP agent running on
a Linux system. Some capabilities of this agent may depend on the configuration
options used by the system administrator when installing the agent. Other
capabilities may depend on the presence of certain operating system features
which are determined during runtime.
As noted before, agent capabilities specifications are relatively static.
Thus, the question is whether an agent capabilities statement indicates
exactly what an agent actually supports at any moment or whether an agent
capabilities statement defines the maximally possible support. Either interpretation
has its problems. The result is that agent capabilities statements are
useful in some but not necessarily all environments.
What should MIB compilers do with agent capabilities specifications?
First, lets define the term MIB compiler. The term is simple, but there
seem to be many interpretations. A MIB compiler should validate that a
collection of MIB modules is syntactically valid, and check that it is
semantically valid as far as possible that can be done by a computer program.
(Note that much of the semantics associated with a MIB module are specified
in text for humans and not for computer programs.) After validation, a
MIB compiler generates output in another format that is directly usable
or is input to another program. For example, a MIB compiler could create
data structure definitions and code stubs to assist in creating an SNMP
agent. Note that the data structures and stub code for one agent implementation
would probably be of little use for other agent implementations. Alternatively,
a MIB compiler could output information that assists a program that graphs
values of objects. There are many uses.
So what should a MIB compiler do with an agent capabilities specification?
Of course, a MIB compiler should validate agent capabilities specifications
like all other SMIv2 constructs. Unfortunately, many MIB compilers ``skip
over'' agent capabilities specifications without any checking to see if
they are valid. They do this because implementors could not figure out
what to do with the agent capabilities specifications. Other MIB compilers
try to use agent capabilities specifications to drive how data structures
and stub code is generated for agents. My experience and belief is the
opposite. That is, the control mechanisms that are used to specify the
implementation characteristics of an agent and used to drive a code generator,
should be used to drive the creation of agent capabilities specifications.
What should a MIB browser do with agent capabilities specifications?
A MIB browser is a program that allows a user to examine and change the
values of MIB objects. It should also allow the creation of new instances
of objects. It is possible for a MIB browser to give hints to its user
about which objects can be accessed based on information from an agent
capabilities specification. However, the benefit seems to be little. I
am not aware of any MIB browser that uses agent capabilities specifications.
I have not seen any agent capabilities specifications from vendors, why
There are three primary reasons why equipment vendors do not ship agent
capabilities specifications with their devices:
It is difficult to get agent implementors to completely document their
implementation. It is ``extra'' work to do after they have finished. And
it is often almost impossible to create agent capabilities specifications
for old agents without re-engineering the agent implementation.
The product marketing managers do not like to publish specifications that
appear to look like bug lists, even when an agent conforms to compliance
Since the agent capabilities are identified by OID values, it is difficult
for users to find the appropriate MIB modules. One of the most common requests
on the SNMP news group is for MIB modules from vendors that define a given
The reviews published in this column represent the opinion of the author(s).
Please contact the author(s) directly if you want to share your comments.
Please contact the editors of The Simple Times if you are interested
to publish your own book review in this column.
The book starts with the usual explanation of TCP/IP and SNMP. Although
the book was published in April 2000, the explanation focuses on SNMPv1
and spends only a few words on SNMPv3. This is a missed opportunity, since
the main tools discussed in the book already support SNMPv3 and many readers
may have questions related to that version.
After the introduction the book continues with some of the core Linux
utilities and tools, such as arp, ifconfig,
ping, tcpdump and
traceroute. For each of these
tools there are about 10 pages of text; this text may be quite useful for
many readers. The following section discusses additional system utilities
and tools, such as arpwatch, ethereal,
xtraceroute. Again, the text is useful, although
some of the examples could be shorter without loss of information.
The remaining two third of the book focuses on SNMP. First there is
a discussion of 80 pages on MIB-II; unfortunately this discussion is very
similar to the text of RFC1213 and similar information is already available
from many other sources. After MIB-II, the book continues with a discussion
of UCD SNMP and SUN's Solstice Enterprise Agents. Although many readers
may find the UCD-SNMP text valuable, it is unclear why a book with the
title ``Red Hat Linux Network Management Tools'' discusses tools for a
Solaris platform. Fortunately, the total text on UCD-SNMP is much larger
than the text on the SUN agent.
After the discussion on agents, the book continues with some of the
most popular Web-enabled tools: mrtg and
there is already many documentation on the Web related to these two packages,
the book is still interesting to read. The last chapters of the book concentrate
on the Linux
control panel and on scotty's
It is unclear why the discussion on the Linux
control panel was
postponed until the end of the book; it would have been more logical to
discuss it earlier. The chapter on
tkined is quite useful; there
are about 50 pages of information which can not easily be found elsewhere.
It is not clear, however, why the Tnm part of the scotty
package has not been discussed. It would have been a good replacement for
the sections on the SUN agent.
The book is surely a valuable source of information for people relatively
new to Linux based network management. What is absolutely missing, however,
are references to sources elsewhere. Although all packages that were discussed
are contained on the two CDs that accompany the book, the book should at
least have included the URLs from where the readers can download the latest
versions of these packages. In fact, except for a few examples like UCD-SNMP,
the book does not even mention the source and authors of these packages
(the publisher even claims that ``the software and information on the diskette
are the property of The McGraw-Hill Companies''!).
Please consult the latest version of Internet Official Protocol Standards.
As of this writing, the latest version is RFC
SMIv1 Data Definition Language
RFC 1155 - Structure
of Management Information
RFC 1212 - Concise
RFC 1215 - A Convention
for Defining Traps
SMIv2 Data Definition Language
RFC 1157 - Simple
Network Management Protocol
RFC 2576 - Coexistence
between SNMP Versions
RFC 2786 - Diffie-Helman
USM Key Management
SNMP Agent Extensibility
SMIv1 MIB Modules
RFC 1213 - Management
Information Base II
RFC 1643 - Ethernet-Like
Interface Types MIB
SMIv2 MIB Modules
RFC 2819 - Remote
Network Monitoring MIB
IANA Maintained MIB Modules
RFC 1270 - SNMP Communication
RFC 1321 - MD5 Message-Digest
RFC 1470 - Network
Management Tool Catalog
RFC 2039 - Applicability
of Standard MIBs to WWW Server Management
RFC 2962 - SNMP Application
Level Gateway for Payload Address Translation
RFC 1187 - Bulk Table
Retrieval with the SNMP
RFC 1224 - Techniques
for Managing Asynchronously Generated Alerts
RFC 1238 - CLNS MIB
RFC 1592 - SNMP Distributed
RFC 1792 - TCP/IPX
Connection MIB Specification
RFC 2593 - Script
MIB Extensibility Protocol
Directory Enabled Networks
This 700+ pages book serves as an authoritative guide for the Directory
Enabled Networks (DEN) technology. Directory enabled networks store network
and service management information in a common repository in an agreed-upon
data format. The book first introduces the object-oriented concepts that
provide the foundation for the Common Information Model (CIM) defined by
the Distributed Management Task Force (DMTF). The second part of the book
explains in detail how the CIM model has been extended to realize the DEN
idea. The last part of the book describes DEN extensions to support policy-based
networking and it presents some case studies how DEN technology is used
by various products.
Inter-Domain Management: Specification Translation & Interaction Translation
This document contains the specification of technologies that enable interworking
between OSI, SNMP and OMG CORBA-based management systems, also known as
the Joint Inter-Domain Management (JIDM) standards. The first part of the
book introduces the Specification Translation rules which describe how
GDMO and SMIv2 data definitions are mapped to CORBA IDL definitions. The
second part of the book covers the Interaction Translation rules which
define how CMIP and SNMP protocol operations can be mapped to CORBA operations.
The Open Group adopted both translation rules as Technical Standard C802.
Red Hat Linux Network Management Tools
This book discusses some of the main network management packages that are
available for the Red Hat as well as other Linux platforms. It explains
how to use utilities like UCD-SNMP,
ntop and the Linux control panel. These utilities, as
well as many others, are included on two CD-ROMs. (For more information,
see the book review elsewhere in this issue.)
Calendar and Announcements
49th Meeting of the
December 11-15, 2000, San Diego, CA, USA
50th Meeting of the
March 19-23, 2001, Minneapolis, MN, USA
51st Meeting of the
August 5-10, 2001, London, England
Conferences and Workshops:
Workshop on IP-oriented Operations
and Management (IPOM 2000)
September 4-6, 2000, Cracow, Poland
Workshop on Distributed
Systems Operations and Management 2000 (DSOM 2000)
December 4-6, 2000, Austin, Texas, USA
Workshop on Policies
for Distributed Systems and Networks (Policy 2001)
January 29-31, 2001, Bristol, England
Integrated Network Management
May 14-18, 2001, Seattle, WA, USA
Exhibitions and Trade Shows:
NetWorld + Interop Atlanta
September 25-29, 2000, Atlanta, USA
NetWorld + Interop Paris
November 6-9, 2000, Paris, France
NetWorld + Interop Sydney
March 7-9, 2001, Sydney, Australia
NetWorld + Interop Las Vegas
May 6-11, 2001, Las Vegas, USA
NetWorld + Interop Tokyo
June 4-8, 2001, Tokyo, Japan
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