| Rfc | 7980 |
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| Title | A Framework for Defining Network Complexity |
|---|
| Author | M. Behringer, A.
Retana, R. White, G. Huston |
|---|
| Date | October 2016 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | INFORMATIONAL |
|---|
|
Independent Submission M. Behringer
Request for Comments: 7980 A. Retana
Category: Informational Cisco Systems
ISSN: 2070-1721 R. White
Ericsson
G. Huston
APNIC
October 2016
A Framework for Defining Network Complexity
Abstract
Complexity is a widely used parameter in network design, yet there is
no generally accepted definition of the term. Complexity metrics
exist in a wide range of research papers, but most of these address
only a particular aspect of a network, for example, the complexity of
a graph or software. While it may be impossible to define a metric
for overall network complexity, there is a desire to better
understand the complexity of a network as a whole, as deployed today
to provide Internet services. This document provides a framework to
guide research on the topic of network complexity as well as some
practical examples for trade-offs in networking.
This document summarizes the work of the IRTF's Network Complexity
Research Group (NCRG) at the time of its closure. It does not
present final results, but a snapshot of an ongoing activity, as a
basis for future work.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7980.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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to this document.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. General Considerations . . . . . . . . . . . . . . . . . . . 5
2.1. The Behavior of a Complex Network . . . . . . . . . . . . 5
2.2. Complex versus Complicated . . . . . . . . . . . . . . . 5
2.3. Robust Yet Fragile . . . . . . . . . . . . . . . . . . . 6
2.4. The Complexity Cube . . . . . . . . . . . . . . . . . . . 6
2.5. Related Concepts . . . . . . . . . . . . . . . . . . . . 6
2.6. Technical Debt . . . . . . . . . . . . . . . . . . . . . 7
2.7. Layering Considerations . . . . . . . . . . . . . . . . . 8
3. Trade-Offs . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1. Control-Plane State versus Optimal Forwarding Paths
(Stretch) . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2. Configuration State versus Failure Domain Separation . . 10
3.3. Policy Centralization versus Optimal Policy Application . 12
3.4. Configuration State versus Per-Hop Forwarding
Optimization . . . . . . . . . . . . . . . . . . . . . . 13
3.5. Reactivity versus Stability . . . . . . . . . . . . . . . 13
4. Parameters . . . . . . . . . . . . . . . . . . . . . . . . . 15
5. Elements of Complexity . . . . . . . . . . . . . . . . . . . 16
5.1. The Physical Network (Hardware) . . . . . . . . . . . . . 16
5.2. Algorithms . . . . . . . . . . . . . . . . . . . . . . . 17
5.3. State in the Network . . . . . . . . . . . . . . . . . . 17
5.4. Churn . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.5. Knowledge . . . . . . . . . . . . . . . . . . . . . . . . 17
6. Location of Complexity . . . . . . . . . . . . . . . . . . . 17
6.1. Topological Location . . . . . . . . . . . . . . . . . . 17
6.2. Logical Location . . . . . . . . . . . . . . . . . . . . 18
6.3. Layering Considerations . . . . . . . . . . . . . . . . . 18
7. Dependencies . . . . . . . . . . . . . . . . . . . . . . . . 18
7.1. Local Dependencies . . . . . . . . . . . . . . . . . . . 19
7.2. Network-Wide Dependencies . . . . . . . . . . . . . . . . 19
7.3. Network-External Dependencies . . . . . . . . . . . . . . 19
8. Management Interactions . . . . . . . . . . . . . . . . . . . 20
8.1. Configuration Complexity . . . . . . . . . . . . . . . . 20
8.2. Troubleshooting Complexity . . . . . . . . . . . . . . . 20
8.3. Monitoring Complexity . . . . . . . . . . . . . . . . . . 20
8.4. Complexity of System Integration . . . . . . . . . . . . 21
9. External Interactions . . . . . . . . . . . . . . . . . . . . 21
10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 22
11. Security Considerations . . . . . . . . . . . . . . . . . . . 22
12. Informative References . . . . . . . . . . . . . . . . . . . 22
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
1. Introduction
Network design can be described as the art of finding the simplest
solution to solve a given problem. Complexity is thus assumed in the
design process; engineers do not ask if there should be complexity,
but rather, how much complexity is required to solve the problem.
The question of how much complexity assumes there is some way to
characterize the amount of complexity present in a system. The
reality is, however, this is an area of research and experience
rather than a solved problem within the network engineering space.
Today's design decisions are made based on a rough estimation of the
network's complexity rather than a solid understanding.
The document begins with general considerations, including some
foundational definitions and concepts. It then provides some
examples for trade-offs that network engineers regularly make when
designing a network. This section serves to demonstrate that there
is no single answer to complexity; rather, it is a managed trade-off
between many parameters. After this, this document provides a set of
parameters engineers should consider when attempting to either
measure complexity or build a framework around it. This list makes
no claim to be complete, but it serves as a guide of known existing
areas of investigation as well as a pointer to areas that still need
to be investigated.
Two purposes are served here. The first is to guide researchers
working in the area of complexity in their work. The more
researchers are able to connect their work to the concerns of network
designers, the more useful their research will become. This document
may also guide research into areas not considered before. The second
is to help network engineers to build a better understanding of where
complexity might be "hiding" in their networks and to be more fully
aware of how complexity interacts with design and deployment.
The goal of the IRTF Network Complexity Research Group (NCRG) [ncrg]
was to define a framework for network complexity research while
recognizing that it may be impossible to define metrics for overall
network complexity. This document summarizes the work of this group
at the time of its closure in 2014. It does not present final
results, but rather a snapshot of an ongoing activity, as a basis for
future work.
Many references to existing research in the area of network
complexity are listed on the Network Complexity Wiki [wiki]. This
wiki also contains background information on previous meetings on the
subject, previous research, etc.
2. General Considerations
2.1. The Behavior of a Complex Network
While there is no generally accepted definition of network
complexity, there is some understanding of the behavior of a complex
network. It has some or all of the following properties:
o Self-Organization: A network runs some protocols and processes
without external control; for example, a routing process, failover
mechanisms, etc. The interaction of those mechanisms can lead to
a complex behavior.
o Unpredictability: In a complex network, the effect of a local
change on the behavior of the global network may be unpredictable.
o Emergence: The behavior of the system as a whole is not reflected
in the behavior of any individual component of the system.
o Non-linearity: An input into the network produces a non-linear
result.
o Fragility: A small local input can break the entire system.
2.2. Complex versus Complicated
The two terms "complex" and "complicated" are often used
interchangeably, yet they describe different but overlapping
properties. The RG made the following statements about the two
terms, but they would need further refinement to be considered formal
definitions:
o A "complicated" system is a deterministic system that can be
understood by an appropriate level of analysis. It is often an
externally applied attribute rather than an intrinsic property of
a system and is typically associated with systems that require
deep or significant levels of analysis.
o A "complex" system, by comparison, is an intrinsic property of a
system and is typically associated with emergent behaviors such
that the behavior of the system is not fully described by the sum
of the behavior of each of the components of the system. Complex
systems are often associated with systems whose components exhibit
high levels of interaction and feedback.
2.3. Robust Yet Fragile
Networks typically follow the "robust yet fragile" paradigm: they are
designed to be robust against a set of failures, yet they are very
vulnerable to other failures. Doyle [Doyle] explains the concept
with an example: the Internet is robust against single-component
failure but fragile to targeted attacks. The "robust yet fragile"
property also touches on the fact that all network designs are
necessarily making trade-offs between different design goals. The
simplest one is "Good, Fast, Cheap: Pick any two (you can't have all
three)", as articulated in "The Twelve Networking Truths" [RFC1925].
In real network design, trade-offs between many aspects have to be
made, including, for example, issues of scope, time, and cost in the
network cycle of planning, design, implementation, and management of
a network platform. Section 3 gives some examples of trade-offs, and
parameters are discussed in Section 4.
2.4. The Complexity Cube
Complex tasks on a network can be done in different components of the
network. For example, routing can be controlled by central
algorithms and the result distributed (e.g., OpenFlow model); the
routing algorithm can also run completely distributed (e.g., routing
protocols such as OSPF or IS-IS), or a human operator could calculate
routing tables and statically configure routing. Behringer
[Behringer] defines these three axes of complexity as a "complexity
cube" with the respective axes being network elements, central
systems, and human operators. Any function can be implemented in any
of these three axes, and this choice likely has an impact on the
overall complexity of the system.
2.5. Related Concepts
When discussing network complexity, a large number of influencing
factors have to be taken into account to arrive at a full picture,
for example:
o State in the Network: Contains the network elements, such as
routers, switches (with their OS, including protocols), lines,
central systems, etc. This also includes the number and
algorithmic complexity of the protocols on network devices.
o Human Operators: Complexity manifests itself often by a network
that is not completely understood by human operators. Human error
is a primary source for catastrophic failures and therefore must
be taken into account.
o Classes/Templates: Rather than counting the number of lines in a
configuration or the number of hardware elements, more important
is the number of classes from which those can be derived. In
other words, it is probably less complex to have 1000 interfaces
that are identically configured than 5 that are configured
completely different.
o Dependencies and Interactions: The number of dependencies between
elements, as well as the interactions between them, has influence
on the complexity of the network.
o Total Cost of Ownership (TCO): TCO could be a good metric for
network complexity if the TCO calculation takes into account all
influencing factors, for example, training time for staff to be
able to maintain a network.
o Benchmark Unit Cost (BUC): BUC is a related metric that indicates
the cost of operating a certain component. If calculated well, it
reflects at least parts of the complexity of this component.
Therefore, the way TCO or BUC is calculated can help to derive a
complexity metric.
o Churn / Rate of Change: The change rate in a network itself can
contribute to complexity, especially if a number of components of
the overall network interact.
Networks differ in terms of their intended purpose (such as is found
in differences between enterprise and public carriage network
platforms) and differences in their intended roles (such as is found
in the differences between so-called "access" networks and "core"
transit networks). The differences in terms of role and purpose can
often lead to differences in the tolerance for, and even the metrics
of, complexity within such different network scenarios. This is not
necessarily a space where a single methodology for measuring
complexity, and defining a single threshold value of acceptability of
complexity, is appropriate.
2.6. Technical Debt
Many changes in a network are made with a dependency on the existing
network. Often, a suboptimal decision is made because the optimal
decision is hard or impossible to realize at the time. Over time,
the number of suboptimal changes in themselves cause significant
complexity, which would not have been there had the optimal solution
been implemented.
The term "technical debt" refers to the accumulated complexity of
suboptimal changes over time. As with financial debt, the idea is
that also technical debt must be repaid one day by cleaning up the
network or software.
2.7. Layering Considerations
In considering the larger space of applications, transport services,
network services, and media services, it is feasible to engineer
responses for certain types of desired applications responses in many
different ways and involving different layers of the so-called
network protocol stack. For example, Quality of Service (QoS) could
be engineered at any of these layers or even in a number of
combinations of different layers.
Considerations of complexity arise when mutually incompatible
measures are used in combination (such as error detection and
retransmission at the media layer in conjunction with the use of TCP
transport protocol) or when assumptions used in one layer are
violated by another layer. This results in surprising outcomes that
may result in complex interactions, for example, oscillation, because
different layers use different timers for retransmission. These
issues have led to the perspective that increased layering frequently
increases complexity [RFC3439].
While this research work is focused on network complexity, the
interactions of the network with the end-to-end transport protocols,
application layer protocols, and media properties are relevant
considerations here.
3. Trade-Offs
Network complexity is a system-level, rather than component-level,
problem; overall system complexity may be more than the sum of the
complexity of the individual pieces.
There are two basic ways in which system-level problems might be
addressed: interfaces and continuums. In addressing a system-level
problem through interfaces, we seek to treat each piece of the system
as a "black box" and develop a complete understanding of the
interfaces between these black boxes. In addressing a system-level
problem as a continuum, we seek to understand the impact of a single
change or element to the entire system as a set of trade-offs.
While network complexity can profitably be approached from either of
these perspectives, in this document we have chosen to approach the
system-level impact of network complexity from the perspective of
continuums of trade-offs. In theory, modifying the network to
resolve one particular problem (or class of problems) will add
complexity that results in the increased likelihood (or appearance)
of another class of problems. Discovering these continuums of trade-
offs, and then determining how to measure each one, become the key
steps in understanding and measuring system-level complexity in this
view.
The following sections describe five such continuums; more may be
possible.
o Control-Plane State versus Optimal Forwarding Paths (or its
opposite measure, stretch)
o Configuration State versus Failure Domain Separation
o Policy Centralization versus Optimal Policy Application
o Configuration State versus Per-Hop Forwarding Optimization
o Reactivity versus Stability
3.1. Control-Plane State versus Optimal Forwarding Paths (Stretch)
Control-plane state is the aggregate amount of information carried by
the control plane through the network in order to produce the
forwarding table at each device. Each additional piece of
information added to the control plane -- such as more-specific
reachability information, policy information, additional control
planes for virtualization and tunneling, or more precise topology
information -- adds to the complexity of the control plane. This
added complexity, in turn, adds to the burden of monitoring,
understanding, troubleshooting, and managing the network.
Removing control-plane state, however, is not always a net positive
gain for the network as a system; removing control-plane state almost
always results in decreased optimality in the forwarding and handling
of packets traveling through the network. This decreased optimality
can be termed "stretch", which is defined as the difference between
the absolute shortest (or best) path traffic could take through the
network and the path the traffic actually takes. Stretch is
expressed as the difference between the optimal and actual path. The
figure below provides an example of this trade-off.
+---R1---+
| |
(aggregate: 192.0.2/24) R2 R3 (aggregate: 192.0.2/24)
| |
R4-------R5
|
(announce: 192.0.2.1/32) R6
Assume each link is of equal cost in this figure and that R6 is
advertising 192.0.2.1/32.
For R1, the shortest path to 192.0.2.1/32, advertised by R6, is along
the path [R1,R2,R4,R6].
Assume, however, the network administrator decides to aggregate
reachability information at R2 and R3, advertising 192.0.2.0/24
towards R1 from both of these points. This reduces the overall
complexity of the control plane by reducing the amount of information
carried past these two routers (at R1 only in this case).
Aggregating reachability information at R2 and R3, however, may have
the impact of making both routes towards 192.0.2.1/32 appear as equal
cost paths to R1; there is no particular reason R1 should choose the
shortest path through R2 over the longer path through R3. This, in
effect, increases the stretch of the network. The shortest path from
R1 to R6 is 3 hops, a path that will always be chosen before
aggregation is configured. Assuming half of the traffic will be
forwarded along the path through R2 (3 hops), and half through R3 (4
hops), the network is stretched by ((3+4)/2) - 3), or .5, a "half a
hop".
Traffic engineering through various tunneling mechanisms is, at a
broad level, adding control-plane state to provide more optimal
forwarding (or network utilization). Optimizing network utilization
may require detuning stretch (intentionally increasing stretch) to
increase overall network utilization and efficiency; this is simply
an alternate instance of control-plane state (and hence, complexity)
weighed against optimal forwarding through the network.
3.2. Configuration State versus Failure Domain Separation
A failure domain, within the context of a network control plane, can
be defined as the set of devices impacted by a change in the network
topology or configuration. A network with larger failure domains is
more prone to cascading failures, so smaller failure domains are
normally preferred over larger ones.
The primary means used to limit the size of a failure domain within a
network's control plane is information hiding; the two primary types
of information hidden in a network control plane are reachability
information and topology information. An example of aggregating
reachability information is summarizing the routes 192.0.2.1/32,
192.0.2.2/32, and 192.0.2.3/32 into the single route 192.0.2.0/24,
along with the aggregation of the metric information associated with
each of the component routes. Note that aggregation is a "natural"
part of IP networks, starting with the aggregation of individual
hosts into a subnet at the network edge. An example of topology
aggregation is the summarization of routes at a link-state flooding
domain boundary, or the lack of topology information in a distance-
vector protocol.
While limiting the size of failure domains appears to be an absolute
good in terms of network complexity, there is a definite trade-off in
configuration complexity. The more failure domain edges created in a
network, the more complex configuration will become. This is
particularly true if redistribution of routing information between
multiple control-plane processes is used to create failure domain
boundaries; moving between different types of control planes causes a
loss of the consistent metrics most control planes rely on to build
loop-free paths. Redistribution, in particular, opens the door to
very destructive positive feedback loops within the control plane.
Examples of control-plane complexity caused by the creation of
failure domain boundaries include route filters, routing aggregation
configuration, and metric modifications to engineer traffic across
failure domain boundaries.
Returning to the network described in the previous section,
aggregating routing information at R2 and R3 will divide the network
into two failure domains: (R1, R2, R3) and (R2, R3, R4, R5). A
failure at R5 should have no impact on the forwarding information at
R1.
A false failure domain separation occurs, however, when the metric of
the aggregate route advertised by R2 and R3 is dependent on one of
the routes within the aggregate. For instance, if the metric of the
192.0.2.0/24 aggregate is derived from the metric of the component
192.0.2.1/32, then a failure of this one component will cause changes
in the forwarding table at R1 -- in this case, the control plane has
not truly been separated into two distinct failure domains. The
added complexity in the illustration network would be the management
of the configuration required to aggregate the control-plane
information, and the management of the metrics to ensure the control
plane is truly separated into two distinct failure domains.
Replacing aggregation with redistribution adds the complexity of
managing the feedback of routing information redistributed between
the failure domains. For instance, if R1, R2, and R3 were configured
to run one routing protocol while R2, R3, R4, R5, and R6 were
configured to run another protocol, R2 and R3 could be configured to
redistribute reachability information between these two control
planes. This can split the control plane into multiple failure
domains (depending on how, specifically, redistribution is
configured) but at the cost of creating and managing the
redistribution configuration. Further, R3 must be configured to
block routing information redistributed at R2 towards R1 from being
redistributed (again) towards R4 and R5.
3.3. Policy Centralization versus Optimal Policy Application
Another broad area where control-plane complexity interacts with
optimal network utilization is QoS. Two specific actions are
required to optimize the flow of traffic through a network: marking
and Per Hop Behaviors (PHBs). Rather than examining each packet at
each forwarding device in a network, packets are often marked, or
classified, in some way (typically through Type of Service bits) so
they can be handled consistently at all forwarding devices.
Packet-marking policies must be configured on specific forwarding
devices throughout the network. Distributing marking closer to the
edge of the network necessarily means configuring and managing more
devices, but it produces optimal forwarding at a larger number of
network devices. Moving marking towards the network core means
packets are marked for proper handling across a smaller number of
devices. In the same way, each device through which a packet passes
with the correct PHBs configured represents an increase in the
consistency in packet handling through the network as well as an
increase in the number of devices that must be configured and managed
for the correct PHBs. The network below is used for an illustration
of this concept.
+----R1----+
| |
+--R2--+ +--R3--+
| | | |
R4 R5 R6 R7
In this network, marking and PHB configuration may be configured on
any device, R1 through R7.
Assume marking is configured at the network edge; in this case, four
devices (R4, R5, R6, R7) must be configured, including ongoing
configuration management, to mark packets. Moving packet marking to
R2 and R3 will halve the number of devices on which packet-marking
configuration must be managed, but at the cost of inconsistent packet
handling at the inbound interfaces of R2 and R3 themselves.
Thus, reducing the number of devices that must have managed
configurations for packet marking will reduce optimal packet flow
through the network. Assuming packet marking is actually configured
along the edge of this network, configuring PHBs on different devices
has this same trade-off of managed configuration versus optimal
traffic flow. If the correct PHBs are configured on R1, R2, and R3,
then packets passing through the network will be handled correctly at
each hop. The cost involved will be the management of PHB
configuration on three devices. Configuring a single device for the
correct PHBs (R1, for instance), will decrease the amount of
configuration management required at the cost of less than optimal
packet handling along the entire path.
3.4. Configuration State versus Per-Hop Forwarding Optimization
The number of PHBs configured along a forwarding path exhibits the
same complexity versus optimality trade-off described in the section
above. The more classes (or queues) traffic is divided into, the
more fine-grained traffic will be managed as it passes through the
network. At the same time, each class of service must be managed,
both in terms of configuration and in its interaction with other
classes of service configured in the network.
3.5. Reactivity versus Stability
The speed at which the network's control plane can react to a change
in configuration or topology is an area of widespread study.
Control-plane convergence can be broken down into four essential
parts:
o Detecting the change
o Propagating information about the change
o Determining the best path(s) through the network after the change
o Changing the forwarding path at each network element along the
modified paths
Each of these areas can be addressed in an effort to improve network
convergence speeds; some of these improvements come at the cost of
increased complexity.
Changes in network topology can be detected much more quickly through
faster echo (or hello) mechanisms, lower-layer physical detection,
and other methods. Each of these mechanisms, however, can only be
used at the cost of evaluating and managing false positives and high
rates of topology change.
If the state of a link change can be detected in 10 ms, for instance,
the link could theoretically change state 50 times in a second -- it
would be impossible to tune a network control plane to react to
topology changes at this rate. Injecting topology change information
into the control plane at this rate can destabilize the control
plane, and hence the network itself. To counter this, most
techniques that quickly detect link-down events include some form of
dampening mechanism; configuring and managing these dampening
mechanisms increases complexity.
Changes in network topology must also be propagated throughout the
network so each device along the path can compute new forwarding
tables. In high-speed network environments, propagation of routing
information changes can take place in tens of milliseconds, opening
the possibility of multiple changes being propagated per second.
Injecting information at this rate into the control plane creates the
risk of overloading the processes and devices participating in the
control plane as well as creating destructive positive feedback loops
in the network. To avoid these consequences, most control-plane
protocols regulate the speed at which information about network
changes can be transmitted by any individual device. A recent
innovation in this area is using exponential backoff techniques to
manage the rate at which information is advertised into the control
plane; the first change is transmitted quickly, while subsequent
changes are transmitted more slowly. These techniques all control
the destabilizing effects of rapid information flows through the
control plane through the added complexity of configuring and
managing the rate at which the control plane can propagate
information about network changes.
All control planes require some form of algorithmic calculation to
find the best path through the network to any given destination.
These algorithms are often lightweight but they still require some
amount of memory and computational power to execute. Rapid changes
in the network can overwhelm the devices on which these algorithms
run, particularly if changes are presented more quickly than the
algorithm can run. Once a device running these algorithms becomes
processor or memory bound, it could experience a computational
failure altogether, causing a more general network outage. To
prevent computational overloading, control-plane protocols are
designed with timers limiting how often they can compute the best
path through a network; often these timers are exponential in nature
and thus allow the first computation to run quickly while delaying
subsequent computations. Configuring and managing these timers is
another source of complexity within the network.
Another option to improve the speed at which the control plane reacts
to changes in the network is to precompute alternate paths at each
device and possibly preinstall forwarding information into local
forwarding tables. Additional state is often needed to precompute
alternate paths, and additional algorithms and techniques are often
configured and deployed. This additional state, and these additional
algorithms, add some amount of complexity to the configuration and
management of the network.
In some situations (for some topologies), a tunnel is required to
pass traffic around a network failure or topology change. These
tunnels, while not manually configured, represent additional
complexity at the forwarding and control planes.
4. Parameters
In Section 3, we describe a set of trade-offs in network design to
illustrate the practical choices network operators have to make. The
amount of parameters to consider in such trade-off scenarios is very
large, and thus a complete listing may not be possible. Also, the
dependencies between the various metrics themselves is very complex
and requires further study. This document attempts to define a
methodology and an overall high-level structure.
To analyze trade-offs it is necessary to formalize them. The list of
parameters for such trade-offs is long, and the parameters can be
complex in themselves. For example, "cost" can be a simple
unidimensional metric, but "extensibility" and "optimal forwarding
state" are harder to define in detail.
A list of parameters to trade off contains metrics such as:
o State: How much state needs to be held in the control plane,
forwarding plane, configuration, etc.?
o Cost: How much does the network cost to build and run (i.e.,
capital expenditure (capex) and operating expenses (opex))?
o Bandwidth/Delay/Jitter: Traffic characteristics between two points
(average, max, etc.)
o Configuration Complexity: How hard is it to configure and maintain
the configuration?
o Susceptibility to Denial of Service: How easy is it to attack the
service?
o Security (Confidentiality/Integrity): How easy is it to
sniff/modify/insert the data flow?
o Scalability: To what size can I grow the network/service?
o Stability: How stable is the network under the influence of local
change?
o Reactivity: How fast does the network converge or adapt to new
situations?
o Extensibility: Can I use the network for other services in the
future?
o Ease of Troubleshooting: Are failure domains separated? How hard
is it to find and correct problems?
o Optimal Per-Hop Forwarding Behavior
o Predictability: If I change a parameter, what will happen?
o Clean Failure: When a problem arises, does the root cause lead to
deterministic failure?
5. Elements of Complexity
Complexity can be found in various elements in a networked system.
For example, the configuration of a network element reflects some of
the complexity contained in this system, or an algorithm used by a
protocol may be more or less complex. When classifying complexity,
"WHAT is complex?" is the first question to ask. This section offers
a method to answer this question.
5.1. The Physical Network (Hardware)
The set of network devices and wiring contains a certain complexity.
For example, adding a redundant link between two locations increases
the complexity of the network but provides more redundancy. Also,
network devices can be more or less modular, which has impact on
complexity trading off against ease of maintenance, availability, and
upgradability.
5.2. Algorithms
The behavior of the physical network is not only defined by the
hardware but also by algorithms that run on network elements and in
central locations. Every algorithm has a certain intrinsic
complexity, which is the subject of research on software complexity.
5.3. State in the Network
The way a network element treats traffic is defined largely by the
state in the network, in form of configuration, routing state,
security measures, etc. Section 3.1 shows an example where more
control-plane state allows for a more precise forwarding.
5.4. Churn
The rate of change itself is a parameter in complexity and needs to
be weighed against other parameters. Section 3.5 explains a trade-
off between the speed of communicating changes through the network
and the stability of the network.
5.5. Knowledge
Certain complexity parameters have a strong link to the human aspect
of networking. For example, the more options and parameters a
network protocol has, the harder it is to configure and troubleshoot.
Therefore, there is a trade-off between the knowledge to be
maintained by operational staff and desired functionality. The
required knowledge of network operators is therefore an important
part in complexity considerations.
6. Location of Complexity
The previous section discussed in which form complexity may be
perceived. This section focuses on where this complexity is located
in a network. For example, an algorithm can run centrally,
distributed, or even in the head of a network administrator. In
classifying the complexity of a network, the location of a component
may have an impact on overall complexity. This section offers a
methodology to find WHERE the complex component is located.
6.1. Topological Location
An algorithm can run distributed; for example, a routing protocol
like OSPF runs on all routers in a network. But, it can also be in a
central location such as the Network Operations Center (NOC). The
physical location has an impact on several other parameters, such as
availability (local changes might be faster than going through a
remote NOC) and ease of operation, because it might be easier to
understand and troubleshoot one central entity rather than many
remote ones.
The example in Section 3.3 shows how the location of state (in this
case configuration) impacts the precision of the policy enforcement
and the corresponding state required. Enforcement closer to the edge
requires more network-wide state but is more precise.
6.2. Logical Location
Independent of its physical location, the logical location also may
make a difference to complexity. A controller function, for example,
can reside in a NOC and also on a network element. Generally,
organizing a network in separate logical entities is considered
positive because it eases the understanding of the network, thereby
making troubleshooting and configuration easier. For example, a BGP
route reflector is a separate logical entity from a BGP speaker, but
it may reside on the same physical node.
6.3. Layering Considerations
Also, the layer of the TCP/IP stack in which a function is
implemented can have an impact on the complexity of the overall
network. Some functions are implemented in several layers in
slightly different ways; this may lead to unexpected results.
As an example, a link failure is detected on various layers: L1, L2,
the IGP, BGP, and potentially more. Since those have dependencies on
each other, different link failure detection times can cause
undesired effects. Dependencies are discussed in more detail in the
next section.
7. Dependencies
Dependencies are generally regarded as related to overall complexity.
A system with less dependencies is generally considered less complex.
This section proposes a way to analyze dependencies in a network.
For example, [Chun] states: "We conjecture that the complexity
particular to networked systems arises from the need to ensure state
is kept in sync with its distributed dependencies."
In this document, we distinguish three types of dependencies: local
dependencies, network-wide dependencies, and network-external
dependencies.
7.1. Local Dependencies
Local dependencies are relative to a single node in the network. For
example, an interface on a node may have an IP address; this address
may be used in other parts of the configuration. If the interface
address changes, the dependent configuration parts have to change as
well.
Similar dependencies exist for QoS policies, access-control lists,
names and numbers of configuration parts, etc.
7.2. Network-Wide Dependencies
Routing protocols, failover protocols, and many others have
dependencies across the network. If one node is affected by a
problem, this may have a ripple effect through the network. These
protocols are typically designed to deal with unexpected consequences
and thus are unlikely to cause an issue on their own. But,
occasionally a number of complexity issues come together (for
example, different timers on different layers), resulting in
unexpected behavior.
7.3. Network-External Dependencies
Some dependencies are on elements outside the actual network, for
example, on an external NTP clock source or an Authentication,
Authorization, and Accounting (AAA) server. Again, a trade-off is
made: in the example of AAA used for login authentication, we reduce
the configuration (state) on each node (in particular, user-specific
configuration), but we add an external dependency on a AAA server.
In networks with many administrators, a AAA server is clearly the
only manageable way to track all administrators. But, it comes at
the cost of this external dependency with the consequence that admin
access may be lost for all devices at the same time when the AAA
server is unavailable.
Even with the external dependency on a AAA server, the advantage of
centralizing the user information (and logging) still has significant
value over distributing user information across all devices. To
solve the problem of the central dependency not being available,
other solutions have been developed -- for example, a secondary
authentication mode with a single root-level password in case the AAA
server is not available.
8. Management Interactions
A static network generally is relatively stable; conversely, changes
introduce a degree of uncertainty and therefore need to be examined
in detail. Also, the troubleshooting of a network exposes
intuitively the complexity of the network. This section proposes a
methodology to classify management interactions with regard to their
relationship to network complexity.
8.1. Configuration Complexity
Configuration can be seen as distributed state across network devices
where the administrator has direct influence on the operation of the
network. Modifying the configuration can improve the network
behavior overall or negatively affect it. In the worst case, a
single misconfiguration could potentially bring down the entire
network. Therefore, it is important that a human administrator can
manage the complexity of the configuration well.
The configuration reflects most of the local and global dependencies
in the network, as explained in Section 7. Tracking those
dependencies in the configuration helps in understanding the overall
network complexity.
8.2. Troubleshooting Complexity
Unexpected behavior can have a number of sources: the configuration
may contain errors, the operating system (algorithms) may have bugs,
and the hardware may be faulty, which includes anything from broken
fibers to faulty line cards. In serious problems, a combination of
causes could result in a single visible condition. Tracking the root
causes of an error condition may be extremely difficult, pointing to
the complex nature of a network.
Being able to find the source of a problem requires, therefore, a
solid understanding of the complexity of a network. The
configuration complexity discussed in the previous section represents
only a part of the overall problem space.
8.3. Monitoring Complexity
Even in the absence of error conditions, the state of the network
should be monitored to detect error conditions ideally before network
services are affected. For example, a single link-down event may not
cause a service disruption in a well-designed network, but the
problem needs to be resolved quickly to restore redundancy.
Monitoring a network has itself a certain complexity. Issues are in
scale; variations of devices to be monitored; variations of methods
used to collect information; the inevitable loss of information as
reporting is aggregated centrally; and the knowledge required to
understand the network, the dependencies, and the interactions with
users and other external inputs.
8.4. Complexity of System Integration
A network doesn't just consist of network devices but includes a vast
array of backend and support systems. It also interfaces a large
variety of user devices, and a number of human interfaces, both to
the user/customer as well as to administrators of the network. A
system integration job is required in order to make sure the overall
network provides the overall service expected.
All those interactions and systems have to be modeled to understand
the interdependencies and complexities in the network. This is a
large area of future research.
9. External Interactions
A network is not a self-contained entity, but it exists to provide
connectivity and services to users and other networks, both of which
are outside the direct control of a network administrator. The user
experience of a network also illustrates a form of interaction with
its own complexity.
External interactions fall into the following categories:
o User Interactions: Users need a way to request a service, to have
their problems resolved, and potentially to get billed for their
usage. There are a number of human interfaces that need to be
considered, which depend to some extent on the network, for
example, for troubleshooting or monitoring usage.
o Interactions with End Systems: The network also interacts with the
devices that connect to it. Typically, a device receives an IP
address from the network and information on how to resolve domain
names, plus potentially other services. While those interactions
are relatively simple, the vast amount of end-device types makes
this a complicated space to track.
o Internetwork Interactions: Most networks connect to other
networks. Also, in this case, there are many interactions between
networks, both technical (for example, running a routing protocol)
as well as non-technical (for example, tracing problems across
network boundaries).
For a fully operational network providing services to users, the
external interactions and dependencies also form an integral part of
the overall complexity of the network service. A specific example
are the root DNS servers, which are critical to the function of the
Internet. Practically all Internet users have an implicit dependency
on the root DNS servers, which explains why those are frequent
targets for attacks. Understanding the overall complexity of a
network includes understanding all those external dependencies. Of
course, in the case of the root DNS servers, there is little a
network operator can influence.
10. Examples
In the foreseeable future, it is unlikely to define a single,
objective metric that includes all the relevant aspects of
complexity. In the absence of such a global metric, a comparative
approach could be easier.
For example, it is possible to compare the complexity of a
centralized system where algorithms run centrally and the results are
distributed to the network nodes with a distributed algorithm. The
type of algorithm may be similar, but the location is different, and
a different dependency graph would result. The supporting hardware
may be the same and thus could be ignored for this exercise. Also,
layering is likely to be the same. The management interactions,
though, would significantly differ in both cases.
The classification in this document also makes it easier to survey
existing research with regards to which area of complexity is
covered. This could help in identifying open areas for research.
11. Security Considerations
This document does not discuss any specific security considerations.
12. Informative References
[Behringer] Behringer, M., "Classifying Network Complexity",
Proceedings of the 2009 Workshop on Re-architecting the
Internet (Re-Arch '09), ACM, DOI 10.1145/1658978.1658983,
December 2009.
[Chun] Chun, B-G., Ratnasamy, S., and E. Eddie, "NetComplex: A
Complexity Metric for Networked System Designs",
Proceedings of the 5th USENIX Symposium on Networked
Systems Design and Implementation (NSDI '08), pp.
393-406, April 2008, <http://usenix.org/events/nsdi08/
tech/full_papers/chun/chun.pdf>.
[Doyle] Doyle, J., Anderson, D., Li, L., Low, S., Roughnan, M.,
Shalunov, S., Tanaka, R., and W. Willinger, "The 'robust
yet fragile' nature of the Internet", Proceedings of the
National Academy of Sciences of the United States of
America (PNAS), Volume 102, Number 41,
DOI 10.1073/pnas.0501426102, October 2005.
[ncrg] IRTF, "IRTF Network Complexity Research Group (NCRG)
[CONCLUDED]", <https://irtf.org/concluded/ncrg>.
[RFC1925] Callon, R., "The Twelve Networking Truths", RFC 1925,
DOI 10.17487/RFC1925, April 1996,
<http://www.rfc-editor.org/info/rfc1925>.
[RFC3439] Bush, R. and D. Meyer, "Some Internet Architectural
Guidelines and Philosophy", RFC 3439,
DOI 10.17487/RFC3439, December 2002,
<http://www.rfc-editor.org/info/rfc3439>.
[wiki] "Network Complexity - The Wiki",
<http://networkcomplexity.org/>.
Acknowledgements
The motivations and framework of this overview of studies into
network complexity are the result of many meetings and discussions
with too many people to provide a full list here. However, key
contributions have been made by John Doyle, Dave Meyer, Jon
Crowcroft, Mark Handley, Fred Baker, Paul Vixie, Lars Eggert, Bob
Briscoe, Keith Jones, Bruno Klauser, Stephen Youell, Joel Obstfeld,
and Philip Eardley.
The authors would like to acknowledge the contributions of Rana
Sircar, Ken Carlberg, and Luca Caviglione in the preparation of this
document.
Authors' Addresses
Michael H. Behringer
Cisco Systems
Building D, 45 Allee des Ormes
Mougins 06250
France
Email: mbehring@cisco.com
Alvaro Retana
Cisco Systems
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
United States of America
Email: aretana@cisco.com
Russ White
Ericsson
144 Warm Wood Lane
Apex, NC 27539
United States of America
Email: russ@riw.us
URI: http://www.ericsson.com
Geoff Huston
Asia Pacific Network Information Centre
6 Cordelia St
South Brisbane, QLD 4101
Australia
Email: gih@apnic.net
URI: http://www.apnic.net