A Framework for Defining Network Complexity
RFC 7980

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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.

Behringer, et al.             Informational                     [Page 1]
RFC 7980                  Complexity Framework              October 2016

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
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.

Behringer, et al.             Informational                     [Page 2]
RFC 7980                  Complexity Framework              October 2016

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