| Internet-Draft | Flooding Reduction Algorithms Framework | February 2025 |
| Przygienda & Hegde | Expires 26 August 2025 | [Page] |
This document introduces a framework to deploy multiple flood reduction algorithms within the same IGP domain in an interoperable fashion.¶
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Scenarios exist where multiple distributed (or centralized) flood reduction algorithms may be deployed simultaneously within an IGP domain. These scenarios necessitate certain agreed on cooperative behaviors between the involved algorithms to ensure the correctness of the overall solution. This is true in both permanent and transient (i.e., migration) deployment cases. Fortunately, existing graph theory concepts allow to provide guidance toward the design of algorithms with the necessary properties to ensure their interoperable coexistence.¶
This document presents the necessary requirements for the involved algorithms and the details of a framework for their interoperable deployment. Although running multiple algorithms simultaneously may not be a preferred operational choice, it is necessary if the migration from one algorithm to another with minimal network disruption is a priority. A migration itself may be caused by the discovery of defects in the deployed algorithms or the deployment of new algorithms that offer improvements.¶
This section outlines a framework that allows the operation of multiple different flood reduction algorithms (called flooding pruners or pruners from here on) in an interoperable fashion.¶
An important observation upfront, which will become clear later in this section, is that full, non-optimized flooding presents a special case of a pruner itself. Normal flooding includes all adjacencies without any pruning, and hence we name it the zero-pruner or zero" for short.¶
This framework permits the use of at most one pruner on each node. It allows to change a specific pruner at any time on any subset of nodes in the network while limiting the impact to the node itself and possibly the re-convergence of a set of nodes within its connected component.¶
A connected component (or component for short) is defined as a subset of nodes running the same pruner (denoted as A) where each of the nodes can be connected to all other nodes by a path that traverses only nodes that run A. Observe that there well may be in the network multiple components that are not connected, but that run the same pruner. We denote a component for pruner P as P|, and if two disjoint components running P are present in the network, we denote those as P|' and P|''.¶
Zero-pruners also build components denoted as Z| and its primes.¶
Another way to visualize components is to consider a network running multiple pruners as "islands running non-zero algorithms" that are connected to each other by components running zero-pruners (i.e. using normal flooding).¶
A pruner may choose within its component a subset of links to flood while making sure that the component remains connected. In other words, after suppressing flooding on some links within the component there must still exist paths consisting of the remaining links that connect each pair of nodes in the component. We use for such remaining links the term flooding connected dominating set or CDS for short (more precisely, a not necessarily loop-free edge dominating set). Such a CDS is colloquially often called flooding topology in context of flood reduction algorithms. A simple spanning tree is an easily visualized special case of a CDS. We denote such a CDS for a component A| as A|*. A|* is often not unique for a component and many different sets of links can be a CDS. Nor is it required that a CDS has to be loop-free since there may be many different paths on the CDS between two nodes in a component. Therefore, it is possible in a most extreme case that each LSP is flooded on a different CDS.¶
To summarize the section above in simple terms, a pruner must choose at least one set of flooding links that guarantees that all information can reach all the nodes in the component.¶
Any flood reduction algorithm expecting to interoperate with other algorithms without understanding their semantics MUST adhere to the following rules. Otherwise, the algorithm cannot be expected to accommodate other algorithms in the network at the same time or is in other words a ship in the night.¶
This document does not consider other approaches that guarantee a pruner property on e.g. a clique. It assumes that such "ship in the night components" can be considered zero-pruners due to their implicit guarantee of correct flooding to nodes that are part of their component where connected to other components.¶
Nodes within a component are free to use any kind of pruner to calculate optimized flooding. Any mode of computation, distributed or centralized, will work fine as long as it adheres to Section 2.1.4.¶
The framework allows but does not assume any centralized instance or election in a component. Computation and communication within each component is completely independent of other components.¶
With the exception of a node having to advertise which pruner is active, no configuration is necessary unless the algorithm itself requires it.¶
A node is free to choose a different pruner or a zero-pruner at any point in time independent of all other nodes. It may end up in another component or become a zero-pruner with the maximum impact consisting of re-computation within two components that see such node leave or join. For a distributed algorithm, it is likely that only the adjoining nodes have to adjust their pruning decisions. That is to say, the framework allows for node-by-node deployment or migration of pruners without networkwide recomputation of optimized flooding. This is obviously critical to the stability of large networks that may not converge within reasonable time if the whole network were to revert to zero-pruning due to networkwide impact. However, such behavior cannot be excluded, for example, in case of election problems due to misconfiguration or topological separation of nodes if the whole network runs a single pruner relying on centralized election. The network itself cannot ensure correctness of a pruner or prevent a pruner having a blast radius of the whole component depending upon the algorithm and signaling used.¶
Although the framework provides extreme operational flexibility when deploying pruners, the most likely scenarios are a node-by-node deployment of a single pruner in addition to a zero-pruner or, if the necessity arises, a node-by-node migration to another pruner.¶
Figure 1 illustrates a network with three pruners running. Two components run pruner A and are denoted as A|' and A|'' and one component runs pruner B. Remaining three components run the zero-pruner algorithm (annotated as Z|', Z|'', and Z|'''). CDSes within components are shown by indicating the links that were pruned from flooding as crossed out. Additionally, the links that are included to connect the CDS of the component following the rules listed in Section 2.1.4 have been made thicker. Despite multiple algorithms and components being present in the network, the complete graph is obviously still covered by the involved CDSes.¶
Figure 1 also illustrates why the overall CDS can easily be more than just a spanning tree of the overall network. A node seeing its neighbor running another algorithm cannot always decide based on local knowledge whether the link should be included in flooding or not. Such a decision could be based on the overall view of the network using some global tie-breaking algorithm. However, due to the potential long flooding paths and one-link minimal cuts, such an algorithm is not considered here but could be proposed in the future.¶
The only signaling necessary is a Sub-TLV of the IS-IS Router Capability TLV-242 that is defined in [RFC7981] with the following format. The Sub-TLV MUST be advertised by a node that is actively running any pruner except a zero-pruner. The absence of this Sub-TLV signifies a node being a 'zero-pruner' or an algorithm behaving within its component in an equivalent fashion while also guaranteeing flooding on links where it connects to other components.¶
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This document outlines framework for modifications to an IGP protocol for operation on high-density network topologies. Implementations SHOULD implement cryptographic authentication compliant to e.g. [RFC5304], and should enable other security measures in accordance with the best common practices for the relevant IGP protocol.¶
The following people have contributed to this draft and are mentioned without any particular order: Jordan Head, Acee Lindem, Raj Chetan and Tony Li.¶