# How to cook a KAT for your pet theory

Kleene algebra with tests is a beautiful, powerful framework for reasoning about programs. You can easily encode conventional While programs into KAT, and KAT enjoys decidable equality. Reasoning with KAT feels like you’re cheating Alan Turing himself: here we are, deciding nontrivial properties of programs!

The gist of KAT is that you write programs using a regular expression like notation: `+` for parallel composition, `;` for sequential, and `*` for iteration. So you might encode:

``````while x > 0:
y += 1
x -= 1``````

As `(xGt0; incY; decX)*; ¬xGt0`, where `xGt0` is a ‘test’ and `incY` and `decX` are ‘actions’. KAT’s equivalence decision procedure can prove that this program is equivalent to any finite unrolling of itself… neat!

NetKAT is the most impactful application of KAT: it’s an influential and successful academic project, and its ideas can already be found in numerous real, production systems. In light of NetKAT’s remarkable success… why don’t we apply KAT more often?

## What’s hard about KAT?

On its own, KAT proves plenty of nice theorems, but none of them reason about particular program behaviors. In the code snippet above, `xGt0`, `incY`, and `decX` are uninterpreted—there’s no relationship between, say `xGt0` and `decX`. That is, you might expect that `¬xGt0`;decX;`¬xGt0` is equivalent to `¬xGt0;decX`, because decrementing a number less than or equal to 0 will yield a number that is also less than or equal to 0. The names of our tests and actions are suggestive, but KAT treats them absractly. If you want to reason about the semantics of your tests and actions, you need to build a custom, concrete KAT. NetKAT reasons about fields on packets, and doing so means building a particular, concrete KAT with particular actions. The original paper spends quite a bit of effort proving this new, custom KAT has a sound, complete, and decidable equivalence checking.

Worse still, KAT’s metatheory is very challenging. To create NetKAT, Nate Foster and company worked through closely related ideas for a few years before Nate joined Cornell and started working with Dexter Kozen, KAT’s progenitor. Only then did they realize that KAT would be a good fit, and they got to work on developing a concrete KAT—NetKAT. Unfortunately, “collaborate with Dexter” is an approach that doesn’t scale.

## How to cook a KAT

In an upcoming PLDI 2022 paper, “Kleene Algebra Modulo Theories: A Framework for Concrete KATs”, Ryan Beckett, Eric Campbell, and I show how to generate a KAT over a given theory, i.e., a set of tests, actions, and their equational theory. We call the approach Kleene algebra modulo theories, or KMT. The paper covers quite a few examples:

• booleans and bit vectors
• monotonic natural numbers
• unbounded sets and maps
• NetKAT

What’s more, our approach allows for higher-order theories, like taking the product of two theories or using finite-time LTL to reason about another theory. (Our approach abstracts and generalizes Temporal NetKAT, which is just a concrete instance of our more general method.)

To build a KMT, you provide primitive tests and actions, along with weakest preconditions relating each pair of test and action. There’s an ordering requirement: a test must be no smaller than its preconditions. With these in hand, we’re able to automatically derive a KAT with good properties in a pay-as-you-go fashion:

• If your theory is sound, the KAT is sound.
• If your theory is complete, the KAT is complete.
• If your theory’s satisfiability checking is decidable, we can derive a decision procedure for equivalence.

I’m particularly excited that our framework is prototype-ready: our code is implemented as an OCaml library, where you define theories as functors. Please try it out—mess around and write your own theories, following our examples. We hope that KMT will significantly lower the bar for entry, making it easier for more people to play around with KAT’s powerful equivalence checking.

## What’s the catch?

There’s more than one way to cook a KAT. KMT generates KATs with tracing semantics, i.e., the exact trace of actions matters. In KAT+B! or NetKAT, later updates override earlier ones, e.g., `x:=false; x:=true ? x:=true`… but KMT will treat these terms differently, because they have different traces. KAT+B! deliberately avoids tracing; NetKAT only traces at predefined points, by means of their `dup` primitive, which marks the current state as historically salient. There’s no deep reason for KMT to use tracing, and we believe KMT can be generalized to support `dup`-like controls for tracing.

The ordering constraint on weakest preconditions is a strong one. Our natural numbers, sets, and maps must be monotonic: they may grow or shrink, but not both. They cannot be compared, e.g., two natural-valued variables `x` and `y` can be compared to constants but not each other.

KMT is also just a prototype. It’s fast for small programs, but it takes dedicated work to make a KAT’s decision procedure efficient enough on more serious examples.

## Why are you talking about cooking KATs?

The greatest POPL paper of all time is Manna and Pnueli 1983, “How to cook a temporal proof system for your pet language”. Why? Just take a look a the first page:

## Update

KMT won a distinguished paper award at PLDI!

# PLVNET

I’m really happy to be part of the first PLVNET, a workshop on the intersection of PL, verification, and networking. I have two abstracts up for discussion.

The first abstract, Temporal NetKAT, is about adding reasoning about packet histories to a network policy language like NetKAT. The work on this is moving along quite nicely (thanks in large part to Ryan Beckett!), and I’m looking forward to the conversations it will spark.

The second abstract, Type systems for SDN controllers, is about using type systems to statically guarantee the absence of errors in controller programs. Fancy new switches have tons of features, which can be tricky to operate—can we make sure that a controller doesn’t make any mistakes when it talks to a switch? Some things are easy, like making sure that the match/action rules are sent to tables that can handle them; some things are harder, like making sure the controller doesn’t fill up a switch’s tables. I think this kind of work is a nice complement to the NetKAT “whole policy” approach, a sort of OpenFlow 1.3+ version of VeriCon with slightly different goals.

Should be fun!

# Concurrent NetCore: From Policies to Pipelines

Cole Schlesinger, Dave Walker, and I submitted a paper to ICFP 2014. It’s called Concurrent NetCore: From Policies to Pipelines. Here’s the abstract:

In a Software-Defined Network (SDN), a central, computationally powerful controller manages a set of distributed, computationally simple switches. The controller computes a policy describing how each switch should route packets and populates packet-processing tables on each switch with rules to enact the routing policy. As network conditions change, the controller continues to add and remove rules from switches to adjust the policy as needed.

Recently, the SDN landscape has begun to change as several proposals for new, reconfigurable switching architectures, such as RMT and FlexPipe have emerged. These platforms provide switch programmers with many, flexible tables for storing packet-processing rules, and they offer programmers control over the packet fields that each table can analyze and act on. These reconfigurable switch architectures support a richer SDN model in which a switch configuration phase precedes the rule population phase. In the configuration phase, the controller sends the switch a graph describing the layout and capabilities of the packet processing tables it will require during the population phase. Armed with this foreknowledge, the switch can allocate its hardware (or software) resources more efficiently.

We present a new, typed language, called Concurrent NetCore, for specifying routing policies and graphs of packet-processing tables. Concurrent NetCore includes features for specifying sequential, conditional and concurrent control-flow between packet- processing tables. We develop a fine-grained operational model for the language and prove this model coincides with a higher level denotational model when programs are well typed. We also prove several additional properties of well typed programs, including strong normalization and determinism. To illustrate the utility of the language, we develop linguistic models of both the RMT and FlexPipe architectures and we give a multi-pass compilation algorithm that translates graphs and routing policies to the RMT model.

# A Balance of Power: Expressive, Analyzable Controller Programming

I just finished reading A Balance of Power: Expressive, Analyzable Controller Programming. It’s an interesting proposal, but I’m writing just to express my satisfaction with the following sentence:

When we hit expressive limits, however, our goal is not to keep growing this language—down that path lies sendmail.cf and other sulphurous designs—but to call out to full-language code.

‘Sulphurous’ indeed. Come for the nonmonotonic interpretation of learning, stay for the colorful prose.