---
layer: to-be
status: designed
code: [mesh-lab]
updated: 2026-08-23
decisions:
  - 02-DECISIONS/0029-the-labs-first-scenario-has-no-pipeline.md
  - 02-DECISIONS/0031-the-lab-provides-the-underlay.md
  - 02-DECISIONS/0016-a-lab-node-is-a-virtual-machine.md
---

# The scenario declaration

A scenario is a **declaration of an underlay**, plus what to put on it. It is the interface
everything in the lab hangs off, so it is worth getting small.

It states what a hosting provider and a home router would provide, and nothing the mesh is
responsible for ([ADR 0031](../../02-DECISIONS/0031-the-lab-provides-the-underlay.md)).

## The shape

```yaml
scenario: published-behind-nat

segments:
  wan:
    cidr: 203.0.113.0/24        # RFC 5737 — never routes on the real internet
  lan:
    cidr: 192.168.1.0/24
    behind: wan                 # NAT; the lab materialises a router

machines:
  anchor:
    segment: wan
    address: 203.0.113.10
  home-server:
    segment: lan
    address: 192.168.1.135
    forwarded: [443]            # reachable from wan through the router
  workstation:
    segment: lan
    address: 192.168.1.250
  laptop:
    segment: detached           # reachable by nothing until attached

place:
  all: [host]
  anchor: [substrate]

snapshot: raised
```

That is a complete bootstrap scenario. Nothing in it mentions the overlay, a hub, peering,
names or certificates — all of which are outcomes to be observed.

## The four parts

**`segments`** — the networks that exist. `behind:` declares NAT, and is the only place a
router comes from: the lab materialises one without being asked, because NAT has to run
somewhere. This is the one implicit machine in an otherwise explicit declaration.

**`machines`** — what sits where. A machine has a segment and an address, and that is nearly
all. `forwarded:` opens a port through the router, which is what makes *published but behind
NAT* reproducible — the case that exists only in production today. `segment: detached` is a
machine on no network, which is how a roaming node is expressed at rest.

**`place`** — what goes inside. `all:` applies to every machine; a machine name overrides for
that machine. This is the only part that differs between the two scenario classes.

**`snapshot`** — names the state once placement finishes, so a run can return to it without
raising everything again. Snapshots are what make repetition cheap, and cheap repetition is
what makes the bootstrap path the inner development loop rather than a ceremony.

## Why the addresses are load-bearing

The routable segment uses RFC 5737 documentation space, and this is not a stylistic choice.

The mesh decides *public versus private* by matching the address. A private range on the
segment meant to be routable makes the hub test as unreachable, and **the mesh silently never
forms** — no error, no failed step, just a mesh that does not exist. Research 004 calls this
the single most important fact in its analysis.

So the format should make this hard to get wrong rather than merely documented: a segment
without `behind:` is a routable segment, and an address in it that is not documentation space
is a declaration error, refused before anything is raised. That is
[ADR 0008](../../02-DECISIONS/0008-a-failed-step-fails-the-job.md) applied to a configuration
file — the failure it prevents is silent, so the check has to be loud.

## The same declaration serves both classes

The bootstrap and full scenarios differ **only in `place:`**
([ADR 0029](../../02-DECISIONS/0029-the-labs-first-scenario-has-no-pipeline.md)). Everything
about the underlay is identical, which is what makes one a strict subset of the other rather
than a fork.

```yaml
# bootstrap — tiers 0 and 1
place:
  all: [host]
  anchor: [substrate]

# full — adds a control plane, a forge, and a module under test
place:
  all: [host]
  anchor: [substrate, control, forge]
module: a-web-service
assert:
  - the service answers on its published name
  - the certificate presented is valid for that name
```

`module:` and `assert:` are meaningless in a bootstrap scenario and absent from one. A
bootstrap scenario's verdict comes from what the host reports about the state it reconciled,
not from an assertion runner — which is why assertion execution is second in the build order,
not first.

## What a scenario deliberately cannot say

- **Overlay addresses, the hub, peer configuration.** Outcomes, not inputs
  ([ADR 0031](../../02-DECISIONS/0031-the-lab-provides-the-underlay.md)).
- **What a machine is in mesh terms** — server or workstation, its site, its names. Mesh
  configuration, established by the mesh.
- **A host's capability profile.** Detected, never declared.
- **Steps.** A scenario is a desired state. Anything expressed as an ordered list of actions
  belongs in the lifecycle, not the declaration.

## Open

- **`user` and `edge` profiles have no scenario.** A lab machine is always privileged, so the
  two profiles that exist for unprivileged and phone-like participation cannot currently be
  exercised. Either the lab grows a way to run the host unprivileged, or those profiles are
  developed against something that is not a virtual machine.
- **Attaching and detaching during a run.** `segment: detached` covers a machine at rest;
  moving one between segments while a scenario is live is what makes a roaming node
  interesting, and that is lifecycle rather than declaration.
- **Where `place:` gets its artifacts from.** Before the mesh is self-hosting these come from
  outside; afterwards from the mesh itself. The declaration should not have to care, which
  suggests a named source rather than a path.
- **Multiple scenarios at once.** Each needs its own segments and addresses. Whether the
  declaration carries absolute addresses, as above, or a template the lab allocates from,
  decides whether two scenarios can run side by side.