hq/03-DESIGN/01-to-be/02-scenario-declaration.md

---
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)).

## What NAT does, and why the design turns on it

A household or office has **one** address the outside world can see, and **many** machines
behind it. Network address translation is what reconciles those.

When a machine inside dials out, the gateway rewrites the packet's source from the private
address to the public one, **remembers the mapping**, and rewrites the replies on the way back.
Four consequences follow, and every one of them shapes this design:

1. **Outbound works; inbound does not.** A mapping exists only because something inside started
   a conversation. Nothing outside can start one — there is no mapping to look up, and no way
   to know which internal machine was meant.
2. **A forwarded port is a permanent mapping made by hand**, in the inbound direction:
   *anything arriving at the public address on 443 goes to this machine.* That is the only way
   a machine behind NAT becomes reachable, and it requires control of the gateway.
3. **Mappings expire.** A gateway forgets one that goes unused. This is why anything holding a
   connection through NAT sends keepalives, and why a mesh that does not is fine until it is
   idle.
4. **From outside, every machine behind the gateway looks like one address.** Identity and
   address stop corresponding.

This is why the mesh dials outward and never inward
([ADR 0001](../../02-DECISIONS/0001-nodes-communicate-over-a-broker.md)), why a hub exists at
all, and why a node's endpoint is something a peer **learns** from arriving packets rather than
something anyone configures.

**Carrier-grade NAT** is the same mechanism applied by an ISP: your own gateway gets a private
address too, and the public one is shared with strangers. Nothing can be forwarded, because the
rule would have to live on equipment you do not own. Common on mobile connections and
increasingly on fixed ones.

## Three positions a machine can be in

The underlay's whole job is to reproduce **where a machine sits relative to the internet**,
because that is what the mesh has to cope with and what only production currently exercises.
There are three positions, and they are genuinely different:

| Position | Reachable from outside | Apparent address | Example |
|---|---|---|---|
| **Attached** | yes, at its own address | its own | a hosted server |
| **Behind a forwardable gateway** | only through a forwarded port, at the *gateway's* address | the gateway's | a machine at home |
| **Behind an unforwardable gateway** | **no** | someone else's, and it changes | a laptop on a café network; anything behind carrier-grade NAT |

The axis is **forwardability, not ownership** — which is worth stating because the obvious
framing gets it wrong. Carrier-grade NAT is *your* connection and is still unforwardable, so it
belongs in the third row alongside the café. What the mesh has to cope with is whether an
inbound mapping can be made, not who owns the equipment.

The third position is the hard one. A machine there can dial out and nothing more: it cannot be
published, its apparent address belongs to a router it does not control, and that address
changes when it moves. Every assumption a mesh makes about reachability breaks there first.

A declaration has to be able to say all three, and to move a machine between them.

## The shape

```yaml
scenario: roaming-and-published

segments:
  internet:
    kind: public                  # stands in for the internet — RFC 5737 addresses
    cidr: 203.0.113.0/24
  home:
    kind: private
    cidr: 192.168.1.0/24
    gateway:
      to: internet
      address: 203.0.113.50       # what the world sees this network as
      nat: true
      forwardable: true           # we control it, so ports can be opened
  elsewhere:                      # a network we do not control
    kind: private
    cidr: 198.51.100.0/24
    gateway:
      to: internet
      address: 203.0.113.80
      nat: true
      forwardable: false          # café wifi, or carrier-grade NAT

machines:
  anchor:
    at: { segment: internet, address: 203.0.113.10 }

  home-server:
    at: { segment: home, address: 192.168.1.135 }
    published:
      - { port: 443, on: home }   # DNAT: 203.0.113.50:443 → 192.168.1.135:443

  workstation:
    at: { segment: home, address: 192.168.1.250 }

  laptop:
    at: { segment: home, address: 192.168.1.98 }

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

snapshot: raised
```

## What each part means, precisely

**`segments`** — a broadcast domain with an address range, and a `kind:`.

`kind: public` marks the segment that stands in for the internet. `kind: private` is everything
else. This is stated rather than inferred, and the earlier version inferred it — *a segment with
no gateway is the internet* — which made an **isolated network inexpressible**: a LAN with no
route out is a private segment with no gateway, and would have been read as the internet and
forced to use documentation addresses. A mesh spanning a site with no internet access is a real
topology, and the model has to be able to say it.

**`gateway:`** — how a segment reaches its parent, and this is where the previous version was
too thin. It carries three facts, and all three are load-bearing:

- `to:` — the parent segment.
- `address:` — **the address the outside world sees this network as.** For a household
  connection this is the public address the ISP hands out. It is not decoration: it is what a
  peer records as the endpoint when a machine here dials out, and what a public name for a
  published machine here resolves to.
- `nat:` — whether addresses are translated. `true` gives the ordinary household case: many
  private machines behind one public address. `false` describes a routed range, where machines
  keep their own addresses and the gateway only forwards.
- `forwardable:` — whether an inbound mapping can be created. Independent of `nat:`, and the
  field that separates a home gateway from carrier-grade NAT. Publishing through a gateway with
  `forwardable: false` is a declaration error, because that is exactly the constraint being
  reproduced.

The lab materialises a machine to be the gateway. That is the one implicit machine in an
otherwise explicit declaration, and it exists because NAT has to run somewhere.

**`machines[].at`** — segment and address. That pair alone determines which of the three
positions a machine is in: on a gateway-less segment it is directly attached; on a segment with
a gateway it is behind one.

**`machines[].published`** — a destination-NAT rule on a named gateway, stated as an outcome
rather than a port list. `{ port: 443, on: home }` means the `home` gateway forwards its own
`203.0.113.50:443` to this machine's `443`. The resulting public endpoint is derivable, which is
the point: a scenario never writes an endpoint down, and the mesh has to discover it.

A machine may be published on **any gateway between it and the internet** — which is how *"our
LAN also has a public IP"* is expressed, and why `on:` names the gateway rather than being
implied. It cannot be published at all on a gateway the scenario models as foreign; attempting
it is a declaration error, because that is precisely the constraint being reproduced.

**`at: detached`** — on no segment. A machine that exists and can reach nothing.

## Moving a machine is a lifecycle operation

`at:` states where a machine *starts*. Moving it is something a run does:

```
move laptop → { segment: elsewhere, address: 198.51.100.23 }
move laptop → detached
move laptop → { segment: home, address: 192.168.1.98 }
```

This is the roaming case made testable, and it is the one that finds the interesting faults.
The same machine, the same identity, three positions in one run: at home where its peers can
reach it directly, on a foreign network where it can only dial out and its apparent address
belongs to a router it does not control, and asleep.

Whether the overlay survives that, re-forms, and is noticed to have changed endpoint is
**observed**, never arranged
([ADR 0031](../../02-DECISIONS/0031-the-lab-provides-the-underlay.md)).

## Why the addresses are load-bearing

The internet 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 a would-be 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.

RFC 5737 reserves three ranges, which is exactly enough for the topology above:

| Range | Used for |
|---|---|
| `203.0.113.0/24` | the internet segment itself — directly attached machines, and gateway addresses |
| `198.51.100.0/24` | a foreign network, so a roaming machine's apparent address is plainly not ours |
| `192.0.2.0/24` | spare — a second foreign network, or a second site |

Private segments use RFC 1918 and can be **byte-identical to production**, because those
addresses mean the same thing everywhere. Only the public side is substituted, and only because
it must be.

The format should make getting this wrong hard rather than merely documented: a segment without
a `gateway:` is a public segment, and an address in it — including a gateway's `address:` — 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.

## Is this general? — the axes a setup can vary along

The question that matters is not *does this cover our mesh*, but **can it express any mesh**.
Audited against the axes a real deployment varies along, the answer is *most, deliberately not
all, and three genuine gaps*.

The standard applied is not "every property a network has". It is **every property that changes
how the mesh behaves**. Bandwidth does not change correctness; MTU does.

| Axis | Values | Expressible | |
|---|---|---|---|
| **Reachability** | attached · forwardable gateway · unforwardable gateway · isolated | yes | the core of the model |
| **Address stability** | static · dynamic · changes mid-run | **partly** | a machine can be *moved*, but an address that changes under it cannot be stated |
| **Gateway depth** | direct · one gateway · nested gateways | **partly** | `to:` chains, so nesting exists; `published:` names one gateway, so forwarding through two does not |
| **Address family** | IPv4 · IPv6 · dual-stack | **no** | `cidr:` is implicitly v4. A v6-only node is a real topology and cannot be written |
| **Interfaces per machine** | one · several | **no** | `at:` is singular. A multi-homed node — on a LAN and a WAN at once — is inexpressible |
| **Path properties** | MTU · latency · loss | **no** | MTU matters: tunnels fragment, and a lower-MTU path is a classic silent failure |
| **Reachability policy** | symmetric · asymmetric | **no** | a firewall dropping inbound while outbound works is different from NAT and behaves differently |
| **Gateway state** | permanent · expiring mappings | **no** | mappings time out; whether keepalives work is untestable without it |
| **Overlapping ranges** | distinct · two sites both on `192.168.1.0/24` | yes | two segments may carry the same range — common, and it breaks routing |
| **Segment count** | one · many · isolated island | yes | after the `kind:` fix above |

### What this says

**Three gaps are real and should be closed**, in this order:

1. **Address family.** A v6-only or dual-stack node is not exotic, and a mesh that assumes v4
   fails there completely rather than partially. This is the largest gap.
2. **Expiring NAT mappings.** Without it, keepalive behaviour is hoped for rather than tested —
   and for a mesh where most nodes sit behind NAT, that is the failure mode most likely to
   appear only after everything has been idle overnight.
3. **MTU.** Tunnels fragment. A path with a smaller MTU produces a connection that establishes
   and then silently drops large packets, which is exactly the shape of fault this whole effort
   exists to stop shipping.

**Two are deliberately out of scope** unless something argues otherwise: latency and loss.
They change performance, not correctness, and a scenario that models them is a network
simulator rather than a fixture.

**Two are partial and probably fine for now:** nested forwarding and mid-run address change.
Both are expressible with small extensions when something needs them, and neither blocks the
bootstrap scenario.

### The honest summary

The model covers **where a machine sits**, which is what the mesh's reachability logic turns
on, and it now covers it completely. It does not yet cover **what the path between machines is
like**, and one of those — address family — is not a refinement but a second world the mesh
would have to work in.

None of this blocks phase 0. A bootstrap scenario is one machine and a pinned bundle, and needs
none of it. But the gaps should be closed before the lab is trusted to say a mesh *works*,
because today it could only say it works over IPv4, on an unconstrained path, against gateways
that never forget.

## 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 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. This is the largest gap.
- **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, and the shape above
  writes addresses absolutely. Whether a scenario carries literal addresses or a template the
  lab allocates from decides whether two can run side by side — and there are only three
  documentation ranges to go round.
- **The three gaps from the audit above** — address family, expiring NAT mappings, MTU — in
  that order. The first is the one that is a second world rather than a refinement.