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

layer, status, code, updated, decisions

layer	status	code	updated	decisions
to-be	in-progress	mesh-lab	2026-08-25	02-DECISIONS/0009-the-lab.md 02-DECISIONS/0009-the-lab.md 02-DECISIONS/0009-the-lab.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 0009).

Public networks are unrelated, and routed rather than bridged

The internet is not a network. It is a very large number of unrelated networks that route to each other, and a machine in one is many hops from a machine in another with no shared broadcast domain between them.

So a scenario does not have an internet segment. It has one public segment per public network, each with its own unrelated prefix, and the lab wires them together through a router, never onto a shared bridge.

That distinction is load-bearing, and putting several public addresses in one prefix would quietly make four things true that are false in reality:

If public addresses share a segment	Reality
machines resolve each other by ARP and talk directly	they are routed, many hops apart
TTL never decrements	every hop decrements it
broadcast and multicast reach across	neither crosses a router
any two are adjacent	adjacency is the exception, not the rule

The third is not hypothetical here. The mesh has already been bitten by multicast name resolution — 00-as-is/01-mesh-and-transport.md records that mesh names are deliberately not multicast names, after a delay and a one-node-only failure mode. A lab where "the internet" is one broadcast domain would let a node discover a peer by multicast that it could never discover in production, and report success.

The rule: kind: public segments are routed to one another and never bridged. It is a property of how the lab wires them, not a field anyone sets, because there is no correct scenario in which two public networks are adjacent.

Which addresses to use

RFC 5737 reserves three ranges and RFC 3849 reserves one v6 prefix. Each public network takes its own, and they are chosen to look nothing like each other — because in reality they would not:

Public network	v4	v6
a hosting provider	192.0.2.0/24	2001:db8:a::/48
a household ISP	198.51.100.0/24	2001:db8:b::/48
a mobile or foreign network	203.0.113.0/24	2001:db8:c::/48

Three is enough for the topologies that matter, and a fourth public network can subnet one of them — an ISP handing out 198.51.100.0/25 and 198.51.100.128/25 to two customers is exactly what really happens.

Private segments still use RFC 1918 and stay byte-identical to production.

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), 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.

Reachability is per address family, not per machine

Adding IPv6 is not a field. It changes the position model, and the reason is worth stating before the syntax.

IPv6 usually has no NAT. A machine behind a household gateway can hold a globally routable v6 address while its v4 address is private and unforwardable. The same machine, at the same moment, is in two different positions at once:

	IPv4	IPv6
a typical machine at home	behind an unforwardable-or-forwardable gateway	attached, directly reachable
a machine on mobile data	behind carrier-grade NAT	often attached, sometimes absent entirely
a machine on an older network	attached or behind NAT	no address at all

So the three positions apply per family, and a machine's reachability is a property of (machine, family) rather than of the machine. A mesh that treats reachability as one fact per node will reach a peer over one family, fail over the other, and report whichever it tried.

That has a direct consequence for what the lab is for: "can these two nodes reach each other" stops being a yes/no question. It is asked once per family, and the interesting answers are the asymmetric ones.

The v6 documentation prefix is 2001:db8::/32 (RFC 3849) — the exact counterpart of the RFC 5737 rule, and load-bearing for the same reason.

The shape

scenario: roaming-and-published

segments:
  hosting:                        # one public network
    kind: public
    cidr: [192.0.2.0/24, 2001:db8:a::/48]

  isp-home:                       # another, unrelated — routed to it, not bridged
    kind: public
    cidr: [198.51.100.0/24, 2001:db8:b::/48]

  home:
    kind: private
    cidr: [192.168.1.0/24, 2001:db8:b:1::/64]
    mtu: 1500
    gateway:
      to: isp-home
      address: [198.51.100.7, 2001:db8:b::7]   # what the world sees this network as
      nat: [v4]                   # v4 is translated; v6 is routed, not translated
      forwardable: true
      mapping_ttl: 120s           # an unused inbound mapping is forgotten after this

  cafe:                           # a network we do not control
    kind: private
    cidr: [10.50.0.0/16]          # v4 only — no v6 offered here at all
    mtu: 1400                     # a tunnelled path, smaller than standard
    gateway:
      to: hosting                 # it reaches the world via a different public network
      address: [192.0.2.200]
      nat: [v4]
      forwardable: false          # café wifi, or carrier-grade NAT
      mapping_ttl: 30s            # aggressive, as carrier NAT tends to be

machines:
  anchor:
    at: { segment: hosting, address: [192.0.2.10, 2001:db8:a::10] }

  home-server:
    at: { segment: home, address: [192.168.1.135, 2001:db8:b:1::135] }
    published:
      - { port: 443, on: home }   # v4 only: 198.51.100.7:443 → 192.168.1.135:443
    inbound: allow                # v6 is routable here, so this decides whether it is reachable

  workstation:
    at: { segment: home, address: [192.168.1.250, 2001:db8:b:1::250] }
    inbound: deny                 # a host firewall: dials out, accepts nothing

  laptop:
    at: { segment: home, address: [192.168.1.98, 2001:db8:b:1::98] }

  border:                         # a machine on two segments at once
    at:
      - { segment: home,    address: [192.168.1.2] }
      - { segment: isp-home, address: [198.51.100.60] }

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

snapshot: raised

What each part means, precisely

segments — a broadcast domain with an address range, and a kind:. A segment is a single broadcast domain, which is precisely why several public networks cannot be one segment.

kind: public marks a segment that stands in for a public network — and there is normally more than one, unrelated to each other. 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 public 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: — which families are translated, as a list. [v4] is the ordinary modern case: v4 translated, v6 routed. [v4, v6] describes a gateway that translates both, which exists and is worth being able to reproduce. [] is 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.
• mapping_ttl: — how long an unused inbound mapping survives. This is what makes keepalive behaviour testable: a mesh that holds a connection through NAT without refreshing it works perfectly until the far side goes quiet for longer than this. Aggressive values reproduce carrier NAT; omitting it means mappings never expire, which no real gateway does.

segments[].mtu — the largest packet the segment carries, defaulting to 1500. Lower values reproduce tunnelled and PPPoE paths. This matters because an overlay adds its own header: a tunnel over a 1400-byte path establishes a connection and then silently drops large packets, which is the shape of fault this whole effort exists to stop shipping.

machines[].inbound — allow or deny, a host firewall. Distinct from NAT and behaves differently: a machine can be perfectly routable and still refuse everything unsolicited, which is the normal state of a v6-addressed machine. Without this, v6 addressing would imply reachability, and it does not.

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.

It is a container, not a virtual machine — a router is scenery rather than something under test, so the fidelity argument that makes a node a virtual machine does not reach it (ADR 0009). What a router must reproduce is kernel behaviour, and a container has the same kernel.

machines[].at — segment and addresses, or a list of them for a machine on several segments at once. Multi-homing is not exotic: it is what a border machine is, and what any node with both a LAN and a WAN interface is. Each entry carries the addresses that machine holds on that segment, one per family.

Position follows from the pair, per family: on a kind: public segment a machine is attached; on a private one it is behind that segment's gateway, unless the gateway does not translate that family — in which case it is attached on that family and behind a gateway on the other.

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 a public network — 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 0009).

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.

Which range goes where is covered above, under public networks are unrelated: one range per public network, chosen to look nothing like each other.

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 0023 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 0009). Everything about the underlay is identical, which is what makes one a strict subset of the other rather than a fork.

# 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 0009).
• 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.

What the model deliberately does not contain

A real network is full of equipment: a modem, a router, switches, access points, controllers. Almost none of it appears here, and the omission is deliberate rather than an oversight.

The test is whether a device changes what an IP packet can do. If two machines can exchange packets, at the same addresses, with the same reachability and the same MTU, whether or not the device exists — then the device is invisible to the mesh, and modelling it would add a fixture with no fault to catch.

Equipment	Modelled?	Why
Switch	no	Moves frames within a segment. Two machines on a switch are two machines on a segment.
Access point	no	Bridges wireless clients onto a segment. A machine on wifi and a machine on cable are the same machine to IP.
Network controller	no	Configures equipment. Its effects appear as segments and policy; it has no packets of its own.
Cabling, PoE, uplink speed	no	Change performance and availability, not reachability.
Router / security gateway	yes — it is the gateway	Translation, forwarding and inter-segment policy all live here.
ISP modem	only in router mode	In bridge mode it is a media converter and invisible. Doing its own NAT, it is a second gateway — and that is double NAT.
VLANs	yes — they are segments	Machines on separate VLANs cannot reach each other except through the router, which is the definition of a separate segment.
Inter-VLAN firewall rules	yes — see below	A rule stopping one segment reaching another is a reachability fact, and the mesh will hit it.

The two entries worth dwelling on are the ones where a common household setup produces something the mesh has to survive.

A modem in router mode gives you double NAT. Your gateway holds a private address from the modem, which holds the public one. Forwarding then requires a rule on both, and one of them may not be configurable. This is expressible as nested segments — to: chains — but publishing through two gateways is not, and remains open.

Segmented networks are the common case, not the exotic one. A router with separate networks for trusted machines, guests and devices is ordinary, and the rules between them are ordinary too. A mesh node on one segment and a mesh node on another are, as far as reachability goes, on different networks that happen to share a gateway.

Segments may share a gateway

Several segments can name the same parent and the same external address. That is one router with several networks behind it, which is what a VLAN-capable gateway is:

segments:
  trusted:
    kind: private
    cidr: [192.168.1.0/24]
    gateway: { to: isp-home, address: [198.51.100.7], nat: [v4], forwardable: true }
  devices:
    kind: private
    cidr: [192.168.30.0/24]
    gateway: { to: isp-home, address: [198.51.100.7], nat: [v4], forwardable: true }

Identical gateway declarations mean one gateway machine, not two. The lab materialises a single router serving both, because that is what the topology being reproduced is — and two routers sharing one address would not work anyway.

Policy between segments

Sharing a gateway does not mean segments can reach each other. What they may do is stated separately, because it is a fact about a pair rather than a property of either:

policy:
  - { from: devices, to: trusted, allow: false }   # devices may not initiate to trusted
  - { from: trusted, to: devices, allow: true }    # the reverse is fine

Default is true between segments behind the same gateway, matching a router with no rules configured. Asymmetry is the normal case and the reason this cannot be a single flag: the useful configuration is almost always one-directional.

This is not the same as machines[].inbound, and conflating them loses a real distinction:

	Enforced by	Blocks
policy	the gateway, between segments	everything crossing, regardless of what the destination thinks
inbound	the machine itself	unsolicited traffic that already reached it

A mesh node behind a policy deny cannot be reached even by a peer that knows exactly where it is — and that is a real topology, not a contrived one.

Worked example — a whole mesh of the ordinary kind

The shape research 004 identified: one machine with a routable address, one publicly named but behind a household connection, one stationary machine on that network, one that roams. Written out completely, with every field the model has.

scenario: the-ordinary-shape

segments:
  # ---- three unrelated public networks. Routed to each other, never bridged. ----

  hosting:                          # where the always-on machine lives
    kind: public
    cidr: [192.0.2.0/24, 2001:db8:a::/48]

  isp-home:                         # the household's uplink
    kind: public
    cidr: [198.51.100.0/24, 2001:db8:b::/48]

  isp-mobile:                       # wherever the roaming machine happens to be
    kind: public
    cidr: [203.0.113.0/24, 2001:db8:c::/48]

  # ---- private networks behind them ----

  home:                             # the household network
    kind: private
    cidr: [192.168.1.0/24, 2001:db8:b:1::/64]
    mtu: 1492                       # PPPoE on the uplink; 1500 if the line is not PPPoE
    gateway:
      to: isp-home
      address: [198.51.100.7, 2001:db8:b::7]
      nat: [v4]                     # v4 translated, v6 routed — the modern default
      forwardable: true             # the household router is ours to configure
      mapping_ttl: 120s

  devices:                          # optional: a segmented network on the same router
    kind: private
    cidr: [192.168.30.0/24]
    gateway:
      to: isp-home
      address: [198.51.100.7, 2001:db8:b::7]   # identical → the SAME gateway machine
      nat: [v4]
      forwardable: true
      mapping_ttl: 120s

  cafe:                             # a network we do not control
    kind: private
    cidr: [10.50.0.0/16]            # RFC 1918 — someone else's private range
    mtu: 1400
    gateway:
      to: isp-mobile
      address: [203.0.113.129]
      nat: [v4]
      forwardable: false            # carrier-grade NAT, or simply not ours
      mapping_ttl: 30s

policy:
  - { from: devices, to: home, allow: false }
  - { from: home, to: devices, allow: true }

machines:
  anchor:                           # routable, nothing in front of it
    at: { segment: hosting, address: [192.0.2.10, 2001:db8:a::10] }
    inbound: allow

  home-server:                      # publicly named, behind the household connection
    at: { segment: home, address: [192.168.1.135, 2001:db8:b:1::135] }
    published:
      - { port: 443, on: home }     # v4 reaches it only through the forward
    inbound: allow                  # and v6 reaches it directly, so this matters

  workstation:                      # on the household network, not published
    at: { segment: home, address: [192.168.1.250, 2001:db8:b:1::250] }
    inbound: deny

  laptop:                           # starts at home; moves during the run
    at: { segment: home, address: [192.168.1.98, 2001:db8:b:1::98] }
    inbound: deny

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

snapshot: raised

What the ISP modem is doing here

Nothing, if it is in bridge mode — which is the ordinary arrangement when the household has its own router. A bridged modem is a media converter: it changes the physical medium and leaves the packets alone, so it creates no IP-level fact and appears nowhere above.

Were it in router mode it would be a second gateway, home would sit behind it rather than behind internet directly, and publishing would need a rule on both — the case the model cannot yet express.

What is deliberately absent

Nothing here mentions overlay addresses, which node is the hub, who peers with whom, any name, or any certificate. Research 004 recorded all of those for this topology, and a scenario must not state them (ADR 0009): they are what the mesh does, and a scenario that supplied them would be certifying its own work.

The absence is the point. Given the declaration above, whether a hub is elected, whether the NATed machine's endpoint is learned, whether the roaming machine re-forms after moving — all of it is observed.

Running it

move laptop → { segment: cafe, address: [10.50.3.23] }      # it leaves the house
move laptop → detached                                      # it sleeps
move laptop → { segment: home, address: [192.168.1.98, 2001:db8:b:1::98] }

One identity, four positions, one run. The mapping_ttl: 30s on cafe means a connection held without refreshing dies while it is out — which is the point of putting a number there.

Note that the laptop's three positions are on three different public networks: at home it appears as 198.51.100.7, at the café as 203.0.113.129, and the machine it is trying to reach is on a fourth. None of them are adjacent, all of them are routed. That is the situation the mesh actually faces.

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.

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, and per family
Address family	IPv4 · IPv6 · dual-stack · neither	yes	cidr: and address: take both; nat: names which families are translated
Interfaces per machine	one · several	yes	at: takes a list
Reachability policy, host	symmetric · asymmetric	yes	inbound: — a routable machine that refuses everything
Reachability policy, network	open · segmented · asymmetric between segments	yes	policy: — inter-segment rules, as a segmented router enforces
Segments per gateway	one · several behind one router	yes	identical gateway declarations mean one gateway machine
Gateway state	permanent · expiring mappings	yes	mapping_ttl:
Path MTU	standard · reduced	yes	segments[].mtu
Overlapping ranges	distinct · two sites both on 192.168.1.0/24	yes	segments may carry the same range
Segment count	one · many · isolated island	yes	kind: distinguishes an island from a public network
Adjacency	same segment · routed · unrelated public networks	yes	public segments are routed, never bridged — so no two are adjacent unless declared so
Gateway depth	direct · one gateway · nested	partly	to: chains, so nesting exists; published: names one gateway, so forwarding through two does not
Address stability	static · dynamic · changes mid-run	partly	a machine can be moved; an address changing under it in place cannot be stated
Path quality	latency · loss · bandwidth	no, deliberately	changes performance, not correctness — modelling it makes a network simulator, not a fixture

What closing the gaps changed

Address family was not a field. It changed the position model: a machine behind a household gateway is typically unforwardable on v4 and directly attached on v6, simultaneously. So reachability is a property of (machine, family), and "can these two nodes reach each other" is no longer a yes/no question — it is asked once per family, and the asymmetric answers are the interesting ones. That distinction did not exist in the model an hour ago and would have been discovered by a mesh failing over one family while reporting the other.

inbound: became necessary because of v6. With NAT, unreachability was implied by the topology. With a globally routable v6 address, a machine is reachable unless something refuses — so refusing has to be sayable, or v6 addressing would silently imply reachability.

nat: became a list rather than a boolean for the same reason: a real gateway translates v4 and routes v6, and a boolean cannot say that.

What remains open, and whether it matters

Two partial axes, both extensible when something needs them, neither blocking: nested forwarding and an address changing in place. A machine can already be moved, which covers the roaming case; what is missing is a lease expiring underneath a machine that stays put.

One deliberate exclusion: path quality. Latency and loss change how fast the mesh is, not whether it is correct. If a timeout turns out to be load-bearing that judgement should be revisited — and it would be revisited by a real failure, which is the right trigger.

The honest summary

The model now covers where a machine sits, what the path between machines is like, and what is permitted between them — across both address families. Together those are what the mesh's reachability logic turns on.

It contains almost no equipment, by the test above: a device that does not change what an IP packet can do has no fault for a scenario to catch. What it does contain is every device that does — which turns out to be the router, and a modem only when the modem is also a router.

What it still does not model is change over time beyond moving a machine, degradation short of failure, and publishing through two gateways at once. The first two are chosen; the third is the one real absence, and it is exactly the double-NAT case.

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.
• Nested forwarding — published: names one gateway, so a machine behind two cannot be published through both.
• An address changing in place, as a DHCP lease expiring under a machine that has not moved.