feat: add ns-3 discrete-event simulation support (ndndSIM)

[markverick]

feat: add ns-3 discrete-event simulation support (ndndSIM)

Summary

This PR adds support for running NDNd inside the ns-3 network simulator as part of the ndndSIM project. It introduces a new sim/ package that bridges NDNd's Go forwarding plane with ns-3's C++ simulation engine via CGo, and makes targeted changes to the forwarding, routing, and sync layers to allow deterministic, single-threaded, discrete-event simulation.

Motivation

NDN simulation is critical for protocol research and evaluation. ndnSIM is the standard ns-3–based NDN simulator, but until now it has been tightly coupled with the C++ NFD forwarder. This patch enables ndndSIM to use NDNd as its forwarding and routing engine, allowing researchers to simulate NDNd's DV routing and SVS sync protocols at scale in a controlled, reproducible environment.

Changes

New sim/ package (~2,200 LOC)

A complete CGo bridge between NDNd (Go) and ns-3 (C++):

File	Purpose
sim/engine.go	SimEngine implements ndn.Engine, translating NDNd's async engine interface into synchronous calls suitable for discrete-event simulation. Manages face dispatch, Interest/Data handler registration, and management command execution.
sim/forwarder.go	SimForwarder wraps fw.Thread with per-node FIB, face table, and route management (AddRoute/RemoveRoute/AddRouteWithOrigin/RemoveRouteWithOrigin).
sim/face.go	SimFace represents a simulated network face backed by ns-3 NetDevices.
sim/fw_face.go	Adapter implementing the fw.Face interface for the simulation layer.
sim/node.go	SimNode ties together the engine, forwarder, DV router, and application-level callbacks per simulated node.
sim/dv.go	Simulation-aware DV router initialization and prefix announce/withdraw helpers.
sim/clock.go	Clock bridge that routes core.NowFunc to ns-3 simulation time.
sim/timer.go	Timer implementation backed by ns-3's event scheduler.
sim/runtime.go	Go runtime configuration and initialization for sim mode.
sim/cgo_export.go	//export functions callable from C++ (node creation, packet injection, face management, DV control, etc.).
sim/ndndsim_sim.h	Shared C header defining callback function pointer types for send/schedule/cancel/time/data-produced/data-received hooks.
sim/doc.go, sim/util.go, sim/cmd/main.go	Package documentation, helpers, and a build entry point.

Forwarding plane hooks (minimal, non-breaking)

File	Change
fw/core/clock.go (new)	Adds core.NowFunc (defaults to time.Now) and core.Now(). All time.Now() calls in the forwarding plane are replaced with core.Now() so simulation can inject ns-3 time.
fw/fw/thread.go	Adds SetFib()/Fib() for per-thread FIB override (defaults to global FibStrategyTable). Adds ProcessPacket() and RunMaintenance() for synchronous single-threaded packet processing. Replaces time.Now() → core.Now().
fw/fw/bestroute.go, fw/fw/multicast.go	time.Now() → core.Now().
fw/table/fib-strategy-tree.go	Adds NewFibStrategyTree() standalone constructor (does not touch global state).
fw/table/pit-cs-tree.go, fw/table/pit-cs.go, fw/table/dead-nonce-list.go	time.Now() → core.Now().
fw/table/rib.go	Adds InitRoot() for standalone RIB table initialization.

DV routing — simulation compatibility

File	Change
dv/dv/router.go	Adds injectable GoFunc, NowFunc, AfterFunc fields to decouple goroutine/timer usage. Adds Init() (non-blocking setup), RunHeartbeat(), RunDeadcheck(), Cleanup(), and Nfdc() for simulation-driven lifecycle. Replaces go f() → dv.GoFunc(f) and time.AfterFunc → dv.AfterFunc throughout.
dv/dv/advert_data.go, dv/dv/advert_sync.go, dv/dv/table_algo.go	go f() → dv.GoFunc(f), time.AfterFunc → engine.Timer().Schedule().
dv/nfdc/nfdc.go	Adds Synchronous mode for direct command execution (no goroutine queue).
dv/table/neighbor_table.go	time.Now() / time.Since() → core.Now().

SVS sync — simulation compatibility

File	Change
std/sync/svs.go	Replaces channel-based event loop with callback-driven design. Adds GoFunc, NowFunc, AfterFunc to SvSyncOpts. Replaces time.Ticker with scheduleTimer() using injectable AfterFunc. Removes recvSv channel; state vectors are now processed inline via GoFunc.
std/sync/svs_alo.go	Removes channel-based run() loop (outpipe/errpipe/publpipe/stop). Publications and errors are delivered directly via GoFunc.
std/sync/svs_alo_data.go	time.AfterFunc → s.opts.Svs.AfterFunc.

Design Principles

1. Zero behavioral change in production — All injected functions default to their standard-library equivalents (time.Now, go f(), time.AfterFunc). Existing users and tests are unaffected.
2. Minimal diff to core packages — The forwarding plane changes are limited to replacing time.Now() with core.Now() and adding opt-in per-thread FIB support. No existing APIs are removed or changed.
3. Deterministic simulation — Goroutines are replaced with GoFunc dispatch and channels are replaced with direct callbacks, enabling single-threaded, deterministic execution under ns-3's discrete-event scheduler.

Testing

These changes have been validated in the atlas-scenarios framework, which runs multi-node grid topologies (2×2 through 6×6) with DV routing and SVS sync under both ndndSIM simulation and Mini-NDN emulation. Both modes produce consistent per-packet byte sizes and convergence behavior when run with matching parameters.

[markverick]

@copilot