JANUS

JANUS — flow-matching unsupervised network anomaly detection over packet sequences.

JANUS is a packet-causal Transformer with two output heads on a shared backbone:

• Continuous Flow Matching head over the (size, IAT, win) packet channels.
• Discrete Flow Matching head over the 6 binary protocol-flag / direction channels.

Trained jointly on benign traffic only (no attack labels at any stage). The deployable scalar score is a Mahalanobis-OAS distance over a 10-d per-flow score vector emitted by the trained model, with the aggregator fit on benign val only — entirely unsupervised end-to-end.

JANUS is the first NIDS method to use Flow Matching as the training paradigm in mixed continuous–discrete state spaces over packet sequences.

Headline results

3-seed mean ± std AUROC. Selection-bias-free Mahalanobis-OAS aggregator on the 10-d JANUS score vector, fit on benign val only.

Within-dataset comparison (AUROC %, mean ± std)

Method	Venue	CIC-IDS2017	CIC-DDoS2019	CIC-IoT2023	ISCXTor2016
Isolation Forest	classical	55.27 ± 0.4 †	—	—	—
OCSVM	classical	59.59 ± 0.6 †	—	—	—
AnoFormer	ICLR'22	63.37 ± 0.7 †	—	—	—
GANomaly	BMVC'18	82.75 ± 5.6 †	—	—	—
RD4AD	CVPR'22	83.78 ± 0.8 †	—	—	—
TSLANet	ICML'24	84.45 ± 1.7 †	—	—	—
ARCADE	—	84.85 ± 2.0 †	—	—	—
MFAD	—	86.02 ± 0.8 †	—	—	—
STFPM	BMVC'21	86.29 ± 1.7 †	—	—	—
MMR	—	89.26 ± 1.2 †	—	—	—
Shafir NF + Shapley	arXiv'26	93.03 ‡	93.00 ‡	72.24 ± 6.08 ★	87.31 ‡
ConMD	TIFS'26	94.43 ± 0.1 †	—	—	—
JANUS (ours)	—	98.26 ± 0.35	99.18 ± 0.05	95.90 ± 0.22	99.09 ± 0.13

† Numbers from ConMD (TIFS'26) Table I; protocol = train 10 K benign / test 5 K + 5 K balanced; 5-seed mean ± std. ‡ Numbers from Shafir et al. (arXiv'26) headline tables; protocol = train 10 K benign / SHAP-selected feature subsets per dataset (single NF). ★ Reproduced by us (3-seed mean ± std, 2-NF ensemble, CSV pipeline, paper-specified 5-feat SHAP subset). Shafir's paper does not publish an AUROC for CIC-IoT2023 — only F1 = 99.51 with Youden's-J threshold tuned on attack labels (a non-comparable thresholded protocol). For threshold-free head-to-head AUROC on this dataset we cite our reproduction.

JANUS is fully unsupervised (benign-only training, no attack labels at any stage) and uses the Mahalanobis-OAS aggregator over its 10-d raw score vector with parameters fit on benign val only.

Thresholded F1 metrics for JANUS across all four datasets are in RESULTS.md Section D.

3×3 cross-dataset transfer matrix

Source (rows) trained on 10K benign of source dataset; target (columns) tested on full target benign + all target attacks. Aggregator fit on target benign val only — no attack labels at any stage. Diagonal italic = within-dataset.

Source ↓ / Target →	CICIDS17	CICDDoS19	CICIoT23
CICIDS17	0.9826 ± 0.0035	0.9690 ± 0.0047	0.8698 ± 0.0031
CICDDoS19	0.9413 ± 0.0212	0.9918 ± 0.0005	0.8767 ± 0.0068
CICIoT23	0.9394 ± 0.0063	0.9030 ± 0.0075	0.9590 ± 0.0022

Ablations (architecture & aggregator)

Two orthogonal ablation axes, each evaluated within-dataset (4 datasets × 3 seeds) and cross-dataset (3×3 transfer × 3 seeds):

• Group A — 7 alternative aggregators on the same JANUS-full sub-score vector (post-processing only; no retraining).
• Group B — 5 architecture variants, each retrained 4 datasets × 3 seeds = 60 runs + 90 cross-evals.

Every load-bearing JANUS design choice has the same shape of ablation curve: small in-distribution cost, large cross-dataset gain.

Component (removed in ablation)	Variant	Within Δ	Cross-mean Δ	Cross-worst Δ
FLOW token (global context)	B1	−0.94	−6.70	−19.97
Packet sequence	B2	+0.15	−23.82	−36.27
Cont/disc head split (drop disc head)	B3	+0.44	−13.14	−25.03
CFM head (drop continuous side)	B4	−2.37	−2.03	−2.86
Joint training of two heads	B5	+0.20	−18.93	−27.54
OAS Mahalanobis aggregator	A1 vs A5	+0.37	−15.88	−27.38

Three ablations (B3 / B5 / A-aggregator) marginally beat JANUS-full at within-dataset evaluation but collapse on at least one cross-dataset transfer direction. The disc head, joint training, and OAS aggregator are deliberate trades: their value is exclusively in cross-dataset robustness.

Full headline summary: artifacts/ablation/ABLATION_SUMMARY.md. Per-variant 3×3 cross matrices: artifacts/ablation/ABLATION_CROSS_B_full.md and artifacts/ablation/ABLATION_TABLE_CROSS_full.md.

Quick start

# Train JANUS on CICIDS2017 (3 seeds available: 42, 43, 44)
cd Mixed_CFM
uv run --no-sync python train.py --config configs/cicids2017_seed42.yaml

# Phase-1 evaluation: per-attack-class AUROC + 10-d score export
uv run --no-sync python eval_phase1.py \
    --model-dir <model_dir> --out-dir <eval_dir>

# Single cross-dataset eval
uv run --no-sync python eval_cross.py \
    --model-dir <src_model_dir> \
    --target-store datasets/<tgt>/processed/full_store \
    --target-flows datasets/<tgt>/processed/flows.parquet \
    --target-flow-features datasets/<tgt>/processed/flow_features.parquet \
    --benign-label normal --n-benign 10000 --n-attack 1000000 \
    --out <result.json>

# 3×3 cross matrix (6 off-diagonal directions × 3 seeds, 2-GPU parallel)
bash ../scripts/aggregate/run_cross_3x3.sh
uv run --no-sync python ../scripts/aggregate/cross_3x3_table.py

JANUS hyper-parameters (locked in Mixed_CFM/configs/<dataset>_seed*.yaml):

T: 64                  # max packet sequence length
d_model: 128
n_layers: 4
n_heads: 4
sigma: 0.1             # within-dataset; cross uses 0.6
lambda_disc: 1.0
use_ot: true           # OT-CFM (Sinkhorn coupling on benign batch)
reference_mode: causal_packets    # Route A: packet-causal attention

Producing the deployable scalar score

eval_phase1.py exports a 10-d per-flow score vector to phase1_scores.npz:

3 continuous-side scores  : terminal_norm, terminal_flow, terminal_packet
7 discrete-side scores    : disc_nll_total + disc_nll_ch{2,3,4,5,6,7}
                            (direction + 5 TCP flags)

The deployable scalar is the Mahalanobis-OAS distance:

d²(s) = (s − μ)ᵀ Σ⁻¹ (s − μ),    where (μ, Σ) come from sklearn.covariance.OAS
                                 fit on benign val ONLY (no attack labels).

Reference implementation: scripts/aggregate/aggregate_score_router.py. It reads artifacts/route_comparison/janus_<ds>_seed*/phase1_scores.npz and artifacts/route_comparison/cross/janus_seed*_<src>_to_<tgt>.npz, then writes artifacts/route_comparison/SCORE_ROUTER.md (within-dataset rows) and artifacts/route_comparison/CROSS_MATRIX_3x3.md (cross matrix, via cross_3x3_table.py).

Tests

uv run --no-sync python -m pytest tests/ Mixed_CFM/tests/ Unified_CFM/tests/

Adding a new dataset

Write one driver at scripts/extract_<name>.py that calls extract_lib.extract_dataset(...) (see scripts/extract_cicids2017.py as the reference template). The driver hardcodes CSV column names, timestamp formats, benign aliases, and drop patterns as module constants, then feeds extract_lib a per-day (canonical_key → [(row_idx, ts_epoch)]) mapping and a per-day pcap file map. The extract pipeline writes all three artifacts (packets.npz, flows.parquet, flow_features.parquet) row-aligned by flow_id = arange(N).

To upgrade an existing artifact pair that lacks flow_features.parquet, run scripts/generate_flow_features.py --packets-npz ... --flows-parquet ... --out ... (or --source-store for sharded stores).

Common gotcha: if CSV timestamps and pcap epochs are in different time zones, extract_lib prints a diagnostic with the recommended --time-offset; rerun with that value.