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Per-Subscriber QoS with CAKE

The gold standard in broadband traffic shaping — applied per subscriber, with accurate overhead compensation.

Why CAKE?​

CAKE (Common Applications Kept Enhanced) is a modern queuing discipline built specifically for broadband. Compared to traditional tc-based shaping (HTB + fq_codel), CAKE provides:

  • Per-flow fairness — A single subscriber's torrent can't starve their VoIP or video
  • Accurate overhead compensation — Correctly accounts for PPPoE, VLAN, and Ethernet framing at the bottleneck link
  • Active Queue Management — Keeps latency low even at full line rate (bufferbloat elimination)
  • Diffserv awareness — Automatically prioritises voice (EF), video (AF41), and interactive traffic
  • Simplicity — One qdisc does the job of HTB + fq_codel + police + classify

No other BNG ships with CAKE-based QoS out of the box.

How It Works​

AthenaBNG's abng-qos daemon manages per-subscriber QoS:

Egress (Download)​

CAKE qdisc applied directly to the subscriber's PPP or VLAN interface:

subscriber traffic → CAKE qdisc → subscriber interface → wire

Ingress (Upload)​

CAKE applied via an IFB (Intermediate Functional Block) device:

wire → subscriber interface → redirect to IFB → CAKE qdisc → forwarding

One IFB device is created per subscriber interface, named ifb-{interface}.

Overhead Compensation​

CAKE's killer feature for ISPs is accurate link-layer overhead compensation. AthenaBNG automatically calculates the correct overhead based on the subscriber's encapsulation:

EncapsulationOverheadMPU
PPPoE over Ethernet34 bytes64
IPoE over Ethernet22 bytes64
PPPoE + 1 VLAN tag38 bytes64
IPoE + QinQ (2 tags)30 bytes64

This means when you shape a subscriber to 50 Mbps, they actually get 50 Mbps of usable throughput — not 50 Mbps minus overhead.

RADIUS-Driven Rates​

Subscriber rates come from RADIUS:

Athena-Rate-Down = 50000000    # 50 Mbps download
Athena-Rate-Up = 20000000      # 20 Mbps upload

Mid-Session Rate Changes (CoA)​

When RADIUS sends a Change of Authorization with new rates, abng-qos updates the CAKE qdisc in place — no packet loss, no session interruption:

tc qdisc change dev ppp0 root cake bandwidth 100mbit ...

The subscriber's speed changes in milliseconds.

Configuration​

qos:
  enabled: true
  default_down: "50mbit"
  default_up: "20mbit"
  cake_options:
    rtt: "20ms"
    ack_filter: true
    split_gst: true
    diffserv4: true
    nat: true

Real-World Impact​

With CAKE QoS, your subscribers experience:

  • Lower latency — Bufferbloat eliminated, gaming and VoIP work great even under load
  • Fair sharing — One device's download doesn't ruin the experience for others on the same connection
  • Accurate speeds — What you sell is what they get, regardless of encapsulation overhead
  • Priority traffic — Voice and video automatically get preferential treatment