In Quality of Service (QoS), both Policing and Shaping are techniques used to manage network traffic, but they have different purposes and behaviors:
1. Traffic Policing:
- Purpose: Traffic policing is used to enforce traffic limits by discarding or marking packets that exceed the allocated bandwidth.
- How it works:
- It monitors the data rate of traffic and drops or re-marks packets that exceed the configured limit.
- If the traffic exceeds the set rate, it does not queue the excess traffic; instead, it discards (drops) or re-marks it immediately.
- Burst handling: Traffic bursts beyond the limit are typically dropped without being buffered, which can cause some packet loss.
- Common Use Cases:
- Applied in environments where traffic exceeding a certain limit must be strictly controlled, like in service provider networks or when enforcing Service Level Agreements (SLAs).
- It’s used for incoming traffic (ingress) or outgoing traffic (egress) where strict bandwidth constraints are necessary.
- Effect on traffic: Can lead to packet loss or degraded performance if traffic exceeds the limit.
- Analogy: Policing is like a traffic cop who stops cars that are speeding, not allowing them to continue.
- Example: A traffic policer might drop packets if traffic exceeds 10 Mbps in a 10 Mbps connection.
2. Traffic Shaping:
- Purpose: Traffic shaping is used to smooth out bursts of traffic and regulate traffic flow so it adheres to a specified rate by delaying (buffering) packets instead of dropping them.
- How it works:
- It buffers excess traffic that exceeds the configured limit, storing it in a queue, and sends it out later when there is available bandwidth.
- By delaying packets, shaping can smooth out traffic bursts over time, ensuring traffic conforms to the rate but without packet loss.
- Burst handling: Bursts of traffic are buffered and sent when bandwidth becomes available, avoiding packet loss as much as possible.
- Common Use Cases:
- Often used on outgoing traffic (egress) to ensure that the flow of traffic does not overwhelm a link, for example, when connecting to slower-speed networks.
- Used for smoothing traffic flow, especially in situations where you don’t want to drop traffic but rather delay it (e.g., in VoIP or video conferencing).
- Effect on traffic: Traffic is delayed (queued) instead of being dropped, leading to more consistent delivery without packet loss, but at the expense of increased latency.
- Analogy: Shaping is like a traffic light at a metered on-ramp, controlling the rate at which cars enter the highway to prevent congestion, but not stopping them permanently.
- Example: Traffic shaping might buffer excess packets if traffic exceeds 10 Mbps on a 10 Mbps connection, but sends them later without dropping.
Key Differences:
| Feature | Policing | Shaping |
|---|---|---|
| Action | Discards or re-marks excess traffic | Buffers excess traffic to send later |
| Packet handling | Drops traffic immediately if it exceeds the rate | Delays traffic to smooth out bursts |
| Traffic type | Typically used for incoming (ingress) traffic | Typically used for outgoing (egress) traffic |
| Burst tolerance | Less tolerant (bursts are dropped) | More tolerant (bursts are buffered and sent later) |
| Latency | Low latency but potential packet loss | Higher latency but less packet loss |
| Common use case | Enforcing SLAs or service limits | Smoothing traffic before sending over a link |
| Impact on network | Can degrade performance by dropping packets | Smooths traffic, ensuring consistent flow but may increase delay |
Summary:
- Policing: Controls traffic by discarding or re-marking traffic that exceeds the limit, leading to potential packet loss.
- Shaping: Controls traffic by buffering and delaying traffic, smoothing out bursts, but adding potential latency without losing packets.
Both are critical in managing bandwidth in different contexts, with shaping often used to ensure smoother traffic, while policing is used to enforce strict limits.
