Quality of Service with DSCP Marking and Linux tc
I implemented QoS by marking latency-sensitive traffic with DSCP and shaping bandwidth using Linux tc with hierarchical token bucket (HTB) classes. Under simulated congestion, priority traffic held low latency while bulk transfers were capped to their allocation.
Objective & Context
Without QoS, bulk transfers starve interactive flows during congestion. This lab classifies traffic with DSCP and enforces per-class bandwidth and priority with HTB, the mechanism behind voice/video prioritization on shared links.
Environment & Prerequisites
- Linux host with tc (iproute2) and iperf3.
- A constrained test link to create congestion.
- Defined traffic classes (priority vs bulk).
flowchart TB
Root[HTB root 100mbit] --> P[Priority class
60mbit, prio 0] Root --> B[Bulk class
40mbit, prio 1] P --> Pf[DSCP EF flows] B --> Bf[Bulk flows]
60mbit, prio 0] Root --> B[Bulk class
40mbit, prio 1] P --> Pf[DSCP EF flows] B --> Bf[Bulk flows]
Step-by-Step Execution
1. Build the HTB hierarchy [PRIVILEGED]
tc qdisc add dev eth0 root handle 1: htb default 30 && tc class add dev eth0 parent 1: classid 1:1 htb rate 100mbit2. Add priority and bulk classes
tc class add dev eth0 parent 1:1 classid 1:10 htb rate 60mbit prio 0 && tc class add dev eth0 parent 1:1 classid 1:30 htb rate 40mbit prio 13. Mark and verify under load with iperf3
iperf3 -c 192.168.10.30 -S 0xb8 && tc -s class show dev eth0class htb 1:10 rate 60Mbit sent 71MB (priority held)
class htb 1:30 rate 40Mbit sent 48MB (bulk capped)Validation & Testing
Generate simultaneous priority and bulk flows and measure latency/throughput per class. Pass criteria: priority class retains low latency and its rate, while bulk is shaped to its ceiling under contention.
Advanced: Troubleshooting
- No shaping effect: tc shapes egress; apply on the correct outbound interface or use IFB for ingress.
- Wrong class match: verify the filter maps DSCP/marks to the intended classid.
- Bursty behaviour: tune
burstandcburstfor HTB.
Key Results
- Held priority-class latency low under simulated link saturation.
- Capped bulk flows to a 40mbit ceiling during contention.
- Classified traffic by DSCP into 2 enforced HTB classes.
- Verified per-class throughput with tc statistics under iperf3 load.