Quality of Service

Advanced Networking
DSCPtraffic shapingtcqueuing disciplines

Objective

Understand QoS concepts, classify traffic, and configure basic traffic shaping on Linux using tc.

Tools & Technologies

  • DSCP
  • traffic shaping
  • tc
  • queuing disciplines

Key Commands

tc qdisc add dev eth0 root tbf rate 100mbit
tc qdisc show dev eth0
tc class add dev eth0 parent 1: classid 1:1 htb rate 50mbit

Architecture Overview

flowchart TD TRAFFIC[All Traffic] --> CLASS{Classification\nDSCP / Port / Protocol} CLASS -->|VoIP\nDSCP EF| Q1[Priority Queue\nLow latency] CLASS -->|Video\nDSCP AF41| Q2[Bandwidth Guaranteed\nQueue] CLASS -->|Data\nDSCP BE| Q3[Best Effort\nQueue] CLASS -->|Bulk/Backup\nDSCP CS1| Q4[Scavenger\nQueue] Q1 --> OUT[Egress\nInterface] Q2 --> OUT Q3 --> OUT Q4 --> OUT style CLASS fill:#1a1a2e,stroke:#00d4ff,color:#e0e0e0 style Q1 fill:#1a1a2e,stroke:#00ff88,color:#e0e0e0

Step-by-Step Process

01
QoS Concepts

QoS prioritizes traffic to ensure latency-sensitive applications (VoIP, video) get bandwidth over bulk traffic.

# DSCP marking values:
# EF (46)  - Expedited Forwarding (VoIP)
# AF41(34) - Assured Forwarding (Video)
# CS3 (24) - Call Signaling
# BE (0)   - Best Effort (default)
# CS1 (8)  - Scavenger (background)
02
Linux Traffic Control (tc)

Linux tc implements queuing disciplines (qdiscs) for traffic shaping.

# Simple rate limiter (token bucket filter)
tc qdisc add dev eth0 root tbf \
  rate 100mbit burst 10mb latency 50ms

# View current qdiscs
tc qdisc show dev eth0

# Remove
tc qdisc del dev eth0 root
03
HTB — Hierarchical Token Bucket

HTB allows bandwidth allocation with guaranteed minimums and shared maximums.

# Root qdisc
tc qdisc add dev eth0 root handle 1: htb default 30
# Parent class
tc class add dev eth0 parent 1: classid 1:1 htb rate 100mbit
# VoIP: guaranteed 10mbit
tc class add dev eth0 parent 1:1 classid 1:10 htb rate 10mbit ceil 100mbit prio 1
# Data: best effort
tc class add dev eth0 parent 1:1 classid 1:30 htb rate 10mbit ceil 100mbit prio 3

Challenges & Solutions

  • tc configuration is not persistent — use /etc/network/interfaces or systemd unit
  • HTB configuration is complex — start with simple tbf for basic rate limiting

Key Takeaways

  • Mark traffic at ingress — remarking at egress is less effective
  • VoIP needs low latency (< 150ms) AND low jitter (< 30ms) — prioritize both