Understanding the Role of Sniffers in QoS Optimization
While tools like tcpdump, Wireshark, or ngrep (commonly referred to as “sniffers”) are primarily designed for capturing and analyzing network traffic, they play a critical supporting role in QoS optimization. Their core function is to help administrators identify traffic patterns, detect bottlenecks, and verify QoS policy effectiveness—rather than directly implementing QoS rules. For example, a sniffer can reveal which applications are consuming the most bandwidth or if VoIP traffic is experiencing excessive jitter, providing the data needed to design targeted QoS strategies.
Step 1: Use Sniffers to Analyze Traffic and Identify Needs
Before configuring QoS, use a sniffer to gather baseline metrics about your network:
udp.port == 5060 for SIP VoIP) or port to isolate critical applications.iftop (real-time per-IP bandwidth) or nload (input/output rates) to find interfaces nearing capacity.ping (to measure latency) or specialized tools like jitter (part of the iputils package) to quantify delay variations.Step 2: Implement QoS with tc (Traffic Control)
The primary tool for QoS in CentOS is tc (part of the iproute2 package), which uses the Linux kernel’s queuing disciplines (qdiscs) to manage traffic. Below is a step-by-step workflow for a common scenario: prioritizing VoIP traffic (EF - Expedited Forwarding) and limiting file transfers (BE - Best Effort).
Install Required Tools:
Ensure tc and iproute2 are installed (pre-installed on most CentOS systems). Verify with:
rpm -q iproute
Configure the Root Queue Discipline:
Use Hierarchical Token Bucket (HTB) to create a hierarchical structure for bandwidth allocation. For example, on interface eth0 (replace with your interface):
sudo tc qdisc add dev eth0 root handle 1: htb default 20
handle 1:: Assigns a unique identifier to the root qdisc.htb: Enables HTB for hierarchical bandwidth management.default 20: Sends unmatched traffic to class ID 1:20 (defined next).Create Parent and Child Classes:
sudo tc class add dev eth0 parent 1: classid 1:1 htb rate 100mbit ceil 100mbit
1:10): 20Mbps guaranteed, priority 1 (real-time).1:20): 50Mbps maximum, priority 2 (best effort).sudo tc class add dev eth0 parent 1:1 classid 1:10 htb rate 20mbit ceil 20mbit prio 1
sudo tc class add dev eth0 parent 1:1 classid 1:20 htb rate 50mbit ceil 100mbit prio 2
Add Filters to Direct Traffic to Classes:
Use filters to match traffic and assign it to the appropriate class. For example:
1:10.sudo tc filter add dev eth0 protocol ip parent 1:0 prio 1 u32 match ip dport 5060 0xffff flowid 1:10
sudo tc filter add dev eth0 protocol ip parent 1:0 prio 1 u32 match ip sport 10000 0xffff flowid 1:10
sudo tc filter add dev eth0 protocol ip parent 1:0 prio 1 u32 match ip sport 20000 0xffff flowid 1:10
1:20.sudo tc filter add dev eth0 protocol ip parent 1:0 prio 2 u32 match ip dport 20 0xffff flowid 1:20
sudo tc filter add dev eth0 protocol ip parent 1:0 prio 2 u32 match ip dport 21 0xffff flowid 1:20
sudo tc filter add dev eth0 protocol ip parent 1:0 prio 2 u32 match ip dport 80 0xffff flowid 1:20
sudo tc filter add dev eth0 protocol ip parent 1:0 prio 2 u32 match ip dport 443 0xffff flowid 1:20
Verify Configuration:
Check the qdisc and class settings with:
sudo tc -s qdisc show dev eth0 # Shows queue statistics
sudo tc -s class show dev eth0 # Shows class usage
Look for dropped packets (indicating congestion) or misclassified traffic.
Step 3: Optimize Kernel and Interface Settings
To ensure QoS policies work effectively, optimize underlying system parameters:
Increase Ring Buffer Size: Reduce packet loss by expanding the NIC’s receive/transmit buffers. Use ethtool (install with yum install ethtool if missing):
sudo ethtool -G eth0 rx 2048 tx 1024 # Adjust values based on your NIC and traffic
Enable Jumbo Frames: For high-throughput networks (e.g., 10Gbps+), increase the MTU to 9000 bytes to reduce overhead. Apply permanently by editing /etc/sysconfig/network-scripts/ifcfg-eth0 (replace eth0 with your interface):
MTU=9000
Then restart the network:
sudo systemctl restart network
Adjust Kernel Buffers: Increase the backlog queue to handle sudden traffic spikes. Add to /etc/sysctl.conf:
net.core.netdev_max_backlog=16384
net.core.rmem_max=16777216
net.core.wmem_max=16777216
Apply changes with:
sudo sysctl -p
Enable QoS on the NIC: Some NICs require explicit QoS enabling. Check your NIC’s documentation for commands like ethtool -K eth0 tx-checksumming off (disables checksum offloading if it interferes with QoS).
Step 4: Monitor and Adjust Policies
QoS is not a “set-it-and-forget-it” task. Regularly monitor traffic and adjust policies to adapt to changing network conditions:
Use iftop/nload: Continuously monitor bandwidth usage to identify new bottlenecks. For example:
sudo iftop -i eth0 -P # Shows real-time bandwidth by IP and port
Analyze Sniffer Data: Periodically capture traffic to check if QoS markings (e.g., DSCP) are being respected. For example, use Wireshark to filter for ip.dscp and verify that VoIP traffic is marked as EF (46).
Adjust Classes/Rates: If a class consistently uses more bandwidth than allocated, increase its ceil value (maximum rate). For example, to allow VoIP traffic to burst up to 30Mbps:
sudo tc class change dev eth0 parent 1:1 classid 1:10 htb rate 20mbit ceil 30mbit
Check QoS Statistics: Use tc -s class show dev eth0 to view packet drops, enqueue counts, and other metrics. High drop rates indicate that the class needs more bandwidth or that the interface is congested.
Key Considerations for Success
mls qos command to enable QoS processing.ceil values should not exceed 100Mbps.
