Bandwidth and latency - the two network metrics

Network performance is measured by two primary metrics: bandwidth (how much data per second?) and latency (how long does a packet take?). Both matter; one may be the bottleneck while the other is fine.

Bandwidth

Bandwidth is the data rate of a network path. Measured in bits per second (bps), kilobits (kbps), megabits (Mbps), or gigabits (Gbps).

For example:

• 1 Gbps Ethernet: 125 MB/s maximum throughput.
• 10 Gbps Ethernet: 1.25 GB/s.
• Internet: varies, often 10-1000 Mbps.

Latency

Latency is the time for a packet to travel from source to destination. Measured in milliseconds (ms) or microseconds (µs).

• LAN: <1 ms typical.
• Metro: 1-10 ms.
• Transcontinental: 50-100 ms.
• Satellite: 500+ ms.

Latency is dominated by:

• Speed of light (geographic distance).
• Hop count (routers, switches).
• Queueing in switches and routers.

Measure bandwidth with iperf3

# Server
iperf3 -s

# Client
iperf3 -c server

Output:

[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.00  sec  1.09 GBytes   935 Mbits/sec    0

• Transfer: total data transferred.
• Bitrate: throughput.
• Retr: retransmissions (0 is ideal).

Measure latency with ping

ping -c 5 server

Output:

64 bytes from server (10.0.0.5): icmp_seq=1 ttl=64 time=0.123 ms
64 bytes from server (10.0.0.5): icmp_seq=2 ttl=64 time=0.156 ms
...

Latency: round-trip time (RTT). <1 ms on LAN, varies by distance for WAN.

Bandwidth vs latency problems

For most applications, latency is more important than bandwidth. A 100 ms latency kills interactive response time even on a 1 Gbps link.

Read /proc/net

cat /proc/net/dev

Output:

Inter-|   Receive                                                |  Transmit
 face |bytes    packets errs drop fifo frame compressed multicast|bytes    packets errs drop fifo colls carrier
  eth0: 1234567  12345    0    0    0     0          0         0  1234567  12345    0    0    0     0    0

Per-interface:

• bytes: total bytes.
• packets: total packets.
• errs: receive errors.
• drop: receive drops (queue full).
• fifo: FIFO overruns.
• frame: frame errors.

High error or drop counts indicate a problem.

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