One link

The simplest network is two nodes and one link. Most real problems, a bad mobile connection or a long satellite hop, are one link, and most tests need no more.

A link has two directions that need not be alike. This one is a mobile access link: more delay and a lower rate going up, a higher rate going down. A stream goes each way, and the up stream is echoed so the round trip is measured:

# file: mobile.yaml
version: 0
name: mobile
seed: 42
duration: 5s
topology:
  nodes: [phone, server]
  links:
    - id: radio
      between: [phone, server]
      forward:                       # phone -> server, up
        delay: 40ms
        jitter: { amount: 5ms, distribution: normal }
        loss: { model: random, p: 1% }
        rate: 10mbit
      reverse:                       # server -> phone, down
        delay: 25ms
        rate: 20mbit
workloads:
  - { id: up, kind: udp-cbr, from: phone, to: server, rate: 1mbit, packet-size: 800b, echo: true }
  - { id: down, kind: udp-cbr, from: server, to: phone, rate: 4mbit, packet-size: 1200b }

import netscenario as ns

result = ns.run(ns.Scenario.load("mobile.yaml"))
up, down = result["up"], result["down"]

# The one-way delay of a stream is what the link says (both ends read the same clock).
# The down stream has no jitter, so no packet is faster than the delay of the link;
# the up stream has a jitter of 5 ms, so its delays are spread around 40 ms
assert down.delay.min >= 0.025
assert 0.030 < up.delay.p50 < 0.050

# The echo is the round trip: up and down together, and lost if either way lost it
print(f"round trip median {up.echo.round_trip.p50 * 1000:.0f} ms")

Three things this shape teaches:

• A bound on a delay has an exact lower end, where the link has no jitter. No packet arrives before the delay of the link (the down stream above), and a test may say so without any room; only the upper end needs room for the machine. With a jitter the delays are spread around the value (the up stream), and the bound is on the percentiles.
• Loss is per direction. The up stream loses 1 %; the down stream loses nothing. A stream does not see the loss of the other way unless it echoes.
• forward is from the first node of between. Put the terminal first and forward is up, the convention of the profile library.

When you do not know the numbers of your link, start from a profile (profile: lte-good, What a link can do) and override what you know.