视频：
https://www.youtube.com/watch?v=lVV4AlgMnHo&list=PLvFG2xYBrYAQCyz4Wx3NPoYJOFjvU7g2Z&index=6

1-5-principle-packetswitching-notes.pdf

Flow:

A collection of datagrams belonging to the same end-to-end communication, e.g. a TCP connection.
e.g:
a voice call consists of many consecutive packets all part of the same communication. We call this sequence of packets a flow. The switch doesn’t need to worry about adding or removing this per-flow state

no per-flow state reuired:

• The switch doesn’t need to worry about adding or removing this per-flow state
• The switches also don’t need to store this state.
• switches don’t have to worry about failures.