This client-side mod adds a window that, when enabled, monitors the size of packets transferred between the server and the client. Recommended for those who swear that it's not their internet that's making their ping bad.

How to use:

Check/Change the keybind (default: Numpad 0).
Press the key to activete the window.
Click Toggle Recording and then enable C→S or S→C or both. (C→S means Client To Server, S→C means Server To Client)
Join a singleplayer/multiplayer world.

How to read:

Example packet: S→C | 10 B | class net.minecraft.network.protocol.game.Clientbound.CustomPayloadPacket→somemod:channel[0]

Direction is Server To Client: S→C
Size is 10 bytes: 10 B
Vanilla packet: net.minecraft.network.protocol.game.Clientbound.CustomPayloadPacket
Custom network channel registered by a mod: somemod:channel
Packet id of 0: [0]

Technical information/limitations:

The mod does not store the packet data, only the size of its content (empty packets are shown as 0 B).
The content of the packet cannot be viewed with this mod.
The units go up to Terrabytes.
1000 bytes = 1 kilobyte, 1000 kilobytes = 1 megabyte, ...
The content size is stored as a long → upper limit = 9,223,372,036,854,775,807 bytes.
The window is written in Swing, may cause problems on other platforms.
Mixins are at a low-level, this may cause instability with other mods that try to modify network traffic.

Network Analyzer