Blocks

Blocks do a lot in Minecraft, given that it's a voxel-based world. In most cases, when a block is placed or mined, the server needs to reply with a block update packet.

If this packet isn't sent, the client just assumes the packet was dropped or the client wasn't allowed to place the block, replacing it with air after a second or two.

Breaking

Packet	Contents
C->S Mine	Telling the server to destroy a block
S->C Block Update	Tells the client a block was updated

Placing

Packet	Contents
C->S Place	Telling the server to place a block
S->C Block Update	Tells the client a block was updated