Protocol Theory – Why Reserved Bits or Bytes Exist in Network Protocols

• If Layer-2 has a checksum, shouldn't this validation be enough?
• Why we are having checksum validation in different OSI Layers?

Simply put, different layers of the OSI model have checksums so you can assign blame appropriately.

• Suppose there is a webserver running on some system (assume TCP port 80, i.e. OSI Layer 4)
• Suppose there is a software error in the Operating System of that webserver that corrupts certain IP payloads. (IPv4 OSI Layer 3)

Case 1: Only use Ethernet checksums

If we only rely on Ethernet (i.e. OSI Layer 2) checksums, then that error would go un-noticed until something crashes or throws an error, because the Ethernet NIC would simply transmit the (already corrupted) data that it received from the Operating System IP stack. For sake of argument, let's assume the TCP payload is corrupted, but the Ethernet checksum is fine.

When the IP stack on the other side receives the Ethernet frame, it unpacks the IP payload and delivers it to the webserver. However, the TCP payload in this packet is corrupted. When the web server crashes from data corruption, the developer has no way to isolate whether this was an IP-level failure or a TCP failure (or perhaps something else farther up the application stack).

Case 2: Layered checksums

However, if TCP, IP and Ethernet all have checksums, we can isolate the layer where the error occurred, and notify the appropriate Operating System or application component of the error.

Here's a link to a Hangout with Vint Cerf (Apr. 2014) where he explains how he thought that this internet was supposed to be an experiment only:

As we were thinking about the Internet (thinking well, this is going to be some arbitrary number of networks all interconnected — we don't know how many and we don't know how they'll be connected), but national scale networks we thought "well, maybe there'll be two per country" (because it was expensive: at this point Ethernet had been invented but it wasn't proliferating everywhere, as it did do a few years later).

Then we said "how many countries are there?" (two networks per country, how many networks?) and we didn't have Google to ask, so we guessed at 128 and that would be 2 times 128 is 256 networks (that's 8 bits) and then we said "how many computers will there be on each network?" and we said "how about 16 million?" (that's another 24 bits) so we had a 32-bit address which allowed 4.3 billion terminations — which I thought in 1974/3 was enough to do the experiment!

I had already posted this as a comment to Jens Link's answer, but I felt it shoud surface a bit more.