Question

Why should file transport protocols compute a checksum on the file data they receive even when using a reliable end-to-end stream transfer protocol like TCP.

Answer 1

First - CheckSum is not always possible, it depends on if the remote server supports the checksum, many do not.

Second - TCP will give reliability not integrity.

Let's assume,
-> Server sent us 10 packets
-> then our client will receive these packets
-> And will acknowledge back to the server that we received 10 packets

But it does not say that all 10 packets are exactly the same as they were sent.
Packets might have changed due to many reasons like,
-> Man in Middle attack
-> Or some other kind of interference

Checksum

-> After receiving all 10 packets
-> We will combine them and generate file

-> And Checksum
-> Which will generate a hash parameter

And if this generated hash parameter is exactly the same as the server provided hash parameter then we will know all 10 packets are not altered.

So, We use checksum for file integrity.
There are many methods use for checksum like,
MD5, SHA-1, SHA-256, and SHA-512....!