design rationale behind eth

[oslfmt]

design rationale behind eth

[fjl]

What exactly does "data validity" mean, and why does requesting the Merkle tree nodes have to do with it?

The "data validity" is defined in the sections about blocks and transactions. Data validity is not related to the merkle trie download.

Is it because just verifying PoW is faster to propagate block while still offering some security, ie, we are making a security vs. speed tradeoff here?

Yes. Validating the PoW is very quick, and coming up with a valid PoW seal is very expensive on mainnet, so it's a good tradeoff. If the block turns out invalid, the node will not accept it.

and don't TCP connections generate extra overhead and time? So if we want to propagate transactions across the network as fast as possible, and as I understand nodes have to make connections first, won't this introduce a lot of delay?

The "eth" network is an unstructured graph of TCP connections between nodes. Basically, it is assumed that nodes will continuously establish connections and maintain them. The connections are "long-lived", i.e. it's not like HTTP with one connection per request. Instead, once a connection is created, it can be maintained until either side decides to end the connection. There is no need to create a connection specifically for relaying a single transaction.