One common problem encountered when using Eth62 & 63 is that on MainNet there is a mix of ETC and ETH nodes all sharing the same network ID. While they have different chain IDs, it’s only the network ID that’s sent as part of the ETH63 STATUS message so connections are established and clients have to do things like explicitly request the DAO block or wait until an invalid header is received post-DAO block which triggers a disconnect. For fast sync these ETC peers are particularly problematic as they can lead to selecting an ETC block as the pivot and then fast sync fails.
Additionally, the difference between chain ID and network ID is a constant source of confusion for users who commonly believe they are the same thing.
I’d propose dropping network ID entirely and using only chain ID which would then be included in the STATUS message. That would allow nodes from different chains to immediately identify the mismatch and reject the connection quickly.