I think remote should be an object so it's more flexible going forward. My initial thought it that it would look like this:

message RemoteBlockResponse {
  string path = 1;
  oneof config {
    RemoteBlockConfigS3 s3 = 2;
    RemoteBlockConfigHTTP http = 3;
  }
}

message RemoteBlockConfigS3 {
  string region = 1;
  string bucket = 2;
  bool requester_pays = 3;
  optional endpoint = 4;
}

message RemoteBlockConfigHTTP {
  string endpoint = 1;
  optional map<string, string> headers = 2;
}

Cool. Makes sense to me.

16? is this arbitrary? if the client thats receiving the blocks is really slow, wouldn't that result in send failures from the spawned task?

I'll add better documentation.

if the client thats receiving the blocks is really slow, wouldn't that result in send failures from the spawned task?

I think the task will just block until the stream is cleared. From the okio::sync::mpsc docs:

    /// # Examples
    ///
    /// In the following example, each call to `send` will block until the
    /// previously sent value was received.
    ///
    /// ```rust
    /// use tokio::sync::mpsc;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let (tx, mut rx) = mpsc::channel(1);
    ///
    ///     tokio::spawn(async move {
    ///         for i in 0..10 {
    ///             if let Err(_) = tx.send(i).await {
    ///                 println!("receiver dropped");
    ///                 return;
    ///             }
    ///         }
    ///     });
    ///
    ///     while let Some(i) = rx.recv().await {
    ///         println!("got = {}", i);
    ///     }
    /// }
    /// ```
    ```
    
    I'm not exactly sure what the Tonic stream is doing under the hood. It may be clearing the buffer into its own buffer for us 🤷 

Instead of these being ...Tx can we spell out Transaction? It keeps it cleaner & had no impact to the traffic on the wire.

Can you add a todo please? And create an issues to implement it. Or do you want to do that before we merge to the master?

Working on it here:
#3116

I can add an issue to be thorough.

For ProtoBlock we want to use protobuf-based encoder and decoder, not Postcard

It should work with reference, cloning block looks like an overkill.