For client side replication: If we receive a replication "packet" from the server, it is denoted by the ID of the entity replicating, and any replicated variables.
If we don't have the ID instantiated locally on the client, we create it and set the replication data.
Of course, I would have to have the server send the full replication because it wouldn't send any previously modified values.
You mention actors that exist already in the level. This isn't a concept that has really applied to me as yet, because I have not considered a scenario where I would want level actors that aren't added depending upon the client connected. Could you possibly elaborate? I saw mention in the Unreal source but I wasn't sure when this would be useful to me.
The question reliability was specific to Unreal (or Tribes) with net priority; I was asking if that is the correct understanding of their model. Out of order delivery was asking how Unreal handles it (although this is related to the typical "How do I order an unreliable protocol"! I wouldn't mind simply dropping out of order packets except when there are delta variables (e.g the difference vector between two velocities) for more precision, which require access to the last state. Additionally, this does not seem to be how Unreal handles it.
My other vague question was about the age of variables. It seems as though there is no specific support for determining the time for which the attributes were valid. This is fine but for extrapolation where I need to know the age of the position and velocity vectors to be able to use EPIC. I think that It would make sense to be able to lookup past states at a particular game tick (a synchronised concept of time) so should I implement this within the replication system, so the networking layer has its packet sequence number and the replication also sends the game tick with the replicated data on top of that protocol?
Let me add some explanation as to why I appear to be so concerned with premature optimisation. I aim at sixty frames per second game frame rate. That leaves 16ms for all processes to execute in. Assuming 1ms is used for the render pipeline on average, that leaves 15ms. That's a lot of time if you write good code, and typically you'll have a fraction of a ms per client. However, this scales geometrically (I believe) with connection of new game-observing clients, as they have to be sent all other actors. Therefore .2 ms may seem small, but when that starts growing with just the scraper logic that seems dangerous. This said, these numbers are merely demonstrating the idea and are not representative of the actual model and this is without relevancy checks.
Seeing as you mentioned your own progress, do you allow any non-actor classes to use replication? or have them handle their own data (e.g the scoreboard, clock)
My final question so far is should the server store the state of all actors every tick? I cannot think immediately of why it would save for replaying, but does Unreal store the "state" of each actor in a lookup by game time? And if so, is it every game tick or every network tick? It seems now would be the time to think about how that would be done, not further down the line.
And thanks for the example source, I'm sure it will be very useful!!