protocol: stop using LastSeen so much

[bradfitz]

Historically, the control plane server has sent updated Node.LastSeen values every 10 minutes. That's a lot of the traffic coming from the server to clients (tailscaled), but it's basically never used. It's used for two things only:

• when TCP connections fail, lastSeen is logged
• when CLI file sends fail, lastSeen is printed to stderr

But we have the Online bool which is more interesting, which is also logged.

We plan to stop assuming lastSeen is present and only mention it in logs/output when it is.

Then we plan to fix the control server to actually do as we'd recently documented LastSeen in 670838c:

        // LastSeen is when the node was last online. It is not                                                                                                                                  
        // updated when Online is true. It is nil if the current                                                                                                                                 
        // node doesn't have permission to know, or the node                                                                                                                                     
        // has never been online.                                                                                                                                                                
        LastSeen *time.Time `json:",omitempty"`

So effectively, LastSeen becomes what the control server calls GoneAt, when the node stopped being online. That changes rarely, rather than every 10 minutes.

[DentonGentry]

Still used in file cp message:

tailscale/cmd/tailscale/cli/file.go

Line 290 in d6dde5a

d := time.Since(*n.LastSeen)

[bradfitz]

As of later today, the control plane server will stop sending LastSeenChange MapResponse updates.

The Node.LastSeen field is still present, but now it doesn't update while the node's connected. It only bumps once on transition from connected to disconnected, saying that last connected seen time.

Closing this as sufficiently done.