Re: More context on ATSSS use case

[Olivier Bonaventure <[e-mail]>]

David,

> 
> 
> [DS] Let me clarify why ATSSS introduces a new point of failure. First 
> here's the common network topology today without ATSSS. In this example 
> the Client is a smartphone, and the user is browsing a website hosted by 
> WebServer.
> 
> [Client] ----- { cellular } ------- [WebServer]
>      \------------{ Wi-Fi   } ------------/
> 
> The client has (at least) two IP addresses: IP_Client_cell and 
> IP_Client_WiFi.
> The WebServer has IP_Server.


My understanding of most of today's QUIC deployments is that the client 
is not connected to the webserver but to a CDN or edge node that serves 
as a proxy to the webserver.

Thus the setup is

[Client] ----- { cellular } ------- [Edge]   ---------[WebServer]
       \------------{ Wi-Fi   } -------/

> Now let's assume the Client is happily browsing using QUICv1 over one 
> interface, if that network goes down, QUIC will automatically migrate to 
> the other network and the web browsing will not fail. The client can 
> send packets to IP_Server on any interface and as long as one interface 
> works they will reach the server.
> 
> Now let's introduce ATSSS:
> 
> [Client] ----- { cellular } ------- [ATSSS Proxy] -------- [WebServer]
>      \------------{ Wi-Fi   } ------------/
> 
> The ATSSS Proxy has IP_ATSSS.
> The packets sent by the client are not sent to IP_Server here, they're 
> sent to IP_ATSSS.
> If the ATSSS proxy becomes unreachable mid connection, the client's 
> connection will break - because it's sending to IP_ATSSS not IP_Server.

The client will detect the failure of the ATSSS proxy and will then be 
able to switch to non-ATSSS to directly reach the webserver/edge node.
Since the client uses the ATSSS for multiple applications, a failure of 
the ATSSS proxy will be quickly detected.

> My point is that in this scenario ATSSS reduces reliability by adding a 
> point of failure.

I don't see a difference between the failure of an Edge node and an 
ATSSS proxy. Both can be deployed with enough redundancy to limit the 
impact of failures. The only difference is that ATSSS nodes will 
probably be deployed by network operators while Edge nodes are deployed 
by CDN providers.

Olivier