Hi David, this depends on the actual use case. Using multipath in a masque-like proxy setup covers multiple scenarios; in the hybrid access scenario it’s throughput, in other cases it can be latency, or a cheaper data subscription. That’s what I tried to explain below.
However, the whole point of ATSSS, as well as other use cases, is to provide the (mobile) operator’s costumer/the user better performance that what you have right now when using only a single path by actually making use of currently unused resources. We can argue what’s the best way to achieve that but you probably need to go to 3GPP and have that this discussion there. I was mainly trying to explain what ATSSS is, what the motivation is, and what the requirements are. Mirja From: David Schinazi <[email protected]> Date: Friday, 23. October 2020 at 23:08 To: Mirja Kuehlewind <[email protected]> Cc: "[email protected]" <[email protected]> Subject: Re: More context on ATSSS use case Hi Mirja, Can you clarify what you mean by "optimize resource usage and therefore also the performance for the user"? 1) What does it mean in networking terms (latency, throughput, etc.)? 2) What does it mean in end-user terms (video loads faster, etc.)? Thanks, David On Fri, Oct 23, 2020 at 12:45 PM Mirja Kuehlewind <[email protected]<mailto:[email protected]>> wrote: Hi all, based on the discussion yesterday I would like to provide some more context for the ATSSS use case and some notes that probably also applies to other proxy based-use cases. First of all, I would like to clearly note that it's the client (UE) that has to request ATSSS support (a Multi-Access (MA)-PDU session) when connecting to the mobile network and it's also the client that starts the QUIC connection to the proxy (hosted in the UPF). Further for each connection that the client starts to some target content server, it can again decide to use the ATSSS setup or not (by otherwise connecting to the server over a single PDU mobile-network-only session). That means the endpoint can locally decide if it wants to only use the mobile link for certain connections instead of any kind of ATSSS service. However, that decision will likely not only depend on the application characteristics but also on e.g. the data subscription, user preferences, or device status. And that brings me to another point: The right scheduling for the use of multiple paths does not only depend on the application characteristics. It's also the network conditions of each link, which to some extend can be measured in the transport if traffic is sent on both/all links, as well as other factors such as user tariff, remaining data volume, or battery status. Yes, this doesn't make the problem easier but we also don't need to solve this problem in a general way. For each of the proxy-based use cases presented yesterday there is a specific network setup with specific characteristics and goals. And often the two links do have quite different but known characteristics which does make the decision easier. For the hybrid access case, you have one DSL and one mobile link and multipath is used for bandwidth aggregation. This setup is usually deployed when the physical line that is serving the DSL doesn't provide sufficient bandwidth and in certain areas upgrading those links would be very costly. In this case the scheduling is clear: you always fill up the DSL first and only use the mobile link when the DSL capacity is exhausted; this can happen for e.g. high quality video streaming. In that case the mobile link usually has a higher latency and you might need to wait a few more seconds before your video starts but I guess that's better than watching the video in low quality. For ATSSS you always have one mobile 3GPP link and one non-3GPP link, usually wifi. And as I said in the chat yesterday, for ATSSS this will probably get first deployed with managed wifi networks, such that are often available today already by mobile operators in certain countries. ATSSS also provides a small number of so called "steering modes" which impacts the scheduling used, as presented by Spencer yesterday. These modes are provided by the network to the client (on the UE) as well as the proxy (hosted in the UPF) and these both tunnel endpoints decide independently which scheduling to use. There are different scenarios for these different steering modes, however, it's rather a small set of options. When selecting these modes the network is able to take additional factors into account such as subscriber data, operator configuration, or also application server provided info, e.g. for cases where there is actually an SLA between the content provider and network operator in place. By default the scheduling could always prefer one link and only switch over when the performance is not sufficient anymore, e.g. the selected network gets loaded. While you can measure the network characteristics, and ATSSS will also rely on measured characteristics when deciding which path to use, the operator of the mobile and wifi networks might actually have some additional knowledge about the current network load (number of connected user, total traffic volume). Further both the UE as well as the UPF in the mobile network might actually have a better view about what's happening on the local link than the far end where the content server sits, e.g. knowing that a user is moving out of coverage. As such the network could for example provide a priority for one path when signaling the steering mode and may also indicate certain threshold values that could be used to make a switching decision. However, for most flows it might be even simpler than that and probably some kind of default mode will be used, e.g. based on lowest delay assuming that delay increases when one link gets congested. Another scenario is that a user might choose a cheaper tariff where as much as possible of the downlink traffic is off-loaded to wifi. This needs to be implemented based on the scheduling in the UPF sitting in the mobile network. Further, as the steering modes are provided on a per flow level, another example scenarios is that bandwidth aggregation is requested for certain traffic flow based on an existing SLA. Please note that in any of these setups there are multiple e2e connection that use the same QUIC tunnel and as just noted each flow can have a different steering mode assigned. This is why simultaneous use of both paths is especially important for proxy-based use cases. All these scenarios benefit from knowledge about the local network conditions to optimize resource usage and therefore also the performance for the user. Hope that helps, Mirja
