RE: More context on ATSSS use case

[Florin Baboescu <[e-mail]>]

Hi David. Inline [FFF2].

-Florin





*From:* David Schinazi [mailto:[e-mail]]
*Sent:* Sunday, October 25, 2020 2:03 PM
*To:* Florin Baboescu <[e-mail]>
*Cc:* QUIC <[e-mail]>
*Subject:* Re: More context on ATSSS use case



Hi Florin, comments inline after [DS].



David



On Sun, Oct 25, 2020 at 1:01 PM Florin Baboescu <
[e-mail]> wrote:

Hi David,

Please see inline [FFF]. Thanks.

-Florin





*From:* David Schinazi [mailto:[e-mail]]
*Sent:* Sunday, October 25, 2020 12:18 PM
*To:* Florin Baboescu <[e-mail]>
*Cc:* QUIC <[e-mail]>
*Subject:* Re: More context on ATSSS use case



Hi Florin,



Thank you for your response, please see comments inline.



David



On Fri, Oct 23, 2020 at 7:54 PM Florin Baboescu <
[e-mail]> wrote:

Hi David,



I see that you are repeating a statement with which I definitely can not
agree “I'm noticing a pattern where no one is able to explain how this will
improve the end-user experience though, so I'm going to assume that this is
beneficial for carriers and not end-users.” So I’ll try to give it a try.
First I thought it was not necessary as there were already some great
presentations by Christoph, Olivier and the guys from Alibaba which should
have provided you with a very good reasoning on benefits for the end user.
I am not going to go again through what they presented.



The Apple and Alibaba presentations were great - but they were about
end-to-end multipath, not about ATSSS. My statement about user experience
is very specific to ATSSS.



               We also had one slide in our presentation, which may have
been overlooked, detailing at least three elements through which a
multipath access solution may improve the overall Quality of Experience for
the end user:

1)      Increased capacity, 2) increased coverage and 3) increased
reliability.



Let’s assume for simplicity an user which would be charged for the amount
of data he/she would use over a cellular network using licensed spectrum
while for all the data exchanged over the WiFi. While the user is under a
good WiFi coverage all his traffic is going to be routed over the WiFi, no
data traffic is going over the cellular. However, when the user is in an
area of limited coverage or the level of interference reaches a certain
threshold the quality of the communication over the WiFi access degrades.
As a result the achievable throughput over the WiFi may get below a certain
threshold. At this moment the WiFi access may not be able to sustain the
throughput the user may require. The user may either switch over to the
cellular (paying a higher penalty) or use both accesses, WiFi and cellular.
When both accesses are used,  all the traffic below a maximum threshold
will go over the WiFi access, while all the leftover traffic will go over
the cellular access.



Thank you - this is exactly the kind of user benefit I was asking for. In
other words: the user can save on monetary costs when watching a high
quality video in this very precise scenario of functional but slow Wi-Fi
while in range of high speed but expensive cellular. We can debate how
common this scenario is and how impactful the solution would be, but I
agree that this is an actual end-user benefit.



In total for this example there are the following cases:

-          User entirely under the WiFi coverage

-          User entirely under the cellular coverage (no WiFi coverage)

-          User under both WiFi and cellular coverage



This solution essentially increases the coverage area for the user
complementing the use WiFi with cellular in zones of poor coverage or no
coverage.  Without it the user would have been left without data access in
areas of no WiFi coverage, or with a high rate of error and limited
throughput access in the areas of poor WiFi coverage or high interference.
The solution increases the reliability, allowing the user to backup the
primary access(in this case WiFi) with a secondary access (cellular).



I don't understand how this increases coverage, can you elaborate? Coverage
is determined by the cellular antennas and Wi-Fi access points available to
me. I can use all of them with QUIC connection migration end-to-end today,
or even plain-old TCP that creates a new connection on IP change - ATSSS
doesn't change the network coverage I have.



[FFF] In the example I provided the primary access was over the WiFi
access. The coverage is increased by augmenting the coverage over the WiFi
with coverage over the cellular. While I do agree with you that QUIC
connection migration may be used in certain scenarios when you completely
switch from one access to another there are still plenty of space left in
which both accesses may be useful to being used. This scenario is not
covered by the connection migration. One example in which something like
this may happen is in areas of high interference or reduce signal strength
on the primary access, while the secondary access may still be throughput
limited.



[DS] Your point here is that there might be value in using two paths. It
doesn't contradict my point that this does not increase coverage. My
definition of coverage is "the geographic area that is covered by networks".



[FFF2] In my opinion the notion of coverage is user dependent and it does
have both a geographical and a temporal dimension. A user may find himself
out of the coverage for a service even without movement for various
reasons. From this point of view I have to admit that I do not quite
understand your definition of coverage.

The point I was trying to make that the area of coverage for a service for
a user may be increased by complementing the primary access of the user (in
my previous example WiFi access) with a secondary access. While I may have
an excellent coverage in my house using WiFi and I can access any video
services , as soon as I step outside of my house the quality of my WiFi
signal degrades rapidly with the distance from the WiFi AP. The application
becomes rate limited and as a result one can safely say that there is no
coverage for me for the video service I described at my desired rate.
However, if I would be able to complement my weak WiFi access outside the
house with a cellular access, then I may be in the position to be able to
run my application at the desired rate. In this context I can say that the
service coverage can be provided outside of the house. Similar argument can
be brought in the context of the temporal dimension for the coverage.



Similarly, I agree that in some circumstances this can increase overall
throughput, but I don't understand how it improves reliability, if anything
I think it'll reduce reliability by adding a new single point of failure in
the network - the ATSSS proxy.



[FFF] I’m not sure what do you mean by arguing that we insert a single
point of failure in the network – the ATSSS proxy. Trying to clarify this
point let me say, in a very simplistic way that the end to end path between
the terminal(UE) and a server(S) is made up of the following:

UE <-> access (cellular/WiFi) <-> UPF <-> S

The 3GPP ATSSS solution is based on the notion of a Multi Access PDN. In
this solution the ATSSS proxy is an on path functionality situated at the
UPF (aka user plane gateway function) with the goal that together with the
ATSSS functionality on the UE it accommodates one or more accesses being
used between the UE and UPF. So from this perspective I do not see how you
can state that we introduces a single point of failure in the network. On
the contrary the increased reliability is provided by augmenting one access
(in my example over WiFi) with a cellular access.



Let me try to provide you with another example. Let’s assume a client IMS
based application. The user uses WiFi as a primary access and would like to
start a high throughput session with a server. On the WiFi side the WLAN
modem may provide the application with “minimum throughput per access
class” that can be achieved at a specific moment. Based on this information
, the application uses IMS to negotiate the end to end pipe to the server
with a “guaranteed” throughput. While the quality of the access over WiFi
remains unchanged and the WiFi load is unchanged the “guaranteed” rate can
be sustained. However, when access characteristics change the value for
this metric may not be sustainable. In this case augmenting the WiFi access
with the cellular use addresses this lack of reliability.



[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.



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.



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



[FFF2] It looks like there is a misunderstanding on the ATSSS solution
here. As I mentioned before the basic element of the ATSSS solution is the
MA PDN session.  In a MA PDN session there is a single global IP
address/prefix associated with the client. All the packets sent by the
client are sent using the client global IP address as a source and the
destination IP of the server. The ATSSS function resides both at the client
and at the associated UPF. All the packets of the PDN session go through
the same associated UPF (which contains the ATSSS functionality). The ATSSS
function at the UE is provided with two local link IP Addresses which are
used to establish the MPQUIC connectivity to the ATSSS function at the UPF.

So in short the ATSSS MPQUIC based solution tunnels IP or Ethernet
packets/frames  between the ATSSS client application on the UE and the
ATSSS application on the UPF. The transport between the UE and UPF is
provided via QUIC encapsulation.



I’ll use the following notation:

IPg = global routable IP address at the UE

IPs = global routable IP address of the Web server

IP1 = local link IP address

IP2 = local link IP address

IP3 = local link IP address

IP4 = local link IP address

*Single Access PDN*

[Client] IPg----- { cellular } ------- [UPF] ------- IPs[WebServer]



*Multiple Access PDN with MPQUIC ATSSS*

[Client]  IPg  [ATSSS Client] IP1----- { cellular } ------- IP3 [ATSSS
Proxy][UPF] --------IPs [WebServer]

                                      IP2 \------------{ Wi-Fi   }
-------------------/IP4





Please note that one may think that there may be another deployment
scenario. *This scenario is NOT covered by the 3GPP ATSSS solution.* For
this scenario I do agree with you that it may introduce a potential point
of failure as the ATSSS node may not be on path. In this scenario the user
creates a single access PDN for Internet and uses the WiFi as a local
offload. For simplicity of the conversation I’ll limit the case to the the
terminal having a global routable address over WiFi (IPg2)and a global
routable address over cellular(IPg1). The ATSSS proxy may also need to
provide the client with a globally routable IP address reachable from the
client via the WiFi access. I not this address IPg3.

[Client]  IPg1  [ATSSS Client] IP1----- { cellular } -------[UPF] -------
IP3 [ATSSS Proxy][UPF] --------IPs [WebServer]

                                      IPg2 \------------{ Wi-Fi   }
---------------------------------------/IPg3



On a side note I would also try to answer a different question. Is the
bandwidth aggregation solution always useful? Based on various companies
contributions in 3GPP it was noticed that there is no benefit for the user
to do bandwidth aggregation when the throughput ratio between the two
accesses exceeds somewhere between (3-5):1.



That's a useful datapoint, thanks! So it is restricting the use benefit
scenario above to only when Wi-Fi and cellular have comparable throughput.

 [FFF] Correct. When we discuss order of magnitude difference using
bandwidth aggregation does not make any sense. IT is like using a straw
when your toilet is overrun.

Another interesting use case addresses one of the limitations of WiFi (
before WiFi6, which uses an OFDMA based access). As many of you know, in
WiFi an user can transmit only after it detects that there is no one else
transmitting at the same time. Because of this when the number of users
served by the same access point increases the quality of the access
decreases, as all the users compete for the same access. In this case the
end user may use the WiFi access for all the downlink traffic while for the
uplink traffic may use the cellular access. This use case improves for the
end user both the capacity for both downlink and uplink as well as the
reliability.



I wouldn't call carrier sense a limitation of Wi-Fi, it's a very reasonable
design choice that's worked incredibly well for us so far. Sending the
uplink and downlink packets on different paths sounds like an interesting
research topic, has this been tried in practice?

[FFF] Yes.



[DS] Great! I'd love to learn more. Do you know if the data is publicly
accessible?



[FFF2] We can take this one offline as I do not think that it belongs in
this thread.

These are just few examples which try to show the benefits of bringing a
multipath solution in the toolbox for both end user as well as network
elements/functions. I hope I brought some more clarity to you.

Regards,

-Florin







*From:* QUIC [mailto:[e-mail]] *On Behalf Of *David Schinazi
*Sent:* Friday, October 23, 2020 6:12 PM
*To:* Mirja Kuehlewind <[e-mail]>
*Cc:* [e-mail]
*Subject:* Re: More context on ATSSS use case



Hi Mirja,



I understand how in some scenarios this could increase throughput.

However, can you clarify how this could improve latency?



I'm noticing a pattern where no one is able to explain how this will

improve the end-user experience though, so I'm going to assume

that this is beneficial for carriers and not end-users. Unfortunately

I don't have the time to go to 3GPP and do this research myself.



David



On Fri, Oct 23, 2020 at 6:07 PM Mirja Kuehlewind <
[e-mail]> wrote:

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 <[e-mail]>
*Date: *Friday, 23. October 2020 at 23:08
*To: *Mirja Kuehlewind <[e-mail]>
*Cc: *"[e-mail]" <[e-mail]>
*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 <mirja.kuehlewind=
[e-mail]> 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