ATSSS

Question 1

What is ATSSS (Access Traffic Steering, Switching and Splitting)?

Answer 1

A service providing network support for multi- access devices(multihomed devices), so you can steer, switch or split paths.

Question 2

What is 3GPP?

Answer 2

A partnership that manages standards like 4G, 5G etc.

Question 3

What are some other proposed techniques for multihoming for mobile devices?

Answer 3

Shim6 (Site multihoming by IPv6 protocol)

Mobile IPv6

Proxy Mobile IPv6

Multipath TCP

Question 4

What are some use cases for multihoming?

Answer 4

Balance load of users’ flows over different available networks

Combine resources provided by the cellular and WLAN networks

Question 5

On a high level, how does the ATSSS provide the service?

Answer 5

Generally the 5G core offers a service (PDU connectivity service) that UE can use to send data to some network (Packet Data Network, or PDN) supporting that protocol.

However with ATSSS service provided in the 5G core the UE can also be provided a multi- access version of the mentioned service.

Question 6

How does ATSSS avoid that the same IP address and port number are used simultaneously by the ATSS and the User Equipment?

Answer 6

The ATSSS User Plane Function may be configured with a pool of IP addresses that it can use in the internet- facing interfaces instead of preserving the Multi- Access IP address assigned to the UE.

Question 7

Which procedures can be done between the 3GPP access and non- 3GPP access with the help of ATSSS?

Answer 7

• Access Traffic steering: Select access network for new data flow and transfer over the selected access network. (best network selection)
• Access Traffic switching: Packets migrate of an ongoing data flow from one access network to another, only one network is in use at the time but session continuity is ensured (seamless handover)
• Access Traffic splitting: Forwarding the packets of a data flow across multiple access networks simultaneously (network aggregation, double the bandwidth for instance!)

Question 8

How does the UE and ATSSS Use Plane Function (UPF) get the rules that say which flows are eligible to use the ATSSS service?

Answer 8

The 5G control plane provides the rules, by mapping them to a Multi- Access PDU session.

Question 9

When are the set of rules delivered to both the UE and the ATSSS UPF?

Answer 9

Once a Multi- Access PDU session has been established?

Question 10

In what modes can traffic of a flow that matches an ATSSS rule be distributed among available access networks?

Answer 10

• Active standby: It will be forwarded via a specific access (“active access”) and switched to another access (“standby access”) when the active access is unavailable.
• Load balancing: It is distributed among available access networks following some distribution ratio, e.g. 70/30.
• Priority based: Accesses are assigned priority levels indicating which is to be used first. Traffic will be steered on the access with a high priority until congestion is detected, then the overflow is forwarded over a low priority access.
• Smallest delay: Traffic is forwarded via the access where the smallest RTT is.

Question 11

How are the necessary measurements for ATSSS operation gathered?

Answer 11

The current network state of each path is needed. Multipath protocols often have the necessary measurements built- in.
If such measurements are not available the solutions is to use a dedicated Performance Measurement Function protocol which provides measurements and reports access availability done by the UE to the UPF.

Question 12

Which mechanisms, normally found in multipath protocols such as MPTCP can be used to control the utilization of the paths?

Answer 12

1. Path manager: Decides when a new subflow needs to be established over a path.
2. Packet Scheduler: Selects the subflow over which next packet will be sent.

Question 13

How can ATSSS use mechanisms found in multipath protocols to apply the Active Standby mode?

Answer 13

path manager tries to use active access and switches to the standby access after certain number of retransmits.

Question 14

How can ATSSS use mechanisms found in multipath protocols to apply the Smallest Delay mode?

Answer 14

Path manager esablishes a subflow over both paths and packet scheduler measures their RTTs and prefers the one with the lowest RTT.

Question 15

How can ATSSS use mechanisms found in multipath protocols to apply Load Balancing?

Answer 15

Use same path manager with a weighted round- robin scheduler.

Question 16

How can ATSSS use mechanisms found in multipath protocols to apply Priorities?

Answer 16

Path manager similar to active- standby one, but reacts faster and a packet scheduler that prefers the high priority path.

Question 17

What is the goal for ATSSS phase 1?

Answer 17

Support for TCP. (by using Multipath TCP and the 0-RTT Convert Protocol to handle TCP traffic)

Question 18

In ATSSS phase 1, what allows the client to use ATSSS even though the server does not support MPTCP?

Answer 18

The client uses MPTCP to reach an MPTCP proxy over one or both access networks. The MPTCP proxy converts MPTCP to TCP by using the Convert Protocol for MPTCP.

Question 19

What is the goal for ATSSS phase 2?

Answer 19

To add non- TCP support as well. The work within 3GPP focuses mostly on using QUIC as the baseline for ATSSS phase 2 at the moment.

Question 20

Why does a solution for QUIC and multihoming like for instance the Convert protocol used for MPTCP not work for QUIC?

Answer 20

Because security is integrated in the protocol and cannot be intercepted.

Question 21

What approaches have they discussed for allowing QUIC as an ATSSS Data Plane protocol? (i.e. how to encapsulate non- TCP traffic into a tunnel between UE and the ATSSS UPF for all traffic, not only TCP or QUIC)

Answer 21

1. Use of QUICv1 with the Unreliable Datagram Extension
2. Use of QUIC with the Unreliable Datagram Extension but with one QUIC connection over each access network
3. Use of a single Multipath QUIC connection extension, with the Unreliable Datagram Extension, over all access networks

Question 22

How does using QUICv1 with the Unreliable Datagram Extension to allows QUIC as an ATSSS Data Plane protocol work?

Answer 22

Only one QUIC connection between UE and ATSSS UPF for a given flow. Can only use one access network at a time. (But switching and steering still supported + CC state is reset on a new path).

Question 23

How does using QUIC with the Unreliable Datagram Extension but with one QUIC connection over each access network to allow QUIC as an ATSSS Data Plane protocol work?

Answer 23

One QUIC connection over each access network. Based on rules given by the network, the UE and ATSSS UPF must select one QUIC connection.

However this makes splitting challenging. (same unreliable flow over different connections + packet reordering likely)

Also similar integrated mechanisms to MPTCP would require complex application.

Question 24

How does using a single Multipath QUIC connection extension, with the Unreliable Datagram Extension, over all access networks to allow QUIC as an ATSSS Data Plane protocol work?

Answer 24

It sill use unreliable datagram extension but also leverages the QUIC multipath proposal.
This means spliting is naturally supported.

Question 25

What is the service provided in the 5G network that allows UE to send data to a Packet Data Network through the 5G network?

Answer 25

The Protocol Data Unit (PDU) Connectivity Service.

Question 26

When are PDU sessions allowing the UE to send data through the 5G network established?

Answer 26

Upon a request from the UE when it connects to that network. (The type of PDU session can be IPv4, IPv6, etc. )

Question 27

How are Multi- access PDU sessions realized?

Answer 27

The ATSSS service provided in the 5G core network control plane and user plane realizes this.