A Message Feedback Method for MPTCP

Introduction With the popularity of mobile devices, most devices have multiple wireless access methods, such as 5G, and Wi-Fi. This has brought a realistic basis for the popularity of MPTCP. However, the current MPTCP can only perceive information such as RTT, but cannot perceive information from edge wireless devices. Therefore, a new subtype of MPTCP options is proposed in this document to transmit cross-layer information of wireless devices . Figure 1 illustrates the integration of the MP_INFO into the wireless devices, where the MPTCP client resides on the UE and the MPTCP server resides on the Server.

Terminology

NR:: New Radio, a new wireless access technology for 5G.
GTP-U Message:: GTP-U (user plane) messages are either user plane messages or signalling messages. User plane messages are used to carry user data packets between GTP-U entities. Signalling messages are sent between network nodes for path management and tunnel management.GTP-U peer: node implementing at least one side of any of the GTP user plane based protocols.
UPF:: User Plane Function.
CQI:: CQI reflects the time-changing link capability.

Design Overview In mobile networks, the performance of wireless devices has a great impact on data transmission. Therefore, by feeding back the information of the wireless device to the MPTCP server, it is convenient for the MPTCP scheduler to schedule traffic to different paths according to the information of the edge device. For the non-3GPP path, the Wi-Fi AP can directly modify the MPTCP data packet to feed back the device information to the MPTCP server. For the 3GPP path, 5G NR periodically feeds back equipment information to UPF through the GTP-U protocol . UPF can directly modify the MPTCP data packet to feed back the device information to the MPTCP server. This document defines the extension of the MPTCP subtype, which allows the MPTCP server schedules traffic to different paths according to the information of edge devices. In addition, this document provides a cross-layer information transfer method for 3GPP and non-3GPP equipment . Section 4 will introduce the transmission method of non-3GPP devices. In Section 5, the transmission method of 3GPP equipment will be introduced. Section 6 will introduce the usage of MP_INFO.

Non-3GPP Device Information The MPTCP server needs to obtain information on non-3GPP devices, such as Wi-Fi AP. This document defines a new MPTCP Option subtype MP_INFO. Non-3GPP devices can transform devices information from wireless devices to the MPTCP server. The format of MP_INFO is shown in Figure 2. It has the following fields:

Device Type:: The type of device, 0x1 represents the non-3GPP path.
Rate:: The send Rate of non-3GPP devices.
Buffer Length:: The buffer length of a non-3GPP device.

3GPP Device Information For the 3GPP path, we need to overcome the difficulty of acquiring 5G NR information. In the 3GPP path, the GTP-U tunnel is responsible for transmitting data between the radio access network and the UPF. So as shown in Figure 3, for the 3GPP path, 5G NR transmits equipment information to UPF through GTP-U . Then, UPF transmits the calculated rate and Buffer to the MPTCP server through MP_INFO. In the 3GPP path, the GTP-U tunnel is responsible for transmitting data between the radio access network and the UPF. 5G NR uses the GTP-U tunnel to periodically send status information to the UPF, e.g. 1 ms. 5G NR only transmits the key information that affects the latency. Note that this information is only transmitted between 5G NR and UPF.

The format of MP_INFO for 3GPP devices is shown in Figure 4. It has the following fields:

Device Type:: The type of device, 0x2 represents the 3GPP path.
Rate:: The send rate of the 3GPP device, can be calculated by CQI.
RLC Buffer:: The sum of RLC buffer length, can be feedback through NR .

MP_INFO Usage MPTCP scheduler can leverage cross-layer information from 5G NR and Wi-Fi AP to predict the transmission delay of data blocks on each path. However, there is a visual gap between this cross-layer information and the MPTCP server, which requires different methods on different paths to transmit cross-layer path information to the MPTCP server. So this document separately introduces the cross-layer information collection for non-3GPP and 3GPP paths. For 3GPP paths, we recommend feeding back the MP-INFO option every 10ms. Because 3GPP equipment generally uses fixed time slot allocation, the time of one frame is 10ms. Non-3GPP devices do not have fixed time slots, so the feedback time can be flexible. And network operator can design their prediction delay methods according to the cross-layer information. By leveraging cross-layer information from edge wireless, the MPTCP server can predict downlink latency and optimize multipath scheduling. For example, By CQI and RLC Buffer information, the MPTCP server can predict the delay of the 3GPP path. By Rate and Buffer Length information, UPF can predict the delay of the non-3GPP path. According to the predicted transmission delay, the MPTCP scheduler can use different strategies to schedule packets. MP-INFO option is an incremental information. In multipath, some paths may do not support this option. For this case, the MPTCP scheduler can fall back to the MinRTT scheduler.

IANA Considerations This draft adds the following subtype numbers to the MPTCP Option Subtypess registry of : Value Symbol Name Reference 0x9 MP_INFO Cross layer information of non-3GPP devices Device Type Value Message 0x1 Non-3GPP Path 0x2 3GPP Path

Security Considerations Since the MP_INFO option requires intermediate devices to modify data packets, some devices may not support this option. Some wireless devices may send incorrect MP-INFO information, so the terminal should confirm the security of the path.