5G Link Aggregation with Multipath TCP (MPTCP)
The unprecedented growth of data traffic and the number of connected devices has made it evident that the current end-to-end host-centric communication paradigm will not be able to meet user demand for massive data rates and low latency. The wireless industry is constantly pushing technology frontiers to cope with this increasing user demand.
The advent of the fifth-generation cellular architecture (5G), along with the evolving LTE and Wi-Fi networks, will boost the ability of the wireless industry to support the new connected reality. The heterogeneous environment, with multiple access networks coexisting, will require end devices to connect to all available wireless access networks to efficiently use the available network resources and spectrum. The use of multi-homing by deploying multi-interface connectivity at the wireless edge of the network has become increasingly prominent. One of the most widely adopted, practically implemented multihoming techniques is Multipath TCP (MPTCP). With successful deployments of MPTCP by some wireless operators aggregating diverse wireless access technologies such as LTE and Wi-Fi, the use of MPTCP has been considered a base feature for 5G.
Multipath TCP (MPTCP)
Traditional TCP is a single-path protocol. An established TCP connection is bound to a specific IP address between the communicating nodes. The wireless industry was motivated to come up with MPTCP because all next-generation networks are multipath (where mobile devices have multiple wireless interfaces), data centers have multiple paths between servers, and multihoming has become the norm.
MPTCP, a proxy-based aggregation solution led by Internet Engineering Task Force (IETF), is simply an overlay network to the underlying IP network. MPTCP is an extension of traditional TCP, ensuring application compatibility (i.e., the ability to run applications on MPTCP that run on TCP) and network compatibility (i.e., the ability to operate MPTCP over any Internet path where TCP operates). MPTCP allows multiple paths to be used simultaneously by a single transport connection.
MPTCP in 5G
MPTCP is now an integral part of 5G mobile networks as a standard feature of 3GPP Release 16. The 3GPP 5G mobile core features Access Traffic Steering, Switching and Splitting (ATSSS) and has officially standardized on MPTCP as a foundational capability. ATSSS allows operators to direct traffic through certain access networks, switch traffic across access networks and aggregate traffic over multiple access networks. Continuous user experience with higher throughout is delivered as the mobile device moves around and among access network technologies such as 5G NR, Wi-Fi and others. The following diagram illustrates how ATSSS is integrated into the 5G mobile core and 5G mobile device.
The user equipment (UE), or mobile device, contains the MPTCP client and ATSSS rules, which instruct the UE how to configure and execute MPTCP operations. The 5G core User Plane Function (UPF) contains the MPTCP proxy. Traffic from applications is directed to the UPF, which then invokes multi-path traffic management toward the UE. 5G RAN and WLAN access networks are portrayed above to carry separate MPTCP traffic flows. The UE provides measurement reports to the UPF such that switching, or traffic aggregation balance decisions made by the UPF, can be done with UE input. This completes the MPTCP user traffic management plane.
The Unified Data Management (UDM) contains the mobile subscriptions, which includes ATSSS as a subscribed feature. The Policy Control Function (PCF) applies policy to traffic flows arranged under the MPTCP user plane as managed by the Session Management Function (SMF).
In summary, MPTCP will be a fully integrated and standard feature within 3GPP Release 16. MPTCP implementation can be enhanced with dual connectivity, software-defined networking and segment routing.
MPTCP with 5G Dual Connectivity (DC)
Introduced in 3GPP Release 15, DC is a feature that allows data exchange between mobile devices and the NR base station, with simultaneous connection to an LTE base station when tight interworking is established between LTE and the 5G NR base station.
The current DC architecture does not support backup and packet duplication to address the latency and out-of-order packet delivery issues with DC. The existing DC algorithm needs enhancements to dynamically select the best available path for a given radio condition considering the ongoing traffic and congestion levels to optimally use each radio link.
MPTCP—composed of path manager, schedular and congestion control mechanism—can address these issues. By integrating MPTCP with the DC and 5G protocol stack to make MPTCP implementation aware of all available network interfaces, the full potential of link aggregation can be realized.
MPTCP Path Control Using Software Defined Networking (SDN)
SDN addresses the issue of out-of-order packet delivery with MPTCP when multiple radio links have varying delays by tracking the available capacity and selecting the best available path considering the varying network conditions. With an SDN-enabled network, an SDN application running on an MPTCP client can monitor data rates on connected paths to identify poor links that increase the number of packets that need reordering. The paths with relatively lower capacity can be removed from link aggregation consideration with MPTCP and can be added back with the availability of sufficiently larger capacity. Using an SDN controller, the capacity over multiple radio links can be estimated, allowing MPTCP to dynamically control the sub-flows.
MPTCP with Segment Routing (SR)
Unlike traditional routers, which forward IP packets by looking up the destination IP address in the IP header and find the best path towards the destination from the routing table, SR leverages the source-based routing model. Similar to labels in Multiprotocol Label Switching (MPLS), segment routing uses segments, which are instructions that a router executes on the incoming packet. With SR, the source router chooses a path to the destination and encodes the path in the packet header as an ordered list of instructions (segments).
The flow allocation mechanism of SDN-based MPTCP solutions increases the forwarding rules, consuming a lot of storage resources. Combining MPTCP and SR for traffic management will limit the storage requirements.
The Role of CableLabs
CableLabs is an active contributor to 3GPP Release 16 work items that leverage MPTCP via ATSSS. CableLabs has worked with our member operators to bring contributions into 3GPP that address traffic bonding to fixed customer premise equipment (CPE) and mobile devices for higher performance and service availability. Other use cases of interest include the continuous user experience across access networks. CableLabs has been active in 3GPP to drive member requirements into work items that leverage ATSSS for the sake of member priority use cases and member requirements are now part of the 5G standard in 3GPP Release 16.