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MEC多接入边缘计算及关键技术

4.2.3 Traffic steering

Traffic steering in MEC refers to the capability of the MEC system to route traffic to the targeted applications in a distributed cloud. Whilst in a generic MEC architecture as defined in [1], traffic steering is controlled by the MEC platform through configuring the data plane via the Mp2 reference point. In a 5G integrated deployment the role of the data plane is delegated to the User Plane Function (UPF). A UPF plays the central role in routing the traffic to desired applications and network functions. In addition to the UPF, there are a few related procedures specified by 3GPP that are used to enable flexible and efficient routing of the traffic to applications. One such procedure is the Application Function (AF) influence on traffic routing as described in clause 5.6.7 of [10]. It allows an AF to influence the selection and re-selection of a local UPF as well as request services to configure the rules to allow the traffic steering to a data network.

The toolset offered by a 5G network can be used by the AF, which, in the case of MEC, maps to Functional Entities (FE) of the MEC system. When a MEC application is instantiated, no traffic is routed to the application until the application is ready to receive traffic and the underlying data plane is configured to route the traffic towards it. This configuration is done by the MEC platform. When deployed in a 5G network, a MEC FE such as a MEC platform, is in the role of a 5G AF towards the 5G core network. It interacts with the PCF to request traffic steering by sending information that identifies the traffic to be steered. The PCF will transform the request into policies that apply to targeted PDU session(s) and provides the routing rules to the appropriate Session Management Function (SMF). Based on the received information, the SMF identifies the target UPF, if it exists, and initiates configuration of the traffic rules there. If no applicable UPF exists, the SMF can insert one or more UPFs in the data path of the PDU session.

In the integrated deployment as described above the data plane functionality of the (generic) MEC architecture is now the responsibility of the UPF. This UPF is influenced by MEC through control plane interactions with 5G core network functions, rather than via a specific reference point that is termed Mp2 in the MEC architecture.

The SMF can also configure the UPF with different options for traffic steering. In the case of IPv4, IPv6, IPv4v6 or Ethernet, the SMF may insert an Uplink Classifier function (UL CL) in the data path. The UL CL can be configured with the traffic rules to forward the uplink traffic towards different targeted applications and network functions, and in the downlink direction it will merge the traffic destined to the UE(s). Alternatively, for PDU Sessions using IPv6 or IPv4v6, and if supported by the UE, the SMF may use the Multi-homing concept for traffic steering. In such a case, the SMF would insert a Branching Point function in the target UPF and configure it to split the UL traffic to a local application instance and the services in the central cloud based on the Source Prefixes of the IP data packets.

The 3GPP 5G system offers a flexible framework for AFs by enabling traffic steering based on a wide set of different parameters. This allows generic traffic rule setting or specific rule setting for certain specific UE(s). The parameters that can be used in traffic steering requests may contain, for instance, information to identify the traffic (DNN, S-NSSAI, AF-Service-Identifier, 5 tuple etc.), a reference ID to preconfigured routing information, a list of DNAIs, information about target UE(s), indication about application relocation possibilities, temporal validity condition (timeframe when routing condition is valid), spatial validity condition (location of UE e.g. geographic area), notification type for user plane management notifications and AF transaction ID (enables modifications to the routing rule).

In addition to selecting the UPF and configuring the traffic steering rules, the 5G system also provides efficient tools for MEC Functional Entities, e.g. tools for a MEC platform, or a MEC orchestrator, to monitor the mobility events that relate to users of MEC application instances in local clouds. MEC FEs can subscribe to user plane path management event notifications from SMF(s), in which case it would receive notifications about path changes, e.g. when the Data Network Access Identifier (DNAI) for a particular PDU Session changes. The MEC management functions can use these notifications to trigger traffic routing configuration or application relocation procedures.

The discussion above assumes that the MEC system, with the relevant Functional Entities supported there, are trusted by the 3GPP network and that policies allow direct access from AFs to the 5G Core Network Functions. There are cases however, where a MEC FE needs to request services from the Network Exposure Function (NEF), for example when MEC is not considered trusted and the policy does not allow direct interaction with the 5G Core NFs. Also, whenever the request targets, or may target multiple PCFs, it would be required to go via the NEF.